The Prophet and the Astronomer by ጌዐዐጕ ርረዐሁዕ

Prologue

Nature from

one

thing

brings

another forth,

And out of death new life is born. LUCRETIUS

T h r o u g h o u t the history of h u m a n k i n d , we can w i t n e s s a persistent l o n g i n g to u n d e r s t a n d a n d m a k e peace w i t h the passage of t i m e . H u m a n s , l i k e all l i v i n g things, are born, g r o w to m a t u r i t y , procreate, a n d perish. But w e , alone, k n o w it. B e i n g conscious of our o w n m o r t a l ity is the u l t i m a t e m i x e d blessing. We create w o r k s of art a n d theories, have children, a n d help those in need, a t t e m p t i n g to produce a legacy that we hope w i l l survive our short lives. A n d yet, death is still the cause of m u c h despair, of cries of injustice a n d tears of pain, our final defeat by nature's p o w e r to create a n d destroy. Religions of the East a n d the W e s t help relieve the fear of d y i n g or the pain of losing a loved one by t u r n i n g death into an event transcending physical reality. S o m e d e s i g n a t e life a n d d e a t h as e q u a l l y important parts of an eternal cycle of existence; others promise eternal life in paradise for those w h o abide by their precepts. In most, r e d e m p t i o n from the specter of death does not come lightly: the transition into a n e w cycle of existence or into eternal life is often punctuated by horrendous

PROLOCUE

c a t a c l y s m s that s h a k e the very foundations of the earth and the sky. T h e D r u i d s believed in a succession of a g e s , each t e r m i n a t e d w h e n the skies fell over their heads; the Zoroastrians believed in a final j u d g ment day, w h e n those of h i g h m o r a l s t a n d a r d s w o u l d be g r a n t e d eternal life w h i l e the w i c k e d of spirit w o u l d be destroyed by floods of fire and molten metal. T h e last book of the N e w Testament, the Revelation to John, tells of the destruction of Earth and the l o w e r heavens by a succession of cosmic disasters, w h i c h include collisions w i t h " b l a z i n g stars," c a u s i n g the S u n to d a r k e n , the Moon to turn bloody red, and the stars to fall from the sky. T h e skies, as active c h a n n e l s of c o m m u n i c a t i o n between God and the people, w e r e w a t c h e d w i t h an intense m i x t u r e of anticipation and fear, since portents of the i m p e n d i n g end could a p p e a r at any m o m e n t . In this book, I explore religion's assimilation of cataclysmic cosmic p h e n o m e n a a n d its influence on scientific thought t h r o u g h the a g e s , from the pre-Socratic philosophers of ancient Greece to m o d e r n - d a y cosmologists. O u r concerns w i t h the stability of our e n v i r o n m e n t , the perpetuation of life on Earth, the reassuring sight of yet another sunrise, or the fate of the cosmos as a w h o l e , have p e r m e a t e d and inspired scientific thought since its early o r i g i n s . If h u g e asteroids did hit Earth in the past, w i l l they hit us a g a i n ? W i l l the S u n shine forever? W i l l the universe exist forever? W h a t is our place in a c h a n g i n g cosmos, w h e r e stars and solar systems a r e constantly created and d e s t r o y e d ? If Earth w i l l be destroyed, can we perpetuate our existence e l s e w h e r e in the universe? L i k e questions about the origin of the universe, those related to the fate of the cosmos a r e deeply i n t e r w o v e n w i t h religious thought. Indeed, I will a r g u e that we create a scientific w o r l d as we do a spiritual o n e â&#x20AC;&#x201D; i n order to overcome fear, to defy time, to u n d e r s t a n d our place in this w o r l d , and to justify our lives. As a research scientist, I find that m u c h of my motivation to study the physical nature of the universe comes from these "big questions," even if my e v e r y d a y routine is filled more w i t h long technical c a l c u l a tions and detailed c o m p u t e r p r o g r a m s than w i t h metaphysical speculation. One of my goals in this book is to h u m a n i z e science, to a r g u e that xii

PROLOGUE

our scientific ideas are very m u c h a product of the cultural and e m o tional e n v i r o n m e n t w h e r e they o r i g i n a t e . It is my hope that the present exploration of apocalyptic ideas in religion a n d

science will

elucidate some of the issues that polarize the science-religion debate, as well as paint a fairly complete picture of scientific thought p e r t a i n i n g to the s k i e s , from o u r o w n p l a n e t to the u n i v e r s e as a w h o l e . D r a w ing on the Book of Daniel, the Book of Revelation, and an i n v e s t i g a tion of apocalyptic sects, art, a n d literature, we will e x a m i n e the formation and evolution of the solar system, the extinction of the dinosaurs, Einstein's g e n e r a l theory of relativity, pulsars and black holes, the big b a n g a n d the i n f l a t i o n a r y u n i v e r s e , all the w a y to the latest ideas in cosmology. C e r t a i n issues are essentially m u l t i d i s c i p l i n a r y in n a t u r e , and a full appreciation of their scope r e q u i r e s that we e x a m i n e their m a n y facets. T h e end of the w o r l d is one of them. T h e m a n y a n s w e r s h u m a n s have proposed t h r o u g h o u t the a g e s a r e expressions of the s a m e universal fears a n d expectations. It is my hope that by e x a m i n i n g some of t h e m we will have a better chance of p r o l o n g i n g our presence on this planet a n d , w h e n the t i m e comes, e l s e w h e r e in this vast universe. T h i s book is w r i t t e n for the nonspecialist, a l t h o u g h I truly hope some of my m o r e specialized colleagues will be attracted by its broader nature; every scientific concept introduced is e x p l a i n e d in jargon-free l a n g u a g e a n d , w h e n e v e r possible, illustrated by i m a g e s a n d a n a l o g i e s from e v e r y d a y p h e n o m e n a ; it is my belief that the essence of their m e s sage can still be c a p t u r e d and its beauty appreciated. I hope that you the reader, after finishing this v o l u m e , w i l l a g r e e w i t h m e . I am confident your effort w i l l be w o r t h w h i l e , for the vistas opened by m o d e r n cosmology are truly magnificent. I should also e m p h a s i z e that it w o u l d be impossible to do complete justice to the rich history of apocalyptic ideas in religion and to the whole of astronomical sciences in one single v o l u m e . In order to k e e p my task (and y o u r s ! ) w i t h i n acceptable d i m e n s i o n s , I have had to exclude several d e s e r v i n g topics, and limit the level of detail w i t h w h i c h some ideas have been treated. I ask my colleagues whose w o r t h y

Mil

PROLOGUE

ideas are not included here to forgive my brevity. As a partial remedy, I a p p e n d a fairly long b i b l i o g r a p h y of popular science books dealing w i t h topics of related interest. My wish is that the w h o l e will be much b i g g e r than the s u m of its parts, and that you will share my joy in part a k i n g in our endless quest for m e a n i n g .

XIV

PART

" T h e Is

E n d N e a r ! "

CHAPTER

The Skies Are Falling / am

all-powerful

Time,

which

destroys

all things.

— B HA CAVAD

GlTA,

I. 3 2

We a r e creatures bound by time. O u r lives have a b e g i n n i n g a n d an end, a finite period of t i m e , w h i c h we hasten to chop into e q u a l segm e n t s — y e a r s , m o n t h s , d a y s , h o u r s — i n the vain hope that t h r o u g h this frantic c o u n t i n g we can s o m e h o w control its passage. But t i m e a l w a y s has the upper h a n d : w e d o g r o w older a n d w e d i e , not k n o w i n g w h e n , not k n o w i n g how. T h i s w e l l - k n o w n fact, w h i c h m a n y people m a y s i m p l y brush aside as obvious, some as too d i s t u r b i n g , or others as just depressing, is the single most f u n d a m e n t a l aspect of our existence. It is w h a t g i v e s m e a n i n g t o being h u m a n . D e a t h gives rise to our y e a r n i n g for p e r m a n e n c e , to our constant s t r u g g l e to create, be it a p a i n t i n g or a family, a m a t h e m a t i c a l theorem or a n e w recipe, s o m e t h i n g that w i l l stay after we g o , s o m e t h i n g beyond the m e r e m e m o r y of our existence in the m i n d s of our friends or relatives. M e m o r i e s fade from g e n e r a t i o n to g e n e r a t i o n . A few y e a r s a g o , w h i l e e x p l o r i n g some forgotten corners of my p a r e n t s ' attic, I b u m p e d into m y g r a n d p a r e n t s ' photo a l b u m s , p a c k e d w i t h h u n d r e d s

THE

PROPHET

AND

THE

ASTRONOMER

of y e l l o w e d photographs of their parties, relatives, friends, celebrations, and speeches, frozen m o m e n t s of a t i m e long gone. " A l l ghosts now," my brother R o g e r i o q u i p p e d , in his u n i q u e sardonic style. L o o k ing at the pictures, I w o n d e r e d how m u c h of that l a u g h t e r , of that w i s d o m , of their m a n y stories, is still alive in the m i n d s of their g r e a t - g r a n d c h i l d r e n . F e e l i n g l i k e the m i s s i n g l i n k of a four-generation c h a i n , I closed the a l b u m s w i t h a d e e p sense of sadness, of h a v i n g lost a part of my o w n history, n o w buried in photos I can't recognize. But w a i t ! Close to the photo a l b u m s there w a s a l a r g e box, m a d e of g o l d e n cardboard. Inside I found dozens of letters my g r a n d p a r e n t s w r o t e to each other, to their relatives in U k r a i n e a n d Russia, to my parents w h e n my father w a s s t u d y i n g in Boston in the early 1950s. E x t r e m e l y excited, I a s k e d L e n o r e Grenoble, a friend in the linguistics d e p a r t m e n t â&#x20AC;&#x201D; a specialist in Eastern European a n d S l a v i c l a n g u a g e s â&#x20AC;&#x201D; to h e l p me translate the letters. But my initial excitement q u i c k l y t u r n e d into disappointment; the letters w e r e painfully boring, full of endless details of e v e r y d a y life. No deep existential m e s s a g e , no deep secret revealed, nothing! It d a w n s on me just h o w selfish we the l i v i n g can be. I w a s not trying to get to k n o w my ancestors better; the letters a n d photos could have helped me w i t h that. W h a t I really w a n t e d w a s to get to k n o w m y s e l f better t h r o u g h t h e m . After all, their history is my history, their lives part of m i n e , w h e r e I g r e w u p , w h o my parents w e r e . But we can't expect the past to define our future completely. Our ancestors' lives a n d lessons m a y teach a n d g u i d e , but we are the ones w h o must m a k e choices. We search for m e a n i n g , for help, for c o m p a n i o n s h i p . We need s o m e t h i n g m o r e than just m e m o r i e s a n d d r e a m s : w e need hope. Perhaps we can defeat the passage of t i m e by e l e v a t i n g ourselves to a s u p e r n a t u r a l level, by t r a n s c e n d i n g life itself. In fact, if we beat t i m e , we m a y be once m o r e w i t h our l o n g - d e p a r t e d loved ones, for it is w h e n time's passage is suspended that life a n d d e a t h m e r g e , a n d the d e a d can coexist w i t h the l i v i n g ; i n i m m o r t a l i t y w e become g o d l i k e . T h u s , w e create the infinite a n d the eternal. Belief soothes a n d justifies. It 4

THE

SKIES

ARE

FALLING

inspires us all, the painter, the teacher, the scientist, the priest, the l a w y e r . A s Saul Bellow once wrote, " W e a r e all d r a w n t o w a r d the same craters of the s p i r i t — t o k n o w w h a t we are a n d w h a t we are for, to k n o w our purpose, to seek g r a c e . " O u r creations in pursuit of the eternal a r e m a n y . In this book we w i l l e x a m i n e some of the w a y s we h u m a n s have attempted to defy our t i m e - b o u n d existence, inspired by a c o m m o n l i n k : the m i x of terror a n d a w e of the s k y above us. Because of the sacred character that all cultures a n d religions attributed to the skies, celestial p h e n o m e n a w e r e often v i e w e d as a manifestation of d i v i n e power, a channel t h r o u g h w h i c h the gods c o m m u n i c a t e d w i t h us. A n d the n e w s from above could be good or bad. In m a n y religions, signs of i m p e n d i n g doom or p u n i s h m e n t often come from the skies, be they celestial objects t h r o w n by a n g r y deities on our homes a n d l a n d , b l a n k e t s of thick d a r k n e s s in the m i d d l e of the day, or floods that d r o w n all but a few chosen ones. In m o r e e x t r e m e apocalyptic texts, falling celestial objects a n n o u n c e the end of all terrestrial life, the end of all e n d s , w h i c h w i l l bring eternal bliss for the virtuous a n d eternal suffering for the sinful. Science, since its origins, has also been inspired by the sky a n d its m y s t e r i e s . F r o m Plato to Einstein, m a n y of the greatest philosophers a n d scientists have studied the sky not just for practical purposes but in an attempt to b r i n g the h u m a n m i n d closer to that of the Creator, the Great C o s m i c O r g a n i z e r , believing that k n o w l e d g e of the natural w o r l d lifted h u m a n k i n d to a h i g h e r m o r a l sphere. T h e pursuit of this k n o w l e d g e t h r o u g h reason w a s t u r n e d into a passionate quest, w o r t h y of a lifelong devotion. As a consequence, our a c c u m u l a t e d scientific k n o w l e d g e o f celestial p h e n o m e n a drove a w a y m a n y ancient s k y related fears a n d beliefs. But in spite of all this progress or, better still, because of it, m a n y n e w c h a l l e n g e s have a p p e a r e d , a n d w i l l k e e p a p p e a r i n g . A scientist m a y refer to the continuous presence of the " m y s t e r i o u s " a s the a m a z i n g creativity o f n a t u r e o r — m o r e c y n i c a l l y — as an expression of our o w n limitations as rational beings, w h i l e a religious person m a y call it a manifestation of the infinite n a t u r e of God. By e x p l o r i n g thus our ageless relationship w i t h the sky, w h e t h e r 5

THE

PROPHET

AND

THE

ASTRONOMER

t h r o u g h faith or reason, or both, we w i l l find that religion a n d science represent different, but c o m p l e m e n t a r y , facets of our s t r u g g l e against t i m e , born of the s a m e q u e s t i n g spirit. T h i s chapter begins w i t h a survey of apocalypticism, d r a m a t i c narratives of the end of the w o r l d a c c o r d i n g to several r e l i g i o n s , from that of the D r u i d s to Christianity. T h e s e timeless stories provide the bare-bones i m a g e r y that w i l l r e a p pear t h r o u g h o u t this book. I call t h e m "archetypes of doom."

Celestial Messages " S u r e l y , our s h a m a n k n e w w h a t h e w a s doing. Every day, h e w o u l d r u n up to the m o u n t a i n , lifting his a r m s t o w a r d the skies a n d c h a n t i n g the sacred h y m n s of our elders, the ones that b r o u g h t us health a n d a plentiful harvest. He k n e w how to talk to the gods t h r o u g h the l a n g u a g e of the night, w r i t t e n in the bright stars a n d the Moon. W h e n m a n y stars fell, the g o d s w e r e s h e d d i n g d i a m o n d tears, s a d d e n e d by our lack of devotion to them. If a hairy star a p p e a r e d one n i g h t and stayed for m a n y moons, s o m e t h i n g bad w o u l d happen, m a y b e even the death of our k i n g . Or worse, a g i a n t sky-serpent could eat the S u n , b r i n g i n g o n eternal night. W e w o u l d b r i n g fruits a n d a n i m a l s a n d clothes to the top of the sacred m o u n t a i n , w h i c h w a s the place closest to our gods, a n d dance a n d chant for as long as our s h a m a n told us to. T h e m o u n t a i n connected the earth a n d the sky." T h i s fictional n a r r a t i v e , w h i c h combines e l e m e n t s of m a n y different ancient cultures w i t h o u t b e i n g specific to any one in particular, is a short allegorical tale of our ancestors' m y s t e r i o u s relationship w i t h the s k i e s . T h e g o d s decide h o w things w i l l be, but w e can plead w i t h t h e m a n d perhaps even c h a n g e their m i n d s if we k n o w h o w to interact w i t h t h e m , speak their l a n g u a g e . T h e s h a m a n , the holy m a n , is the interpreter of the gods, the decipherer of their intentions as w r i t t e n in the skies. H i s actions go both w a y s , from the g o d s to the tribe a n d from the tribe to the gods. He possesses the k n o w l e d g e needed to u n d e r s t a n d the gods, being the b r i d g e to the u n k n o w n . As such, he is m o r e than

THE

SKIES

ARE

FALLING

h u m a n , existing in a w o r l d s o m e w h a t parallel to ours, a m a g i c reality h o v e r i n g between the n a t u r a l and the s u p e r n a t u r a l . As I tried to illustrate in this short n a r r a t i v e , the s h a m a n ' s m a i n power arises from his " a b i l i t y " to read the skies, the m e s s a g e s the gods send t h r o u g h the stars a n d other celestial apparitions. ( T h e " h a i r y star" is supposed to represent a comet, the serpent e a t i n g the S u n , a solar eclipse.) T h u s , the skies are v i e w e d as a sort of holy scroll, w h i c h the gods use to c o m m u n i c a t e w i t h people via the s h a m a n . We are not q u i t e sure exactly w h e n this tradition started, but it is clear that d i v i n a t i o n based on the skies w a s a l r e a d y prevalent w i t h the Babylonians w e l l before 2000 B.C.E. Astrology w a s a translation of this scroll, describing h o w the skies influence crops a n d i n d i v i d u a l s , b r i n g on a plentiful harvest or a flood, victory in w a r s or the death of a k i n g . T h e r e g u l a r i t y of the h e a v e n l y motions provided the basis for a coherent interpretation of the seasons, their cyclic repetition b u i l d i n g a sense of t r a n q u i l i t y a n d control. T h e S u n returns every day, the full Moon in about 28 d a y s , the s u m m e r solstice in 365 d a y s , a n d so on. But the skies w e r e not a l w a y s predictable. Inexplicable celestial events did happen, a n d i n v a r i a b l y brought terror to the population. T h i s is true for the o v e r w h e l m i n g majority of c u l t u r e s t h r o u g h o u t history. In the Old Testament, we can find some very e a r l y e x a m p l e s of this " s k y - l i n k e d " terror in the Book of Exodus, w h i c h tells the story of the captivity of the J e w s a n d their escape from Egypt a r o u n d 1300 B.C.E., u n d e r the g u i d a n c e of Moses. T w o of the ten p l a g u e s sent by God to force the pharaoh into freeing the J e w s are directly related to the skies. T h e seventh p l a g u e b r o u g h t a storm of hail a n d t h u n d e r that spread fire a n d destruction t h r o u g h o u t Egypt: " T h e r e w a s hail w i t h fire flashing c o n t i n u a l l y in the midst of it, such heavy hail as has never fallen in all the l a n d of E g y p t since it b e c a m e a nation" (Exodus 9:24). T h e ninth p l a g u e brought a thick b l a n k e t of d a r k n e s s that p a r a l y z e d the w h o l e k i n g d o m : "So Moses stretched out his h a n d t o w a r d heaven, and there w a s dense d a r k n e s s in all the land of Egypt for three d a y s " (Exodus 10:22). L e a v i n g aside the fruitless debate of w h a t truly h a p pened w i t h Egypt's w e a t h e r d u r i n g those t i m e s , it is clear that to the

THE

PROPHET

AND

THE

ASTRONOMER

a u t h o r s of Exodus the skies w e r e symbolically l i n k e d to the punitive actions of God. T h i s l i n k is m o r e i m p o r t a n t than a n y speculation about the physical causes of those events, in p a r t i c u l a r w h e n we r e a l i z e that they d r e w on a text w r i t t e n some six h u n d r e d y e a r s after the events took place. T h e m e a n i n g here is in the symbol, not in the facts. Because of their s e e m i n g unpredictability, comets, eclipses, and meteor s h o w e r s w e r e interpreted as m e s s a g e s from the gods, either a d m o n i s h i n g a people or a n n o u n c i n g a tragic c o m i n g event; m o r e often than not, we fear w h a t we don't u n d e r s t a n d . R e l i g i o n s a n d countless folkloric tales tend to link such apparitions w i t h possible d i s asters that follow or precede t h e m , from a k i n g ' s death to the end of the w o r l d . T h e first s u r v i v i n g reference to a comet, from a C h i n e s e sentence from the fifteenth century B.C.E., connects its a p p e a r a n c e w i t h a series of m u r d e r s c o m m i t t e d by a political leader: " W h e n C h i e h e x e cuted his faithful counselors, a comet a p p e a r e d . "

A n o t h e r C h i n e s e text, from three h u n d r e d y e a r s later, also relates a comet to the d e a d l y actions of a political leader: " W h e n K i n g W u w a n g w a g e d a p u n i t i v e w a r a g a i n s t K i n g C h o u , a comet a p p e a r e d w i t h its tail pointing t o w a r d the people of Y i n . "

Reference to comets, from about the s a m e t i m e , can be found as

F I G U R E I : Excerpt from the first catalog of comets, ca. 300 B.C.E., the Mawangdui sill{.

THE

SKIES

ARE

FALLING

well in a few B a b y l o n i a n fragments produced d u r i n g the reign of N e b u c h a d n e z z a r I: " W h e n a comet reaches the path of the S u n , Gan-ba 3

will b e d i m i n i s h e d ; a n uproar will happen t w i c e . . . . " W h a t e v e r "Ganba" m e a n t to the B a b y l o n i a n s , it clearly w a s not a good thing to have d i m i n i s h e d . Mournful events on Earth w e r e thus m a g i c a l l y m i r r o r e d by celestial p h e n o m e n a . H o w e v e r , these accounts of d r a m a t i c events l i n k e d to comets are fairly m i l d c o m p a r e d w i t h their role in a p o c a l y p tic n a r r a t i v e s , prophetic stories about the end of all ends, the t i m e w h e n our i n d i v i d u a l histories are blended into a single universal history. F e a r of the skies a n d fear of the i m p e n d i n g end have been l i n k e d for thousands of years. F a r t h e r to the west, the fabled D r u i d s , religious leaders of some Celtic tribes that spread across Europe starting a r o u n d the sixth century B.C.E., supposedly believed the end w o u l d come from the skies. One of the most quoted R o m a n sources on the D r u i d s comes from G a i u s J u l i u s C a e s a r (100-44 B.C.E.), the g r e a t R o m a n g e n e r a l a n d dictator w h o c o n q u e r e d G a u l a n d tried to b r i n g the C e l t s , w i t h the exception of those in Ireland, into the pax Romana. In book 4 of his De Bello Gallico, C a e s a r w r o t e , " T h e D r u i d s officiate at the w o r s h i p of the gods, r e g u l a t e public a n d private sacrifices, a n d g i v e r u l i n g on all religious questions. L a r g e n u m b e r s of y o u n g m e n flock to them for instruction, 4

and they are held in g r e a t honour by their people." T h e D r u i d s are portrayed as the r e l i g i o u s priesthood of the C e l t s , noble a n d proud, but also as " b a r b a r i c " a n d " s a v a g e , " no doubt because of their central role in performing h u m a n sacrifices. C a e s a r w a s also impressed by the D r u i d s ' belief in i m m o r t a l i t y , w h i c h he c l a i m e d e x p l a i n e d the famed Celtic bravery in battle: "A lesson w h i c h they t a k e p a r t i c u l a r pains to inculcate is that the soul does not perish, but after death passes from one body to another; they think that this is the best incentive to bravery, because it teaches m e n to d i s r e 5

g a r d the terrors of d e a t h . " S u c h an attitude c l e a r l y illustrates how r e l i g i o u s belief in the i m m o r t a l i t y of the soul serves as an antidote to our helplessness in the face of the inevitability of death. Since death m a r k s the end of our physical existence, a n y attempt at o v e r c o m i n g it w i l l 9

THE

PROPHET

AND

THE

ASTRONOMER

have to i n v o k e a s u p e r n a t u r a l reality that posits some sort of i m m o r t a l ity, be it of the soul or of the body itself, as w i t h the E g y p t i a n s . M o d e r n religions, of course, a r e no exception. C a e s a r m e n t i o n s the D r u i d s ' k n o w l e d g e of a n d interest in the skies: " T h e y also hold long discussions about the h e a v e n l y bodies a n d their m o v e m e n t s , the size of the U n i v e r s e a n d of the Earth, the physical constitutions of the w o r l d a n d the p o w e r s a n d properties of the gods."

M o d e r n evidence seems to indicate that the C e l t s or the D r u i d s did not build S t o n e h e n g e , the a w e - i n s p i r i n g c i r c u l a r m o n u m e n t of h u g e u p r i g h t stones in S a l i s b u r y P l a i n , southern E n g l a n d , or others l i k e it spread

around

Great

Britain,

dating

back

about

3000

B.C.E.

A l t h o u g h the true identity of the b u i l d e r s of these m e g a l i t h i c structures r e m a i n s a mystery, they w e r e incorporated into Celtic c u l t u r e , as astronomical observatories a n d sacred places for rituals. T h e perfect a l i g n m e n t of the S u n w i t h strategically placed stones at the s u m m e r solstice shows that their positions w e r e not accidental. T h e stones clocked the motions of the skies, the r e a l m of the g o d s , a n d w e r e thus at once useful and holy. T h e s e m o n u m e n t s can be thought of as a reenactment on Earth of the celestial motions, a place w h e r e a parallel m a g ical reality w a s m a d e real t h r o u g h ritual. Given the C e l t s ' a t t i t u d e t o w a r d life after death, a n d the connection they m a d e b e t w e e n the sky a n d m a g i c , w e m i g h t w o n d e r w h a t they feared the most. A c c o r d i n g to A r r i a n , a G r e e k w h o lived a r o u n d 100 C.E., this question w a s a s k e d by none other than A l e x a n d e r the Great, w h e n he met the Celts on the b a n k s of the D a n u b e in 335â&#x20AC;&#x201D;334 B.C.E. " W e fear only that the sky w i l l fall on our heads," w a s the solemn 7

a n s w e r by the Celtic chieftains. T h e apocalyptic i m a g e r y of the Celts, as reconstructed by f r a g m e n t a r y evidence, envisioned the end of the w o r l d as initiated by the collapse of the sky itself, followed by fire a n d w a t e r s w a l l o w i n g up the earth a n d the destruction of all m e n . A n e w heaven a n d earth a n d a n e w race of m e n w o u l d then reappear. T h e s e i m a g e s , found in m a n y other invocations of the end, depict n a t u r e runn i n g w i l d , l e a v i n g h u m a n k i n d completely helpless a n d terrified by its o v e r w h e l m i n g p o w e r a n d , more important, r e n d e r i n g physical life

THE

SKIES

ARE

FALLING

impossible. A n e w k i n d of life is necessary, a s u p e r n a t u r a l life, an e x i s tence beyond physical time.

The Bridge to Eternity Several of the l i n k s connecting sky a n d doom that later played a crucial role in J e w i s h a n d C h r i s t i a n apocalyptic n a r r a t i v e s o r i g i n a t e d in the ancient civilizations of the M i d d l e East. B a b y l o n i a n cataclysmic floods, w h i c h cleansed h u m a n k i n d of evil, as they d i d in the story of Noah; a paradisiacal l a n d for d e s e r v i n g E g y p t i a n souls k n o w n as the enchanted fields; or the Zoroastrian concept of the end of t i m e at the j u d g m e n t d a y â&#x20AC;&#x201D; t h e s e are but a few e x a m p l e s of the belief in an afterlife, often h e r a l d e d by g r e a t destruction. W i t h o u t being e x h a u s t i v e , we w i l l follow some of these ideas, w h i c h presaged the g r a n d apocalyptic visions of the Book of Daniel and the Book of Revelation. No other c u l t u r e has been so careful w i t h its d e a d bodies as the E g y p t i a n ; a happy afterlife d e p e n d e d on the body's state of preservation, a n d that explains w h y the E g y p t i a n s took m u m m i f i c a t i o n techniques

seriously.

Initially,

only

the

pharaohs,

being

divine

themselves, w e r e entitled to a happy afterlife. H o w e v e r , d u r i n g the b r e a k d o w n of p h a r a o n i c authority ( 2 2 0 0 - 2 0 0 0 B.C.E.), the belief g r e w that an afterlife existence w a s the right of most, a d e m o c r a t i z a t i o n of i m m o r t a l i t y . It w a s in this period that one of the oldest, if not the oldest, detailed conception of the afterlife b e c a m e very popular, that of the k i n g d o m of Osiris, the lord of the d e a d . T h e k i n g d o m ' s location w a s uncertain; some accounts placed it loosely in the north, others in S y r i a , and still others even m o r e remotely, in the M i l k y W a y , "the g r e a t w h i t e N i l e of the sky." T h e k i n g d o m w a s a paradisiacal place, w h e r e the l a n d w a s fertile, the g r a i n g r e w h i g h , and the chosen ones could sit u n d e r the trees, w a t c h i n g their slaves do all the w o r k , w h i l e they played g a m e s , and ate a n d d r a n k w i t h their s m i l i n g friends a n d spouses. Of course, not everyone w a s g r a n t e d access to such a pleasant (perhaps a bit b o r i n g ? ) existence:

THE

PROPHET

AND

THE

ASTRONOMER

j u d g m e n t h a d to be passed before the soul w a s a l l o w e d into these "enchanted fields." As is depicted in an illustration from the Book of the Dead a n d m a n y other funeral p a p y r i , Osiris himself presided over the trial, a cross-examination of just how virtuous the soul h a d been (see figure 1 in insert). A n u b i s , the j a c k a l - h e a d e d g u a r d i a n god of the cemetery, b r i n g s the soul to Osiris, w h o sits stoically on his throne, flanked on one side by Isis a n d on the other by their sister N e p h t h y s . T h e soul i m m e d i a t e l y recites all the t h i n g s it didn't do: I k n e w no w r o n g , I d i d no evil thing. . . . I a l l o w e d no one to hunger. I caused no one to w e e p . I d i d not m u r d e r . I d i d not c o m m a n d to m u r d e r

I d i d not c o m m i t a d u l t e r y . . . . I d i d not

load the w e i g h t of the balances. . . . I d i d not interfere w i t h the god in his p a y m e n t s . I am purified four times, I am p u r e . . . .

T h i s negative confession is repeated forty-two t i m e s , to each of the j u d g e s representing the provinces of Egypt. A n u b i s then places the d e a d person's heart on a scale, w e i g h i n g it against an ostrich feather. If the soul is light, it is a l l o w e d to enter the enchanted fields. O t h e r w i s e , retribution is swift. In some n a r r a t i v e s , it is destroyed by a strange being, w i t h the head of a crocodile a n d a body that is a h y b r i d of a lion a n d a hippopotamus. In other accounts, it is t h r o w n into a fiery hell, w h e r e it suffers terrible p u n i s h m e n t s . We can identify several moral notions that reappear in m a n y other religions: the dos a n d don'ts of a righteous existence, the j u d g m e n t of the soul by the gods, a n d r e w a r d in paradise or p u n i s h m e n t a n d torture in a fiery hell. Over a period of more than t w o thousand y e a r s , a n d after m a n y cycles of invasion a n d d o m i n a t i o n , there w a s no doubt w i d e s p r e a d religious cross-fertilization in the M i d d l e East. But for a clear ethical distinction between good a n d evil, as w e l l as several other basic e l e m e n t s that w o u l d be crucial parts of the J e w i s h , C h r i s t i a n , and Islamic faiths, we must consider a religion that arose in the eastern l i m its of the M i d d l e East, Zoroastrianism.

THE

SKIES

ARE

FALLING

Zoroaster, w h o s e n a m e is a G r e e k corruption of the ancient Iranian n a m e Z a r a t h u s t r a , "the one w h o p l o w s w i t h c a m e l s , " w a s the son of a l a n d o w n e r w h o lived in w h a t is today the region a r o u n d western A f g h a n i s t a n a n d Iran. H i s birth is loosely d a t e d to 660 B.C.E. or so, a l t h o u g h there is g r e a t uncertainty; some sources date it m u c h earlier, a r o u n d 1000 B.C.E., w h i l e others place it as late as the first half of the sixth century B.C.E. A c c o r d i n g to tradition, he w a s a k i n d a n d goodn a t u r e d y o u n g m a n w h o d e c i d e d , at the a g e of twenty, to leave his parents

and

the

wife

they

chose

for

him,

of r e l i g i o u s

e n l i g h t e n m e n t . G r e e k sources indicate that, frustrated by the a n s w e r s from people he met in his travels, Zoroaster spent seven y e a r s l i v i n g in complete silence a n d isolation in a cave (extended to t w e n t y y e a r s a n d eating only cheese by later R o m a n sources!) until the m a g i c m o m e n t of revelation a r r i v e d , w h e n he w a s thirty. In his vision, the a r c h a n g e l Vohu M a n a h (Good T h o u g h t ) , a p p e a r i n g as a figure " n i n e t i m e s as l a r g e as a m a n , " invited h i m to leave his body behind a n d travel as a d i s e m b o d i e d soul, to meet A h u r a M a z d a (the W i s e L o r d ) a n d his court of a n g e l s . A h u r a M a z d a then instructed him on the doctrines a n d intricacies of the true r e l i g i o n , p r o c l a i m i n g Zoroaster to be his prophet. Zoroaster's religion w a s a m o r a l l y r i g i d m o n o t h e i s m , in w h i c h A h u r a M a z d a w a s the only true God. M a z d a w a s the creator of the w o r l d a n d of m a n , s u p r e m e in p o w e r a n d in v a l u e : according to Zoroaster, a l t h o u g h not to some of his followers, Lord M a z d a w i l l e d all t h i n g s into being. Because the traditions of Zoroastrianism w e r e transmitted orally for almost e i g h t h u n d r e d y e a r s (written versions of the Avesta, their sacred text, w e r e first produced only d u r i n g the third or fourth century C.E.), there is considerable discussion as to w h a t the o r i g i n a l teachings of Zoroaster w e r e . Nevertheless, we k n o w that he believed in the constant conflict b e t w e e n good a n d evil as a pervasive aspect of m a n a n d n a t u r e . T h i s conflict o r i g i n a t e d w h e n M a z d a created the w o r l d , g i v i n g freedom of choice to its h u m a n inhabitants. An i n d i v i d u a l can choose his o w n path, in e x e r t i n g his free w i l l . T h i s is a brilliant solution to the problem of evil: it shifts the responsibility of goodness from God to 13

THE

PROPHET

AND

THE

ASTRONOMER

m a n , w i t h o u t d i m i n i s h i n g his omnipotence. M a n ' s soul is thus the seat of a constant battle b e t w e e n good a n d evil. But people must be a w a r e of the consequences of their choices: evil does not go u n p u n i s h e d . For the "Day of F i n a l J u d g m e n t " w i l l come, w h e n M a z d a w i l l finally triu m p h over evil. As w i t h the C e l t s , there w a s here a conception of the end of times, an eschatology rich w i t h i m a g e r y that r e a p p e a r e d in later apocalyptic n a r r a t i v e s . A c c o r d i n g to Zoroaster, in the end of d a y s the d e a d will be resurrected a n d , together w i t h the l i v i n g , subjected to an ordeal of fire a n d floods of molten m e t a l . But w h i l e the evil ones w i l l be b u r n e d a n d destroyed by the rivers of molten m e t a l , the good a n d virtuous w i l l have nothing to fear, the incandescent metal feeling l i k e cool m i l k to their s k i n s â&#x20AC;&#x201D; a moral l a v a . In another version, each soul must be j u d g e d i n d i v i d u a l l y shortly after death, a n d must a w a i t its destiny until the end of time. T h e soul is to pass t h r o u g h the " B r i d g e of the Separator," w h i c h crosses over the abyss of hell, stretching o u t w a r d t o w a r d p a r a dise. D u r i n g the crossing, the soul's list of deeds is read a n d j u d g m e n t is passed. If good p r e v a i l s over evil, the "pointing of the h a n d " (probably M a z d a ' s ) will be t o w a r d paradise; o t h e r w i s e , the h a n d w i l l point d o w n , to the fiery hell below. T h i s version of the end of t i m e is perhaps the first apocalyptic n a r rative w i t h a single a n d final end, not followed by other b e g i n n i n g s or rebirths, as is the case in flood stories or in religions such as H i n d u i s m , w h e r e the w o r l d is re-created in cycles. Zoroastrianism i n a u g u r a t e s a n e w era of religious thought w i t h a linear history, the w o r l d h a v i n g one b e g i n n i n g a n d one end. T h e end of t i m e becomes also the end of the conflict between the t w o opposites, the final a n d eternal victory of good over evil. I m m o r t a l i t y is achieved both by the righteous a n d by the sinful, the first l i v i n g forever in p a r a d i s e w h i l e the others a r e tortured forever in hell. T h i s u n f o r g i v i n g n a t u r e of Zoroastrianism, w i t h its e m p h a s i s on eternal bliss or d a m n a t i o n , w o u l d be t a k e n over by C h r i s t i a n i t y ' s v i e w s of the end. We create gods so that we can aspire to an abstract ideal of goodness, w h i c h we call sacred. T h i s w a s as true for the followers of

THE

SKIES

ARE

FALLING

Zoroaster as it is for m a n y of us today. It is our w a y of d i r e c t i n g our lives a n d g i v i n g m e a n i n g to our actions; civilization w o u l d have been impossible w i t h o u t such an ideal. But this i m p u l s e t o w a r d the sacred is also our w a y of t r a n s c e n d i n g ourselves. For after we create g o d s we aspire to be l i k e them. H u m a n s a r e spiritual beings, constantly y e a r n ing to establish a relationship w i t h the m y s t e r i o u s , the u n k n o w n . If you a r e an atheist, m a y b e you enjoy horror movies, or other fictional connections w i t h the s u p e r n a t u r a l . If you are an atheist w h o hates a n y thing remotely related to the s u p e r n a t u r a l , m a y b e you w o r s h i p y o u r tennis match or the football g a m e on S u n d a y s , or the constant quest for perfection at sports or at w o r k , w h i c h , as a consequence, becomes e n s h r i n e d w i t h ritualistic details. M u c h of religious ritual is an i m i t a tion of the actions of g o d s or God, an attempt to reach the s u b l i m e , a pursuit of the e x t r a o r d i n a r y , of the u n r e a l , of the eternal. J e w s rest on the Sabbath because their L o r d rested on the seventh day, w h i l e C h r i s tians w a s h someone's feet on M a u n d y T h u r s d a y , the T h u r s d a y before Easter, just as Jesus w a s h e d the disciples' feet. By being l i k e g o d s , we suspend the passage of time, joining the eternal. It is an ironic p a r a d o x that the very consciousness that g i v e s us our a w a r e n e s s of the passage of t i m e , a n d thus m a k e s us h u m a n , needs to defeat this a w a r e n e s s . O u r k n o w l e d g e that life has a b e g i n n i n g a n d an end, that it is b r a c k e t e d w i t h i n a short period of time, pulls us in t w o opposite directions; either to accept death as the end of existence or to transcend it. T h i s , in a nutshell, is the essence of the h u m a n p r e d i c a m e n t â&#x20AC;&#x201D; t o k n o w there is an end to physical life.

Dreams of Perfection Early G r e e k philosophers despised the pantheon of g o d s adored by the people; they w e r e too h u m a n , suffering from too m a n y m o r a l frailties. H o w could one aspire to be l i k e that? At a r o u n d the sixth century B.C.E., a search for the One, for the f u n d a m e n t a l reality of things, c h a n g e d forever the intellectual history of the West. T h a l e s saw e v e r y -

THE

PROPHET

AND

THE

ASTRONOMER

t h i n g as water, A n a x i m e n e s as air, H e r a c l i t u s as fire, a n d A n a x i m a n d e r as an abstract, i n t a n g i b l e substance that p e r m e a t e d

reality. Plato

believed in the D e m i u r g e , the rational cosmic artisan, as an absolute principle of pure Good, w h o stood above all other m i n o r deities. H i s illustrious disciple Aristotle, at least in his earlier w o r k , did a w a y w i t h all m i n o r gods a n d deities, but needed to postulate the existence of the P r i m e Mover, the initiator of all motions in the cosmos, w h i l e h i m s e l f b e i n g devoid of motion. ( S o m e call h i m the U n m o v e d Mover.) T h u s , we see in ancient Greece a move t o w a r d a m o n i s m , side by side w i t h that of the Zoroastrians in Iran a n d that of another S e m i t i c tribe of the M i d d l e East, the J e w s . T h e G r e e k philosophers aspired to a rational perfection, w h i l e the others aspired to a m o r a l perfection. H o w e v e r , these t w o parallel monistic currents forked into t w o very different paths, a n d e l e m e n t s of Plato's a n d Aristotle's theological ideas w e r e incorporated into a monotheistic religion only w i t h the a d v e n t of C h r i s t i a n i t y , a few centuries later. A s i d e from its influence on C h r i s t i a n theology, G r e e k monistic t h o u g h t is of g r e a t i m p o r t a n c e to the d e v e l o p m e n t of m o d e r n science, in p a r t i c u l a r after the rediscovery of G r e e k texts in Europe d u r i n g the R e n a i s s a n c e , courtesy of the A r a b s . M o d e r n physical science is built upon an abstract structure based on f u n d a m e n t a l l a w s that dictate the behavior of n a t u r a l systems. T h e s e l a w s a r e discovered both by inspect i o n â&#x20AC;&#x201D; t h a t is, by a p a i n s t a k i n g c o m p a r i s o n of data w i t h m a t h e m a t i c a l m o d e l s â&#x20AC;&#x201D; a n d by d e d u c t i o n , w h i c h starts w i t h a theoretical foundation a n d then m a k e s specific predictions s u b s e q u e n t l y contrasted a n d c o m pared w i t h observational d a t a . One then a r r i v e s at a m o d e l , resting on a specific set of f u n d a m e n t a l l a w s , that describes a host of n a t u r a l phen o m e n a . T h i s p r o c e d u r e is self-correcting a n d self-improving: m o d els, a n d the l a w s they a r e based upon, are c o n t i n u a l l y s c r u t i n i z e d by the scientific c o m m u n i t y , the goal b e i n g the explicit u n d e r s t a n d i n g of their l i m i t a t i o n s a n d w e a k n e s s e s . Once the scientific c o m m u n i t y a r r i v e s at a consensus, the model is accepted and used, w i t h i n its l i m i t s of validity, to describe as m a n y p h e n o m e n a as it possibly can. For e x a m p l e , the three l a w s of motion, as stated by Isaac N e w t o n in 16

THE

his

1687

book

Mathematical

Principles

SKIES

ARE

FALLING

of Natural Philosophy,

accurately

describe motions at speeds f a m i l i a r in our e v e r y d a y life, such as those of cars, a i r p l a n e s , a n d spaceships. H o w e v e r , they fail to describe motions close to the speed of light (186,000 m i l e s per second), or on very small spatial scales, such as those of m o l e c u l e s or a t o m s . Newton's l a w s of motion, a n d a n y other l a w of n a t u r e , have been s h o w n to be valid since the d a w n of t i m e (or a l m o s t ) and a n y w h e r e in the universe; as such, they a r e said to be universal l a w s . Science, at its most fundamental level, can be thought of as the pursuit of these l a w s , w h i c h describe different levels of physical reality, from the microscopic to the macroscopic. B e i n g a true heir of G r e e k m o n i s m , science a i m s at an abstract rational perfection, a l t h o u g h its practitioners k n o w that true perfection is u n a t t a i n a b l e ; a continuous process of self-refinement propels science f o r w a r d . To a scientist, nothing could be m o r e central than to p a r t a k e in this ritual of discovery. H e n c e , a clear parallel between the g o a l s of science a n d those of religion e m e r g e s from this discussion. Both a i m at s o m e t h i n g beyond the o r d i n a r y , an abstract ideal of perfection, w h i c h transcends the h u m a n d i m e n s i o n . A n d yet, science cannot provide the emotional comfort that religion provides to so m a n y people, w h i l e religion cannot provide the rational d i m e n s i o n that science provides. T h e fact that they have s o m e t h i n g in c o m m o n does not m e a n that they have the s a m e purpose. You don't ask a physicist or a chemist for solace w h e n s o m e one y o u love d i e s , at least not as a professional, a n d you don't (or shouldn't) ask a rabbi or a priest for explanations of q u a n t u m m e c h a n ics (unless, of course, the rabbi or the priest is a physicist, a l w a y s a possibility). We need both pursuits, a n d closing our door to either leads only to confusion. Just as it is absurd to say that Earth is six thousand y e a r s old, it also does not m a k e sense to say that science has all the a n s w e r s , or even that it is capable of h a v i n g all the a n s w e r s . S o m e q u e s tions simply don't belong to science.

THE

PROPHET

AND

THE

ASTRONOMER

Visions of D o o m and R e d e m p t i o n T h e m a n y apocalyptic t h e m e s a n d i m a g e s found i n ancient B a b y l o n i a n , E g y p t i a n , a n d Zoroastrian beliefs c r y s t a l l i z e in the Book of D a n i e l , w r i t t e n w h e n the J e w s w e r e suffering terrible r e l i g i o u s persecution from the S y r i a n s d u r i n g the second century B.C.E. C o n s i d e r e d to be the first truly apocalyptic text, the Book of Daniel powerfully describes the i m p e n d i n g end as seen by the prophet in visions c h a r g e d w i t h sociopolitical s y m b o l i s m . It also offers a clear l i n k b e t w e e n doom a n d the d i s ruption of the sky a n d its n a t u r a l order, w h i c h w o u l d reappear in several C h r i s t i a n apocalyptic texts. As we w i l l see, some of the i m a g e r y found in the Book of Daniel w a s used d u r i n g the seventeenth century by preachers a n d n a t u r a l philosophers a l i k e , as they i n v o k e d the fear a n d the science of the "End of T i m e . " Before I discuss the apocalyptic i m a g e r y of the text itself a n d its celestial l i n k s , it is i m p o r t a n t to survey the historical b a c k g r o u n d that prompted its w r i t i n g . In 332 B.C.E., A l e x a n d e r the Great defeated D a r i u s III of Persia, i n i tiating G r e e k e x p a n s i o n i s m t o w a r d A s i a a n d Africa. S u r r o u n d e d by H e l l e n i c c u l t u r e , the J e w s of Palestine a n d the Diaspora responded in different w a y s ; w h i l e some w e r e excited by G r e e k theater, philosophy, sports, a n d poetry, others w e r e disturbed by this foreign influence. T h e G r e e k s w e r e q u i t e tolerant of J e w i s h religion; some even participated in their religious rites, p l a c i n g the Jewish god Y a h w e h (or Iao, as they called h i m ) side by side w i t h Zeus a n d Dionysus in their list of high deities. T h e J e w i s h ritual, w i t h its absence of statues a n d e m p h a s i s on oratory, m u s t have s e e m e d to t h e m a variation on a philosophical debate, the s y n a g o g u e as a substitute for the school of philosophy. A l e x a n d e r ' s early death in 323 B.C.E. led to one h u n d r e d y e a r s of invasions by rulers of his prior provinces, the S e l e u c i d s ( S y r i a n s ) and the Ptolemies ( E g y p t i a n s ) . In 198 B.C.E., the S e l e u c i d s finally w o n control a n d t h i n g s w e r e peaceful for a w h i l e , until K i n g A n t i o c h u s IV Epiphanes, w h o r e i g n e d from 175 to 164 B.C.E., d e c i d e d to unleash a policy of r e l i g i o u s intolerance, i m p o s i n g the cult of Zeus on the b e w i l 18

THE

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d e r e d J e w s . In D e c e m b e r 167, A n t i o c h u s erected an a l t a r to Zeus in the T e m p l e of J e r u s a l e m , o r d e r i n g different sacrifices to be performed there, such as the k i l l i n g of pigs, to the horror of the J e w s . T h e t u r n i n g point c a m e w h e n an e l d e r J e w i s h priest n a m e d M a t t a t h i a s w a s forced by a S y r i a n c o m m i s s i o n e r to participate in a sacrifice for Zeus in the v i l l a g e of M o d e i n . M a t t a t h i a s ended up k i l l i n g the commissioner, u n l e a s h i n g a strong revolt against the S y r i a n d o m i n a t i o n . T h e p r o m i n e n t figure in this r e m a r k a b l e story w a s one of M a t tathias's five sons, J u d a s M a c c a b e u s , whose b a n d s of J e w i s h soldiers, to the complete astonishment of the S y r i a n s , defeated four of their a r m i e s a n d forced a fifth to retreat. In 165 B.C.E., J u d a s m a n a g e d to reconquer J e r u s a l e m , c l e a n s i n g the T e m p l e from the H e l l e n i c " a b o m i n a t i o n s " a n d rescuing J u d a i s m in Palestine. T h e r e followed a period of peace a n d i n d e p e n d e n c e that lasted until 63 B.C.E., w h e n a civil w a r between different factions a m o n g the J e w s , the S a d d u c e e s a n d the Pharisees, ushered in the R o m a n d o m i n a n c e of Palestine. It w a s d u r i n g the reign of Antiochus IV Epiphanes, a period of great chaos a n d pain to the Jewish people, that the Book of Daniel w a s written, probably between 167 a n d 164 B.C.E. In it we find the first of several J e w ish apocalyptic texts that became quite popular between 150 B.C.E. and 100 C.E. Although it is difficult to trace the exact roots of the rich and fantastic i m a g e r y used in the Book of Daniel, it has been a r g u e d that m a n y of the elements of Jewish apocalypticism display Zoroastrian influences.

T h e Book of D a n i e l , it is clear, w a s w r i t t e n as a m o r a l e booster to the J e w s , an antidote to their sufferings u n d e r a despotic a n d castrating foreign ruler. It is a book of hope, of final t r i u m p h for those w h o r e m a i n loyal to their faith and to their God t h r o u g h difficult times. Daniel appears as a special person, a visionary capable of d e c i p h e r i n g d r e a m s a n d of s u r v i v i n g in a den of lions for a w h o l e night. H i s p o w e r s come from his faith, from his devotion to the one a n d only God. T h e first six chapters are set in the past, d u r i n g the B a b y l o n i a n rule, a n d demonstrate

how

tyrants

who

place

themselves

above

God

are

destroyed by their o w n vanity, a lesson also t a u g h t in Exodus: no one can confront the J e w i s h God.

THE

PROPHET

AND

THE

ASTRONOMER

After the first six chapters established Daniel's p o w e r s a n d his intim a t e a l l i a n c e w i t h God, the final six turn to the apocalyptic. In chapter 7, Daniel sees God, "the A n c i e n t One took his throne, his clothing w a s w h i t e as snow, and the hair of his head l i k e pure wool; his throne w a s fiery flames, a n d its w h e e l s w e r e b u r n i n g fire" (7:9). God w a s surr o u n d e d by fire a n d by a court of thousands w h o served a n d stood by h i m . " T h e court sat in j u d g m e n t , a n d the books w e r e o p e n e d " (7:10). D a n i e l then sees

one l i k e a h u m a n b e i n g c o m i n g w i t h the clouds of heaven. . . . T o h i m w a s g i v e n d o m i n i o n a n d g l o r y a n d k i n g s h i p , that all peoples, nations, a n d l a n g u a g e s should serve h i m . H i s d o m i n ion is an e v e r l a s t i n g d o m i n i o n that shall not pass a w a y , a n d his k i n g s h i p is one that shall never be destroyed. ( 7 : 1 3 - 1 4 )

We can easily discern e l e m e n t s of the apocalyptic n a r r a t i v e s of the Zoroastrians, w i t h God a n d his court passing j u d g m e n t a n d a m e s sianic figure i n a u g u r a t i n g a n e w a g e of peace a n d prosperity for the people of Israel. In chapter 8, D a n i e l relates another vision, of a fierce battle b e t w e e n the r a m ( M e d e s a n d Persians) a n d the goat (Greek e m p i r e ) . After the goat destroys the r a m , four horns sprout from its h e a d (the dissolution of A l e x a n d e r ' s e m p i r e ) a n d one of them (the reign of A n t i ochus IV E p i p h a n e s ) " g r e w as h i g h as the host of heaven. It threw down to

the earth

some of the host and some of the stars,

and trampled on

them "

(Daniel 8:10, my italics). T h i s i m a g e , l i n k i n g the disruption of political a n d cosmic order, is a recurrent one in apocalyptic n a r r a t i v e s . Stars a n d other celestial objects tend to come d o w n from the skies in t i m e s of trouble. T h e vision e n d s w i t h the a r c h a n g e l Gabriel r e a s s u r i n g Daniel that this evil k i n g w i l l "be b r o k e n , a n d not by h u m a n h a n d s . " T h e book concludes w i t h the prediction of the fall of Antiochus's k i n g d o m a n d the final r e d e m p t i o n of the people of Israel. T h i s revelation comes to Daniel from a s u p e r n a t u r a l narrator, w i t h "body of beryl, face l i k e l i g h t n i n g , eyes l i k e f l a m i n g torches, a r m s a n d legs l i k e the g l e a m of 20

THE

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b u r n i s h e d bronze, a n d the sound of his w o r d s l i k e the roar of a m u l t i t u d e " (10:6). T h i s narrator tells Daniel of the a r r i v a l of M i c h a e l , the g r e a t prince, the protector of y o u r people. . . . T h e r e shall be a t i m e of a n g u i s h , such as has never occurred since nations first c a m e into existence. But at that t i m e y o u r people shall be d e l i v e r e d , e v e r y o n e w h o is found w r i t t e n in the book. M a n y of those w h o sleep in the dust of the earth shall a w a k e , some to everlasting life, a n d some to s h a m e a n d everlasting contempt. (12:1â&#x20AC;&#x201D;2)

T h i n g s w i l l thus get worse before they can get better. T h e day of j u d g m e n t w i l l also be the d a y of the resurrection, w h e n the d e a d w i l l arise a n d join the l i v i n g in either eternal bliss or eternal d a m n a t i o n (see figu r e 2 in insert). T h e story has a tragic twist: peace a n d redemption can come to the J e w s only t h r o u g h a s u p e r n a t u r a l action, w h e n God h i m self w i l l t a k e m a t t e r s into his h a n d s a n d destroy the e n e m i e s of Israel. As the text m a k e s clear, this can happen only at the end of history, the end of time as we k n o w it, w h e n the dead a n d the l i v i n g can coexist. T h e r e is no salvation in " r e a l " t i m e . Daniel is a b r i d g e to d i v i n e k n o w l e d g e , the interpreter of t h i n g s to come. If the visions are fantastic a n d o t h e r w o r l d l y , it is because they m u s t be so; no religious i m a g e r y based on concrete reality w o u l d have such an effect on people's i m a g i n a t i o n . After all, we are s p e a k i n g of events w i t h an epic m o r a l scope, the perpetual end to suffering, the final vindication of goodness. It is the end of h u m a n existence as we k n o w it, the b e g i n n i n g of the reunion of m a n w i t h God, as it h a d been long ago, w h e n God a n d m a n w a l k e d side by side, before the expulsion from the G a r d e n of Eden. T h e apocalyptic n a r r a t i v e foretells the t r a n s formation of the faithful a n d virtuous into g o d l i k e creatures a n d the devious a n d perverse into d e v i l - l i k e ones. It m u s t have been very h a r d to resist its a l l u r i n g power; to m a n y people, it still is. T h e n a r r a t i v e invited people to participate in a g r a n d scheme laid d o w n by God h i m self, to e n d u r e evil times as they w e r e short, a n d never to g i v e up hope

THE

PROPHET

AND

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ASTRONOMER

that vindication w a s at h a n d a n d that it w o u l d be definitive. In fact, put into context, people's sufferings w e r e to be v i e w e d as a necessary part of the w h o l e thing, g i v i n g it m e a n i n g , p l a c i n g it w i t h i n the unfolding of the events to come. As Saint L a c t a n t i u s w r o t e a few centuries later, "virtue is the ability to e n d u r e h a r d t h i n g s . " T h e m o r e y o u can e n d u r e suffering, the m o r e v i r t u o u s you a r e , a n d the more w o n derful w i l l be your r e w a r d s in the hereafter. W h a t a g r e a t recipe for social p a r a l y s i s ! E n d u r e y o u r suffering w i t h virtue a n d g r a c e , have faith, a n d God w i l l t a k e care of the rest for you. It is no w o n d e r that these concepts w e r e elevated to absurd levels d u r i n g the M i d d l e A g e s , w h e n pestilence, despair, a n d lack of l e a d e r s h i p reached absurd levels. T h e d a r k e r the t i m e s , the m o r e people need hope, the m o r e they need to have faith in their r e d e m p t i o n . T h e d a r k e r the t i m e s , the m o r e v u l nerable people are, the m o r e r e a d i l y they fall prey to blind fanatic i s m â&#x20AC;&#x201D; a tendency that should not be overlooked in our o w n day.

R e v e l a t i o n s o f the E n d T h e apocalyptic ideas put forward in the Book of D a n i e l percolated across several other J e w i s h texts a n d also inspired early C h r i s t i a n i t y , w h e n it too w a s a persecuted sect. T h e R o m a n occupation of Palestine, w h i c h started in 63 B.C.E., e n d e d t r a g i c a l l y in 70 C.E. w i t h the destruction of the Second T e m p l e in J e r u s a l e m a n d a n e w exile of the J e w s . T h e reversal o f w h a t w a s a n u n u s u a l l y tolerant r e l i g i o u s policy c a m e after a radical J e w i s h resistance g r o u p k n o w n as the Zealots openly opposed the R o m a n d o m i n a n c e , h o l d i n g the R o m a n a r m i e s back for four y e a r s . T h e Zealot resistance w a s e v e n t u a l l y o v e r t h r o w n by the R o m a n e m p e r o r Vespasian, the T e m p l e w a s b u r n e d to the g r o u n d , a n d , t o complete the h u m i l i a t i o n , J e r u s a l e m w a s r e n a m e d A e l i a C a p i tolina. By that time, t h o u g h , another religion w a s q u i c k l y s p r e a d i n g t h r o u g h Palestine a n d beyond, based on the t e a c h i n g s of a n e w prophet, Jesus of N a z a r e t h . T h e t u r n i n g point in Jesus' life c a m e w h e n he w a s about thirty. 22

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( T h e r e is an uncertainty of a few y e a r s because of contradictory d a t i n g in the Gospels.) An ascetic k n o w n as John the Baptist, probably from the h i g h l y spiritual J e w i s h sect k n o w n as Essene, w a s s u m m o n i n g the masses: "Repent!

For the K i n g d o m of H e a v e n has come n e a r ! "

( M a t t h e w 3:2). John w a s convinced that the e n d w a s i m m i n e n t a n d that only those w h o repented their sins w o u l d stand a chance w h e n j u d g m e n t c a m e . T h e r e w a s g r e a t expectation a m o n g the J e w s that the Messiah w a s c o m i n g , a n d that he w a s a n o r m a l m a n , a descendant of K i n g D a v i d , founder of J e r u s a l e m . Receptivity is k e y to the success of any religion. People from all over Palestine c a m e to listen to John. T h o s e w h o chose to follow h i m w e r e then bathed in the river J o r d a n , to w a s h a w a y their sins, an Essene purification rite. John instructed his followers to share their clothes a n d food w i t h those in need a n d to be g e n e r o u s to their neighbors. It w a s after his baptism that Jesus h a d his vision: " | T h e j h e a v e n s w e r e opened to h i m a n d he s a w the Spirit of God d e s c e n d i n g l i k e a dove and a l i g h t i n g on h i m . A n d a voice from heaven said, ' T h i s is my Son, my beloved Son, w i t h w h o m I am w e l l p l e a s e d ' " ( M a t t h e w 3:16â&#x20AC;&#x201D;17). John's apocalyptic m e s s a g e lies at the very heart of C h r i s t i a n i t y a n d w a s preached by Jesus himself, a l t h o u g h w i t h some i m p o r t a n t c h a n g e s . For Jesus, r e d e m p t i o n w a s open to all; the k i n g d o m of the F a t h e r w a s for all righteous m e n , not only J e w s . We should k e e p in m i n d that the first of the Gospels, that of M a r k , w a s w r i t t e n some seventy y e a r s after Jesus' d e a t h , a r o u n d 30 C.E., a n d that distortions of deeds a n d s a y i n g s , as w e l l as later editorial c h a n g e s , must have t a k e n place. It is not u n l i k e l y that the conflict b e t w e e n Jesus and J e w s w a s d u l y e x a g g e r a t e d in later texts, for p r o p a g a n d i s t i c reasons. T h i s w o u l d certainly have w i d e n e d the split b e t w e e n J u d a i s m a n d a n e w e m e r g i n g religion, attracting those u n h a p p y w i t h the J e w i s h religious l e a d e r s h i p a n d its q u a r r e l i n g factions. All

three

synoptic

Gospels

(from

synoptikps,

"seen

together,"

M a t t h e w , M a r k , a n d L u k e ) i n c l u d e Jesus' description o f the a r r i v a l o f the "End of the A g e , " w h e n the Messiah w i l l come d o w n from heaven in a cloud. In all three, the signs of the end a r e clearly cosmic signs, doom c o m i n g from the skies: " I m m e d i a t e l y after the suffering of those

THE

PROPHET

AND

THE

ASTRONOMER

d a y s the sun will be d a r k e n e d , a n d the moon w i l l not g i v e its light; the stars will fall from heaven, a n d the powers of heaven w i l l be s h a k e n " ( M a t t h e w 24:29). Essentially the same descriptions can be found in M a r k 13:24â&#x20AC;&#x201D;25 a n d in L u k e 21:25, a l t h o u g h L u k e is less explicit about specific cosmic events. T h e apocalyptic n a r r a t i v e from the Book of Daniel a n d other intertestamental texts a n n o u n c i n g the end of time a n d the a r r i v a l of the Messiah is c o m b i n e d , at the very birth of C h r i s tianity, w i t h signs of doom c o m i n g from the skies. T h e s e prophecies create a state of constant anxiety w i t h r e g a r d to cosmic events; every shooting star, every eclipse, every comet or unexpected celestial event m a y be interpreted as being part of the d o o m s d a y prophecy, the harb i n g e r of the end to come. As a result, people look up to the sky w i t h a m i x t u r e of hope a n d terror, c o m m e n s u r a t e w i t h the level of social despair; in quiet times (no doubt q u i t e r a r e ! ) , the skies a r e u s u a l l y less threatening. For those w i t h a g u i l t y conscience, celestial events g i v e life to the s a y i n g "God is w a t c h i n g y o u , " w a r n i n g that y o u r sins w i l l not pass u n p u n i s h e d . For the suffering pious, the skies offer the hope of a better afterlife a n d the celestial signs are e a g e r l y a w a i t e d . In either case, God's message of doom a n d hope is w r i t t e n in the stars. Eschatological v i e w s m a d e a p p e a r a n c e s in several of the Apostolic letters (for e x a m p l e , in 2 Peter 3:3â&#x20AC;&#x201D;13 a n d in 1 John 2:18), but reached an absolute c l i m a x in the Revelation to John, the last book in the N e w Testament canon. (In fact, its inclusion in the N e w T e s t a m e n t w a s a very controversial issue in the e a r l y church.) A c c o r d i n g to historians of religion, Revelation w a s probably w r i t t e n a r o u n d 95 C.E. by a prophet n a m e d John, the leader of a C h r i s t i a n circle in Ephesus, on the western coast of T u r k e y . After the fall of J e r u s a l e m in 70 C.E., the teachings of Jesus q u i c k l y spread across the western a n d northern M e d i t e r r a n e a n coast all the w a y to R o m e , t h r o u g h the courageous actions of Apostles such as Paul, B a r n a b a s , a n d m a n y others. T h e s e d e p a r t u r e s from the "old faith," that is, the respected J u d a i s m , w e r e looked upon as trouble by the R o m a n s , a manifestation of impiety. For t h e m , to c l a i m that Plato, Aristotle, and A l e x a n d e r the Great w e r e sons of a god w a s q u i t e acceptable. But to c l a i m that a Jew w h o d i e d a terrible death in a far-

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ARE

FALLING

a w a y land w a s not only the son of a god, but the Son of God, w a s sheer lunacy. As a result, there w a s w i d e s p r e a d persecution a n d severe p u n ishment of C h r i s t i a n s at least since 64 C.E., d u r i n g Nero's rule, w h i c h included exile or death. " T h e C h r i s t i a n s to the l i o n s ! " w a s a c o m m o n cry. Quite possibly, John's p r o m i n e n c e saved h i m from being fed to the [ions, a n d he w a s banished to the island of P a t m o s , near Ephesus, his birthplace. Revelation w a s w r i t t e n as a response to the R o m a n a n t i - C h r i s t i a n sentiment a n d its threat to the survival of the c h u r c h . L i k e previous apocalyptic texts, it e n c o u r a g e s the faithful to e n d u r e the persecutions and the sufferings, e x h o r t i n g m a r t y r d o m w h i l e p r o m i s i n g the a r r i v a l f the end, w h e n justice w i l l prevail a n d goodness be vindicated. H o w ever, the l a n g u a g e and symbols used by John, s o m e t i m e s q u i t e beautiful a n d poetic a n d s o m e t i m e s absolutely terrifying, are by far the most powerful found in apocalyptic n a r r a t i v e s . T h e s u b l i m e is m i x e d w i t h the horrific, w i t h the clear intent of scaring a n d m o v i n g people, to a w a k e n them to the C h r i s t i a n faith—if, that is, they a r e interested in eternal salvation. T h e d o c u m e n t is brilliant p r o p a g a n d a , still h o l d i n g its p o w e r as the b a n n e r of m a n y apocalyptic sects today, sometimes with tragic consequences. After c o n g r a t u l a t i n g some churches for their w o r k a n d c e n s u r i n g others for slackness, John proceeds to relate a series of visions in three cycles, each w i t h a set of seven symbols (specifically, the seven seals of the scroll, chapters 6—7; the seven a n g e l s b l o w ing their t r u m p e t s , chapters 8-10; the seven b o w l s of God's w r a t h , chapters 15—16). T h e n a r r a t i v e is interspersed w i t h visions of God on his celestial throne, of horrible beasts of all sorts r a m p a g i n g the earth, of violent cosmic c a t a c l y s m s , of the battle b e t w e e n the a r c h a n g e l Michael and the d r a g o n ( S a t a n ) , a n d of the victory of C h r i s t over the Antichrist, i n a u g u r a t i n g Christ's t h o u s a n d - y e a r k i n g d o m on Earth (the doctrine k n o w n as c h i l i a s m ) . After this period, S a t a n is a g a i n liberated from the "bottomless pit," a n d the final battle b e t w e e n good a n d evil t a k e s place, m a r k i n g the end of t i m e a n d the a p p e a r a n c e of a n e w heaven and a n e w earth, the N e w J e r u s a l e m . Every one of the visions is punctuated by cosmic c a t a c l y s m s , s i g n i 25

THE

PROPHET

AND

THE

ASTRONOMER

fying not only the absolute p o w e r of God over n a t u r e but also his use of nature's p o w e r to express his w r a t h . In chapter 6:12â&#x20AC;&#x201D;14, w h e n the L a m b (Jesus) opened the sixth seal, "there c a m e a great e a r t h q u a k e ; the sun b e c a m e black as sackcloth, the full moon b e c a m e l i k e blood, a n d the stars of the sky fell to the earth as the fig tree drops its w i n t e r fruit w h e n s h a k e n by a g a l e " (see figure 3 in insert). After the seventh seal w a s opened, an a n g e l "took the censer [he w a s h o l d i n g in front of God's throne] a n d filled it w i t h fire from the altar a n d t h r e w it on the earth; a n d there w e r e peals of thunder, r u m b l i n g s , flashes of l i g h t n i n g , a n d an e a r t h q u a k e " (Revelation 8:5). T h e i m a g e of a censer filled w i t h fire being t h r o w n on Earth is strongly evocative of a comet or a l a r g e shooting star. T h e n a r r a t i v e then shifts from the seven seals to the seven t r u m p e t s , all b l o w n by a n g e l s , one at a time. Each s o u n d i n g of a t r u m p e t b r i n g s a m o r e horrific disaster on Earth, in an attempt to coerce the sinful to repent. W h e n "the third a n g e l b l e w his t r u m p e t , a g r e a t star fell from heaven, b l a z i n g l i k e a torch, a n d it fell on a third of the rivers a n d on the springs of w a t e r " (8:10). H e r e we have another description inspired by a falling celestial object c a u s i n g terrible d a m a g e on impact. A l l cosmic events associated w i t h apocalyptic n a r r a t i v e s w e r e unpredictable; in the cultures that g e n e r a t e d t h e m , there w a s a n a t u r a l division of celestial p h e n o m e n a into the cyclic, the ones they k n e w w o u l d return r e g u l a r l y , a n d the r a n d o m , the ones that a p p e a r e d suddenly. C o m e t s a r e s o m e w h a t peculiar here, since m a n y of t h e m do reappear cyclically, albeit u s u a l l y w i t h q u i t e long orbital periods. T h e most famous of our cometary visitors, H a l l e y ' s comet, has a period of a p p r o x i m a t e l y seventy-six y e a r s (see figure 4 in insert). To predict the return of a comet w i t h o u t k n o w l e d g e of the w o r k i n g s of g r a v i t y r e q u i r e s the existence of a sustained astronomical culture for prolonged periods of t i m e , s o m e t h i n g quite rare in antiquity. W i t h the exception of the B a n t u - K a v i r o n d o people of Africa, the idea that comets do in fact reappear at r e g u l a r intervals h a d to w a i t until the e a r l y eighteenth century, w h e n E d m o n d H a l l e y applied Isaac N e w ton's theory of universal g r a v i t a t i o n to c o m e t a r y motion. A n d , as we w i l l see, even then c o m e t a r y science w a s deeply e n m e s h e d in r e l i g i o u s

THE

SKIES

ARE

FALLING

d o o m s d a y rhetoric, apocalyptic fears, a n d apocalyptic science interw o v e n by popular c u l t u r e . T h e n a t u r e of w h a t constitutes a terrifying celestial apparition c h a n g e d over the ages; the m o r e science could q u a n t i t a t i v e l y describe the skies, the less terrifying several apparitions became. T h u s , eclipses are n o w predictable to the second, comets a r e observed sometimes y e a r s before they become visible w i t h the n a k e d eye (or even w h e n they a r e n ' t ) , and meteors, bits of rock that fall onto the earth a n d are ignited by friction w i t h the atmosphere, a.k.a. shooting stars, if not predictable, inspire m o r e w o n d e r than fear. Unless, of course, they a r e big bits of rock, too big to be vaporized by the a t m o s p h e r e and end up hitting the g r o u n d , in w h i c h case they a r e called meteorites. T h e s e " r a n d o m " celestial events invariably found a place in religious apocalyptic n a r r a tives. Since the skies w e r e controlled by the gods or by God, they had to be c o n v e y i n g a d i v i n e m e s s a g e . A n d since God u s u a l l y d i d not interfere w i t h w o r l d affairs unless things w e r e g o i n g badly, these messages w e r e , for the most part, signs of a n g e r or u p c o m i n g trouble. T h i s notion carved itself deeply into the psyche of the W e s t e r n w o r l d . Even though the final intent of apocalyptic n a r r a t i v e s w a s the promise of justice and peace to the faithful, the skies w e r e t i n g e d w i t h fear. T h e passage into eternal life or d a m n a t i o n w a s to be h e r a l d e d by celestial chaos: hell comes from the heavens.

CHAPTER

Heaven's Alarm to the W o r l d What

if this present

were

the

worlds

last

â&#x20AC;&#x201D; JOHN

night? DONNE

( 1 5 7 2 - 1 6 3 1 )

T h e title of this chapter comes from a 1681 sermon by Increase M a t h e r ( 1 6 3 9 - 1 7 2 3 ) , president of H a r v a r d C o l l e g e a n d "teacher of a church in Boston in N e w E n g l a n d . "

Increase w a s the son of the P u r i t a n

R i c h a r d Mather, w h o sailed for A m e r i c a in 1635 to preach w h a t he could not at h o m e , a n d father of the famous missionary Cotton Mather. He firmly believed that unexpected cosmic events, a n d the c a l a m i t i e s preceding a n d following t h e m , w e r e messages from a very u n h a p p y God to the c r o w d of sinners d o w n below. In fact, the full title of the sermon reads,

Heavens Alarm to the

World.

Or A SERMON Wherein is shewed, T h a t Fearful S i g h t s a n d S i g n s i n H e a v e n A r e the Presages of g r e a t C a l a m i t i e s at h a n d .

THE

PROPHET

AND

THE

ASTRONOMER

T h e message of the sermon w a s clear: to u r g e m e m b e r s of the c o n g r e gation to repent their sins, because cosmic signs of the end w e r e revealing themselves as prophesied in several biblical texts, from J e r e m i a h to Joel to Revelation, w h i c h he quoted profusely a m i d his i n f l a m e d rhetoric. A believer in the literal m e a n i n g of the Bible's w o r d s , M a t h e r s a w the skies as a stage w h e r e God enacted his w r a t h . T w o y e a r s after his 1681 sermon, he published his Cometography, an encyclopedic s u r v e y of all recorded (and some i n v e n t e d ) comets, "from the B e g i n n i n g of the 2

W o r l d unto this present year, 1683," a n d their associated c a l a m i t i e s . T h u s , in 984 C.E., five y e a r s before the passage by H a l l e y ' s comet, "a B l a z i n g S t a r w a s seen; a n E a r t h q u a k e , W a r s , P l a g u e , F a m i n [ e ] s fol3

l o w e d , the E m p e r o r a n d the Pope d i e d . " In 1005 C.E.,

a B l a z i n g Star horrible to behold w a s seen f l a m i n g in the H e a v e n in the S p r i n g t i m e for the space of thirteen d a y s a n d then a g a i n in October. T h e r e followed the most fearful P l a g u e c o n t i n u i n g for three y e a r s . In some places w h i l e s t the l i v i n g w e r e b u r y i n g the d e a d , they d r e w their last breath, a n d w e r e t h r o w n into the G r a v e w i t h those w h o m they i n t e n d e d to leave there.

C o u n t l e s s variations on these basic t h e m e s m a y be found in the Reverend M a t h e r ' s survey. T h e comet of 1680, the one that no doubt inspired his sermon, a p p e a r e d , as D o n a l d K. Y e o m a n s r e m a r k e d in his chronological history of comets, d u r i n g "the zenith of c o m e t a r y superstition": "in G e r m a n y alone there w e r e n e a r l y 100 tracts published. Of 5

these, only four w e r e w r i t t e n to q u i e t superstitious fears." A m e d a l struck in G e r m a n y to c o m m e m o r a t e the a p p e a r a n c e of the c o m e t carried this inscription: " T h e star threatens evil t h i n g s : trust in God w h o 6

w i l l turn t h e m to g o o d . " T h e stern R e v e r e n d M a t h e r w a s not a l o n e in his beliefs. In the late seventeenth century, comets provoked the same fears as t w o thousand y e a r s before; they w e r e t a k e n to be signs of i m p e n d i n g doom, of disease a n d d e a t h of i m p o r t a n t r u l e r s , of w a r s a n d famines, of chaotic n a t u r a l events beyond anyone's control. T h u s , in

HEAVEN'S

ALARM

THE

WORLD

the m i n d s of most people, comets w e r e not a c c i d e n t a l l y correlated w i t h these tragic events: they w e r e sent d o w n from heaven as messengers of an a n g r y God. T h e y w e r e religious entities, p e r f o r m i n g a function prescribed by sacred texts. In other w o r d s , comets w e r e not n a t u r a l , but supernatural, phenomena. Inspiring as it w a s to preachers of doom, the comet of 1680 w a s d e s tined to become a t u r n i n g point. In 1687, Isaac N e w t o n established a q u a n t i t a t i v e method devised for c o m p u t i n g the motions of celestial objects, following observations of this very comet. In the next chapters, we w i l l discuss the scientific i m p o r t a n c e a n d repercussions of N e w t o n ' s theory of g r a v i t a t i o n to cometary science a n d to astronomy in g e n e r a l . But n o w I w a n t to explore a m o r e obscure topic, n a m e l y , N e w t o n ' s motivations for s t u d y i n g the physics of the heavens. N e w t o n spent a l a r g e a m o u n t of t i m e t r y i n g to date biblical events. Not just d a t i n g them, but, as one m i g h t expect from N e w t o n , precisely d a t i n g t h e m . He w a n t e d to quantify prophecy, fitting it w i t h i n history. In his Observations lished

upon

the

Prophesies

posthumously

of Daniel and the Apocalypse 1733,

wrote

that

John,

pub-

prophecies

of St.

offer

"convincing a r g u m e n t that the w o r l d is ruled by p r o v i d e n c e . " T h a t is, w i t h the proper q u a n t i t a t i v e a p p r o a c h â&#x20AC;&#x201D; t h e science he w a s inventi n g â&#x20AC;&#x201D; i t w a s possible to actually interpret the Bible as a c a l e n d a r of events, those in the past a n d those still to come, as revealed by God to the prophets. T h e c l i m a x of this l i n e a r unfolding of prophecies w a s , if not as an exact prediction at least as a certainty, the eventual a r r i v a l of the j u d g m e n t day. I confess my surprise w h e n I first encountered this side of N e w t o n ' s w o r k . After a l l , how could the m a n w h o seems the e m b o d i m e n t of rationality a n d cool scientific j u d g m e n t be involved in pursuits that so clearly m i x science w i t h superstition? T h e a n s w e r is that, for N e w t o n , as for m a n y (but not a l l ) of his contemporaries, there w a s no clear-cut distinction b e t w e e n science a n d religion. In his view, the scientist's (or, to use the more a p p r o p r i a t e term for the t i m e s , the n a t u r a l philosopher's) search for a q u a n t i t a t i v e description of n a t u r a l p h e n o m e n a w a s part of a g r a n d e r quest, that of d e c i p h e r i n g God's plan, or m i n d : the

THE

PROPHET

AND

THE

ASTRONOMER

scientist w a s a decoder of God's w r i t i n g . No w o n d e r N e w t o n has been 8

called "the last of the m a g i c i a n s . " A n d yet, N e w t o n w a s careful not to publicize his w o r k on biblical chronology or alchemy. W a s he afraid of exposing h i m s e l f to the ridicule of some of his less m y s t i c a l l y m i n d e d colleagues? Quite possibly. It is certainly w e l l - k n o w n that N e w t o n had a g r e a t aversion to a c a d e m i c disputes, partly because of his character a n d partly because of some painful experiences early in his career. I can only i m a g i n e the e n o r m o u s satisfaction a n d moral e n c o u r a g e m e n t N e w t o n extracted from his secret labors, no doubt believing that w h e n the end c a m e his efforts w o u l d be more than vindicated. A l t h o u g h over fifteen centuries had passed since John of Patmos received a n d d u l y recorded his Revelation, in N e w t o n ' s time belief in the end w a s as strong as ever. E x t r e m e l y resilient, these beliefs d i d not disappear w i t h the onset of the so-called E n l i g h t e n m e n t of the e i g h teenth century, w h e n r a t i o n a l i s m reigned s u p r e m e , a l t h o u g h m a n y people considered them foolish old superstitions. A n d they certainly haven't d i s a p p e a r e d today. Just think of the proliferation of apocalyptic sects throughout the w o r l d , s o m e t i m e s w i t h tragic consequences, as in the 1993 incident between the Branch D a v i d i a n s a n d the U . S . g o v e r n m e n t in W a c o , Texas, or in the 1997 suicide of t h i r t y - n i n e m e m b e r s of the Heaven's Gate order in S o u t h e r n California. Or the all-pervasive apocalyptic i m a g e r y in the m e d i a , referring to real or fictional threats such as a nuclear holocaust, alien invasions, collisions w i t h asteroids a n d comets, or global w a r m i n g . Not to mention g e n e r a l i z e d end-ofc i v i l i z a t i o n - a s - w e - k n o w - i t social panics, such as the so-called Y2K c o m p u t e r threat, the " m i l l e n n i u m bug." In this instance, countless books w e r e w r i t t e n and w e b sites selling survival kits created, in w h a t a m o u n t e d to an a m a z i n g l y profitable techno-farce. But skeptic that I a m , I backed up all my computers in late D e c e m b e r and took my family to Rio for the N e w Year, w h e r e at least we w o u l d all be w a r m . W i t h o u t a doubt, eschatological visions a n d apocalyptic fears are very m u c h a part of our technological w o r l d , of our " a g e of science," the ancient rhetoric m e r e l y being recast into m o d e r n parlance. Floods

HEAVEN'S

ALARM

THE

WORLD

m a y now come from global w a r m i n g , pestilence from a vicious biological w a r f a r e , the poisoning of the soil, air, a n d w a t e r from industrial pollution. S o m e observers m a y even say that the very d e v e l o p m e n t of science brought d o o m s d a y closerâ&#x20AC;&#x201D;that now, for the first t i m e in our history, we don't have to w a i t for God to decree the end: we can do that ourselves, since we hold the k e y to our collective oblivion. W h a t w a s once found in sacred texts is n o w a palpable reality, forcing us to reevaluate how to deal w i t h our collective survival or, m o r e to the point, how to e n s u r e it. But my more hopeful v i e w is that by e x a m i n ing the reasons for the persistence of apocalyptic thought in our culture, we w i l l stand a better chance of r e l e g a t i n g it to just that, c u l t u r e â&#x20AC;&#x201D; a n d , as a result, have a better u n d e r s t a n d i n g of our o w n humanity.

The Importance of Being Vague V a g u e n e s s can be an e x t r e m e l y powerful p r o p a g a n d i s t w e a p o n ; by i n s i n u a t i n g w i t h o u t c o m m i t t i n g , by s u g g e s t i n g w i t h o u t defining, by creating fantastic i m a g e s that inspire a n d horrify w i t h o u t offering a clear interpretation of their m e a n i n g , you are certain to attract the attention of m a n y people. If your message e n s u r e s there is a r e w a r d for those w h o follow you a n d your ideas, some sort of relief to the t r i b u l a tions of life, y o u r success is practically g u a r a n t e e d . Better yet, because you leave so m u c h room for interpretation, extracting the detailed m e a n i n g (or m e a n i n g s ) of your m e s s a g e can turn into an obsession for m a n y g e n e r a t i o n s to come. So it w a s w i t h the Book of Revelation. T h e stakes w e r e h i g h , eternal salvation o r d a m n a t i o n . T h e r e w a s m u c h d i s a g r e e m e n t a m o n g the e a r l y c h u r c h fathers over h o w to distill John's visions. S o m e of this confusion w a s d u e to exogenous factors, such as natural c a l a m i t i e s a n d invasions by G e r m a n tribes a n d later by M u s l i m a r m i e s , w h i l e some w a s d u e to e n d o g e n o u s factors, such as schisms a n d p o w e r s t r u g g l e s w i t h i n the c h u r c h . In times of g r e a t distress, a literal

THE

PROPHET

AND

THE

ASTRONOMER

r e a d i n g of Revelation w a s favored, w h e r e a s q u i e t e r times n u r t u r e d a more abstract approach. T h e s e disputations w e r e fed by the structure a n d style of the text, w h i c h cleverly lent itself to so m a n y different r e a d i n g s . Its v a g u e n e s s w a s its virtue. One of the first C h r i s t i a n apocalyptic sects inspired by the powerful rhetoric of Revelation a p p e a r e d in 156 C.E. in A n a t o l i a , T u r k e y . A m a n n a m e d M o n t a n u s declared h i m s e l f the incarnation of the Holy Ghost, w h o , a c c o r d i n g to the F o u r t h Gospel, w a s supposed to be the voice of t h i n g s to come. M o n t a n u s w a s soon joined by a band of ecstatics, w h o fervently believed in the i m m i n e n t c o m i n g of the N e w K i n g d o m ; the h e a v e n l y J e r u s a l e m w a s about to descend from the skies. T h e extent to w h i c h people believed in such visions a n d prophecies should not be u n d e r e s t i m a t e d : T e r t u l l i a n of C a r t h a g e (ca. 160â&#x20AC;&#x201D;after 220), the greatest theologian in the W e s t at the t i m e , after j o i n i n g the m o v e m e n t declared that in J u d e a a w a l l e d city, no doubt the N e w J e r u s a l e m , had been seen floating in the sky every m o r n i n g for forty d a y s !

T h e m o v e m e n t inspired a m i l i t a n c y that w a s to be characteristic of m a n y future m i l l e n a r i a n sects, m o v e m e n t s that believed in the i m m i nent arrival of the Messiah. It served a deep emotional function for those w h o joined, t r a n s f o r m i n g the suffering and the persecuted into a g e n t s of c h a n g e , into k e y p l a y e r s in God's g r a n d scheme for h u m a n i t y , the apocalyptic d r a m a . It m a d e C h r i s t i a n s into the chosen ones, a role previously reserved for the J e w s . It also created a k i n d of apocalyptic blindness, w h e r e all m e a s u r e of reality w a s lost to the g r a n d e r goal of the mission. T h e n a t u r a l a n d the s u p e r n a t u r a l w e r e completely intert w i n e d : if holy cities could descend from the skies to fulfill prophecies, w h y not comets a n d other celestial p h e n o m e n a ? It w a s S a i n t A u g u s t i n e ( 3 5 4 - 4 3 0 ) , the A l g e r i a n - b o r n bishop of H i p p o , w h o defined the s t a n d a r d s for i n t e r p r e t i n g apocalyptic a n d other biblical texts. In The City of God, A u g u s t i n e a r g u e d that R e v e l a tion w a s to be read a l l e g o r i c a l l y , that the s t r u g g l e b e t w e e n good a n d evil w a s a l r e a d y present in people's lives, a n d that the c h u r c h of Christ was the K i n g d o m of God on Earth; it had a r r i v e d a l r e a d y . T h e r e w a s

HEAVEN'S

ALARM

THE

WORLD

no use in t r y i n g to decipher God's logic in h u m a n t e r m s , by predicting w h e n a n d how the last j u d g m e n t w o u l d a r r i v e . D a t i n g the apocalypse w a s a superfluous task, l e a d i n g necessarily to a m b i g u i t y . T h i s is not to say that A u g u s t i n e did not believe in the last j u d g ment; he surely did a n d u r g e d people "not to be slow to turn to the Lord . . . for H i s w r a t h shall come w h e n you k n o w not."

T h e believer

is to live in a state of constant apocalyptic angst, w h e r e every action counts t o w a r d the last j u d g m e n t , w h i c h m a y be just a r o u n d the corner or m i l l e n n i a a w a y . It w a s an e x t r e m e l y clever w a y of s e c u r i n g the m e a n i n g of the m e s s a g e from Revelation w i t h o u t d i s c r e d i t i n g the c h u r c h in case prophecies failed to m a t e r i a l i z e . Augustine

w a s also concerned

with

the

interpretation

of the

w o r l d ' s end, w h i c h seemed to be described differently in Old a n d N e w T e s t a m e n t books. In particular, he w a s distressed by some H e b r e w texts, w h i c h predicted the end of both heaven a n d Earth, as in P s a l m s 102: 2 5 - 2 6 : " L o n g a g o You laid the foundation of the earth, a n d the heavens are the w o r k of your h a n d s . T h e y w i l l perish, but You w i l l e n d u r e . " To counter this v i e w of utter destruction of the cosmic framew o r k , A u g u s t i n e offered the t a m e r C h r i s t i a n view, in w h i c h only the w o r l d a n d the l o w e r part of the heavens perish, thus preserving the sanctity a n d eternity of the upper heavens, the r e a l m of God a n d the righteous. O t h e r w i s e , w h e r e w o u l d the souls go w h e n the end c a m e ? He strengthens his a r g u m e n t by q u o t i n g from M a t t h e w 24:29, that "the stars will fall from heaven, a n d the p o w e r s of heaven w i l l be s h a k e n , " as m e a n i n g that if the stars f a l l / r o w heaven, heaven r e m a i n s up there: " T h i s expression, then, is either figurative, as is more credible, or this p h e n o m e n o n w i l l t a k e place in this lowest h e a v e n . "

proceeds to support this interpretation by q u o t i n g from V i r g i l ' s epic poem The Aeneid, finished a r o u n d 19 B.C.E., w h i c h tells of the fall of Troy. Aeneas's father, Anchises, is refusing his son's u r g i n g to leave his palace a n d the c r u m b l i n g Troy before it is too late. To help h i m out of his q u a n d a r y , Anchises a s k s for a sign from the gods a n d is promptly g r a n t e d q u i t e a display:

THE

PROPHET

AND

THE

ASTRONOMER

T h e old m a n had h a r d l y spoken w h e n from our left c a m e A s u d d e n crash of thunder, a n d a shooting star slid d o w n T h e sky's d a r k face, d r a w i n g a trail of light behind it. We w a t c h e d the star as it g l i d e d high over the palace's roof, A n d b l a z i n g a path, b u r i e d its brightness d e e p in the woods of Ida; w h e n it w a s g o n e , it left in its w a k e a long furrow Of light, a n d a s u l p h u r o u s s m o k e spread w i d e l y over the terrain. T h a t d i d convince m y father.

V i r g i l a n d S a i n t A u g u s t i n e w e r e basing their ideas on the Aristotelian v i e w that meteors, as w e l l as comets, belonged to the l o w e r heavens, b e i n g in fact e q u a t e d w i t h atmospheric p h e n o m e n a , as in meteorology. A t m o s p h e r i c , but not n a t u r a l p h e n o m e n a ! To both V i r g i l a n d A u g u s tine, the connection b e t w e e n these celestial apparitions a n d n a t u r a l p h e n o m e n a w a s never there in the first place, because they w e r e Godsent entities, s e r v i n g a w e l l - d e f i n e d s u p e r n a t u r a l purpose. S t a r s a n d comets w e r e p l a y t h i n g s in the h a n d s of the gods, p l u c k e d from the l o w e r heavens as dictated by Aristotle's cosmic scheme, w h e r e c h a n g e w a s relegated to the s u b l u n a r y sphere, w i t h the h i g h e r spheres being perfect a n d i m m u t a b l e . T h e b o u n d a r i e s b e t w e e n the m a g i c a l a n d the real w e r e b l u r r e d , creating g r e a t fear a n d confusion; all cosmic p h e n o m e n a w e r e attributed to s u p e r n a t u r a l causes. In a w o r l d w h e r e evil w a s a palpable presence a n d temptation w a s e v e r y w h e r e , only the c h u r c h could offer g u i d a n c e a n d support. Every aspect of life (and d e a t h ) revolved a r o u n d religion a n d superstition.

Wars of Redemption T h e eagerness to believe runs d e e p in the m i n d s of the discontent. In spite of S a i n t A u g u s t i n e , apocalyptic expectations w e r e very m u c h a part of people's lives d u r i n g the early M i d d l e A g e s . Even after C o n J6

HEAVEN'S

ALARM

THE

WORLD

stantine the Great, ruler of the eastern w i n g of the R o m a n E m p i r e , k n o w n as the B y z a n t i n e E m p i r e , e m b r a c e d C h r i s t i a n i t y in 324 C.E., w a v e s of invasions by G e r m a n i c a n d Asian tribes, as w e l l as internal corruption, kept e r o d i n g Rome's former g l o r y a n d

frustrated

the

attempts to restore it. C h r i s t i a n poets of the late fourth century, such as P r u d e n t i u s (348â&#x20AC;&#x201D;after 405), w o u l d express their despair in g l o o m y verses c h a n t i n g the c r u m b l i n g cosmic order,

One day the heavens w i l l be rolled up as a book, T h e sun's revolving orb w i l l fall upon the earth, T h e sphere that r e g u l a t e s the m o n t h s will crash in r u i n .

T h i s alternation of stable a n d unstable periods e n d e d in the m i d - s i x t h century. In the 570s the bubonic p l a g u e m a d e its first entry into the West, following a trail of devastation that started in A s i a M i n o r a n d ran through the M i d d l e East. R e n e w e d G e r m a n i c invasions, this t i m e under

the

half-pagan

L o m b a r d s , b r o u g h t horror to the northern

region of the Italian peninsula. T h e u r b a n populations s h r a n k , p o w e r w a s d e c e n t r a l i z e d , a n d isolationism b e c a m e the rule. Self-proclaimed messiahs a p p e a r e d e v e r y w h e r e , s o m e t i m e s a m a s s ing h u g e followings. Most stories have a s i m i l a r pattern; a m a n or, occasionally, a w o m a n has some sort of epiphany, a n d escapes into the w i l d e r n e s s for a period of meditation. T h e person e m e r g e s from the woods transformed, e n l i g h t e n e d , a n d starts c a l l i n g h i m s e l f the n e w Messiah, or at least God's messenger. People flock a r o u n d this person, some being t h r o w n into a state of religious frenzy by the prophet's preaching. T h e n the m i r a c l e s start: the chronically ill are healed just by touching the holy m a n , the blind see a g a i n , the d e a f hear, a n d the m u t e speak, w h i l e s t r a n g e celestial p h e n o m e n a confirm the prophet's m i s sion. T h e w h o l e action is a ritual r e e n a c t m e n t of w h a t happened w h e n Christ w a s a l i v e , transporting people to a holy t i m e , p r o m i s i n g them justice everlasting. Hopes soar that this is indeed the Second C o m i n g . But t i m e passes a n d the Second C o m i n g doesn't come. In some cases, people's patience ran out a n d the prophet e n d e d up exiled or often-

THE

PROPHET

AND

THE

ASTRONOMER

t i m e s k i l l e d . Or the m o v e m e n t a s s u m e d m i l i t a n t d i m e n s i o n s , becoming a revolutionary force w i t h i n the poor a n d the socially displaced, t h r e a t e n i n g the establishment. T h e m o v e m e n t w o u l d e v e n t u a l l y be crushed by the c h u r c h or nobility or by an a l l i a n c e of the t w o , its l e a d ers s l a u g h t e r e d a n d their m a n g l e d bodies (or parts of t h e m ) d i s p l a y e d publicly as a w a r n i n g a g a i n s t future insurrections. But apocalyptic hopes w o u l d not go a w a y this easily. F a i t h is fed not by a rational search for truth but by a passionate belief in its d i v i n e revelation. In the g r u e s o m e early M i d d l e A g e s , the s u p e r n a t u r a l reality offered by the c h u r c h w a s the only g l i m m e r of hope. People's perception of reality w a s i n d i s t i n g u i s h a b l e from the fantastic. As the historian N o r m a n C o h n a r g u e s in The Pursuit of the Millennium, the more revolutionary aspects of messianic m o v e m e n t s c a m e later, a r o u n d the eleventh century, as a result of the d e v e l o p m e n t of c o m m e r c e a n d the consequent increase in the u r b a n

concentration of

w e a l t h . U p until then, European society w a s mostly a g r a r i a n , obeying age-old rules, w h i c h bound the lords a n d their serfs in a covenant of m u t u a l d e p e n d e n c e a n d security. T h e s e a g r a r i a n n u c l e a r societies enjoyed

some

precarious

but comforting stability, and

not m u c h

c h a n g e w a s expected or hoped for. W i t h the d e v e l o p m e n t of trade, mostly because of the e m e r g e n c e of the textile industry, the peasantry confronted a n e w reality, w h e r e w e a l t h w a s possible to those either skilled e n o u g h or c u n n i n g e n o u g h to succeed in the m a r k e t p l a c e . T e m p t a t i o n settled in, a n d m a n y m i g r a t e d to the n e w townships, s h a k i n g off their vows of servitude in search of n e w f o u n d d r e a m s , w h i c h , however, w e r e rarely fulfilled. T h e i n d i v i d u a l felt left behind, a useless a p p e n d a g e to society. W h e n all seemed lost, there a p p e a r e d a savior, p r o m i s i n g a n e w life to those w h o followed h i m , i n c l u d i n g relief from their pain a n d suffering for all eternity in God's paradise. Q u o t i n g profusely a n d enthusiastically from the Bible, the self-appointed prophet, u s u a l l y a person w i t h strong personal m a g n e t i s m a n d great eloquence, q u i c k l y w o n the hearts a n d souls of the desperate. T h e people of the M i d d l e A g e s , o v e r w h e l m e d by very bleak conditions, lived in "a m o r e or less constant state of apocalyptic expecta38

HEAVEN'S

tion."

ALARM

THE

WORLD

M u c h has been said about how these expectations exploded into

g e n e r a l i z e d panic a r o u n d the y e a r 1000 C.E., the "terrors of the y e a r 1000." H o w e v e r , a more careful a n a l y s i s has s h o w n that we just do not have e n o u g h evidence to d e t e r m i n e w h a t truly h a p p e n e d a r o u n d 9 9 9 â&#x20AC;&#x201D; w h e t h e r people's g e n e r a l state of m i n d w a s any worse than in other y e a r s . M a n y of the reports of Europe's c o m i n g to a halt as the fateful hour a p p r o a c h e d , w h i c h i n c l u d e tales of horrible famines a n d even c a n n i b a l i s m , are probably e x a g g e r a t i o n s concocted later for propagandistic purposes. One of the b e s t - k n o w n sources from the t i m e is the travel d i a r y of the F r e n c h n o m a d i c m o n k R a d u l p h u s Glaber, a.k.a. R a l p h the B a l d , w h o paints a picture of absolute social chaos a n d despair, t r i g g e r e d by a series of c a t a c l y s m i c events, r a n g i n g from a violent eruption of M o u n t Vesuvius in Italy to w i d e s p r e a d pestilence a n d famine. Referring to the situation in Italy, G l a b e r w r i t e s ,

In those d a y s also, in m a n y regions, the horrible famine compelled m e n to m a k e their food not only of u n c l e a n beasts a n d c r e e p i n g t h i n g s , but even of men's, w o m e n ' s , a n d children's flesh, w i t h o u t r e g a r d even of k i n d r e d ; for so fierce w a x e d this h u n g e r that g r o w n - u p sons d e v o u r e d their mothers, a n d m o t h ers, forgetting their m a t e r n a l love, ate their b a b e s .

Glaber's account w a s not the only one p a i n t i n g the terrors of the y e a r 1000. C e n t u r i e s later, the preacher of the cosmic doom we met e a r l i e r in this chapter, Increase Mather, w r o t e in his Cometography of

an h o r r e n d o u s comet seen a p p e a r i n g in the m o n t h of D e c e m ber, followed by a most terrible e a r t h q u a k e . A n d g r e a t w a r s between the e m p e r o r of Constantinople a n d the B u l g a r i a n s . .. . A l s o a g r i e v o u s famine attended this comet, a n d a p r o d i g i o u s d r o u g h t , so as that both M e n a n d Cattle d i e d of t h i r s t .

The

recent

Cometography

the

American

astronomer

Kronk m a k e s no m e n t i o n of this " h o r r e n d o u s c o m e t . "

Gary

THE

PROPHET

AND

THE

ASTRONOMER

T h e year 1000, if not a y e a r of s i n g u l a r despair in European history, w a s certainly the b e g i n n i n g of a n e w era. M a y b e the c h a n g e s that foll o w e d the turn of the m i l l e n n i u m w e r e the product of a collective sigh of relief from the postponement of the end. Pope U r b a n II q u i c k l y seized the opportunity, s u m m o n i n g the faithful to jointly t a k e up a r m s in w h a t c a m e to be k n o w n as the First C r u s a d e â&#x20AC;&#x201D; t h a t m o m e n t o u s a t t e m p t to r e g r o u p Christianity, liberate the Eastern E m p i r e from the M u s l i m s , a n d stop the w a r s r a v a g i n g most of feudal Europe. H i s famous 1095 appeal at C l e r m o n t sent w a v e s across Europe. N o b l e m e n a n d c o m m o n e r s a l i k e v o w e d t o serve C h r i s t e n d o m . T h e crusade w a s transformed into a battle for spiritual salvation a n d , of course, a little m a t e r i a l a n d political g a i n on the side. It g a l v a n i z e d the m i l l e n a r i a n expectations of the poor, w h o left in droves to m a r c h to J e r u s a l e m in search of holiness. B e a r i n g the cross s e w n on their c r u d e outfits, the a r m y of Christ, blinded by its lofty goals and c h a r i s m a t i c prophet l e a d ers, " l i b e r a t e d " J e r u s a l e m by m a s s a c r i n g every m a n , w o m a n , a n d child w h o refused i m m e d i a t e conversion. T h a t i n c l u d e d not only M u s l i m s but also thousands of J e w s u n l u c k y e n o u g h to be on the path of the "liberators." One account includes a horrifying scene of horses w a d i n g "in blood up to their k n e e s , n a y up to the b r i d l e . "

The killings were

all justified by faith, as is often the case in "holy w a r s , " believed to be a necessary part of the mission in b r i n g i n g about everlasting peace. T h e religion that preached brotherly love w a s turned into a d e m o n i c device of destruction. A n d w h a t is worse, in the m i n d s of the "holy" m u r d e r ers, the end perfectly justified the m e a n s : the blood of the sinner is the sanctified w i n e of the just.

T h e M a s q u e o f the R e d D e a t h T h e church g r e w stronger. Gothic c a t h e d r a l s w e r e erected i n F r a n c e a n d G e r m a n y , the crusaders kept fighting for C h r i s t e n d o m a n d salvation in the East and the Iberian peninsula, w h i l e the clergy exerted its d o m i n a n c e over the souls of people and k i n g s . As c o m m e r c e e x p a n d e d , 40

HEAVEN'S

ALARM

THE

WORLD

m a n y peasants flocked to the g r o w i n g cities in search of jobs a n d protection. Europe w a s c a u g h t completely u n p r e p a r e d for the c h a l l e n g e s that lay a h e a d : g r o w i n g c o m m e r c e a n d lack of urban sanitation also m e a n t the rapid spread of disease a n d poverty. Confined to small villages or f a r m i n g c o m m u n i t i e s in the early M i d d l e A g e s , the pestilence a s s u m e d epic proportions in the centuries to come. In 1186, an astrological forecast brought n o r m a l activity in m u c h of Europe to a halt; a major planetary conjunction w a s to t a k e place in the constellation of L i b r a . Since L i b r a is an " a i r " sign, the conjunction w a s interpreted as a sign of, a m o n g other evils, a cataclysmic w i n d s t o r m l i k e none ever seen before. E n s u i n g e a r t h q u a k e s w e r e to complete the destruction. G e r m a n chronicles relate that people d u g u n d e r g r o u n d caves w h e r e they intended to hide d u r i n g the storms, w h i l e several special services w e r e held in churches to appease the panic-stricken population. It w a s predicted that in sandy regions, such as E g y p t a n d Ethiopia,

entire

cities

would

disappear.

astrologer

named

C o r u m p h i r a wrote,

T h e conjunction about to t a k e place, w h a t e v e r others m a y say, signifies to m e , if God so w i l l s , the m u t a t i o n of k i n g d o m s , the superiority of the F r a n k s , the destruction of the S a r a c e n i c race, w i t h the superior blessedness of the religion of Christ, a n d its special exaltation, together w i t h longer life to those w h o shall be born hereafter.

T h e forecast translates w e l l the overall emotional c l i m a t e in Europe, w h e r e people of all levels of society w e r e m o r e than w i l l i n g to e m b r a c e ny prophecy p r o m i s i n g radical c h a n g e . A g a i n s t this b a c k g r o u n d of fervent but unfocused expectation, the C a l a b r i a n m o n k J o a c h i m of Fiore (ca. 1135â&#x20AC;&#x201D;1202) b r o u g h t about a r e n e w e d convergence of apocalyptic prophecy w i t h his prognostications of an i m p e n d i n g n e w a g e , complete w i t h an estimate of dates. A skilled n u m e r o l o g i s t , Joachim used the Bible as a predictive instrument, w h i c h d i v i d e d history into three eras, i m p r i n t s in t i m e of the Holy T r i n i t y : the Old Testament, or 41

THE

PROPHET

AND

THE

ASTRONOMER

the a g e of the law, w a s the a g e of the Father, w h e n God's heavy h a n d ruled over the world's affairs; the N e w Testament, or the a g e of g r a c e , w a s the a g e of the Son, m a r k e d by the establishment of the Catholic C h u r c h ; the n e w a g e to come w a s the a g e of the Holy Ghost, an a g e of r e n e w e d spirituality, w h e n C h r i s t i a n i t y ' s loftiest ideals w o u l d finally be realized and h u m a n k i n d w o u l d live in a p e r m a n e n t state of ecstatic religious contemplation a n d purity. One h u n d r e d y e a r s after Joachim's death, the g r e a t Italian poet Dante A l i g h i e r i placed h i m in p a r a d i s e . Joachim's intentions w e r e far from revolutionary or subversive; he had the blessings of three popes and w a s w i d e l y a d m i r e d . A n d yet, his ideas g e n e r a t e d a series of m o v e m e n t s that threatened the stability of the church. H i s c o m p l e x calculations w e r e based on S a i n t M a t t h e w , w h o said forty-two g e n e r a t i o n s passed between A b r a h a m a n d Christ. A s s u m i n g that the s a m e period of forty-two g e n e r a t i o n s separated the second and third eras, and e s t i m a t i n g each g e n e r a t i o n to last thirty y e a r s , Joachim placed the b e g i n n i n g of the third era s o m e w h e r e between 1200 and 1260. T i m e w a s r u n n i n g out. History, t h r o u g h the eyes of prophecy, becomes the foreseer of t h i n g s to come, a reversal of its n o r m a l role of l o o k i n g at past events. T h e people of the late M i d d l e A g e s s a w life as the unfolding of the g r e a t apocalyptic d r a m a , each i n d i v i d u a l identifying w i t h one of several characters as he or she saw fit: saints and m a r t y r s , a n g e l s and devils, messiahs and Antichrists. It w a s as if reality t u r n e d into a h u g e w a v e , w h i c h inexorably d r a g g e d existence b u b b l i n g and foaming a l o n g w i t h it, until the inevitable crash at the beach. T h i s is w h e n the flagellants a p p e a r e d . S u r g i n g t h r o u g h Italy a r o u n d 1260 and m o v i n g north to G e r m a n y , these bands of penitent people took the s a y i n g "no pain no g a i n " to its e x t r e m e : to t h e m , selfinflicted physical pain w a s a p u r g i n g of the soul, a declaration of religious

fervor

and

martyrdom,

one-way

ticket

salvation

j u d g m e n t d a y (see figure 5 in insert). T h e flagellants w e r e the chorus line of the apocalyptic d r a m a , e n a c t i n g a terrifying choreography of despair. I m a g i n e a p a r a d e of h u n d r e d s , sometimes thousands of people w e a r i n g w h i t e hooded robes w i t h red crosses s e w n on the front and

HEAVEN'S

ALARM

THE

WORLD

back, c a r r y i n g b a n n e r s a n d candles w h i l e s i n g i n g r e l i g i o u s h y m n s o f repentance, m a r c h i n g from v i l l a g e to v i l l a g e , u r g i n g passersby to join in if they w a n t e d to be saved from eternal doom. After a r r i v i n g in a village, the flagellants w o u l d circle its m a i n c h u r c h or s q u a r e a n d start their ritual of self-flogging, u s i n g leather w h i p s i n d e n t e d w i t h iron spikes, c r y i n g a n d w a i l i n g deliriously for eternal salvation w h i l e blood squirted from their m a n g l e d bodies. T h e peak of the flagellant m o v e m e n t coincided w i t h , or w a s a response to, one of the most h o r r e n d o u s episodes in the history of h u m a n k i n d , the Black Death. T h i s e p i d e m i c of bubonic fever, w h i c h apparently o r i g i n a t e d in C e n t r a l

A s i a a n d first a p p e a r e d in the

c r o w d e d port cities of southern Italy in 1347, spread l i k e brushfire through the c o u n t r y s i d e a n d across northern Europe, k i l l i n g about a third of its population in u n d e r t w o y e a r s . It is estimated that over twenty-five m i l l i o n people d i e d in Europe a n d another fifty m i l l i o n in Asia, m o r e than d u r i n g the t w o w o r l d w a r s of the twentieth century. T h e signs that such devastation w a s possible w e r e plentiful. In 1340, a p l a g u e k i l l e d about fifteen thousand people in Florence. P l a g u e s a n d pestilence w e r e r e a p p e a r i n g in overpopulated cities w i t h g r o w i n g fury; a total lack of sanitation, practically nonexistent habits of personal h y g i e n e , a n d a tendency to accept disease as a chastising of sinners did not help the situation. T h e Black Death w a s r e g a r d e d by m a n y as a second flood, perfectly consistent w i t h the a p p r o a c h i n g end. To lend further credence to its role in the apocalyptic d r a m a , the everwatchful perceived an a b u n d a n c e of cosmic signs. T h e chronicler Giovanni

Villani,

his

Florentine

Stories

(1348),

blamed

the

1340

Florentine p l a g u e on a comet, w h i c h a p p e a r e d at the end of M a r c h in the constellation of Virgo. In the Chronicle of Jean de Venette (1368) of Gaul, the same comet, w i t h "tail a n d rays extended t o w a r d the east a n d north," w a s "thought to be a presage of w a r s a n d tribulation to come in the k i n g d o m . "

The

H u n d r e d Y e a r s ' W a r ( 1 3 3 7 - 1 4 5 3 ) had just

started its periodic w a v e s of devastation. V i l l a n i recorded another comet in 1347, w h i c h w a s to b r i n g on the death of k i n g s . In the Decameron, G i o v a n n i

Boccaccio described the a r r i v a l of the B l a c k

THE

PROPHET

AND

THE

ASTRONOMER

Death in Florence, uncertain w h e t h e r the e p i d e m i c a p p e a r e d "through the operation of the h e a v e n l y bodies or because of our o w n inequities w h i c h the just w r a t h of God sought to correct." To most people, the t w o w e r e related. F r e n c h texts tell of a star that hovered over Paris d u r i n g A u g u s t , w h i l e the G e r m a n Nuremberg Chronicles (1493) m e n tion a " h a i r y star" that r e m a i n e d visible for two m o n t h s (see figure 6 in insert). T h a t these celestial objects could also be seen in other times did not detract from their collective evocation of i m p e n d i n g doom. In the m e a n t i m e , corpses a c c u m u l a t e d in fetid piles faster than they could be b u r i e d , w h i l e w i d e s p r e a d f a m i n e c o m p o u n d e d the devastation caused by the merciless killer. As the panic increased, so d i d the c r o w d s of parading

flagellants,

whose

infected

open

wounds

promoted

the

spread of the bacillus. C r i e s of " B r i n g out your d e a d ! B r i n g out your d e a d , " followed by the sad tolling of bells, echoed in every t o w n and v i l l a g e . W h i l e people died l i k e flies in the streets, m e m b e r s of the h i g h c l e r g y a n d the aristocracy b a r r i c a d e d themselves behind the fortified w a l l s of their castles, w a i t i n g for the w a v e of death to pass by. In his allegorical tale inspired by the Black Death, " T h e M a s q u e of the Red Death," E d g a r A l l a n Poe (1809-1849) w r i t e s of the t r a g i c end of Prince Prospero a n d his aristocratic guests, w h o a r r o g a n t l y believed their lives w e r e protected from the e p i d e m i c by their h i g h e r social status. T h e y danced a n d dined w i t h i n the w a l l s of Prospero's castle w h i l e the peasants died outside, b e g g i n g the indifferent prince for protection. Death m a k e s an entry into the castle d u r i n g a m a s q u e r a d e : " H i s vesture dabbed in bloodâ&#x20AC;&#x201D;and his broad brow, w i t h all the features of the face, w a s b e s p r i n k l e d w i t h the scarlet horror." T h e d a n c i n g a n d l a u g h ter stop a n d d r e a d spreads t h r o u g h the castle, as one by one Prospero's guests fall to the g r o u n d , the once bustling rooms turned into ghostly chambers:

A n d n o w w a s a c k n o w l e d g e d the presence of the Red Death. He had come l i k e a thief in the night. A n d one by one dropped the revellers in the b l o o d - b e d e w e d halls of their revel, a n d died each in the d e s p a i r i n g posture of his fall. A n d the life of the 44

HEAVEN'S

ALARM

THE

WORLD

ebony clock w e n t out w i t h that of the last of the g a y . A n d the flames of the tripods expired. A n d D a r k n e s s a n d Decay and the Red Death held i l l i m i t a b l e d o m i n i o n over a l l .

R e - c r e a t i n g H e a v e n and Hell Faced w i t h the horrors of the Black Death a n d the succession of d e v a s tating w a r s a n d famines, people u n d e r s t a n d a b l y y e a r n e d for salvation, seeing their t r a g i c lives w i t h i n the f r a m e w o r k of a g r a n d e r apocalyptic plot. T h i s transposition of biblical a n d m y t h i c characters into real life found d r a m a t i c expression in the artistic output of the times: a p o c a l y p ticism w a s very pervasive, an integral part of the lives of everyone, not just the clergy a n d the fanatic. Great w o r k s of l i t e r a t u r e from the late M i d d l e A g e s , such as Dante's

The Divine Comedy

(1321) and

Chaucer's

The Canterbury

Tales

(ca. 1370), a n d from the Renaissance, such as Spenser's The Faerie Queene (ca. 1590) a n d Milton's Paradise Lost (1667), abound w i t h apocalyptic references, m i x i n g the fantastic w i t h the real, b r i n g i n g c h a r a c ters from politics and c l e r g y a n d often the poets themselves into the n a r r a t i v e . M e d i e v a l a n d Renaissance painters did the same: apocalyptic iconography b e c a m e one of the m a i n sources of inspiration to m a n y artists of the t i m e , from the great Italian m a s t e r s Giotto di Bondone, L u c a S i g n o r e l l i , a n d M i c h e l a n g e l o to the G e r m a n s M a t t h i a s G e r u n g and Albrecht Durer. In their p a i n t i n g s , frescoes, a n d e n g r a v i n g s , we find both aspects of the apocalyptic n a r r a t i v e , the t r a g i c a n d the hopeful, often combined. I m a g e s of the last j u d g m e n t , or the fall of the Antichrist, are so a b u n d a n t that I can mention only a few representative e x a m p l e s . S i g n o r e l l i ' s a m a z i n g frescoes in the chapel of S a n B r i z i o in Orvieto's c a t h e d r a l , depicting the Antichrist p r e a c h i n g , display several stages of the apocalyptic d r a m a . We see cosmic signs of the end of the w o r l d , the resurrection of the d e a d , the c o n s i g n m e n t of sinners to hell a n d the ascent of the just to heaven. In the fresco Deeds and Sins of the Antichrist, a g r o u p of D o m i n i c a n s a r g u e behind Antichrist, w h o is

THE

PROPHET

AND

THE

ASTRONOMER

p r e a c h i n g from a pedestal w h i l e the devil w h i s p e r s in his ear. T h e g r o u p of listeners to his right includes Dante, S i g n o r e l l i himself, his m e n t o r F r a A n g e l i c o , a n d other important Florentines of the t i m e (see figure 7 in insert). T h e cosmic a n d the c o n t e m p o r a r y a r e clearly fused, l e n d i n g reality to the prophetic n a r r a t i v e . In the Vatican's Sistine C h a p e l , the Last Judgment by M i c h e l a n g e l o (finished ca. 1541), portrays the saved a n d the d a m n e d side by side, light a n d d a r k n e s s b a l a n c i n g the t w o parts of the fresco in a profusion of i m a g e s of p u n i s h m e n t a n d bliss. T h e G e r m a n s w e r e not as subtle. Diirer's collection of e n g r a v i n g s from 1498, The Apocalypse, is full of the most horrific i m a g e r y , l a r g e l y built upon d r a m a t i c cosmic events. In The Opening of the Fifth and Sixth Seals, a s h o w e r of stars falls over the heads of terrified sinners below, w h i l e a d a r k e n e d S u n and a bloodied, a n g r y - l o o k i n g Moon decorate the l o w e r skies. H i g h above the chaos, we see the celestial throne surrounded by m a r t y r s a n d a n g e l s , on their faces an expression of beatit u d e a n d peace. H i g h m e m b e r s of the Catholic c l e r g y a r e depicted a m o n g the sinners below (see figure 8 in insert). T h i s collection of e n g r a v i n g s predates the start of the Reformation by only nineteen y e a r s . It w a s in 1517 that M a r t i n L u t h e r nailed the ninety-five theses on the door of the church in W i t t e n b e r g , c o n d e m n i n g , a m o n g other t h i n g s , the p o w e r of priests to pardon sins t h r o u g h confession and absolution. T h e C a t h o l i c C h u r c h a n d its l e a d e r s w e r e seen by m a n y as completely corrupt a n d decadent; the reformists c o n d e m n e d popes a n d bishops w i t h i l l e g i t i m a t e c h i l d r e n , their w e a l t h a n d a r m i e s blatantly at odds w i t h a religion that preached h u m i l i t y a n d peace. T h e r e w a s a proliferation of horrifying i m a g e s of popes as Antichrists, popes crucifying Christ, devils e x c r e t i n g popes, popes as devils, a n d so on. In the l i t e r a t u r e of the late M i d d l e A g e s , we find s i m i l a r messages, albeit u s u a l l y presented in a s o m e w h a t m o r e discreet format. Dante's Divine

Comedy

and

Chaucer's

Canterbury

Tales

are

structured

pil-

g r i m a g e s . A l t h o u g h C h a u c e r ' s often comic masterpiece is m u c h lighter in content than Dante's journey through the afterlife, it nevertheless

4(,

HEAVEN'S

ALARM

THE

WORLD

concludes w i t h the a r r i v a l at C a n t e r b u r y â&#x20AC;&#x201D; w h i c h represents " H e a v enly J e r u s a l e m " â&#x20AC;&#x201D; w h e r e resides the "holy blissful m a r t y r , q u i c k to g i v e his help to them w h e n they w e r e sick." C h a u c e r treats the m e m b e r s of the clergy a m o n g the p i l g r i m s w i t h obvious sarcasm: of the m o n k , w h o had a "special license from the Pope," he w r i t e s ,

S w e e t l y he heard his penitents at shrift W i t h pleasant absolution, for a gift. He w a s an easy m a n in p e n a n c e - g i v i n g W h e r e he could hope to m a k e a decent l i v i n g .

T h e c y n i c i s m directed at the church gets worse as the thirty-one pilg r i m s tell their stories. Dante's rhetoric is more subtle and m o r e somber, a l t h o u g h no less efficient. T h e r e a r e several i m a g e s borrowed straight

from

Revelation, as

well

as c o n d e m n a t i o n s of popes as

Antichrists. T h e poet often a s s u m e s the role of the prophet, d e n o u n c ing the ills of society and clergy as he climbs from the searing heat of hell to the b l i n d i n g light of paradise. Dante's personal life and tribulations are painfully reflected in his poem. If in real life he w a s exiled from his beloved Florence because of political i n t r i g u e , in the poem he faces another exile, that of the afterlife. If in real life his love for a w o m a n called Beatrice w a s c o n d e m n e d by social convention, in the poem it is Beatrice w h o g u i d e s h i m t h r o u g h paradise. Dante kept his love secret a n d never actually touched the w o m a n he so fervently desired, w h o , m u c h to his despair, d i e d very y o u n g . T h e poem joins his u n c o n s u m m a t e d love for Beatrice, shared by all u n q u e n c h e d lovers, w i t h the u n c o n s u m m a t e d love for God, w i t h w h i c h every C h r i s t i a n wrestles. In so doing, Dante masterfully blends the t w o m a i n concerns of his times, true love in life and salvation in death. A profusion of art has been created a r o u n d the t h e m e of loss, in particular the death of a loved one. T h e helplessness of losing someone dear as a result of uncontrollable factors, the despair of k n o w i n g we will never touch or be touched by this person a g a i n or, as the t w e n t i e t h -

THE

PROPHET

AND

THE

ASTRONOMER

century Italian w r i t e r P i r a n d e l l o said of his mother, of k n o w i n g that person no longer w i l l be able to t h i n k of you is, s o m e t i m e s , too m u c h to bear. Death sharpens all the corners we desperately try to round out by our h u m a n i t y . W h a t can we do but create our o w n poetic justice, crying out our despair in poems a n d p a i n t i n g s , s y m p h o n i e s a n d p l a y s . S o m e t i m e s the stories e n d in tragedy, as in S h a k e s p e a r e ' s Romeo and Juliet (ca. 1595), w h i c h so blatantly explores the inevitability of fate. T h i s t h e m e r e e m e r g e s in his Sonnet 14, full of cosmic s y m b o l i s m :

Not from the stars do I my j u d g e m e n t pluck, A n d yet m e t h i n k s I have astronomy; But not to tell of good or evil l u c k , Of p l a g u e s , of d e a r t h s , or seasons' q u a l i t y ; Nor can I fortune to brief m i n u t e s tell, Pointing to each his thunder, rain a n d w i n d , Or say w i t h princes if it shall go w e l l By oft predict that I in heaven find. But from thine eyes my k n o w l e d g e I d e r i v e , A n d , constant stars, in t h e m I read such art As truth a n d beauty shall together thrive, If from thyself to store thou w o u l d s t convert. Or else of thee this I prognosticate; T h y end is truth's a n d beauty's doom a n d d a t e .

T h e sonnet y i e l d s a double blow on our vain search for truth a n d beauty. W h i l e d e r i d i n g beliefs in the p o w e r s of astrology, or in his ability as an astrologer, S h a k e s p e a r e l a m e n t s his despair, k n o w i n g that the truth a n d beauty he sees in his lover's e y e s â&#x20AC;&#x201D; t h e poet's a s t r o n o m y â&#x20AC;&#x201D; a r e e p h e m e r a l , as they cannot be as eternal as stars. He finds solace in his verse: only t h r o u g h creativity can we defeat death, as the closing lines of Sonnet 19 defiantly proclaim:

Yet, do thy worst, old T i m e ! Despite thy W r o n g , My love shall in my verse ever live y o u n g . 48

HEAVEN'S

ALARM

THE

WORLD

A n o t h e r great classic of late English Renaissance l i t e r a t u r e , John M i l ton's Paradise Lost, finished in 1667, delves deeply into i m m o r t a l i t y a n d salvation, resorting often to cosmic s y m b o l i s m . Milton w a s the son of a Protestant father w h o had in turn been disinherited by his o w n fervently Catholic father. As the tensions of the Reformation ran d e e p in the family, Milton sided w i t h his father, b e c o m i n g an a r d e n t critic of the Catholic C h u r c h , constantly c o n d e m n i n g its corruption a n d e q u a t ing the pope w i t h the Antichrist. To M i l t o n , a n d to the reformists, the fact that the Catholic C h u r c h g a v e its followers the choice of absolution t h r o u g h confession a n d repentance w a s unacceptable. T h i s structure g a v e e n o r m o u s p o w e r to the c l e r g y a n d w a s at the heart of its c o r r u p tion; as C h a u c e r ' s m o c k e r y of the p i l g r i m M o n k m a d e clear, priests w e r e m o r e than h a p p y to e x c h a n g e absolution a n d eternal salvation for a gift or two. T h e t h e m a t i c structure of Milton's masterpiece poem, Paradise Lost, revolves a r o u n d the first choice ever m a d e in ( J u d e o C h r i s t i a n ) history, the choice by A d a m to taste the forbidden fruit from the Tree of K n o w l e d g e , thereby contradicting God's orders, a n d the chain of events this choice u n l e a s h e d . A d a m ' s choice w a s clearly a bad one, its p u n i s h m e n t being h u m a n mortality. As we read in the opening lines

of Paradise Lost,

Of m a n s First Disobedience, a n d the Fruit Of that F o r b i d d e n Tree, whose mortal tast[e] B r o u g h t Death into the W o r l d , a n d all our w o e , W i t h loss of Eden, till one g r e a t e r M a n Restore us, a n d regain the blissful S e a t . . . .

We are all p a y i n g for A d a m a n d Eve's transgression, at least until we reembrace eternity at the end of time. Both the sinner a n d the virtuous regain their lost i m m o r t a l i t y , the former in the torments of hell and the latter in paradise. W h a t happens after death d e p e n d s on the choices we m a k e in life. Milton's m e s s a g e c a m e c h a r g e d w i t h cosmic signs. In his travels after finishing his studies at C a m b r i d g e University, he met G a l i l e o , 49

THE

PROPHET

AND

THE

ASTRONOMER

" g r o w n old, a prisoner to the Inquisition for t h i n k i n g A s t r o n o m y othe r w i s e than the F r a n c i s c a n a n d D o m i n i c a n licensers t h o u g h t . "

His

hatred of the Catholic C h u r c h w a s reinforced by seeing this g r e a t scientist fall prey to F r a n c i s c a n s a n d D o m i n i c a n s , the t w o g r o u p s that, four centuries e a r l i e r h a d been considered the messengers of the n e w a g e predicted by J o a c h i m of Fiore. T h e true story of Galileo's conviction by the Inquisition is m u c h subtler, as I e x p l a i n e d in my book The Dancing Universe. H o w e v e r , Milton's visit to Galileo w a s an expression of his interest in the c h a n g e s o c c u r r i n g in the science of the heavens. In Paradise Lost, Milton blends the science of the cosmos w i t h the cosmic r e l i g i o u s fantasy of Revelation, i n v o k i n g sulfurous storms a n d fiery cataracts d e s c e n d i n g from a " F i r m a m e n t of H e l l , " red l i g h t n i n g , and d a r k n e s s , side by side w i t h references to the Sun's relative position w i t h respect to several constellations. In the closing lines, he c o m p a r e s God's s w o r d of l i g h t to a comet:

T h e brandisht S w o r d of God before t h e m b l a z ' d Fierce as a C o m e t ; w h i c h w i t h torrid heat, A n d v a p o u r as the L y b i a n a i r dust, Began to parch that t e m p e r a t e C l i m e , (book 12, lines 6 3 3 - 3 6 )

F r o m D a n t e to M i l t o n , from S i g n o r e l l i to Diirer, the artistic output of the late M i d d l e A g e s a n d the Renaissance reflects the obsessive fascination that apocalyptic t h e m e s , a n d their l i n k to celestial p h e n o m e n a , exerted on European society. T h e s e w o r k s created a d a t a b a n k of cosm i c apocalyptic i m a g e s , a c o m m o n set of g r a p h i c representations and literary symbols of eschatological t h e m e s that have been an integral part of the collective i m a g i n a t i o n of h u m a n k i n d ever since. As we w i l l soon see, they g r e a t l y influenced early astronomical thought, because the central quest of the n a t u r a l philosopher w a s to u n r a v e l God's m e s sages w r i t t e n in the skies. A n d , perhaps surprisingly, they continue to influence astronomical thought today, even if dressed in the q u a n t i t a tive l a n g u a g e of m o d e r n science.

HEAVEN'S

ALARM

THE

WORLD

Apocalypse Now! We still, and a p p a r e n t l y at an accelerated rate, speak of eternal salvation and d a m n a t i o n , of a fiery hell below and a paradise above, of cataclysmic events as p u n i s h m e n t s from God, of unpredictable cosmic p h e n o m e n a as the h e r a l d s of doom or, as in a profusion of e n o r m o u s l y popular recent movies, of comets a n d asteroids as the i n s t r u m e n t s of doom themselves. A 1983 G a l l u p poll disclosed that 62 percent of A m e r icans had "no doubt" that Christ will return to Earth s o m e t i m e in the future. In 1992, about 53 percent believed that the return w a s i m m i nent and that it w o u l d be followed by the cataclysmic events connected with the destruction of evil as prophesied in the Bible. In C a n a d a , a 1993 poll revealed that, no less than 30 percent of the respondents believed it w i l l happen w i t h i n the next h u n d r e d y e a r s . O u r collective eschatological i m a g i n a t i o n is as active as ever, even if its s y m b o l i s m has been g r e a t l y influenced by science. We still fear w h a t we cannot u n d e r stand and control; thus, as scientific progress shifts our ignorance about n a t u r e to different areas, we also shift our fears. In order to illustrate how this shifting operates, we w i l l briefly e x a m i n e the tragic history of some apocalyptic sects a n d their beliefs, past a n d present. D u r i n g the early y e a r s of the Reformation, w i d e s p r e a d feelings of despair and distrust s p a w n e d an a b u n d a n c e of apocalyptic sects. People w e r e desperate for salvation at a n y cost. T h e signs w e r e there for all to see, from the accusations of popes and Catholic prelates as being Antichrists, to famines, p l a g u e s , w a r s , invasions, a n d , of course, comets and other u n u s u a l cosmic p h e n o m e n a . In 1524, astrological forecasting of a flood to be " c a u s e d " by a conjunction of all planets in Pisces w a s t a k e n so seriously that a m a g i s t r a t e from the p a r l i a m e n t of Toulouse ordered an a r k to be built on top of a nearby hill. T h e very r u p t u r i n g at the seams of the Catholic C h u r c h by the Protestant "rebellion" w a s seen by m a n y as a sign of the a p p r o a c h i n g end. T h e Protestant reformers M a r t i n L u t h e r a n d P h i l i p p M e l a n c h t h o n ( 1 4 9 7 - 1 5 6 0 ) w e r e con-

THE

PROPHET

AND

THE

ASTRONOMER

vinced the end w o u l d happen before the

1600s. In 1522, L u t h e r

preached a sermon on the Second C o m i n g , d e c l a r i n g that the i m p e n d ing doom w a s written in the skies: We see the S u n to be d a r k e n e d and the Moon, the Stars to F a l l , m e n to be distressed, the w i n d s a n d w a t e r s to m a k e a noise, a n d w h a t e v e r else is foretold of the L o r d , all of them come to pass as it w e r e together. Besides, we have seen not a few C o m e t s , having the form of the Cross, i m p r i n t e d from heaven. . . . H o w m a n y other S i g n s also, a n d u n u s u a l impressions, have we seen in the H e a v e n s , in the S u n , Moon, Stars, R a i n - b o w s a n d s t r a n g e A p p a r i t i o n s , in these last four y e a r s ? Let us at least a c k n o w l e d g e these to be S i g n s , a n d S i g n s of some g r e a t a n d notable change.

As countless prophets a p p e a r e d e v e r y w h e r e , the Protestant u p s u r g e inspired m o r e dissenting sects than even the reformist leaders could digest. A m o n g these, the most d r i v e n w e r e the Anabaptists, w h o repud i a t e d both the Catholics a n d the L u t h e r a n s , c o m m a n d i n g their foll o w e r s to rebaptize. Occasionally, one w o u l d be seen r u n n i n g n a k e d t h r o u g h the streets of A m s t e r d a m a n d c r y i n g , " W o e , woe, w o e the w r a t h of God!" T h e 1525 peasant revolts in G e r m a n y w e r e believed to be the battle of the saints d u r i n g the last d a y s â&#x20AC;&#x201D; u n t i l they w e r e c o m pletely crushed a n d their Anabaptist leaders executed. T h e r e t u r n of H a l l e y ' s comet in 1531 c o m p o u n d e d the g r i m expectations. In 1534, the G e r m a n town of Miinster w a s s h a k e n by a storm of Anabaptist m i l i tancy. W i t h i n a w e e k of p r e a c h i n g in the streets, m o r e than fourteen h u n d r e d people w e r e rebaptized. A y o u n g Dutch preacher n a m e d Jan Bockelson, k n o w n as John of L e i d e n , caused a frenzy of apocalyptic fanaticism w h e n e v e r h e spoke. W o m e n , i n c l u d i n g m a n y n u n s w h o q u i t their convents to "convert" to the n e w faith, w o u l d u n d e r g o fits of hysteria, t h r o w i n g themselves on the g r o u n d , s c r e a m i n g , k i c k i n g , a n d foaming at the mouth. E v e n t u a l l y the Anabaptists i n v a d e d the town hall, causing a mass e x o d u s of the L u t h e r a n majority. T h e y invited all

HEAVEN'S

neighboring

Anabaptists

join

ALARM

them

THE

Miinster,

the

WORLD

"New

J e r u s a l e m , " w h i c h alone w o u l d be saved from destruction at the last 'ays. Soon the Anabaptist g o v e r n m e n t t u r n e d to terror; it confiscated property, banned all books except the Bible, a n d tortured a n d executed dissenters. John of L e i d e n instituted p o l y g a m y , a n d declared h i m s e l f k i n g of N e w J e r u s a l e m , none other than the Messiah foretold by Old Testament prophets. F u n d e d by heavy taxation, the n e w court q u i c k l y g r e w s u m p t u o u s , w h i l e the people of M i i n s t e r experienced abject poverty a n d privation. "Not to w o r r y , " John w o u l d tell his disciples, "soon you too w i l l be covered by the gold a n d silver of p a r a d i s e . " At that point, the town w a s a l r e a d y u n d e r siege by an a r m y of m e r c e n a r i e s o r g a n i z e d by Miinster's former Catholic bishop. He m a n a g e d to get help from m a n y local rulers, a n d in J a n u a r y 1535 a complete blockade w a s instituted, slowly starving the inhabitants of N e w J e r u s a l e m . In a few months, as the promised bread d i d not fall from heaven, all a n i m a l s w e r e eaten, from dogs a n d horses to rats a n d h e d g e h o g s . T h e n people started to consume g r a s s , moss, a n d the bodies of the d e a d . T h e k i n g r e m a i n e d completely oblivious, until bodies began to litter the streets. Finally, he a l l o w e d those w h o w i s h e d to leave t o w n to do so, a l t h o u g h no better l u c k a w a i t e d them outside the fortified w a l l s : they all met g r u e s o m e deaths at the h a n d s of the m e r c e n a r i e s , w h o d i d not spare w o m e n , c h i l d r e n , or the elders. In J u n e 1535, the t o w n w a s finally t a k e n over, most of its battered population k i l l e d in a massacre that lasted several d a y s . John w a s chained a n d p a r a d e d by the bishop on a leash, ordered to act l i k e a bear for the a m u s e m e n t of m o c k i n g c r o w d s . He w a s brought back to Miinster early in 1536 a n d , w i t h t w o other Anabaptist leaders, w a s publicly tortured to death w i t h red-hot irons. It is said that the k i n g did not utter a w o r d or m a k e a m o v e m e n t until his death. T h e three bodies w e r e then left to rot in iron cages suspended from a tower of S a i n t L a m b e r t ' s C h u r c h , a very g r u e s o m e sight still there to be seen today. T h i s h o r r e n d o u s episode sounds at first l i k e a historical curiosity, a s i n g u l a r tale of apocalypticism run crazy. But it isn't. T h e last few cen53

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turies have seen m a n y s i m i l a r instances w h e r e a m i l l e n a r i a n movement, initially w i t h perfectly pacific intent, w a l l e d itself off from the outside w o r l d , its m e m b e r s psychologically (and s o m e t i m e s p h y s i c a l l y ) enslaved by a c h a r i s m a t i c l e a d e r bent on t a k i n g his self-appointed prophetic mission to the end. T h e results have almost a l w a y s been tragic. F r o m the point of v i e w of the " w a l l e d - i n " sectarians, the u n a v o i d a b l e clash w i t h the "outside w o r l d " is the battle of A r m a g e d don, their m a r t y r d o m a passport to heaven. F r o m the point of v i e w of the outside w o r l d , the sect represents a threat to society, fueled by its a n a r c h i c mistrust of the g o v e r n m e n t a n d backed up by the illegal possession of firearms and w e a p o n r y . S k i p p i n g a few centuries of history, I w i l l mention just a few recent e x a m p l e s of apocalyptic sects that bring back to life e l e m e n t s of the Miinster insurrection of 1534â&#x20AC;&#x201D;35. In 1993, the " w a l l e d - i n " tradition of Miinster w a s t r a g i c a l l y revived in the conflict b e t w e e n the Branch D a v i d i a n s of W a c o , Texas, and the forces of the U . S . g o v e r n m e n t . T h e i r leader, David Koresh, w a s to be the opener of the seven seals of Revelation 6 and 7, to explain their m e a n i n g , and to lead his followers to heaven, transported in God's flying saucers, the m o d e r n - d a y version of God's chariots of fire of P s a l m 68:17. After a fifty-one-day siege, w h i c h led to the death of several federal agents, the w h o l e structure h o u s i n g the sect m e m b e r s w e n t up in flames, k i l l i n g 73 m e n , w o m e n , and children, together w i t h Koresh. Fifteen y e a r s before the W a c o tragedy, 913 followers of J a m e s W a r r e n Jones, a c h a r i s m a t i c preacher w h o fled the United States to found his o w n w a l l e d - i n N e w J e r u s a l e m , a.k.a. Jonestown, in G u y a n a , c o m m i t t e d collective suicide by d r i n k i n g c y a n i d e - l a c e d K o o l - A i d . N u c l e a r holocaust w a s one of his favorite themes." T h e mass suicides of the Branch D a v i d i a n s a n d of the inhabitants

* A s I w a s putting the final touches on this chapter, another apocalyptic sect m e t w i t h a tragic end, this time in U g a n d a , A f r i c a . T h e n u m b e r of bodies c o n s u m e d by the fire that b u r n e d their place of w o r s h i p keeps climbing, and has surpassed even the J o n e s t o w n count, w i t h o v e r 924 dead. A p p a r e n t l y , m a n y of the deaths w e r e not v o l u n t a r y , but violently inflicted upon the victims, probably those w h o tried to flee f r o m the b u r n i n g c h u r c h .

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f Jonestown t r a g i c a l l y illustrate the d e a d l y p o w e r of apocalyptic lore, he reader should not be q u i c k to dismiss these acts as the follies of m a d d e n e d r e l i g i o u s r a d i c a l s , or the product of ignorant superstition. W e l l - e d u c a t e d people, perfectly in tune w i t h their t i m e s , often succ u m b to apocalyptic p a r o x y s m s . T h e visit of H a l l e y ' s comet in 1910 w a s greeted w i t h a m i x of anxiety a n d h u m o r o u s e n t h u s i a s m . Harper's eekly published a d r a w i n g w i t h the caption " W a i t i n g for the End of e W o r l d , " echoed by m a n y counterparts in G e r m a n y . Other cosmic signs corroborated the g r i m expectations of the i n c r e a s i n g l y terrified population: u n u s u a l w e a t h e r , m o r e comets, meteor s h o w e r s , sunspots, w e s o m e d i s p l a y s of a u r o r a borealis, a n d even the death of a k i n g , d w a r d VII of E n g l a n d . A l t h o u g h there w e r e no predictions of a collision w i t h Earth, astronomers noted that the comet's tail, rich in cyanogen g a s , w o u l d pass t h r o u g h Earth. Terror ensued w h e n , a m o n g thers, the F r e n c h astronomer C a m i l l e F l a m m a r i o n pointed out that he m i x t u r e of the comet's c y a n o g e n w i t h the h y d r o g e n in our a t m o s phere w o u l d produce the d e a d l y prussic acid. Gas m a s k s a n d "comet p i l l s " w e r e q u i c k l y sold out, churches filled up w i t h people desperate to p u r g e their sins before " i m p a c t , " doors w e r e sealed up, and panic k y r o c k e t e d . On M a y 18, 1910, the New Yorf( Times reported that "teror occasioned by the near approach of H a l l e y ' s comet has seized hold of a l a r g e part of the population of C h i c a g o . " N e w York w a s no exception; m a n y of its citizens could be seen p r a y i n g on their k n e e s in p a r k s and streets, w h i l e "several religious processions took place in different parts of the c i t y . "

In F r a n c e , after a n i g h t l o n g vigil c h a r g e d w i t h a n x -

iety, people e m b r a c e d a n d danced in the streets after the authorities declared the " d a n g e r " to be over. Of course, the trace a m o u n t s of c y a n o g e n g a s in the comet's very diffuse tail w e r e never a true d a n g e r but a clear illustration of apocalyptic fear inspired by irresponsible apocalyptic science. In contemporary society, apocalyptic jargon incorporates science's latest findings a n d c u t t i n g - e d g e technology. If angels are not c o m i n g from heaven, then flying saucers are, populated by benevolent, a n g e l like aliens, such as the ones featured in Steven Spielberg's Close Encoun55

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ters of the Third Kind or his extraterrestrial emissary of love in E.T.

comparison can be m a d e between the alien in E.T. a n d Jesus himself. After all, the alien could perform countless m i r a c l e s , i n c l u d i n g bringing dead things back to life, c o m m u n i c a t e telepathically, fly a n d m a k e others fly, resurrect w i t h a big red beating heart s h o w i n g t h r o u g h his chest (as in m a n y depictions of C h r i s t ) , a n d , finally, ascend back to heaven in one of God's m o d e r n "chariots of fire." W h a t w e r e once s u p e r n a t u r a l p o w e r s attributed to gods are n o w " n a t u r a l " powers attributed to aliens, expressing a deep connection between religious symbolism a n d our age-old expectations inspired by scientific ideas. H e n c e Spielberg's g e n i u s , of tapping into our collective expectations of redemption, u s i n g the m o d e r n l a n g u a g e of science a n d its fictions. Extraterrestrial a n g e l s w e r e also believed to be the saviors of another apocalyptic sect, k n o w n as H e a v e n ' s Gate. H e r e too the story e n d e d in t r a g e d y (at least from the point of v i e w of the "outside w o r l d " ) , w h e n t h i r t y - n i n e m e m b e r s w e r e found d e a d on a ranch near S a n Diego, C a l i f o r n i a . T h e precursor event for the collective suicide w a s the passage of the comet H a l e - B o p p d u r i n g the w i n t e r a n d spring of 1997. T h e sect's leader, M a r s h a l l A p p l e w h i t e , or "Do," believed that his soul a n d those of his followers w e r e to be hoisted from their earthly existence by a flying saucer c o m i n g from the " L e v e l above H u m a n , " w h i c h w a s h i d i n g behind the comet. T h e i r destiny w a s to share a bodiless eternity in the " K i n g d o m of H e a v e n , " a m o n g the stars a n d planets. U n l i k e those of most other apocalyptic sects, the m e m b e r s of Heaven's Gate w e r e not left behind by m o d e r n technology; rather, they w e r e deeply u n h a p p y w i t h w h a t technology a n d m o d e r n life had to offer. T h e apocalyptic t h r e a d they shared w i t h other sects could be found in their deep criticism of w h a t they s a w as a corrupt w o r l d , w h o s e relig i o u s institutions h a d been seized by Lucifer, or " L u c y , " as Do l i k e d to call S a t a n . T h e i r expressed d i s l i k e for their bodies m a t c h e s w e l l their connection w i t h the Internet, a high-tech version of a d i s e m b o d i m e n t ritual: only y o u r m e s s a g e â&#x20AC;&#x201D; y o u r s p i r i t â&#x20AC;&#x201D; t r a v e l s across the w e b , connecting w i t h other "spirits" i n far-removed l i n k s . W h e n a n a m a t e u r 56

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astronomer " s a w " an unidentified point of light (the l o n g - a w a i t e d r e d e e m i n g U F O , w h i c h w a s soon a f t e r w a r d revealed to be the star S A O 141894) t r a i l i n g the comet, Do k n e w the t i m e had come. On March 23, d u r i n g the comet's closest approach w i t h Earth, the first w a v e of fifteen suicides took place, soon followed by the rest. A p p a r ently, they all d i e d w i l l i n g l y a n d happy, e a g e r to e m b r a c e their n e w form of existence.

T h u s S p o k e the S c i e n c e A p o l o g i s t It seems s o m e w h a t paradoxical that at the start of a n e w m i l l e n n i u m , d u r i n g w h a t we p r o u d l y refer to as the scientific a g e , m a n y people still look back to the prophecies of Revelation w i t h such fearsome a w e . T h e r e is g r o w i n g c y n i c i s m t o w a r d science, a sense of b e t r a y a l , of promises u n r e a l i z e d . After all, w a s not science supposed to be the n e w redeemer, the a c c u m u l a t e d k n o w l e d g e of the w o r l d , our s h i n i n g s w o r d to w a r d off the threats of u n p r e d i c t a b l e n a t u r e ? We get cures for m y r iad diseases only to discover new, incurable ones; we create n e w technologies that supposedly m a k e life easier a n d m o r e pleasant, only to spend more hours than ever at w o r k . Even worse, technology a d v a n c e s so fast that it is v i r t u a l l y impossible for most of us to k e e p up, and a vast "technological u n d e r c l a s s " is e m e r g i n g , reminiscent of the socially d i s placed rural m i g r a n t s in the m e d i e v a l cities. We can send a m a n to the Moon (or could, w h e n it w a s politically relevant) but cannot feed most of the world's population. We c o n s u m e the natural resources of our planet w i t h an appetite w o r t h y of one of Daniel's apocalyptic beasts, feeding our endless g r e e d for m a t e r i a l goods w i t h o u t l o o k i n g back at the devastation we often leave behind. A n d all this t h a n k s to "science"! So goes the credo of the discontent. N o w I must w e a r the robes of the Science Apologist a n d refute all the above accusations. "First a n d foremost, science does not promise redemption. Science is a h u m a n invention preoccupied w i t h u n d e r s t a n d i n g the w o r k i n g s of nature. It is a body of k n o w l e d g e about the universe a n d its m a n y =57

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inhabitants, l i v i n g a n d nonliving, a c c u m u l a t e d t h r o u g h a process of constant testing a n d refinement k n o w n as the scientific m e t h o d . W h a t the practice a n d study of science does provide is a path back to n a t u r e , a w a y of r e i n t e g r a t i n g ourselves w i t h the w o r l d a r o u n d us. In so d o i n g , it teaches us that the essence of n a t u r e â&#x20AC;&#x201D; f r o m the i n a n i m a t e to the a n i m a t e â&#x20AC;&#x201D; i s c h a n g e a n d transformation, that life a n d death a r e intert w i n e d in a cosmic chain of being. It w a s the ' d e a t h ' of a nearby star that t r i g g e r e d the formation of our S u n , w h e r e life b e c a m e possible in at least one m e m b e r of its court of planets a n d moons. If there w a s life near that original d y i n g star, it w a s destroyed w i t h it, the same w a y life h e r e w i l l be destroyed w h e n our S u n b u r n s out. T h i s d a n c e of creation a n d destruction is constantly h a p p e n i n g t h r o u g h o u t the universe, l i n k ing our histories, our lives a n d deaths, to a l a r g e r cosmic chain of t r a n s formation. As such, every l i n k is important, from w h a t we create a n d destroy in life to w h a t we leave behind. Science m a y not offer eternal salvation, but it offers the possibility of a life free from the spiritual slavery caused by an irrational fear of the u n k n o w n . It offers people the choice of s e l f - e m p o w e r m e n t , w h i c h m a y contribute to their spiritual freedom. In t r a n s f o r m i n g m y s t e r y into c h a l l e n g e , science a d d s a n e w d i m e n s i o n to life. A n d a n e w d i m e n s i o n opens m o r e paths t o w a r d selffulfillment." T h u s spoke the Science Apologist. "Second, science does not d e t e r m i n e w h a t is to be done w i t h its a c c u m u l a t e d k n o w l e d g e : we do. A n d this decision often falls into the h a n d s of politicians, chosen by society, at least in a democracy. T h e b l a m e for the d a r k e r uses of science must be shared by all of us: A r e we to b l a m e the inventor of g u n p o w d e r for all the d e a t h s from gunshots a n d explosives? Or the inventor of the microscope for the d e v e l o p m e n t of biological w a r f a r e ? W e , the scientists, have the duty to m a k e clear to the public w h a t we do in our labs, a n d w h a t consequences, good or bad, our inventions m a y have for society at l a r g e . But there is no such t h i n g as 'the scientists' as a g r o u p that shares a set of m o r a l s or v i e w s , or the b l a m e for the uses a n d abuses of science. T h e r e is, I w o u l d l i k e to believe, a c o m m o n set of g o a l s , to better u n d e r s t a n d the w o r l d a n d our place in it a n d , yes, to i m p r o v e our living conditions a n d health. It has

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been a r g u e d that defense b r i n g s peace, that the a c c u m u l a t i o n of an arsenal of destruction w a r d s off further a r m e d conflicts, at least l a r g e scale ones. We created a w a r w i t h o u t w i n n e r s . T h u s , m a n y in the defense and w e a p o n s industries see themselves as w a r d e n s of the peace and not creators of w e a p o n s of destruction. Personally, I see the need to collect w e a p o n s to g u a r a n t e e the peace as a sad confirmation of our collective stupidity." T h u s spoke the Science Apologist. " F i n a l l y , science has not betrayed our expectations. T h i n k of a w o r l d w i t h o u t antibiotics, computers, televisions, a i r p l a n e s , and c a r s â&#x20AC;&#x201D; a w o r l d in w h i c h we are all back in the forests and fields w h e r e we c a m e from, l i v i n g w i t h no technological comfort. H o w m a n y of us w o u l d be ready or w i l l i n g to do it? C a n you see yourself l i v i n g in some cave or p r i m i t i v e hut, h u n t i n g for food, physically fighting constantly for s u r v i v a l ? T h e r e is m u c h hypocrisy in the criticism of science a n d of w h a t it has done to us a n d to the planet. We d i d it all ourselves, through our choices, creativity, a n d g r e e d . It is not by s l o w i n g d o w n scientific research or its teaching, t h r o u g h legislation or censorship, that we will c h a n g e the i n e q u i t i e s of a technological society; that is surely a o n e - w a y ticket back to the M i d d l e A g e s . W h a t is needed is universal access to the n e w technologies, an 'internetization' of society at l a r g e , coupled w i t h a w i d e s p r e a d effort to p o p u l a r i z e science, its creations, and its consequences. Only a society w e l l versed in scientific issues will be able to dictate its o w n destiny, from the preservation of the n a t u r a l e n v i r o n m e n t to the m o r a l choices of genetic research. " W h a t h a p p e n e d to telephones a n d televisions w i l l also happen to the newest technologies; they w i l l become (almost) u n i v e r s a l l y a v a i l able. But there will a l w a y s be a l a g t i m e , and this d e l a y ostracizes m u c h of the l o w e r - i n c o m e population or those w i t h o u t access to the latest innovations. F r o m this 'access g a p ' is born the m o d e r n version of a technological

underclass,

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information a n d its production and dissemination, even though they m a y use it, as in the case of the H e a v e n ' s Gate sect. A n x i e t i e s soar, and the idea that conspiratorial g r o u p s are plotting to take over the w o r l d becomes plausible: movies such as Wag the Dog describe the fabrication 59

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of realities by the n e w s m e d i a ; h u g e l y p o p u l a r sci-fi television shows such as X-Files have plots based on a secret conspiracy i n v o l v i n g a partn e r s h i p b e t w e e n sectors of the U . S . g o v e r n m e n t a n d a l i e n s . T h o s e fears a r e contrasted w i t h the i m a g e s of the 'successful people,' the beautiful stars, the rich a n d famous, the ones w i t h access to all the n e w technotoys you can possibly find. As a result, m a n y people feel used a n d useless, m e r e spectators in a g a m e they can never play. T h i s situation, w i t h i n l i m i t s , can be c o m p a r e d to w h a t w a s h a p p e n i n g in Europe in the thirteenth century, w h e n a l a r g e u r b a n u n d e r c l a s s w a s developing. N o w a n d then, a c h a r i s m a t i c leader a p p e a r s , p r o m i s i n g salvation a n d r e d e m p t i o n , a n e w life for those that follow h i m or her. N o w a n d then, e x t r e m i s t r e l i g i o u s m o v e m e n t s are born, often b l e n d i n g C h r i s t i a n eschatology w i t h technological a n d p a r a m i l i t a r y e l e m e n t s , w h e r e the m e m b e r s of the g r o u p see themselves as the a g e n t s of c h a n g e , the key p l a y e r s in the great apocalyptic d r a m a . T h e 'access g a p ' w i d e n s into an abyss, a n x i e t y becomes anger, a n d a h u n g e r for justice blinds any vestige of social m o r a l s ; in their m i n d s , the final crusade is starting, a n d it m u s t be fought to the e n d . "Science, l i k e m u c h else, is completely helpless a g a i n s t this form of r e l i g i o u s e x t r e m i s m . T h e r e w i l l a l w a y s be people w h o find no other path to spiritual salvation but that offered by an a l l - o r - n o t h i n g k i n d of logic. Nevertheless, I believe there is hope. Science a n d religion should not be pitted a g a i n s t each other, but seen as t w i n paths to a better life. T h e i r c o m p l e m e n t a r i t y springs from their c o m m o n source, our fascination w i t h questions beyond our control a n d u n d e r s t a n d i n g . T h e y both express our a w e w i t h t h i n g s that are b i g g e r (and s m a l l e r ) than ourselves, a t t e m p t i n g to e x p a n d our vision of the w o r l d w i t h i n a n d w i t h o u t . T h e i r methods are certainly different, as are their i m m e d i a t e g o a l s , but most people need both. It w o u l d be a m i s t a k e to t h i n k that society could a d v a n c e w i t h a p u r e l y a n a l y t i c a l or w i t h a purely faithbased approach to existence." T h u s spoke the Science Apologist.

A P T E R

Making Worlds

All coming into being is mixture,

all perishing dissolution. ANAXAGORAS (CA.

CLAZOMENAE

5 O O - 4 2 8

B.C.E.)

T h e r e w a s g r e a t c o m m o t i o n i n A t h e n s that s u m m e r m o r n i n g . T h e y e a r w a s 354 B.C.E., a n d the t h i r t y - y e a r - o l d philosopher Aristotle h a d told his fellow m e m b e r s of the A c a d e m y that a g r e a t debate w a s being o r g a n i z e d in Delphi, h o m e of the famed a n d m y s t e r i o u s oracle. " W h y D e l p h i ? " a s k e d J u b o x u s , one of Aristotle's pupils, g i v e n to long contemplative bouts, inspired by n o t h i n g m o r e than the w h i s p e r i n g w i n d or the spirals of a seashell. S o m e historians consider h i m a follower of the philosopher P y t h a g o r a s , even t h o u g h most of these n u m b e r mystics had d i s a p p e a r e d by then. " H a ! " e x c l a i m e d Aristotle w i t h o u t h i d i n g his disdain. "Delphi has been chosen for being considered, at least by i g n o rant fools, a m a g i c a l place, w h e r e the spirits of the old 'physicists' m a y speak to us t h r o u g h the oracle." " F a n t a s t i c ! " e x c l a i m e d Juboxus, t r e m bling w i t h anticipation. "I w o u l d not get too excited w i t h all this hocus-pocus," said Popicas, a n o t h e r of Aristotle's pupils, m o r e focused and intense than J u b o x u s , but b a r e l y able to conceal his secret fascination w i t h the u n k n o w n . In his s p a r e time, he w a s keen on d e s i g n i n g

THE

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AND

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ASTRONOMER

impossible cities that defied all principles of then k n o w n architecture. Popicas w a s a visionary urbanist, w h o openly defied most G r e e k architects' blind slavery to the straight line. "Sinuosity is the w a y to re-create the w o r l d in our cities, not b o x y - l o o k i n g things! H a v e you ever seen a boxy cloud? W h a t is the line traced by the w i n g t i p of a s w a l l o w ? T h a t should be the outline of our roofs!" Popicas w a s w h a t we could call a theoretical architect, an artist m o r e than a builder. Little did he k n o w that his style w a s to be finally i m p l e m e n t e d , some t w e n t y - t h r e e hundred y e a r s after his death, by the Spanish architect Antoni G a u d i and others. T h e trip to Delphi w a s c h a r g e d w i t h philosophical discussion. An experienced mentor, Aristotle w o u l d provoke a r g u m e n t s a m o n g his pupils by stating an impossible proposition. One of his favorites w a s " T h e S u n is a hot rock." Since students love to prove their mentor w r o n g — a n d this is an i m p o r t a n t step in l e a r n i n g , for it promotes selfconfidence a n d dispels the m y t h of infallibility of a u t h o r i t y — t h e response w o u l d be a u t o m a t i c . Everyone had his o w n w a y of disproving the master's statement, a n d a lively debate w o u l d e n s u e w h i l e Aristotle patiently listened, n o d d i n g constantly, interjecting occasionally, a n d l o o k i n g m i l d l y impressed w i t h their creativity. L i k e most mentors, he l e a r n e d m o r e from his pupils than he cared to a d m i t . "But, Master," said Popicas, "you y o u r s e l f told us that the Moon a n d all celestial objects above it, i n c l u d i n g the S u n , a r e m a d e of a substance different from the four e l e m e n t s of our w o r l d — e a r t h , water, w i n d a n d fire. Is it not called the fifth essence, or e t h e r ? " To this J u b o x u s replied, " T h e S u n , being an object of the heavens, is a perfect sphere. As such it cannot be m a d e of the coarse m a t e r i a l of e a r t h l y things. It h u m s its o w n m e l o d i e s as it circles us in its celestial orb. Oh, how I w i s h I could hear it s o m e d a y ! " "Hey, J u b o x u s , m a y b e if you just stayed in Delphi forever, y o u w o u l d end up h e a r i n g it," q u i p p e d Popicas. T h e y accelerated their c l i m b t o w a r d Delphi, nested on the southern slopes of fabled M o u n t Parnassus. Others w e r e a l r e a d y w a i t i n g in the splendorous T e m p l e of A p o l l o , w h e r e T a l i a s , the oracle, w a s to conduct the seance-debate. T h e m e e t i n g in Delphi w a s to center on one question of g r e a t inter-

MAKING

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est to Greek philosophers, the p l u r a l i t y of w o r l d s : A r e there m a n y w o r l d s l i k e our o w n in the cosmos, or a r e we a l o n e ? T h i s question had been a hot topic of debate since the t i m e of the first "physicists," a g r o u p of people w h o , starting w i t h T h a l e s of M i l e t u s d u r i n g the sixth century B.C.E., tried to u n d e r s t a n d the w o r k i n g s of n a t u r e w i t h i n nature, without i n v o k i n g the actions of gods or God. Aristotle h a d a great interest in their ideas, a n d a good fraction of his w r i t i n g r e v i e w s and criticizes t h e m . T h e y a r e n o w k n o w n as the pre-Socratic philosophers, a l t h o u g h some of t h e m lived just after the birth of Socrates, a r o u n d 470 B.C.E. As Aristotle, Juboxus, Popicas, a n d other m e m b e r s of the A c a d e m y a r r i v e d , the place w a s a l r e a d y p a c k e d . People c a m e from as far a w a y as S y r a c u s e in southern Italy a n d Ephesus in the west. T h e seating w a s a r r a n g e d w e d g e - s h a p e d a r o u n d the altar, w h e r e the oracle w a s to p r e side over the ceremony. M a g n i f i c e n t statues of the g o d s , painted to look eerily real, flanked the center stage, flowers a n d fruits a d o r n i n g their m a r b l e feet. Excitement a n d the s t e a m i n g of herbs m a d e the a i r thick a n d moist, s p a r k s r e a d y to start flowing from h e a d to head. M u s i cians played ancient m e l o d i e s , said to have been composed by P y t h a g o ras himself. Outside, vendors sold retsina a n d bread soaked w i t h olive oil. Molten lead clouds s w i r l e d furiously o v e r h e a d , w h i l e a s t r a n g e mist e m e r g e d from the valley, as if escaping from an open w o u n d on the earth's crust. T h a n k s to the a m a z i n g p o w e r s of T a l i a s , the g r e a t oracle, two h u n d r e d y e a r s of k n o w l e d g e w e r e to coexist, the passage of t i m e m a g i c a l l y suspended. Philosophers from the P y t h a g o r e a n school w e r e to debate w i t h those of the atomistic school a n d those w i t h Plato a n d his followers, to the d e l i g h t a n d instruction of the astonished a u d i e n c e . All voices flowed from the s a m e source, the oracle w h o alone could tunnel in a n d out of the w o r l d of the spirits. Aristotle entrusted Juboxus w i t h t a k i n g notes, w h i c h w e r e to be discussed later in the A c a d e m y . As the beautiful T a l i a s finally a p p e a r e d on the altar, the a u d i e n c e fell silent, transfixed by her m a g n e t i c presence a n d majestic attire. H e r hair w a s black as ebony, a n d her skin m i l k y w h i t e . Over her w h i t e tunic, she w o r e a h u g e p e n d a n t encrusted w i t h a s h i n i n g ruby,

THE

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AND

THE

ASTRONOMER

supposedly the key to the other w o r l d . R u b b i n g his eyes constantly and pinching his s o m e w h a t p l u m p belly, a n a w e d J u b o x u s could h a r d l y concentrate on his task. T h e text below, The Stone of Aegospotami, is w h a t Juboxus m a n a g e d to jot d o w n on that m e m o r a b l e e v e n i n g , c o m plete w i t h c o m m e n t a r y a d d e d later.

T h e Stone of Aegospotami T a l i a s c l i m b e d on top of the famous c r a c k e d stones a n d took a long w h i f f of the y e l l o w sulfurous vapors that seeped out. W h e n her face e m e r g e d , she looked different, her eyes s h i n i n g w i t h an u n e a r t h l y light. S h e w a s not T a l i a s a n y m o r e , but c l a i m e d to be s p e a k i n g for A n a x i m a n d e r , the successor of T h a l e s , w h o m our master called the first philosopher. T h e y lived about t w o h u n d r e d years a g o [550 B.C.E.] in the t o w n of M i l e t u s , off the coast of L y d i a [ T u r k e y ) . A n a x i m a n d e r said,

Destruction a n d coming-to-be follow each other through the infinity of time. F r o m the Boundless come into being all the heavens a n d the w o r l d s w i t h i n them. A n d the source of all existing things is w h e r e destruction, too, happens, according to necessity; for injustice

they

pay

according

penalty to

the

and

retribution

assessment

each

other for

their

of Time.

Talias's distorted voice faded, a n d her face c h a n g e d back to n o r m a l . A g r o w i n g b u z z could be h e a r d from the a m a z e d a u d i e n c e . So this w a s A n a x i m a n d e r ' s teaching: that the cosmos is eternal a n d e v e r y t h i n g in it comes from this abstract q u a n t i t y called Boundless a n d goes back to it in endless cycles. A nice d y n a m i c a l picture of n a t u r e , if you ask me. T h i s business of "penalty a n d retribution" fits neatly into his picture, for there must be a balance b e t w e e n creation a n d destruction. T h i s is h o w i n j u s t i c e â&#x20AC;&#x201D; t h e i m b a l a n c e â&#x20AC;&#x201D; i s avoided as t i m e passes. But people w e r e interested mostly in A n a x i m a n d e r ' s use of the w o r d " w o r l d s " in the p l u r a l . Did he m e a n there w e r e m a n y coexisting w o r l d s out there

MAKING

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in the cosmos, being created a n d destroyed all the t i m e , or that our world has been t h r o u g h m a n y cycles of existence, flowing back into the Boundless just to r e e m e r g e a g a i n in the fullness of t i m e ? A l t h o u g h it is tempting to defend the first position, evidence points t o w a r d the second: that A n a x i m a n d e r thought only of our w o r l d ' s being created a n d destroyed in cycles. For h i m , the S u n , Moon, a n d stars w e r e not other " w o r l d s , " but fire escaping from holes in cosmic w h e e l s s u r r o u n d i n g the Earth. It is c u r i o u s , t h o u g h , his idea of the Earth g o i n g through m a n y existences. H o w d i d h e e n v i s a g e our w o r l d ' s destruction? M a y b e he shared the i m a g e the g r e a t Plato later w r o t e in his Timaeus, w h e n e x p l a i n i n g how n a t u r a l disasters of terrible consequences happen from time to time:

There is a story,

that once upon

time Phaeton,

the son

of Helios,

having yoked the steeds in his father's chariot, because he was not able to drive them, in the path of his father, burnt up all that was upon the Earth,

and was himself destroyed by a

thunderbolt.

Now this has the

form of a myth, but really signifies a declination of the bodies moving in upon

the

heavens

the

Earth

around the which

Earth,

and a great

recurs after long

conflagration

of things

intervals.

Another connection comes to m i n d here. I r e m e m b e r that A n a x i m a n der's disciple A n a x i m e n e s conjectured that the " n a t u r e of the heavenly bodies is fiery, a n d that they have a m o n g t h e m certain earthly bodies that a r e carried round w i t h t h e m , being invisible." About a h u n d r e d years after A n a x i m e n e s , s i m i l a r ideas w e r e echoed in the teachings of Diogenes of A p o l l o n i a . He also believed

that m a n y such bodies

revolved unseen in the sky. It sounded to me l i k e a very plausible suggestion, and I b r o u g h t it up to the a u d i e n c e . M a y b e these invisible bodies are the ones that fall on Earth, c a u s i n g the destruction Plato mentions? Master Aristotle w a s q u i c k to explain that, for h i m , heavenly bodies stay in the heavens; that rocks that sometimes fall on Earth c a m e from the g r o u n d a n d not from the sky, being first removed by some violent force, l i k e a storm, h u r r i c a n e , or volcano eruption, a n d

THE

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then r e t u r n i n g to w h e r e they belong. S o m e of us w e r e not as convinced, especially g i v e n the size of the famous "stone of A e g o s p o t a m i , " w h i c h fell there some 113 y e a r s a g o |467 B.C.E.]. In fact, this event inspired the ideas of Diogenes I mentioned above. But Master Aristotle r e m i n d e d us that the fury of n a t u r e should not be u n d e r e s t i m a t e d . W i t h that, Talias returned to the altar a n d took another w h i f f of the foul-smelling stuff. H e r body t r e m b l e d , possessed by invisible w a v e s , a n d , as she spoke, she uttered the w o r d s of A n a x a g o r a s of C l a z o m e n a e :

S o m e say I predicted the fall of the stone of A e g o s p o t a m i . Of course, I d i d no such thing, a l t h o u g h it honors me that people think I k n o w the w a y s of the heavens. But some things I do k n o w , that the Sun gives the Moon its brightness, and the stars, their being

revolution as

ing mass,

air

pass and

which

beneath ether

is infinite in

the cold and the dark came while part off by

the of the

the

were

Earth.

being

The

separated

number.

the force

these

and speed.

things

where went

rotated

Their speed

from

came the

into

surround-

The dense and the moist and

together here,

rare and the hot and the dry ether.

world off

thus like

the

Earth

now

is,

outwards

the further

[theyI

were

separated

the

speed

of nothing

that now exist among men, but it is altogether many times as fast.

I l i k e the idea that the S u n g i v e s light to the Moon; Master Aristotle d i s a g r e e s , for he t h i n k s that all heavenly bodies, S u n , Moon, stars, a n d planets, a r e m a d e of ether a n d g e n e r a t e their o w n light. But my favorite w a s A n a x a g o r a s ' s m e c h a n i s m for the formation of Earth. A p r i m o r d i a l rotation separated the solid matter, w h i c h falls to the center, from the hot stuff, w h i c h drifted o u t w a r d . T h e rotation g e n e r a t e d a "force" that facilitated the compression of m a t t e r seeds into l a r g e r solid bodies. As far as I k n o w , this w a s the first t i m e someone proposed such a m e c h a n i s m , a n d that alone deserves a lot of credit. T h e rest of the a u d i e n c e w a s not so impressed. But then a g a i n , some ideas, even if correct, take t i m e to flourish. I think this is one of them. T a l i a s , l o o k i n g paler than she a l r e a d y did before the ceremony, 68

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once m o r e c l i m b e d the steps t o w a r d the sacred stones. W h o m else could she possibly be " r e c e i v i n g " ? I never thought the life of an oracle was this h a r d ! S h e e m e r g e d from the vapors w i t h an odd smile on her face. We then k n e w , even before she spoke, that she w a s D e m o c r i t u s , the atomistic philosopher k n o w n as the " l a u g h i n g one." T h i s is w h a t "he" said: E v e r y t h i n g is in constant motion in the void. T h e r e are i n n u m e r a b l e w o r l d s , w h i c h differ in size. In some w o r l d s there is no sun and moon, in others they are l a r g e r than in our w o r l d , a n d in others m o r e n u m e r o u s . In some parts they a r e a r i s i n g , in others failing. T h e y are destroyed by collision one w i t h another. T h e r e a r e some w o r l d s devoid of l i v i n g creatures or plants or any moisture.

T h e r e w a s the idea of the cataclysmic collisions of w o r l d s once a g a i n ! L o o k i n g over my shoulder, I saw Master Aristotle s q u i r m i n g on his seat. I m a g i n e that, other w o r l d s w i t h l i v i n g creatures a n d plants! Democritus or, rather, his "voice" then e x p l a i n e d how these w o r l d s were formed, also through the s w i r l i n g of matter, s o m e w h a t l i k e A n a x a g o r a s , but in m u c h more detail. He a n d L e u c i p p u s believed everything w a s m a d e of indivisible atoms m o v i n g in the void, w h i c h , through the revolutions of a k i n d of vortex, w o u l d c o n g r e g a t e , l i k e to like. It w a s s o m e t h i n g l i k e a rotating disk of matter, w h e r e earthy atoms w o u l d concentrate in the center forming Earth, w h i l e others w o u l d rotate a r o u n d it, c o a g u l a t e w i t h their neighbors, a n d ignite to become the heavenly bodies, after their original moisture d r i e d up. A lot of the details w e r e m i s s i n g , but this w a s the g e n e r a l picture, at least w h a t I could capture of it. T a l i a s collapsed to the g r o u n d , a n d w a s taken to her private c h a m b e r s by other priestesses. It w a s t i m e for us to d r i n k retsina, a r g u e some m o r e , a n d dance to joyous m u s i c . As we w a l k e d outside, we could not help looking u p w a r d , the countless stars saluting us for our intellectual c o u r a g e . Or m a y b e they w e r e m o c k i n g our i g n o r a n c e ? 69

THE

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AND

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Evil Also Rises We can only m a r v e l at the pre-Socratic i m a g i n a t i o n . A l t h o u g h , from our m o d e r n perspective, some of their ideas w e r e q u i t e off the m a r k , others w e r e s t r a n g e l y prescient a n d a r e still w i t h us, in one form or another. Today, the k e y w o r d to describe the natural w o r l d is " c h a n g e , " from its smallest components to the u n i v e r s e as a w h o l e . Even the idea of e q u i l i b r i u m , w h i c h seems to i m p l y the absence of c h a n g e , is often related to a d y n a m i c balance of i n p u t a n d output; t h i n k of l i v i n g systems, from single-cell bacteria to w h a l e s , w h i c h continuously t a k e a n d g i v e back to their s u r r o u n d i n g e n v i r o n m e n t . S o m e t i m e s the timescale for these c h a n g e s is so long, c o m p a r e d w i t h h u m a n scales, that we are fooled into t h i n k i n g of stability. But m o u n t a i n s c h a n g e , a n d stars move in the heavens. L e u c i p p u s a n d D e m o c r i t u s called the basic constituents of matter atoms; we call them e l e m e n t a r y particles, but the idea that matter is m a d e up of f u n d a m e n t a l constituents is essentially the same. A n d from the fictional p i l g r i m a g e to Delphi we l e a r n e d that some preSocratics e n v i s a g e d a cosmos w i t h m a n y coexisting w o r l d s , some of them invisible to us, some empty, a n d others populated by a n i m a l s a n d plants. T h e atomists, possibly i n s p i r e d by A n a x a g o r a s a n d Diogenes, w e n t so far as to suggest a d y n a m i c a l m e c h a n i s m for the creation a n d destruction of w o r l d s : w o r l d s are created from the continuous a g g r e gation of atoms i n d u c e d by the rotation of a proto-disk of m i x e d matter, w h i l e w o r l d s are destroyed by collisions w i t h other w o r l d s . T h e s e processes a r e r a n d o m and purposeless, o c c u r r i n g in the "fullness of t i m e . " In the w o r d s of the R o m a n poet L u c r e t i u s , " N e v e r suppose the 3

a t o m s had a p l a n . " T h e s e ideas, as we w i l l see, a r e incredibly m o d e r n . But Aristotle w o u l d have none of it. H i s system of the w o r l d w a s radically different from that of the atomists. C h a n g e w a s relegated to the s u b l u n a r y sphere, the heavens a n d its bodies b e i n g u n c h a n g e a b l e , m a d e of ether, the fifth essence (see figure 9 in insert). T h i s division of the cosmos into t w o r e a l m s forced A r i s t o t l e to devise a very ingenious, if not fanciful, theory of comets a n d shooting stars. Since they w e r e 70

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clearly t i m e - d e p e n d e n t events, they had to belong to the earthly r e a l m , w h e r e the four e l e m e n t s , w i t h their combinations a n d motions, promoted the c h a n g e we observe. H i s solution w a s to consider these celestial events as b e l o n g i n g in the upper a t m o s p h e r e , the "outermost part of the terrestrial w o r l d w h i c h falls below the circular motion [ l u n a r y 4

s p h e r e | . " To Aristotle, comets a n d shooting stars w e r e meteorological p h e n o m e n a , caused by exhalations "of the right consistency" that rose up a n d w e r e ignited by the dryness of the upper skies, being subsequently c a r r i e d a r o u n d by the c i r c u l a r motion of the nearby l u n a r y sphere. R a p i d , intense fires that b u r n e d q u i c k l y w e r e the cause of the 5

"'shooting' of scattered 'stars.' " A comet, by contrast, w a s produced w h e n a "fiery principle," intense e n o u g h to sustain its b u r n i n g for a long t i m e a n d t a m e e n o u g h not to be so bright, met the right k i n d of exhalation rising from Earth. T h e shapes d e p e n d e d on the b u r n i n g patterns of the exhalation: "If [the e x h a l a t i o n ] diffused e q u a l l y in every side the star is said to be fringed, if it stretches in one direction it is called b e a r d e d . "

T h e bearded exhalation " e x p l a i n e d " the tails of

comets. Aristotle even related the presence of fiery comets to w e a t h e r patterns such as d r o u g h t s a n d strong w i n d s . L i k e m a n y of his successors, he v i e w e d comets as portents, w h i c h could be used in w e a t h e r prognostication.

illustrated

his

argument

with

the

meteorite

A e g o s p o t a m i , w h i c h he supposed had been swept up by w i n d s caused by

the

presence

of a

comet:

"For

instance,

when

the

stone at

Aegospotami fell out of the a i r â&#x20AC;&#x201D; i t had been c a r r i e d up by a w i n d a n d fell d o w n in d a y t i m e â&#x20AC;&#x201D; t h e n too a comet h a p p e n e d to have a p p e a r e d in 7

the w e s t . " Aristotle's theory of comets and shooting stars w a s consistent w i t h his dismissal of the atomists' case for the existence of other, E a r t h - l i k e w o r l d s . Since these w o r l d s w e r e necessarily m a d e of the four basic e l e m e n t s , they just could not fit into Aristotle's scheme of concentric spheres, w h e r e every e l e m e n t moved n a t u r a l l y in the r e a l m w h e r e it belonged; other e a r t h l y w o r l d s w o u l d all c o a g u l a t e into this one, just as a stone falls back to the g r o u n d , w h e r e it belongs. We see, then, a split of opinions concerning the existence of other m a t e r i a l

PROPHET

AND

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w o r l d s into m a i n l y t w o g r o u p s : Aristotle a n d his followers e x p l a i n i n g comers, shooting stars, a n d other sudden celestial apparitions as meteorological p h e n o m e n a , Earth being the only m a t e r i a l w o r l d subject to c h a n g e ; a n d the atomists, d e f e n d i n g the existence of other material w o r l d s , coexisting w i t h our o w n , w h i c h sometimes m a y become visible as comets or shooting stars, or even collide w i t h us. For Aristotle, comets foretold w e a t h e r - r e l a t e d c a t a c l y s m s ; for the atomists, entire w o r l d s m i g h t be destroyed by colliding w i t h other w o r l d s , w h i c h circ l e — s o m e t i m e s i n v i s i b l y — i n the heavens. Aristotle's v i e w s on comets, l i k e m u c h else he wrote, w a s to g r e a t l y influence astronomical thought well into the Renaissance. One discord a n t voice worth m e n t i o n i n g for its great intellectual c o u r a g e w a s that of Seneca (ca. 4 B.C.E.—65 C.E.), the Spanish-born R o m a n orator and Nero's tutor, whose Natural Questions a r g u e d for a cosmic origin of comets. He shared m a n y of the opinions of a pre-Socratic k n o w n as A p o l l o n i u s of M y n d o s (fourth century B.C.E.), w h o c l a i m e d ,

M a n y comets a r e planets . . . a celestial body on its o w n , l i k e the S u n a n d the Moon. . . . A comet cuts t h r o u g h the upper regions of the universe a n d then finally becomes visible w h e n it reaches the lowest point of its orbit. . . . S o m e a r e bloody, m e n a c i n g — they c a r r y before t h e m the o m e n of bloodshed to c o m e .

Seneca a g r e e d w i t h A p o l l o n i u s that comets w e r e celestial objects, but not that they w e r e planets. After a l l , he a r g u e d , we can see stars t h r o u g h a comet's tail but not through planets. He believed they w e r e objects of their o w n , " a m o n g Nature's p e r m a n e n t creations," to be approached w i t h the reverence of things divine: "God has not m a d e all t h i n g s for man.'"' W i t h incredible foresight, he speculated that comets m i g h t return in their celestial orbits, albeit at "vast i n t e r v a l s , " a n d predicted, " T h e t i m e w i l l come w h e n d i l i g e n t research over very long periods w i l l bring to light t h i n g s w h i c h now lie h i d d e n . . . . T h e r e will come a t i m e w h e n our descendents will be a m a z e d that we d i d not k n o w t h i n g s that are so plain to t h e m . " 72

Seneca's lucid defense of a

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rational approach to the study of n a t u r a l p h e n o m e n a is t e m p e r e d by his belief that comets served a d i v i n e purpose, w h i c h people could use for forecasting the future, if only they understood the message: " T h e roll of fate is unfolded on a different principle, sending a h e a d e v e r y w h e r e indications of w h a t is to come, some f a m i l i a r to us, others u n k n o w n . " " He believed thece w a s a d i v i n e cosmic plan, to w h i c h comets belonged. If we had access to its m e a n i n g , we could read the future l i k e an open book. Even those t h i n k e r s w h o c l a i m e d to defend a rational approach to the w o r l d could not s h a k e themselves loose from the a l l u r e of cometl i n k e d prognostication. Seneca d i e d five years before the destruction of the Second T e m p l e in J e r u s a l e m , w h e n C h r i s t i a n i t y started to spread its seeds w e s t w a r d . F r o m his lessons, the l i n k b e t w e e n comets a n d prognostication w o u l d s u r v i v e a n d be most influential t h r o u g h o u t the M i d d l e A g e s . O r i g e n of A l e x a n d r i a (ca. 1 8 5 - 2 5 3 ) , an e a r l y defender of an e v e n h a n d e d , allegorical interpretation of Revelation w h o lived a h u n d r e d y e a r s before S a i n t A u g u s t i n e , suggested that the S t a r of Bethl e h e m , seen d u r i n g Jesus' birth, w a s a comet or a meteor. He a r g u e d that, a l t h o u g h such objects u s u a l l y prognosticated the fall of dynasties and the outbreak of war, they m i g h t also signal an auspicious event. T h e e a r l y - f o u r t e e n t h - c e n t u r y Italian master Giotto di Bondone, the first painter to truly a n i m a t e C h r i s t i a n figures, seems to have shared this point of v i e w ; his 1304 Adoration of the Magi d i s p l a y s the S t a r of Bethlehem as a comet. In fact, his model w a s H a l l e y ' s comet, w h i c h Giotto saw in 1301 (see figure 10 in insert). Origen's optimistic t a k e on comets w a s by far the exception. If Giotto indeed placed the comet as a positive sign, others interpreted it as a r e m i n d e r of the forthcoming j u d g m e n t . C h i e f a m o n g t h e m w a s T h o m a s A q u i n a s (ca.

1225-1274), w h o not only discredited the

cometary n a t u r e of the S t a r of B e t h l e h e m (comets do not a p p e a r d u r ing d a y t i m e , w a s one of his a r g u m e n t s ) but even attached d o o m s d a y significance to a n y comet: "On the seventh d a y all the stars, both planets and fixed stars, w i l l throw out fiery tails l i k e comets," he w r o t e .

Not m u c h w a s a d d e d to cometary theory or the discussion of other 75

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w o r l d s d u r i n g the M i d d l e A g e s . O n l y in the fifteenth century w i l l we see a c h a n g e , if not in f o r m u l a t i n g n e w ideas about comets, at least in m a k i n g better m e a s u r e m e n t s of their positions. Even t h o u g h this increase of interest w a s t r i g g e r e d , not s u r p r i s i n g l y , by astrology, the e m p h a s i s on accuracy is historically important. A curious exception to the contagious a l l u r e of star-based prophecy is one w o r k of Johannes M i i l l e r (1436—1476) of K o n i g s b e r g , k n o w n as R e g i o m o n t a n u s . In his short treatise on comets k n o w n as The Sixteen Problems

the

Magnitude,

Longitude,

and

True

Location

of Comets,

R e g i o m o n t a n u s explicitly avoided l i n k i n g his ideas on h o w to m e a s u r e distances a n d properties of comets to astrology, about w h i c h he also w r o t e at l e n g t h . He w a s interested in o b t a i n i n g actual n u m b e r s describing comets, such as the sizes of their h e a d a n d tail, their distance to Earth a n d v a r i a b l e a p p e a r a n c e , a m o n g others. H i s style w a s very m o d e r n in that it w a s concise, g e o m e t r i c a l , a n d devoid of astrological speculation. U s i n g a m e t h o d for m e a s u r i n g distances of celestial objects k n o w n as p a r a l l a x , R e g i o m o n t a n u s a l l e g e d l y estimated that the comet of 1472 w a s at least 1,822 m i l e s a w a y .

1 3

Since he w a s off by q u i t e

a bit (the a v e r a g e distance to the Moon is r o u g h l y 240,000 m i l e s ) , he found no r e a s o n — a n d did not w a n t a n y — t o c h a l l e n g e the Aristotelian belief that comets w e r e sublunary. H o w e v e r , his p i o n e e r i n g use (or at least proposed use) of p a r a l l a x m a r k e d a g i a n t step t o w a r d a m o r e precise astronomy, because only accurate m e a s u r e m e n t s w i l l force the revision of deeply i n g r a i n e d preconceptions. It is simple to see h o w p a r a l l a x w o r k s : place a finger right in front of y o u r nose, a n d look at it w i t h the left eye only a n d then w i t h the right e y e only; you will see y o u r finger " m o v e " w i t h respect to objects in the b a c k g r o u n d . N o w stretch y o u r a r m a n d do the same; y o u r finger will still move, but m u c h less. If you had a very long a r m , it w o u l d move even less. T h e displacement a n g l e of y o u r finger w i t h respect to a fixed object in the b a c k g r o u n d is called parallax. T h e s a m e is true for a celestial object: the closer it is to us, the more it w i l l be displaced w i t h respect to b a c k g r o u n d constellations, w h e n seen from different points on Earth's surface (see figure 11 in insert). T h e m e a s u r e d p a r a l l a x of the

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comet of 1472 w a s 6 degrees. Its actual v a l u e is estimated w i t h m o d e r n techniques at 3"—that is, 3 arc seconds, or

/3600

of a d e g r e e — c l e a r l y

impossible to m e a s u r e w i t h the n a k e d eye or the instruments of the time. T h e smallest a n g l e we can discern w i t h the n a k e d eye is about 1'—an arc m i n u t e , or Veo of a d e g r e e . D u r i n g the sixteenth century, t w o voices c o n c e r n i n g comets a n d other celestial apparitions w e r e h e a r d side by side: w h i l e religious l e a d ers and d o o m s d a y "prophets" fed even m o r e superstitions a n d fear, a g r o w i n g n u m b e r of astronomers started p a y i n g m o r e attention to d a t a and less to astrological prognostication. T h i s is not to say that debates about the physical n a t u r e of comets a n d the existence of other w o r l d s were w i d e s p r e a d in the 1500s; the reputation of comets as h a r b i n g e r s of doom w a s stronger than ever a n d mostly d w a r f e d any interest in a

FIGURE

2 :

Anonymous engraving depicting mysterious celestial phenomena, Heidel-

berg (1622). Left , two illustrations of multiple suns surrounded by rainbows; right, a comet and a cross-shaped apparition. All these phenomena were considered to be bad omens. (WOP-\. Courtesy Adler Planetarium and Astronomy Museum, Chicago.)

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q u a n t i t a t i v e description of celestial events. T h e a p p a r i t i o n of a comet in 1577 prompted A n d r e a s C e l i c h i u s , bishop of A l t m a r k , a l e a d i n g L u t h e r a n reformer, to offer the following reflection on its n a t u r e a n d purpose:

T h e thick s m o k e of h u m a n sins, rising every day, every hour, every m o m e n t , full of stench a n d horror, before the face of God, a n d b e c o m i n g g r a d u a l l y so thick as to form a comet, w i t h curled a n d plaited tresses, w h i c h at last is k i n d l e d by the hot a n d f i e r y a n g e r o f the S u p r e m e H e a v e n l y J u d g e .

T h e Aristotelian m e c h a n i s m for the formation of comets, exhalations rising from Earth i g n i t e d in the upper parts of the atmosphere, is transformed by apocalyptic rhetoric into evil rising from the souls of sinners a n d ignited by the w r a t h of God. In 1579, A n d r e a s D u d i t h , a H u n g a r i a n cleric a n d h u m a n i s t , replied that " i f comets w e r e caused by the sins of m o r t a l s , they w o u l d never be absent from the s k y . "

doubt! T h e debate on the theological n a t u r e of comets is to be contrasted w i t h the discovery, in the 1530s, that a comet's tail a l w a y s points a w a y from the S u n . Progress w a s slow but present.

Cosmic Boils It w a s the g r e a t D a n i s h astronomer T y c h o B r a h e ( 1 5 4 6 - 1 6 0 1 ) w h o d e l i v e r e d the first serious blow to the Aristotelian cosmic order. In 1543, C o p e r n i c u s published his treatise placing the S u n at the center of the cosmos, the planets plastered to concentric spheres a r o u n d it; as the spheres t u r n e d , so d i d the planets. At first sight, Copernicus's move w a s e x t r e m e l y d a r i n g a n d courageous for the times. H o w e v e r , in his m i n d , forward a n d b a c k w a r d t h i n k i n g w e r e deeply e n m e s h e d : his Renaissance-inspired aesthetical o r d e r i n g principle, of h a v i n g a cosmic a r r a n g e m e n t c o m m e n s u r a t e w i t h the planetary orbital p e r i o d s â&#x20AC;&#x201D; M e r cury, the shortest, first from the S u n , S a t u r n , the longest, l a s t â&#x20AC;&#x201D; w a s 76

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backed by an attempt to satisfy the Platonic ideal of celestial objects m o v i n g in circles w i t h constant velocities."' T h e so-called C o p e r n i c a n revolution w a s to depend on the efforts of Copernicus's successors, namely, Tycho B r a h e , Johannes Kepler, and Galileo Galilei, r e a c h i n g its c l i m a x in the w o r k of Isaac N e w t o n d u r i n g the late seventeenth century. As we shall see, these a d v a n c e s in the u n d e r s t a n d i n g of celestial m e c h a n i c s initiated only a very g r a d u a l c h a n g e in the w i d e s p r e a d belief that comets and other unexpected celestial visitors w e r e d i v i n e messengers. T h e rationally based description of their motions led to a reinterpretation of their purpose; instead of being m e r e tokens of God's w r a t h , they w e r e seen as necessary a g e n t s of c h a n g e , w h i c h shaped cosmic history

and

its cataclysmic

transformations.

A l t h o u g h doom

became progressively m o r e r a t i o n a l i z e d , the religious symbolism of comets and meteors r e m a i n e d very m u c h a l i v e , albeit d i s g u i s e d by a scientific l a n g u a g e of q u a n t i t a t i v e precision. On N o v e m b e r 11, 1572, a " n e w star" flared in the constellation of Cassiopeia (the one that looks l i k e a slanted W).* T h e eyes of most astronomers in Europe (and e l s e w h e r e ) w e r e riveted i m m e d i a t e l y on this n e w and e x t r a o r d i n a r y object. " W h a t w a s it? W h e r e w a s i t ? " investigators a s k e d . T h e first a n s w e r s to these t w o questions w e r e , of course, Aristotelian: it is a sort of vapor condensation, h o v e r i n g below the l u n a r y sphere. S o m e (erroneous) p a r a l l a x m e a s u r e m e n t s seemed to confirm this. But T y c h o w a s not convinced. He m e t h o d i c a l l y observed the star until it faded out of sight in M a r c h . H i s conclusions w e r e c r y s tal clear: the n e w star w a s not a planet, since it did not move a n d , u n l i k e planets, w a s far from the zodiac, the belt of t w e l v e constellat i o n s â&#x20AC;&#x201D; p o p u l a r from h o r o s c o p e s â&#x20AC;&#x201D; w h e r e the planets p a r a d e . It w a s also not a comet, since comets m o v e w i t h respect to the fixed stars. Moreover, because the star failed to show any p a r a l l a x , it w a s certainly farther a w a y than the Moon. C o n t r a r y to Aristotelian belief, the skies

* N o w a d a y s , we call this event a s u p e r n o v a explosion, w h i c h m a r k s the end of a large star's life cycle. In part 3, we will discuss s u p e r n o v a e in some detail.

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could, after all, c h a n g e . Tycho's observations, of u n p r e c e d e n t e d accuracy, w e r e published in a book, De Nova Stella ( T h e N e w S t a r ) , w h i c h also contained astrological interpretations of the event; after a l l , these w e r e transitional times. Other a s t r o n o m e r s , such as T h o m a s D i g g e s in E n g l a n d a n d M i c h a e l M a s t l i n i n G e r m a n y , a g r e e d w i t h T y c h o , publ i s h i n g their o w n observations, w h i c h they c l a i m e d offered support to the C o p e r n i c a n system. E c h o i n g R e g i o m o n t a n u s ' s m o d e r n i t y , they abstained from i n c l u d i n g astrology in their treatises. A few decades later, the poet John Donne's w o r d s illustrated h o w s l o w l y astronomical discoveries percolated t h r o u g h society,

W h o v a g r a n t transitory comets sees, W o n d e r s because t h e y ' r e rare; but a n e w star W h o s e motion w i t h the f i r m a m e n t a g r e e s , Is m i r a c l e ; for, there no n e w things a r e .

1 7

S o m e t i m e s a felicitous conspiracy of t h i n g s happens at the right t i m e to the r i g h t people. T h e skies w e r e q u i t e g e n e r o u s to Tycho, w h o relentlessly pushed f o r w a r d his observational p r o g r a m based on a c c u r a t e and methodical observations. On N o v e m b e r 13, 1577, w h i l e casting a net to catch some fish before sunset, T y c h o looked west a n d s a w a " b r i g h t star w h i c h a p p e a r e d as distinct as V e n u s . " At night, he m e a s u r e d the comet's tail to be over 21 d e g r e e s long, positioned at the " e i g h t h house," w h i c h , a c c o r d i n g to Tycho's interpretation, signified forthcoming pestilence. Once it m o v e d to the ninth house, the house of religion, Tycho predicted the rise of n e w sects. W h i l e busy p o n d e r i n g the astrological significance of the celestial n e w c o m e r , T y c h o also d e t e r m i n e d that, l i k e the n e w star in Cassiopeia, the comet h a d no p a r a l l a x , being thus beyond the l u n a r y sphere. He estimated it to be at least 230 Earth radii a w a y (Earth's r a d i u s is 6,378 k i l o m e t e r s ) . C o m e t s w e r e finally let loose from the Aristotelian c h a i n s that kept t h e m anchored to Earth; they w e r e n o w free to r o a m the cosmos a n d shatter the crystalline spheres, w h i c h c a r r i e d the planets in their o r b i t s â&#x20AC;&#x201D; a n d a n y t h i n g else on their paths.

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Tycho's conclusions that comets w e r e far beyond the Moon did very little to assuage the w i d e s p r e a d belief in t h e m as messengers of doom. Superstition about the comet of 1680, m o r e than one h u n d r e d y e a r s later, w a s as g r e a t as ever. In 1684, John E d w a r d s (1637â&#x20AC;&#x201D;1716), a w e l l k n o w n C a l v i n i s t from C a m b r i d g e , w r o t e in his Cometomania that, apart from the comet's usual i m a g e as h a r b i n g e r s of death, scarcity, famine, a n d the death of k i n g s , these "funeral torches to light K i n g s to their tombs" also had a h e a l i n g function for the heavens, b e i n g l i k e cosmic boils that collected corrupt m a t t e r a n d expelled it out, as pus from a wound. Comets were

a long Collection of corrupt a n d filthy m a t t e r . . . as in a M a n ' s putrid H u m o u r s often g a t h e r in one part. . . . A n d these . . . e x c r e m e n t a l h u m o u r s b r e a k i n g out, the A e t h e r (like the Body of M a n ) is thereby kept S o u n d a n d H a l e . . . . T h e S u n a n d other L u m i n a r i e s fare the better for the expulsion of this gross stuff w h i c h w o u l d o t h e r w i s e over-run t h e m . . . .

An opinion split h a d developed d u r i n g the m i d - 1 6 5 0 s , mostly in England a n d F r a n c e , c o n c e r n i n g the relation between comets a n d prognostication. Astrology w a s d e c r e a s i n g in popularity, in p a r t i c u l a r a m o n g the r u l i n g a n d e d u c a t e d classes, w h o c l a i m e d it responsible for social unrest a n d the c h a l l e n g i n g of the established order; rebels felt fortified by messages of the fall of k i n g s , a n d prognostication often t u r n e d into self-fulfilling prophecy. Astrology a n d related secular prophecy w e r e perceived as potential revolutionary w e a p o n s of g r e a t importance; if God w r o t e his w i l l in the stars a n d the stars said the k i n g w i l l die, then he m u s t die in order to fulfill God's w i l l . S o , let's go kill h i m ! D a v i d Gregory, of Oxford University, expressed this negative sentiment t o w a r d astrology w h e n he wrote in 1686:

We prohibit Astrology to t a k e place in our A s t r o n o m y , since it is supported by no solid f u n d a m e n t , but stands on the utterly r i d i c u l o u s opinions of certain people, opinions that are so 79

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framed as to promote the attempts of m e n t e n d i n g to form factions.

A n d yet, he a l l o w e d that comets m i g h t promote n a t u r a l disasters if they ever c a m e into contact w i t h Earth, their tails tainted w i t h noxious vapors that could poison our a t m o s p h e r e a n d l i v i n g beings. W h i l e most of the population still n u r t u r e d the i m a g e of comets as s u p e r n a t u ral portents, m e s s e n g e r s of God's w r a t h , the e d u c a t e d classes d e e m e d this v i e w " v u l g a r , " accepting only their n a t u r a l influences on Earth: l i k e m u c h else in E n g l a n d , opinions on comets a n d celestial p h e n o m ena became related to class.

The Rationalization of Doom Isaac N e w t o n w a s c e r t a i n l y listening to the debate on w h e t h e r comets w e r e portents, "cosmic boils," or just n a t u r a l p h e n o m e n a . T h e comet of 1680 a p p e a r e d " t w i c e , " in N o v e m b e r a n d December, a n d N e w t o n carefully followed its path, filling his notebook w i t h c o m m e n t s a n d d r a w i n g s . A l t h o u g h John F l a m s t e e d , the astronomer royal of E n g l a n d , insisted that the t w o a p p a r i t i o n s w e r e the s a m e comet at different points in its orbit, N e w t o n i n i t i a l l y rejected that c l a i m . At the t i m e he, l i k e E d m o n d H a l l e y , thought that comets obeyed l a w s distinct from those of planets; he leaned t o w a r d the rectilinear-path hypothesis proposed by Kepler e a r l i e r in the seventeenth century (see figure 12 in insert). H o w e v e r , by 1683, N e w t o n , g r o w i n g i n c r e a s i n g l y dissatisfied w i t h the poor fit b e t w e e n theory a n d d a t a , h a d completely c h a n g e d his m i n d . D u r i n g the fall of 1684, a n d three comets later ( H a l l e y ' s in 1682, followed by t w o others in 1683 a n d 1684), N e w t o n put f o r w a r d his De Motu, a short d o c u m e n t in w h i c h he proposed that the motions of p l a n ets a n d comets w e r e g o v e r n e d by a universal l a w of g r a v i t a t i o n that v a r i e d w i t h the inverse s q u a r e of the distance b e t w e e n the celestial objects a n d the S u n . C o m e t s , N e w t o n s u r m i s e d , m o v e d in curved paths, some of t h e m closed. T h i s inspired H a l l e y , a devoted follower of

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Newton's ideas, to conclude in 1705 that the 1682 comet w a s the s a m e as that of 1531 a n d 1607, describing a h i g h l y elongated elliptical orbit with a period of 75.5 y e a r s . In 1687, upon H a l l e y ' s persistent pressure, N e w t o n finally finished his Principia, one of the greatest a c h i e v e m e n t s of the h u m a n intellect. In successive editions, N e w t o n w o u l d perfect his cometary theory w i t h n e w observational evidence, such as H a l l e y ' s , successfully predicting their orbital paths across the skies. T h e m y s t e r y of cometary orbits w a s finally solved. After a p p l y i n g his theory to several different comets in book 3 of the Principia, N e w t o n put forward a curious idea c o n c e r n i n g the fate a n d purpose of comets:

So, fixed stars, that have been g r a d u a l l y w a s t e d by the light a n d vapors emitted from them for a long time, m a y be recruited by comets that fall upon t h e m ; a n d from this fresh supply of n e w fuel those old stars, a c q u i r i n g n e w splendor, m a y pass for n e w stars. Of this k i n d are such fixed stars as a p p e a r on a s u d d e n , a n d shine w i t h wonderful brightness at first, a n d a f t e r w a r d s vanish by little a n d little. S u c h w a s the star w h i c h a p p e a r e d in Cassiopeia's C h a i r .

" N e w stars," such as the one observed by Tycho in 1572, w e r e just old stars that got their fuel replenished by absorbing falling comets! T h e gravitational pull from stars w o u l d g r a d u a l l y attract orbiting comets, until they could not but fall on t h e m . N e w t o n then continued his m u s ings on cosmic replenishment, e x t e n d i n g his vision to planets:

T h e vapors w h i c h arise from the sun, the fixed stars, a n d the tails of comets, m a y meet at last w i t h , a n d fall into, the a t m o s pheres of planets by their g r a v i t y , a n d there be condensed a n d turned into w a t e r a n d h u m i d spirits; a n d from thence, by slow heat, pass g r a d u a l l y into the form of salts, a n d s u l p h u r s , a n d tinctures, a n d m u d , a n d clay, a n d sand, a n d stones, a n d coral, a n d other terrestrial substances.

THE

PROPHET

FIGURE

AND

THE

ASTRONOMER

Diagram depicting the orbit of the comet of 1680, from Newton's

Principia

<1687). The comet's elliptical orbit is approximated by a parabola in the vicinity of the Sun.

T h e importance and beauty of N e w t o n ' s scheme can scarcely be overestimated. H i s description of a constant ebbing and flowing of m a t e r i a l substances across the heavens expressed an o r g a n i c , a l c h e m i c a l vision of the w o r l d . N e w t o n envisioned the cosmos as a w e b spun by w a n d e r i n g comets, w h i c h are responsible for the r e n e w a l of stars a n d planets a n d for the g e n e r a t i o n of substances that w i l l u l t i m a t e l y support life. Moreover, he implicitly a s s u m e d a s i m i l a r m a t e r i a l composition of all planets, m a d e of "terrestrial substances" obtained from stellar and c o m e t a r y " v a p o r s " cooked over slow heat (a reference to the slow b u r n ing of alchemical e x p e r i m e n t s ) : in Newton's vision, there w a s only one cosmic chemistry. But this is only half of the story. T h e other half comes from N e w t o n ' s belief in a teleological role for comets. C o m e t s w e r e God's tools, used for c r e a t i n g , r e n e w i n g , and d e s t r o y i n g w o r l d s . In his m i n d , cosmic p h e n o m e n a served a d i v i n e purpose. H i s physics e x p l a i n e d motions on Earth a n d in the heavens, his a l c h e m y the o r g a n i c interconnectedness of the w h o l e cosmos, and his theology justified the motions p r o m o t i n g these m a t e r i a l e x c h a n g e s as part of God's plan. It all fitted perfectly. T h e r e w a s no room here for c h e a p prophecy, w h i c h N e w t o n deeply despised: " T h e folly of S2

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interpreters has been, to foretell t i m e s a n d t h i n g s by this Prophecy | Hook of Revelation |, as if God d e s i g n e d to m a k e t h e m Prophets. By this rashness they have not only exposed themselves, but b r o u g h t Prophecy also into c o n t e m p t . " " T h e m a r r i a g e of physics w i t h theology h a d the clear goal of justifying God's actions through rational m e c h a n i s m s , in order to predict not when things w o u l d happen but how God operates in the w o r l d . N e w ton had no doubt that all the prophecies of Revelation w o u l d be fulfilled—some a l r e a d y had been, he a r g u e d — b u t he had no interest in predicting w h e n . As we r e m a r k e d before, for N e w t o n the fulfillment of prophecy w a s evidence for God's presence in the w o r l d : "the event of things predicted m a n y ages before will then be a convincing a r g u ment that the w o r l d is g o v e r n e d by P r o v i d e n c e . "

F r o m this we can

conclude that N e w t o n saw the n a t u r a l philosopher as a n e w k i n d of prophet—or better, the only k i n d of p r o p h e t — a n interpreter of God's w o r k as it w a s manifest in n a t u r e . In N e w t o n ' s m i n d , science a n d religion w e r e one indissoluble w h o l e . In k e e p i n g w i t h the spirit of Revelation, N e w t o n devised t w o m e c h a n i s m s for the destruction of w o r l d s by comets: direct i m p a c t — w h i c h H a l l e y later took u p — a n d falling into the S u n , promoting h u g e fires that w o u l d c o n s u m e Earth a n d its l i v i n g creatures. " H e could not say w h e n this comet 11680J w o u l d d r o p into the Sun . . . but w h e n e v e r it d i d , it w o u l d so m u c h increase the heat of the S u n , that this Earth w o u l d be burnt, a n d no a n i m a l s in it could survive," w r o t e N e w t o n ' s n e p h e w - i n - l a w John Gonduitt, after a famous fireside chat they h a d in 1725.

In N e w t o n ' s scheme of the w o r l d , history w a s punctuated by

catastrophes promoted by collisions w i t h comets through the a g e n c y of God, in w h a t m i g h t be called a causal theology. Inadvertently, N e w t o n redressed n e g a t i v e popular beliefs on comets a n d other celestial objects w i t h the l a n g u a g e o f science; a l t h o u g h t h e r e w a s n o l o n g e r room for "local" forecasting, l i n k i n g comets to the death of r u l e r s or famines and p l a g u e s , the l a w s of n a t u r e a l l o w e d for the continued belief in comets as a g e n t s of d o o m , the ushers of a n e w a g e , just as prophesied in Revelation.

THE

PROPHET

AND

THE

ASTRONOMER

If N e w t o n kept his thoughts mostly secret, r e v e a l i n g them only to a few select friends, H a l l e y w o u l d be the s p o k e s m a n of doom in the robes of the scientist. He had successfully d e m o n s t r a t e d that N e w t o n ' s physics e x p l a i n e d the orbits of comets, predicting the return of those w i t h closed orbits. To a r r i v e at this conclusion, H a l l e y h a d to p a i n s t a k i n g l y reconstruct the trajectories of comets from astronomical observations and c o m p a r e them to predictions from N e w t o n ' s theory, a truly a m a z i n g feat. But even before his discovery, H a l l e y h a d a r g u e d that comets played a cataclysmic role in Earth's h i s t o r y â&#x20AC;&#x201D; i n particular, that they e x p l a i n e d events described in the Old Testament. H i s favorite w a s a n a t u r a l theory of the d e l u g e , caused by "the casuall Choc of some transient body, such as a C o m e t or the l i k e . "

2 5

A c c o r d i n g to H a l l e y , the

collision w o u l d have a b r u p t l y tilted Earth's axis a n d d i u r n a l rotation, the u n b a l a n c e c a u s i n g major flooding all over the planet. He w e n t so far as to conjecture that an impact crater should exist in the C a s p i a n S e a ! As we w i l l see later, impact craters are today the s m o k i n g g u n s of such cataclysmic events. H o w e v e r , influenced by a "secret friend" w h o s e opinion he g r e a t l y respected (probably N e w t o n ) , he pondered h o w Noah's A r k could have survived such a mess. By 1696, he had decided that the impact b r o u g h t not the d e l u g e but the end of a former w o r l d , h a p p e n i n g thus before Creation; H a l l e y believed, as d i d some pre-Socratics, in a succession of former w o r l d s , each destroyed by an impact w i t h a celestial body. H a l l e y closed his famous 1705 essay on comets by a r g u i n g for the possibility that such impacts could occur; as an e x a m p l e , he cited the comet of 1680, w h i c h passed very close to Earth. In a thinly disguised d r a m a t i c tone, he left open w h a t the consequences of such impacts m i g h t be: "But w h a t m i g h t be the C o n s e q u e n c e s of so near a [ p r o x i m i t y ) ; or of a Contact; or, lastly, of a Shock of the Celestial Bodies, ( w h i c h is by no m e a n s impossible to come to pass) I leave to be discuss'd by the S t u d i o u s of Physical M a t t e r s . "

Just in case, in another publication,

H a l l e y a s k s for God's help to avert such c a t a c l y s m s : " M a y the great good God avert a shock or contact of such g r e a t Bodies m o v i n g w i t h

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such forces . . . lest this most beautiful order of t h i n g s be intirely destroyed a n d r e d u c e d into its antient c h a o s . "

T h e N e w t o n - H a l l e y ideas concerning impacts w i t h celestial bodies provoked all sorts of responses, from c h u r c h leaders to w r i t e r s a n d social chroniclers. Jonathan Swift, in his Gulliver's Travels (1726), sarcastically translated this a n x i e t y to the d i m i n u t i v e L a p u t a n s , w h o p a n icked at the approach of a d o o m s d a y comet w i t h i n thirty-one y e a r s , the time predicted by H a l l e y for the return of the 1682 comet. Preachers used it for their o w n purposes, u r g i n g sinners to repent w h i l e there w a s still time, n o w that comets had been proven to be God's i n s t r u m e n t s of apocalyptic doom: science v a l i d a t e d

prophecy.

William

Whiston,

w h o m N e w t o n n o m i n a t e d his successor to the L u c a s i a n C h a i r of M a t h e m a t i c s at C a m b r i d g e , w e n t as far as to suggest, in 1717, that comets w e r e a perfect place for hell. A l t e r n a t i n g spells of terrible cold (when far from the S u n ) a n d heat ( w h e n near the S u n ) , comets w e r e a "place of t o r m e n t "

for the sinful, d o o m e d

to circle the

heavens

throughout eternity. F o l l o w i n g W h i s t o n ' s l e a d , Cotton Mather, w h o m we met in chapter 2, preached that comets w e r e former planets that God's w r a t h transformed into orbiting hells: comets w e r e hell in the heavens.

The Genesis of Worlds T h e m a t h e m a t i c a l e l e g a n c e a n d predictive accuracy of N e w t o n i a n science, w i t h its solution to the long-lasting m y s t e r y of celestial orbits a n d motions, g a v e n a t u r a l philosophers a n e w sense of e m p o w e r m e n t . A rational description of n a t u r e w a s now possible, based on the a p p l i c a tion of physical principles to n a t u r a l p h e n o m e n a . N a t u r e w a s an open book, w a i t i n g to be explored by sagacious scientific m i n d s . To m a n y natural philosophers, there w a s no limit to w h a t could be a c c o m plished. T h i s inflated confidence ushered in the r a t i o n a l i s m of the eighteenth century, w h e r e every aspect of n a t u r e a n d life w a s believed

THE

PROPHET

AND

THE

ASTRONOMER

to be explicable by the application of a set of g e n e r a l principles or l a w s to the w o r l d , society, a n d the i n d i v i d u a l . If, for N e w t o n , God w a s a necessary presence in the cosmos, a sort of m e c h a n i c w h o kept celestial motions in check against perturbations in their orbits caused by g r a v i tational attraction, for the n e w rationalists God's role w a s relegated to that of Creator. Once the m a t e r i a l w o r l d h a d been created by divine power, its evolution unfolded u n d e r the strict jurisdiction of physical l a w s . T h e cosmos b e c a m e a w a t c h , and God the w a t c h m a k e r . Ironically, the very precision of N e w t o n ' s science u n d e r m i n e d his d e m a n d for God's continuous presence in the w o r l d . T h e task of the natural philosopher w a s (and is), after discovering these l a w s , to apply them to the m y r i a d p h e n o m e n a of n a t u r e . In doing so, he w a s b r i n g i n g himself closer to the m i n d of the Creator, his intelligence an atom of God's. N e w t o n left one f u n d a m e n t a l question untouched: the o r i g i n of the planets or, m o r e g e n e r a l l y , of the solar system itself. In a letter to R i c h a r d Bentley, a chaplain to the bishop of Worcester interested in a p p l y i n g N e w t o n ' s ideas to prove the existence of God, N e w t o n confessed he had no idea w h y one single body should e m i t light at the center w h i l e other o p a q u e bodies orbit a r o u n d it. "I do not think [this a r r a n g e m e n t ] explicable by m e r e natural causes, but am forced to ascribe it to the counsel a n d contrivance of a voluntary A g e n t . "

Later

on, N e w t o n attributed to the s a m e a g e n t the initial push that set the planets into their solar orbits. N e w t o n ' s followers w o u l d be m u c h m o r e a m b i t i o u s , a n d w o u l d try to u n d e r s t a n d the origin of the solar system s i m p l y in t e r m s of m e c h a n i c a l principles. T h e F r e n c h m a n Rene Descartes (1596â&#x20AC;&#x201D;1650), w h o died w h e n N e w t o n w a s eight y e a r s old, used to say, "Give me matter a n d I w i l l 29

construct a w o r l d out of i t . " A l t h o u g h l a c k i n g in a precise, q u a n t i t a tive formulation, Descartes's m o d e l of the cosmos w a s the first to appeal to a mechanistic description. He envisioned an infinite universe w i t h an infinite n u m b e r of stars s u r r o u n d e d by planets, as in our solar system. T h e stars w e r e the centers of vortices of a diffuse, fluid matter, w h i c h filled all space, s o m e w h a t l i k e the w h i r l p o o l s a r o u n d d r a i n s of

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an e m p t y i n g bathtub, or in turbulent rivers. T h e c i r c l i n g motion of the vortices d r a g g e d the planets in their orbits. Descartes i m a g i n e d a cosmos structured l i k e a q u i l t , each piece housing a star a n d its s u r r o u n d ing vortex. Planets w i t h moons h a d their o w n small vortices, the pattern being thus repeated on a s m a l l e r scale, an idea that r e a p p e a r e d later. A c c o r d i n g to Descartes, this ordered structure w i t h its c i r c u l a r motions w a s the initial w o r k o f God, w h o s e w i s d o m w a s manifest throughout n a t u r e . T h e r e w e r e three different k i n d s of matter: the fluid m a t t e r of the vortices, the " s p e e d y " m a t t e r that composed the S u n and stars, a n d the "coarse" m a t t e r that composed the planets. T h e rotating fluid vortex provided a s e p a r a t i n g force b e t w e e n the t w o other types of m a t t e r as a k i n d of cosmic centrifuge, the p l a n e t a r y m a t t e r being pulled out w h i l e the stellar m a t t e r s o m e h o w sank to the center. A n a x a g o r a s ' s ideas found an echo here some t w o thousand y e a r s later. A l t h o u g h N e w t o n criticized m a n y of Descartes's ideas, i n c l u d i n g the existence of cosmic vortices, some of t h e m surely influenced later m o d els of solar system formation. More than a n y t h i n g , Descartes r e n d e r e d plausible the possibility that the g e o m e t r i c a r r a n g e m e n t of the cosmos w a s a m e n a b l e to rational thought. Immanuel Kant (1724-1804), whose great mind was imprisoned in a d i m i n u t i v e a n d frail body, is k n o w n mostly for his w o r k on philosophy a n d metaphysics, a n d for an obsessive devotion to d a i l y w a l k s in a street n o w k n o w n as Philosopher's W a l k , in his native Konigsberg, G e r m a n y . It is said that Kant w a s so punctual that people set their clocks according to his w a l k i n g schedule. In his youth, Kant w a s fascinated m a i n l y by the a r r a n g e m e n t of the cosmos a n d by N e w t o n i a n science. He w a s a firm believer in the deistic notion of God as C r e a t o r of the w o r l d a n d its physical l a w s a n d m a n as the decoder of its m e c h a n ics, as befit a true e i g h t e e n t h - c e n t u r y rationalist. W h e n he w a s thirtyone, still s t r u g g l i n g to get a professional post, Kant w r o t e a treatise entitled

Universal

Natural

History

and

Theory

of the

Heavens

(1755),

w h e r e he proposed a rather a m b i t i o u s description of the a r r a n g e m e n t of the cosmos, as well as of the origin of the solar system. Kant w a s so

THE

PROPHET

AND

THE

ASTRONOMER

F I G U R E 4 : Illustration of Descartes's vortices surrounding stars. Each vortex marks a polygonal domain, as in a sort of cosmic quilt. A comet travels from vortex to vortex, causing turbulence in the cosmic fluid. (Rene Descartes, L e M o n d e [Paris 1664].) (Reproduced from Sara Schechner Genuth, Comets, Popular Culture, and the Birth of Modern Cosmol ogy [Princeton: Princeton University Press, 1997[, p. 112.)

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confident in the value of his w o r k that he dedicated the treatise to F r e d e r i c k the Great, k i n g of Prussia, hoping the k i n g ' s patronage w o u l d secure h i m an a c a d e m i c position. But fate w a s u n k i n d to Kant, and his publisher w e n t b a n k r u p t just before d i s t r i b u t i n g the freshly printed book. Most copies d i s a p p e a r e d , a n d F r e d e r i c k never got to see his. However, shortly thereafter, a r e v i e w w a s published in a literary m a g a z i n e a n d Kant's ideas started to be discussed in a c a d e m i c circles. Kant's goal w a s to provide a c o m p e l l i n g explanation for k n o w n facts r e g a r d i n g the solar system, on the basis of N e w t o n i a n mechanics and gravity. T h a t is, he w a n t e d to show that these facts are a consequence of a d y n a m i c formation process resulting from the motions of matter u n d e r the influence of various forces. H e r e are some observational facts p e r t a i n i n g to the solar system that Kant set out to e x p l a i n : (a) T h e planetary orbits lie nearly on a plane intersecting the S u n ' s equator. T h a t is, the solar system is q u i t e flat. M o r e precisely, the "thickness" of the disk is about '/>o of the d i a m e t e r of Pluto's orbit. (Pluto w a s not k n o w n in Kant's t i m e ; but the flattened shape of the solar system w a s . ) One can r o u g h l y picture this as a laser disk bisecting a P i n g - P o n g ball (not to scale!), the planets b e i n g little dots on the laser disk m o v i n g in orbits a r o u n d the center. C o m e t s , however, depart from this rule, h a v i n g orbits at a r b i t r a r y inclinations w i t h respect to the orbital plane of the planets, (b) All planets go a r o u n d the S u n in the same c o u n t e r c l o c k w i s e direction as seen from above Earth's North Pole. T h i s is also the s a m e direction in w h i c h the S u n rotates about its axis. In 1751, Kant b e c a m e a w a r e of ideas from the E n g l i s h m a n T h o m a s W r i g h t ( 1 7 1 1 - 1 7 8 6 ) , the first to propose a specific shape for the M i l k y Way. A c c o r d i n g to W r i g h t , the M i l k y W a y w a s an infinite collection of stars flattened between t w o infinite planes, s o m e w h a t l i k e a cosmic sandwich. Kant conjectured that there w a s a d e e p connection b e t w e e n W r i g h t ' s model of the M i l k y W a y a n d the flattened shape of the solar system: both w e r e formed in the s a m e w a y , by the s a m e physical processes, the M i l k y W a y a n d its stars being just a l a r g e r version of the

THE

PROPHET

AND

THE

ASTRONOMER

S u n a n d its planets. In fact, Kant proceeded to propose that the m a n y " n e b u l a e " that had recently been observed w e r e m e r e l y collections of stars l i k e the M i l k y W a y , s o m e t h i n g that w o u l d be confirmed only in 1924, by the A m e r i c a n astronomer E d w i n H u b b l e . ( S o m e w e r e indeed g a l a x i e s , w h i l e others w e r e g a s clouds.) In Kant's view, the universe w a s filled w i t h g a l a x i e s , each composed of i n n u m e r a b l e stars, each star s u r r o u n d e d by its court of planets, a n d some planets s u r r o u n d e d by their court of moons. T h e same pattern repeated itself from the largest distances in the cosmos d o w n to Earth a n d its Moon. Kant saw this magnificent cosmic s y m m e t r y as a clear manifestation of God's design. T h e structure b e i n g set, it w a s necessary to devise a m e c h a n i c a l principle to explain h o w these shapes e m e r g e d in t i m e . Kant assumed that, in the b e g i n n i n g , the infinite universe w a s filled w i t h scattered particles, the basic constituents of all matter. He paid tribute to the atomists

and

their

primordial

chaos,

writing

that

the

ideas of

L u c r e t i u s — w h o m he d e a r l y l o v e d — a n d of his predecessors Epicurus, D e m o c r i t u s , a n d L e u c i p p u s , "had m u c h r e s e m b l a n c e " to his o w n .

3 0

revived the atomists' (and in spirit, if not in detail, Descartes's) general idea of vortices, a r g u i n g that they arose from the competition between an i n w a r d fall—present in Epicurus's system a n d e x p l a i n e d by N e w ton's g r a v i t a t i o n a l force—and a s i d e w a y s motion, w h i c h he erroneously attributed to a repulsive force, as in the "elasticity of vapors, the effluences of strong s m e l l i n g bodies, a n d the diffusion of all spirituous m a t t e r s . "

H o w e v e r fond of the atomists' physics, Kant d i d criti-

cize their a t h e i s m , c l a i m i n g that a universe w i t h o u t God's counsel w o u l d never have ordered from its initial chaos. A c c o r d i n g to Kant, initial overdensities in the p r i m o r d i a l chaotic matter started to attract m o r e matter, in a s i m p l e infall motion determ i n e d by gravity. As m o r e matter accreted, the " r e p u l s i v e " forces a m o n g the particles pushed them s i d e w a y s , g e n e r a t i n g a g e n e r a l circular motion, or vortex. W h i l e l a r g e r q u a n t i t i e s of particles fell to the center, forming the S u n , s m a l l e r s w i r l i n g particles in the vortex combined to form the planets. Kant believed that the m a t e r i a l composition

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F I G U R E 5 : The eccentricity is the ratio of the distance between the two foci and the length of the larger axis of the ellipse. For a circle, the distance between the two foci is zero and so is its eccentricity.

of the vortex w a s such that the closer to the center, the denser and more massive the particles. T h u s , as one moved a w a y from the center, the planets had l a r g e r q u a n t i t i e s of l i g h t e r particles, w h i c h had more variations in their motions. As a consequence, the eccentricity of the planetary orbits, that is, their deviations from a perfect circle, increased with their distance from the S u n . C o m e t s , the outermost inhabitants of the solar system, w e r e the ones w i t h the lightest components a n d thus the most eccentric orbits. Kant even conjectured that there w o u l d be other, more eccentric planets beyond S a t u r n . He w a s , of course, d e a d right about the existence of other planets, but d e a d w r o n g about their increasing eccentricities, as you can verify from table 1. (Note, in particular, that N e p t u n e has a nearly c i r c u l a r orbit.)

THE

PROPHET

AND

THE

ASTRONOMER

PLANET

ECCENTRICITY

Mercury

0.206

Venus

0.007

Earth

0.017

Mars

0.093

Jupiter

0.048

Saturn

0.054

Uranus

0.047

Neptune

0.009

Pluto

0.249

T A B L E I: Eccentricities of solar system planets.

Kant's u n i v e r s e w a s l a r g e l y built out of his u n c a n n y intuition. H i s m e c h a n i s m for the formation of the solar system followed the logical application of physical principles, w h i c h , as we have briefly seen, h a d a few basic flaws. As a consequence, some of his ideas w e r e far-fetched, such as his notion of the i n c r e a s i n g eccentricities of the outer planets, or his belief in an absolute center of the cosmos, to w h i c h all other seconda r y centers w e r e h i e r a r c h i c a l l y related. To Kant's credit, he w a s the first to a d m i t that there w e r e very few q u a n t i t a t i v e calculations in his w o r k , a l t h o u g h he c l e a r l y expected that his ideas w o u l d be confirmed by a more precise m a t h e m a t i c a l formulation. A n d , as we w i l l soon see, some w e r e . Q u a n t i t a t i v e precision aside, Kant's rhetoric expressed his passion for n a t u r e a n d his firm belief in the p o w e r of reason to face its most p u z z l i n g m y s t e r i e s . H i s eloquence reached a c l i m a x w h e n he speculated about the eternal cycles of creation a n d destruction that p u n c t u a t e the vast cosmic distances. T h e s a m e forces that create w o r l d s out of i n a n i m a t e m a t t e r will e v e n t u a l l y cause their destruction, a s , in the "fullness of t i m e , " they collapse into the massive center of their orbits. A c c o r d i n g to Kant, once t i m e has run its course (a r a t h e r v a g u e statement), a g r a d u a l falling of p l a n e t a r y a n d cometary m a t t e r into the central sun w i l l t r i g g e r a "conflagration," w h i c h w i l l rescatter their constituent particles t h r o u g h the vastness of space they once occupied. 92

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F r o m this r e n e w e d chaos, small overdensities w i l l a g a i n accrete m o r e n e i g h b o r i n g matter, a n d a n e w sun w i l l be born, together w i t h its court of planets. Kant believed these beautiful cycles of creation a n d d e s t r u c tion to be the very essence of n a t u r e , filling the eternity of time. Only God a n d the eternal soul w e r e above these m a t e r i a l processes of birth and death:

'Kyft

W h e n we follow this Phoenix of N a t u r e , w h i c h b u r n s itself only in order to revive a g a i n in restored youth from its ashes, through all the infinity of times a n d spaces . . . w i t h w h a t reverence the soul r e g a r d s even its o w n being, w h e n it considers that it is destined to survive all these t r a n s f o r m a t i o n s !

Pierre S i m o n de L a p l a c e (1749â&#x20AC;&#x201D;1827), a brilliant m a t h e m a t i c i a n a n d physicist, took over w h e r e Kant left off. H i s approach to science w a s quite different from Kant's, being based on a solid m a t h e m a t i c a l description of physical p h e n o m e n a . A p p a r e n t l y , he w a s not even a w a r e of Kant's w o r k . By a refined application of N e w t o n i a n m e c h a n i c s to the motions of Jupiter a n d S a t u r n , L a p l a c e w a s able to show that their m u t u a l g r a v i t a t i o n a l attraction w o u l d not affect their orbits a r o u n d the S u n , h i n t i n g at the l o n g - t e r m stability of the solar system; even t h o u g h planets tug at each other, there w a s no d a n g e r of their r u n n i n g off track. He used this result to a r g u e for the safety of life on Earth, at least from the point of v i e w of orbital instabilities. As a consequence, if the solar system could r e g u l a t e itself t h r o u g h its o w n motions, there w a s no need to i n v o k e God's constant presence, as N e w t o n d i d ; to L a p l a c e , God w a s not a player in celestial m e c h a n i c s . H i s stability a n a l y s i s relied on a crucial physical concept, that of the conservation of a n g u l a r m o m e n t u m . Since a n g u l a r m o m e n t u m w i l l be a r e c u r r i n g t h e m e from here on, let us t a k e a m o m e n t to r e v i e w the basic ideas behind it. We start by first considering linear m o m e n t u m , w h i c h is defined simply as the product of a body's m a s s a n d its velocity. So, a bicycle m o v i n g on a straight line at 20 miles per hour a n d an e i g h t e e n - w h e e l e r at 20 miles per hour m a y have the same velocity, but have very differ93

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ent m o m e n t a , because of their l a r g e mass difference. L i n e a r m o m e n t u m is a m e a s u r e of an object's tendency to r e m a i n in its state of linear motion; b r a k e s have to w o r k m u c h h a r d e r to stop an e i g h t e e n - w h e e l e r than a bicycle. Descartes w a s the first to clearly suggest that, in the absence of friction or other losses, the total linear m o m e n t u m is conserved w h e n t w o or m o r e bodies interact. T h a t is, the s u m of all the i n d i v i d u a l m o m e n t a is the same before a n d after a collision. You can v i s u a l i z e this in collisions involving billiard balls, as they hit each other a n d careen w i t h different velocities in different directions; a l t h o u g h the conservation is not perfect, because of friction a n d t h e r m a l losses (for e x a m p l e , the fact that you hear the collision m e a n s that some e n e r g y is being lost d u r i n g the shock), it's pretty close for our purposes. An even better e x a m p l e is the g a m e of air hockey, w h e r e both friction w i t h the table and the rolling of the balls are l a r g e l y e l i m i n a t e d , i m p r o v i n g m o m e n t u m conservation. A n g u l a r m o m e n t u m is an extension of this idea for motions involving rotation. I m a g i n e y o u are s p i n n i n g a stone at the end of a string: the a n g u l a r m o m e n t u m of the stone is g i v e n by the product of its mass, its a n g u l a r velocity (the rate at w h i c h it is rotating, say, 30 d e g r e e s per second), a n d the s q u a r e of its distance from the center. T h u s , the longer the string or the a n g u l a r velocity of the stone, the l a r g e r its a n g u l a r m o m e n t u m , a m e a s u r e of the tendency of a body to r e m a i n in circular motion. T h a t , in the absence of friction, a n g u l a r m o m e n t u m is conserved is a concept we a r e all f a m i l i a r with: w h e n an ice-skater does her pirouettes, she usually starts w i t h her a r m s outstretched. T h i s sets her initial a n g u l a r m o m e n t u m , w h i c h d e p e n d s o n her a r m ' s length. W h a t happens as she b r i n g s her a r m s in? S h e spins faster a n d faster, until she reaches her m a x i m u m a n g u l a r speed w i t h her a r m s folded on her chest, to the enthusiastic applause of the c r o w d . T h i s speeding up is easily seen as a consequence of a n g u l a r m o m e n t u m conservation: since the total a n g u l a r m o m e n t u m is conserved, it must be the same at the b e g i n n i n g , as the s k a t e r spins w i t h her a r m s stretched out, a n d at the end, w i t h her a r m s folded. H e r mass staying the s a m e , the only w a y to

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keep a n g u l a r m o m e n t u m constant as her a r m s retreat ( s m a l l e r distance to the center) is to increase her a n g u l a r velocity. If you have never seen a pirouetting ice-skater, try the following e x p e r i m e n t : find a rotating stool, a n d spin sitting on it w i t h y o u r a r m s stretched out. W h i l e still spinning, bring them closer to your chest a n d notice how you spin faster. Voila! You have d e m o n s t r a t e d the conservation of a n g u l a r momentum.

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R e t u r n i n g to L a p l a c e â&#x20AC;&#x201D; h e believed that the ordered a r r a n g e m e n t of the solar system, w i t h the planets s p i n n i n g in t a n d e m w i t h i n a n a r row plane a n d in the s a m e direction as the S u n a r o u n d its a x i s , w a s the result not of chance but of solid physical principles. He proposed the so-called nebular hypothesis, a c c o r d i n g to w h i c h the Sun's atmosphere once extended all the w a y t o w a r d w h e r e today we find the outermost planet (in Laplace's d a y s , U r a n u s ) . So, u n l i k e Kant, L a p l a c e started not w i t h some p r i m o r d i a l universal chaos but w i t h the p r i m i t i v e S u n surrounded by a diffuse, f l u i d l i k e , a n d fiery a t m o s p h e r e . In fact, he refrained from a p p l y i n g his ideas to other stars or to the cosmos as a w h o l e , l i m i t i n g h i m s e l f to the solar system. He further a s s u m e d this vaporous mass to be slowly rotating a r o u n d the S u n , thus establishing its initial total a n g u l a r m o m e n t u m . H o w e v e r , he d i d not venture to explain the origin of the vaporous mass or its a n g u l a r m o m e n t u m . A c c o r d i n g to L a p l a c e , as the solar p r i m o r d i a l a t m o s p h e r e w a s pulled by its o w n g r a v i t y t o w a r d the center, it also g r a d u a l l y cooled off ( m o d ern physics predicts the opposite for a contracting g a s cloud, a serious problem w i t h Laplace's ideas). Because of the conservation of a n g u l a r m o m e n t u m , as it b e c a m e smaller, the contracting gaseous nebula rotated faster, l i k e a cosmic ice-skater. T h e combination of contraction with faster rotation caused the nebula to b u l g e a r o u n d its equator, becoming flatter the faster it spun. R o t a t i n g p i z z a d o u g h does the same. L a p l a c e conjectured that, at a certain point d u r i n g the contraction, the outer portion of this rotating mass broke off from the rest, as its attraction to the central S u n w a s balanced by the centrifugal force

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caused by its motion. T h i s created a ring of m a t t e r detached from the rest of the further contracting mass, until another, s m a l l e r ring w a s created a n d so on (see figure 13 in insert). W i t h i n each of these concentric rings, m a t e r i a l particles slowly collided a n d coalesced into large m a s s i v e bodies, w h i c h g r e w to become the present planets of the solar system. Variations in the m a t e r i a l s r u n n i n g a r o u n d the ring, that is, rocky versus volatile substances, e x p l a i n e d the differences in the m a t e rial composition of the planets. T h i s l a r g e r pattern repeated itself on a s m a l l e r scale: each protoplanet, at a certain point d u r i n g its evolution, also a c q u i r e d r i n g s of matter a r o u n d it. T h e s e , upon further cooling, b e c a m e the planet's satellites or, s o m e t i m e s , diffuse r i n g s m a d e of s m a l l e r separate bodies, as in S a t u r n . Laplace's influence as a scientist w a s very w i d e s p r e a d : "Before the i m p o s i n g greatness of L a p l a c e all bow" w a s the w a t c h w o r d a r o u n d scientific circles in the e a r l y nineteenth c e n t u r y .

C o n t r a r y to g e n e r a l

belief, science is not i m m u n e to power-based opinions: fashions a n d trends abound! H o w e v e r , its a d v a n t a g e over other areas of h u m a n activity is that, sooner or later, opinions supported by a r g u m e n t s based on p o w e r a n d not on substance a r e o v e r t h r o w n : a w r o n g hypothesis, no matter w h o proposed it, will hold for only so long (unless, of course, it is so far removed from the reality probed by e x p e r i m e n t s a n d observations that it can survive for a long t i m e ) . S l o w l y but surely, several questions e m e r g e d c o n c e r n i n g the plausibility of Laplace's scenario. Scientists, especially outside of F r a n c e , questioned the fact that he did not offer a m e c h a n i s m for the o r i g i n of the S u n a n d its atmosphere, or their overall rotation. A l s o , at that time it w a s k n o w n that at least two of the satellites of U r a n u s orbited b a c k w a r d , that is, h a d a retrograde orbital motion, w h i c h contradicted the uniform rotation i m p l i e d by Laplace's hypothesis. F u r t h e r m o r e , it w a s not clear how m a t t e r in the r i n g s coalesced into very l a r g e planets, it being m o r e plausible to a s s u m e they w o u l d at most form small bodies spread in a belt, such as the one found b e t w e e n M a r s a n d Jupiter, the so-called asteroid belt. C o m e t s w e r e also a problem: since they have very eccentric orbits, m a k i n g a r b i t r a r y a n g l e s w i t h the orbital plane of the planets, L a p l a c e

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w a s forced to conjecture that they d i d not belong to the solar system, being small n e b u l a e w i t h dense k e r n e l s crisscrossing the cosmos from star to star. F i n a l l y , the S u n rotates m o r e slowly than a straight application of conservation of a n g u l a r m o m e n t u m w o u l d predict. Given the importance of l i n k i n g the d e v e l o p m e n t of astronomy with religious a n d popular c u l t u r e , before we move on to m o d e r n theories of the genesis of w o r l d s , it is worth l o o k i n g a bit further into Laplace's v i e w s on comets. Since comets w e r e free-roaming cosmic travelers, they could sometimes get caught by the S u n ' s gravity, l i k e a fish by a net, occasionally l o c k i n g into the periodic orbits established by Halley. L a p l a c e used his celestial m e c h a n i c s as a w e a p o n against the irrational fear of comets still r a m p a n t a m o n g l a r g e sectors of the population. Or such w a s his intention. He showed that the tails of comets w e r e so rarefied that their total masses w e r e s m a l l e r than that of a small hill; their effects upon b r u s h i n g Earth w e r e completely n e g l i g i ble. (Nevertheless, as we have seen, terror still spread l i k e the p l a g u e w h e n H a l l e y ' s comet's tail brushed Earth in 1910, c a r r y i n g its " d e a d l y " cyanogen.) L a p l a c e praised "the light of science [that] has dissipated the vain terrors w h i c h comets, eclipses, and m a n y other p h e n o m e n a inspired in the a g e s of i g n o r a n c e . "

He also a r g u e d that, a l t h o u g h pos-

sible, collisions w i t h comets w e r e e x t r e m e l y rare. T h i s being the case, his scientific honesty forced h i m to a d d , " T h e small probability of this circumstance m a y , by a c c u m u l a t i n g d u r i n g a long succession of a g e s , become very g r e a t . "

T h a t is, cosmic collisions, science w a s confirm-

ing, do happen. F r o m the perspective of popular culture a n d religion, L a p l a c e and his colleagues a p p e a r e d to be c o n f i r m i n g d o o m s d a y prophecies, w e a r i n g the robes of rational science: the temptation to prognosticate, even if justified by a sense of scientific integrity, w a s just too great. L a p l a c e w e n t on to describe the effects of such a collision, without t r y i n g to m i n i m i z e its apocalyptic consequences:

T h e [Earth's] axis and motion of rotation changed, the waters . . . precipitate themselves t o w a r d s the n e w equator; the g r e a t e r part of m e n and a n i m a l s d r o w n e d in a universal d e l u g e , or 97

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destroyed by the violence of the shock g i v e n to the terrestrial globe; w h o l e species destroyed; all the m o n u m e n t s of h u m a n industry r e v e r s e d .

H e r e is a short parable on p o w e r and fear, w h i c h some of you m a y identify w i t h . You go to a doctor w i t h a red spot on y o u r s k i n . After a cursory e x a m i n a t i o n , he says, his eyes g l e a m i n g w i t h the look of those w h o can see death w i t h i n life: "Listen, there is a very small chance that y o u r skin rash is a c t u a l l y an e a r l y s y m p t o m of a d e v a s t a t i n g flesh-eating disease, that could basically d e v o u r you w i t h i n a couple of d a y s . " " W o w , Doc, t h a n k s ! I w o u l d never have thought of it," you say, sweat r u n n i n g d o w n y o u r forehead. I am sure you w i l l not sleep w e l l for the next t w o nights. In the midst of the e n s u i n g panic, the c a u t i o n a r y w o r d s " a very small c h a n c e " q u i c k l y lose their m e a n i n g . F e a r q u i c k l y b l u r s sensibility. After an a g o n i z i n g w e e k , your rash d i s a p p e a r s a n d y o u realize you w e r e not the o n e - i n - a - m i l l i o n case. But you could have been, r i g h t ? Laplace's

ambiguity

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c o m e t s â&#x20AC;&#x201D; r a r e but f a t a l â&#x20AC;&#x201D; r e m i n d s me of a 1967 R o m a n Polanski movie, The Fearless Vampire Killers. It tells the story of a professor w h o sets off for T r a n s y l v a n i a w i t h his assistant (played by P o l a n s k i ) to prove his theories about the existence of v a m p i r e s a n d , w h i l e he's at it, to exterm i n a t e the r e m a i n i n g ones. After e n c o u n t e r i n g q u i t e a few of t h e m , i n c l u d i n g a J e w i s h v a m p i r e w h o l a u g h s defiantly w h e n the professor threatens h i m w i t h the cross ("Oy, you got the w r o n g v a m p i r e ! " ) , his assistant falls prey to the i n n k e e p e r ' s d a u g h t e r - t u r n e d - v a m p i r e , played by S h a r o n Tate. T h e professor k n o w s nothing of this a n d , after causing major havoc in the castle w h e r e the v a m p i r e s hide, escapes w i t h his c o n t a m i n a t e d assistant a n d his v a m p i r e girlfriend back to western Europe. T h e m o v i e closes w i t h the line " A n d t h a n k s to Professor A b r o m s i u s , the evil he sought to e x t e r m i n a t e w i l l spread t h r o u g h o u t the w o r l d . " Laplace's rhetoric e n d e d up d o i n g the s a m e .

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The Genesis of Worlds, circa A . D . 2 0 0 1 In the t w o h u n d r e d or so y e a r s since L a p l a c e developed his n e b u l a r hypothesis, astronomers have proposed a variety of ideas that either tried to fill the g a p s of his scenario or to set forth very different ones. T h e r e have been catastrophic formation models, w h e r e planets a r e c h u n k s of matter y a n k e d from the S u n by the near-encounter w i t h another star; g i a n t protoplanet scenarios, w h e r e the planets themselves are the product of condensing n e b u l a r m a t e r i a l , clouds broken off from the g i a n t nebula that became the solar system; and planetesimal scenarios, w h e r e the planets w e r e formed by a g r a d u a l a c c u m u l a t i o n of m a t e r i a l . T h i s last scenario is the one w i d e l y accepted today. In order to explain the formation of the solar system, we must start at m u c h l a r g e r spatial scales a n d a long time a g o , w i t h the formation of our o w n g a l a x y . ( F i g u r e 14 in insert shows a spiral g a l a x y s i m i l a r to ours.) Present estimates place the g e n e r a l era of g a l a x y formation around t w e l v e billion y e a r s a g o , w h e n the universe itself w a s at the tender a g e of one or t w o billion years. (For now, we will t a k e the a g e of the universe to be fourteen billion years.) In older versions of g a l a x y formation scenarios (the top-down scenarios), the M i l k y W a y e m e r g e d from the contraction of a g i a n t n e b u l a â&#x20AC;&#x201D; a s Kant s u g g e s t e d â&#x20AC;&#x201D; w i t h a mass of a few h u n d r e d billion suns. Its initial composition w a s very simple, mostly h y d r o g e n and h e l i u m , the t w o p r e d o m i n a n t chemical elements in the universe, a p p e a r i n g in the proportion of 3:1; for each four atoms, three w e r e h y d r o g e n and one h e l i u m . Recent studies suggest instead that l a r g e g a l a x i e s , such as the M i l k y W a y , formed t h r o u g h a succession of m e r g e r s of smaller n e b u l a e , in a sort of cosmic c a n n i b a l ism. In any case, the chemistry of the m e r g i n g n e b u l a e follows the 3:1 rule. T h e details of g a l a x y formation r e m a i n very m u c h a topic of debate in the astronomical c o m m u n i t y . S m a l l overdensities, regions w h e r e the a m o u n t of matter in a g i v e n volume exceeded the a v e r a g e v a l u e across the nebula, l i k e clots in 99

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bread d o u g h , t r i g g e r e d the contraction of h y d r o g e n / h e l i u m clouds because of their o w n self-gravity. T h e m o r e these regions contracted, the hotter and denser they b e c a m e . After a few m i l l i o n y e a r s , the temperature at the core of the denser regions w a s so h i g h that a process k n o w n as nuclear fusion started to occur: nuclei of h y d r o g e n atoms (one proton) w o u l d fuse into nuclei of h e l i u m a t o m s (two protons and t w o neutrons), l i b e r a t i n g e n o r m o u s a m o u n t s of energy. T h e largest of these early stars, thousands of times m o r e massive than the S u n , started to shine w i t h t r e m e n d o u s fury, their existence h a n g i n g on the balance between its o w n g r a v i t a t i o n a l contraction and the e n e r g y liberated outw a r d by nuclear fusion, implosion versus explosion. But g r a v i t y never rests. In the end, it a l w a y s wins,' and the h u g e stars collapsed over themselves. As the s h r i n k i n g outer regions of the star hit the very dense core, they recoiled w i t h a bang, ejecting l a r g e a m o u n t s of matter across the interstellar m e d i u m . In part 3, we will discuss the life cycles of stars in detail. For now, it is only important to k e e p in m i n d that these supermassive stars had relatively short lives, s p e w i n g huge a m o u n t s of matter into the p r i m o r d i a l interstellar m e d i u m as they convulsed into oblivion. About 3 percent of this matter w a s composed of heavier chemical e l e m e n t s , such as carbon, o x y g e n , and nitrogen, cooked by the continuation of nuclear b u i l d u p , w h i c h had started with h y d r o g e n g o i n g into h e l i u m . After billions of y e a r s , these short-lived supermassive stars seeded the early g a l a x y w i t h tiny g r a i n s of matter, w h i c h w e r e g o i n g to be key i n g r e d i e n t s d u r i n g the formation of our solar system. As several n e b u l a e m e r g e d on their w a y to b e c o m i n g the Milky W a y , matter a c c u m u l a t e d at the galactic center and its beautiful spirala r m structure b e c a m e m o r e and more defined.* Scattered across our proto-galaxy, s m a l l e r overdense g a s clouds of different sizes, shapes,

* Part of the debate c o n c e r n i n g the m e c h a n i s m of galaxy f o r m a t i o n is related to the apparent fragility of spiral galaxies, such as the M i l k y W a y , against violent m e r g e r s . F o r us, it is sufficient to assume that w h a t e v e r the details of galaxy f o r m a t i o n , overdensities that e v o l v e d to become solar systems w e r e present about five billion years ago.

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masses, and t e m p e r a t u r e s orbited a r o u n d the center, c o l l i d i n g w i t h or passing near each other, w h i l e being seeded by g r a i n s s p e w e d by the dying s u p e r m a s s i v e stars in their neighborhoods. A bit over 4.5 billion years a g o , one of these clouds began to contract, possibly t r i g g e r e d by a near collision w i t h another cloud, by its passage t h r o u g h an overdense region of a g a l a c t i c a r m , or by a nearby stellar explosion. T h i s cloud of hydrogen a n d h e l i u m , s p r i n k l e d w i t h the h e a v y - e l e m e n t seeds spread throughout the g a l a x y , w o u l d become our solar system. As the rotating solar nebula contracted a n d flattened, the t e m p e r a ture at its core started to increase, as h a p p e n s w i t h supermassive stars. However, since the nebula w a s considerably less massive, the h y d r o g e n fusion process there w a s not as furious; after a very l u m i n o u s initial stage that lasted a few d o z e n million y e a r s , the core cooled off a n d settled into a more gentle h y d r o g e n b u r n i n g pace, w h i c h w o u l d sustain itself for about ten billion years. T h e S u n is in its m i d d l e a g e now. During its early stage, it looked l i k e an overheated star s u r r o u n d e d by a fairly flat gaseous d i s k , s p r i n k l e d w i t h h e a v y - e l e m e n t dust a n d g a s left over from previous stellar explosions (see figure 15 in insert). As it cooled d o w n , some of the matter in the gaseous disk condensed into solid g r a i n s , w h o s e sizes r a n g e d from microns (one m i l l i o n t h of a meter) to m i l l i m e t e r s . Since different substances solidify at different temperatures, specific k i n d s of solid g r a i n s w e r e m o r e c o m m o n at different distances from the S u n , from the inner regions ( t e m p e r a t u r e s of a few thousand d e g r e e s ) to the cooler outskirts of the disk (a few degrees above absolute zero). We are familiar w i t h the fact that different substances freeze at different t e m p e r a t u r e s ; if you place a bottle of water in the freezer, the w a t e r w i l l turn to ice, but v o d k a w i l l become more viscous, w i t h o u t freezing. L i k e w i s e , different substances w i l l solidify at v a r y i n g distances from the S u n , as represented in the figure below. It is this distribution of different solid m a t e r i a l s across the solar nebula that d e t e r m i n e s the m a t e r i a l composition of the planets. Once the distribution of g r a i n s is established, the process of planet buildup occurs in three stages. First, the small g r a i n s act as seeds for the condensation of m o r e g r a i n s a r o u n d t h e m . T h i s is w h a t happens 101

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w i t h rain as w e l l ; dust or soot in the air attracts w a t e r vapor molecules a n d helps them release heat. As the clusters of molecules cool, they condense into l a r g e l i q u i d droplets that fall to the g r o u n d â&#x20AC;&#x201D; t h a t is, it rains. T h i s is w h y we sometimes see planes " s p r i n k l i n g " clouds w i t h salts, in the hope that it w i l l speed up the condensation process and produce the m u c h needed rainfall. Back to planet formationâ&#x20AC;&#x201D;solid g r a i n s of m a t e rial g r o w to become small c l u m p s of matter, g r a i n s sticking to g r a i n s , as snow sticks to s n o w b a l l s . T h e l a r g e r the c l u m p s , the l a r g e r their surface area a n d the m o r e g r a i n s stick to t h e m . T h i s process of accretion continues for a w h i l e , until the c l u m p s g r o w into objects a few h u n d r e d k i l o m e t e r s across. At this point, the c l u m p s are massive e n o u g h for their gravity to start helping the collection of nearby material; the process speeds up further, a n d the c l u m p s g r o w to the size of small moons, or p l a n e t e s i m a l s , c o m p l e t i n g the first stage of planetary b u i l d u p . D u r i n g the second stage of planet formation, lasting about one h u n d r e d m i l l i o n y e a r s , the combination of motion a n d m u t u a l g r a v i t a tional attraction caused all sorts of collisions a m o n g the m i l l i o n s of p l a n e t e s i m a l s orbiting the central mass. In this treacherous environment, the rich b e c a m e richer, as small planetesimals w e r e either destroyed or absorbed onto l a r g e r ones. C o m p u t e r s i m u l a t i o n s show that this c l e a n u p process also m a d e the orbits of the l a r g e r planetesim a l s m o r e circular. Essentially, the objects that survived this process of collisions a n d b o m b a r d m e n t s b e c a m e the nine planets of the solar system. T h e r e is also the asteroid belt, between Mars a n d Jupiter, a collection of millions of p l a n e t e s i m a l s that never formed into a planet, from h u g e C e r e s , w i t h a d i a m e t e r of 913 k i l o m e t e r s , to r o u g h l y 28 million rocks l a r g e r than a football field. T h e reason for this failure is probably Jupiter's h u g e g r a v i t a t i o n a l pull, w h i c h m a d e their collisions too violent, preventing the s t i c k i n g needed for efficient g r o w t h . T h e outer planets, from Jupiter on to Neptune (Pluto is a special case), entered a third stage of planetary buildup, s w e e p i n g up l a r g e q u a n t i t i e s of g a s present in the outer regions of the solar nebula. We k n o w that these planets are very different in composition from the "terrestrial" planets, that is, M e r c u r y , V e n u s , Earth, a n d M a r s : inner 102

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F I G U R E 6: Different substances solidify at different distances from the Sun, as is indicated in the diagram. At large distances (Jupiter and up), there is a marked absence of rocky materials.

planets are rocky, w h e r e a s outer planets a r e gaseous. We can u n d e r stand these differences if we briefly revisit the formation process, focusing on the outer regions of the solar nebula. Since the t e m p e r a t u r e s there were lower, more gases could solidify, and this abundance of frozen matter j u m p - s t a r t e d the accretion process forming the p l a n e t e s i m a l s : the outer planets g r e w l a r g e r because their s u r r o u n d i n g m e d i u m w a s richer. T h i s b u i l d u p m a t e r i a l w a s m a i n l y in the form of c o m p o u n d s of w a t e r vapor ( H 2 O ) , a m m o n i a ( N H 3 ) , a n d m e t h a n e ( C H 4 ) , w h i c h could not solidify in the w a r m e r regions closer to the S u n (H stands for h y d r o g e n , O for o x y g e n , N for n i t r o g e n , a n d C for carbon). T h e outer planetesimals g r e w l a r g e e n o u g h to accrete the h y d r o g e n - r i c h n e b u l a r g a s a r o u n d t h e m , until they reached their e n o r m o u s sizes. T h e icy p l a n e t e s i m a l s left over from this accretion process w e r e 103

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flung either into the inner regions of the solar system or out, by the e n o r m o u s g r a v i t a t i o n a l pull of the gaseous g i a n t s . A vast cloud of these small icy objects exists in the outskirts of the solar s y s t e m â&#x20AC;&#x201D; t h e Oort cloud, n a m e d after the Dutch astronomer Jan Oort, w h o first proposed its existence in the 1950s. In fact, this is the largest of t w o cloud n u r s eries of comets, w h e r e these objects, icy debris left over from the form a t i o n of the outer planets, m o v e in their orbits a r o u n d the S u n . T h e other, k n o w n as the K u i p e r belt, is located outside Neptune's orbit. Once in a w h i l e , as a result of rare collisions, or close encounters w i t h n e i g h b o r s or the p r o x i m i t y of a nearby star or a g a s cloud, an icy ball becomes destabilized a n d is s u c k e d into the inner solar system: a comet is born. As the comet approaches the S u n , i n c r e a s i n g t e m p e r a t u r e s w a r m up its m a t e r i a l s a n d start v a p o r i z i n g t h e m , one by one. T h e point at w h i c h a g i v e n m a t e r i a l b u r n s is d e t e r m i n e d by the a m b i e n t t e m p e r a t u r e , that is, by the comet's distance from the S u n . T h e different compositions of the comets account for the differences in colors observed in their tails, w h i c h a r e nothing m o r e than v a p o r i z e d m a t e rial pushed back by the pressure c o m i n g from the solar w i n d , an outw a r d flow of fast-moving c h a r g e d particles produced by the S u n . T h e m o d e r n account of the formation of the solar system, based on the condensation of the p r i m o r d i a l solar nebula a i d e d by interstellar dust, m a t c h e s most, but not a l l , of its observed properties. It does not e x p l a i n w h y certain planets (Venus, U r a n u s , a n d Pluto) rotate a r o u n d their axis opposite to the Sun's rotation; or w h y U r a n u s ' s axis of rotation is so t i l t e d â&#x20AC;&#x201D; a l m o s t 90 d e g r e e s â&#x20AC;&#x201D; w i t h respect to the plane of the ecliptic; or w h y M a r s has such a rarefied a t m o s p h e r e ; or w h y Earth has so m u c h w a t e r ; a n d so on. T h e s e odd properties a r e e x p l a i n e d by i n v o k i n g processes that occurred mostly after the formation of the planets. M o r e to the point, as these properties depart from the apparent o r d e r of the solar system, their causes are attributed to catastrophic p h e n o m e n a of v a r i o u s sorts, from meteoric a n d c o m e t a r y b o m b a r d m e n t s to collisions w i t h satellites or l a r g e asteroids. T h u s , in order to complete our m o d e r n description of the solar system, we m u s t come full circle to its catastrophic side. 104

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O u r present u n d e r s t a n d i n g of solar system d y n a m i c s incorporates cataclysmic p h e n o m e n a in its e v e r y d a y l a n g u a g e . It leads to the inescapable conclusion that destruction is part of creation; that cosmic events unfold irrespective of any m o r a l j u d g m e n t on our part. It is neither right nor w r o n g for a comet to collide w i t h a planet: it just h a p pens w h e n their paths meet in the vastness of the cosmos. T h r o u g h o u t the a g e s , religions have charted our beliefs, inspired by the a w e s o m e forces of n a t u r e , seen as manifestations of d i v i n e power; for e x a m p l e , the rains that g a v e us food also caused d e v a s t a t i n g floods, comets m a y herald the birth of Jesus or the i m p e n d i n g p u n i s h m e n t of sinners. To a large extent, m o d e r n science has c h a n g e d our attitude t o w a r d n a t u r a l phenomena; w h a t w a s once e x p l a i n e d a w a y by d i v i n e causes is n o w seen as a consequence of w e l l - u n d e r s t o o d physical causes. C o n t r a r y to w i d e s p r e a d popular belief, this does not m e a n that m o d e r n science took a w a y the m a g i c a l beauty of celestial p h e n o m e n a . Quite the oppositeâ&#x20AC;&#x201D;our u n d e r s t a n d i n g of the w o r k i n g s of the cosmos only a d d s to our appreciation of how a w e - i n s p i r i n g it truly is. Moreover, it teaches us that, if we a r e wise e n o u g h to monitor the c o m i n g s a n d g o i n g s of our racing celestial neighbors, so that we can act in our defense if needed, we w i l l be able to r e m a i n on this planet for a very long time. A s s u m i n g , of course, that we w i l l also k e e p an eye on ourselves, a n d on our o w n destructive power. T h e alternative is to join the cycle of creation a n d destruction that perpetuates itself across the universe, a n d burn in a "conflagration," as countless other planets have a n d w i l l . R e l i g i o u s prophecies translated our fear of the skies into a y e a r n i n g for eternal salvation; falling stars m a r k e d the end of history a n d the b e g i n n i n g of eternity.

Science confirmed

the possibility that the

destructive side of these prophecies m a y very w e l l be fulfilled one day. But it also g i v e s us, perhaps paradoxically, hope for salvation. To see why, we turn to a discussion of catastrophism in the solar system, the m e e t i n g g r o u n d of apocalyptic fears a n d apocalyptic science.

105

CHAPTER

Impact! â&#x20AC;˘

The Committee believes that it is imperative that the detection rate of Earth-orbit-crossing asteroids

must

increased substantially,

and that

the means to destroy or alter the orbits of asteroids when they threaten collision

should be defined and agreed upon â&#x20AC;&#x201D; U.S. N A S A

MULTIYEAR

HOUSE

internationally. OF

REPRESENTATIVES,

AUTHORIZATION

ACT

1990

T h e early history of the solar system w a s m a r k e d by violent encounters between the g r o w i n g planets a n d s m a l l e r p l a n e t e s i m a l s . T h i s w a s the " c l e a n i n g house" era, w h i c h lasted until 3.9 billion y e a r s a g o , w h e n y o u n g planets traveled in an e n v i r o n m e n t filled w i t h debris left over from the accretion process. T h e i r satellites, of course, w e r e not spared the b o m b a r d m e n t . T h e surface of the Moon, pocked w i t h craters of all sizes (over thirty thousand of t h e m ) , offers testimony to its violent b e g i n n i n g s . S e v e r a l satellites of the g i a n t planets display the same tortured surfaces. V e n u s is covered by such a thick b l a n k e t of noxious fumes that direct v i e w i n g of its surface is impossible. H o w e v e r , r a d a r i m a g e s by the Magellan spacecraft exposed over nine h u n d r e d craters, r a n g i n g from a few k i l o m e t e r s in d i a m e t e r to the g i a n t M e a d crater, some 280 k i l o m e t e r s . M e r c u r y ' s surface is l i k e w i s e covered w i t h craters. Earth surely did not escape this e a r l y b o m b a r d m e n t , a l t h o u g h these ancient craters have mostly been erased by intense erosion promoted by continuous atmospheric a n d seismic activity. Of the few 107

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k n o w n craters, most a r e fairly recent; for the celestial b o m b a r d m e n t , if less intense n o w than d u r i n g the solar system's first billion y e a r s , is still very m u c h a d i s t u r b i n g reality. An e x a m p l e is Meteor crater in northern A r i z o n a , w h i c h has a d i a m e t e r of 1.2 k i l o m e t e r s a n d a depth of 200 meters (see figure 16 in insert). T h e m a i n reason we can still see this p a r t i c u l a r crater is its very y o u n g a g e , about fifty thousand y e a r s . C o l l i sions w i t h asteroids a n d comets are not relegated to the very distant past. A l t h o u g h rarer now, they can happen a n y t i m e . It w a s only in the 1960s that E u g e n e S h o e m a k e r , a l e a d i n g authority on i m p a c t g e o physics, proved c o n v i n c i n g l y that the A r i z o n a crater w a s caused by the i m p a c t of an iron-rich meteorite about 50 meters in d i a m e t e r > T h e a l t e r n a t i v e e x p l a n a t i o n at the t i m e held that the crater w a s the result of some violent volcanic activity. S h o e m a k e r a n d his collaborators found s a m p l e s of h i g h - p r e s s u r e glassy rock a n d deformed geological structures produced by the t r e m e n d o u s violence of the impact, a n d that finding put the debate to rest. Picture a rock the size of a fifteen-story b u i l d i n g hitting the g r o u n d at 11 k i l o m e t e r s per second ( r o u g h l y 25,000 m i l e s per h o u r ) . Impact physics is about m o m e n t u m a n d e n e r g y transfer. Both of them m u s t be the s a m e before a n d after the impact: w h a t the celestial offender gives, Earth receives. Because m o m e n t u m is mass times velocity, a n d Earth's m a s s is so m u c h l a r g e r than a n y asteroid or comet ( r o u g h l y three billion t i m e s l a r g e r for a 5 - k i l o m e t e r - w i d e a s t e r o i d ) , the c h a n g e in Earth's m o m e n t u m as a result of a collision is perfectly n e g l i g i b l e : an impact w i l l not k n o c k Earth out of its orbit, just as you cannot move a truck by hitting it w i t h a baseball. W i t h energy, however, t h i n g s are very different. T h e a m o u n t of e n e r g y before i m p a c t â&#x20AC;&#x201D; t h e e n e r g y of motion, or kinetic energy, of the i n c o m i n g p r o j e c t i l e â&#x20AC;&#x201D; m u s t be e q u a l to the e n e r g y after impact, that is, the e n e r g y dissipated on the g r o u n d a n d atmosphere a r o u n d it. Now, the k i n e t i c e n e r g y is proportional to the mass times the square of the projectile's velocity. It is the square of the velocity that m a k e s all the difference, especially w h e n the velocities a r e of tens of thousands of m i l e s per hour. You can verify this by t h r o w i n g rocks 108

of about the s a m e size into a pond, v a r y i n g their velocity at each t h r o w a n d w a t c h i n g the results. T h e h a r d e r you t h r o w the rocks, the m o r e of a disturbance y o u cause on the surface of the pond (see figure 17 in insert). You see the kinetic e n e r g y of the rock b e i n g dissipated by concentric w a t e r w a v e s p r o p a g a t i n g o u t w a r d from the point of impact; you see the recoiling w a t e r r i s i n g up into the a i r a n d falling back a g a i n , w a t e r drops s p r e a d i n g a r o u n d a l a r g e area; a n d , for very h a r d t h r o w s , you see the rock penetrate into the bottom of the pond. A l t h o u g h the velocity p l a y s a k e y role in d e t e r m i n i n g the d a m a g e d u e to an impact, the mass is also c l e a r l y important. Everybody l a u g h s w h e n little five-year-old Johnny does his cannonball dive on the pool, but w h e n 200-pound U n c l e L o u i e does the s a m e , nobody finds it very funny. Earth is s p r i n k l e d d a i l y w i t h over a ton of micrometeorites the size of a g r a i n of sand or smaller. T h e y a r e the shooting stars, s t r e a k i n g the n i g h t skies w i t h a feeble light, as they a r e v a p o r i z e d by friction with the atmosphere. W h e n falling rocks reach the size of a fist or a bit larger, all the a t m o s p h e r e can do is slow them d o w n , a n d an impact occurs. T h e s e are the meteorites we see in m u s e u m s and p l a n e t a r i u m s . As for little J o h n n y and U n c l e L o u i e , it should be q u i t e clear w h i c h of the two has a g r e a t e r chance of hitting the bottom of the pool. Back to Meteor crater. K n o w i n g the velocity a n d size of the rock, we can estimate the i m p a c t e n e r g y to have been e q u i v a l e n t to the simultaneous collision of about one billion 20-ton t r u c k s m o v i n g at 100 miles per hour. Since we cannot pack one billion t r u c k s into 50 meters (the estimated size of the rock), we can use another a n a l o g y ; the impact energy w a s e q u i v a l e n t to the detonation of 20 to 40 m e g a t o n s of T N T (mega = m i l l i o n ) . Since a l a r g e h y d r o g e n bomb deploys about 1 m e g a ton of T N T , the i m p a c t that forged Meteor crater is e q u i v a l e n t to dozens of h y d r o g e n bombs e x p l o d i n g together (without the radioactivâ&#x20AC;˘ty, of course). T h e g r o u n d a r o u n d the i m p a c t w a s instantly v a p o r i z e d , together w i t h most of the asteroid. A b o u t 175 m e g a t o n s of rock w e r e lifted by the impact, only to rain d o w n over an a r e a of about 9.5 k i l o 1

meters from g r o u n d zero. T h e collision created a shock w a v e called an "tr blast, w i t h w i n d s of over 1,000 k i l o m e t e r s per hour. Even as far as 40

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k i l o m e t e r s from the impact site, w i n d s w e r e still of h u r r i c a n e force. Fossils indicate that at the time of impact, d u r i n g the last ice a g e , the a r e a w a s populated w i t h m a m m o t h s , mastodons, g i a n t g r o u n d sloths, a n d other h u g e m a m m a l s . It is h a r d to i m a g i n e that a n y of these a n i m a l s could have survived w i t h i n a r a d i u s of at least 20 k i l o m e t e r s . If a s i m i l a r impact w e r e to occur in a metropolitan region today, millions of people w o u l d die in a flash. A n d this is only one of the little g u y s . W i d e s p r e a d geological s u r v e y s from land and space d u r i n g the past t w o decades led to the discovery of several impact craters. T h e total n o w stands at about 150 and is g r o w i n g by 5 or so a year. T h e crater sizes and times of impact vary w i d e l y . T h e H a v i l a n d crater, in Kansas, and the Sobolev crater, in Russia, are only about one thousand y e a r s old, w h e r e a s the Vredefort crater, in South Africa, is t w o billion years old. T h e H a v i l a n d crater is 15 meters w i d e , w h e r e a s a h u g e crater in S u d b u r y , Ontario, could potentially be 250 k i l o m e t e r s w i d e . T h e conclusion is clear: Earth has been b o m b a r d e d by l a r g e objects in the past and not so distant past, just l i k e all other bodies of the solar system. Scientists use expressions such as "cosmic pinball," "cosmic shooting g a l l e r y , " "target Earth," and so forth to illustrate the fact that collisions a r e an integral part of life in the solar system. Doomsday scenarios, once part of religious discourse, are now vested w i t h scientific precision. We now k n o w that a 1 0 - k i l o m e t e r - w i d e object fell over the Y u c a t a n peninsula in the G u l f of Mexico sixty-five million y e a r s ago, obliterating over 40 percent of life on Earth, d i n o s a u r s included. Other mass extinctions have been related to impact craters. T h i s n e w evidence has forced us to reformulate the w a y we think about evolution of life on Earth, our o r i g i n s , a n d , u l t i m a t e l y , our survival as a species. Extensive k n o w l e d g e about cosmic impacts has s h a k e n our beliefs in a s o m e w h a t docile, g r a d u a l evolution of our planet and its life forms; instead, we now see an evolution punctuated by major catac l y s m s , w h i c h has pushed the "survival of the fittest" idea to its limit. T h e species w i t h the highest odds of survival w e r e those best adapted not only to their e n v i r o n m e n t but also to abrupt major c h a n g e s in that

110

IMPACT!

e n v i r o n m e n t . T h e d i n o s a u r s r e i g n e d s u p r e m e over l a n d , air, and w a t e r for over 150 m i l l i o n y e a r s , until hell broke loose 65 million years ago. W h i l e the reptiles w e r e w i p e d out of existence, u n a b l e to adapt to the harsh n e w conditions a n d the disruption of the food c h a i n , small m a m m a l s and birds s u r v i v e d . T h e s e m a m m a l s evolved, a n d t w o million y e a r s a g o the first h o m i n i d s a p p e a r e d . We o w e our existence partly to that single horrific impact event. Given today's evidence, it is hard to i m a g i n e w h a t else w o u l d have kept the d i n o s a u r s from still being a r o u n d . A l t h o u g h we m a y be justly terrified by the thought of another major impact h a p p e n i n g in the future, we should also r e m e m b e r our debt to chance. Collisions destroy and collisions create.

Geology Turns Catastrophic As an illustration of a true d o o m s d a y scenario, we w i l l e x a m i n e in some detail the fated collision that w i p e d out the d i n o s a u r s sixty-five million

years ago.

The

story of h o w geologists,

paleontologists,

chemists, and physicists c a m e to the conclusion that about 40 percent of life on Earth w a s indeed k i l l e d by a h o r r e n d o u s collision w i t h a comet or a s t e r o i d — w e a r e still not quite certain w h i c h — i s in itself a w o n d e r ful illustration of how science w o r k s . T h e hunches, the false clues, the brilliant intuition of some, the severe resistance of others, the a c c u m u lation of evidence, the fierce d e b a t e s — a l l converged on an eventual acceptance ( w h i c h is still not u n a n i m o u s ) of a n e w idea, w h i c h , as we have seen, is also very old. M a n y of the d e t a i l s are told in W a l t e r A l v a r e z ' s book T. Rex and the Crater of Doom, w h i c h is w e l l worth 2

r e a d i n g . W a l t e r , a geologist, and his father, L u i s A l v a r e z , a Nobel Prize—winning particle physicist, played a key role in u n v e i l i n g the m a n y clues l e a d i n g to the inescapable conclusion that the extinction of all dinosaurs coincided w i t h the collision of a 1 0 - k i l o m e t e r - w i d e body m the G u l f of Mexico. T h e record of this " d o o m s d a y " collision, and of other collisions, is written in the geological strata of Earth, l a y e r after

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layer. To q u o t e W a l t e r A l v a r e z , "fluids a n d gases forget, but rocks r e m e m b e r . " T h e c h a l l e n g e w a s to decipher the m e s s a g e e n g r a v e d in their m e m o r y . Geologists a n d paleontologists had long been a w a r e of a fairly s h a r p b o u n d a r y s e p a r a t i n g the C r e t a c e o u s a n d the T e r t i a r y eras 65 m i l lion y e a r s a g o , the so-called K.T boundary. H o w e v e r , before the 1960s, very few s a m p l e s of this b o u n d a r y w e r e a c t u a l l y a v a i l a b l e for study. A n d there w a s no reason to look for m o r e . Since C h a r l e s Lyell published his influential Principles of Geology in the early

1830s, geology

h a d been d o m i n a t e d by the idea that c h a n g e s on Earth occur slowly. L y e l l m a d e the curious extrapolation that, since the l a w s of n a t u r e do not c h a n g e in space a n d t i m e , w h i c h is w h a t m a k e s science possible, processes on Earth should be very g r a d u a l , or u n i f o r m . T h i s u n i f o r m i t a r i a n doctrine, as it b e c a m e k n o w n later, a s s u m e d that the very slow rate at w h i c h t h i n g s c h a n g e d on Earth h a d a l w a y s beert the s a m e . A c c o r d i n g to this "classical" view, the role of the geologist w a s to m a p the different strata, l e a r n i n g about Earth's history as it w a s recorded on rocks a t various h e i g h t s a n d depths. T h i s g r a d u a l i s t v i e w g a i n e d t r e m e n d o u s i m p e t u s d u r i n g the late

1960s, w h e n plate tectonics

b e c a m e w i d e l y accepted as the d r i v i n g m e c h a n i s m s h a p i n g Earth's crust: nothing seems m o r e g r a d u a l than the slow drifting of continentc a r r y i n g plates by a few c e n t i m e t e r s a y e a r (about the speed at w h i c h y o u r nails g r o w ) over eons of t i m e . In this view, the most c a t a c l y s m i c p h e n o m e n a Earth could w i t n e s s w e r e volcanic eruptions, c l i m a t e c h a n g e s such as ice a g e s , a n d l a r g e - s c a l e floods. Evolutionary biology e m b r a c e d geological g r a d u a l i s m . C h a r l e s D a r w i n w a s certainly influenced by Lyell's ideas w h e n he took off on his famous expedition onboard the H M S Beagle from 1831 to 1836. W h e n the Origin of Species

was

published

1859, g r a d u a l i s m

was

a s s u m e d to be a central tenet of evolution: the c h a n g e s a l t e r i n g a given species w e r e slow, mostly destructive. A l t h o u g h Alfred Russel W a l l a c e proposed s i m i l a r i d e a s , it w a s D a r w i n w h o provided most of the evidence a n d p u r s u e d its consequences. R a r e beneficial c h a n g e s helped the l u c k y species, as it a g g r e s s i v e l y competed w i t h others for resources 112

IMPACT!

and survival. L o n g n e c k s a l l o w e d giraffes to reach the tender h i g h e r leaves, spots helped leopards hide better from their prey (and giraffes from their predators), and so on. N a t u r a l selection, as this process b e c a m e k n o w n , w a s a g r a d u a l process, l e a d i n g to the eventual survival of the fittest. T h e key i n g r e d i e n t m i s s i n g in D a r w i n ' s theory w a s genetics. Once genetics and evolution w e r e c o m b i n e d , and D N A w a s identified in the 1950s, the theory of evolution based on natural selection w a s u n d e r stood as a consequence of the faulty transmission of genetic m a t e r i a l from one g e n e r a t i o n to the next. Specific traits of the progeny result from countless variations d u r i n g the fusion of genetic m a t e r i a l from their parents. Most variations a r e uneventful; puppies all look a l i k e , apart from the location of this or that spot, or the color of an eye. Once in a w h i l e , however, a g i v e n piece of D N A , a g e n e , m a y suffer a mutation, a c h a n g e in its m o l e c u l a r a r r a n g e m e n t . T h i s m a y be caused by different sorts of h i g h - e n e r g y radiation (x r a y s , g a m m a r a y s , cosmic-ray particles, natural r a d i o a c t i v i t y ) or by chemical d a m a g e . T h i s m u t a t i o n m a y be passed on to the i n d i v i d u a l ' s offspring. Most m u t a t i o n s cause severe i m p a i r m e n t or death. H o w e v e r , on very rare occasions, a m u t a tion m a y a c t u a l l y benefit the i n d i v i d u a l ' s survival. As the m u t a t e d fitter o r g a n i s m

reproduces, its beneficial traits m a y pass on to its

offspring. For l a r g e r a n i m a l s , w h i c h reproduce fairly slowly, it m a y take m i l l i o n s of y e a r s for a species to m u t a t e into an entirely n e w species, a rate w o r t h y of geological g r a d u a l i s m . W i t h i n this g e n e r a l f r a m e w o r k of g r a d u a l geological and biological changes, mass extinctions w e r e also believed to be g r a d u a l ; species don't simply disappear. Several factors, often in combination, m a y lead to the slow d i s a p p e a r a n c e of a g i v e n species, from c l i m a t i c c h a n g e s to volcanic activity to i m b a l a n c e s in the predator-prey chain. T h u s , it w a s widely held that d i n o s a u r s d i s a p p e a r e d over millions of y e a r s , g r a d u ally petering out before the KT boundary. T h e s a m e w a s believed to be true of the a m m o n i t e s , m a r i n e invertebrates related to the beautiful chambered n a u t i l u s of today (see figure 18 in insert). In an ideal w o r l d , one can trace the rise and fall of a g i v e n species

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by e x a m i n i n g the frequency w i t h w h i c h its fossils a r e found at different depths; from the bottom u p , as a species g r a d u a l l y rises to p r o m i nence, its fossil record becomes richer. As the species declines, its fossils become rarer, until they vanish altogether, s o m e w h a t l i k e a smooth w a v e of bones i m p r i n t e d over vertical l a y e r s of rock. An abrupt mass extinction w o u l d be c h a r a c t e r i z e d by a very sharp decline in the n u m ber of fossils of several species. In practice, of course, things a r e not so simple. A p a r t from being q u i t e r a r e , fossils are not laid d o w n r e g u l a r l y , a n d can easily be destroyed over m i l l i o n s of y e a r s . To m a k e things worse, the rarer the fossil, the m o r e g r a d u a l the extinction process a p p e a r s . T h i s is the so-called S i g n o r - L i p p s effect; instead of a clearly d e l i n e a t e d w a v e of fossils, one has a flatter w a v e , whose peak does not stand out m u c h . It w a s thus reasonable for most paleontologists to a r g u e that the reason w h y a sharp decline in the n u m b e r of d i n o s a u r fossils before the KT b o u n d a r y h a d not been found w a s that it wasn't there. H o w e v e r , only a finer c o m b i n g of the sediments w o u l d truly d e t e r m i n e the sharpness of the extinction a n d its lofcation relative to the K T boundary.

^^N.

In c a m e the catastrophists. In 1971, the paleontologist Dale Russell a n d the astrophysicist W a l l a c e T u c k e r bravely suggested that the d i n o s a u r s w e r e k i l l e d by a nearby supernova explosion, a h u g e ejection of matter a n d radiation m a r k i n g the end of the n u c l e a r fusion cycle of 3

m a s s i v e stars. Russell a n d T u c k e r conjectured that the explosion could h a v e induced severe c l i m a t i c c h a n g e s for w h i c h the d i n o s a u r s w e r e not p r e p a r e d . Other papers followed, a n a l y z i n g in more detail the conseq u e n c e s of such a violent event for life on Earth. T h e problem w a s that astronomers could not find the r e m n a n t s of a supernova that detonated about sixty-five m i l l i o n y e a r s a g o in our cosmic neighborhood; the m u r d e r w e a p o n w a s missing. T h e r e w a s also no evidence of the explosion i m p r i n t e d on the KT boundary. As we have seen, matter ejected d u r i n g stellar explosions travels across vast cosmic distances, seeding the interstellar m e d i u m w i t h heavy e l e m e n t s . S o m e of it should have been deposited on Earth, l i k e sprinkles on a c a k e . L u i s A l v a r e z conjectured that a nearby supernova w o u l d have left an a b n o r m a l a m o u n t of

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p l u t o n i u m 2 4 4 â&#x20AC;&#x201D; a n isotope of p l u t o n i u m forged d u r i n g the hellish explosionâ&#x20AC;&#x201D;over the terrestrial surface. H o w e v e r , no excess of pluton i u m 244 w a s found, r u l i n g out a supernova explosion as a viable cause for the KT mass extinction. T h e g r a d u a l i s t s in the scientific c o m m u nity, the vast majority at that point, had the first l a u g h . W h i l e catastrophist geologists and physicists w e r e busy t r y i n g to figure out w h a t w o u l d be the telltale substance pointing t o w a r d a catastrophic event in the KT boundary, t w o paleontologists c h a l l e n g e d g r a d u a l i s m in their o w n field. In 1972, Niles E l d r e d g e and Stephen Jay Gould proposed that evolution w a s far from monotonic, that the fossil record suggests the existence of periods of rapid c h a n g e s in the n u m b e r of species, spikes of evolutionary activity, interspersed w i t h long periods of g r a d u a l c h a n g e .

T h e i r suggestion of punctuated equilibrium

was

not w e l c o m e d by the g r a d u a l i s t s ; the debate still goes on, a l t h o u g h the evidence compiled over the last thirty y e a r s indicates that there w e r e indeed periods w h e n n e w species boomed into existence at an a m a z ingly fast pace. P u n c t u a t e d e q u i l i b r i u m is a perfect evolutionary c o m promise between g r a d u a l i s m and catastrophism; an abrupt c h a n g e in the e n v i r o n m e n t , caused by the impact w i t h an extraterrestrial object or by some other local or, m o r e rarely, global event, w o u l d forcefully redefine the notion of fitness. Countless species, previously perfectly well a d a p t e d , w o u l d find themselves u n a b l e to survive u n d e r the s u d denly imposed n e w conditions, w h i l e others w o u l d thrive. T h e abrupt c h a n g e in e n v i r o n m e n t a l conditions w o u l d promote a period of intense evolutionary c h a n g e , a q u i c k boiling of the genetic soup, until a r e n e w e d state of e q u i l i b r i u m w a s reached, lasting until the next abrupt change. W h i l e these catastrophic ideas w e r e mostly being neglected by the scientific c o m m u n i t y , L u i s and W a l t e r A l v a r e z and their c o w o r k e r s w e r e busily r e t h i n k i n g their strategy. After a few false starts, L u i s A l v a r e z c a m e up w i t h a plausible scenario: an extraterrestrial object w o u l d be rich in i r i d i u m , an e l e m e n t that is very rare at Earth's surface, being mostly concentrated in its molten m e t a l l i c core. A sufficiently violent impact w o u l d vaporize most of the invader and the bedrock 115

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a r o u n d it, l a u n c h i n g a h u g e a m o u n t of i r i d i u m - r i c h dust into the a t m o s p h e r e . Volcanic eruptions do s o m e t h i n g similar, on a s m a l l e r scale: the 1883 eruption of K r a k a t o a in Indonesia, s p r i n k l e d so m u c h dust into the atmosphere that sunsets in L o n d o n w e r e r e d d e r for months, w h i l e the global t e m p e r a t u r e dropped by half a d e g r e e C e l sius. T h e dust acts as a reflector of sunlight, cooling the planet u n d e r neath. L u i s A l v a r e z reasoned that t w o effects should follow a h u g e impact. First, there should be an excess of i r i d i u m s p r i n k l e d over the KT boundary, brought by the extraterrestrial killer. Second, the dust from the impact w o u l d block s u n l i g h t so efficiently that the w o r l d w o u l d turn completely d a r k a n d t e m p e r a t u r e s w o u l d p l u n g e b e l o w freezing.

T h i s is

precisely

the abrupt w i d e s p r e a d e n v i r o n m e n t a l

c h a n g e needed to promote global mass extinctions. T h e A l v a r e z t e a m found excess i r i d i u m in one k n o w n s a m p l e of the KT b o u n d a r y from Italy. T h e n in another from D e n m a r k . A g e o l ogist from H o l l a n d , Jan S m i t , had independently found the " i r i d i u m a n o m a l y " in a KT s a m p l e from S p a i n . In J u n e 1980, the A l v a r e z t e a m published its paper on the extraterrestrial origin for the KT extinction, 5

using the i r i d i u m a n o m a l y as k e y e v i d e n c e . A veritable d e l u g e of papers followed. Just d u r i n g the 1980s, over t w o thousand papers w e r e published for or against extraterrestrial causes for mass extinctions! T h e clincher, the discovery of the impact crater, w a s a n n o u n c e d only in 1991. By that t i m e , over one h u n d r e d KT sites h a d been s h o w n to be rich in i r i d i u m , c o n f i r m i n g the global scope of tne~<lisaster. An i m p a c t such as this w o u l d have left an e n o r m o u s crater. W h e r e w a s it? F i n d ing a 6 5 - m i l l i o n - y e a r - o l d crater is not easy, especially on a planet w h e r e almost 70 percent of the surface is covered by water. T h e odds of havi n g an impact over an a r e a accessible to exploration w e r e s m a l l â&#x20AC;&#x201D; t h e opposite of w h a t we w o u l d w i s h of an impact today. Even s m a l l e r w e r e the odds of h a v i n g the crater preserved after m i l l i o n s of y e a r s of erosion a n d seismic activity. A g a i n , the m e m o r y of rocks c a m e into play. T h e impact, it w a s later found, happened either very near or just w i t h i n the w a t e r l i n e of

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IMPACT!

the Yucatan p e n i n s u l a in the G u l f of Mexico. T h e e n o r m o u s violence of the collision caused a t s u n a m i estimated to have g r o w n to at least fifty a n d possibly a few h u n d r e d m e t e r s in height. Such a g i g a n t i c w a v e w i p e d out most of the G u l f coast of the U n i t e d States a n d M e x ico, as well as the C a r i b b e a n , c a r r y i n g w i t h it all sorts of debris. W o r k ing b a c k w a r d , geologists found the Sediment layer brought by the g i a n t t s u n a m i right on the KT boundary, at sites in T e x a s a n d H a i t i . If the t s u n a m i c a r r i e d sediments to T e x a s a n d H a i t i , they reasoned, the impact must have occurred in the G u l f of M e x i c o . S h a r i n g information w i t h geologists from Petroleos M e x i c a n o s ( P E M E X , the M e x i c a n oil c o m p a n y ) , the A m e r i c a n geologist A l a n H i l d e b r a n d a n d collaborators located the site of impact: the C h i c x u l u b crater, from the M a y a n w o r d m e a n i n g "tail of the d e v i l , " w i t h a d i a m e t e r of r o u g h l y 170 k i l o m e t e r s . It took a 1 0 - k i l o m e t e r - w i d e object, t r a v e l i n g at r o u g h l y 20 k i l o m e t e r s per second to d i g such an e n o r m o u s hole. It is hard to i m a g i n e a m o r e horrific event on a p l a n e t a r y scale. Rocks have ordered crystalline structures. H o w e v e r , w h e n boiled or v a p o r i z e d by the t r e m e n d o u s e n e r g i e s of the impact a n d then r a p idly cooled, they fail to r e a r r a n g e themselves into r e g u l a r patterns, becoming glassy, or a m o r p h o u s , in structure. After p a i n s t a k i n g search and a n a l y s i s , glassy m a t e r i a l s w e r e identified in s a m p l e s obtained from several places a r o u n d g r o u n d zero; their a g e s nicely m a t c h e d the estim a t e d time of impact at sixty-five m i l l i o n y e a r s ago. S o m e of the c r u cial w o r k that connected the glassy m a t e r i a l s w i t h the KT b o u n d a r y w a s done d o w n the hall from my office at D a r t m o u t h by P a g e C h a m berlain and my friend Joel B l u m , n o w at the U n i v e r s i t y of M i c h i g a n . In 1993, Joel participated in a debate on the fate of dinosaurs w i t h C h u c k D r a k e , another D a r t m o u t h professor, w h o nevertheless leaned t o w a r d g r a d u a l i s m . T h e o v e r w h e l m i n g evidence for the impact at Yucatan must have m a d e D r a k e ' s job very h a r d . In his defense, h o w ever, it should be pointed out that debates such as this, w h e r e evidence for a n d against a g i v e n theory is discussed a n d criticized, a r e crucial for the w e l l - b e i n g of science. I think it is fair to say that Joel w o n this one.

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Dinosaur

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Doom

T h e extraterrestrial k i l l e r , estimated to have been about 10 k i l o m e t e r s w i d e , approached Earth at about 20 k i l o m e t e r s per second, crossing most of the atmosphere in less than t w o seconds. T h e air in front of it w a s so severely compressed that it reached t e m p e r a t u r e s four to five times that of the Sun's surface, c r e a t i n g a b l i n d i n g flash of light a n d a sonic boom the likes of w h i c h has not been heard since then (see figure 19 in insert). T h e e n e r g y released by the impact w a s e q u i v a l e n t to an a b s u r d 100 m i l l i o n m e g a t o n s of T N T , ten thousand times m o r e than all the t h e r m o n u c l e a r bombs a v a i l a b l e at the height of the C o l d W a r detonated together. T h e consequences of d u m p i n g such an e n o r m o u s a m o u n t of e n e r g y in so short a t i m e are terrifying; the celestial k i l l e r carved into the ocean floor a hole about 40 k i l o m e t e r s d e e p a n d 200 k i l o m e t e r s w i d e , i m m e d i a t e l y v a p o r i z i n g all the w a t e r a n d rock it found in its w a y . W a v e s of boiling w a t e r a n d rocks rushed into the g i g a n t i c hole as 100-meter t s u n a m i s propagated o u t w a r d . E a r t h q u a k e s of u n h e a r d - o f strength, r e a c h i n g 10 on the Richter scale, shook the g r o u n d for h u n d r e d s of k i l o m e t e r s , c a u s i n g the coastline to c r u m b l e , w h i l e feeding further t s u n a m i s . W h e n we t h r o w stones into a l a k e , there is a l w a y s an initial d i s p l a c e m e n t of water, followed by a central p e a k , w h i c h rises higher, the faster you t h r o w the stone or the m o r e massive it is. T h e impact w a s so violent that v a p o r i z e d rock, debris, and w a t e r w e r e thrust u p w a r d in a p l u m e that w e n t h a l f w a y to the Moon, only to fall back a g a i n , c a u s i n g an u m b r e l l a of destruction thousands of k i l o m e t e r s w i d e . Rocks behaved literally l i k e fluids, g r o u n d zero m a r k i n g the site of a g i a n t bull's-eye s u r r o u n d e d by c i r c u l a r u n d u l a t i o n s that w e r e i m p r i n t e d on the boiling bedrock. A n y living thing w i t h i n m a n y h u n d r e d s of k i l o m e t e r s from the point of impact w a s i m m e d i a t e l y v a p o r i z e d . F a r t h e r out, at a few thousand k i l o m e t e r s , a n i m a l s saw a b l i n d i n g flash of light followed by an uncontrollable s h a k i n g of the g r o u n d , as the seismic w a v e s passed one after another. T h e ejecta falling back on Earth from space caused the 118

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next w a v e of devastation. T h e y reentered the atmosphere w i t h such fury that the sky w a s literally set a b l a z e , i g n i t i n g continent-size fires that broiled a n y t h i n g in their path. Giant t s u n a m i s completed the i m m e d i a t e devastation. Most parts of the U n i t e d States a n d M e x i c o w e r e completely destroyed in a matter of hours. T h e heavens b r o u g h t hell to Earth. F a r t h e r a w a y , in Europe, Africa, South A m e r i c a , a n d Asia, the i m m e d i a t e effects w e r e less d r a m a t i c . T h o s e regions had to w a i t for the secondary effects of the impact, w h i c h w e r e slower, but no less effective, k i l l e r s . An astonishing a m o u n t of dust w a s lifted into the a t m o s phere by the impact, as if a m i l l i o n volcanoes had erupted together. As this dust spread t h r o u g h the upper a t m o s p h e r e b l o c k i n g the sunlight, Earth turned cold a n d d a r k for months. Very cold a n d d a r k . So d a r k that you could not have seen y o u r h a n d in front of you, a n d so cold as to reach subfreezing t e m p e r a t u r e s . T h e plants a n d a n i m a l s u n l u c k y e n o u g h to s u r v i v e the near-instant death from the impact faced a g o n i z i n g h u n g e r because of the food chain's collapse. Still, there w a s more to come. As rains w a s h e d d o w n the dust, the t e m p e r a t u r e started to increase. M u c h of the g r o u n d u n d e r the impact w a s m a d e of l i m e stone, a rock rich in c a l c i u m carbonate. T h e heat released by the impact decomposed the carbonate, w h i c h combined w i t h o x y g e n to form carbon dioxide, a g r e e n h o u s e g a s very familiar to us; Earth w e n t from e x t r e m e l y cold to e x t r e m e l y hot, as the excessive carbon d i o x i d e a n d suspended w a t e r vapor trapped the heat from the S u n near the g r o u n d , c a u s i n g an intense g r e e n h o u s e effect, w h i c h lasted for possibly thousands of y e a r s . To complete the horror, m u c h of the nitrogen a n d sulfur found in rocks a r o u n d the impact a r e a combined w i t h o x y g e n a n d h y d r o g e n to form h u g e a m o u n t s of sulfuric a n d nitric acids, w h i c h rained d o w n for decades. Given the assault from fire to cold and d a r k n e s s to heat a n d acid rain, it is r e m a r k a b l e that a n y t h i n g actually survived this true apocalypse, w h i c h far surpassed Saint John's most terrifying visions. But several species d i d , a n d we are here as their descendants. If the history of life on Earth, in all its m y r i a d forms, can be understood as an e x p e r i 119

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m e n t in evolutionary genetics orchestrated by n a t u r a l selection, the e m e r g e n c e of intelligent life seems to be the result of a chance occurrence, an odd event that a p p a r e n t l y w o u l d be e x t r e m e l y h a r d to d u p l i cate e l s e w h e r e . T h e long reign of the dinosaurs lasted for over 150 m i l l i o n y e a r s a n d s h o w e d no signs of w e a k e n i n g until it w a s t e r m i nated almost instantaneously. T h e i r success a n d longevity as a species m a k e it h a r d to a r g u e for the necessity of intelligent life at the top of the evolutionary chain; the d i n o s a u r s ' l i m i t e d brains seem to have been q u i t e e n o u g h to ensure their d o m i n a n c e . H a d it not been for the celestial i n t r u d e r a n d the drastic e n v i r o n m e n t a l c h a n g e s that it caused, the d i n o s a u r s w o u l d q u i t e possibly still rule, w h i l e m a m m a l s w o u l d be insignificant. It is a h u m b l i n g thought. If, indeed, intelligence is not a necessary consequence of evolution, a r e we alone as t h i n k i n g beings in this vast cosmos, the odd products of a b i z a r r e evolutionary q u i r k ? A l t h o u g h we w i l l be able to a n s w e r this question only by probing forâ&#x20AC;&#x201D;or being probed b y â&#x20AC;&#x201D; l i f e e l s e w h e r e , it is m u c h easier to a r g u e for extraterrestrial life than for extraterrestrial intelligent life. On the other h a n d , it is also h a r d to conceive of a u n i verse w i t h billions of g a l a x i e s , each w i t h billions of stars, w h e r e any k i n d of evolutionary q u i r k can be so rare as to happen only once. I prefer to t h i n k of a u n i v e r s e filled w i t h intelligent life, even if spread s o m e w h a t sparsely, some of it a d v a n c e d e n o u g h to have found solutions to the m a n y problems that p l a g u e our society, such as w a r , famine, and hatred. S o m e colleagues a r g u e , q u i t e reasonably, that if intelligent life existed in our g a l a x y , it w o u l d by n o w have colonized it entirely; the M i l k y W a y , being over ten billion y e a r s old a n d "only" 100,000 l i g h t - y e a r s across, w o u l d be p a c k e d full w i t h intergalactic cruisers. M a y b e they c a m e a h u n d r e d m i l l i o n y e a r s a g o , s a w the d i n o s a u r s , a n d left in disgust, never to come back. A n o t h e r popular idea is that we are the results of their seeding, c h i l d r e n of e x t r a t e r r e s trial explorers of yonder. If that is true, we all suffer from a bad case of cosmic parental a b a n d o n m e n t . A g a i n , I prefer to t h i n k that this a s s u m e d expansionist need is not an i m p e r a t i v e of intelligent life. Perhaps one of the characteristics of the a l i e n s ' w i s d o m is precisely to have

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found balance w i t h i n their b o u n d a r i e s , as opposed to the "destroy a n d e x p a n d " policy we seem to p u r s u e so m u c h . H o w could we possibly p r e s u m e to u n d e r s t a n d motive in an a d v a n c e d alien psyche if we h a r d l y u n d e r s t a n d our o w n ? It is t e m p t i n g to d r a w p a r a l l e l s b e t w e e n these i m a g i n e d e x t r a t e r restrial beings a n d the m i r a c u l o u s visions of saints, a n g e l s , a n d g o d s , popular in m a n y religions across the globe. S o m e of us believe they are "up there" s o m e w h e r e , w i s e a n d distant, a m o d e l of w h a t we w a n t or should become. S o m e persons w a n t to believe so m u c h that they c l a i m to have seen these celestial visitors, h o v e r i n g over some obscure country road or in a secluded d e s e r t â&#x20AC;&#x201D; a n g e l s or, m o r e often, d e m o n s that a p p e a r to be as c u r i o u s about us as we a r e about them. It is very unfortunate that these a l l e g e d visitors never seem to w a n t to establish a n y sort of serious contact w i t h scientists or political a u t h o r i t i e s , w h o could try to establish a true d i a l o g u e . In fact, they never seem to leave a n y concrete proof of their visitations. T h e i r elusiveness m a k e s it hard for most scientists, i n c l u d i n g this one, to g i v e c r e d e n c e to these s i g h t i n g s . A n d this is not because we a r e closed m i n d e d . Quite the contrary, as C a r l S a g a n a r g u e d in his book The Demon-Haunted

World, no one

w o u l d be m o r e d e l i g h t e d a n d excited to confirm the existence of e x t r a terrestrial life than the scientists w h o d e d i c a t e their lives to s t u d y i n g 6

the universe a n d its m y s t e r i e s . Unfortunately, until that d a y comes, or until we m a n a g e to find it ourselves, intelligent life w i l l r e m a i n an odd evolutionary q u i r k , the end product of a rare celestial collision, a n d we will have to k e e p g u e s s i n g how our cosmic c o l l e a g u e s look a n d t h i n k up there in the heavens.

Apocalypse N o w ? Apologies for the sensationalistic rhetoric, but I believe a w a k e - u p call is badly needed. W h a t k i l l e d the d i n o s a u r s can k i l l us too. Yes, scientists have become the n e w (or at least another class of) prophets of doom. We have followed the e a r l y steps of N e w t o n , H a l l e y , a n d 121

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L a p l a c e to confirm the real possibility of a collision w i t h a celestial object. To bring m a t t e r s closer to our a g e , let me tell t w o stories of recent collisions, one here on Earth a n d the other on Jupiter. Early in the m o r n i n g of J u n e 30, 1908, the sky above the basin of the Stony T u n g u s k a River in central Siberia w a s sliced in h a l f by a b l a d e of light. An object believed to have been an asteroid about 30 m e t e r s in d i a m e t e r , t r a v e l i n g at 20,000 meters per second, exploded at a height of about 8 k i l o m e t e r s above the g r o u n d , w i t h an e n e r g y e q u i v a lent to 15 m e g a t o n s of T N T , almost a thousand bombs l i k e the one dropped over H i r o s h i m a . A m a n called Semenov, w h o w a s q u i e t l y sitting by his porch near a t r a d i n g station a few k i l o m e t e r s from the blast, w a s t h r o w n 6 meters a w a y a n d k n o c k e d unconscious. He later recalled that the heat blast w a s so intense that he thought his shirt w a s b r a n d e d onto his chest. T h e tent over a sleeping family w a s lifted into the air a n d dropped d o w n w i t h such incredible force that t w o people fainted a n d all suffered bad bruises. T h e g r o u n d shook, a n d hot w i n d s traveling at high speeds b u r n e d a n d flattened 2,100 s q u a r e k i l o m e t e r s of forest (see figure 20 in insert). T h e superposition of t w o consecutive blasts, the supersonic blast from the object tearing through the atmosphere a n d the explosion of the asteroid itself, caused the k n o c k e d - d o w n trees to a s s u m e a c u r i o u s pattern, r e s e m b l i n g the open w i n g s of a butterfly, a d a r k poetic touch. At the v i l l a g e of V a n a v a r a , about 60 k i l o m e t e r s a w a y from the blast point, several roofs collapsed, w i n d o w s w e r e b r o k e n , a n d the i n c r e d u lous inhabitants s a w a h u g e m u s h r o o m - s h a p e d cloud of s m o k e shoot up into the air, as if a g i a n t conjurer w e r e p l a y i n g w i t h the elements. W e a t h e r stations in G e r m a n y detected a pressure w a v e that circled the globe t w i c e , w h i l e passengers in a train 500 k i l o m e t e r s a w a y s a w and then heard the event. In western Europe, people observed a peculiar g l o w in the night s k y ; even as far a w a y as C a l i f o r n i a , the soot from the blast d a r k e n e d the skies for w e e k s . As w i t h the impact that carved Meteor crater in the U n i t e d States 50,000 y e a r s earlier, had this event h a p p e n e d over a densely populated region, m i l l i o n s of people w o u l d h a v e d i e d . T h i s is the famous T u n g u s k a event, an eye-opening

ill

IMPACT!

r e m i n d e r that these i m p a c t s are not entirely t h i n g s of the remote past. Even m o r e terrifying, it w o u l d be e x t r e m e l y difficult to detect such a small celestial i n t r u d e r w i t h e n o u g h lead t i m e to evacuate the target a r e a . A collision l i k e this could happen a n y t i m e , a n y w h e r e . T h e T u n g u s k a event is child's play c o m p a r e d w i t h the devastating collision between Jupiter a n d the f r a g m e n t e d comet S h o e m a k e r - L e v y 9 between July 16 and 22, 1994. T h i s w a s the best-documented, -phot o g r a p h e d , -observed, a n d -discussed collision in astronomical history; a very rare event that we w e r e fortunate to have the right technology to observe in great detail. H a d it h a p p e n e d only a couple of decades back, we w o u l d have been confined to distant photos t a k e n by Earth-based telescopes. T h e H u b b l e Space Telescope a n d the Galileo spacecraft, w h i c h w a s on its w a y to Jupiter, provided spectacularly detailed photos, w h i c h b e c a m e instant classics, icons of a n e w "in y o u r face" astronomy, w h i c h blends itself w i t h pop c u l t u r e t h r o u g h television a n d , m o r e recently, the Internet: people could w a t c h the collisions in the comfort of their homes, w h i l e m u n c h i n g on their snacks a n d staring at their TV screens (see figures 21 a n d 22 in insert). It w a s one of these occasions w h e n terror a n d e x c i t e m e n t get tangled by a w e â&#x20AC;&#x201D; e x c i t e m e n t at being l u c k y e n o u g h to see such a fantastic celestial event, terror at the thought of w h a t w o u l d have h a p p e n e d to little Earth had it been the target of the d e a d l y b o m b a r d m e n t . It is fitting that this comet w a s discovered by the S h o e m a k e r s , Gene and C a r o l y n , a n d by D a v i d L e v y â&#x20AC;&#x201D; a l l of t h e m fierce comet and asteroid hunters c o m m i t t e d to s c a n n i n g the skies for possible threats. W h a t C a r o l y n identified as a " s q u a s h e d " comet w a s later confirmed to be t w e n t y - t h r e e fragments of one single comet that w a s ripped apart by Jupiter's e n o r m o u s g r a v i t a t i o n a l pull, flying together like a string of d e a d l y pearls stretching over h u n d r e d s of thousands of kilometers. L i k e most comets, this one w a s a d i r t y s n o w b a l l , a four-billion-year-old piece of debris left over from the formation of the solar system. Its fara w a y orbit b e c a m e d e s t a b i l i z e d by a rare close encounter or collision with another body, or by passing close to a nearby star or interstellar gas cloud, m a k i n g it p l u n g e t o w a r d the S u n , crossing Jupiter's path. 123

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T h e comet r e m a i n e d in this new, e l o n g a t e d orbit until last century, w h e n it finally got too close to Jupiter; a collision w a s u n a v o i d a b l e , each fragment crashing into the planet's u p p e r a t m o s p h e r e at over 200,000 k i l o m e t e r s per h o u r â&#x20AC;&#x201D; o v e r 25,000 m e g a t o n s of T N T per fragment. Jupiter is eleven times l a r g e r than Earth a n d three h u n d r e d times more massive. It does not have a solid surface, a l t h o u g h d e e p d o w n it m a y have a dense solid core. One can think of it as a g i a n t g a s ball, w h i c h g r o w s denser n e a r e r its center. W h a t w o u l d these cometary fragments do to a celestial body l i k e this? To m a k e a n a l y s i s easier, each f r a g m e n t w a s assigned a letter in order of impact. F r a g m e n t A hit Jupiter's upper a t m o s p h e r e just before 4 P . M . E D T on J u l y 16. As the f r a g m e n t blew up in a g i a n t fireball, a p l u m e of ejecta rose to m o r e than 3,000 k i l o m e t e r s above the point of impact. As the debris c a m e c r a s h i n g d o w n , it left a d a r k spot r o u g h l y a t h i r d of Earth's size. A n d that w a s just the first collision. T h i s pattern w a s repeated for each of the fragments observed (a few w e r e lost): initial fireball, rising p l u m e of m a t e r i a l , debris c r a s h i n g d o w n , devastation s p r e a d i n g over l a r g e a r e a s . F r a g m e n t G, one of the largest, left a bruise r o u g h l y the size of Earth. T r u e , an impact such as this is very r a r e , especially here on Earth; Jupiter's e n o r m o u s g r a v i t y acts l i k e a k i n d of cosmic v a c u u m cleaner for w a n d e r i n g comets a n d asteroids, d e c r e a s i n g the chances that they w i l l hit us. But the u n a v o i d a b l e conclusion from the S h o e m a k e r - L e v y 9 event is that they can hit us, w i t h d e v a s t a t i n g effects. We d i d not conjecture this event, or reconstruct its consequences from g a t h e r e d evidence: we saw it! W h a t are we to do?

S e a r c h i n g for K i l l e r s First, we must k n o w w h a t is out there. Collisions can happen w i t h either asteroids or comets, w h i c h , as we have seen, a r e t w o very different k i n d s of objects. Asteroids, found mostly in a region b e t w e e n M a r s a n d Jupiter k n o w n as the asteroid belt, a r e leftover rocky planetesi124

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m a l s , w h i c h could not coalesce into a l a r g e r planet, because of Jupiter's constant g r a v i t a t i o n a l t u g g i n g . C o m e t s , also debris from the formation of the solar system, consist mostly of frozen gases. A l a r g e n u m b e r of t h e m are found in the Oort cloud, located w e l l outside Neptune's orbit, some fifty thousand t i m e s more distant from the S u n than Earth is. T h e s e are the long-period comets, w h i c h - m a y t a k e a n y w h e r e from h u n d r e d s of thousands to millions' of y e a r s to complete one single orbit a r o u n d the S u n . We tend to spot t h e m w h e n they "light u p " a r o u n d Jupiter. T h e Oort cloud m a y have trillions of these frozen objects. In 1951, G e r a r d Kuiper, w o r k i n g then at the Y e r k e s Observatory of the U n i v e r s i t y of C h i c a g o , suggested that another comet belt existed just beyond Neptune's orbit, w h i c h b e c a m e k n o w n as the K u i p e r belt. T h e objects in this belt orbit the S u n in r o u g h l y circular paths thirty to one h u n d r e d t i m e s more distant than Earth. Over sixty of these objects have been found so far, a n d some estimates put their n u m b e r in the billions. In fact, Pluto is considered by m a n y to belong to this belt a n d to be m o r e l i k e a h u g e comet than l i k e a bona fide planet. C l y d e T o m b a u g h , w h o discovered Pluto in 1930, calls it " K i n g of the K u i p e r belt." T h e s e are the short-period comets, w i t h orbits u n d e r t w o h u n d r e d y e a r s , a l t h o u g h some of the Oort cloud comets can also be captured in orbits passing close to the S u n ; it is often q u i t e difficult to tell w h i c h of the t w o belts a g i v e n short-period comet comes from. In order for comets a n d asteroids to become a threat, they must s o m e h o w be n u d g e d off their distant orbits. For asteroids, the d i s t u r bance could be d u e to a collision (or near-collision) w i t h a neighbor or to a passage too close to Jupiter over several orbits; at every pass, the asteroid receives a g r a v i t a t i o n a l jolt, very m u c h l i k e a child p u m p i n g on a s w i n g . T h e s a m e w a y that p u m p i n g at the right t i m e propels the child h i g h e r a n d higher, w h e n e v e r an asteroid passes sufficiently near Jupiter, it gets pushed to a more unstable orbit until finally it b r e a k s loose of the asteroid belt. F o r t u n a t e l y for the child, the s w i n g is fastened to its supporting frame a n d she won't fly off. For comets in the Oort cloud, w h o s e h u g e size m a k e s collisions between neighbors rare, the disturbance is more l i k e l y to be caused by the g r a v i t a t i o n a l pull of a 125

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nearby star or of an interstellar g a s cloud. In the K u i p e r belt, w h e r e collisions are still very rare, the pull from the m a s s i v e outer planets, especially N e p t u n e , m a y cause comets to d e r a i l . Once the orbit of a comet or asteroid is sufficiently disturbed, one of t w o t h i n g s will h a p pen: it will get pushed either o u t w a r d or i n w a r d , t o w a r d the S u n . T h e latter case is of most concern to us, since some of these i n v a d e r s from beyond m a y settle into Earth-crossing orbits, that is, their n e w paths a r o u n d the S u n m a y intercept ours, increasing the chances of a collision at some point in t i m e . Asteroids that settle into Earth-crossing orbits are k n o w n as Apollo asteroids, after the first Earth-crossing asteroid, the 8 - k i l o m e t e r - w i d e A p o l l o , discovered in 1932, then "lost," a n d found a g a i n in 1980 by chance, w h e n it c a m e w i t h i n 9 million k i l o m e ters of Earth (around thirty times the Earthâ&#x20AC;&#x201D;Moon distance), pretty close by astronomical s t a n d a r d s (see figure 23 in insert). N o w that w e k n o w w h a t the t h r e a t e n i n g celestial bodies a r e a n d w h e r e they a r e located, we must get an idea of their n u m b e r a n d sizes. For comets, this is an impossible task; there are too m a n y " d i r t y ice b a l l s " at the Oort cloud a n d K u i p e r belt, a n d they a r e too small a n d too far for direct detection. T h e typical size of a comet n u c l e u s is a few k i l o m e t e r s , l a r g e e n o u g h for an extinction-level impact but very h a r d to spot at distant orbits. For e x a m p l e , comet S w i f t - T u t t l e , a w e l l known

Earth-crosser

responsible

for

the

annual

Perseid

meteor

shower, is about 25 k i l o m e t e r s in d i a m e t e r , more than t w i c e as l a r g e as the object that k i l l e d the d i n o s a u r s . A n o t h e r w o r r i s o m e property of comets is the r a n d o m n e s s of their orbits; w h e r e a s asteroids rotate a r o u n d the S u n in the s a m e direction as Earth, comets m a y rotate in the opposite direction. As we k n o w , hitting s o m e t h i n g head-on is m u c h worse than being hit from behind; S w i f t - T u t t l e flies by w i t h a speed of 61 k i l o m e t e r s per second; an i m p a c t w o u l d be e q u i v a l e n t to one billion H i r o s h i m a bombs, a true d o o m s d a y event. F o r t u n a t e l y , calculations indicate that the comet is safely out of synch w i t h us; w h e n it visits us a g a i n , the nearest approach will occur on A u g u s t 14, 2126, at a nice long distance of 24 million k i l o m e t e r s . H o w e v e r , w h e n these calculations are run f o r w a r d far e n o u g h , a very close encounter is pre126

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dieted for 3044, w h e n the comet w i l l pass by at only 1.5 million k i l o m e ters, about four times the distance to the Moon. Since cometary orbits m a y shift because of hot-gas emission from their nuclei, it is hard to be sure about this prediction; very small orbital d i s p l a c e m e n t s can m a k e a h u g e difference after a long time. Moreover, since we k n o w the comet's relative speed to Earth, we see that it bas a w i n d o w of only about 3.5 m i n u t e s to hit u s , the t i m e for an object t r a v e l i n g at 61 k i l o m e t e r s per second to hit a m o v i n g target w i t h Earth's d i a m e t e r (12,756 k i l o m e ters). A n d 3.5 m i n u t e s is a very small fraction out of an orbit longer than a century: the odds for a collision are very small. But how small is small e n o u g h for comfort? For asteroids, the situation is s o m e w h a t better, since we k n o w r o u g h l y how m a n y a n d how l a r g e . T h e r e are three g i a n t asteroids, C e r e s , Pallas, a n d Vesta, w i t h d i a m e t e r s of 913, 580, a n d 540 k i l o m e ters, respectively. T h e n there a r e about a d o z e n or so w i t h d i a m e t e r s over 200 k i l o m e t e r s . We believe that most, if not all, asteroids w i t h d i a m e t e r s of 100 k i l o m e t e r s or m o r e a r e k n o w n a n d cataloged, as w e l l as about 50 percent of the asteroids l a r g e r than 10 k i l o m e t e r s in d i a m e ter. In fact, m o r e than 9,000 asteroids have been n u m b e r e d a n d n a m e d at the t i m e of this w r i t i n g . Of these, m o r e than 220 are l a r g e r than 100 k i l o m e t e r s , a n d estimates put the n u m b e r s of rocks l a r g e r than 10 k i l o m e t e r s at over 10,000. As we go d o w n in sizes, the n u m b e r s c l i m b fast; there a r e over 752,000 l a r g e r than 1 kilometer, a n d about 28 m i l l i o n l a r g e r than a football field. T h e asteroid that caused the T u n g u s k a event belonged to this last g r o u p of l i g h t w e i g h t s . Given the e n o r m o u s distances involved and the relatively small d i a m e t e r of Earth, the l i k e lihood of a collision m a y be s m a l l . On the other h a n d , the n u m b e r of potential strikers is quite l a r g e . Not that the asteroid belt is l i k e a c r o w d e d interstate. If we w e r e to build a scale model of the solar system, w h e r e the S u n is s h r u n k into a twelve-inch globe, Earth w o u l d be the size of a m a r b l e at 100 feet from the globe, M a r s a pepper seed at 160 feet, a n d Jupiter a g o l f ball at about 550 feet. Now, the mass of all the millions of asteroids put together a m o u n t s to about half the Moon's mass, a n d corresponds to a g r a i n of 127

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sand in this scale model. To get an idea of how c r o w d e d the asteroid belt is, smash a g r a i n of sand into m i l l i o n s of bits a n d spread it between the orbits of M a r s a n d Jupiter, located between 160 a n d 550 feet from the S u n . Each asteroid w o u l d be one thousand t i m e s s m a l l e r than an a m o e b a ; there w o u l d be plenty of room for one trillion billion of t h e m , a l t h o u g h there are only tens of m i l l i o n s â&#x20AC;&#x201D; t h e asteroid belt is mostly e m p t y space. T h e i m a g e s we see in movies or d r a w i n g s of spaceships d o d g i n g s w a r m i n g asteroids are incorrect, even if exciting. Still, once in a long w h i l e collisions between neighbors do happen, a n d they a r e l a u n c h e d t o w a r d the inner solar system. Earlier I said that estimates put the n u m b e r of asteroids l a r g e r than 10 k i l o m e t e r s at over ten thousand a n d those l a r g e r than 1 k i l o m e t e r at almost one m i l l i o n . Of these, Gene S h o e m a k e r a n d collaborators estim a t e d that at least t w o thousand are Earth-crossers, that is, potential hitters. In the e a r l y 1970s, the S h o e m a k e r s , w o r k i n g for the U . S . Geological S u r v e y , and Eleanor H e l i n , from the Jet Propulsion Laboratory in P a s a d e n a , led t w o t e a m s of a s t e r o i d - h u n t i n g astronomers. T h e i r strategy w a s fairly s i m p l e : one can find asteroids by p h o t o g r a p h i n g the n i g h t sky w i t h a telescope at r e g u l a r intervals; since they a r e rocky, they reflect s u n l i g h t just l i k e planets, s h o w i n g up as dots of light moving against the b a c k g r o u n d stars. An asteroid near Earth will a p p e a r to m o v e faster a g a i n s t the stars than one farther a w a y . If y o u have a small telescope a n d have ever tried l o o k i n g at Jupiter a n d its moons, you can i m a g i n e how h a r d it is to spot rocks only a few k i l o m e t e r s w i d e and m a n y m i l l i o n s of miles a w a y . In 1989, t w o m e m b e r s of a spin-off survey p r o g r a m called P A C S ( P a l o m a r Asteroid C o m e t S u r v e y ) discove r e d an asteroid about half a k i l o m e t e r w i d e a few hours after it brushed by at a distance of only about 600,000 k i l o m e t e r s , just 1.5 times the distance to the Moon. T h e asteroid, n o w n a m e d A s c l e p i u s , w o u l d have caused severe d a m a g e to the w h o l e planet. It w i l l probably hit us some day, l i k e countless other Earth-crossers. S h o e m a k e r ' s a n d H e l i n ' s pioneering efforts w e r e far from enough; there are just too m a n y Earth-crossers out there. In the early 1980s, Robert M c M i l l a n a n d T o m G e h r e l s , from the U n i v e r s i t y of A r i z o n a , 128

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created the S p a c e w a t c h Project: a dedicated 36-inch telescope at Kitt P e a k , A r i z o n a , a r m e d w i t h e x t r e m e l y sensitive light detectors k n o w n as c h a r g e d - c o u p l e d devices (the C C D s you see in video c a m e r a s ) , c a p a 7

ble of finding six h u n d r e d asteroids in one good n i g h t . A n o t h e r prog r a m flourished for a w h i l e in A u s t r a l i a , but h a d its funds cut in 1996. In fact, of the several o r i g i n a l p r o g r a m s started in the seventies a n d e i g h t i e s , only S p a c e w a t c h is still operational. Even w i t h its a d v a n c e d a u t o m a t e d search technology, it w o u l d be h u n d r e d s of y e a r s before it could track most of the Earth-crossers, a l u x u r y we cannot afford. In the early nineties, a l a r g e r international effort, k n o w n as S p a c e g u a r d Survey, w a s proposed, following a request from the U . S . C o n gress to N A S A to e x a m i n e the impact question. T h e idea w a s to have a n e t w o r k of dedicated telescopes, w h i c h jointly could track over a h u n dred thousand m a i n - b e l t asteroids per month, as well as rarer Earthcrossing comets, w a y before w h a t we can do w i t h current technology (but no more than about a y e a r or t w o before i m p a c t ) . W i t h six 2-meter telescopes, this w a r n i n g system w o u l d cost about $50 million to build and $15 million a y e a r to run, a modest s u m c o m p a r e d w i t h w h a t is spent in countless research projects w o r l d w i d e . U n d e r the original plan, S p a c e g u a r d could reduce to a couple of decades w h a t w o u l d instead take centuries w i t h current search tools. In the m e a n t i m e , an o r g a n i z a t i o n called S p a c e g u a r d F o u n d a t i o n , w i t h h e a d q u a r t e r s in R o m e , Italy, w a s established by the international astronomical c o m m u nity to integrate observations a n d data a n a l y s i s from observatories a r o u n d the w o r l d . T h e goals set by S p a c e g u a r d are a m b i t i o u s : to have 90 percent of all near-Earth asteroids ( N E A s ) l a r g e r than one k i l o m e ter, t r a c k e d by 2010. T h i s r e q u i r e s an eightfold increase over c u r r e n t search rates. A l t h o u g h S p a c e g u a r d has not m a t e r i a l i z e d as o r i g i n a l l y proposed, there a r e c u r r e n t l y three N E A searching p r o g r a m s , apart from the ongoing S p a c e w a t c h , w h i c h have been q u i t e successful: L I N E A R , from the Massachusetts Institute of Technology Lincoln Laboratory and the U . S . A i r Force; N E A T ( N e a r Earth Asteroid T r a c k i n g ) carried out by the Jet Propulsion Laboratory a n d the U . S . A i r Force a n d 129

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h e a d e d by Eleanor H e l i n ; a n d L O N E O S , from the L o w e l l Observatory in Massachusetts. Together, these p r o g r a m s d o u b l e d the n u m b e r of k n o w n N E A s since 1998; as of A u g u s t 2000, we k n e w of about 1,050 N E A s , of w h i c h 400 are l a r g e ones, that is, w i d e r than 1 kilometer. Recently, N E A T astronomers have revised the estimate of the n u m b e r of N E A s l a r g e r than one k i l o m e t e r , l o w e r i n g the old figure of about 2,000 objects to u n d e r 1,000, a 50 percent cut in potential hitters." T h a t b e i n g the case, we a r e r o u g h l y h a l f w a y t o w a r d the S p a c e g u a r d g o a l , not all that bad. One m a y l e g i t i m a t e l y question, however, w h e t h e r this goal is truly foolproof. Statistics a r e often quoted both to raise a n d to l o w e r the r i s k s of a collision. Events such as the one in T u n g u s k a or the one that carved Meteor crater are e s t i m a t e d to happen once every century. Objects b e t w e e n 100 m e t e r s a n d 1 k i l o m e t e r w i d e m a y hit once every 100,000 y e a r s . Objects l a r g e r than 1 k i l o m e t e r , the ones capable of global d a m a g e , m a y hit once every 10 m i l l i o n y e a r s . A first look at these n u m b e r s

F I G U R E J: Diagram of asteroid impact energy

(bottom)

andfrequency

(vertical)

versus

size of the object (top). A Tunguska-class event, at about 30â&#x20AC;&#x201D;50 meters, happens on the average once every century, with an energy of tens of megatons. The thicker vertical line marks impacts with global consequences.

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tells you not to w o r r y : the odds for collisions w i t h l a r g e objects a r e very small. H o w e v e r , these n u m b e r s a r e obtained by a v e r a g i n g over k n o w n impacts on Earth a n d on the Moon, a n d from our current inventory of Earth-crossing objects.

They do not represent a prediction,

but an expecta-

tion. A n d , as we k n o w , even w h e n the odds are small, people w i n lotteries. It w o u l d be q u i t e foolish to rest the l u t u r e of civilization (at least of countless lives) on the feeble assurance of small odds. It is a matter not of w h e t h e r a serious collision w i l l happen but of w h e n .

Fighting Doom in Outer Space On M a y 18, 1996, the h e a d l i n e of the Boston Globe read, " T h a n k heavens, h u g e asteroid is only passing by." T h e asteroid, m e a s u r i n g 500 meters across, brushed by at 446,000 k i l o m e t e r s from Earth, a w h i f f more than the distance to the Moon. It w a s the largest object passing so close ever recorded, a n d it w a s discovered only five d a y s before its nearest point of contact. I m a g i n e w h a t M a r t i n L u t h e r or Increase Mather, w h o m we met in chapter 2, w o u l d have m a d e of this: "God has sent us a sign! S i n n e r s , repent before the m i l l e n n i u m comes, or face his w r a t h a n d spend eternity in the pits of hell." W h a t w a s first confined to ancient religious texts, prophecies of fiery rocks falling from the skies and b r i n g i n g w i d e s p r e a d destruction, has become a l e g i t i m a t e branch of astronomy. D u r i n g the last t w o d e c a d e s of the last century, Earth scientists confirmed that a h u g e impact k i l l e d the d i n o s a u r s , C o n g r e s s u r g e d N A S A to seriously g a u g e the impact threat, a n d cosmic collisions (like that between S h o e m a k e r - L e v y 9 a n d Jupiter) a n d near-misses w e r e a c t u a l l y seen a n d recorded in g r e a t detail. In m a n y people's m i n d s , old apocalyptic fears w e r e a w a k e n e d , a n d by none other than the scientists themselves; yes, astronomers have become prophets of d o o m . But a true prophet w o u l d not just foretell d o o m s d a y but also offer a path to salvation: a c c o r d i n g to astronomers, salvation could come from destroying or deflecting an i n c o m i n g celestial k i l l e r in outer space. 131

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H o l l y w o o d did not w a s t e a n y time. If real asteroids w e r e not about to hit us, at least a b a r r a g e of impact-disaster movies w a s . In 1997, the A m e r i c a n television n e t w o r k N B C produced the two-part Asteroid, w h e r e Earth braces for impact w i t h t w o asteroids, a s m a l l e r one that hits Kansas City, Missouri, c a u s i n g d a m a g e over a 2 0 0 - m i l e r a d i u s , a n d the h u g e Eros, a 6 - k i l o m e t e r - w i d e monster. A secret laser g u n (no doubt inspired by the flawed S t r a t e g i c Defense Initiative project) w a s to blast the asteroid to pieces. T h o u s a n d s of fragments end up r a i n i n g d o w n all over Earth, c a u s i n g terrible, w i d e s p r e a d d a m a g e . T h e largest fragment, 80 meters across (more than t w i c e as l a r g e as the T u n g u s k a stone) falls right on D a l l a s , w i p i n g out the w h o l e city. Blasting asteroids is, in g e n e r a l , a very bad strategy. A l t h o u g h the m o v i e had several flaws (one being that a l t h o u g h over 70 percent of Earth's surface is covered by w a t e r , the l a r g e fragm e n t s tend to fall over A m e r i c a n cities), it does alert the population to t w o crucial facts: that collisions w i t h Earth can h a p p e n , a n d that we can do s o m e t h i n g to protect ourselves. To a certain extent, disaster movies serve as a sort of unconscious rehearsal for confronting some of our hidden fears; we see it h a p p e n i n g in a movie, k n o w i n g all the time it can also happen in reality, an e l e m e n t of plausibility l a c k i n g in horror movies. As such, disaster movies fabricate an "almost reality," a w e s o m e in its tragic p o w e r but still e v e r y d a y e n o u g h that a u d i e n c e s can identify w i t h it. P l a n e s crash, s k y s c r a p e r s burn, e a r t h q u a k e s s h a k e the land, a n d volcanoes erupt in the real w o r l d a n d in the movies. A n d now the n e w c o m e r to the list of plausible accidents is "death by impact," c h a n g ing the destruction scale from local to global. Fiction a n d science m i x w i t h great d r a m a t i c impact. O n l y in the Bible can we also find such violence associated w i t h cosmic collisions. In the s u m m e r of 1998, A m e r i c a w a s hit by t w o h u g e blockbuster movies e x p l o r i n g the consequences of g l o b a l - k i l l i n g collisions, Deep Impact and Armageddon.

the more

interesting Deep Impact,

featuring

M o r g a n F r e e m a n as an African A m e r i c a n president, a 7 - m i l e - w i d e comet w a s d u e to hit w i t h i n t w o y e a r s . A g a i n , a secretly designed spacecraftâ&#x20AC;&#x201D;called, a p p r o p r i a t e l y e n o u g h , The Messiahâ&#x20AC;&#x201D;is deployed to 132

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meet a n d destroy the comet. Its mission is to l a n d on the comet, d i g a hole into its core, i m p l a n t nuclear explosives there, a n d detonate t h e m remotely, in the hope of blasting the comet into m a n y small a n d h a r m less fragments. Unfortunately, the blast succeeds only in splitting the comet into t w o pieces, both still h u r t l i n g t o w a r d Earth. (A good i l l u s tration of conservation of linear m o m e n t u m . ) T h e s m a l l e r one, m e a s uring

1.5 m i l e s , hits the A t l a n t i c , d e v a s t a t i n g all eastern North

A m e r i c a n coastal cities a n d p r e s u m a b l y a lot of Africa a n d Europe as well. Since a mission n a m e d Messiah cannot possibly fail, in a desperate last attempt its c r e w decides to collide head-on w i t h the l a r g e r 6 - m i l e w i d e piece, detonating its last four nuclear bombs: m a r t y r s d y i n g to save civilization. Then

c a m e Armageddon,

typical

action-packed

crowd

pleaser, w h e r e o r d i n a r y people, Bruce W i l l i s a n d his c r e w of oil d i g g e r s , become heroes u n d e r e x t r a o r d i n a r y circumstances. T h i s time it w a s an absurdly l a r g e asteroid "the size of T e x a s " (no such asteroid w o u l d have been a r o u n d without being noticed) to hit in only eighteen d a y s ! W i l l i s a n d his c r e w a r e hoisted onto the h e l l i s h - l o o k i n g metallic monster asteroid ("Dr. Seuss's worst n i g h t m a r e , " cries a c r e w m e m b e r ) , complete with d a r k s p i r a l i n g peaks, to d i g a hole a n d plant nuclear explosives inside. A r m a g e d d o n â&#x20AC;&#x201D; w h e r e good a n d evil have their final b a t t l e â&#x20AC;&#x201D; i s the asteroid itself, a flying piece of hell, confronted by the crew of a spaceship that resembles an a n g e l ; the religious symbolism is quite obvious. A g a i n , the mission is accomplished at the last m i n u t e by an act of heroic m a r t y r d o m . We can now go back to a m o r e sober a n a l y s i s of w h a t could be done if one of the N E A searches spotted an object t r a v e l i n g t o w a r d us. W i t h present-day or near-future t r a c k i n g technology, there a r e three possible scenarios, w h i c h call for different approaches. One possibility w o u l d be to find a fairly " s m a l l " asteroid, u n d e r 500 meters w i d e , d u e to impact w i t h i n a few months. Being relatively small a n d faint, an object l i k e this could pop up on very short notice, as we have l e a r n e d d u r i n g the past few y e a r s . Unless we had an intercepting mission c o m pletely ready, our only option w o u l d be to track its orbit as precisely as 133

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possible, find the i m p a c t d a t e a n d site, a n d evacuate if necessary. A second possibility is s i m i l a r to the first one, but w i t h m u c h more serious consequences; to spot a l o n g - p e r i o d comet w i t h one or t w o y e a r s ' notice. T h e m a i n difference here is size: comets a r e on the a v e r a g e a few k i l o m e t e r s w i d e , b r i n g i n g the impact into the g l o b a l - c a t a c l y s m category. W i t h o u t an i n t e r c e p t i n g mission ready to go, we could do little m o r e than stock up food, seeds, a n d provisions a n d build u n d e r g r o u n d shelters w i t h at least one or t w o y e a r s of functionality. T h e s e m e a s u r e s w o u l d probably g u a r a n t e e the survival of our species. It w o u l d be q u i t e difficult to d e c i d e w h o w o u l d go into the shelters, a problem b r o u g h t up in the m o v i e Deep Impact. T h e last possibility, by far the most favorable, is to detect a l a r g e asteroid w i t h a few d e c a d e s notice, as is S p a c e g u a r d ' s goal. H e r e , even if we did not have a n y defenses set u p , we w o u l d have a m p l e t i m e to prepare. T a k e n together, these three scenarios indicate the need to intensify our search capabilities in order to have as m u c h lead t i m e as possible, certainly no less than a few decades. T h i s translates into b u i l d i n g m o r e telescopes d e d i c a t e d to searching for n e a r - E a r t h objects (asteroids a n d comets that pose a threat), preferably w i t h l a r g e r m i r rors (2 meters plus) so as to catch fainter objects, be they s m a l l e r or just farther a w a y . As for d e v e l o p i n g the technology to intercept a n d someh o w get rid of the celestial k i l l e r , here t h i n g s get more complicated, as science becomes directly e n m e s h e d w i t h m i l i t a r y strategy.

Prepare or Else! Ideally, we w o u l d have such g r e a t search capabilities that most threate n i n g objects w o u l d be spotted w i t h a few decades or more to spare, as in case three above. T h i s is the joint goal of all present search p r o g r a m s a n d others planned for the near future. T h e crucial question, however, r e m a i n s : Is this "react only if u n d e r threat" approach e n o u g h ? Spaceg u a r d ' s o r i g i n a l d o c u m e n t c l e a r l y states that a survey based on several

IMPACT!

2-meter telescopes w o u l d have a success rate of about 75 percent; that is, we could still miss one out of every four a p p r o a c h i n g objects. Even if this w e r e an acceptable m a r g i n of risk for asteroids and short-term comets, recall that long-period comets a r e found only as they get close to Jupiter, g i v i n g us at best a few y e a r s ' a d v a n c e notice. I w o u l d a r g u e that our present search p r o g r a m s are still w a n t i n g ; the stakes are just too h i g h . If we w e r e to t a k e the search for potential cosmic hitters to a n e w level of m u c h needed precision, we could perhaps follow F r e e m a n Dyson's suggestion and develop space-borne telescopes c o m b i n i n g very w i d e fields and powerful optical fiber technology, the heir to the w o n derful H u b b l e Space Telescope. T h e s e tools w o u l d , in principle, be capable of detecting very faint and distant objects, increasing our lead 9

time from d e c a d e s to c e n t u r i e s . T h i s very p r u d e n t strategy w o u l d also g r e a t l y benefit m a n y branches of astronomy, from planetary to extragalactic. Intercepting missions cost m u c h more than space-borne telescopes a n d , w i t h luck and reliable telescopes, could be avoided for centuries or even m i l l e n n i a . A serious d i l e m m a presents itself at this point. If there is no plan to fund such w i d e - f i e l d orbiting telescopes, should there be a contingency plan for a r e n d e z v o u s w i t h an i n c o m i n g celestial k i l l e r ? Such a plan w o u l d be tied up w i t h national defense and security, being coordinated jointly by the U . S . A i r Force and N A S A . As I w r i t e these lines, President Bush had just decided to g i v e the g o - a h e a d to the National Missile Defense ( N M D ) system, an e x t r e m e l y costly p r o g r a m (estimates place it in the $60â&#x20AC;&#x201D;100 billion r a n g e ) , w h i c h is supposed to intercept incoming missiles from rogue nations, such as Iraq, L i b y a , or North Korea. T h e scientific c o m m u n i t y o v e r w h e l m i n g l y opposes the project, not only idealistically but technically; it is easy to a r g u e that such defense systems are bound to fail, since they will not be able to differentiate between b o m b - c a r r y i n g missiles and increasingly sophisticated decoys, w h i c h w o u l d be released w i t h the a t t a c k i n g missiles. Experts c l a i m that decoys a n d other c o u n t e r m e a s u r e s w o u l d q u i c k l y saturate the defense system. Moreover, if a terrorist nation really w a n t e d to attack

135

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the U n i t e d States, it could do so w i t h chemical a n d biological w e a p o n s , w h i c h could be d e l i v e r e d in F e d e r a l Express p a c k a g e s , as opposed to missiles. (Recent events have painfully shown that terrorism does not need missile technology to be effective.) T h e political consequences of such a unilateral d e v e l o p m e n t are very serious, as nations l i k e C h i n a h a v e promised to respond to the creation of an N M D system by increasing their o w n n u c l e a r arsenals: w h a t should defend creates m o r e tension a n d possible g r o u n d s for future conflict. In the H o l l y w o o d movies I discussed, the intercepting missions w e r e all improvised spin-offs from A m e r i c a n defense projects, not u n l i k e the N M D . Instead of b u i l d i n g a missile defense system u n i l a t e r a l l y (or almost), perhaps we should create a truly international collaboration

for

the detection

and

interception

of r o g u e asteroids a n d

c o m e t s â&#x20AC;&#x201D; a L e a g u e for P l a n e t a r y Defense, dedicated to the construction of l a r g e telescopes a n d an a r r a y of s u r v e y i n g a n d intercepting m i s sions, designed for different situations. If the threat is global, we should react globally, as w i t h more i m m e d i a t e global threats l i k e pollution. Unfortunately, we tend to act w h e n the threat is all too i m m i n e n t . T h i s m a y w o r k s o m e t i m e s , but w i t h a celestial killer, it won't. T h e probability of a serious impact is certainly very s m a l l , and nobody should take to the streets s c r e a m i n g " T h e End is N e a r ! " on the basis of this or any other book on the subject. However, the odds are there, as the S h o e m a k e r - L e v y 9 episode g r u e s o m e l y r e m i n d e d us. I t a k e it as a w a r n i n g sign, a scientific e q u i v a l e n t of the religious "Repent or else," w h i c h could go as " P r e p a r e or else." We have the chance to do something, a n d w e should. Suppose astronomers find an i n c o m i n g asteroid or comet. W h a t should we do? T h e first response is usually, " W e l l , we fly out there a n d blast the b u g g e r into s m i t h e r e e n s w i t h our n u k e s . " Not a good idea, even though w i t h current technology it m i g h t be our only choice. Explosions transfer a h u g e a m o u n t of e n e r g y but not in a specific d i r e c tion. If several bombs explode near or inside an asteroid or comet, m a n a g i n g to break it into pieces, s o m e t h i n g that d e p e n d s crucially on

136

IMPACT!

the composition of the object, the odds are q u i t e h i g h that some or most of the pieces w o u l d continue to move in the s a m e direction, r a i n ing d o w n on Earth w i t h devastating effects. W i t h o u t e n o u g h detailed k n o w l e d g e of the celestial object's composition a n d geological structure for us to be able to predict h o w it w o u l d fragment, explosions are quite a g a m b l e . For e x a m p l e , spme asteroids a r e put together as " r u b ble piles," s o m e w h a t loose and porous assemblages of s m a l l e r parts. T h e s e asteroids a r e n a t u r a l l y more resilient to impact, a n d m a y absorb the e n e r g y of an explosion m u c h better than h a r d , rocky ones. H o w ever, nuclear detonations or just a head-on collision w i t h a l a r g e spacecraft m a y be able to deflect smaller objects, those posing only local threats. Unless they a r e z o o m i n g t o w a r d a densely populated area, it is an open question w h e t h e r any action in space is justified. T h e best approach is to transfer m o m e n t u m to the object, deflecting it as far from Earth as possible: a small deflection at l a r g e distances causes a l a r g e miss, as any billiards player k n o w s . T h i s is because m o m e n t u m transfer involves direction, and can push the object a w a y from a colliding trajectory w i t h Earth. Just as a u x i l i a r y rockets are used to position a spacecraft into different orbits, we can e n v i s a g e different m e c h a n i s m s to g e n t l y n u d g e the celestial object from its k i l l i n g path. One of them involves i m p l a n t i n g rockets on the surface of the object, transforming it into a giant spacecraft, w h i c h could then be g u i d e d into a safe trajectory (see figure 24 in insert). Such a project w i l l doubtless face t r e m e n d o u s technical c h a l l e n g e s . For e x a m p l e , w h a t powerful rockets w o u l d these be a n d w h a t fuel w o u l d they use? T h e first a n s w e r is, a g a i n , nuclear power. H o w e v e r , the d e v e l o p m e n t of such a d v a n c e d n u c l e a r technology goes against the c u r r e n t trend t o w a r d nuclear d i s a r m a m e n t . Also, placing n u c l e a r p o w e r in space has m a n y potential detractors, because accidents could have serious e n v i ronmental consequences. T h e first of these problems could possibly be resolved w i t h a truly international collaboration, but the second one r e m a i n s . (Unless the asteroid is rich in u r a n i u m , w h i c h could then be locally m i n e d a n d processed into nuclear fuel.)

137

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PROPHET

AND

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ASTRONOMER

A n o t h e r idea is to use the object's o w n m a t e r i a l s to push it a w a y . A d i g g i n g device could be developed that w o u l d p u m p m a t t e r from the object's surface a n d expel it w i t h l a r g e velocities in a g i v e n direction, s o m e w h a t l i k e a h u g e hose. H o w to power such m a c h i n e s , k n o w n as mass drivers, w i t h o u t u s i n g n u c l e a r fuel m i g h t be a difficult problem. T h e cleanest solution is, of course, solar energy. H o w e v e r , this r e q u i r e s installing h u g e collectors on the surface of the object, a n d their efficacy w o u l d be restricted to interceptions sufficiently close to the S u n that e n o u g h p o w e r could be collected. To m a k e things m o r e complicated, both asteroids a n d comets t u m b l e q u i t e e r r a t i c a l l y as they move along, m a k i n g a steady collection of p o w e r for even a few hours impossible. F u r t h e r m o r e , the surfaces of these objects, especially comets a p p r o a c h ing the inner solar system, a r e notoriously hostile, w i t h violent jets of v a p o r i z e d m a t t e r popping all over the place, "Dr. Seuss's worst n i g h t m a r e . " It m i g h t be q u i t e h a r d to install a n d m a i n t a i n such facilities for a sufficiently long time. P r o b l e m s a n d c h a l l e n g e s n o t w i t h s t a n d i n g , we must plan a h e a d . I have no doubt that if sufficient resources a n d b r a i n p o w e r a r e put together, solutions to most problems will be found, just as they w e r e for the m a n u f a c t u r e of the atom bomb by scientists of the M a n h a t t a n Project. An e x p a n d e d fleet of space-borne w i d e - f i e l d telescopes a n d survey spacecraft w o u l d g i v e us plenty of t i m e to explore the i n c o m i n g object's composition a n d decide on the best course of action. T h e recent success o f N A S A ' s N E A R ( N e a r Earth Asteroid R e n d e z v o u s ) mission, i n w h i c h a spacecraft operated from Earth orbited a n d l a n d e d on an asteroid for the first t i m e , 433 Eros (not the one in the m o v i e Asteroid), g i v e s us plenty of confidence that we have at least the p r e l i m i n a r y technology needed for interception (see figure 25 in insert). Different cont i n g e n c y plans should be d e s i g n e d for action on different objects. Since the pioneering w o r k of N e w t o n , H a l l e y , a n d L a p l a c e , we have learned not only to foresee the d a n g e r s associated w i t h cosmic collisions but also that we can do s o m e t h i n g about them. As I have tried to show, d u r i n g the last few decades we have a m a s s e d convincing evidence that such collisions occur all over the solar system, Earth 138

IMPACT!

i n c l u d e d - In the w o r d s of Increase Mather's s e r m o n title, they a r e " h e a v e n ' s a l a r m to the w o r l d . " S h o u l d we just put this information b e h i n d us and pray to God that no such horrendous events ever o c c u r ' N o w , we should t a k e a step back from our self-centered position a n d recall Newton's w o n d e r f u l l y l i b e r a t i n g vision of cosmic r e n e w a l a n d destruction. C o m e t s , he a r g u e d , w e a v e the universe together, feedi n g spent worlds with their genercHis vapors. We h a v e seen h o w life on Earth has been stirred into spurts of a m a z i n g l y fast-paced g r o w t h by such cosmic collisions and their aftereffects. T h e end of an era m a r k s the b e g i n n i n g of another, creation following destruction, in a d a n c e l a r g e l y orchestrated by celestial d y n a m i c s . We h a v e l e a r n e d to love its beauty, inventing time to follow its c a d e n c e in w a y s we can u n d e r s t a n d a n d quantify. But if we t a k e the next step, peering out of the solar s y s tem, we find that our beloved S u n is just an o r d i n a r y star, fated to also become one of the spent w o r l d s N e w t o n hoped comets could resuscias

tate. A l > i t i

s o

- A s w e m o v e outside our i m m e d i a t e cosmic

neighborhood, we should invoke another vision, that of Kant, in w h i c h stars and their courts of planets a r e constantly being created a n d destroyed in the vastness of space, l i k e the m y t h i c bird P h o e n i x . S o , we t a k e off the stars now, w h e r e one d a y we must find o u r n e w h o m e s , to

if we are smart enough to survive the m a n y forthcoming cosmic collisions in the ensuing five billion y e a r s before the S u n evolves into a n e w stage and life on Earth collapses forever.

139

CHAPTER

F i r e in the S k y

When thou hast risen they live, When thou settest they die; For thou art length of life thyself, Men

live through

â&#x20AC;&#x201D; CHANT

thee. THE

SUN IN

FROM

THE

ANCIENT

REIGN

EGYPT

(CA.

AKHENATON 1370

B.C.E.)

We all g a t h e r e d on the ship's upper deck, c a r r y i n g our c a m e r a s a n d protective lenses; it w a s A u g u s t 11, 1999, total solar eclipse day, the last of the m i l l e n n i u m . T h e aptly n a m e d Stella Solaris (Solar S t a r ) w a s flanked by three other l a r g e ships, w h i c h together brought thousands of people from all over Europe and the U n i t e d States to witness w h a t is surely one of the most a w e s o m e n a t u r a l events. M a n y w e r e "eclipse g r o u p i e s , " addicts

w h o travel

across

the globe

h u n t i n g for

total

eclipses, t r y i n g to q u e n c h their thirst for the unfathomable. My w i f e , Kari, a n d I w e r e "eclipse v i r g i n s , " for our k n o w l e d g e t h r o u g h books and photos w a s m u c h b i g g e r than our experience of the "real thing." It w a s our good fortune that I w a s chosen to lead a g r o u p of D a r t m o u t h a l u m n i on this cruise, w h i l e Kari, an a l u m n a herself and very skilled at social d y n a m i c s , w o r k e d to m a k e sure everyone w a s happy. O u r most recent encounter w i t h an eclipse had been only a partial one, visible from N e w H a m p s h i r e d u r i n g the spring of 1994. " W a i t until you see this," w a s w h a t we heard over and over a g a i n from several " g r o u p i e s , " 143

THE

PROPHET

AND

THE

ASTRONOMER

as the ship sailed the Black Sea w a t e r s in search of the optimal position, clear of clouds a n d w e l l w i t h i n the u m b r a , the cone-shaped s h a d o w cast by the Moon as it blocks the sunlight. I had seen a total eclipse before, w h e n I w a s seven, h o l d i n g my n a n n y ' s h a n d a n d s l u r p i n g on a Popsicle g i v e n to me by " S e u " A l e x a n d r e , the corner Popsicle vendor w i t h a quixotic countenance. We w e r e planted right u n d e r my b u i l d i n g , a block a w a y from C o p a c a b a n a Beach, s u r r o u n d e d by a h u g e c r o w d of fellow "eclipse v i r g i n s . " ( T h e eclipse w a s a c t u a l l y partial in Rio. To my seven-year-old i m a g i n a t i o n , however, it w a s as g r a n d as the real thing, a n d registered as such.) I r e m e m b e r mostly the reverent silence that befell every one of us as the Moon blocked more a n d m o r e of the S u n , as if our voices had been fed by the w a n i n g sunlight: even though it w a s m i d m o r n i n g , the birds stopped chirping, a n d the traffic in the busy streets froze as if by m a g i c (a true m i r a c l e in Rio, repeated only d u r i n g W o r l d C u p g a m e s ) . H i g h above, a d a r k disk s u r r o u n d e d by a diffuse y e l l o w i s h light looked to me l i k e the e y e of a very a n g r y a n d powerful God, not too happy w i t h w h a t he s a w below. "Is the S u n d y i n g ? " I a s k e d , e c h o i n g age-old fears that inspired countless m y t h s . After a couple of m i n u t e s of agony, a thin a n d gracefully c u r v e d line of bright light a p p e a r e d from behind the d a r k d i s k , forcing me to look d o w n , a big smile of relief stamped across my face. L i g h t w o n after all; God g a v e us another chance. I w a s t h i n k i n g of these v a g u e but powerful m e m o r i e s as we w a i t e d for the "first encounter," w h e n the disk of the Moon just touches the golden solar d i s k , initiating the eclipse. W o u l d I be as scared a n d a w e d as w h e n I w a s seven? Or w o u l d all these years of science have trained my m i n d to look at n a t u r e w i t h the steadier eye of reason? We w e r e told to w a t c h for the beautiful " d i a m o n d ring," w h i c h happens just before totality (and just after) w h e n the d a r k l u n a r disk is s u r r o u n d e d by a ring of light s p i k e d by an incredibly intense b e a m , the " d i a m o n d " (see figure 26 in insert). We should also strain our eyes to find solar prominences, bursts of red f l a m i n g g a s that shoot up from the solar surface, w h i c h a r e h a r d to see unless the g l a r e from the solar disk is

144

FIRE

THE

SKY

blocked. Lastly, as d a y l i g h t fades a w a y d u r i n g totality, we should look for stars a n d planets, especially M e r c u r y a n d V e n u s , a n d for the translucent corona, the tenuous outer l a y e r s of the solar atmosphere. All this in one m i n u t e a n d t w e n t y - t w o seconds, the t i m e of totality for this eclipse. If the relatively short t i m e for totalrty w a s not ideal, at least we w e r e l u c k y w i t h the weather, w h i c h w a s perfect, a n d w i t h the t i m e of the eclipse, w h i c h happened a r o u n d noon, w h e n the S u n w a s h i g h up in the sky. I w a s s o m e w h a t stressed out by the m a n y things I w a s supposed to do a n d look for, w h i c h also included t a k i n g pictures of the eclipse. As the Moon proceeded to block over h a l f of the solar disk, the d a y l i g h t d i m m e d q u i t e considerably, t i n g i n g e v e r y t h i n g w i t h a v a g u e g r a i n i n e s s , as if reality w e r e about to dissolve right in front of our eyes. A touch of angst i n v a d e d me a n d , from w h a t I could gather, most people a r o u n d m e ; the exceptions w e r e those experienced eclipse-goers w h o , j u g g l i n g telescopes, zoom lenses, a n d m a n y different c a m e r a s , w e r e too busy to w a x existential. At this point, the Moon had mostly covered the S u n , a n d we prepared for the show. A d i a m o n d r i n g exploded a r o u n d the d a r k disk of the Moon, even more spectacular than I expected, its incredibly intense light etching its patterns onto our m e m o r i e s forever. I looked a r o u n d ; the sky had turned into a majestic m e t a l l i c blue, a n d t w o dots of light w e r e barely visible a r o u n d the d a r k e n e d S u n : V e n u s a n d elusive M e r c u r y . W e had a 360-degree v i e w of the horizon, w h i c h turned into a pink band, as if the sunset w e r e c o m i n g from all points in space at once. T h e breatht a k i n g l y beautiful combination of colors and d a r k n e s s b r o u g h t tears to my eyes, a n d I w a s i n v a d e d by a completely irrational reverence for what w a s h a p p e n i n g ; there w a s no room for the steady eye of reason now. Kari, s t a n d i n g by my side, w a s pretty m u c h in an ecstatic trance. Before I followed her, I m a n a g e d to t a k e a couple of pictures. A n d then it w a s t i m e to look up. T h e r e w a s the eye of God a g a i n , staring at us d o w n below. To the Persians, the S u n w a s the eye of O r m u z d ; to the H i n d u s , the eye of

THE

PROPHET

AND

THE

ASTRONOMER

V a r u n a . To the ancient G r e e k s , it w a s the eye of Zeus; to the early T e u tons, the e y e of W o d a n (or O d i n ) . I w a s i n v a d e d by a soft sense of terror, at once p r i m a l and s u b l i m e , as the silver-blue d a r k n e s s engulfed us a l l , a light from the w o r l d beyond, a light from death: here I w a s , thirty-three y e a r s later, staring once m o r e at eternity. A total solar eclipse is a h i g h l y subjective experience, each person's reaction no doubt a m i r r o r of w h a t he or she carries inside. F i l l e d w i t h a w e , I could easily understand w h y eclipses w e r e v i e w e d w i t h such horror in so m a n y past cultures; the event's p r i m a l e n e r g y e p i t o m i z e s the archetypal s t r u g g l e between good and evil, light and d a r k n e s s . To some, a d r a g o n w a s eating the S u n ; to others, a serpent. But to all, the event s y m b o l i z e d our indebtedness to the S u n , the life-giving god. M o r e than that, it s y m b o l i z e d our complete d e p e n d e n c e on the S u n , our fear that if it goes, we go w i t h it. T h e threat of a d a r k e n e d S u n m i r r o r e d the threat of the end, a symbol readily adopted by the apocalyptic n a r r a tives we a n a l y z e d e a r l i e r on. (Recall Revelation 6:12-14, w i t h the d a r k ened S u n , and its depiction by L u c a S i g n o r e l l i shown in figure 3 in the insert.) As I hope to have m a d e clear in the first t w o parts of this book, science, in its thematic d e v e l o p m e n t , incorporates trends from popular beliefs and religion. For e x a m p l e , the end of Earth, prophesied by several apocalyptic texts as being promoted by cosmic c a t a c l y s m s ordered by a n g r y g o d s , found its expression in a scientific a n a l y s i s of the possibilities of future collisions w i t h asteroids and comets. Of course, this is not a o n e - w a y road, a n d scientific trends a r e also incorporated by relig i o u s discourse; one recent e x a m p l e is the tragic end met by m e m b e r s of the Heaven's Gate sect. In the present chapter, we w i l l k e e p following this trend, investigating how w o r s h i p of the S u n in so m a n y cultures, and the preoccupation w i t h its stability, filtered from popular beliefs into scientific discourse. T h e question of how and for h o w long a star shines is as pressing for a m o d e r n - d a y astrophysicist as it w a s for the Lord Inca.

146

FIRE

Sun

THE

SKY

Worshipers

T h e S u n must have been adored since m e n first r e a l i z e d that their survival chances w e r e e n h a n c e d if they g a t h e r e d into g r o u p s . T h e r e is such a rich profusion of religious rituals^ m y t h s , a n d l e g e n d s dedicated to the Sun t h r o u g h o u t r e c o r d e d history that it is safe to say that every religion, past or present, has had a role for the Sun in its structure. A m o n g its m a n y h u m a n o r partly h u m a n incarnations, the S u n w a s A p o l l o for the G r e e k s , T h o r for the V i k i n g s , H o r u s , Osiris, or at times A m o n - R e or A t u m for the E g y p t i a n s , the S u n goddess A m a t e r a s u for the Japanese, w h i l e , in early H i n d u i s m it w a s represented by S u r y a h , 1

Savitar, and V i s h n u . It w o u l d be a futile task to present here a c o m p r e hensive o v e r v i e w of S u n w o r s h i p i n g t h r o u g h the a g e s , a l t h o u g h the topic is truly fascinating. Instead, we w i l l e x a m i n e three illustrations of Sun w o r s h i p i n g , in Egypt, Japan, a n d South A m e r i c a , e x p l o r i n g how they express the complex relationship between ourselves, the vast cosmos we inhabit, a n d the passage of t i m e — o r , m o r e specifically, how we cope with our perplexity at being able to ponder the eternal, only to s u c c u m b to d e a t h . For the E g y p t i a n s , there w e r e t w o gates in the sky, one to the east, w h e r e the S u n god c a m e out e v e r y d a y , a n d one to the west, w h e r e he retired for his descent into the u n d e r w o r l d . T h i s d i u r n a l motion of the Sun w a s v i e w e d w i t h a g r e a t sense of mystery, the d i s a p p e a r a n c e of the Sun god m a r k i n g his defeat to the d e m o n s of the u n d e r w o r l d , w h e r e a s his reappearance at d a w n m a r k e d his victorious return. T h i s cycle of death a n d resurrection w a s m i r r o r e d by the h u m a n soul, for it too had to battle the d e m o n s of the u n d e r w o r l d before it could ascend triumphantly

back

life.

During

the

first

and

second

dynasties

(3100—2600 B.C.E.) the p h a r a o h w a s a god—specifically, the incarnation of H o r u s , the a l l - p o w e r f u l , life-giving S u n . T h i s identification of the pharaoh with an omnipotent deity certainly contributed to the centralization of power in ancient Egypt. Echoes of this doctrine can be found in m a n y other c u l t u r e s — f o r e x a m p l e , in Japan, w h e r e the e m p e r o r is a

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direct descendant o f A m a t e r a s u , the Sun g o d d e s s , a n d w i t h F r e n c h absolutism, w h i c h r e a c h e d a c l i m a x d u r i n g the r e i g n of L o u i s XIV, the " S u n K i n g , " the longest in E u r o p e a n history, l a s t i n g s e v e n t y - t w o y e a r s ! It is no small m e a s u r e of m a n ' s a r r o g a n c e to justify his political p o w e r t h r o u g h the omnipotence of the S u n . Back to Egypt, d u r i n g the Old K i n g d o m ( 2 6 0 0 - 2 2 0 0 B.C.E., also k n o w n a s the P y r a m i d A g e ) , the most a d o r e d g o d w a s R e , the chief S u n g o d , the "father" o f the p h a r a o h . E g y p t i a n w o r s h i p w a s firmly centered on the S u n , even though the related d e i t y c h a n g e d in t i m e a n d space. Osiris ("the one w h o sees c l e a r " ) w a s the most beloved of the solar deities, associated both w i t h the setting S u n , w h e n he d e s c e n d e d into the u n d e r w o r l d , a n d w i t h the S u n above. H e w o u l d thus connect the w o r l d above w i t h the w o r l d below, life a n d d e a t h , the central tenet of E g y p t i a n religion m i r r o r i n g the d a i l y m o t i o n of the S u n . T h i s adoration of the S u n reached its p e a k d u r i n g the r e i g n of the p h a r a o h A k h e n a t o n {akhen, pious; aton, solar g l o b e ) , w h o c o u r a g e o u s l y declared all deities false, i m p o s i n g a m o n o t h e i s t i c cult of the S u n all over Egypt. T h e m o d e r n A m e r i c a n composer P h i l i p Glass w r o t e a n opera inspired by this u n i q u e episode in E g y p t i a n history, w h i c h no doubt sent monotheistic w a v e s across the M i d d l e East. But it w a s to be very short-lived. A k h e n a t o n ' s successor, his s o n - i n - l a w T u t a n k h a t e n , c h a n g e d his n a m e to T u t a n k h a m o n , r e i n s t a t i n g the cult of A m o n - R e a n d the plethora of other E g y p t i a n deities. In this reinstated polytheistic cult, the S u n continued to reign s u p r e m e , u n t i l C h r i s t i a n i t y overt h r e w its " p a g a n " adoration. P e r h a p s the most poetic of all expressions of r e v e r e n c e to the Sun can be found in the S h i n t o ( " T h e W a y of the G o d s " ) r e l i g i o u s tradition of Japan. A c c o r d i n g to one of the versions of the S h i n t o m y t h , the Kojiki, or C h r o n i c l e of A n c i e n t Events, the J a p a n e s e i s l a n d s w e r e the creation o f the m a l e a n d female p r i m a l g o d s , I z a n a g i a n d I z a n a m i . After the p r i m o r d i a l chaos w a s separated into h e a v e n a n d ocean, I z a n a g i descended on the F l o a t i n g B r i d g e of H e a v e n (possibly a rainb o w ) a n d stirred the w a t e r s together into an i s l a n d . I z a n a m i then

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joined h i m , a n d bore the eight islands of J a p a n a n d several other deities, until she died g i v i n g birth a n d w e n t to the u n d e r w o r l d . T e r r i bly d i s t r a u g h t , I z a n a g i followed her, hoping to bring his companion back. But he w a s too late; she w a s a l r e a d y decomposing a n d felt s h a m e d to be seen in this state by her former partner. Embittered, she sent horrible monsters to chase I z a n a g i out of the u n d e r w o r l d . After m a n y a d v e n t u r e s , he escaped arid w e n t to bathe in the ocean, hoping to cleanse h i m s e l f from the evils of the u n d e r w o r l d . F r o m the w a s h e d filth of I z a n a g i ' s left eye w a s born the most revered of all Japanese deities, A m a t e r a s u , the S u n goddess. Out of his right eye c a m e T s u k i y o m i , the Moon god, w h o proceeded to join A m a t e r a s u in heaven. A m a t e r a s u sent her g r a n d s o n N i - n i - g i to rule over the islands for her, c o m m a n d i n g h i m in w o r d s most Japanese c h i l d r e n learn in school: "This

L u x u r i a n t - R e e d - P l a i n - L a n d - o f - F r e s h - R i c e - E a r s shall be the 2

land w h i c h thou shalt r u l e . " Later, Ni-ni-gi's great-grandson, J i m m u Tenno, the first h u m a n emperor, took over the province of Y a m a t o , on the central island of J a p a n , a n d set up the empire's capital there in 660 C.E. T h u s , the Shinto m y t h establishes the holiness both of the Japanese islands, a creation of the gods, a n d of the i m p e r i a l family, whose m e m bers are direct descendants of the S u n goddess, A m a t e r a s u . In the Shinto tradition, the S u n is w o r s h i p e d as the essence of all that is holy. T h e most revered shrine in J a p a n , the G r a n d Imperial S h r i n e at Ise, is dedicated to A m a t e r a s u . W i t h i n the inner shrine of the l a r g e t e m p l e , there h a n g s a beautiful mirror, the most precious of all " d i v i n e I m p e r ial regalia." It m a y seem strange that a m i r r o r occupies such a h i g h place in Japanese tradition. H o w e v e r , its s y m b o l i s m , as e x p l a i n e d in a m y t h of great poetic beauty, expresses the hope that the S u n w i l l a l w a y s shine and that d a r k n e s s w i l l never overcome light. A m a t e r a s u ' s brother, the storm god S u s a - n o - w o , w a s c a u s i n g major havoc below, r u i n i n g rice f i e l d s and polluting the w a t e r s . T h e f i n a l s t r a w c a m e w h e n S u s a - n o wo destroyed the roof of A m a t e r a s u ' s w e a v i n g hall w i t h a thunderbolt and t h r e w a "horse from h e a v e n " t h r o u g h it. A m a t e r a s u ' s l a d i e s - i n -

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w a i t i n g w e r e so terrified that they all died of fright. In disgust, A m a t e r a s u retired to a cave in heaven a n d shut the e n t r a n c e behind her. W i t h the S u n h i d d e n a w a y , the w o r l d q u i c k l y sank into d a r k n e s s , a m y t h i c a l re-creation of a total eclipse. A n d w i t h d a r k n e s s c a m e decadence, as evil spirits could now run free across the w o r l d a n d cause all sorts of mischief. T h e "eight m i l l i o n g o d s " w e r e so w o r r i e d that they devised a plan to get A m a t e r a s u out of the cave. T h e y brought a sacred s a k a k i tree to its e n t r a n c e , a n d h u n g m a n y artifacts on it, i n c l u d i n g an eight-foot-long mirror. T h e n , U z u m e , the phallic g o d d e s s , started to d a n c e to the s i n g i n g of the deities, w h o l a u g h e d a n d s c r e a m e d w i t h premeditated

joy.

T h e noise m a d e A m a t e r a s u

very curious:

she

stepped out of the cave, expressing her surprise at all this joy a n d l a u g h t e r i n her absence. " W e l l , " a n s w e r e d U z u m e , d i r e c t i n g the m i r ror t o w a r d A m a t e r a s u , " w e a r e celebrating the a p p e a r a n c e of another b e a u t y to rival y o u r s . " As A m a t e r a s u g l a n c e d at the m i r r o r , h a r d l y believing her eyes, another god ran behind her a n d stretched a rope, the shimenawa, across the cave's entrance to stop her from g o i n g back in. T h e land below shone bright, a n d all gods rejoiced in A m a t e r a s u ' s return. As the m y t h o l o g y expert Joseph C a m p b e l l has r e m a r k e d , if the C h r i s t i a n cross is the "symbol of the mythological passage into death, 3

the shimenawa is the simplest sign of the resurrection." Both represent a b o u n d a r y b e t w e e n t w o w o r l d s — t h e w o r l d of the l i v i n g a n d the w o r l d of the d e a d , the w o r l d of light a n d the w o r l d of d a r k n e s s . For our final cross-cultural foray into S u n w o r s h i p i n g , we go to South A m e r i c a , specifically to P e r u ca. 1100 C.E., w h e r e the Incas had established a S u n - w o r s h i p i n g e m p i r e as g r a n d i o s e as the E g y p t i a n or the Japanese. L i k e the early pharaohs, the Lord Inca w a s a direct descendant of the S u n god, the S u p r e m e Creator, w h o m they called Inti ( L i g h t ) . E v e r y t h i n g u n d e r the s k y — t h e l a n d , the people, the g o l d — b e l o n g e d to the L o r d Inca, a de facto theocracy. T h e ruler personified the S u n on Earth (see figure 27 in insert)., b r i n g i n g his d i v i n e presence to the e v e r y d a y lives of its subjects. Every aspect of life for the Incas revolved a r o u n d S u n w o r s h i p i n g ;

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v i l l a g e s w e r e built w i t h an unobstructed v i e w of the east so that its inhabitants could join in w o r s h i p every sunrise. Of all the m a n y deities of the Incas, only the S u n had a t e m p l e in every city. T h e magnificence of these temples is legendary, perhaps not m a t c h e d by those of any other nation on Earth (see figure 27 in

insert). Gold w a s e x t r e m e l y

special to the Incas, since it w a s believed to hold a m y s t e r i o u s relation 1

w i t h the S u n ; the n u g g e t s found in the m o u n t a i n s w e r e thought to be tears from the S u n god himself. In order to express their reverence, the Incas decorated their temples w i t h e n o r m o u s a m o u n t s of gold. In C u z c o , their most sacred city, founded by the l e g e n d a r y first L o r d Inca, M a n c o C a p a c , ca. 1100, there stood the Palace of Gold, as the Great T e m p l e o f the S u n w a s k n o w n . T h e w h o l e b u i l d i n g , a n enorm o u s structure, w a s s u r r o u n d e d by a thick frieze of gold six inches w i d e . T h e s a m e friezes adorned each of the internal rooms. A h u g e g o l d e n disk, w i t h the e n g r a v e d face of the S u n god a n d studded w i t h precious stones, a d o r n e d the m a i n altar-facing doors opening to the east. At certain times of the year, the disk reflected the s u n l i g h t so p o w erfully that it a p p e a r e d the Sun god h i m s e l f w a s visiting his temple. L i k e the D r u i d s , w h o , as we saw in chapter 1, w e r e also S u n w o r shipers, the Inca priesthood performed h u m a n sacrifices. T h e blood of the victim, preferably that of a y o u n g v i r g i n , w a s s m e a r e d over m o u n taintop sacrificial stones so as to g l i m m e r in all its gory redness d u r i n g sunrise. T h e rituals m a d e sure the S u n god w o u l d be sufficiently pleased w i t h his subjects; o t h e r w i s e , the w o r l d w o u l d p l u n g e into lifedestroying d a r k n e s s . T h i s d a r k n e s s finally c a m e in 1535, w i t h the m u r derous p l u n d e r i n g by the Spanish a r m i e s of Francisco P i z a r r o , a n d the several revolts a n d disputes that followed. A m e r c e n a r y soldier a p p a r ently stole the golden d i s k , the most revered object in the Palace of Gold, only to subsequently lose it d u r i n g a d r u n k e n bout of g a m b l i n g . To a l a r g e extent, most of the S u n - w o r s h i p i n g religions w e r e (and are) concerned w i t h one central t h e m e â&#x20AC;&#x201D; t h a t the preservation of life d e p e n d s on the stability of the S u n , its cyclic motions, and the light a n d w a r m t h it generates. W o r s h i p of the S u n w a s an expression of the peo-

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pie's indebtedness to its l i f e - g i v i n g p o w e r s a n d the u n d e r l y i n g fear that one d a y it could just stop s h i n i n g . T h i s is w h y total eclipses w e r e deeply terrifying in so m a n y cultures; they represented a t e m p o r a r y loss of s u n l i g h t d u r i n g d a y t i m e , a t e m p o r a r y defeat of the S u n god to the d e m o n s of d a r k n e s s . A n d since this "defeat" does happen for a short span of time, w h y not p e r m a n e n t l y ? T h e s e fears a r e deeply i n g r a i n e d in our collective m i n d , even if now they a r e redressed in poetic rather than in r e l i g i o u s a w e , as a n y witness to a m o d e r n - d a y eclipse can testify, a n d as I hope to have convinced you w i t h my o w n testimony. T h e feeling of e x h i l a r a t i o n as we w a t c h the sunlight r e e m e r g e from behind the d a r k e n e d l u n a r disk is o v e r w h e l m i n g a n d irrational. To this, I w i l l now a d d the also beautiful, but rational, description of how stars shine, a n d how one day the S u n w i l l indeed cease to be able to support life on Earth.

Solar Chemistry M a n y G r e e k philosophers brushed aside S u n w o r s h i p i n g as superstitious nonsense; Aristotle c h a m p i o n e d the v i e w that the S u n , l i k e all other celestial l u m i n a r i e s , w a s m a d e of a fifth k i n d of matter, different from the four basic " e l e m e n t s " that composed all t h i n g s m a t e r i a l here on E a r t h â&#x20AC;&#x201D; a i r , water, earth, a n d fire. T h i s w a s the "quintessence," or ether, the m a t t e r that w a s no matter, because it w a s not subject to the transformations u n d e r g o n e by usual matter. Ether w a s u n c h a n g i n g a n d eternal, a n d all t h i n g s m a d e of it n a t u r a l l y m o v e d in circles; hence the circular motion of the S u n , stars, a n d planets a r o u n d Earth. Dism a n t l i n g the v i e w that celestial l u m i n a r i e s a n d objects of Earth w e r e m a d e of different stuff w o u l d t a k e over t w e n t y centuries. T h e first blow w a s d e l i v e r e d by Galileo, w h o in 1613 published his Letters on Sunspots, w h e r e he correctly a r g u e d , a g a i n s t a Jesuit priest n a m e d Christoph Scheiner, that sunspots w e r e blemishes on the Sun itself a n d not planets orbiting close to it. T h u s , the S u n w a s not a perfect globe of ether but had imperfections that a c t u a l l y c h a n g e d over 152

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time; the sunspots m o v e d about its surface l i k e acne. In spite of Galileo's observations, not m u c h progress w a s m a d e in u n d e r s t a n d i n g the S u n ' s m a t e r i a l composition until early in the nineteenth century, w h e n the G e r m a n lens m a k e r Joseph F r a u n h o f e r accidentally discovered h u n d r e d s of black lines in the solar spectrum. Up until then, it w a s believed that the S u n had a perfectly continuous spectrum, obtained by m a k i n g s u n l i g h t pass through a p r i s m , the colors smoothly shifting from the deepest violet to red, as w i t h a rainbow. T h e black lines represented colors that w e r e mysteriously missing, as if some capricious god w e r e filtering colors selectively, perhaps to e m b r o i d e r a n e w robe for A m a t e r a s u ; at the t i m e , this w a s as plausible an e x p l a n a tion as any. By the m i d - n i n e t e e n t h century, it w a s clear that gaseous samples of chemical e l e m e n t s , w h e n heated u p , g l o w e d very selectively as w e l l , each e l e m e n t s h i n i n g w i t h a specific set of colors, its o w n spect r u m . T h e heat that w a r m s the g a s is reprocessed a n d reemitted in personalized r a i n b o w s â&#x20AC;&#x201D; h y d r o g e n w i t h l a r g e a m o u n t s of violet; s o d i u m , of y e l l o w ; neon, of reds and oranges; a n d so forth. T h e reverse w a s also shown to be true. W h e n light w i t h a continuous spectrum passes through a cool g a s s a m p l e of a g i v e n chemical element, the e l e m e n t absorbs the exact s a m e colors it emits w h e n it is hot: s o d i u m absorbs yellow, neon reds a n d o r a n g e s , a n d so on. W h e n the e l e m e n t is "cold," it thus eats certain colors, a n d w h e n it is hot, it emits those same colors, as figure 28 in the insert illustrates. On the basis of these observations, it b e c a m e possible to assemble a catalog of spectra b e l o n g i n g to different chemical e l e m e n t s . T h i s catalog could then be used to explain the m i s s i n g lines in the solar spect r u m . If we suppose, q u i t e reasonably (and correctly), that the solar interior is m u c h hotter than its outer l a y e r s , the continuous spectrum generated w i t h i n w o u l d have certain lines "eaten a w a y " by w h a t e v e r chemical e l e m e n t s are found in its cooler outer l a y e r s . In reality, the situation is a bit subtler; the S u n can be thought of as h a v i n g several different l a y e r s , l i k e a g i a n t onion (see figure 8). T h e light that we see (i.e., the radiation emitted by the S u n in the visible portion of the s p e c t r u m ) , which defines the solar "surface," comes from the photosphere, w h e r e

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F I G U R E 8 : The many layers of the Sun and their respective radii. (Not shown are the outlying transition zone and the solar corona. Figure not to scale.)

the t e m p e r a t u r e is about 6000 d e g r e e s Kelvin.* Even though the photosphere is m a d e up mostly of just a few chemical e l e m e n t s , each w i t h its very o w n s i g n a t u r e spectral lines, the total spectrum at the photosphere is continuous, w i t h no d a r k lines. At 6000 d e g r e e s , all chemical elem e n t s are in their gaseous state. H o w e v e r , the atoms of these elements a r e very tightly p a c k e d , since the Sun's a v e r a g e density is 1.4 l a r g e r than that of l i q u i d water. So, we have a g a s , but a very congested one. On top of the congestion, the h i g h t e m p e r a t u r e causes the a t o m s to j i g g l e w i l d l y ; at the atomic and m o l e c u l a r levels, t e m p e r a t u r e is u n d e r stood as motion, and the h i g h e r the t e m p e r a t u r e , the faster the atoms m o v e and collide w i t h each other. T h e s e fierce collisions destroy the

* A quick note on t e m p e r a t u r e scales. In physics, we don't like the u n n a t u r a l F a h r e n h e i t scale. Instead, we use mostly the K e l v i n or the Celsius scale. T h e Celsius scale sets the freezing point of w a t e r at 0 degrees a n d the boiling point at 1 0 0 degrees, a v e r y reasonable choice. To go from the Celsius to the K e l v i n scale, you simply add 2 7 3 degrees. T h u s , 0 degrees Celsius means 273 K e l v i n , w h i l e - 2 7 3 Celsius m e a n s 0 K e l v i n , the so-called absolute z e r o .

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coherence of the i n d i v i d u a l spectra, creating a j u m b l e d optical mess, w h i c h shows up as a continuous spectrum w i t h no d a r k lines. As the light from the photosphere passes t h r o u g h the thinner a n d cooler chromosphere, selective filtering finally occurs, p r o d u c i n g the d a r k lines observed by Fraunhofer. C o m p a r i n g the different spectral lines in the "spectral catalog" w i t h the" missing lines in Fraunhofer's solar spectrum, we can infer wTiich chemical e l e m e n t s m a k e up the Sun's cooler outer l a y e r s . In the early 1920s, the Indian astrophysicist M e g h n a d S a h a played a key role in solving the e x t r e m e l y complicated p u z z l e of the spectral lines, obtaining the chemical composition of the S u n . But S a h a w e n t a step further a n d helped d e t e r m i n e not only the chemical e l e m e n t s that m a k e up the S u n but their proportions as w e l l . In order to u n d e r s t a n d how he a n d others d i d this, it is important to recall how atoms are o r g a n i z e d .

T h e first w o r k i n g model of the atom w a s proposed by the Danish physicist Niels Bohr in 1913. A l t h o u g h it relies on several simplifying assumptions, it does incorporate some of the essential features of m o d ern atomic theory, a n d it will serve us w e l l . T h e r e is a heavy and very small nucleus, formed by the positively c h a r g e d protons a n d the electrically neutral neutrons. (Bohr didn't k n o w about the neutrons in 1913, since they w e r e discovered only in 1932, by Sir J a m e s C h a d w i c k . ) N e g a t i v e l y c h a r g e d electrons move a r o u n d the nucleus at very specific orbits, or levels. T h e y cannot be in between t w o orbits, just as we c a n not be in b e t w e e n t w o steps of a l a d d e r (see figure 9). T h e lowest orbit, the first r u n g of the ladder, is the " g r o u n d state" of the a t o m , w h i l e h i g h e r r u n g s a r e called excited states. Since the positively c h a r g e d nucleus attracts the n e g a t i v e l y c h a r g e d electrons, you must supply e n e r g y to the electron in order to move it to h i g h e r orbital levels. T h i s e n e r g y comes in little packets of electromagnetic radiation called photons; each color of light is m a d e of photons w i t h a characteristic energy. T h e same is true for invisible types of electromagnetic radiation, such as x r a y s , infrared, ultraviolet, radio, a n d so on. In order for an electron to move up a r u n g , it needs energy. It gets this e n e r g y by absorbing a photon of the exact e n e r g y difference between the t w o r u n g s ; the elec-

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THE BOHR ATOM

F I G U R E 9 : Emission and absorption of radiation (photons) according to Bohr's model of the atom. To jump a level or more, the electron must absorb a photon of the precise energy difference between the two levels. To go down a level or more, the electron releases a photon of the energy difference between the two levels.

tron " e a t s " the photon, becomes more energetic, a n d happily j u m p s up an orbit. S o m e t i m e s the r e q u i r e d photon is in the visible part of the s p e c t r u m , a n d sometimes it is not. T h e converse is also true; in order for an excited electron to come d o w n a rung, it spits out a photon w i t h e n e r g y equal to the e n e r g y difference between the t w o r u n g s . It is also possible to have j u m p s or, better, transitions b e t w e e n f a r a w a y orbits, a l t h o u g h some restrictions apply. As a g e n e r a l r u l e , transitions over m a n y r u n g s r e q u i r e m o r e energetic photons not necessarily w i t h i n the visible part of the s p e c t r u m , such as ultraviolet or x - r a y photons. We now u n d e r s t a n d the o r i g i n of the emission a n d absorption spectra of different e l e m e n t s in a m u c h deeper w a y ; if y o u shine w h i t e l i g h t (continuous s p e c t r u m ) at a g i v e n e l e m e n t , there w i l l be photons 156

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of all possible e n e r g i e s from red to violet (and the invisible ones) hitting the a t o m s of the e l e m e n t . Since each e l e m e n t has a fixed n u m b e r of protons a n d electrons, it w i l l also have a u n i q u e set of possible orbits for the electrons; each e l e m e n t has its o w n ladder, w i t h r u n g s of different heights. So, the electrons of each e l e m e n t w i l l "eat" those photons c o r r e s p o n d i n g to the very specifi6 transitions b e t w e e n its orbits, letting the other photons pass t h r o u g h . T h e electrons then c l i m b up in their orbits, a n d photons end up b e i n g missed at the other end—-the d a r k lines typical of absorption spectra. If y o u instead have an excited a t o m a n d let it be, it w i l l e v e n t u a l l y relax d o w n to its g r o u n d state by e m i t t i n g photons c o r r e s p o n d i n g to the specific j u m p s of its electrons d o w n their very o w n l a d d e r of o r b i t s — t h e emission s p e c t r u m . Now, if a very energetic i n c o m i n g photon hits an a t o m , or if a t o m s are colliding at h i g h e n o u g h t e m p e r a t u r e s (as in the photosphere), it is possible for the atomic electrons to be not only p u m p e d up to h i g h e r e n e r g y levels but k i c k e d out altogether; in this case, we say the atom gets "ionized." For each electron that gets k i c k e d out, the a t o m — n o w called an i o n — g a i n s a net positive c h a r g e . In the case of h y d r o g e n , w i t h its single electron orbiting a single proton, the ionization leaves just a proton z o o m i n g a r o u n d . Ionized h y d r o g e n does not have an emission spectrum, because there is no electron to eat up passing photons. N o w we can go back to S a h a a n d the chemical composition of the S u n . S a h a r e a l i z e d that at 6000 d e g r e e s Kelvin, practically all h y d r o g e n a t o m s are ionized a n d should not produce any hydrogen-specific absorption lines in the F r a u n h o f e r spectrum. A n d yet, there they a r e ! S a h a correctly d e d u c e d that the only explanation for this is that the Sun has h u g e a m o u n t s of h y d r o g e n so that, even though most are ionized, the ones left over eat up e n o u g h photons to m a k e up the observed d a r k lines typical of h y d r o g e n absorption. He then repeated this a r g u m e n t for other e l e m e n t s , w i t h different n u m b e r s of electrons a n d r e q u i r i n g different ionization e n e r g i e s . For e x a m p l e , to strip one electron from the h e l i u m atom (two protons a n d two electrons) takes a little less than twice the e n e r g y to ionize h y d r o g e n , w h i l e to strip the other takes a bit over four times as m u c h . To m a k e things even more exciting, ioniza-

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tion c h a n g e s the spectra of e l e m e n t s ; thus, the spectrum of H e ( h e l i u m + +

less one electron) is different from that of H e . (Or of H e , h e l i u m less t w o electrons. In fact, should H e

+ +

have an absorption s p e c t r u m ? ) *

T h e studies of S a h a a n d others revealed that the S u n is m a d e up of 91.2 percent h y d r o g e n a n d 8.7 percent h e l i u m (in today's n u m b e r s ) . T h e leftover 0.1 percent is composed of several other e l e m e n t s , i n c l u d ing o x y g e n , carbon, nitrogen, a n d silicon. T h e truly r e m a r k a b l e point here is that the c h e m i s t r y of the S u n is as f a m i l i a r to us as the chemistry of Earth; the same c h e m i c a l e l e m e n t s we find here a r e found there, albeit in very different proportions. As we have seen, l a r g e a m o u n t s of h y d r o g e n and h e l i u m are found in Jupiter a n d the other g i a n t g a s planets, but not in the inner rocky planets, l i k e Earth. Establishing the chemical d o m i n a n c e of h y d r o g e n in the S u n offers a very important clue as to how it shines. A l t h o u g h the d a r k lines in the solar spectrum g i v e direct evidence only of the composition of the Sun's absorbing outer l a y e r s , it is w i d e l y thought that the above n u m bers hold true for most of the S u n , w i t h the exception of its inner core. A n d that's w h e r e the real action is.

Solar Alchemy We start by g o i n g back to our description of the S u n as h a v i n g several l a y e r s , l i k e a g i a n t onion (figure 8). T h e three innermost l a y e r s are by far the largest ones, starting w i t h the solar core, w h i c h has a r a d i u s of 200,000 k i l o m e t e r s . H e r e the solar e n g i n e w o r k s at full blast, a true n u c l e a r hell, w h e r e t e m p e r a t u r e s reach 15 m i l l i o n d e g r e e s Kelvin. T h e enormous energy generated, packed

in h i g h l y energetic

photons

(mostly g a m m a r a y s ) , flows o u t w a r d t h r o u g h the next t w o l a y e r s , the radiation zone and the convection zone. T h e different n a m e s stem from the different m e c h a n i s m s by w h i c h e n e r g y is transported out-

* Of course not! T h e r e are no electrons to absorb any photons, right?

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w a r d t h r o u g h these t w o regions. In the inner part of the r a d i a t i o n zone, as the n a m e indicates, the heat from the core travels o u t w a r d t h r o u g h r a d i a t i v e processes: the t e m p e r a t u r e s near a n d a r o u n d the core a r e so h i g h that all a t o m s are ionized t h r o u g h their fierce collisions w i t h one another a n d do not have any electrons to absorb the outg o i n g photons a n d j u m p to h i g h e r o r b i t s t j n s t e a d , the i o n i z e d electrons move freely about, c o l l i d i n g with the o u t g o i n g photons a n d s l o w i n g them d o w n , as in an obstacle course. As we move o u t w a r d from the center, the t e m p e r a t u r e drops a n d m o r e a n d m o r e a t o m s m a n a g e to retain their electrons a n d thus can absorb photons. In fact, by the end of the radiation zone, some 300,000 k i l o m e t e r s a w a y from the core (see figure 8 ) , all photons get c a p t u r e d ; its outer e d g e is a l m o s t perfectly o p a q u e [photons get b l o c k e d ] ) . Since we do see radiation c o m i n g out of the S u n , this cannot be the end of the story. W h e r e the radiation zone ends, the convection zone begins; here heat is transported o u t w a r d very m u c h as in a boiling pot of soup; hot g a s "floats" u p w a r d , cools off, a n d sinks back d o w n , in a cycle of heat e x c h a n g e , or convective motion, from the solar interior to its surface a n d back d o w n . T h e sizes of these rolling convection cells vary from 30,000 k i l o m e t e r s deep in the convection zone to a " m e r e " 1,000 k i l o m e t e r s at its end. Once they reach this size, some 200,000 k i l o m e t e r s a w a y from the radiation zone, the g a s is so rarefied that it cannot support convective motion a n y longer; photons begin to radiate o u t w a r d , transporting the heat a w a y from the S u n ; this is the photosphere that we see. It m a y t a k e a photon m i l l i o n s of y e a r s to travel from the Sun's inner core to the photosphere. F r o m there, it t a k e s only 8.3 m i n u t e s for it to catch our eyes here on Earth. N o w that we k n o w h o w heat escapes the solar interior, we focus on h o w it is a c t u a l l y produced. T h e process by w h i c h the S u n g e n e r a t e s its a m a z i n g a m o u n t of heat is k n o w n as n u c l e a r fusion; the Sun's l u m i nosity or power, the total e n e r g y it radiates per second, is 4 x 1 0

watts,

that is, 4 followed by 26 zeros w a t t s , e q u i v a l e n t to 100 billion 1-megaton n u c l e a r bombs. C o m p a r e this w i t h y o u r l i v i n g room light bulb, at 100 watts. M o r e than that, the S u n has been g e n e r a t i n g this a m o u n t of 159

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e n e r g y for about five billion y e a r s a n d w i l l continue to do so, at a fairly steady rate, for another five billion years. W h e r e does all this e n e r g y c o m e from? To a n s w e r this question, we leave electrons behind a n d p l u n g e into the atomic n u c l e u s . T h e state of m a t t e r that exists in the solar core is k n o w n as a p l a s m a , m a d e of a t o m s stripped of all their electrons as a result of their violent m u t u a l collisionsâ&#x20AC;&#x201D;that is, bare nuclei a n d electrons flying a r o u n d i n d e p e n d e n t l y . In its solid state, m a t t e r has its a t o m s a r r a n g e d in r e g u l a r crystalline structures, such as cubic or p y r a m i d a l shapes. If we try to s q u e e z e a solid into a s m a l l e r v o l u m e , the electric repulsion b e t w e e n electrons from its a t o m s w i l l m a k e t h e m repel each other fiercely, g i v i n g a solid its r i g i d structure. In p l a s m a s , since the electrons a r e all g o n e , the nuclei can be s q u e e z e d into d i s tances ten thousand t i m e s smaller, and still move w i t h relative ease. T h i s i n t i m a c y entices nuclei to fuse, creating different k i n d s of nuclei. T h e s e processes, w h e r e t w o or m o r e a t o m i c nuclei fuse into another n u c l e u s , are k n o w n as fusion n u c l e a r reactions. To u n d e r s t a n d the basic rules of fusion, we must first learn about a t o m i c accounting. Each chemical e l e m e n t is c h a r a c t e r i z e d by the n u m b e r of protons in its n u c l e u s , k n o w n as the a t o m i c n u m b e r , a n d represented by the letter Z. T h u s , h y d r o g e n , the simplest, has only 1 proton ( Z = l ) , h e l i u m h a s 2 (Z=2), l i t h i u m has 3 (Z=3), u r a n i u m has 92 (Z=92), a n d so forth. T h e other component of the n u c l e u s is the neutron, w h i c h can come in different n u m b e r s . For e x a m p l e , w h e r e a s n o r m a l h y d r o g e n has no neutrons in its n u c l e u s , it can a p p e a r in v a r i a n t forms w i t h 1 or 2 neutrons. T h e s e v a r i a n t forms are called isotopes. To m a k e things simple, chemists l i k e to represent e l e m e n t s a n d their m a n y isotopes by specifying the n u m b e r of protons plus neutrons (the a t o m i c w e i g h t , A) in their nucleus. T h e notation is as follows: 'H is n o r m a l h y d r o g e n , with 2

1 proton ( A = l ) ; H is its isotope k n o w n as d e u t e r i u m , w i t h 1 proton 3

a n d 1 neutron (A=2); H is its isotope t r i t i u m , w i t h 1 proton a n d 2 neutrons (A=3). One m o r e e x a m p l e :

2 3 5

U is a rare isotope of u r a n i u m w i t h

143 neutrons. (You get this by subtracting 92, the a t o m i c n u m b e r of u r a n i u m , that is, the n u m b e r of protons in its n u c l e u s , from 235.)

160

F I G U R E I : Painting of the judgment, dating from about 1025 B.C.E. Princess Entui-ny appears before Osiris, god of the dead. Maat, or Truth, with the crown made of an ostrich feather, stands at the extreme left. (Courtesy of Erich Lessing/Art Resource, New Yorl(.)

F I G U R E 2: Luca Signorelli's depiction of the resurrection, in the San Brizio Chapel, in Orvieto. (Courtesy of Photo Scala, Florence.)

F I G U R E 3 : Luca Signorelli's powerful depiction of the Apocalypse, from the San Brizio Chapel in Orvieto. All cosmic symbols from Revelation 6:12-14are present: the blackened Sun, the blood-colored Moon, and the raining stars (all to the right of the picture). (Courtesy of Photo Scala , Florence.)

FIGURE

4 :

Depiction of Halley's comet in the Bayeux Tapestry of1066. This comet was

said to herald the arrival of William the Conqueror in 1066. L e f t , some people are baffled at the celestial apparition; r i g h t , an emissary is breaking the bad news to King Harold. (Giraudon/Art Resource, New Yorl{, authorized by the city of Bayeux.)

F I G U R E 5 : Sodoma's fresco in the Monte Oliveto Maggiore Abbey, Tuscany, depicting Saint Benedict exorcising a demon from a monk^ by flagellation. T o p , the exorcised demon is surrounded by smoke; c e n t e r , another demon is at worl{ seducing an unsuspecting mon\. (Courtesy of Photo Scala, Florence.)

FIGURE

Illustration by Herman Gall (1556) depicting Halley's comet and the havoc

wreaked by its apparition. Top right, note the strange conjunction of the Moon and Sun, a clear sign of impending apocalyptic doom. (Reproduced from William Hess, Himmels-

u n d Naturerscheinungen in Einblattdrucken des XV bis XVIII Jahrhunderts [Nieuwkpop: B. de Graaf, 19731.)

FIGURE

7 : Luca Signorelli's Deeds and Sins of the Antichrist, in the San Brizio

Chapel, Orvieto. {Courtesy of Photo Scala, Florence.)

8 :

Albrecht Diirer's Opening of the Fifth and Sixth Seals (1498). All a

symbols of doom are clearly used as elements to enhance the dramatic message found ii Revelation.

(Potter Palmer Collection,

1956.960.)

FIGURE

9: Illustration from the Nuremberg Chronicles (1495), depicting Aristotle's

cosmos as seen by the church. Earth in the center is surrounded by the four elements, Moon, Mercury, Venus, Sun, the rest of the planets, and the sphere of the fixed stars. The outermost sphere is the Primum Mobile, surrounded by the Kingdom of God. (Courtesy Adler Planetarium and Astronomy Museum, Chicago.)

F â&#x20AC;˘ o u RE

,0:

G.otto di Bondone, Adoration of the Magi (1304), Scrovegn, Chapel ,n

Padua. The Star of Bethlehem (center top) is depicted as a comet, inspired by Halley's comet. (Copyright AlinarilAn Resource, New Yor%.)

F I G U R E I I : Diagram illustrating parallax. For a comet very far away, the angle between the two beams would be much smaller than for a comet nearby.

FIGURE

12:

Illustration depicting a scientific debate over cometary paths, from

Johannes Hevelius's C o m e t o g r a p h i a (1668). L e f t , Aristotle holds a diagram with comets as sublunary phenomena; Hevelius, s e a t e d , argues that comets come from the atmospheres of other planets and move in curved paths; r i g h t , Kepler insists comets move in rectilinear paths. (Courtesy Adler Planetarium and Astronomy Museum, Chicago.)

F I G U R E 1 3 : Sketch of Laplace's model for the formation of the solar system: (a) initial solar nebula dense in center and extending to the outskirts of solar system; (b) rings rich in gas and solid particles breaks offfrom the flattened disl{ because of angular momentum conservation; (c) planets form from accretion of material in rings.

F I G U R E 1 4 : Photo of the NGC 4414 Spiral Galaxy taken by the Hubble Space Telescope. (Courtesy of NASA.)

F I G U R E I 6: The Meteor crater, with a diameter of 1.2 kilometers and a depth of 200 meters, carved by the impact of an iron-rich meteorite about fifty thousand years ago in the state of Arizona. (Copyright AP/Wide World Photos.)

F I G U R E 1 5 : Formation of the solar system from a contracting nebula (a): after flattening into a dis\ ( b ) , the accretion ofmaterialforms planetesimals (c-e), which grow to become the planets (0-

FIGURE

1 7 : A rock, thrown on a pond and the outgoing waves the impact generates.

Note how the disturbance caused by the rock, varies with the energy of the impact.

F I G U R E I 8 : An ammonite fossil, displaying its spiral structure, similar to today's chambered nautilus.

(Copyright heBlanclPaleoplace.com.)

F I G U R E 1 9 : An artist's representation of the fateful impact at Chicxulub. (By Don Davis, courtesy of NASA.)

F I G U R E 2 1 : The twenty-three fragments of comet Shoemaker-Levy 9 flying in formation toward Jupiter, as seen by the Hubble Space Telescope. (Courtesy Space Telescope Science Institute.)

F I G U R E 2 0 : Photo of the flattened woods at the Tunguska site. (Copyright AP/Wide World Photos.)

F I G U R E 2 2 : Photomontage of fragments hitting Jupiter. F r o m b o t t o m ( s m a l l b r i g h t d o t ) u p , we see one and then two dark^ shocks waves spreading outward on Jupiter's upper atmosphere; their sizes are comparable to F,arth's diameter. (Courtesy NASA/JPL/Caltech.)

F I G U R E 2 3 : Diagram of an Earth-crossing (or Apollo) asteroid. Also shown is an Armor asteroid, crossing Mars's but not Earth's orbit, and the two clusters of Trojan asteroids, locked into the same orbit as Jupiter's.

FIGURE

24:

Diagram illustrating several ways proposed to date to deflect incoming cos-

mic fillers: (a) implanted rockets; (b) nuclear warhead blast; (c) collision with spacecraft; (A) laser or microwave blasting; (e) mass driver; (f) solar sailing (momentum from the Sun's radiation pushes it away, as wind pushes an umbrella).

F I G U R E 2 5 : An artist's rendition of the NEAR Shoemaker spacecraft as it orbits asteroid 433 Eros. (Courtesy of NASA.)

F I G U R E 2 6 : The eclipse of August 11, 1999: the diamond ring. (Photo by George T. Keene.)

FIGURE Michael}.

27:

Inca main ceremonial complex in Machu Picchu, Peru. (Photo by

P. Scott/Stone.)

F I G U R E 2 8 : Emission and absorption spectra. A source producing a continuous spectrum (such as a light bulb) will have some of its lines absorbed as its radiation passes through a cloud of cool gas ( t o p s e q u e n c e ) . If instead, one examines the spectrum from a single warm gas, it will reveal its very particular emission lines ( b o t t o m r i g h t s e q u e n c e ) .

FIGURE 2 9 :

The Stingray nebula, the youngest known planetary nebula, as seen by the

Hubble Space Telescope. (Courtesy Space Telescope Science Institute.)

F I G U R E 3 0 : In 1987, a supernova detonated in the Large Magellanic Cloud, a small satellite galaxy sometimes visible with the naked eye in the Southern Hemisphere. The progenitor star, being fifteen times heavier than the Sun, put on a spectacular display of the amazing power liberated in such events: at its peak,

supernova reached a luminosity of 100 million Suns, outshining all the other stars in the Large Magellanic Cloud combined. The arrow points to the location of the progenitor star. (Copyright Anglo-Australian Observatory. Photo by David Malm.)

F I G U R E 3 1 : Two images of the Crab Nebula, the most famous historical supernova, which flared in 1054. Left, an image obtained from an Earth-based telescope (the 5-meter Hale telescope) at Mount Palomar Observatory. Right, an image from the Hubble Space Telescope, magnifying the portion within the square of the left picture. There are two small stars at the center; the one to the left is a pulsar, a rapidly rotating neutron star. (Courtesy Space Telescope Science Institute.)

F I G U R E 3 2 : Formation of a Type I supernova. A white dwarf in a binary system accretes matter from its companion until the degenerate electron pressure can no longer support it against collapse. The collapsing star fuses heavy elements at a furious pace until it detonates as a supernova.

F I G U R E 3 3 : The pulsar as a cosmic lighthouse. Beams of radiation are accelerated by the pulsar's magnetic field, which is not aligned with its rotation axis. As the beams sweep through Earth, we detect a very periodic signal.

FIGURE

34:

Map by Olaus Magnus (sixteenth century) depicting the Maelstrom, l o w e r

r i g h t . (Reproducedfrom Giorgio de Santillana and Hertha von Dechend, H a m l e t ' s M i l l [Boston: Gambit, 1969], p. 91.)

F I G U R E 3 5 : Map of temperature fluctuations in the cosmic microwave background obtained by the COBE satellite. The fluctuations are only on the order of one hundred thousandth of a degree. (Courtesy NASA.)

F I G U R E 3 6 : Computer simulation showing the frothy nature of the large-scale structure of the universe. Reddish points denote larger concentrations of matter, such as galaxies and galaxy clusters. (Courtesy G. L. Bryan and M. L. Norman, National Center for Supercomputer Applications.)

F I G U R E 3 7 : Two maps of the cosmic microwave backgroundfluctuations. T o p left, the COBE map, with an accuracy of about ten degrees. B o t t o m r i g h t , the BOOMERANG map, with an accuracy of one degree. (COBE map courtesy NASA; BOOMERANG map courtesy BOOMERANG

Collaboration.)

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A l t h o u g h there are only 92 n a t u r a l l y o c c u r r i n g c h e m i c a l e l e m e n t s , there a r e thousands of isotopes. D u r i n g the first t w o decades of the t w e n t i e t h century, the N e w Z e a l a n d physicist Ernest Rutherford r e a l i z e d the greatest d r e a m of the alchemists, to achieve e l e m e n t t r a n s m u t a t i o n . G r a n t e d , he w a s not trying to c h a n g e lead into g o l d , restricting h i m s e l f to less profitable, but no less f u n d a m e n t a l , transformations between chemical elements. T h i s can be done in t w o w a y s ; one is by fusing their nuclei, a n d the other is by b r e a k i n g t h e m apart. In either case, the reacting nuclei must come very close together t h r o u g h collisions, w h i c h m e a n s they must overcome their m u t u a l electric repulsion or barrier; in other w o r d s , n u c l e a r reactions r e q u i r e a lot of input e n e r g y so that the nuclei can "touch" in spite of their positive electric c h a r g e s . I put "touch" in quotation m a r k s to stress that nuclei a r e to be thought of as small deformable distributions of matter, m o r e l i k e sponges than hard b i l l i a r d balls. In 1919, Rutherford a n n o u n c e d that he had succeeded in c h a n g i n g nitrogen (Z=7, A= 14) into an isotope of o x y g e n (Z=8, A= 17) by b o m b a r d i n g the nitrogen w i t h nuclei of h e l i u m (Z=2, A=4). T h i s n u c l e a r reaction can be represented as follows:

As in any n u c l e a r reaction involving e l e m e n t t r a n s m u t a t i o n , there is a shuffling b e t w e e n protons a n d neutrons; notice how the s u m of the atomic w e i g h t s is preserved. ( T h e total atomic w e i g h t A=18 on both sides.) Of the t w o protons from the h e l i u m nuclei, one is "eaten" by the nitrogen, c h a n g i n g it into o x y g e n , a n d the other is expelled (the h y d r o gen nucleus). T h i s p a r t i c u l a r n u c l e a r reaction consumes more e n e r g y than it liberates. H o w e v e r , w i t h it Rutherford proved that the n a t u r e of chemical e l e m e n t s is not w r i t t e n in stone but is a m e n a b l e to c h a n g e , as the alchemists d r e a m t , so long as they are exposed to the right conditions, of w h i c h the alchemists couldn't have d r e a m t . Soon after Rutherford's initial n u c l e a r t r a n s m u t a t i o n s , it b e c a m e quite clear that n u c l e a r reactions could liberate vast a m o u n t s of energy. 161

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Lighter nucleus

Lighter nucleus F I G U R E I O : Schematic diagram of nuclear fission. A neutron hits a uranium nucleus, splitting it into smaller nuclei and creating a few extra neutrons.

T h i s is true for n u c l e a r b r e a k d o w n reactions (or fission reactions), such as those o c c u r r i n g in a t o m i c bombs a n d n u c l e a r reactors, a n d for n u c l e a r fusion reactions, as in h y d r o g e n bombs a n d the interior of stars. Even though we are interested in fusion reactions here, I cannot let you m o v e on w i t h o u t at least briefly e x p l a i n i n g fission reactions. After all, they g a v e us both nuclear p o w e r stations a n d the infamous p o w e r to destroy life on Earth m a n y t i m e s over, irreversibly c h a n g i n g the collective history of h u m a n k i n d . In a typical fission reaction, a very heavy n u c l e u s is hit by a neutron a n d split into t w o m e d i u m - s i z e nuclei, liberating a lot of e n e r g y in the process. T w o isotopes a r e particu l a r l y important, u r a n i u m 235, w h i c h w e met above, a n d p l u t o n i u m 239. T h e reason for their i m p o r t a n c e is that the splitting of these heavy nuclei produces not only m e d i u m - s i z e nuclei but also a few free neutrons. T h e s e neutrons act l i k e bullets a n d hit other heavy n u c l e i , splitt i n g t h e m a n d c r e a t i n g m o r e neutrons that w i l l split m o r e nuclei, and so on. T h i s is w h a t we call a chain reaction; a very fast one becomes an a t o m i c b o m b , w h e r e a s slower ones g e n e r a t e e n e r g y in a controlled w a y , as in nuclear p o w e r stations.

T h e secret behind the power liberated in fusion reactions is the transformation of m a t t e r into energy. As Einstein proposed early in the

162

1900s, there is a relationship between the mass (m) of an object a n d its 2

e n e r g y (ÂŁ), encapsulated in the famous equation E = mc , w h e r e c is the speed of light in empty space, 300,000 k i l o m e t e r s per second. W h a t does this formula, perhaps the most popular but also the most misquoted in all of physics, actually m e a n ? It m e a n s that we can think of mass as stored e n e r g y ; in the same w a y that we &an store e n e r g y in a spring by s q u e e z i n g it a n d then releasing ttie,stored e n e r g y by letting the spring go, any massive body can have its mass released as e n e r g y â&#x20AC;&#x201D; i n the form of electromagnetic r a d i a t i o n â&#x20AC;&#x201D; u n d e r the proper circumstances. Fortunately, those circumstances are exotic e n o u g h that we don't see people or dogs t u r n i n g into a flash of h i g h l y energetic photons; if we d i d , it w o u l d be the last thing we w o u l d ever see: if we could convert 1 k i l o g r a m of matter into e n e r g y in one second, it w o u l d g e n e r a t e about 1 0

watts of power! However, the conditions u n d e r w h i c h mass turns into energy are routinely created in e x p e r i m e n t s involving h i g h l y energetic particle collisions and, of course, in the interior of stars. T h e converse is also true, " e n e r g y " can turn to matter; it is possible for h i g h l y energetic photons, the bullets of e l e c t r o m a g n e t i c radiation, to spontaneously create particles of matter. Because photons a r e electrically neutral, a n d electric c h a r g e is conserved in particle reactions, the material products of their d e m i s e must also a d d up to total zero c h a r g e . A typical e x a m p l e is the transformation of photons into electronpositron pairs. Positrons a r e electrons w i t h positive c h a r g e , also k n o w n as the electron's antiparticle. A c c o r d i n g to the l a w s of particle physics, every particle of matter has a partner antiparticle of " a n t i m a t t e r " ; the electron has the positron, the proton the antiproton, a n d so on. T h u s , a typical m a t t e r - e n e r g y transformation can be represented as

particle + antiparticle <-> photons ( e n e r g y )

So, if you shook h a n d s w i t h your antibeing, you w o u l d both disintegrate into a burst of g a m m a - r a y photons, w h i c h w o u l d probably destroy

a big c h u n k

of the U n i t e d

States.

For electron-positron

encounters, the a m o u n t of e n e r g y is m o r e modest since the masses 163

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involved a r e m u c h smaller. In any case, I w a n t to reassure you that there is n o t h i n g to f e a r â&#x20AC;&#x201D; y o u won't be hit by a w a n d e r i n g bunch of antiparticles. M a t t e r is m u c h m o r e a b u n d a n t than a n t i m a t t e r in the universe; a n t i m a t t e r particles a r e e x t r e m e l y rare, a p p e a r i n g profusely only at the heart of very energetic particle collisions, such as those promoted by m o d e r n particle accelerators. H a d it been o t h e r w i s e , we w o u l d not be here discussing solar physics. In fact, we w o u l d n ' t be here at all. T h e basic l a w controlling these massâ&#x20AC;&#x201D;energy transformations is that the total a m o u n t of mass a n d e n e r g y m u s t be the s a m e before a n d after; if some mass is lost, it turns up as e n e r g y a n d vice versa, but the total sum of the t w o (in appropriate units) must be conserved. Fusion reactions m a k e full use of this g e n e r a l m a s s - e n e r g y conservation; the m a s s of the fused nucleus is s m a l l e r than the masses of the fusing nuclei, w i t h the difference b e i n g converted into some sort of energy. T h u s , a g e n e r a l fusion reaction can be w r i t t e n as follows:

nucleus-1 + nucleus-2 â&#x20AC;&#x201D;> nucleus-3 + e n e r g y

T h i s surplus of e n e r g y p o w e r s the S u n a n d other stars, a l t h o u g h the types of fusion reactions w i l l d e p e n d on the stars' core t e m p e r a t u r e s , w h i c h are d e t e r m i n e d by their masses. For modest stars l i k e the S u n , the core t e m p e r a t u r e can reach about 15 million d e g r e e s Kelvin, h i g h e n o u g h to initiate the so-called proton-proton chain, the first r u n g in the nuclear fusion b u i l d u p . T h e simplest of all nuclei is the h y d r o g e n n u c l e u s , w i t h its single a n d lonely proton. C l e a r l y , if t w o protons a r e to fuse into something else, they m u s t o v e r c o m e their electric repulsion. But even if they are m o v i n g e x t r e m e l y fast, so as to collide h a r d w i t h one another, w h a t m a k e s t h e m stick? At these very small distances, another fundamental force of n a t u r e comes into play, the strong nuclear force. It is this force, w h i c h acts only w i t h i n n u c l e a r distances, but is about a h u n d r e d times stronger than the electric repulsion b e t w e e n protons, that fuses nuclei together. Incidentally, it is also the strong force that g l u e s the atomic 164

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nucleus into a w h o l e s o m e l u m p , w i t h all its r e p e l l i n g protons a n d electrically indifferent neutrons. T h e strong force's short r a n g e e x p l a i n s w h y there a r e only n i n e t y - t w o stable atoms in n a t u r e ; a n y more protons, a n d the strong force could not overcome their electric repulsion and g l u e them together w i t h i n a small n u c l e a r - s i z e v o l u m e . To recap, n u c l e a r fusion takes a d v a n t a g e of the m a t t e r - e n e r g y conservation l a w , as matter is converted into e n e r g y d u r i n g the fusion process. In order for nuclei to fuse, they must be m o v i n g at e x t r e m e l y high velocities so as to overcome their m u t u a l electrical repulsion, a n d get sufficiently close to a l l o w the strong n u c l e a r force to perform its t r a n s m u t a t i o n trick. T h e first r u n g in the fusion l a d d e r is also the s i m plest, t w o protons fusing into a n u c l e u s of the h y d r o g e n isotope d e u 2

t e r i u m ( H ) , as shown in figure 11. T h e "fusion" here a c t u a l l y involves the t r a n s m u t a t i o n of a proton into a neutron. T h e r e a r e t w o other reaction products: a positron, the "antielectron" we encountered above, a n d a particle called neutrino, w h i c h , l i k e the photon, has no electric c h a r g e (hence the n a m e , "little n e u t r o n " ) a n d has at most a tiny m a s s ( w e still don't k n o w for s u r e ) , at least m i l lions of times s m a l l e r than the electron's, if any. N e u t r i n o s have the r e m a r k a b l e property of h a r d l y ever interacting w i t h other particles; in fact, they easily travel across the S u n a n d out to space at the speed of

F I G U R E I I : Two protons fuse into a deuteron, the nucleus of the hydrogen isotope deuterium. A neutrino and a positron are also created during the fusion process.

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light or very close to it. Not so for the positrons. Recall that the p l a s m a at the solar core is rich both in h y d r o g e n nuclei (the protons) and in electrons. Therefore, the freshly minted positrons meet a very fast death as they collide w i t h their matter partners, d i s i n t e g r a t i n g into h i g h l y energetic g a m m a - r a y photons. In short, the first r u n g in the nuclear fusion chain consists of t w o protons fusing into a deuteron (the nucleus of d e u t e r i u m ) a n d p r o d u c i n g g a m m a - r a y photons a n d neutrinos. T h e s e t w o particles carry a w a y the e n e r g y s u r p l u s of the fusion process. T h e next r u n g in the l a d d e r is to fuse the n e w l y m a d e deuterons 3

w i t h another proton to m a k e the next-heaviest isotope, H e , the lightest isotope of h e l i u m , w i t h t w o protons a n d one neutron in its nucleus:

H + 'H —> ' H e + e n e r g y

A g a i n , the e n e r g y comes out in the form of g a m m a - r a y photons. T h e next and final step in the proton-proton chain is the actual fusion of a 4

h e l i u m nucleus, H e . T h i s can be accomplished in a n u m b e r of differ3

ent w a y s , but the most efficient is to fuse t w o H e isotopes, produced in the p r e c e d i n g step:

H e + H e —> H e + 'H + 'H + e n e r g y

T h e net balance of the three-step reaction is as follows: for each H e we 3

used three h y d r o g e n nuclei ( ' H ) , a n d we needed t w o H e to produce 4

the final H e . T h i s last step also produces a s u r p l u s of t w o h y d r o g e n nuclei. So, we used a total of six h y d r o g e n nuclei but got t w o back from the h e l i u m fusion. T h e total h e l i u m fusion reaction can be s u m m a r i z e d , i n c l u d i n g the t w o neutrinos from step one (performed t w i c e ) , as follows:

4 ( ' H ) —> H e + e n e r g y + 2 neutrinos T h e e n e r g y liberated at the core as g a m m a rays flows into the radiation zone, w e a k e n i n g a l o n g its w a y until it reaches the convection zone, 166

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w h e r e it is boiled up to the solar surface, r a d i a t i n g in the visible a n d infrared. T h e S u n will r e m a i n in this h y d r o g e n - b u r n i n g phase, also k n o w n as the m a i n sequence, for most of its life cycle, r o u g h l y for another five billion years. At that point, the continuous fusion of h y d r o g e n nuclei at the core will deplete its reserves, and the S u n will have to fuse something else to generate e n o u g h energy to'contain the inexorable gravitational crunching of its outer layers. T h i s is the b e g i n n i n g of the h e l i u m - b u r n ing cycle, the b e g i n n i n g of the end for the S u n and stars of similar mass and composition. I w i l l n o w tell this tale of self-cannibalism, w h e r e stars literally devour their o w n entrails in order to survive.

Red Giants A star's fate is sealed by its mass; very massive stars, m u c h m o r e m a s sive than the S u n , must s t r u g g l e m u c h h a r d e r to contain the i n w a r d pull of their o w n gravity. L i k e any star, their only w e a p o n is their o w n matter, w h i c h must burn by fusion at a furious pace to release the enormous e n e r g y needed to detain their collapse. In the e n d , their l a r g e masses c o m p r o m i s e their longevity; the most massive stars live only a few tens of m i l l i o n s of y e a r s before encountering a violent death. We met t h e m before, d u r i n g our discussion of the formation of the solar system, in chapter 3; their r e m a i n s s p r i n k l e d our neighborhood w i t h heavier elements and the small g r a i n s needed for the formation of planetesimals. T h e y m a y also have caused the initial gravitational instability of the progenitor h y d r o g e n cloud, w h i c h contracted to become our solar system. At the other e x t r e m e , we find small cold stars called "red d w a r f s , " w h i c h burn so slowly that they m a y live for trillions of y e a r s . T h e S u n is in between these t w o e x t r e m e s , a star that can fuse h y d r o g e n into h e l i u m t h r o u g h the proton-proton chain for about ten billion y e a r s . T h e question, then, is w h a t happens after these ten billion y e a r s of fairly steady b u r n i n g . In order to follow the evolution of a star t h r o u g h its final stages, it 167

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is useful to k e e p in m i n d the interplay between the t w o m a i n forces d e t e r m i n i n g its fate. W h i l e the attractive force of g r a v i t y w a n t s to s q u e e z e the star into the smallest possible v o l u m e , the heat g e n e r a t e d by this very s q u e e z i n g creates a counterpressure that balances the star's collapse. So, in order to survive as a stable hot ball of g a s , the star must k e e p producing heat, w h i c h comes from fusing h y d r o g e n nuclei at its core. C l e a r l y , this d a n g e r o u s g a m e of c o n s u m i n g its o w n entrails for survival cannot last forever. For now, we w i l l focus on w h a t happens w i t h stars l i k e the S u n , w h i c h have fairly low masses. As we w i l l see in the next chapter, the story c h a n g e s q u i t e d r a m a t i c a l l y for stars of masses l a r g e r than r o u g h l y eight times that of the S u n â&#x20AC;&#x201D; t h e r e a l m of supernova explosions and their exotic r e m n a n t s , neutron stars and black holes. But before we get to that, we w i l l use m o d e r n astrophysical ideas to foretell the S u n ' s t r a g i c fate, w h i c h , as the E g y p t i a n s , Incas, Japanese, a n d m a n y other peoples have k n o w n for m a n y centuries, is of great consequence to our o w n . For about 10 billion y e a r s , h y d r o g e n fuses into h e l i u m , creating e n o u g h e n e r g y to balance g r a v i t y ' s crush. As h e l i u m becomes more a b u n d a n t at the center, h y d r o g e n becomes rarer. At its present a g e , the S u n has converted only about 5 percent of its total m a s s into h e l i u m . As this a m o u n t increases, so w i l l the Sun's luminosity, that is, its total p o w e r output. In 1.1 billion y e a r s , the luminosity w i l l be 10 percent l a r g e r than today; in 3.4 billion y e a r s , 40 percent larger. T h i s extra a m o u n t of p o w e r w i l l have serious consequences for Earth's c l i m a t e , creating first a "moist greenhouse"effect and then a true " r u n a w a y g r e e n h o u s e " effect. Possibly, clouds m a y d e l a y these effects s o m e w h a t , but the prospect for life on Earth beyond r o u g h l y 1 billion y e a r s is q u i t e g r i m . T h e rapid increase in t e m p e r a t u r e w i l l melt the polar ice caps, c a u s i n g the oceans to rise a n d flood all coastal areas. T h e tempera t u r e w i l l rise some m o r e , and the oceans w i l l boil, t h i c k e n i n g the a t m o s p h e r e w i t h dense clouds of steam. L u r k i n g behind the thick vapor, a faint S u n w i l l seem to be m o c k i n g an Earth that looks m u c h l i k e Venus. T h e a m o u n t of h e l i u m will k e e p g r o w i n g , a n d in five billion years 168

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it will reach a crisis level, as practically no h y d r o g e n nuclei (protons) will survive at the Sun's core; most h y d r o g e n b u r n i n g w i l l happen in a shell s u r r o u n d i n g the core. T h i s is w h e n serious trouble begins; if h e l i u m fused into something else, the Sun's d o o m w o u l d be postponed, since the heat released w o u l d act to stop any further contraction. T h e problem is that h e l i u m nuclei, w i d i t w b protons, are h a r d e r to fuse because of their l a r g e r m u t u a l electric repulsion. In fact, the fusion of h e l i u m requires t e m p e r a t u r e s on the order of h u n d r e d s of millions of d e g r e e s , as opposed to the ten or so million of h y d r o g e n fusion. T h u s , w h e n the core becomes h e l i u m d o m i n a t e d , it is too cold (by a factor of ten) to induce h e l i u m fusion; gravity, of course, takes a d v a n t a g e of this l o w e r i n g of the g u a r d , s q u e e z i n g the h e l i u m core further. T h i s causes

FIGURE

12:

The path to the red giant stage. As the helium-dominated core contracts

and heats up, hydrogen burns even more furiously in the surrounding shell. The enormous pressure generated there pushes the star's nonburning envelope outward.

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the t e m p e r a t u r e at a n d a r o u n d the core to increase, h e a t i n g up even m o r e the h y d r o g e n in its s u r r o u n d i n g shell, w h i c h begins to burn at a furious pace. T h e star enters a schizophrenic phase, w h e r e the "cold" h e l i u m core continues to shrink a n d heat up w h i l e its h y d r o g e n envelope burns a n d e x p a n d s , pushing the n o n b u r n i n g l a y e r s o u t w a r d (see figure 12). T h i s is the red g i a n t phase, w h e n the S u n w i l l g r o w to one h u n d r e d times its n o r m a l size (the " g i a n t " in red g i a n t ) . T h i s g r o w t h w i l l cause its surface t e m p e r a t u r e to decrease, as a result of the t h i n n i n g of the g a s in its outer l a y e r s , w h i c h will radiate in the cooler red end of the visible spectrum (the " r e d " in red g i a n t ) . A powerful red g i a n t can be seen w i t h the n a k e d eye in the constellation Orion, the one easily identified by the "belt" of three a l i g n e d stars. T h e star corresponding to Orion's " r i g h t s h o u l d e r " is a red s u p e r g i a n t called Betelgeuse, one of the brightest stars in the night sky. ( W e w i l l soon see w h a t the "super" stands for.) After a careful look, you w i l l notice that it does indeed t w i n k l e red. W h e n the Sun reaches the p e a k of its red g i a n t phase, about one h u n d r e d million y e a r s after depleting all h y d r o g e n at the core, its size w i l l e n g u l f M e r c u r y ' s orbit, a n d it w i l l blast about 30 percent of its mass into space. To m a k e things worse for our distant descendants, if a n y are still a r o u n d then, the Sun's luminosity, the power it g e n e r a t e s , will become thousands of times l a r g e r than w h a t it is today. If there w a s any form of life that survived all the p r e - r e d - g i a n t - p h a s e instabilities and greenhouse effects, none will r e m a i n after it. Eventually, the atmosphere w i l l also boil a w a y a n d stars w i l l shine at d a y t i m e side by side w i t h the S u n ; w h a t we call the sky w i l l be literally gone, a n d the planet (let's not call it Earth a n y m o r e ) will be b o m b a r d e d w i t h lethal ultraviolet r a d i a tion. Rocks w i l l m e l t a n d the planet's surface w i l l boil l i k e soup, as the Sun's outer layers brush close by. We can only hope our descendants w i l l be s m a r t e n o u g h to have left Earth w e l l before its fiery e n d , w h e n hell descends from heaven, an apocalyptic i m a g e w o r t h y of Revelation. But the swollen red S u n is not about to g i v e up. It w i l l fight the i n w a r d pull of g r a v i t y until the end, w i t h the only w e a p o n it h a s â&#x20AC;&#x201D; t h e fusing of w h a t e v e r is at its core. 170

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W h i l e the outer l a y e r s of the star e x p a n d o u t w a r d driven by the e n o r m o u s heat g e n e r a t e d by the h y d r o g e n - b u r n i n g shell, its h e l i u m core, devoid of any pressure-producing heat, continues to shrink slowly. W h e n the core density reaches a s t a g g e r i n g 100 million k i l o g r a m s per cubic meter (that is, a cube one meter a side w e i g h s 100 thousand tons) the t e m p e r a t u r e finally rises^to the 100 million d e g r e e s Kelvin r e q u i r e d for h e l i u m to fuse into carbon a n d reignite the inner solar fire. Carbon fusion happens in t w o steps; first t w o h e l i u m nuclei 4

( H e ) fuse into the h i g h l y unstable b e r y l l i u m 8 ( B e ) . Before this isotope of b e r y l l i u m can b r e a k d o w n , it encounters another h e l i u m nuclei a n d 1 2

fuses w i t h it into carbon ( C ) . T h e two-step fusion reaction can be schematically represented as follows:

H e + H e —> B e + e n e r g y

l 2

B e + H e —» C + e n e r g y

Because h e l i u m 4 nuclei w e r e traditionally called alpha particles, this fusion reaction, using three h e l i u m 4 nuclei, is k n o w n as the triplealpha process. But carbon fusion is not the only process t r i g g e r e d by core densities of h u n d r e d s of m i l l i o n s of k i l o g r a m s per cubic meter; recall that d u r ing the h y d r o g e n - b u r n i n g stage the core w a s rich not only in h y d r o g e n nuclei but also in electrons stripped from their parent atoms. T h e s e electrons finally get fed up w i t h all this s q u e e z i n g a n d start to react. According to q u a n t u m physics, every subatomic particle is characterized by a set of n u m b e r s that label all its relevant properties. C a l l e d quantum

numbers,

they

include

information

about

the

particle's

energy (for e x a m p l e , in w h i c h e n e r g y level of the atom the electron is), its a n g u l a r m o m e n t u m (how fast it rotates about the atomic n u c l e u s ) , and its intrinsic a n g u l a r m o m e n t u m or spin, w h i c h , using a r o u g h image, tells how the particle rotates about its o w n axis, c o m p a r e d w i t h a fixed direction in space, say, the up direction. (Picture a top that can spin only at fixed a n g l e s w i t h respect to the up direction; each a n g l e specifies a rotation state of the " q u a n t u m top," or particle.) Once this

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set of n u m b e r s is g i v e n , we can describe w h a t the electron is d o i n g — that is, we can specify its q u a n t u m state. One of the problems w i t h Bohr's formulation of the a t o m — w h e r e , as we have seen, electrons m o v e about the nucleus in discrete orbits—is that it didn't explain w h a t prevented all the electrons from just bunching together at the lowest e n e r g y state, closest to the n u c l e u s . If electrons could a g g r e g a t e there, all h i g h e r - e n e r g y atomic levels w o u l d be e m p t y a n d atoms w o u l d not be able to combine w i t h other atoms to form molecules; a m o n g other things, the c h e m i s t r y of life w o u l d not have been possible. It turns out that electrons a r e very a n t i g r e g a r i o u s particles a n d cannot coexist at the s a m e q u a n t u m state. For atoms, this m e a n s that electrons fill up the a v a i l a b l e discrete orbits, l i k e m a r b l e s placed in pairs on the steps of a staircase; if they are s q u e e z e d too close to each other by some outside force, they respond by c o u n t e r i m p o s i n g a pressure k n o w n as electron d e g e n e r a c y pressure. It is this pressure that w i l l help support the h e l i u m core against further collapse. T h i s antig r e g a r i o u s behavior of the electrons a n d m a n y other particles, i n c l u d ing protons a n d neutrons, is k n o w n as the Pauli exclusion principle, after W o l f g a n g P a u l i , w h o proposed it in the mid-1920s. As w i t h a spring m a t t r e s s , the h e a v i e r the sleeper, the h a r d e r the springs m u s t w o r k to resist the d o w n w a r d pressure caused by his w e i g h t . L i k e w i s e , the d e g e n e r a t e electrons w i l l try to resist the g r a v i tational s q u e e z i n g of the h e l i u m core by the outer l a y e r s of the star. To do this, they w i l l h a v e to m o v e faster a n d faster—the h i g h e r their speed, the h i g h e r the counterpressure they can produce. T h i n k of w a t e r f l o w i n g out of a hose a n d h o w the faster it flows, the h i g h e r the pressure it exerts on objects. T h e r e is a l i m i t to h o w m u c h g r a v i t a tional s q u e e z i n g d e g e n e r a t e electrons can stand; at some point, their speeds w i l l approach the speed of l i g h t a n d , as we w i l l soon see, other effects c o m e into play. For now, t h o u g h , the i m p o r t a n t property of this electron pressure is that it does not d e p e n d on t e m p e r a t u r e ; as h e l i u m fusion b e g i n s at the a g o n i z i n g Sun's core a n d t e m p e r a t u r e s start rising, most of the g r a v i t y - o p p o s i n g pressure comes from d e g e n erate electrons, as opposed to t h e r m a l pressure. T h i s c h a n g e s the

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physics at the core q u i t e d r a m a t i c a l l y : d u r i n g n o r m a l b u r n i n g , w h e n the star's core heats up because of fusion, it e x p a n d s , cools, a n d contracts until a reasonable fusion rate is found that properly balances the g r a v i t a t i o n a l s q u e e z i n g w i t h o u t b u r n i n g too m u c h fuel. A thermostat in an air conditioner is a reasonable a n a l o g y , because it turns the e n g i n e on a n d off to k e e p the temperahure at a set v a l u e . H o w e v e r , since d e g e n e r a t e electrons do not respond to t e m p e r a t u r e c h a n g e s , w h e n h e l i u m fusion raises the core t e m p e r a t u r e , there is no corres p o n d i n g expansion a n d cooling, as if the star's thermostat w e r e broken; the t e m p e r a t u r e shoots up, setting a b o m b - l i k e pace of h e l i u m b u r n i n g , k n o w n as a h e l i u m flash. After a few hours of out-of-control h e l i u m b u r n i n g , the t e m p e r a t u r e rises to a point w h e r e t h e r m a l p r e s sure finally b e g i n s to d o m i n a t e a g a i n . T h e star's thermostat c l i c k s back on, a n d h e l i u m b u r n s to carbon at a m o r e m o d e r a t e pace, albeit still very fast, for a few tens of m i l l i o n s of y e a r s . A g a i n , this relative peace cannot last for long. T h e now carbon-rich core gets depleted of h e l i u m , and the star falls into a s i m i l a r cycle: the carbon core is too cold for carbon fusion a n d starts to s h r i n k u n d e r its own g r a v i t y , w h i l e its s u r r o u n d i n g h e l i u m shell keeps b u r n i n g , b e i n g itself s u r r o u n d e d

the

hydrogen-burning

shell

that

previously

enclosed the h e l i u m core (see figure 13). Just as before, the s h r i n k i n g of the carbon core causes an increase in t e m p e r a t u r e , w h i c h heats u p even m o r e the h e l i u m - a n d h y d r o g e n b u r n i n g shells. All this heat causes the outer l a y e r s of the star to e x p a n d o u t w a r d , in a d r a m a t i c repeat of the red g i a n t phase. But the t e m p e r a tures a r e now m u c h h i g h e r than before, and the convulsing star g r o w s even l a r g e r than before, s w a l l o w i n g e v e r y t h i n g past the orbit of M a r s . Earth w i l l be no m o r e . If the core t e m p e r a t u r e c l i m b e d to about 600 m i l l i o n d e g r e e s Kelvin, carbon could start fusing into h e a v i e r elements, a n d the release of heat w o u l d support the star's collapse for a bit longer. But as the core density reaches a s t a g g e r i n g 10 billion k i l o g r a m s per cubic m e t e r â&#x20AC;&#x201D; a c h e r r y - s i z e c h u n k of this stuff w o u l d w e i g h one ton on E a r t h â&#x20AC;&#x201D; t h e electrons once m o r e start to react against any further s q u e e z i n g , their 173

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F I G U R E 1 3 : The path to a red supergiant. The helium at the core fuses into carbon. The shrinking carbon core is surrounded by three layers: the helium-burning layer, the hydrogen-burning layer, and an expanding nonburning envelope.

pressure stopping the contraction of the carbon core. As a result, the t e m p e r a t u r e never rises e n o u g h for carbon fusion to begin in earnest: the g r a v i t a t i o n a l contraction of the carbon core gets balanced by the electron d e g e n e r a c y pressure, a q u a n t u m effect b a l a n c i n g an astronomical object, a true m a t c h i n g of the physics of the very big a n d that of the very s m a l l . A l t h o u g h a bit of o x y g e n does get fused at the inner e d g e of the h e l i u m - b u r n i n g shell, the star is pretty m u c h spent now.

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T h e S h e d d i n g o f the V e i l T h e story is not yet over. It is h a r d to i m a g i n e how an object w i t h core t e m p e r a t u r e s of about 300 m i l l i o n d e g r e e s K e l v i n , densities of billions of k i l o g r a m s per cubic meter, and s u r r o u n d e d by h e l i u m and h y d r o g e n - b u r n i n g shells w o u l d just rest in peace. As the core adjusts to its electron-supported

equilibrium,

the

burning

the

inner

shells

becomes very unstable. A series of h e l i u m flashes occurs in the inner h e l i u m shell, liberating so m u c h e n e r g y that the star literally starts to pulsate violently, its outer layers becoming looser w i t h each convulsion. After about one m i l l i o n y e a r s of this agony, the star expels its outer l a y ers into space at speeds of tens of k i l o m e t e r s per second. T h e " S u n , " or what's left of it, has n o w t w o separate parts; a very hot carbon core, s u r r o u n d e d by a thin l a y e r w h e r e h e l i u m is fused into carbon and o x y g e n , a n d a thin, a p p r o x i m a t e l y spherical veil of the cooler ejected matter, w h i c h covers roughly the size of the solar system. ( T h e m o r e technical term is "envelope," but I prefer the i m a g e of a veil.) T h i s veil of g l o w i n g gases, one of the most beautiful sights in the universe, is k n o w n as a planetary nebula (see figure 29 in insert). Of course, planetary nebulae have nothing to do w i t h planets; the term o r i g i n a t e d in the eighteenth century, w h e n astronomers identified these colorful objects w i t h low-resolution telescopes and w e r e tricked into associating them w i t h colorful planets or, as in S a t u r n , with r i n g s a r o u n d them, the veils of other planets. As time goes by, the ejected veil moves farther a w a y from the core, e n r i c h i n g the interstellar m e d i u m w i t h h y d r o g e n , h e l i u m , carbon, a n d o x y g e n . A true cosmic recycler of matter, the d y i n g star sows the seeds for n e w stars to be born, echoing Kant's P h o e n i x - l i k e vision of d e a t h and rebirth of celestial objects. W h i l e the veil ventures into outer space, the leftover carbon core, unable to fuse a n y t h i n g else, starts to slowly cool off. It is a small and compact object, r o u g h l y the size of Earth a n d w i t h a mass about h a l f that of the S u n ; its stability against further g r a v i t a t i o n a l contraction is 175

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g u a r a n t e e d by the stubborn d e g e n e r a t e electrons. T h e l i n g e r i n g h i g h temperatures

make

the core's

surface

shine

white

hot, a l t h o u g h

because of its small size, its luminosity (total p o w e r g e n e r a t e d ) is quite small; for these t w o reasons, small size a n d white-hot surface, these compact objects are k n o w n as w h i t e d w a r f s , hot e m b e r s fading after a cosmic fire. T h e cooling of w h i t e d w a r f s takes billions of y e a r s , a t i m e c o m p a r a b l e to the a g e of the universe; as the surface t e m p e r a t u r e drops, its g l o w becomes increasingly d i m , until it finally slides into d a r k oblivion. T h i s g l o o m y scenario is reserved for stars of masses c o m p a r a b l e to the Sun's. L a r g e r - m a s s stars w i l l crush the electrons at the core to such an extent that not even their powerful d e g e n e r a c y pressure w i l l be able to halt the star's g r a v i t a t i o n a l implosion. T h i s is the r e a l m of e x t r e m e astrophysics, of supernovae, neutron stars, a n d black holes, w h e r e we n o w turn.

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/ had a dream, which was not all a dream. The bright sun was extinguished, Did wander darkling in

and the stars

the eternal space,

Rayless, and pathless, and the icy Earth Swung blind and blackening in

the moonless air. â&#x20AC;&#x201D; BYRON,

"DARKNESS"

( I 8 I 6)

T h r o u g h o u t h u m a n c u l t u r e , i n l e g e n d s , m y t h s , a n d fables, there i s manifest a g r e a t fascination w i t h fantastic j o u r n e y s , the exploration of u n k n o w n l a n d s , e a r t h l y or b e y o n d . In these o t h e r w o r l d l y r e a l m s , flesh-and-blood heroes m e e t g o d s , d r a g o n s , a n d u n i c o r n s , battle evil spirits, a n d descend into the fiery pits of hell. Moses c l i m b e d M o u n t Sinai to meet God; Jason, the l e a d e r of the A r g o n a u t s , w h i l e l o o k i n g for the fabled G o l d e n F l e e c e battled fire-breathing bulls a n d sowed a d r a g o n ' s teeth; H e r c u l e s , one of Jason's c r e w m e m b e r s , descended into H a d e s a n d c a p t u r e d C e r b e r u s , the t h r e e - h e a d e d dog that g u a r d e d its e n t r a n c e ; K i n g A r t h u r a n d the K n i g h t s of the R o u n d T a b l e searched for the H o l y G r a i l ; a n d so on. Each of these fantastic j o u r n e y s has an e l e m e n t of self-transcendence, a contact w i t h a h i g h e r r e a l m of b e i n g , w h e r e the real m i x e s w i t h the u n r e a l . T h e hero r e t u r n s deeply t r a n s formed by his e x p e r i e n c e s , not the flesh-and-blood person that started the v o y a g e but a d e m i g o d , a leader, one w h o has a c q u i r e d some secret k n o w l e d g e not s h a r e d b y c o m m o n m o r t a l s , one w h o inspires h i g h e r 177

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m o r a l i t y . It is the fantastic e l e m e n t of these n a r r a t i v e s that p e r m i t s the d e e p transformation of the self that the t r a v e l e r u n d e r g o e s . But fantasy not only transforms; it also captivates. W e , the a u d i e n c e , i m m e r s e ourselves in the heroes' m a g i c a n d , in so doing, also become heroes, l e a r n i n g as we m a r v e l ; the n a r r a t i v e s s y m b o l i z e our search for k n o w l e d g e and m e a n i n g , w h i c h m a y be revealed to us if we a r e brave e n o u g h to probe the u n k n o w n , to behave l i k e heroes. It is not very difficult to e x p a n d this a r g u m e n t to i n c l u d e the search for scientific k n o w l e d g e . Scientists also w r e s t l e w i t h the u n k n o w n in search of e n l i g h t e n m e n t ; in order to e x p a n d scientific k n o w l e d g e , we m u s t probe into u n k n o w n territory, a process of exploration that oftentimes requires g r e a t intellectual c o u r a g e . L i k e the m y t h i c heroes, w e transform ourselves t h r o u g h our search, l e a r n i n g to face the a w e s o m e creativity of nature w i t h d e e p h u m i l i t y . In this chapter, we w i l l e m b a r k on a fantastic journey dictated not by the heroic legends of the distant past but by the intellectual c o u r a g e of a few astrophysicists w h o d a r e d to t a k e the path w h e r e science itself becomes a w i z a r d , capable of c r e a t i n g the most a m a z i n g realities, on a par w i t h a n y m y t h i c a l p i l g r i m a g e . A j o u r n e y into a black hole, those whirlpools of space-time, is an intellectual p i l g r i m a g e w o r t h y of Jason or A r t h u r ; the traveler, even if only t r a v e l i n g w i t h his m i n d , e m e r g e s deeply transformed, his vision of the cosmos forever c h a n g e d . For the universe is populated by the most b i z a r r e creations, w h i c h d e m a n d that our very notions of space and time be deeply revised. To see w h y , we start our journey w h e r e we stopped last, by discussing the fate of stars heavier than the S u n , the progenitors of neutron stars a n d black holes.

The Unbearable Heaviness of Being T h e S u n is a fairly light star. W i t h i n the m a i n s e q u e n c e â&#x20AC;&#x201D; t h e stage d u r i n g stellar evolution in w h i c h h y d r o g e n fusion produces the energy b a l a n c i n g the star a g a i n s t its o w n g r a v i t y â&#x20AC;&#x201D; s t a r s can have masses rang178

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ing from one-tenth of the solar mass to t w e n t y times it, placing the S u n in the featherweight class. We have seen h o w the S u n w i l l end its life in a d r a m a t i c split into t w o objects, a veil of gases t r a v e l i n g o u t w a r d at tens of k i l o m e t e r s per second, and a hot and dense w h i t e dwarf, w h e r e d e g e n e r a t e fast-moving electrons create e n o u g h pressure to contain the inexorable crush of gravity. Stars l i k e the S u n can be thought of as cosmic recyclers of m a t e r i a l ; in go"*hydrogen a n d a bit of h e l i u m , a n d out go

h y d r o g e n , h e l i u m , carbon, a n d o x y g e n . D u r i n g the second half of

the twentieth century, it became clear that the lightest chemical e l e m e n t s â&#x20AC;&#x201D; h y d r o g e n , h e l i u m , l i t h i u m , and their isotopesâ&#x20AC;&#x201D;are mostly " p r i m o r d i a l , " that is, they w e r e produced d u r i n g the early stages of the universe's history. Since some of these elements are the basic i n g r e d i ents needed to m a k e stars, they had to come from s o m e w h e r e else. But w h a t about all the other chemical e l e m e n t s ? W h e r e do sulfur, s o d i u m , c a l c i u m , silicon, m a n g a n e s e , g o l d , and u r a n i u m come from? T h e a n s w e r is, perhaps, not m u c h of a surprise: from stars heavier than the S u n . T h e fusion p a r o x y s m that drove the S u n to its end w i l l continue and produce heavier e l e m e n t s in m o r e massive stars. In a very true sense, the chemistry of the universe is d e t e r m i n e d by stellar evolution. We should t h i n k of stars not as m e r e pale dots of light scattered a r o u n d g a l a x i e s but as e n g i n e s that g i v e the cosmos its rich chemical diversity, our existence being just one of their m a n y by-products. Stars lighter than about eight solar masses will settle into w h i t e d w a r f s , because the core t e m p e r a t u r e s w i l l never rise above the 600 m i l l i o n degrees needed to fuse heavier elements. T h e r e is just not e n o u g h mass in the outer layers to compress the core to h i g h e r t e m p e r atures. To go back to the sleeper-on-the-mattress analogy, this sleeper is just too light to really squeeze the mattress d o w n . Stars heavier than eight solar masses w i l l be hot e n o u g h at their core to continue w h e r e the S u n stops; instead of halting fusion at carbon (and a bit of o x y g e n ) , there w i l l be a progressive b u i l d u p , based on a series of different fusion reactions. T h e most c o m m o n involve the successive capture of h e l i u m 4 nuclei: carbon 12 captures h e l i u m to form o x y g e n 16 (at 200 million d e g r e e s ) ; o x y g e n 16 captures h e l i u m to 179

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form neon 20; neon 20 captures h e l i u m to form m a g n e s i u m 24; m a g n e s i u m 24 captures h e l i u m to form silicon 28. If you guessed that there is a pattern here, you a r e right. Each n e w l y fused n u c l e u s has four more protons than its predecessor; t w o protons a n d t w o neutrons from h e l i u m 4, w h i c h get converted into protons. Because of the efficiency of these h e l i u m - c a p t u r e reactions, " m u l t i p l e s of four" c h e m i c a l e l e m e n t s a r e the most a b u n d a n t in the universe. But clearly they a r e not the only ones. Other e l e m e n t s are fused as single protons a n d neutrons a r e freed from their parent nuclei a n d then absorbed by others. T h i s pattern of successive fusion by fours is m i r r o r e d in the very structure of the star; if stars l i k e the S u n can be thought of as h a v i n g an o n i o n - l i k e structure, w i t h a carbon core s u r r o u n d e d by shells of h e l i u m a n d h y d r o g e n b u r n i n g (figure 13), here we w i l l have a l a r g e r n u m b e r of shells f o l l o w i n g the multiples-of-four h i e r a r c h y : from inside out, silicon 28, m a g n e s i u m 24, neon 20, o x y g e n 16, carbon 12, h e l i u m 4, a n d h y d r o g e n , as i n d i c a t e d in figure 14. W h e n the star fuses silicon 28, another process starts to compete w i t h h e l i u m c a p t u r e , called photodisintegration, the b r e a k d o w n of heavier nuclei by heat. T h e star's core is n o w at an a b s u r d 3 billion d e g r e e s , a n d the radiation ( g a m m a - r a y photons) associated w i t h this heat is so intense that it can b r e a k d o w n heavier nuclei as easily as we can c r u m b l e a s u g a r cube. For e x a m p l e , silicon 28 photodisintegrates into seven h e l i u m 4 nuclei. T h i s s u r p l u s of h e l i u m 4 is a c t u a l l y very good n e w s , because it breathes n e w life into the h e l i u m - c a p t u r e process, a l l o w i n g heavier nuclei to get built up, all the w a y to nickel 56 (see figure 15). At this point, s o m e t h i n g q u i t e different h a p p e n s , w h i c h w i l l r a d i c a l l y alter the fusion-burning cycle of the star; nickel 56 is an unstable isotope of nickel ( w i t h 28 protons) a n d d e c a y s very q u i c k l y into cobalt 56 ( w i t h 27 protons). But cobalt 56 is also unstable, a n d it d e c a y s into the very stable iron 56 (with 26 protons). I say very stable because iron 56 has the most stable n u c l e u s in n a t u r e . T h i s m e a n s that it has the most tightly b o u n d a r r a n g e m e n t of protons a n d neutrons of a n y of the e l e m e n t s ; iron 56 has the hardest n u c l e u s to b r e a k apart. Not even the furious g a m m a - r a y photons c o r r e s p o n d i n g to t e m p e r a t u r e s of a few billion

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F I G U R E 1 4 : A slice through a highly evolved star, with mass greater than eight solar masses. Note the onion structure, with heavier elements burning closer to the center at higher and higher temperatures. The star is about to detonate as a supernova.

d e g r e e s can do a n y t h i n g w i t h iron 56; it is as if the s u g a r cube t u r n e d rock-solid. T h e net result of this sequence of reactions is that the core q u i c k l y becomes iron rich. T h i s sequence of fusion steps is quite frantic; the heavier the e l e ment, the shorter the duration of the fusion process. As the star struggles against the crush of g r a v i t y , it b u r n s w h a t e v e r fuel it has a v a i l a b l e , heavier a n d heavier nuclei at h i g h e r a n d h i g h e r t e m p e r a t u r e s . In a p p r o x i m a t e n u m b e r s , a star t w e n t y times as massive as the S u n b u r n s h y d r o g e n for ten m i l l i o n y e a r s (the heavier the star, the shorter its m a i n - s e q u e n c e b u r n i n g stage), h e l i u m for one m i l l i o n y e a r s , carbon for one thousand y e a r s , o x y g e n for a year, a n d silicon for a w e e k . T h e iron 181

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core evolves w i t h i n less than a day. Iron gets fused at the core not just from the decay of nickel 56 but also from the direct fusion of t w o silicon 28 nuclei, the closest l a y e r to the iron core. On top of this, other unstable heavy nuclei also end up as iron. As m o r e iron a c c u m u l a t e s at the core, the star finds itself in a terrible p r e d i c a m e n t ; being so stable, the fusion of iron 56 a c t u a l l y consumes e n e r g y instead of l i b e r a t i n g it. T h e iron core stops p r o v i d i n g the heat pressure that is badly n e e d e d to stop its o w n g r a v i t a t i o n a l c r u n c h ; q u i t e suddenly, the star loses its foundation a n d starts collapsing upon itself. As the core t e m p e r a t u r e rises to about 10 billion d e g r e e s Kelvin, the g a m m a - r a y photons become so energetic that they can split even the iron nuclei. In fact, at these t e m p e r a t u r e s , photodisintegration runs w i l d a n d every n u c l e u s at the core gets d e m o l i s h e d into protons a n d neutrons; in fractions of a second, the photons u n d o w h a t took m i l l i o n s of y e a r s to build t h r o u g h fusion. T h e once a l m i g h t y star is being r e d u c e d to n u c l e a r rubble from the inside out. P a r a d o x i c a l l y , photodisintegration costs e n e r g y ; it's not easy to b r e a k d o w n tightly bound nuclei into their constituents. T h i s u s a g e of e n e r g y cools off the core, further r e d u c i n g its t h e r m a l pressure a n d o p e n i n g the w a y for m o r e g r a v i t a t i o n a l compression by its outer l a y e r s ; the star's heaviness becomes u n b e a r a b l e . Electron d e g e n e r a c y pressure, so useful for lighter stars, is helpless here; as the s q u e e z i n g mounts, protons a n d electrons a r e s m a s h e d together w i t h such force that they m o r p h into neutrons a n d neutrinos: proton + electron â&#x20AC;&#x201D;> neutron + 182

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neutrino. T h i s furious particle m u t a t i o n at the core happens in less than one second! T h e i m p e r v i o u s neutrinos carry a w a y more energy, a n d the collapse proceeds at an even faster pace. T h e core is now a g i a n t contracting ball of neutrons, w h i c h , because of their zero electric c h a r g e , can be s q u e e z e d m u c h closer to each other than electrons can. If for w h i t e d w a r f s the core densities ate on the order of h u n d r e d s of thousands of g r a m s per cubic centimeter (a few tons per cubic inch), for these neutron balls they can reach h u n d r e d s of trillions of g r a m s per cubic centimeter (billions of tons per cubic inch); a m o u n t a i n squeezed into a s u g a r cube! At these incredible densities, even the electrically inert neutrons react; they are s q u e e z e d so close together that, as w i t h electrons for w h i t e d w a r f s , their d e g e n e r a c y pressure finally starts to counterbalance the central core's collapse. T h i s transition from indifference to d e g e n e r a c y pressure is not smooth; the core's outer l a y e r s , still not converted, fall over the d e g e n erate neutrons w i t h a v e n g e a n c e . F u l l of d e g e n e r a c y pride, the neutron core acts as a brick w a l l , r e b o u n d i n g a n y t h i n g that hits it; the rebounding m a t t e r causes a h u g e shock w a v e that propagates o u t w a r d at speeds of 50,000 k i l o m e t e r s per second, one-sixth the speed of light. H e r e two t h i n g s can happen. One possibility is that the e n o r m o u s violence of the shock w a v e literally rips the star apart, s p e w i n g its outer layers into space in w h a t is k n o w n as a core-collapse supernova explosion, a Type II supernova, one of the most spectacular a n d energetic events in the universe. ( W h y Type II? You w i l l find out very soon.) For a few d a y s , a supernova m a y shine brighter than its host g a l a x y , w i t h billions of stars (see figure 30 in insert). T h e e n o r m o u s release of e n e r g y through its outer layers cooks e l e m e n t s heavier than iron; the heavier chemistry of the universe is the s i g n a t u r e of the d y i n g star's last convulsions. A n o t h e r possibility is that the shock w a v e reverses itself on its w a y out and that the outer l a y e r s of the star come back c r a s h i n g d o w n on the core. In this case, not even the n e u t r o n s can hold the star back, a n d the core collapses into a black hole, an object w i t h such intense g r a v i t y that not even light can escape its g r a v i t a t i o n a l g r i p . But this is not a tale only of decay a n d destruction. Five billion years a g o , a nearby supernova

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q u i t e possibly t r i g g e r e d the collapse of the h y d r o g e n cloud that b e c a m e our solar system. T h e explosion s p r i n k l e d our progenitor cloud w i t h the chemical e l e m e n t s needed for life to develop on Earth. Explosions destroy a n d explosions create. T h e blast from a supernova m a y leave behind a h i g h l y compacted neutron ball, w h i c h is a p p r o p r i a t e l y called a neutron star, a star of n u c l e a r matter, w i t h a m a s s c o m p a r a b l e to the Sun's but w i t h a r a d i u s of only about 10 k i l o m e t e r s . B i z a r r e as they a r e , neutron stars still strike me as the last bastions of normalcy, w h e r e physical l a w s , even if in e x t r e m e circumstances, can be applied a n d m e a n i n g be extracted from them. T h e situation c h a n g e s w i t h black holes. Of course, astrophysicists have studied the properties of black holes in g r e a t detail. But in spite of all their success, or because of it, the study of black holes has also raised m a n y questions, w h i c h , as we w i l l see, r e m a i n very m u c h unresolved. Before we p l u n g e into a more detailed study of the properties of neutron stars a n d black holes, let us briefly visit the history of supernovae explosions, a n d u n d e r s t a n d w h y there a r e t w o types of possible supernova blasts, Types I a n d II.

N e w Stars T h a t A r e Old S u p e r n o v a e have baffled sky w a t c h e r s for m i l l e n n i a . C h i n e s e records from 185 C.E. tell of a n e w star in the constellation C e n t a u r u s that shone for t w e n t y months, d i s a p p e a r i n g as m y s t e r i o u s l y as it appeared. A n o t h e r w a s recorded on 393 C.E. Most certainly, the court astrologers v i e w e d these events w i t h m u c h trepidation, as celestial portents of hard times to come. M o d e r n astronomers have identified the r e m n a n t s of these explosions, wisps of g a s r a p i d l y e x p a n d i n g t h r o u g h the interstellar m e d i u m . In 1006, the skies once a g a i n featured a n e w light; this t i m e , not just C h i n e s e but also A r a b i c , E u r o p e a n , a n d Japanese sky g a z e r s recorded the u n u s u a l event, a n e w star that flared in the constellation L u p u s a n d w a s so b r i g h t as to be visible d u r i n g the d a y for months a n d at night for three y e a r s , initially even r i v a l i n g the Moon.

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T h e most famous of all supernovae flared in 1054 in the constellation of T a u r u s . An entry in the records of the Imperial Observatory of P e k i n g reads,

In the first y e a r of the period C h i h h a , the fifth moon, the d a y C h i - c h o u [July 4, 1054], a great star a p p e a r e d a p p r o x i m a t e l y several inches southeast of T i e n - K u a n [i.e., the star Zeta T a u r i ] . After m o r e than a y e a r it g r a d u a l l y b e c a m e invisible.-'

T h i s supernova flared d u r i n g the day for three w e e k s , a n d at night for t w o y e a r s . A cave p a i n t i n g in C h a c o C a n y o n , N e w Mexico, attributed to the A n a s a z i , shows q u i t e clearly the bright star close to the w a n i n g crescent Moon, precisely as it a p p e a r e d in the skies on July 5, s h i n i n g b r i g h t e r than Venus. T h e stellar explosion left a beautiful e x p a n d i n g r e m n a n t k n o w n as the C r a b N e b u l a , first cataloged by the French astronomer C h a r l e s Messier in the 1770s u n d e r the symbol M l â&#x20AC;&#x201D; t h e first object of the Messier catalog of celestial objects. Little did he k n o w that the core of the nebula nested another r e m n a n t of the explosion, a rapidly rotating neutron star discovered only in 1968, spinning at an a m a z i n g thirty revolutions per second (see figure 31 in insert). I m a g i n e an object m a d e of neutrons, not m u c h l a r g e r than M o u n t Everest, w i t h a mass c o m p a r a b l e to the Sun's, a n d spinning a r o u n d its axis thirty times a second! H o w can such an object exist? T w o other historical supernovae w e r e e x t r e m e l y important to the development of W e s t e r n science, those of 1572 a n d 1604. We encounered the supernova of 1572 in chapter 3, the " n e w star" in the constellation of Cassiopeia seen by T y c h o Brahe on the e v e n i n g of N o v e m b e r 11, as he w a l k e d back from his a l c h e m y laboratory. A l t h o u g h the supernova w a s seen by m a n y other European astronomers, it w a s Tycho's detailed q u a n t i t a t i v e a n a l y s i s that conclusively d e m o n s t r a t e d the star to be farther a w a y than the Moon, thus d e l i v e r i n g a severe blow to the Aristotelian v i e w of an u n c h a n g e a b l e cosmos. Johannes epler, the visionary G e r m a n astronomer w h o first formulated the ws of planetary motion, recorded w i t h g r e a t e x c i t e m e n t the super-

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nova of 1604, w h i c h a p p e a r e d conspicuously close to a near-conjuction of M a r s a n d Jupiter; all three celestial objects w e r e located w i t h i n a very small region of the sky, a sign c h a r g e d w i t h the gloomiest astrological significance, w h i c h Kepler a n d m a n y others w e r e q u i c k to point out. In spite of his strong spiritual (and l u c r a t i v e ) attraction to astrology, Kepler w a s very m u c h a m a n of a transitional era, w h e n accuracy a n d precision of astronomical m e a s u r e m e n t w a s first being combined w i t h serious scientific q u e s t i o n i n g r e g a r d i n g the physical causes of natural p h e n o m e n a . As his former e m p l o y e r T y c h o had done before, Kepler e m p h a s i z e d the great distance to the n e w celestial l u m i nary, w h i c h , u n l i k e planets, d i d not move w i t h respect to other stars. It is an interesting irony that, a l t h o u g h supernovae have been historically associated w i t h n e w stars, they a c t u a l l y m a r k the end of a star's e x i s tence, being " n e w stars" that are really old a n d d y i n g . T h e " n e w star" m i s n o m e r is a consequence of observational l i m i t a t i o n s ; w i t h the n a k e d eye or small telescopes, it w a s (and even today still is) hard to realize that the " n e w star" w a s a c t u a l l y there all a l o n g , just not as obviously visible as others because of its distance (and thus faintness) or of obscuration by interstellar dust. D a t i n g back to the first recorded supernovae, almost t w o thousand years of astronomical research w e r e necessary for scientists to reach this conclusion. T h e supernovae described above, created w h e n the shock w a v e from the r e b o u n d i n g core rips apart the outer l a y e r s of the a g o n i z i n g star, are not the only k i n d possible. T h i s e x p l a i n s the " T y p e II" title I used earlier, i m p l y i n g that there must be a T y p e I. For T y p e I supernovae, the e n o r m o u s explosion a c t u a l l y takes t w o p l a y e r s , t w o stars that travel a r o u n d each other in a k i n d of cosmic m e r r y - g o - r o u n d , bound together by their m u t u a l g r a v i t a t i o n a l attraction. T h e s e systems of t w o stars are called binary systems a n d a c t u a l l y m a k e up most of the stars in our g a l a x y ; our S u n is an atypical loner. Let us, then, i m a g i n e a b i n a r y system w h e r e one of the stars became a w h i t e dwarf. If the t w o stars are close together, the w h i t e d w a r f s g r a v i t a t i o n a l pull on its c o m p a n i o n w i l l be so intense that it w i l l be able to suck the m a t t e r in its outer l a y e r s , l i k e a cosmic d r a i n (see figure 32 186

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in insert). T h i s process, k n o w n as mass accretion, is l i k e l y to occur if the c o m p a n i o n star is also near its death, say, w i t h i n its red g i a n t phase. T h e w h i t e d w a r f w i l l a c c u m u l a t e h y d r o g e n a n d h e l i u m i n its outer layer, w h i c h m a y ignite explosively into t h e r m o n u c l e a r fusion, ejecting h u g e a m o u n t s of h y d r o g e n into space. T h e s e explosions, less powerful than supernovae, are k n o w n as n o v a e \ T h e y don't disrupt the stability of the w h i t e d w a r f a n d can a c t u a l l y recur several t i m e s . To a c t u a l l y turn a nova into a supernova, the w h i t e d w a r f must accrete e n o u g h m a t t e r for its mass to g r o w above the critical v a l u e of 1.4 solar masses, k n o w n as the C h a n d r a s e k h a r limit. Above this m a s s , the electron d e g e n e r a c y pressure cannot support the star any longer, a n d the g r a v i tational s q u e e z i n g causes the pressure a n d t e m p e r a t u r e at the core to soar, r e a w a k e n i n g the fusion chain. C a r b o n fuses into heavier e l e m e n t s at a frantic pace e v e r y w h e r e w i t h i n the w h i t e dwarf, c u l m i n a t i n g w i t h an e n o r m o u s explosion of power c o m p a r a b l e to (and sometimes even g r e a t e r t h a n ) the supernovae produced by the collapse of heavy stars. To d i s t i n g u i s h between T y p e I a n d Type II supernovae, astronomers e x a m i n e the spectrum emitted by the e x p l o d i n g objects. Since a Type I starts w i t h carbon fusing into heavier e l e m e n t s , its spectrum w i l l be very poor in h y d r o g e n . T h e opposite is true of T y p e II supernovae, w h o s e h y d r o g e n - r i c h outer layer is d u l y recorded in their spectra.

An A b s u r d Idea T h e attentive reader must have noticed that I m e n t i o n e d a critical mass for w h i t e d w a r f s of 1.4 solar masses without e x p l a i n i n g w h e r e this odd n u m b e r comes from. T h e discovery of the stability limit of w h i t e d w a r f s has a very interesting story, well w o r t h w h i l e retelling here. D u r i n g the 1920s and 1930s, the astronomy scene in E n g l a n d w a s d o m inated by the t o w e r i n g figure of S i r A r t h u r S. Eddington. In his 1925 book

The Internal Constitution of Stars, E d d i n g t o n raised the issue of

the stability of w h i t e d w a r f s , w h i c h , to h i m , w a s a g r e a t mystery. L i k e all astrophysicists in the early 1920s, he believed that the stability of a

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star, any star, despite the g r a v i t a t i o n a l s q u e e z i n g of its outer l a y e r s could be d u e only to the pressure caused by the very hot atoms in its central region. If a star cools d o w n , its heat pressure w i l l decrease a n d the g r a v i t a t i o n a l s q u e e z e w i l l increase. As this h a p p e n s , the star gets s m a l l e r a n d matter at the core gets hotter a g a i n , creating m o r e heat pressure until a n e w state of balance is achieved. T h e n the star starts to cool, and the w h o l e process is repeated. T h i s sequential s h r i n k i n g truly p u z z l e d Eddington; if, for each n e w state of e q u i l i b r i u m , the cooling star is smaller, w i l l there come a point w h e n the star s h r i n k s into " n o t h i n g " a n d d i s a p p e a r s ? E d d i n g t o n refused to accept this radical idea (actually s i m i l a r to w h a t happens w h e n a black hole forms, as we w i l l soon see) a n d offered a ( w r o n g ) w a y out, conjecturing that the only other counterpressure to balance the star is the electric repulsion of the s q u e e z e d adjacent atoms, the s a m e that g i v e s rocks their solidity. T h e problem w i t h Eddington's suggestion is that it r e q u i r e d the stellar matter to behave l i k e rocks, that is, to have r o c k l i k e densities of a few g r a m s per cubic centimeter, w h i c h w a s tens of thousands of times less dense than

white dwarfs

w e r e then

said

to be.

Eddington

expressed his i n d i g n a t i o n w i t h these h u g e densities in strong w o r d i n g : "I think it has g e n e r a l l y been considered proper [ w h e n discussing 3

u l t r a - h i g h - d e n s i t y stars) to a d d the conclusion ' w h i c h is a b s u r d . ' " At the time, astronomers k n e w that the brightest star in the night sky, S i r ius, a c t u a l l y has a very small a n d dense c o m p a n i o n , k n o w n as S i r i u s B. T h e i r m e a s u r e m e n t s indicated that S i r i u s B has a mass e q u i v a l e n t to 0.85 solar masses a n d a r a d i u s of 18,780 k i l o m e t e r s . T h e n u m b e r s today a r e 1.1 solar masses a n d a r a d i u s of 5,500 k i l o m e t e r s , g i v i n g S i r i u s B a m e a n density of three m i l l i o n g r a m s per cubic centimeter, a v a l u e even m o r e " a b s u r d " than that prevalent in Eddington's day. He conceded that the w h i t e d w a r f s w e r e "a c u r i o u s problem a n d one m a y m a k e fan4

ciful suggestions as to w h a t a c t u a l l y will h a p p e n . " A n d fanciful they w e r e indeed. T h e first step t o w a r d the resolution of Eddington's w h i t e d w a r f p a r a d o x w a s put forward by the British astrophysicist R. H. F o w l e r in 1926, in an article titled "On Dense Matter." U s i n g a b r a n d - n e w devel188

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opment in q u a n t u m m e c h a n i c s , Pauli's exclusion principle (see chapter 5), F o w l e r brilliantly pointed out that Eddington's a r g u m e n t for the stability of w h i t e d w a r f s on the basis of classical physics w a s flawed; at the e n o r m o u s densities at the cores of w h i t e d w a r f s , electron d e g e n e r acy pressure, a n d not heat, provides the r e q u i r e d support. F o w l e r conc l u d e d that the physics of the very ^mall had a crucial role in the physics of compact (high density'.+ small size = compactness) astronomical objects, c h a n g i n g forever the course of astrophysical research. F a r a w a y in M a d r a s , India, a teenager n a m e d S u b r a h m a n y a n C h a n d r a s e k h a r , still w o r k i n g t o w a r d his bachelor's d e g r e e , fell in love w i t h this n e w astrophysics. T h e s e w e r e also the d a y s w h e n Einstein's g e n e r a l theory of relativity, a r a d i c a l l y n e w w a y of u n d e r s t a n d i n g g r a v ity, w a s receiving g r e a t a c c l a i m , some of its predictions being confirmed by none other than E d d i n g t o n himself. W h a t could be m o r e exciting for an e m e r g i n g physicist than to w o r k on a topic that m i x e d the t w o great n e w theories of the t i m e ? C h a n d r a s e k h a r ' s d r e a m c a m e true in 1930 w h e n , at the a g e of n i n e teen, he sailed to E n g l a n d to p u r s u e g r a d u a t e studies at C a m b r i d g e University, the h o m e of his idols, F o w l e r a n d E d d i n g t o n . He d i d not waste any t i m e . D u r i n g the e i g h t e e n - d a y trip, he decided to e x p a n d on F o w l e r ' s w o r k ; there w e r e lots of open questions, most notably how the d e g e n e r a t e electrons w o u l d respond to an increase in the s q u e e z i n g caused by a l a r g e r m a s s . W o u l d their d e g e n e r a c y pressure increase so as to counterbalance the m o u n t i n g gravitational c r u n c h ? W a s there a limit to how m u c h g r a v i t a t i o n a l s q u e e z i n g the electrons could t a k e ? T h e a n s w e r s a r e yes a n d yes. As the outer layers of the w h i t e d w a r f s q u e e z e the d e g e n e r a t e electrons, they w i l l respond by m o v i n g faster and faster in their " e x c l u s i v e " cells, k i c k i n g i n d i g n a n t l y against the imposed loss of space. C h a n d r a s e k h a r r e a l i z e d that at densities c o m p a rable to that of S i r i u s B, their velocities w e r e about 57 percent the speed of light. Einstein had d e m o n s t r a t e d in 1905 that no massive body could reach the speed of light, electrons i n c l u d e d . In his special theory of relativity, he predicted that a n y body that is accelerated to speeds close to the speed of l i g h t w i l l respond by increasing its m a s s ; that is, fast

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motion m a k e s things heavier. At 57 percent the speed of light, some c h a n g e s , even if s m a l l , a r e to be expected. T h e fact that we don't notice these effects, because of the very low speeds of e v e r y d a y l i f e â&#x20AC;&#x201D; m u c h s m a l l e r than 300,000 k i l o m e t e r s per secondâ&#x20AC;&#x201D;does not m e a n they are not observed in e x t r e m e situations, for e x a m p l e , in e x p e r i m e n t s i n v o l v i n g h i g h l y energetic particle collisions. In the interior of w h i t e d w a r f s , the extra e n e r g y d u e to all the s q u e e z i n g is partially transformed into mass a n d not into l a r g e r speeds, w i t h catastrophic consequences for the star's stability. Even though at the time there w a s no theory m e r g i n g q u a n t u m m e c h a n i c s w i t h special relativity, C h a n d r a s e k h a r w a s able to d e m o n s t r a t e by m e a n s of a p p r o x i m a t e a r g u m e n t s that w h i t e d w a r f s could not be stable if their masses w e r e above 1.4 solar masses; the extra e n e r g y spent m a k i n g the electrons " h e a v i e r " c o m p r o m i s e d their ability to counteract the g r a v i tational s q u e e z i n g , c a u s i n g the star to i m p l o d e . So, at nineteen, C h a n drasekhar

predicted

that

white

dwarfs

could

not

have

masses

e x c e e d i n g 1.4 solar masses, a truly r e m a r k a b l e result. But C h a n d r a s e k h a r had an uphill battle to fight. He s u m m a r i z e d his results in t w o papers, one about slow d e g e n e r a t e electrons a n d the other about fast electrons and his predicted mass l i m i t , a n d sent t h e m to F o w l e r for publication. F o w l e r published the first one, about normal w h i t e d w a r f s , but w a s suspicious of C h a n d r a s e k h a r ' s results for relativistic electrons. After a few months, C h a n d r a s e k h a r decided to subm i t his mass limit calculations to the A m e r i c a n publication The Astrophysical journal. T h e paper w a s accepted, but, q u i t e to his shock, nothing else h a p p e n e d after its publication; the astronomical c o m m u nity decided to ignore his results on the critical mass for w h i t e d w a r f s . S i n c e he had to finish his Ph.D., for the next three y e a r s C h a n d r a s e k h a r d e c i d e d to t a k e on less controversial subjects. Doctorate in h a n d , he r e s u m e d the attack. Inspired by some c o l l e a g u e s , he decided to check the masses of ten k n o w n w h i t e d w a r f s to prove that they w e r e all below his critical m a s s limit. It w a s not definite proof, but certainly strong circumstantial e v i d e n c e that his theory w a s consistent w i t h observation. After m a n y d a y s of h a r d w o r k , w h i c h i n c l u d e d using a

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d e r i v e d from his theory, C h a n d r a s e k h a r got w h a t he w a n t e d ; all ten stars had masses below the 1.4 solar mass limit, the masses behaving in excellent a g r e e m e n t with his theory. He w a s now convinced that the astronomical community would have to accept his results. Not so easy! C h a n d r a s e k h a r w a s openly confronted by Eddington in a c h a r g e d night at the Royal Astronomical Society of London, w h e r e , to his surprise, they both*"were scheduled to g i v e presentations; a l t h o u g h Eddington had been following C h a n d r a s e k h a r ' s progress closely (even the computer was his), he never mentioned his o w n w o r k on the subject. After C h a n d r a s e k h a r presented his results, it w a s Eddington's turn. He criticized C h a n d r a s e k h a r ' s w o r k , d e c l a r i n g that "there should be a law of N a t u r e to prevent a star from behaving in 5

this absurd w a y ! " It was very h a r d for him to accept the implications of C h a n d r a s e k h a r ' s results, that collapsing stars could lead to a b r e a k d o w n of the l a w s of physics. His p o i n t — w h i c h w a s w r o n g — w a s that C h a n d r a s e k h a r ' s meshing of relativity and q u a n t u m mechanics w a s incorrect a n d thus so w e r e his conclusions. Eddington proposed his o w n combination of the two theories, with the result, not surprisingly, that w h i t e d w a r f s could be as massive as they h a d to be; they w e r e the corpses of a n y mass star. C h a n d r a s e k h a r instinctively k n e w that Eddingon's ideas were wrong; but w h a t could he do, w h e n the greatest astrophysicist of the time challenged a recent Ph.D.? T h e astronomical c o m m u n i t y , without better g u i d a n c e , listened to authority, something that happens more often in science than it is thought. Controversy is part of e v e r y d a y life for scientists, since skepticism is the only attitude that protects science against fraud a n d c h a r l a t a n i s m . H o w e v e r , m a n y controversies, in the absence of e n o u g h data or observational evidence, are settled on the basis of authority, at least at first. W h a t saves scientists from stubbornly supporting an erroneous theory is the continuous scrutiny to w h i c h ideas, even those accepted as correct, a r e submitted. Sooner or later, an idea initially dismissed as w r o n g m a y e m e r g e as the correct explanation, even though this m a y sometimes t a k e decades. T h i s w a s the case with C h a n d r a s e k h a r ' s mass limit. C h a n d r a s e k h a r

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w a s so burned out by the w h o l e episode that he d i d not w o r k on w h i t e d w a r f s for thirty y e a r s . In 1982, he w a s a w a r d e d the Nobel P r i z e in physics for his r e m a r k a b l e contributions to a s t r o p h y s i c s , a n d in particu l a r for his prediction of a critical mass for w h i t e d w a r f s . Scientists, b e i n g only h u m a n , m a k e m i s t a k e s . F o r t u n a t e l y , w e have our coll e a g u e s to set us straight, a process that is as necessary as it is painful.

The Last Bastions of Normalcy T h e implications o f C h a n d r a s e k h a r ' s m a s s l i m i t w e r e clear; i f w h i t e d w a r f s cannot have masses above 1.4 solar m a s s e s , w h a t h a p p e n s w h e n the stellar core is m o r e m a s s i v e than t h a t ? T h e r e a r e countless stars m u c h more massive than the S u n , up to ten or t w e n t y t i m e s m o r e m a s sive. S u r e l y their collapse a n d supernova d e t o n a t i o n w o u l d leave a core heavier than 1.4 solar m a s s e s . We have seen that a n o t h e r state of matter is expected to e m e r g e as the core pressure m o u n t s b e y o n d the w h i t e d w a r f limit, w h e n protons a n d electrons a r e s q u e e z e d together into neutrons at densities of h u n d r e d s of t r i l l i o n s of g r a m s per cubic centimeter. T h e s e neutron stars w e r e first c o n j e c t u r e d by the controversial (and brilliant) a s t r o n o m e r F r i t z Z w i c k y i n collaboration w i t h W a l t e r B a a d e , both w o r k i n g at C a l t e c h in the m i d - t h i r t i e s , the t i m e of the C h a n d r a s e k h a r - E d d i n g t o n confrontation. S i m i l a r i d e a s w e r e also proposed by the great R u s s i a n physicist Lev L a n d a u , e v e n before Z w i c k y ; it is said that L a n d a u t h o u g h t of the possibility of n e u t r o n starsâ&#x20AC;&#x201D;or at least neutron cores in s t a r s â&#x20AC;&#x201D; t h e s a m e d a y that C h a d w i c k announced the discovery of the n e u t r o n in 1932. B u t L a n d a u d i d not publish his results until 1937, a n d then as a d e s p e r a t e a t t e m p t to h e l p h i m escape the w i d e s p r e a d p u r g e s u n d e r w a y in S t a l i n ' s r e g i m e . U n f o r t u n a t e l y , his plan did not w o r k ; he w a s imprisoned the next y e a r , a n d subjected to horrible t r e a t m e n t until his release in 1939. A l t h o u g h L a n d a u did recover from his y e a r in prison, to the point of d o i n g pioneer w o r k in l o w - t e m p e r a t u r e physics a n d r e c e i v i n g a Nobel P r i z e for it, he was

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never the same a g a i n ; if Stalin's K G B failed to destroy his g e n i u s , it surely destroyed his spirit. In a classic paper from 1934, Z w i c k y a n d B a a d e proposed that supernovae a r e astronomical objects different from novae, a n d that they flare d u r i n g a star's transformation into a neutron star. T h e y even coined the n a m e neutron star. However^ prescient Z w i c k y a n d B a a d e w e r e , they left the crucial question u n a n s w e r e d : If w h i t e d w a r f s have a mass limit, w h a t about neutron stars?. A n d if neutron stars have a mass limit, w h a t h a p p e n s next? T h e transition from d e g e n e r a t e electrons to d e g e n e r a t e neutrons w a s far from trivial. W h e r e a s electrons interact by their electric repulsion, neutrons interact by the strong nuclear force, w h i c h posed several theoretical difficulties at the time a n d , to a certain extent, even today. Nevertheless, t w o other C a l t e c h physicists had e n o u g h k n o w l e d g e of n u c l e a r physics to develop, at least a p p r o x i mately, the basic theory behind neutron stars: R i c h a r d C h a c e T o l m a n and J. Robert O p p e n h e i m e r . T h e latter w a s to become the w e l l - k n o w n leader of the M a n h a t t a n Project, w h i c h built the A m e r i c a n a t o m i c bomb a few y e a r s later. As w a s his style, O p p e n h e i m e r enlisted a student, George Volkoff, to w o r k out the details. H i s approach w a s q u i t e logical; since they did not k n o w m u c h about h o w the strong n u c l e a r force acts on u l t r a d e n s e neutrons, they should first obtain the critical mass l i m i t for neutron stars a s s u m i n g that there is no strong force at all. T h e y found that, in e absence of strong n u c l e a r forces, neutron stars do indeed have a critical mass, a n d only of 0.7 solar masses. T h i s w a s q u i t e an u n e x pected result. After a l l , if neutron stars have a mass l i m i t , the idea of black holes h a d to be t a k e n q u i t e seriously; a star that had a neutron core heavier than the critical limit w o u l d k e e p on collapsing, w i t h nothing to stop it from its t e r m i n a l implosion. In c a m e T o l m a n , w i t h his estimates of how neutrons should behave if the strong n u c l e a r force between t h e m w a s either h i g h l y attractive or repulsive. Once these t w o hypothetical l i m i t s w e r e factored in, the o r i g i n a l estimate c h a n g e d . But not by m u c h . T h e trio c o n c l u d e d â&#x20AC;&#x201D; c o r r e c t l y , we now k n o w w i t h con-

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fidence—that neutron stars do have a critical mass between half and a few solar masses. After fifty y e a r s of observations, a n d w i t h m u c h m o r e sophisticated k n o w l e d g e of the strong nuclear force, astrophysicists have concluded that neutron stars do have a l i m i t between 1.5 and 3 solar masses, not far from the O p p e n h e i m e r - T o l m a n - V o l k o f f prediction. W h a t we still do not u n d e r s t a n d is w h y the observed neutron stars, now in the h u n d r e d s , all seem to have a r o u n d 1.4 solar masses. It w a s only in 1967 that neutron stars w e r e first observed. Jocelyn Bell (now Bell B u r n e l l ) w a s w o r k i n g t o w a r d her Ph.D. at C a m b r i d g e University, u n d e r the supervision of A n t h o n y H e w i s h . H e r research consisted of developing and testing radio telescopes d e s i g n e d to m e a s u r e emissions from distant r a d i o sources. Just as the S u n radiates strongly in the visible and infrared parts of the e l e c t r o m a g n e t i c spect r u m , different astronomical sources m a y r a d i a t e m a i n l y at different frequencies, r a n g i n g from the low-frequency (and l o w - e n e r g y ) radio w a v e s t o the h i g h - f r e q u e n c y (and h i g h - e n e r g y ) g a m m a rays. T h u s , w e m a y "see" sources that are a c t u a l l y invisible to our eyes t h r o u g h specially d e s i g n e d telescopes, sensitive to different k i n d s of radiation. Jocelyn's task w a s e x t r e m e l y tedious—to m a n u a l l y check h u n d r e d s of meters of tape w i t h the recorded d a t a from the radio telescope, w h i c h g a v e the intensity of the received radiation as time w e n t by. Research is not a l w a y s — i n fact, s e l d o m — g l a m o r o u s , often involving detailed a n a l y s i s of h u g e a m o u n t s of d a t a , or fixing m i s t a k e s in computer p r o g r a m s thousands of lines long, or p l o w i n g through pages and pages of calculations. But it is all (almost a l w a y s ) w o r t h w h i l e in the end, w h e n results are finally o b t a i n e d — e s p e c i a l l y results such as those obtained by Jocelyn. S h e found an i n t r i g u i n g signal in her d a t a , a peak in the intensity of received radiation, that w a s repeated w i t h incredible accuracy every 1.33730133 seconds, s o m e w h a t l i k e a very steady heartbeat in an e l e c t r o c a r d i o g r a m . T h e radio source w a s e m i t t i n g pulses of radiation as r e g u l a r l y as the ticktack of a clock! Not k n o w i n g w h a t these w e r e , Bell B u r n e l l and H e w i s h decided to n a m e the object L G M , for Little Green M e n ; after all, astronomical sources w e r e believed to be l a r g e , g e n e r a t i n g radiation of low frequency, typical of slowly mov-

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ing matter. T h e possibility that a source could be at once so r e g u l a r a n d fast t r i g g e r e d the i m a g i n a t i o n of the astronomical c o m m u n i t y a n d the popular press, a n d the Little Green M e n hypothesis w a s taken q u i t e seriously by a n u m b e r of people. M a y b e the s i g n a l s w e r e indeed prod u c e d by a distant intelligent c i v i l i z a t i o n , e a g e r to establish contact w i t h other intelligences in the universe, y Soon after this s h o c k i n g discovery, Bell Burnell a n d H e w i s h found other rapid sources of r a d i o pulses, w i t h different periods, w h i c h b e c a m e k n o w n as pulsars; so m u c h for the L i t t l e Green M e n hypothesis. T h e theorists F r a n c o Pacini a n d T h o m a s Gold suggested that these a m a z i n g pulse sources w e r e rapidly rotating neutron stars; as planets rotate about their axis, so do stars a n d pretty m u c h e v e r y t h i n g else in the universe. If you recall our discussion of a n g u l a r m o m e n t u m in chapter 3, as a s p i n n i n g c h u n k of m a t t e r contracts, it spins faster. T h i s will also be the case for a rotating massive star u n d e r g o i n g g r a v i t a tional collapse; if the star rotates about its a x i s once every couple of w e e k s , as heavier m a i n - s e q u e n c e stars are observed to do, by the t i m e its core s h r i n k s into a neutron core 20 k i l o m e t e r s in d i a m e t e r it w i l l rotate a few t i m e s per second or faster. No e a r t h l y m a t t e r could w i t h stand the e n o r m o u s centrifugal forces from this m a d rotation; but ultradense neutrons can, at least up to periods of about a thousandth of a second, or a millisecond. At those incredible rotational velocities not even d e g e n e r a t e neutrons can stay together. Fast rotation is a n a t u r a l outcome of the w a y neutron stars a r e formed. But w h a t about the r a d i o pulses? T h e y are g e n e r a t e d by m a t ter b e i n g s w i r l e d a r o u n d in the h i g h l y focused a n d intense m a g n e t i c fields that are also produced d u r i n g the formation of neutron stars. O r d i n a r y stars, along w i t h m a n y planets, i n c l u d i n g Earth, have l a r g e scale m a g n e t i c fields. Just as the star's mass density at the core increases with its collapse (same a m o u n t of mass in a s m a l l e r v o l u m e i m p l i e s larger d e n s i t y ) , so does its m a g n e t i c field; as the star collapses, its m a g netic field is also s q u e e z e d into a s m a l l e r v o l u m e , r e a c h i n g e n o r m o u s values. Contraction a m p l i f i e s not only the g r a v i t a t i o n a l pull of a star but also its m a g n e t i c field; we m a y t h i n k of contraction as a focusing

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m e c h a n i s m for the m a g n e t i c field, just as w h e n we focus light u n d e r a microscope, b r i n g i n g the rays "closer together" for a better i m a g e . T h e resulting fields are a m a z i n g ; the typical m a g n e t i c field of a neutron star is trillions of times l a r g e r than Earth's field. A person w e a r i n g a belt w i t h a steel b u c k l e close to a pulsar w o u l d be flung a w a y at supersonic speeds. (If he got too close to the star, he w o u l d also be flattened thinner than a sheet of paper by the h u g e g r a v i t a t i o n a l pull at a neutron star's surface. Not a very hospitable e n v i r o n m e n t . ) M o r e often than not, the m a g n e t i c field is not a l i g n e d w i t h the rotation axis of the star. An e x a m p l e of a m i s a l i g n m e n t between t w o " a x e s " familiar to us is a lighthouse, w h e r e the h i g h l y focused b e a m of light m a k e s a right a n g l e w i t h the rotation a x i s , w h i c h points up. In fact, the lighthouse a n a l o g y is e x t r e m e l y useful w h e n p i c t u r i n g a pulsar; substitute for the beam of light the m a g n e t i c field axis, a n d you have a rapidly rotating m a g n e t i c field, a l t h o u g h for pulsars the a n g l e s are s m a l l e r than 90 d e g r e e s (see figure 33 in insert). Electrically c h a r g e d particles left over from the parent star a r e accelerated to very high e n e r g i e s by the rotating m a g n e t i c field, e m i t t i n g radiation w i t h i n a n a r r o w cone a l i g n e d w i t h the m a g n e t i c field axis, just l i k e light c o m i n g out from a lighthouse. If we happen to be at the right spot, this radiation w i l l pass by us as the m a g n e t i c axis sweeps across the cosmos. A n d if a r a d i o telescope (or other telescopes, l i k e x-ray, g a m m a - r a y , a n d even sometimes optical) points in the direction of the b e a m , it w i l l detect r e g u l a r pulses every t i m e the b e a m sweeps its surface. T h i s m e c h a n i s m correctly explains the m y s t e r y of the Little Green M e n ; some pulsars are so incredibly accurate that their period is predicted to c h a n g e by only a few seconds over m i l l i o n s of y e a r s , w h i c h m a k e s them far m o r e accurate than the best a t o m i c clocks on Earth. T h e r e have been only a few direct associations b e t w e e n pulsars and supernova r e m n a n t s , most notably the one that flared in the C r a b Nebula in 1054, six thousand l i g h t - y e a r s from Earth (see figure 31 in insert). A s t r o n o m e r s using both radio a n d optical telescopes identified a very fast pulsar there, s p i n n i n g thirty-three times per second, faster than our eyes can detect. T h e same w a y that the mass d e t e r m i n e s the 196

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fate of a star, the m a g n i t u d e of the m a g n e t i c field d e t e r m i n e s the fate of a pulsar; the pulsar slows d o w n as it dissipates e n e r g y through its rotation, and it practically stops after a few tens of m i l l i o n s of y e a r s . T h u s , the fastest pulsars a r e also the youngest, as is the case of the C r a b pulsar, not even one thousand years old. T h e relatively short life span of pulsars m a y e x p l a i n the lack of more matches between the h u n d r e d s of k n o w n supernova r e m n a n t s a n d t h e h u n d r e d s of k n o w n pulsars; the pulsars a r e a l r e a d y " e x t i n g u i s h e d , " turned into practically motionless neutron stars. Or they w e r e never formed d u r i n g the violent supernova explosion, w h i c h either tore the neutron core apart or created a black hole instead. A n o t h e r explanation is that the pulsars m a y very well be there, but b e a m i n g at a n g l e s that s i m p l y miss us. Finally, supernova explosions m a y not be perfectly spherical; if there is a small a s y m m e t r y in the core collapse, the neutron star m a y recoil from the center of the explosion at speeds of several h u n d r e d k i l o m e t e r s per second. T h e supernova r e m n a n t a n d the pulsar w o u l d then be at odd locations, m a k i n g it hard for astronomers to m a t c h the t w o . T h e r e are m a n y things we still do not u n d e r s t a n d about neutron stars a n d pulsars. N e u t r o n matter at the core of a neutron star m a y assume forms that are at the e d g e of our present u n d e r s t a n d i n g of h i g h - e n e r g y particle physics. T h e r e have been several e x t r e m e l y interesting proposals to model matter at densities of thousands of trillions of g r a m s per cubic centimeter, but they r e m a i n speculative; it is very hard to test physics at these densities! In any case, the question of most interest to us, w h e t h e r neutron stars have a critical mass, w a s a n s w e r e d approximately by T o l m a n , O p p e n h e i m e r , a n d Volkoff just before the Second W o r l d W a r and w i t h m u c h h i g h e r precision over the past t w o decades; neutron stars do have a critical m a s s s o m e w h e r e b e t w e e n 1.5 and 3 solar masses. T h a t m e a n s m o r e massive cores w i l l continue their collapse indefinitely. As the core contracts further, g r a v i t y w i l l g r o w progressively stronger, in a r u n a w a y process that will create the most fascinating object in the u n i v e r s e , a black hole.

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Gravity Redefined In order to appreciate fully the r e m a r k a b l e properties of black holes, we must explore the physics that dictates their behavior. Before we p l u n g e into our o w n journey of exploration, we need to spend some t i m e p r e p a r i n g for the trip. T h e h u r r i e d r e a d e r m a y s k i p to the last p a r a g r a p h , w h e r e I s u m m a r i z e , very briefly, the m a i n lessons that must be l e a r n e d . H o w e v e r , I suggest that you all t a k e a d e e p breath a n d stick w i t h me for the next few pages, for the price is w e l l w o r t h p a y i n g . L a t e in 1915, after s t r u g g l i n g w i t h confusion, exciting results, a n d false starts on a n d off for eight y e a r s , A l b e r t Einstein, then only thirtysix years old, published his w o r k on a n e w theory of g r a v i t y k n o w n as 6

the g e n e r a l theory of relativity. T h e theory m a r k e d a s h a r p d e p a r t u r e from the then u n i v e r s a l l y accepted explanation of g r a v i t a t i o n a l attraction, proposed by N e w t o n in 1687. In the N e w t o n i a n theory, the g r a v i tational attraction b e t w e e n t w o bodies acted "at a distance," t h r o u g h a force proportional to the product of their masses a n d inversely proportional to the s q u a r e of their distance. N e w t o n w a s n ' t clear about h o w t w o bodies could interact w i t h o u t touching, or w h a t m y s t e r i o u s m e c h a n i s m w i t h i n the bodies g e n e r a t e d the attraction; he preferred to "feign no hypotheses," b e i n g q u i t e content w i t h h a v i n g d e v e l o p e d a framew o r k that could describe q u a n t i t a t i v e l y the motions of celestial a n d terrestrial objects u n d e r the influence of g r a v i t y . I w o u l d l i k e to pause for a second a n d m a k e an i m p o r t a n t r e m a r k about the structure of physical theories. M a n y people, inside a n d outside the classroom, have expressed to me their suspicions about N e w ton's " e x p l a n a t i o n " of g r a v i t y . " R e a l l y , " they say, " h o w can physicists c l a i m they understand g r a v i t y if they don't even k n o w w h y massive bodies attract each o t h e r ? " W e l l , we m a y not u n d e r s t a n d why, but we u n d e r s t a n d how. A n d to build a description of the physical w o r l d it is not critical to u n d e r s t a n d the w h y s of n a t u r a l p h e n o m e n a ; the how is good e n o u g h . It is i m p o r t a n t to k e e p in m i n d that scientific e x p l a n a tions are built to describe the p h e n o m e n a w e observe in n a t u r e , after w e

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m a k e certain assumptions about w h a t it is we w a n t to describe. L i k e all that we create, science has its limitations, w o r k i n g q u i t e well w i t h i n its set b o u n d a r i e s . T h i s does not m e a n that questions such as the o r i g i n of the g r a v i t a t i o n a l attraction of masses are outside the scientific d i s course; m a y b e one day we w i l l u n d e r s t a n d it. But it does m e a n that w h i l e we don't, we can still m a k e plenty of progress u n d e r s t a n d i n g how g r a v i t y shapes the natural vvorjd. Newton's theory w a s built upon a rigid definition of space and t i m e ; in order to describe physical p h e n o m e n a , he a s s u m e d that space a n d t i m e a r e absolute entities, indifferent to the presence of an observer. T h i s m e a n s that we can picture absolute space as the a r e n a w h e r e things happen, a sort of stage for natural phenomena; as we observe them unfolding, the stage r e m a i n s the same. M o r e specifically, it m e a n s that people m o v i n g at different velocities m e a s u r e the same distances between two pointsâ&#x20AC;&#x201D;say, the length of a stickâ&#x20AC;&#x201D;irrespective of their relative velocity. (For now, we limit the discussion to constant velocities.) "Not so," Einstein w o u l d have said. T h e reason w h y we think the two m e a s u r e m e n t s g i v e the same result is that our motions are very slow c o m p a r e d w i t h the speed of light. Consider the following situation: one person is in a train m o v i n g at constant velocity w i t h respect to another person s t a n d i n g at a station. Inside the train, there is a 1-meterlong stick, as m e a s u r e d by the person in the train. W h a t w o u l d the person at the station m e a s u r e ? A shorter stick! T h e faster the train moves, the shorter the stick will appear to be, especially for speeds close to the speed of light in empty space, 300,000 k i l o m e t e r s per second. T h u s , a c c o r d i n g to Einstein, a n d this he k n e w a l r e a d y in 1905, observers in relative motion will m e a s u r e different lengths; space is not as rigid as N e w t o n thought. T h e same holds for the passage of time. If the t w o observers in relative motion a r e m e a s u r i n g the t i m e interval for a p a r t i c u l a r event, they will also get different results. As an illustration, consider a g a i n t w o observers, one at a train station a n d the other t r a v e l i n g in the train at some constant speed close to the speed of light. ( T h i s is an i m a g i n a r y train, of course!) T h i s very fancy train has a glass ceiling a n d is 199

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Two observers, one at a train station ( A ) and one traveling in a train (B)

moving at constant velocity, measure the time it takes for the light from two flashlights to reach them. Whereas for the observer at the station the two flashlights flash simultaneously, the observer in the train sees the front light pulse before the one from the back^.

e q u i p p e d w i t h t w o l a r g e flashlights fastened to its top at opposite ends so as to face each other, as in figure 16. T h e glass c e i l i n g a l l o w s the passenger inside to see w h a t is h a p p e n i n g at the top; she sits exactly at their m i d p o i n t . T h e flashlights w e r e d e s i g n e d to flash together once every so often. H e r e is the e x p e r i m e n t : as s h o w n in the figure, as the train approaches the station, the t w o flashlights flash together precisely w h e n the observer on the platform is at their midpoint. He m a k e s a note of h o w long it took for the t w o flashes to hit h i m , each c o m i n g from one side, a n d he concludes they took the s a m e exact t i m e ; for h i m , the flashing w a s s i m u l t a n e o u s . "Not so," says the passenger in the t r a i n . "I also m e a s u r e d the t i m e it took for the t w o flashes to hit me, a n d conclude that the flash from the front took a shorter t i m e to hit m e . " Now, this is q u i t e unexpected; t w o events that are s i m u l t a n e o u s for one observer a r e not so for another! T h i s e x p e r i m e n t has very broad implications; w e describe natural p h e n o m e n a by m e a s u r i n g h o w they behave in space a n d t i m e , that is, 200

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w h e r e a certain object is w h e n . If m e a s u r e m e n t s d e p e n d on the relative motion of observers, unless we establish a set of rules so that they can c o m p a r e their results, we w o u l d n ' t be able to m a k e head or tail of the natural w o r l d , at least for things m o v i n g close to the speed of light. T h i s " d i c t i o n a r y " is precisely w h a t Einstein provided in 1905, w i t h his special theory of relativity. His conclusions a r e a consequence of a s s u m i n g that the speed of light is the fastest speed possibleâ&#x20AC;&#x201D;the fastest speed at w h i c h information can t r a v e l â&#x20AC;&#x201D; a n d that it is a l w a y s the same, even if it is emitted by a m o v i n g source. T h i s is w h e r e light differs from other objects; if y o u are in a car at 60 m i l e s per hour a n d throw a ball at 20 miles per hour in the same direction you are m o v i n g , the ball's speed will be 80 m i l e s per hour w i t h respect to the g r o u n d . But not light; it w i l l a l w a y s move at the same speed! If one could travel faster than light, w h i c h one can't, our nicely ordered reality where causes precede effects w o u l d go up in smoke; we could, in principle, travel b a c k w a r d in time, toward the past. T h e bizarre effects of length contraction and loss of simultaneity are consequences of this limiting speed. Another effect is "time dilation," the fact that fastmoving clocks tick slower. If the passenger in the train had a clock with her, the observer on the platform w o u l d "see" it ticktock slower. "But how do you k n o w this really h a p p e n s ? " you m a y justifiably ask. Because the predictions from special relativity have been confirmed over and over again in laboratory experiments and in observations involving fast-moving elementary particles, w h e r e the speed of light is not such an enormous speed. Our perceptions of the natural world, i m m e r s e d as they are in our slow-moving everyday reality, are quite myopic. Revolutionary as the predictions of special relativity w e r e , Einstein k n e w they w e r e just the tip of the iceberg. After all, most motions we observe in n a t u r e do not happen at constant speeds but involve c h a n g ing speeds, that is, acceleration. S o m e h o w , a complete theory of relativity should be able to incorporate accelerated motion. At this point enters w h a t Einstein dubbed "the happiest thought" of his life, w h i c h occurred to h i m as early as 1907. Lost in contemplation one day, he asked h i m s e l f an a p p a r e n t l y innocent question, w h i c h I p a r a p h r a s e : 201

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" I m a g i n e a person falling from a h u g e height. As she is falling, she feels w e i g h t l e s s ; falling e l i m i n a t e s the force of g r a v i t y ! " Let me elaborate on this i m a g e t h r o u g h a m o r e concrete e x a m p l e , a trip d o w n a fast elevator in a h i g h - r i s e . You get in on the top floor, squeezed a m o n g l a w y e r s , stockbrokers, and office staff. Feet planted on the floor, you feel your full w e i g h t , as Earth pulls you d o w n . ( A n d you pull Earth up, but she doesn't seem to care m u c h . ) As the elevator starts its descent, you feel lighter, a q u e a s y feeling in y o u r stomach; most people are familiar w i t h this experience. S u d d e n l y , you hear a loud noise; the cables snap, and the c r o w d e d box falls vertically d o w n , accelerating at a constant rate. A m i d the s c r e a m i n g , you notice that y o u r feet, a n d that of your fellow passengers, are not touching the g r o u n d any longer; you a r e free-falling w i t h the elevator and feel w e i g h t l e s s , very m u c h l i k e astronauts in a spaceship. Gravity, or at least the feel of it, is gone. (Of course, g r a v i t y is w h a t is m a k i n g you and the elevator fall at the same rate.) Fortunately, w h e n the elevator reaches the tenth floor, the e m e r gency b r e a k i n g system k i c k s in; e v e r y o n e hits the g r o u n d w i t h a v e n g e a n c e , feeling m u c h heavier for a few floors, until the elevator slows d o w n to its comfortable constant speed by the second floor. Your n o r m a l w e i g h t is back and all e n d s w e l l . T h i s d r a m a t i c experience illustrates h o w acceleration can m i m i c the effects of gravity. If the elevator is not m o v i n g (or m o v i n g at a constant speed), Earth pulls you d o w n and the floor pulls you " u p " by the s a m e a m o u n t a n d you feel y o u r n o r m a l w e i g h t . As the elevator begins its descent, it m u s t accelerate from rest, that is, w i t h respect to Earth; this results in a decrease of the net force that you (your feet) feel and thus a decrease in y o u r perceived w e i g h t . T h e l a r g e r the d o w n w a r d acceleration of the elevator, the l i g h t e r you feel; it is as if y o u r feet could not k e e p up w i t h the elevator's floor. W h e n the elevator free-falls, the " u p " force effectively d i s a p p e a r s , a n d you feel w e i g h t l e s s , free-falling w i t h the elevator. T h e situation is reversed w h e n the b r a k e s start to slow d o w n the elevator; the floor pushes h a r d against your feet, and you feel heavier. Reasoning a l o n g s i m i l a r lines, Einstein c o n c l u d e d that it is not pos202

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sible a priori to truly distinguish between accelerated motion and the acceleration caused by g r a v i t y ; inside the elevator, the c h a n g e s in acceleration feel just l i k e c h a n g e s in w e i g h t , w h i c h is how y o u r body (your total mass) responds to Earth's gravity. H a d you not k n o w n that you w e r e in an elevator on Earth, you could have a r g u e d that some m a d scientist w a s toiling w i t h Earth's g r a v i t y , m a k i n g it w e a k e r and stronger at w i l l . T h e fact that accelerated motion can m i m i c g r a v i t y is k n o w n as the principle of equivalence. T h u s , a g e n e r a l

theory of rela-

tivity that incorporates accelerated motion m u s t also be a theory of gravity. By the t i m e Einstein a r r i v e d at the final formulation of his theory, it had t a k e n a surprising turn; he r e a l i z e d that there w a s a n e w w a y of t h i n k i n g about g r a v i t y , q u i t e different from the action at a distance that c h a r a c t e r i z e d Newton's theory. To Einstein, the g r a v i t a t i o n a l pull a massive body exerts on others could be e x p l a i n e d as a c u r v a t u r e of space a r o u n d the body; the more massive the body, the m o r e accentuated the c u r v a t u r e a r o u n d it and thus the stronger its g r a v i t a t i o n a l pull. T h i s can be illustrated by a t w o - d i m e n s i o n a l a n a l o g y , w h i c h , although l i m i t e d , has its merits. I m a g i n e a perfectly flat mattress. If you throw small m a r b l e s on it, they will move in straight lines at con-

ic u R E

1 7 : Illustration depicting a mattress, which is deformed by a heavy ball placed

> its surface. Notice how the trajectories of two marbles that pass by the ball are different, depending on their proximity to the ball.

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stant speeds (see figure 17). N o w place a l a r g e lead ball at the center of the mattress; the surface of the mattress w i l l curve a r o u n d the ball, the c u r v a t u r e being l a r g e r the closer to the ball. If you t h r o w a m a r b l e close to the ball, its path w i l l no longer be straight, for it will move in a c u r v e d g e o m e t r y ; thus, the motion of the m a r b l e is influenced by the distortion the lead ball causes on the mattress. F u r t h e r m o r e , this d i s tortion w i l l cause the m a r b l e to accelerate. Einstein e q u a t e d this acceleration w i t h the distortion of the geometry, pointing out that, for small e n o u g h masses, even as l a r g e as planets, the motions w o u l d closely m a t c h the predictions of N e w t o n i a n gravity.-In other w o r d s , the effects of c u r v a t u r e predicted by g e n e r a l relativity w i l l be relevant only for very l a r g e masses, such as stars. T h e rigid space of N e w t o n i a n physics w a s gone for good; Einstein a d d e d plasticity to space, w h i c h deformed in response to l a r g e masses placed in it. But space w a s not the only one of the t w o N e w t o n i a n absolutes that g e n e r a l relativity revised. T h e flow of t i m e w i l l also be modified by the presence of l a r g e massive b o d i e s â&#x20AC;&#x201D; t h e m o r e c u r v e d the space, the slower the passage of t i m e . On Earth, this effect is pretty m u c h unnoticeable; the difference between the (slower) flow of time at the floor and at the ceiling of a room (gravity is w e a k e r as you move a w a y from E a r t h ) is less than one part in a thousand trillion. H o w e v e r , close to a very massive star, or a compact object such as a w h i t e d w a r f or a neutron star, the effect is m u c h larger. A n d close to a black hole, time comes practically to a standstill, as we w i l l soon see. T h a t strong g r a v ity slows d o w n the passage of time is not easy to see. H e r e is one illustration of this p h e n o m e n o n , w h i c h , l i k e m a n y others, g i v e s only a g e n e r a l idea of w h a t truly goes on. I m a g i n e a box full of atoms, which a r e e m i t t i n g radiation at a certain frequency as m e a s u r e d on Earth. Say the strongest emission line is a tone of blue. Since frequency is just a count of h o w m a n y w a v e crests pass by a point per second, we can think of the atoms as clocks. N o w you place the box close to a neutron star and go far a w a y before you check the emission lines c o m i n g out of it. M o v i n g far a w a y g u a r a n t e e s that, u n l i k e the a t o m s in the box, you

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are not being strongly influenced by the g r a v i t a t i o n a l field of the star. T h e blue emission line is gone, and you see instead an o r a n g e line. If you place the box closer to the star, the line turns redder. You conclude that the stronger the g r a v i t a t i o n a l field, the more the radiation is shifted t o w a r d the r e d â&#x20AC;&#x201D; t h a t is, the s m a l l e r its frequency and thus its energy. Close to the neutron star, the photons have to w o r k m u c h h a r d e r to escape the stronger gravitational pull, w a s t i n g some of their e n e r g y as they " c l i m b " up, l i k e c h i l d r e n t r y i n g to c l i m b up a slide "the w r o n g w a y " ; the steeper the slide the h a r d e r the c l i m b . T h i s effect is k n o w n as gravitational redshift. Since I a r g u e d that you m a y think of vibrating atoms as clocks, a strong g r a v i t a t i o n a l field decreases the freq u e n c y of its emitted radiation and thus the flow of t i m e ; a "second" m e a s u r e d by a clock h o v e r i n g near the star lasts longer than one m e a s ured far from it, i n d e p e n d e n t l y of the k i n d of clock being used. We can thus i m a g i n e a g r a v i t a t i o n a l field so strong that photons lose all their energy t r y i n g to c l i m b out of i t â&#x20AC;&#x201D; l i g h t gets effectively trapped by the gravitational fieldâ&#x20AC;&#x201D;and t i m e comes to a standstill; this is the exotic reality in the neighborhood of a black hole. Let me s u m up. Einstein e q u a t e d g r a v i t y w i t h the c u r v a t u r e of space a r o u n d a massive body. T h e effect is q u i t e n e g l i g i b l e for light masses, but becomes i m p o r t a n t for massive stars a n d even more so for very compact objects such as neutron stars, w h e r e g r a v i t y ' s pull is one h u n d r e d thousand times stronger than at the Sun's surface. As small masses are placed in the neighborhood of a l a r g e r mass, the distortions of space a r o u n d it w i l l cause their motions to deviate from w h a t is predicted by the N e w t o n i a n theory. A n o t h e r r e m a r k a b l e consequence of Einstein's n e w theory of g r a v i t y is the s l o w i n g d o w n of clocks in strong gravitational fields. T h i s effect is related to the redshifting of emission lines from atoms; photons must w o r k h a r d e r to escape the g r a v i t a tional pull of strong fields, losing e n e r g y and thus decreasing their frequency, according to the predictions of g e n e r a l relativity. In short, strong g r a v i t y bends space a n d slows time. We are n o w ready to explore black holes, the cosmic m a e l s t r o m s .

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D e s c e n t i n t o the M a e l s t r o m : Prologue D a r k w a t e r is o m i n o u s ; I k n o w excellent s w i m m e r s w h o refuse to dive into the d a r k w a t e r s of a l a k e . T h e r e is a sensation of loss of control, of not k n o w i n g w h a t could be u n d e r n e a t h , just w a i t i n g to get you. If the w a t e r s of Loch Ness w e r e crystal clear, there w o u l d be no stories of a monster in them. T h e m y s t e r y is in w h a t you cannot see, but only i m a g i n e . A n d if there is motion in the water, t h i n g s just get worse. Especially if this motion is a w h i r l p o o l , d r a g g i n g e v e r y t h i n g that comes too close to it d o w n its deep, d a r k throat t o w a r d . . . w h o k n o w s w h a t ? M e d i e v a l a n d Renaissance chronicles tell of a g i a n t whirlpool off the coast of N o r w a y , referred to as the gurges mirabilis,

the w o n -

d r o u s a n d terrifying M a e l s t r o m , the umbilicus maris, navel of the sea.

T h e p o w e r o f this whirlpool w a s l e g e n d a r y . S o m e believed the M a e l strom to be the m a i n g a t e controlling the ebbing a n d f l o w i n g of the tides of all oceans, w h i c h w e r e connected by u n d e r g r o u n d tunnels. A t h a n a s i u s Kircher ( 1 6 0 1 - 1 6 8 0 ) , the G e r m a n Jesuit a n d scholar somet i m e s called the "last Renaissance m a n , " described in his Mundus Subterraneus (1665) how "every whirlpool formed a r o u n d a central rock: a g r e a t cavern opened beneath; d o w n this cavern the w a t e r rushed; the w h i r l i n g w a s produced as in a basin e m p t y i n g t h r o u g h a central h o l e "

(see figure 34 in insert). T h i s seventeenth-century description has m a n y of the central e l e m e n t s that will help us v i s u a l i z e w h a t happens near a black hole: w h i r l p o o l s forming a r o u n d a central object; a great cavern into w h i c h m a t t e r rushes; a central hole t h r o u g h w h i c h things seem to disappear. In Norse m y t h o l o g y , as in several others,

the M a e l s t r o m w a s

believed to be the passage to the w o r l d of the d e a d , a tunnel connecting the l i v i n g a n d the nonliving. W h o e v e r v e n t u r e d — o r w a s sucked—• d o w n its monstrous throat w o u l d never come back a l i v e . W e l l , one person d i d come back from the depths of the w h i r l p o o l , the tortured n a r r a t o r of E d g a r A l l a n Poe's short story, "A Descent into the M a e l -

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strom," a m a n w h o s e jet-black h a i r t u r n e d completely w h i t e d u r i n g his horrifying experience. H o w long his p l u n g e into the w h i r l i n g abyss lasted we are not told, because the fisherman's w a t c h stopped t i c k i n g as soon as he crossed the i m a g i n a r y line of no return, the e d g e of the s w i r l i n g current. But he does tell us how terror c h a n g e d to a w e as he faced the stark beauty of the p h e n o m e n o n , a n d r e a l i z e d "how m a g n i f i 1

cent a thing it w a s to die in such" a m a n n e r , a n d how foolish it w a s in me to t h i n k of so paltry a consideration as my o w n i n d i v i d u a l life, in v i e w of so wonderful a manifestation of God's power." After this cathartic revelation, our n a r r a t o r fell prey to an irresistible desire to k n o w w h a t lies beyond the d a r k abyss, a feeling I e q u a t e to the curiosity that drives scientists a n d explorers to probe u n k n o w n w o r l d s , be they part of our physical reality or an i m a g i n a r y one: "After a little w h i l e I became possessed w i t h the keenest curiosity about the w h i r l itself. I positively felt a wish to explore its depths, even at the sacrifice I w a s g o i n g to m a k e ; a n d my principal g r i e f w a s that I should never be able to tell my old c o m p a n i o n s on shore about the m y s t e r i e s I should see." It is t i m e our n a r r a t i v e starts, a fictional descent into a black hole, w h e r e i n some of the physical properties of these s t r a n g e objects w i l l be illustrated w h i l e others w i l l m e r e l y be fantasized.

D e s c e n t i n t o the M a e l s t r o m : A Fantasy In my y o u n g d a y s , w h e n t i m e seemed to be an endless c o m m o d i t y , I used to build spaceships out of scrap parts of old ones. My intent w a s not so m u c h to build a functional interstellar c r u i s i n g m a c h i n e as to pay h o m a g e to the pioneer d a y s of space exploration, w h e n , thousands of years a g o , we first a t t e m p t e d to leave our o r i g i n a l solar system in search of a l t e r n a t i v e h o m e s . In retrospect, we should have listened to the scientists w h e n they forecast the catastrophic consequences of climatic c h a n g e o w i n g to global w a r m i n g . It w a s thus a hobby of m i n e to

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travel from planet to planet l o o k i n g for old spaceship repositories, w h e r e all sorts of bits a n d pieces could be found. On one of my travels in search of a rare gyroscope for a 2180 M a r s L a n d e r , I found "Mr. Strom's Rocket Parts," an e n o r m o u s h a n g a r littered w i t h countless spaceship parts. W h i l e I w a s consulting the store's virtual stock-scann i n g device to search for the gyroscope, Mr. Strom h i m s e l f c a m e to greet m e . I had h e a r d of Mr. S t r o m before, as h a d m a n y other space buffs; he w a s famous t h r o u g h o u t the g a l a x y for c l a i m i n g to be the one w h o c a m e closest to a black hole, a story that, to most people, w a s just that, a story. L i k e m a n y before m e , I a s k e d Mr. S t r o m to tell me his story. As he stared at m e , t r y i n g to read my true intentions, I r e a l i z e d how devastating it must be to have your story discredited by everyone, to be a fors a k e n hero, whose feats no one believes. It w a s no w o n d e r his face looked l i k e a fortress abused by the h a n d s of m a n a n d time, crisscrossed by m a n y w r i n k l e s that told of countless m i s a d v e n t u r e s a n d secret pains. H i s eyes truly m a d e me s h u d d e r â&#x20AC;&#x201D; t w o d a r k pools w h e r e only the deepest sadness could s w i m , t w i n abysses l u r i n g you deeper a n d deeper into oblivion, l i k e m i n i a t u r e black holes. "I w a s c o m m a n d e r of a fleet built to explore the c o m p l e x astrophysical x - r a y source k n o w n as C y g n u s X - l , " he started. " S i n c e the 1970s, over three m i l l e n n i a a g o , this w a s suspected to be a binary star system six thousand l i g h t - y e a r s a w a y from Earth. T h e t w o m e m b e r s of the binary system, thought to be a blue g i a n t star about 20â&#x20AC;&#x201D;30 solar masses a n d a black hole about 7 - 1 5 solar masses, orbited so close together that the black hole frantically s u c k e d matter from its h u g e companion, very m u c h as an e m p t y i n g d r a i n sucks w a t e r into a s p i r a l i n g oblivion. T h i s m a d s w i r l i n g heated the infalling stellar matter to e n o r m o u s temperatures, p r o d u c i n g the x rays observed on Earth by astronomers. Even though observations indicated that the s m a l l e r object of the pair had a mass m u c h l a r g e r than the m a x i m u m mass for neutron stars, it was still not clear w h e t h e r or not it w a s a black hole. Since other attempts to identify it as such h a d failed, the L e a g u e of Planets decided that the only w a y to k n o w for sure w a s to go there.

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" T h e fleet consisted of three vessels, each under the c o m m a n d of a S t r o m , a great honor to my family, dedicated to space travel and exploration as we had been for m a n y centuries. I led the vessel n a m e d CXI, my m i d d l e brother led CX2, and the youngest led CX3. H a d I k n o w n what w a s about to happen, I w o u l d have m a d e sure the three of us w e r e all together in one v e s s e l . . . . I will spare you the details of how the m i s sion w a s prepared, and how, after rrj'any problems with our h y p e r r e l a tivistic plasma drive, we finally arrived to w i t h i n one light-month of our destination. [For fictional purposes, let us assume that, somehow, in three thousand years it w i l l be possible to cover vast interstellar d i s tances in reasonable times.] It w a s a majestic sight the l i k e s of which I had never seen; through our telescopes, we could see an enormous hot blue star, being d r a i n e d of its substance by an invisible hole in space! All you could see w a s the matter s w i r l i n g a r o u n d this very small region, e m i t t i n g all sorts of radiation as it plunged into the d a r k abyss, electromagnetic shrieks of despair if you ask m e . " W e w e r e instructed to fly Indian file toward the black hole, k e e p ing a very l a r g e distance from each other, my youngest brother first, my m i d d l e brother second, a n d I last. We k n e w that, from a distance, a black hole behaves l i k e any other massive object, that the differences predicted by general relativity happen only fairly close to it. We also k n e w that every black hole has an i m a g i n a r y l i m i t i n g sphere around it k n o w n as the 'event horizon,' w h i c h m a r k s the distance from which not even light could escape. T h i s distance is also k n o w n as the S c h w a r z s c h i l d r a d i u s , since it w a s Karl S c h w a r z s c h i l d w h o in 1915, just months after Einstein published his n e w theory of gravity, first came up w i t h a solution to the problem of the gravitational field around a massive body. Every object, celestial or not, if compacted enough, can turn into a black hole w i t h its o w n event horizon; for the good old Sun to turn into a black hole, it has to be s h r u n k into a ball with a r a d i u s of 3 kilometers, its S c h w a r z s c h i l d radius, w h i l e a h u m a n must be s h r u n k to 10 trillion trillionths of a centimeter [ 1 0

2 3

cm]."

Yeah, y e a h , " I said, my impatience b e t r a y i n g my excitement, "I also took intro physics in junior high. Please go on with your story!" 209

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"Very w e l l , " Mr. S t r o m w h i s p e r e d , his dark, eyes inexorably p u l l i n g me in. " M y youngest brother's ship, the CX3, w a s to approach the hole, sending us periodic light flashes of a g i v e n frequency; we w e r e to follow at a distance, m e a s u r i n g the frequency of the radiation emitted by my brother's ship as w e l l as the time interval b e t w e e n the flashes, a n d then c o m p a r e them w i t h the theoretical predictions for g r a v i t a t i o n a l redshift a n d t i m e delay.* T h e three vessels p l u n g e d to a distance of 10,000 k i l o m e t e r s from the hole [by comparison, M e r c u r y is at an a v e r a g e distance of 58 m i l l i o n k i l o m e t e r s from the S u n ) ; w h i l e CXI a n d CX2 hovered at that distance, my brother closed in to 100 k i l o m e t e r s from the hole. He w a s instructed to send us infrared radiation; but we detected only l o w - f r e q u e n c y [invisible] radio w a v e s . T h e g r a v i t a t i o n a l redshift

formula

was

indeed

correct.

Furthermore,

the

intervals

b e t w e e n t w o pulses increased q u i t e perceptibly; t i m e w a s flowing slower for my brother, as v i e w e d from our distant ships. He p l u n g e d to the d a n g e r o u s l y close distance of 10 k i l o m e t e r s from the hole, only 7 from the event horizon; this w a s the closest distance the [fictionally s t u r d y ] ship could stand, g i v e n the e n o r m o u s tidal forces a r o u n d the hole, w h i c h stretch e v e r y t h i n g into spaghetti. A F r e n c h astrophysicist once c o m p a r e d the stretching forces on a person s t a n d i n g up at the event horizon of a 10-solar-mass black hole to w h a t he w o u l d feel if he w e r e h a n g i n g from the Eiffel T o w e r w i t h the entire population of Paris suspended from his a n k l e s .

Ouch! A n y w a y , from that close

orbit, my brother w a s to send pulses of visible light, but all we detected w e r e even l o w e r - f r e q u e n c y r a d i o w a v e s ; we could not see my brother's ship any longer, a n d I started to feel very uneasy about this whole thing. Yes, the theory w a s c o r r e c t â&#x20AC;&#x201D; a ship falling into a black hole will become invisible to people in a more distant ship (us) because of the redshifting of light. T h a t also m e a n t that we w o u l d never be able to

* F o r simplicity, I will take the mass of the black hole in C y g n u s X - l to be 1 solar mass, although we k n o w it to be m u c h larger, s o m e w h e r e a r o u n d 10 solar masses. ( W e are not sure at present, i.e., 2 0 0 2 C.E.)

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actually see a star collapsing into a black hole, since it w i l l become invisible before it meets its end. A related effect w a s the s l o w i n g of time. As my youngest brother approached the black hole, the radiation pulses w e r e a r r i v i n g at increasingly long intervals. T h u s , not only could we not see him a n y m o r e ; we w o u l d also have to w a i t an enorm o u s a m o u n t of time to receive any Vnessage from h i m . T h i s confirmed the prediction that, indeed,.for a distant observer the collapse of a star w o u l d t a k e forever. Of course, for the u n l u c k y traveler w h o freefalls into the black hole, nothing u n u s u a l w i t h the passage of time w o u l d happen, as is e x p l a i n e d by the e q u i v a l e n c e principle; g r a v i t y is neutralized

free

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However,

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stretched." "Yes, I k n o w that," I said, shifting nervously on my chair. "Please do g o on!" "But the theory said nothing about steering a spaceship a r o u n d the enormous g r a v i t a t i o n a l pull of a rotating black hole, w h i c h m a k e s the horrible currents a r o u n d the fabled M a e l s t r o m look l i k e w a v e s in a child's pool. T h e orbital instabilities w e r e c o m p o u n d e d by the enormous a m o u n t of turbulence created by the s w i r l i n g m a t t e r of the blue giant. Still, the radio signals from my brother kept c o m i n g in, albeit at very l a r g e intervals. H i s instructions w e r e to return to us after completing t w o orbits a r o u n d the hole. M o r e than that, and the spaceship would not be able to take the major tidal stresses and the continuous bombardment of material falling into the rotating abyss. Another m e s sage c a m e in, w h i c h we decoded as s a y i n g , ' M S . Bottleâ&#x20AC;&#x201D;End.' My m i d d l e brother, a g r e a t fan of ancient gothic l i t e r a t u r e , understood it i m m e d i a t e l y ; the message w a s a s k i n g us to read the last p a r a g r a p h of Edgar A l l a n Poe's short story ' M S . F o u n d in a Bottle':

Oh, horror upon horror! . . . We a r e w h i r l i n g d i z z i l y , in i m m e n s e concentric circles, round and r o u n d the borders of a g i g a n t i c a m p h i t h e a t r e , the s u m m i t of whose w a l l s is lost in the d a r k n e s s a n d the distance. But little time w i l l be left me to pon-

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d e r upon m y d e s t i n y ! T h e circles r a p i d l y g r o w s m a l l — w e a r e p l u n g i n g m a d l y w i t h i n the g r a s p o f the w h i r l p o o l . . . o h God! And—going down! My youngest brother w a s d r a g g e d in by the furious rotation of space itself a r o u n d the black hole. H o w ironic that w o r d s w r i t t e n in 1833 w o u l d be so true three thousand years later, a r o u n d an object too terrifying even for Poe's i m a g i n a t i o n . " Tears of l o n g i n g a n d helplessness flooded Mr. Strom's d e e p d a r k pools. But they vanished as q u i c k l y as they a p p e a r e d . "I decided to try a n d find my youngest brother. W h o k n o w s ? M a y b e the theory w a s w r o n g , a n d it w a s possible to p l u n g e w i t h i n the event horizon a n d still s o m e h o w escape. T h e theory predicted that, once you cross the event horizon, the only possible m o v e m e n t is straight into the center of the hole, the so-called central s i n g u l a r i t y . No m a t t e r w h a t you do to escape, y o u can't. It is as if inside the event horizon space becomes u n i d i r e c t i o n a l , all roads pointing t o w a r d the center, just as t i m e is unidirectional outside the hole, a l w a y s pointing t o w a r d the future. At the s i n g u l a r i t y , the gravitational pull is infinitely strong. Now, as you k n o w , w h e n e v e r a physical theory predicts that some q u a n t i t y is infinite, it is often also predicting its failure to e x p l a i n w h a t is truly g o i n g on. In a sense, the s i n g u l a r i t y s i g n a l s the b r e a k d o w n of g e n e r a l relativity, a n d is b e g g i n g for s o m e t h i n g else to come in. M a n y physicists believe that this s o m e t h i n g else is q u a n t u m m e c h a n i c s or, better, a theory that w e d s q u a n t u m m e c h a n i c a l ideas to g e n e r a l relativity. T h i s idea a p p e a r e d d u r i n g the twentieth century a n d is still with u s , unproved but on m u c h firmer ground.* Since we k n o w that the behavior of matter at atomic a n d subatomic scales is very different from that at h u m a n scales, we predict that the properties of space and t i m e w i l l also c h a n g e r a d i c a l l y at e x t r e m e l y small scales, m u c h smaller

* I believe and hope that in three thousand years we will k n o w h o w to w e d the t w o theories! H o w e v e r , for d r a m a t i c r e a s o n s — a n d to avoid speculations that will probably be incorrect am assuming the question w i l l still be open that far in the f u t u r e .

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than an e l e m e n t a r y particle. T h i s is w h e r e a theory of quantum gravity comes in, t r y i n g to m a k e sense of w h a t h a p p e n s to space a n d t i m e at e x t r e m e l y small distances. But for us, h u m a n - s i z e black hole explorers, these q u a n t u m effects are of no use; as we p l u n g e in, the g r a v i t a t i o n a l monorail w i l l crash us into w h a t e v e r lies at the core of the hole, a n d we w i l l not be able to d e v i a t e from it, or fcend s i g n a l s outside to tell our story. Unless, of course, the holeSs rotating. A n d this one w a s . " My ears p e r k e d u p , for I sensed that s o m e t h i n g wonderful w a s about to be revealed to m e . I k n e w that the c r u s h i n g central s i n g u l a r i t y existed only for static black holes, those that do not rotate. Since rotation is u b i q u i t o u s in the universe, most black holes w o u l d form from rotating stars a n d w o u l d also rotate. Now, I also k n e w that rotation has the effect of p r y i n g the central s i n g u l a r i t y open, distorting it into a ring.

D u r i n g the t w e n t i e t h century, physicists conjectured t h a t â&#x20AC;&#x201D;

m a t h e m a t i c a l l y , at l e a s t â&#x20AC;&#x201D; i t w a s possible to construct solutions of Eintein's e q u a t i o n s w h e r e a rotating black hole t e r m i n a t e s not in a central i n g u l a r i t y but in a throat connecting to a w h i t e hole, its exact opposite! W h a t the black hole t a k e s in, the w h i t e hole spits out. If they exist, w h i t e holes w o u l d be sources of m a t t e r a n d radiation at another point f the universe or even another universe, a l t h o u g h here fanciful specuations blur proper j u d g m e n t more a n d more; try as we w a n t , not every o m p e l l i n g l y beautiful m a t h e m a t i c a l result has a place in n a t u r e . T h e xistence of w h i t e holes has r e m a i n e d u n c l e a r for all these y e a r s , as no onclusive proof or observation has been offered so far. T h e throat conecting a black hole to a w h i t e hole ( m a y b e a " w h i t e source" w o u l d be more accurate, if less a p p e a l i n g , n a m e ) is called an Einstein-Rosen ridge or, w h e n connecting t w o different points in our universe, a ormhole (see figure 18). In fact, back in the 1980s, the astronomer arl S a g a n w r o t e a novel, Contact, in w h i c h the heroine c o m m u n i c a t e d ith an a d v a n c e d alien intelligence t h r o u g h w o r m h o l e connections. Inspired by this a n d other stories, we have spent three thousand y e a r s looking for w o r m h o l e s a n d t r y i n g to figure out h o w to k e e p their mouths open; for the theory also says that k e e p i n g a w o r m h o l e m o u t h n requires very exotic k i n d s of matter, w h i c h exert a sort of "anti-

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g r a v i t y " effect on o r d i n a r y matter. C o u l d Mr. Strom's experience shed some light on this theoretical impasse? "I decided to risk my life a n d go into the m a e l s t r o m after my brother," Mr. S t r o m continued. " M y m i d d l e brother w a i t e d in a safe distant orbit a r o u n d the black hole. He w a s to report to base w i t h i n three d a y s , irrespective of w h a t h a p p e n e d to m e . As I p l u n g e d into the hole, the w h i r l i n g of space d r a g g e d me in the w a y w a t e r d r a g s a ship into a whirlpool.* T h e combination of e n o r m o u s g r a v i t a t i o n a l pull a n d furious b o m b a r d m e n t of radiation a n d particles took a toll on my ship; but its fuselage m i r a c u l o u s l y â&#x20AC;&#x201D; w h a t else could it be but a m i r a c l e ? â&#x20AC;&#x201D; s u r v i v e d , as I d i d , t h a n k s to the once controversial a n t i c r u n c h shield. I couldn't see a thing, but my brain w a s p r o d u c i n g all sorts of i m a g e s a n d sounds, as if all my neurons decided to fire in unison. I s a w my past life a n d w h a t I believed w o u l d be my future life. But this i m a g e bifurcated into countless others, each spun by a choice I m a d e a l o n g the w a y , possible destinies w r a p p e d into an instantaneous g l i m p s e . S t r a n d s of t i m e w e r e rolled up in a ball, w h i c h I could hold in the p a l m of my h a n d , w h i l e space convulsed into infinitely m a n y shapes a n d forms coexisting in one single point. My m i n d envisioned all that w a s i m a g i nable, possible or impossible. It b e c a m e clear to me that w h a t we call impossible d e p e n d s on w h e r e we place the b o u n d a r i e s of reality. A n d at that m o m e n t , reality h a d no b o u n d a r i e s . I saw my d e a d relatives a n d those w h o w e r e not yet born; I s a w myself, as an a d u l t , t a l k i n g to my mother, even t h o u g h she died w h e n I w a s a child. As she sadly apolog i z e d for her absence, for not b e i n g able to see me g r o w , or to g i v e me her love, she b e c a m e the child a n d I the a d u l t ; a n d for the first t i m e I understood she h a d no choice. We w e r e s t a n d i n g in the l i v i n g room of our old h o m e but also flying over deserts a n d s w i m m i n g u n d e r w a t e r . I s a w m y s e l f b e i n g born, a n d m y mother s a w m e d y i n g . O u r i m a g e s m e r g e d into a knot, w h i c h b e c a m e a tear in my mother's e y e . As I raced to e m b r a c e her, desperate for her touch after so m a n y y e a r s , the

* Caution: W h a t follows is completely fictional. It is a playful g a m e inspired by speculations in general relativity.

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spaceship w a s flooded by the most intense light I had ever experienced. It w a s too late. My stretched-out h a n d s passed t h r o u g h her body as she vanished into bright nothingness. I w a s alone a g a i n , blinded by w h i t e ness, my innermost hopes defeated. T h e black hole had fed on my d r e a m s and m o c k e d my pain.

"I felt an e n o r m o u s push, as if the spaceship w e r e being coughed up by a giant. T h e last thing I remernber w a s hitting my head against the w a l l . I must h a v e r e m a i n e d unconscious for q u i t e a w h i l e , but could not be certain, because all my clocks w e r e d a m a g e d . W h e n I looked in the mirror, I could h a r d l y believe w h a t I s a w ; my hair h a d turned c o m pletely w h i t e a n d my face w a s covered w i t h w r i n k l e s I didn't have m o m e n t s ( m o m e n t s ? ) ago. I checked my location in the computer a n d r e a l i z e d that, somehow, I r e e m e r g e d t w o thousand l i g h t - y e a r s a w a y from C y g n u s X - l ! T h e only possible explanation w a s that I traveled t h r o u g h a w o r m h o l e that s o m e h o w w a s kept open inside the black hole a n d w a s tossed out by a w h i t e hole at a f a r a w a y point in space. I had g l i m p s e d eternity and contacted my d e a d loved ones, I had held infinity in my h a n d s , but w a s u n a b l e to find my youngest brother. A n d yet, to this d a y I am certain that he also s u r v i v e d , that he r e e m e r g e d s o m e w h e r e in this universe, t a k i n g a different path through the w o r m hole. I feel that we r e m a i n connected by t h r e a d s l e a d i n g back to the m a e l s t r o m , a place w h e r e all the choices we m a d e coexist w i t h those we didn't, w h e r e all our I's are possible." I w a s deeply moved by Mr. Strom's n a r r a t i v e . Its truth w a s i r r e l e vant to me. S o m e h o w , the i m a g e of all my life coexisting in one point in space a n d t i m e b e c a m e more important than its plausibility. T h e black hole p i l g r i m a g e , for it w a s a p i l g r i m a g e , w a s an expression of Mr. Strom's innermost desires, a search for the totality of his self; science had g i v e n h i m the possibility of c o n t e m p l a t i n g immortality. Even beyond that, I r e a l i z e d that science a l l o w e d h i m to i m a g i n e a place w h e r e time could flow in a n y sense, or m a n y senses at once, or not at all. W h o k n o w s ? M a y b e this place really exists, a n d Mr. Strom's n a r r a tive is factual. Or, if it doesn't, one d a y we m a y create it. For it is precisely at the cutting e d g e of k n o w l e d g e that i m a g i n a t i o n is most

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F I G U R E I 8 : T o p : A two-dimensional illustration of an Einstein-Rosen bridge, connecting first two points in hypothetical different universes, or two points in our own. B o t t o m : A wormhole connecting two points in our universe. In both cases, one goes in through a black, hole and out through a white hole.

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crucial; w h a t w a s once only a d r e a m m a y very well become a reality tomorrow.

D e s c e n t i n t o the M a e l s t r o ^ m : Postscript We have traveled far into the r e a l m of the scientific u n k n o w n , w h e r e w h a t we can t a k e for certain blurs w i t h m a t h e m a t i c a l speculation. O u r present k n o w l e d g e of w h i t e d w a r f s , neutron stars, and black holes m a y not be complete, but it certainly is very extensive. T h i s k n o w l e d g e has transformed the w a y we see the cosmos a n d its m a n y w o r l d s , confirming once more the a m a z i n g a n d limitless creativity of nature. Kant w o u l d have been ecstatic to see his i m a g e of the "Phoenix of N a t u r e , " of repeating cosmic cycles of creation a n d destruction, a n i m a t e d so spectacularly in the birth a n d death of stars. But he w o u l d perhaps have been i n c r e d u l o u s to see that ideas d a t i n g back to his contemporaries John Michell ( 1 7 2 4 - 1 7 9 3 ) a n d L a p l a c e , of stars so massive as to be capable of s w a l l o w i n g their o w n light, w e r e also vindicated. A n d he w o u l d have been even m o r e i n c r e d u l o u s to learn that those objects could turn N e w t o n i a n notions of absolute space a n d t i m e , so precious to his philosophy, inside out. T h e event horizon separates our universe, or w h a t we can observe, from the central s i n g u l a r i t y of a black hole, protecting us from w h a t ever lies inside. T h i s protection has been d u b b e d cosmic censorship, as if n a t u r e itself w e r e protecting our l i m i t e d reasoning abilities from the b i z a r r e reality that lies w i t h i n . T h e possibility of a c t u a l l y h a v i n g a spaceship travel through the t h r o a t l i k e s i n g u l a r i t y of a rotating black hole, as did our hero Mr. S t r o m , or t h r o u g h a w o r m h o l e in space, is at best very remote. T h e possibility that this traveler w o u l d a c t u a l l y r e e m e r g e unscathed s o m e w h e r e in the universe is even m o r e i m p l a u s i ble. We don't k n o w w h e t h e r these cosmic shortcuts indeed exist, a n d , if they do, how they could be kept open, a n d w h e t h e r they could be a

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passage to s o m e w h e r e else in space a n d in time. But i g n o r a n c e a n d i m p l a u s i b i l i t y should never stop us from speculating, so long as the speculation is g r o u n d e d on solid scientific reasoning. T h i s is w h a t m a n y astrophysicists w o r k i n g on black holes do, a n d it is t h a n k s to their intellectual boldness a n d creativity that we have l e a r n e d so m u c h about these fascinating objects. We don't k n o w w h e t h e r science can provide secret passages to other realities, l i k e the closet in The Lion, the Witch, and the Wardrobe, by C. S. L e w i s , or the tree in Alice in Wonderland, by L e w i s C a r r o l l . But if a scenario can be reasoned t h r o u g h the l a w s of physics, the effort is w o r t h w h i l e . As the n a r r a t o r of our fictional tale said, "it is precisely at the cutting e d g e of k n o w l e d g e that i m a g i n a t i o n is most crucial; w h a t w a s once only a d r e a m m a y very well become a reality tomorrow." We now believe that most g a l a x i e s nest a g i a n t black hole in their center, monsters of m i l l i o n s or even billions of solar masses. A black hole w i t h three m i l l i o n solar masses, l i k e the one we believe exists in the core of the M i l k y W a y , has an event horizon of about 8 million k i l o m e t e r s , t w e n t y t i m e s s m a l l e r than the distance b e t w e e n the Sun a n d Earth, tiny in comparison w i t h the g a l a x y . T h u s , we don't have to w o r r y about being s u c k e d d o w n this black hole a n y m o r e than of being d r a g g e d d o w n by a whirlpool in the C a r i b b e a n w h i l e s w i m m i n g in R i o . W i t h h u n d r e d s of billions of g a l a x i e s out there, each w i t h hund r e d s of billions of stars, we do have an e n o r m o u s s a m p l e of black holes to study, from s m a l l - s o l a r - m a s s ones to true cosmic g i a n t s . A n d study t h e m we w i l l , as several missions are being p l a n n e d w i t h orbiting x - r a y telescopes to i m a g e k n o w n black holes in u n p r e c e d e n t e d detail, u p g r a d i n g the c u r r e n t C h a n d r a satellite from N A S A . W i t h i n two d e c a d e s , we w i l l k n o w m u c h m o r e about these objects and their properties. W i l l we k n o w w h a t lies inside the event horizon, or w h e t h e r w o r m h o l e s do exist? T h a t is h a r d to guess. My o w n feeling is that at most we w i l l k n o w better how plausible our present speculations conc e r n i n g the interior of black holes and cosmic shortcuts are a n d that we w i l l create m a n y m o r e guesses. But then a g a i n , I could be w r o n g . . . We w i l l now t a k e the last logical step of our v o y a g e , from stars and 21*

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black holes to the universe as a w h o l e . In discussing the possible end of Earth and of our S u n , we encountered a n e w physics, w h e r e space a n d t i m e g a i n a plasticity that reaches a m i n d - b o g g l i n g c l i m a x inside black holes. But planets, people, a n d stars exist within the universe a n d t a k e part in its history. A n d w h a t history is this? We are confident that the big bang model of m o d e r n cosmology ^provides at least a broad-brush description of the u n i v e r s e , frofh'i,ts b e g i n n i n g to its "end." In the last part of this book, I w i l l present our current ideas about the fate that a w a i t s the universe. After a l l , this fate is, in a very d e e p sense, our o w n . W i l l it end, as the A m e r i c a n poet Robert Frost once w r o t e , in fire or in ice? For the first t i m e in history, we believe we a c t u a l l y k n o w the a n s w e r . Or almost. T h i s cosmological discussion of possible ends w i l l t a k e us back full circle to our o p e n i n g reflections on t i m e a n d i m m o r tality. U n d e r the ever v i g i l a n t eye of science, of course.

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Some say the world will end in fire, Some say in ice. â&#x20AC;&#x201D;

R O B E R T

F R O S T

R i g h t behind the Physics a n d A s t r o n o m y B u i l d i n g at D a r t m o u t h , surrounded by t o w e r i n g pine trees, there is a life-size bronze statue of the A m e r i c a n poet Robert Frost. He sits q u i e t l y on top of some rocks (granite, of course), notepad on his lap and eyes lost in thought. F e w poets, if any, have captured the essence of life in rural northern N e w E n g l a n d , w i t h its h a r d s h i p s a n d stark beauty, as d i d Frost. T h e bronze celebrates Frost's g e n i u s a n d brief passage t h r o u g h D a r t m o u t h , w h e r e he w a s a student for a short period. W h e n my d a u g h t e r , Tali, w a s about two, I used to t a k e her for w a l k s a r o u n d these w o o d s . S h e w a s completely e n c h a n t e d by this shining, motionless m a n , w h o looked so real a n d yet w a s not. Every t i m e we w e n t b a c k , Tali w o u l d i m m e d i ately dart t o w a r d the statue, sit on his lap, a n d kiss his c h e e k s . " W h y did you kiss the s t a t u e ? " I w o u l d ask in that idiotic a d u l t tone, c o m pletely devoid of m a g i c . "Because he looks so sad, D a d d y . Is it because he can't m o v e ? " To m e , Tali's actions c a p t u r e d the essence of our perception of time; t i m e is motion, is h o w we describe c h a n g e . A motion-

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less w o r l d , a w o r l d w i t h o u t t i m e is a sad w o r l d , fated to never reinvent itself. T i m e is the absence of perfection. T h i s , to m e , is w h a t the notion of p a r a d i s e , w i t h its absence of t i m e , i m p l i e s ; perfection is changeless a n d thus timeless. Eternity is not just an infinitely long time but the absence of t i m e , the absence of c h a n g e , the absence of r e n e w a l , "nevere n d i n g present," as S a i n t A u g u s t i n e wrote. It is decreed in the Old a n d N e w Testaments that t i m e w i l l come to an end w h e n good defeats evil. W h i l e they both coexist, t i m e flows f o r w a r d , inexorably. We are here, now, d e a l i n g w i t h this battle, t r y i n g to m a k e sense of our m i s t a k e s and choices. W h a t w o u l d life be if we m a d e no m i s t a k e s a n d thus needed no forgiveness? W o u l d we still be h u m a n ? I, for one, am not r e a d y to g i v e u p time. Frost, l i k e most of us, t h o u g h more eloquently, w o n d e r e d about the end of time. H i s w e l l - k n o w n poem " F i r e a n d Ice" a p p e a r e d in Harper's Magazine in 1920, t w o y e a r s before the Russian meteorologist t u r n e d cosmologist A l e x a n d e r A l e x a n d r o v i c h F r i e d m a n n obtained the first cosmological solutions s h o w i n g that the u n i v e r s e as a w h o l e could indeed c h a n g e in t i m e , g r o w i n g or contracting, a n d a c t u a l l y e n d i n g in fire or in ice. Frost's inspiration probably c a m e from c o m b i n i n g t w o m o r e Earth-bound e n d s , the apocalyptic scenario of C h r i s t i a n i t y â&#x20AC;&#x201D; falling stars a n d a l l - c o n s u m i n g fire and d o o m â&#x20AC;&#x201D; w i t h the desolation of a N e w E n g l a n d frigid w i n t e r a n d the possibility of a n e w ice age. H o w e v e r , Frost's description could h a r d l y have been m o r e appropriate for the cosmology of his d a y s , eerily timely a n d prophetic. In 1917, two y e a r s after p u b l i s h i n g his g e n e r a l theory of relativity, Einstein showed that his e q u a t i o n s a p p l i e d not only to stars but to the u n i v e r s e as a w h o l e ; just as we need the m a t t e r distribution of a star to obtain the g e o m e t r y of space inside a n d a r o u n d it, if we k n o w the m a t t e r distribution in the universe we can obtain its geometry. T h a t m u s t have been a very powerful feeling, w h e n Einstein first r e a l i z e d that he could a c t u a l l y d e t e r m i n e the g e o m e t r y of the u n i v e r s e t h r o u g h physics and mathematics. D u r i n g the past e i g h t y y e a r s , cosmology has g o n e t h r o u g h some very deep transformations, c h a n g e s , a n d revisions. Most important, it

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has become an e x p e r i m e n t a l science, as opposed to a m e r e l y m a t h e m a t ical exercise inspired by g e n e r a l physical concepts. T h e desktop u n i verses of the e a r l y twentieth century, inspiring as they w e r e , a r e g o n e for good, t h a n k s to an e n o r m o u s joint effort of cosmologists a n d astronomers. But even though their w o r k has solved m a n y conund r u m s , it has also g i v e n rise to m a n y mpre. We are n o w in a p a r a d o x i cal a g e w h e r e our successful observations have introduced t r e m e n d o u s uncertainties about the cosmos. We don't k n o w w h a t most of the universe is m a d e of. We don't even k n o w w h e t h e r its d o m i n a n t e n e r g y contribution comes from m a t e r i a l particles or from some sort of diffuse " v a c u u m e n e r g y " being p u l l e d from the q u a n t u m w o r l d . W e also don't k n o w the universe's fate a n d m a y not even be able to predict it. A n d yet, we have never k n o w n as m u c h about the universe as we do n o w ! T h i s strange state of affairs r e m i n d s me of the " c r i s i s " in physics d u r i n g the late nineteenth century, w h e n w i d e s p r e a d theoretical confid e n c e â&#x20AC;&#x201D; b o r d e r i n g on a r r o g a n c e â&#x20AC;&#x201D; w a s being u n d e r m i n e d by a series of e x p e r i m e n t a l discoveries that just didn't fit the classical w o r l d v i e w so carefully constructed since the d a y s of N e w t o n . A n e w revolution w a s about to happen, the q u a n t u m revolution that profoundly c h a n g e d our view of the atomic a n d subatomic w o r l d s , redefined our destructive capabilities (through the use of n u c l e a r e n e r g y ) , a n d , to a l a r g e extent, t r i g g e r e d the information revolution of the late twentieth century, w i t h 1

its semiconductors, lasers, a n d d i g i t a l devices. It is q u i t e possible that we a r e about to witness a s i m i l a r revolution in our u n d e r s t a n d i n g of the cosmos. S o m e scientists even say that we are a l r e a d y in the midst of it. O u r confusion couldn't be m o r e s t i m u l a t i n g ! In this chapter you will see why.

The

Cosmos,

circa

1945

T w e n t i e t h - c e n t u r y cosmology d i d not a d v a n c e at a steady pace. Periods of focused activity w e r e followed by times of relative c a l m a n d isolated interest. It is fair to say that, until the 1960s, most physicists 225

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v i e w e d cosmology w i t h g r e a t suspicion. It is also fair to say that, unfortunately, this attitude r e m a i n s intact in some circles, a l t h o u g h these are r a p i d l y d w i n d l i n g . T w o reasons mostly account for this suspicion. First, cosmology a s k s the big q u e s t i o n s â&#x20AC;&#x201D; " H o w did the w o r l d come to b e ? " or " W i l l the universe e n d ? " â&#x20AC;&#x201D; w h i c h have traditionally been the province o f r e l i g i o u s a n d metaphysical i n q u i r y . T h i s u n a v o i d a b l e link has long d i s t i n g u i s h e d cosmological research from m o r e " d o w n to e a r t h " questions related to the behavior of localized systems, w h e t h e r subatomic particles, crystals, or black holes. Second, until the m i d 1960s there w a s an a p p a l l i n g lack of data b a c k i n g up or contradicting the m a n y desktop universes proposed e a r l i e r on. H o w could cosmologists c l a i m to be d o i n g science w i t h o u t h a v i n g their hypotheses tested? After all, w i t h o u t observational evidence, the only g u i d a n c e we have in b u i l d i n g m o d e l s and hypotheses is m a t h e m a t i c a l e l e g a n c e and physical intuition, w h i c h a r e crucial but not definitive. W h a t m a y be m a t h e m a t i c a l l y c o m p e l l i n g m a y not necessarily correspond to physical reality. In all honesty, cosmologist that I a m , I m u s t a d m i t that the skepticism on the part of most of the a c a d e m i c c o m m u n i t y w a s partly justified. But not any longer. Einstein's pioneering cosmological research w a s based on a g e n e r a l principle still crucial to cosmology today, the cosmological principle: on the a v e r a g e , the universe looks the same e v e r y w h e r e . In other w o r d s , all points in the universe a r e essentially e q u i v a l e n t . T w o i m a g e s often used are the perfectly smooth surfaces of a ball a n d of a very large s q u a r e table, both e x a m p l e s of t w o - d i m e n s i o n a l g e o m e t r i e s w h e r e all points are e q u a l l y important. (For the table, we must be sufficiently far from the e d g e s , since they are special. Only if the table w e r e infinitely long in all directions w o u l d its flat g e o m e t r y truly satisfy the cosmological principle.) A q u i c k g l a n c e at a moonless night sky shows that this principle should not be applied to the local cosmic neighborhood we can see w i t h our n a k e d eye; stars are distributed u n e v e n l y and do not hint at a cosmic homogeneity. To apply the cosmological principle, we m u s t move t o w a r d m u c h l a r g e r distances, not of a few tens of lighty e a r s , but of h u n d r e d s of m i l l i o n s of l i g h t - y e a r s . In fact, the precise dis-

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tance scale at w h i c h we can a c t u a l l y say the universe is on the a v e r a g e the s a m e has steadily g r o w n , as i n c r e a s i n g l y l a r g e complex struct u r e s â&#x20AC;&#x201D; h u g e clusters of g a l a x i e s a n d h u g e e m p t y

regions called

v o i d s â&#x20AC;&#x201D; w e r e discovered d u r i n g the past t w o decades. But we n o w t h i n k we have a r r i v e d at a consensus that, at these e n o r m o u s distances, the universe can indeed be interpreted a*s being h o m o g e n e o u s . In practice, we can i l l u s t r a t e r j i e c o s m o l o g i c a l principle by m e a n s of the vegetable soup approach; if we put a bunch of vegetables in a cooking pot filled w i t h water, spices, beets, carrots, potatoes, a n d the l i k e , the final soup w i l l be very i n h o m o g e n e o u s , that is, l u m p y . So, to m a k e it h o m o g e n e o u s , we put the cooked v e g g i e s in a blender until they are transformed into a smooth puree. T h e final c r e a m y soup w i l l have the same a m o u n t of m a t t e r (mass) as the l u m p y soup, but w i t h the crucial difference that it w i l l satisfy the inside-the-pot cosmological principle, that all points w i t h i n the soup a r e essentially e q u i v a l e n t . Einstein put the universe in a blender, so to speak, a n d a s s u m e d that the g e o m e t r y of the cosmos w o u l d be d e t e r m i n e d by the a m o u n t of m a t t e r w i t h i n it, distributed h o m o g e n e o u s l y as in the smooth vegetable soup. T h i s s i m plification a l l o w e d h i m to find a solution to his e q u a t i o n s describing a static universe. W h y static? In 1917, Einstein h a d no reason to suppose that the universe w a s a d y n a m i c entity that could a c t u a l l y evolve in time: the expansion of the universe w a s not conclusively discovered until 1929. Einstein w a s , of course, very proud of his solution, even though the r e q u i r e m e n t of h a v i n g a static cosmos forced h i m to nclude an extra term into his e q u a t i o n s , w h i c h he called " n e g a t i v e ressure." (Reader, t a k e note of this, because n e g a t i v e pressure w i l l ome back full force in l a t e - t w e n t i e t h - c e n t u r y cosmology.) We n o w -all this term a cosmological constant, a m a t h e m a t i c a l l y a l l o w e d contribution to the e q u a t i o n s of g e n e r a l relativity that acts as a k i n d of antigravity, p u s h i n g the g e o m e t r y apart. R e m e m b e r that the matter (and thus e n e r g y ) distribution d e t e r m i n e s the g e o m e t r y . Einstein needed the cosmological constant in order to stabilize his universe; a static u n i verse filled w i t h a h o m o g e n e o u s distribution of m a t t e r w o u l d spontaneously collapse upon itself because of its o w n g r a v i t y . T h e existence

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a n d n a t u r e of this i n e l e g a n t intruder, w h i c h efficiently performed the b a l a n c i n g act needed in Einstein's universe, r e m a i n s one of the greatest m y s t e r i e s of m o d e r n physics. F r i e d m a n n freed Einstein's universe from the c h a i n s of i m m o b i l i t y by a l l o w i n g m a t t e r to c h a n g e in time. A n d since g e n e r a l relativity states that m a t t e r d e t e r m i n e s the g e o m e t r y , if m a t t e r c h a n g e s in t i m e so does the geometry. Before discussing F r i e d m a n n ' s solutions, I will introduce a concept crucial to cosmology, the energy density. T h e q u i c k , dirty, a n d , for us, sufficient definition is that e n e r g y density is the total e n e r g y d i v i d e d by the v o l u m e that contains it; if you have a certain a m o u n t of e n e r g y in a cubic box a n d you decrease the sides of the box by half, the e n e r g y density g r o w s by a factor of eight;* in a s m a l l e r box, the e n e r g y is s q u e e z e d together a n d hence its density is higher. If you double the sides of the box, the e n e r g y density decreases by the same factor of eight.

Thus,

e x p a n d i n g box

the e n e r g y density

decreases, w h i l e in a contracting box it increases. T h e key question, w h i c h we w i l l revisit as we trace the d e v e l o p m e n t of m o d e r n cosmology, is w h a t contributes to the e n e r g y density of the universe. T h e r e are three possible contributions to the e n e r g y density of the universe, w h i c h m a y a p p e a r i n d i v i d u a l l y or together. T h e r e m a y be a radiation contribution, consisting of photons, the particles of electrom a g n e t i c radiation we encountered before, a n d a n y other particles w i t h no mass or very low m a s s , such as neutrinos. Matter also con2

tributes, since it carries e n e r g y in its m a s s — t h r o u g h the E = mc relat i o n — a n d in its motion; here we must t h i n k of m a t t e r not just in the form of g a l a x i e s a n d stars but also in a more e l e m e n t a r y form, that is, b r o k e n d o w n to m o r e f u n d a m e n t a l constituents. W h a t these constituents a r e — p r o t o n s ,

neutrons, or s o m e t h i n g

more exotic—will

d e p e n d on w h a t k i n d of m a t t e r fills the universe. F i n a l l y , the cosmological constant, or some other diffuse form of e n e r g y that m i m i c s its effects, m a y also contribute to the g e o m e t r y of the universe.

* H e r e is w h y : the density (D) of e n e r g y (E) in a box of v o l u m e V is D = E/V. F o r a cubic box of K

size L, K= L . T h u s , if you h a l v e the side, D = E/(LflY. Since 2 = 8, D gains a factor o f 8.

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Several desktop universes w e r e found d u r i n g the 1920s and 1930s, by solving Einstein's e q u a t i o n s w i t h v a r y i n g k i n d s of contributions to the e n e r g y density. A p a r t from F r i e d m a n n , other key p l a y e r s in this g a m e w e r e A r t h u r Eddington, w h o m w e met i n chapter 6 , Georges Lemaftre, a priest c u m cosmologist from B e l g i u m , and the Dutch W i l l e m de Sitter. T h e key broad-brush feature of these solutions (some subtleties w i l l be dealt w i t h later) is that they exhibited t w o possible behaviors for the g e o m e t r y of the universe: eternal expansion or a cyclic

sequence of e x p a n d i n g a n d contracting eras, the so-called

Phoenix universe. (Kant w o u l d have enjoyed this t r e m e n d o u s l y ! ) In the absence of a cosmological constant, the k e y p a r a m e t e r d e t e r m i n i n g the fate of the u n i v e r s e — w h a t my colleagues L a w r e n c e Krauss from Case W e s t e r n Reserve University a n d M i c h a e l T u r n e r from the U n i versity

of C h i c a g o aptly a n d

eschatologically

called

the

relation

between g e o m e t r y and d e s t i n y — i s the e n e r g y density. T h e cosmological e q u a t i o n s d e t e r m i n e a critical v a l u e for the total e n e r g y d e n s i t y — t h a t is, the s u m of radiation, matter, a n d other contrib u t i o n s — w h i c h , in the absence of a cosmological constant, seals the fate of the universe; they put all contributions to the e n e r g y density of the universe into Einstein's cosmic blender, so as to m a k e it h o m o g e neous throughout. T h e a l l - i m p o r t a n t critical e n e r g y density is about one a t o m of h y d r o g e n in a cube of r o u g h l y h a l f a meter a side. If the total e n e r g y density is above this v a l u e , the g r a v i t y of the universe will eventually pull it back a n d cause it to collapse into a "big crunch"; if the e n e r g y density is below this v a l u e , the universe will k e e p e x p a n d i n g forever. It will also e x p a n d forever if the e n e r g y density is exactly equal to the critical v a l u e . A very useful a n a l o g y is a rocket being l a u n c h e d from Earth. T h e e n g i n e s thrust the rocket up, a n d Earth's g r a v i t y pulls it back d o w n . C l e a r l y , if the rocket reaches a h i g h e n o u g h velocity, it will k e e p g o i n g u p a n d w i l l e v e n t u a l l y escape Earth's g r a v i t a t i o n a l pull. (It w i l l still be pulled by Earth's g r a v i t y , but to very small effect, since g r a v i t y d e c a y s w i t h the s q u a r e of the distance.) O t h e r w i s e , it w i l l eventually reverse its motion a n d come c r a s h i n g d o w n on the g r o u n d ; the closer the rocket is to this critical velocity (called escape velocity), 229

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the h i g h e r it will travel before it starts falling back. You can easily verify this by t h r o w i n g a rock u p w a r d w i t h different speeds; the g r e a t e r the speed, the higher the rock travels before c o m i n g d o w n . In fact, if y o u can t h r o w it at 11 k i l o m e t e r s per second, about 25,000 m i l e s per hour, the rock w i l l escape into outer space! Now, i m a g i n e that the rocket can travel only at a fixed velocity. T h i s rocket w i l l be able to escape planets only if its velocity is e q u a l to or g r e a t e r than the planet's escape velocity. For e x a m p l e , a rocket that can n a r r o w l y escape from Earth w i l l be e q u a l l y successful escaping from M a r s , but w i l l fail m i s e r a b l y t r y i n g to escape from Jupiter. T h e lesson is simple; the l a r g e r the planet's g r a v i t a t i o n a l pull, the h a r d e r it is to escape from its surface. T h i s is w h e r e the a n a l o g y w i t h the u n i verse gets more interesting. W h y does the universe e x p a n d in the first place? After all, the rockets have e n g i n e s to push t h e m up. T h e reason is that in very early times the universe w a s so small that m a t t e r a n d radiation w e r e s q u e e z e d to a b s u r d l y h u g e pressures a n d t e m p e r a t u r e s . T h i n k of s q u e e z i n g d o w n some very powerful springs a n d letting g o â&#x20AC;&#x201D; i t w a s m u c h l i k e that w i t h the universe. But this is not the w h o l e story. A p a r t from the e n e r g y density, the e q u a t i o n s that control the expansion of the universe have a second t e r m , d e t e r m i n e d by the g e o m e t r y of space. T h e r e a r e three possibilities: a flat g e o m e t r y , a t h r e e - d i m e n s i o n a l a n a l o g u e of the flat table we discussed above; a closed g e o m e t r y , a t h r e e - d i m e n s i o n a l a n a l o g u e of the surface of the ball (it is pretty m u c h impossible for h u m a n s to visualize this, hence the t w o - d i m e n s i o n a l e x a m p l e s ) ; a n d finally, an open g e o m e try, w h i c h in two d i m e n s i o n s we can v i s u a l i z e as a s a d d l e , c u r v i n g in t w o opposite directions. For a flat g e o m e t r y , the spatial c u r v a t u r e term is absent from the cosmological e q u a t i o n s a n d the universe e x p a n d s at its escape velocity. For curved spaces, however, it is there a n d it m a k e s a very big difference. If the g e o m e t r y is closed, the u n i v e r s e will never be able to e x p a n d forever; eventually, the c u r v a t u r e t e r m w i l l d o m i n a t e a n d w i l l force the universe to collapse on itself. I m a g i n e a t t a c h i n g a spring to the rocket; as it tries to escape the g r a v i t a t i o n a l pull, the spring e v e n t u a l l y gets stretched to its m a x i m u m length a n d starts

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p u l l i n g the rocket back d o w n . L i k e w i s e , a closed universe w i t h o u t a cosmological constant will reach a m a x i m u m r a d i u s a n d collapse upon itself. A closed g e o m e t r y seals a destiny of collapse. If the g e o m e t r y is open, the universe w i l l be u n d e r d e n s e and will e x p a n d forever; the open g e o m e t r y gives an extra " k i c k " to the e x p a n sion, the exact reverse from the overderrse closed-geometry case. Back 1

to our rocket a n a l o g y : the spring attached to the rocket w a s m a n u f a c tured to k e e p stretching indefinitely as it e x p a n d s , g e t t i n g w e a k e r as it gets longer, boosting the rocket's escape. An open g e o m e t r y seals a destiny of continuous expansion (see figure 19). To s u m m a r i z e , in the absence of a cosmological constant, there is a one-to-one correspondence between the e n e r g y density of the universe, its g e o m e t r y , and its fate:

U n d e r c r i t i c a l density => open g e o m e t r y => expansion Critical density => flat g e o m e t r y => expansion (borderline case) Overcritical density => closed g e o m e t r y => eventual collapse

But these, of course, w e r e all desktop universes, since physicists d u r i n g the 1920s had no conclusive observation as to the d y n a m i c a l behavior of the cosmos. T h i s situation c h a n g e d d r a m a t i c a l l y in 1929, w h e n the A m e r i c a n astronomer E d w i n H u b b l e discovered that the universe is e x p a n d i n g in accordance w i t h a very simple rule; the farther a w a y the objectâ&#x20AC;&#x201D;for e x a m p l e , a g a l a x y â&#x20AC;&#x201D; t h e faster it recedes from us. In order to come to this a m a z i n g conclusion, H u b b l e needed to m e a s u r e t w o crucial n u m b e r s : the distance and the velocity of the g a l a x y relative to us. For the velocity, he used the Doppler effect, the fact that the pitch of a w a v e rises if its source is m o v i n g t o w a r d us (or we t o w a r d the w a v e ' s source) and falls if its source is m o v i n g a w a y from us (or we a w a y from the source). We are f a m i l i a r w i t h the sound w a v e s version of the Doppler effect from our urban and h i g h w a y experiences w i t h sirens and horns. In astronomy, we m e a s u r e the c h a n g e s in the emission spectra of g a l a x i e s as c o m p a r e d w i t h those of s i m i l a r nearby objects; if the 231

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"Big C r u n c h "

Time

F I G U R E 1 9 : The three possible fates of the universe, according to Friedmann's cosmology. An overdense universe has closed geometry and recollapses; a critical universe has flat geometry and expands forever; an underdense universe has open geometry and also expands forever. For Friedmann's cosmology, geometry determines destiny.

g a l a x y is m o v i n g a w a y from us, the radiation it e m i t s will be shifted t o w a r d l o w e r frequencies. We say that light from a receding object is redshifted, because tones of red light have the lowest frequencies of the visible spectrum. If, on the other h a n d , the g a l a x y is a p p r o a c h i n g us, its spectrum w i l l be shifted to h i g h e r frequencies, or blueshifted. T h i s is the case, for e x a m p l e , of our n e i g h b o r i n g g a l a x y A n d r o m e d a , located about 2.5 million l i g h t - y e a r s a w a y . But how could A n d r o m e d a be a p p r o a c h i n g us, if in an e x p a n d i n g universe all objects at l a r g e distances should be g e t t i n g a w a y from one a n o t h e r ? You m a y think of the cosmological expansion as a river flow, c a r r y i n g w i t h it all sorts of floating objects. H o w e v e r , here a n d there t w o objects m a y get tangled on a local eddy, r e m a i n i n g t e m p o r a r i l y detached from the overall w a t e r flow. In the case of pairs or clusters (self-gravitating g r o u p s ) of g a l a x i e s , their local g r a v i t a t i o n a l attraction, if they are sufficiently close

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to each other, o v e r w h e l m s the overall cosmic expansion, c a u s i n g their emission spectra to be blueshifted. Keep in m i n d that the expansion of the universe is not l i k e an explosion, w h e r e e v e r y t h i n g flies a w a y from a c o m m o n center, but more l i k e a stretching of the g e o m e t r y of space. T h i n k of a rubber sheet s t u d d e d w i t h small rigid d i s k s ; if the sheet is stretched, the distances between txje disks w i l l a u t o m a t i c a l l y increase, as the sheet's stretching carries them a w a y , but the d i s k s w i l l k e e p their original shape. T h e same holds for g a l a x i e s a n d other objects drifting in the cosmic flow. So m u c h for velocities. W h a t about distances? H u b b l e k n e w he n e e d e d w h a t astronomers call a standard c a n d l e , an astronomical object w i t h a well-defined luminosity, w h i c h can be identified easily in f a r a w a y g a l a x i e s . Since, by definition, the " c a n d l e " emits the s a m e a m o u n t of light w h e r e v e r it is a n d its brightness drops as the s q u a r e of the distance, if we m e a s u r e the candle's l u m i n o s i t y in the f a r a w a y g a l a x y , we can obtain its distance and hence the g a l a x y ' s distance as w e l l . In a terrestrial analogy, we can i m a g i n e placing several identical candles at v a r y i n g distances from our home; by m e a s u r i n g the d r o p in luminosity of each candle, we can obtain its distance from us. H u b b l e identified stars called C e p h e i d s , w h i c h have a w e l l - k n o w n variability in their brightness, in several nearby g a l a x i e s . In fact, it w a s by m e a n s of C e p h e i d s that he solved

problem

that h a d

been

plaguing

astronomers for centuries: Is the M i l k y W a y the only g a l a x y in the universe, or are there other "island u n i v e r s e s " out there, as Kant had e n v i sioned? Incredible as it m a y sound, it w a s only in 1924 that H u b b l e s h o w e d that the universe contains countless ( w e l l , h u n d r e d s of billions) of g a l a x i e s of many different shapes a n d sizes. But to extract his velocity-distance relation H u b b l e had to look d e e p into the cosmos, a n d even the 100-inch telescope at M o u n t W i l s o n Observatory in C a l i fornia w a s not powerful enough to find the requisite C e p h e i d s . H u b b l e then took the next logical step a n d looked for the brightest stars in each of the distant galaxies, w h i c h , of course, w e r e easier to see. A s s u m i n g that these powerful icons had a p p r o x i m a t e l y the s a m e brightness,

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H u b b l e could use t h e m as s t a n d a r d candles a n d thus extract the d i s tances to g a l a x i e s far e n o u g h a w a y . T h e choice of c a n d l e s varies, but this t e c h n i q u e is still extensively used today. Hubble's discovery of the cosmic expansion c h a n g e d the course of cosmology. H e r e w a s d a t a that conclusively proved that the universe is indeed e x p a n d i n g , as h a d been conjectured in some of the desktop m o d e l s of the 1920s. Einstein tossed a w a y his static universe a n d the cosmological constant in disgust; the universe w a s a d y n a m i c a l entity, its g e o m e t r y being stretched in all directions. But the cosmological constant w o u l d not go a w a y so easily. An obvious consequence of Hubble's discovery is that, if the universe is e x p a n d i n g , it w a s s m a l l e r in the past. U s i n g his velocity-distance relation, H u b b l e estimated that the u n i verse reached a point of e n o r m o u s density two billion y e a r s ago. In other w o r d s , H u b b l e e s t i m a t e d the a g e of the u n i v e r s e , the t i m e since the initial " b a n g " that pushed the g e o m e t r y , matter, a n d radiation outw a r d . T h e r e w a s a problem, though. It w a s k n o w n then that Earth w a s older than t w o billion y e a r s . H o w could Earth be older than the u n i v e r s e ? Solutions w e r e q u i c k l y proposed, some of them by E d d i n g ton a n d L e m a i t r e , i n v o k i n g the cosmological constant; for the right choices of the cosmological constant, it is possible to m a k e the universe go through a "coasting phase," in w h i c h the distances b e t w e e n objects h a r d l y c h a n g e w h i l e t i m e k e e p s passing. T h e s e coasting solutions a l l o w for a m u c h older universe w i t h distances s i m i l a r to w h a t we m e a s u r e today, " s o l v i n g " the e m b a r r a s s i n g a g e p r o b l e m . In 1952, W a l ter B a a d e â&#x20AC;&#x201D; w h o , as we saw in chapter 6, jointly proposed w i t h Z w i c k y the existence of s u p e r n o v a e â&#x20AC;&#x201D; r e f i n e d Hubble's distance m e a s u r e m e n t s a n d showed that the universe w a s comfortably older than Earth. T h e cosmological constant could be dropped a g a i n . W i t h the o u t b r e a k of the Second W o r l d W a r , most cosmological research h a d come to a halt, as physicists focused on more i m m e d i a t e questions, such as survival, defense, a n d , unfortunately, attack.

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Consistent as they w e r e w i t h the cosmic expansion found by H u b b l e , the desktop universes of the 1920s, a n d 1930s focused on w h a t we could call the g e o m e t r y of the cosmos and not on the physical properties of the matter that fdled it. L e m a i t r e , . w h o had an u n c a n n y physical intuition, w a s the first to r e a l i z e that the t w o , g e o m e t r i c a l a n d m a t e r i a l history, cannot be separated; one is inextricably related to the other. As a result, it w a s crucial to k n o w w h a t the stuff w a s that w a s s q u e e z e d to very h i g h t e m p e r a t u r e s a n d pressures early on in the cosmic history. C o u l d the properties of this p r i m o r d i a l m a t t e r be understood from well-established physical principles? In other w o r d s , could cosmologists reconstruct the cosmic history by c o m b i n i n g g e n e r a l relativity w i t h atomic a n d n u c l e a r physics? Questions of this sort l a u n c h e d the second phase of m o d e r n cosmology, w h e r e the focus c h a n g e d from devising m a t h e m a t i c a l m o d e l s that described the m a n y possible cosmic g e o m e t r i e s to physical models that incorporated the a l l - i m p o r t a n t influence of the m a t e r i a l constituents of the cosmos in its history. D u r i n g the early 1930s, L e m a t t r e proposed his prescient " p r i m e v a l a t o m " model, w h e r e he envisioned a universe evolving from the radioactive decay of a g i a n t atom: in the b e g i n n i n g , there w a s the atom. It w a s h i g h l y unstable, a n d , as soon as it c a m e into existence ( L e m a t t r e did not m a k e clear how this h a p p e n e d ) , it fissioned into m a n y pieces, and these fissioned into m a n y more. F r o m these fragments, electrons, h e l i u m nuclei, protons, a n d other particles rushed out. By conjecturing that the fragmentation resulted in an increase in v o l u m e , L e m a t t r e related the decay of the p r i m e v a l atom to an e x p a n d i n g universe. He w a s w e l l a w a r e of the l i m i t a t i o n s of his ideas, w h i c h he considered more a vision than a theory. Nevertheless, he w e n t as far as p r e d i c t i n g the existence of "fossil r a y s , " some form of radiation left over from the initial stages of disintegration a n d expansion. Little d i d he k n o w that fossil rays w e r e indeed g o i n g to be discovered d u r i n g the mid-1960s. Eddington, Lemattre's Ph.D. adviser, w a s not happy. ( B e t w e e n 235

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C h a n d r a s e k h a r , L e m a t t r e , a n d others, he sure had to deal w i t h q u i t e a few creative y o u n g s t e r s ! ) He had an aversion to the idea of an abrupt b e g i n n i n g of the u n i v e r s e . " T h e most satisfactory theory w o u l d be one w h i c h m a d e the b e g i n n i n g not too unaesthetically abrupt," he w r o t e in The Expanding Universe, published in 1933.

He w e n t on to suggest a

universe w h e r e the initial state w a s g i v e n by Einstein's static solution, the one held steady by the cosmological constant. E d d i n g t o n had shown that this solution w a s unstable against small fluctuations, l i k e a ball b a l a n c i n g on a n a r r o w fence, w h i c h w o u l d fall if touched lightly. T h u s , he proposed that the universe started as an Einsteinian universe a n d stayed such for an i n d e t e r m i n a t e a m o u n t of time: " T h e p r i m o r d i a l state of things w h i c h I picture is an even distribution of protons and electrons, e x t r e m e l y diffuse a n d filling all (spherical) space, r e m a i n i n g nearly balanced for an e x c e e d i n g l y long t i m e until its inherent instabil3

ity p r e v a i l s . " T h u s , in Eddington's universe, t i m e starts to tick only w h e n m o u n t i n g instabilities t a k e over Einstein's universe. T h e s e instabilities g r a d u a l l y g a t h e r force a n d e v e n t u a l l y propel the universe into its e x p a n d i n g phase, consistent w i t h Hubble's observations. It is somew h a t c u r i o u s that the notion of a universe w i t h a timeless past that "spontaneously" transitions into an e x p a n d i n g era m a d e a c o m e b a c k in the 1980s, in the h a n d s of the physicists J a m e s H a r t l e a n d Stephen H a w k i n g , A l e x V i l e n k i n , a n d A n d r e i L i n d e , as a consequence of a p p l y i n g q u a n t u m m e c h a n i c s to cosmology. But we w i l l save this d e v e l o p m e n t for later. A y o u n g Russian physicist n a m e d George G a m o w , w h o w o r k e d w i t h F r i e d m a n n until his u n t i m e l y death in 1925, w a s listening very attentively to the debates on the p r i m o r d i a l history of the universe. T r a i n e d as a q u a n t u m physicist, G a m o w b e c a m e a m a s t e r of a p p l y i n g atomic a n d n u c l e a r physics to cosmology. He w a s also a master of irreverence, whose p u n s shook the rigidity of q u i t e a few scientific circles. A p p a r e n t l y , he p a r t i c u l a r l y enjoyed a r r i v i n g in C a m b r i d g e with his very loud motorcycle, w h i c h he m a d e sure roared louder the closer he w a s to "senior" faculty. D u r i n g the 1930s, he devoted h i m s e l f to stellar astrophysics, pioneering m a n y of the ideas that e v e n t u a l l y led H a n s

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Bethe, W i l l i a m Fowler, F r e d H o y l e , a n d others to obtain the m a i n fusion reactions that e n s u r e a star's balance a g a i n s t g r a v i t y , as well as its explosive e n d i n g s .

G a m o w recognized that the p r i m o r d i a l universe shared m a n y properties w i t h stars: h i g h t e m p e r a t u r e s a n d pressures, matter dissociated into its most basic constituents, an6Sa continuous s t r u g g l e against g r a v i t y ' s pull. But he w a s also a w a r e of a key difference: u n l i k e stars, w h i c h r e m a i n fairly static d u r i n g their h y d r o g e n - b u r n i n g stage, the universe e x p a n d s in t i m e . A n d as it does so, both the t e m p e r a t u r e a n d the density of its m a t e r i a l contents decrease. R i c h a r d T o l m a n , the C a l tech physicist w h o helped O p p e n h e i m e r w i t h his neutron star a n d gravitational collapse m o d e l s , h a d studied the t h e r m a l behavior of the e x p a n d i n g universe in detail d u r i n g the 1930s. U s i n g T o l m a n ' s e q u a tions, inspired by L e m a i t r e ' s p r i m e v a l atom, a n d a p p l y i n g his k n o w l edge of nuclear physics, G a m o w proposed a cosmological model in w h i c h the universe started as a very hot a n d dense soup of p r i m o r d i a l matter, mostly consisting of electrons, protons, neutrons, neutrinos, and, of course, a lot of photons (radiation). L i k e L e m a i t r e a n d his p r i m e v a l atom, G a m o w also declined to speculate on w h e r e the soup came from. G a m o w reasoned that initially the heat w a s so intense that every time neutrons a p p r o a c h e d protons to fuse into a nucleus of d e u t e r i u m or h e l i u m , h i g h l y energetic photons w o u l d break their n u c l e a r bonds. A n d if the strong n u c l e a r force had no chance a g a i n s t these furious photons, the m u c h w e a k e r electric attraction between electrons a n d protons w a s completely ineffective. But as the universe e x p a n d e d a n d cooled below a few thousand billions of d e g r e e s , the radiation b e c a m e less energetic, until it w a s no longer strong e n o u g h to prevent nuclear bonding. T h i s is the b e g i n n i n g of the "nucleosynthesis e r a , " the period between one second a n d three m i n u t e s after the "bang," w h e n light nuclei w e r e h i e r a r c h i c a l l y fusedâ&#x20AC;&#x201D;from h y d r o g e n (protons) to d e u t e r i u m , to t r i t i u m , to h e l i u m 3, to h e l i u m 4, a n d so on. O r i g i n a l l y , j a m o w proposed that all chemical e l e m e n t s w e r e cooked up in the furnace of the p r i m o r d i a l universe. At the other e x t r e m e , H o y l e a n d 237

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others a r g u e d that this w a s not the case, m a i n t a i n i n g instead that nucleosynthesis occurred exclusively d u r i n g stellar e v o l u t i o n â&#x20AC;&#x201D; n o prim o r d i a l nuclear a l c h e m y ! Both w e r e right a n d w r o n g ; G a m o w w a s partly right since light nuclei (up to l i t h i u m 7) w e r e fused p r i m o r d i a l l y , a n d H o y l e w a s partly right since all heavier e l e m e n t s a r e synthesized in stars. T h e y w e r e both w r o n g in hoping that a single physical m e c h a nism could e x p l a i n all the c h e m i s t r y of the universe. H o y l e had a very different cosmology in m i n d , the so-called steadystate model, w h i c h posited that the universe never c h a n g e d in time, b e i n g thus eternal a n d uncreated. In order to reconcile an u n c h a n g i n g universe w i t h the observed cosmological expansion, H o y l e , H e r m a n n Bondi, a n d T h o m a s Gold, all from C a m b r i d g e University, proposed in 1948 that m a t t e r could be created out of nothing (creatio ex nihilo) in such a w a y as to balance the decrease in density caused by the e x p a n sion. H e n c e the n a m e steady-state. L i k e E d d i n g t o n , the C a m b r i d g e trio had a serious aversion to the notion of a universe that mysteriously a n d abruptly a p p e a r e d at some point back in t i m e ; it w a s too close to the J u d e o - C h r i s t i a n creation event, too close for their intellectual comfort. T h e s a m e n e g a t i v e reaction w a s w i d e s p r e a d in the Soviet Union, if not a m o n g all the physicists, at least a m o n g the political authorities. W i t h his collaborators R a l p h A l p h e r a n d Robert H e r m a n , G a m o w improved his calculations to the point that they m a d e two key predictions. One w a s that the universe should contain about 75 percent h y d r o g e n a n d 24 percent h e l i u m (according to m o d e r n m e a s u r e m e n t s ) a n d a few other l i g h t isotopes cooked d u r i n g the first three m i n u t e s . T h a t is, apart from some h e l i u m , only about 1 percent of the m a t t e r in the universe is a c t u a l l y cooked in stars. T h e other prediction w a s that the universe should be bathed in radiation, w h i c h should have a current t e m p e r a t u r e of 5 d e g r e e s Kelvin, about â&#x20AC;&#x201D;450 F a h r e n h e i t . T h i s radiation, suggestively s i m i l a r to w h a t L e m a i t r e called fossil rays, appeared w h e n the universe cooled e n o u g h , to r o u g h l y 6000 degrees Kelvin, e n a b l i n g electrons a n d protons to bind a n d form hydrogen atoms some 300,000 years after the bang. After this time, photons were not energetic e n o u g h to interfere w i t h the electrical attraction between

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F I G U R E 2 0 : Most important events in cosmic history. We will discuss them in this chapter and the next. (Times are approximate.)

electrons a n d protons, a n d w e r e free to r o a m across the universe. In order for f a r a w a y objects to be visible to us (optically or in any w a v e l e n g t h ) , the radiation (photons) they e m i t m u s t be able to reach us; that is, it m u s t travel fairly u n i m p e d e d . T h u s , we can think of this a l l important c h a n g e in the properties of the universe, called decoupling, as a transition from a p r i m o r d i a l era of o p a c i t y â&#x20AC;&#x201D; w h e n photons w e r e so tightly bound to electrons a n d protons that they could not propagate far before hitting t h e m â&#x20AC;&#x201D; t o an era of transparency, w h e r e we are until today, some fourteen billion y e a r s later. In other w o r d s , the d e c o u p l i n g transition m a r k s a n absolute observational b o u n d a r y beyond w h i c h w e annot probe directly t h r o u g h telescopes of a n y sort. A n y information >ut the u n i v e r s e before d e c o u p l i n g m u s t come indirectly, t h r o u g h clues left over from these very early times. One e x a m p l e of such a clue ÂŤs precisely the light c h e m i c a l e l e m e n t s , cooked w h e n the universe w a s between a second and three m i n u t e s old. T h a t their a b u n d a n c e w a s redicted by the big b a n g model a n d later confirmed by observations 239

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g i v e s us confidence that we do indeed u n d e r s t a n d the physics of the universe at this very e a r l y a g e , a truly r e m a r k a b l e a c h i e v e m e n t . But we w a n t to go back even further, as close to the initial b a n g as possible. Perhaps other clues are out there, w a i t i n g to be discovered, m a y b e even encoded s o m e h o w in the radiation left over from decoupling. T h i s is w h a t cosmologists have been t r y i n g to do since r o u g h l y the 1980s, as we will soon see. T h e fact that G a m o w ' s model m a d e t w o testable predictions that could in principle be observed should not be u n d e r e s t i m a t e d . T h i s is precisely w h a t d i s t i n g u i s h e s a physical theory from p u r e m a t h e m a t i c a l speculation, the possibility of c o n f i r m i n g or rejecting a theory's predictions

through

m e t i c u l o u s observations.

Unfortunately, d u r i n g the

1950s, no one seemed to be t a k i n g G a m o w ' s ideas of p r i m o r d i a l n u c l e a r fusion seriously e n o u g h to m o u n t a consistent e x p e r i m e n t a l search. T h e t u r n i n g point c a m e in 1964, w h e n t w o g r o u p s of astrophysicists built r a d i o a n t e n n a s capable of c a p t u r i n g the radiation left over from d e c o u p l i n g , one k n o w i n g l y a n d the other u n k n o w i n g l y . In Princeton, a g r o u p led by Robert D i c k e w a s b u i l d i n g an a n t e n n a designed to test G a m o w ' s (and Alpher's a n d H e r m a n ' s ) prediction a n d m e a s u r e the t e m p e r a t u r e of the leftover radiation. Interestingly, the Princeton g r o u p w a s not a w a r e of G a m o w ' s prediction, h a v i n g rederived it independently, t h a n k s mostly to the w o r k of a y o u n g theorist n a m e d J a m e s Peebles. Not too far from Dicke's lab, A r n o P e n z i a s a n d Robert W i l s o n , w o r k i n g for Bell Telephone Laboratories, w e r e searching for radio s i g n a l s from a supernova r e m n a n t ten thousand lighty e a r s from Earth. G i v e n the l a r g e distance from the source, the signals P e n z i a s a n d W i l s o n could hope to c a p t u r e w e r e very w e a k . It w a s thus crucial that all possible sources of interference be e l i m i n a t e d or taken into account. But try as they m i g h t , a persistent b a c k g r o u n d noise, similar to an a n n o y i n g hiss in a radio, w o u l d not go a w a y . T h e y w e r e even careful e n o u g h to shoo a w a y a couple of pigeons that had happily nested inside their horn-shaped a n t e n n a . After they cleaned up the "dielectric substance" left in a b u n d a n c e by the pigeons ( w h o said research is a l w a y s g l a m o r o u s ? ) , the noise w a s still there. Even worse, it 240

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seemed to be c o m i n g from all directions in the s k y ; no m a t t e r w h e r e they pointed the a n t e n n a , the hissing w a s still there. F l u s t e r e d , they contacted B. F. B u r k e , a r a d i o astronomer from MIT, w h o told t h e m about Dicke's research at Princeton. T h e m y s t e r y of the hissing noise w a s solved, ironically not by the g r o u p that w a s looking for it; P e n z i a s a n d W i l s o n w e r e detecting the photons left over from d e c o u p l i n g , w h i c h b e c a m e k n o w n , for lack of a m o r e inspired name,

the

cosmic

microwave

background

radiation.

"Background"

since the photons are e q u a l l y ( h o m o g e n e o u s l y ) spread t h r o u g h o u t the cosmos, w i t h a t e m p e r a t u r e of 2.726 d e g r e e s Kelvin (according to present-day m e a s u r e m e n t s ) . R e m a r k a b l y , recent observations showed that this t e m p e r a t u r e is the s a m e at different points in the sky to one part in one h u n d r e d thousand! T h a t is, if you point an a n t e n n a s o m e w h e r e in the sky, m e a s u r e the t e m p e r a t u r e of the radiation, a n d then repeat the operation s o m e w h e r e else, the t w o t e m p e r a t u r e s w i l l a g r e e to 1/100,000 of a d e g r e e . " M i c r o w a v e " indicates that the m e a s u r e d photon d i s t r i b u tion p e a k s at frequencies typical of m i c r o w a v e s , about ten thousand m e g a h e r t z ( w a v e l e n g t h s of a few c e n t i m e t e r s ) .

T h e discovery by P e n z i a s a n d W i l s o n m a r k e d a t u r n i n g point in the history of cosmology; it put an end to the long battle between the steady-state a n d the big b a n g m o d e l s , at least in the m i n d s of most cosmologists; H o y l e (until his death in 2001) a n d a few collaborators still insist that it is possible to e x p l a i n the m i c r o w a v e b a c k g r o u n d w i t h i n a steady-state model. H o w e v e r , their e x p l a n a t i o n s a r e e x t r e m e l y contrived a n d u n n a t u r a l , starting w i t h the cornerstone assumption of the steady-state model, the continuous creation of m a t t e r out of nothing, w h i c h , to this cosmologist, is even m o r e o u t r a g e o u s than a universe w i t h a history.

Short

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Time:

Part

T h e discovery of the cosmological expansion and its most direct consequence, a universe with a beginning, forced scientists to confront the issue 241

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of time and its passage. T h e debate over the nature of time w a s not new to science, having originated in ancient Greece with the pre-Socratic philosophers, w h o basically split into two camps: those, like Heraclitus of Ephesus, w h o defended the central role of transformation in natureâ&#x20AC;&#x201D;the cosmos of b e c o m i n g â&#x20AC;&#x201D; a n d those, like P a r m e n i d e s of Elea, w h o posited the inherent immutability of what is truly f u n d a m e n t a l â&#x20AC;&#x201D; t h e cosmos of being. A compromise of sorts w a s achieved by the atomists Leucippus a n d Democritus, w h o held that the fundamental material entities of the w o r l d , the atoms, w e r e eternal a n d indestructible, but that they combined and bound to promote the changes we see in nature. Aristotle proposed another compromise, w i t h his division of the cosmos into two realms, the sublunar w o r l d of change and material transformation, and the supralunar world of ethereal, unchangeable objects. Saint A u g u s t i n e g a v e m u c h thought to the question of time. In particular, he w o r r i e d about h o w to reconcile t i m e a n d the C r e a t i o n . A n d don't we still, even if dressed in the robes of science? After a l l , a universe w i t h a b e g i n n i n g raises the question of how it c a m e into being; if not through the action of an a l l - p e r v a d i n g , all-powerful God, then h o w ? C a n science, a h u m a n invention that seeks a logical order in the patterns a n d cycles we observe in n a t u r e , a n s w e r such a question? T h e crucial difference b e t w e e n a religious a n d a scientific approach to the question of the origin of e v e r y t h i n g is that religion a n s w e r s it from w i t h o u t , t h r o u g h the s u p e r n a t u r a l action of a God or gods, w h i l e science must a n s w e r it as it a n s w e r s every other question, from w i t h i n , as the consequence of a chain of n a t u r a l causes. T h e creation of the universe is, for religion a n d science a l i k e , a question of the first cause, the cause that t r i g g e r e d existence. Aristotle called it the Primum Mobile, the U n m o v e d Mover, the Being located at the outermost sphere of the cosmos, from w h o m all mention o r i g i n a t e s . Cosmologists e u p h e m i s t i cally prefer to call it a problem of "initial conditions," a n d m u c h recent w o r k in cosmology deals w i t h w a y s of t r y i n g to bypass such a d i l e m m a . A u g u s t i n e proposed that t i m e a p p e a r e d w i t h the C r e a t i o n , a r g u i n g that to ask w h a t w a s before the C r e a t i o n is nonsense because there w a s no "before" before t i m e existed:

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My a n s w e r to those w h o ask " W h a t w a s God doing before he m a d e heaven a n d e a r t h ? " is not " H e w a s p r e p a r i n g Hell for people w h o pry into m y s t e r i e s . " T h i s frivolous retort has been m a d e before now, so we are told, in order to e v a d e the point of the question. . . . [God is] the M a k e r of all t i m e . If, then, there w a s a n y t i m e before [God] m a d e heaven a n d earth, h o w can a n y o n e say that [God w a s ] idle? [God] m u s t have m a d e that t i m e , for t i m e could not elapse before [God] m a d e it.

T h e absence of t i m e before the big b a n g is accepted by cosmology as well. T h e big b a n g has a lot in c o m m o n w i t h the space-time s i n g u l a r i ties we have seen exist inside black holes; it also m a r k s the b r e a k d o w n of our classical description of space a n d t i m e by m e a n s of the l a w s of general relativity. If we play the cosmic movie b a c k w a r d , from today to the earliest m o m e n t s of the universe's existence, we find matter c o m pressed to e n o r m o u s densities, as d u r i n g the final stages of stellar collapse. T h e same q u a n t u m behavior we s a w to be important near a black hole s i n g u l a r i t y w i l l be important near the cosmological s i n g u larity; our notions of time a n d space s i m p l y d i s i n t e g r a t e into a q u a n tum foam of possible coexisting times a n d spatial g e o m e t r i e s , a soup of tangled histories that go e v e r y w h e r e a n d n o w h e r e . T h u s , we also c a n not talk about time "before" the big bang, because there is no flow of time to talk about. H o w the universe m a y have possibly transitioned from a p r i m o r d i a l q u a n t u m

foam of g e o m e t r i e s into a universe

e n d o w e d w i t h a classical flow of time goes back to the determination of the "initial conditions" that set the cosmos in motion. T h e key idea is to combine g e n e r a l relativity a n d q u a n t u m m e c h a n i c s , a n d try to m a k e sense of a universe that incorporates properties a k i n to atoms a n d nuclei. We w i l l have occasion to briefly m e n t i o n some of these ideas below. Before w e move on, though, w e should s u m m a r i z e w h a t w e have l e a r n e d so far, at least in r e g a r d to time. Relativity a d d e d plasticity to the classical notion of time, posited by N e w t o n to m a r c h inexorably f o r w a r d , a l w a y s at the same rate, i r r e spective of w h o is m e a s u r i n g it a n d of w h e r e it is being m e a s u r e d . If 243

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DIAGRAM OF D I S T A N C E SCALES

F I G U R E 2 1 : Diagram of distances, from a proton to the observable universe.

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ICE

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t w o observers m o v i n g w i t h respect to each other m e a s u r e an event occurring in t i m e , they w i l l not a g r e e on its d u r a t i o n , the a m o u n t of d i s a g r e e m e n t g r o w i n g w i t h their relative velocity; fast-moving clocks tick slower. T h e y w i l l also not a g r e e if one of them is in a stronger g r a v i t a t i o n a l field; clocks in strong g r a v i t a t i o n a l fields also tick slower. We don't notice these differences in our e v e r y d a y lives, because our speeds are too small c o m p a r e d w i t h the speed of light, and the v a r i a tions on the g r a v i t a t i o n a l field a r o u n d Earth's surface cause n e g l i g i b l e c h a n g e s to the flow of time. Science has e x p a n d e d our perception of time well beyond w h a t our senses allow; there is a host of different time flows, l u r k i n g behind our naive sensorial notion of a steady c l o c k w o r k . Relativity refined time, p l u c k i n g it from the rigid N e w t o n i a n flow, so indifferent to our presence, and rendered it manifold. But this local plasticity of time is only part of the story. P e r h a p s paradoxically, t h r o u g h the discovery of an e x p a n d i n g universe, m o d e r n cosmology restored the idea of a cosmic flow of time, a global, universal t i m e , indifferent to local idiosyncrasies. We thus apparently describe reality w i t h t w o times: one local, w h i c h we perceive a n d m e a s u r e and w h i c h may, in e x t r e m e circumstances, be affected by motion or by local g r a v i t a t i o n a l variations; and the other global, cosmological, w h i c h flows completely oblivious to ourselves, our concerns a n d m e a s u r e m e n t s . T h i s indifference of cosmological time goes w a y beyond h u m a n scales, stretching t o w a r d e x t r a g a l a c t i c scales, and even to the scales of the largest self-gravitating structures, superclusters of g a l a x i e s . T h e cosmological expansion does not stretch objects bound by local forces, such as atoms, people, planets, stars, solar systems, g a l a x i e s , or clusters of g a l a x i e s ; it applies to space itself in the largest of scales, the space b e t w e e n g a l a x i e s and clusters of g a l a x i e s , of distances over tens of m i l l i o n s \ ) f l i g h t - y e a r s (see figure 21). A n d yet, in spite of their apparently enormous differences, the two times, local and cosmological, a r e really only one, t i c k i n g forward, together. It is this universality of time that l i n k s us to the rest of the cosmos. As we g r o w old, so does the S u n and so does our g a l a x y and so does the universe. T h e relevant scales are, of course, completely differ246

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ent, one h u n d r e d years being a very long life for us but not m u c h for a h y d r o g e n - b u r n i n g star a n d even less to the u n i v e r s e . In 1856, the Germ a n physicist H e r m a n n von H e l m h o l t z m a d e a very somber prediction, w h i c h some consider the gloomiest prediction in the history of 6

science: the universe is d y i n g , r u n n i n g d o w n the w a y we h u m a n s do. Of course, H e l m h o l t z k n e w nothing of the cosmological expansion, d i s covered m u c h later by Hubble. But he k n e w that, a c c o r d i n g to the second l a w of t h e r m o d y n a m i c s (the first l a w says that e n e r g y must be conserved), all isolated s y s t e m s â&#x20AC;&#x201D; s y s t e m s that cannot e x c h a n g e e n e r g y w i t h other s y s t e m s â&#x20AC;&#x201D; b e c o m e more disordered as t i m e goes by. T h e flow of time, being irreversible, is deeply related to this degeneration; it sets the r h y t h m of decay. A q u a n t i t y called entropy w a s devised to m e a s u r e this g r o w t h of disorganization; the m o r e d i s o r g a n i z e d a system, the higher its entropy, a n d the h a r d e r to extract a n y t h i n g useful from it. T h e second l a w of t h e r m o d y n a m i c s can be stated in a few different w a y s , w h i c h a r e all e q u i v a l e n t . In its simplest version, it says s o m e t h i n g q u i t e obvious, that heat a l w a y s flows from a hot spot to a cold one. T h i s we k n o w from countless practical observations, but let's i m a g i n e putting an ice cube in a glass of water, w h i c h cannot e x c h a n g e heat w i t h its s u r r o u n d i n g s , a perfect thermos; we see the ice cube m e l t i n g a n d not the w a t e r freezing. In fact, the " s y s t e m " (the ice cube a n d the w a t e r in the g l a s s ) reaches a final t e m p e r a t u r e above the m e l t i n g point of water. T h e ice melts, the w a t e r cools a bit, a n d once they reach an appropriate m e a n t e m p e r a t u r e , called the e q u i l i b r i u m t e m p e r a t u r e , nothing else happens. An i m a g i n a r y observer in the w a t e r glass finds that time stops flowing

when

the

ice

cubeâ&#x20AC;&#x201D;water system

reaches

its final

equilibrium

state.

In other w o r d s , t i m e flows only w h e n systems a r e not in t h e r m o d y n a m i c e q u i l i b r i u m . H e l m h o l t z extrapolated this s i m p l e idea to the universe as a w h o l e . Since there is nothing outside the universe, the universe being all there is, it can be considered a closed system. As such, it will also obey the second l a w a n d decay in time; stars will k e e p r a d i a t i n g their heat to outer space, w h i c h w i l l get progressively hotter as the overall entropy keeps g r o w i n g . Eventually, an e q u i l i b r i u m situation w i l l be reached w h e r e the t e m p e r a t u r e w i l l on the a v e r a g e be 247

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e v e r y w h e r e the same a n d , just as w i t h the ice cubeâ&#x20AC;&#x201D;water system, nothing else will happen; time will effectively stop, a n d the universe will die a heat death. H e l m h o l t z predicted the end of t i m e as an unavoidable consequence of the second l a w of t h e r m o d y n a m i c s . H e l m h o l t z ' s a r g u m e n t w a s a terrible blow to h u m a n k i n d ' s selfesteem. T h e d e v e l o p m e n t of science d u r i n g the eighteenth a n d nineteenth centuries, w i t h its rational interpretation of natural p h e n o m e n a , w a s followed, at least in the West, by a progressive a b a n d o n m e n t of religion. D u r i n g the M i d d l e A g e s a n d the Renaissance, religious beliefs responded to most of the anxieties of life a n d , most important, of death. T h e r e w a s a d i v i n e purpose to our terrestrial existence, w h i c h started w i t h the C r e a t i o n t h r o u g h the action of God a n d w o u l d a g a i n end through his actions, at the j u d g m e n t day. T h e comfort that w a s to be found in faith g r a d u a l l y g a v e w a y to a g r o w i n g secularization of W e s t ern thought, r e a c h i n g a crisis level w i t h H e l m h o l t z ' s prediction of the end of t i m e , w h i c h had nothing beatific about it. T h e historian of religion S. G. F. Brandon expressed this w o r r y clearly w h e n he wrote, "For W e s t e r n t h i n k e r s there can be no m o r e u r g e n t task than that of resolving this d i l e m m a , a n d , if possible, of p r o d u c i n g an a d e q u a t e phi7

losophy of history, i.e., of the m e a n i n g of man's life in t i m e . " Since science had m u c h to do w i t h b r i n g i n g on this crisis, I think it w o u l d be e x t r e m e l y irresponsible of us to turn our backs to this issue a n d just k e e p p l u g g i n g a w a y a t o u r theories and speculations. A l t h o u g h m a n y scientists have a d e e p aversion to philosophical issues, we w o u l d do a disservice to society if we neglected to clean up our own mess, so to speak. T h e question then is . . . H o w ? S o m e of my c o l l e a g u e s resolve this difficulty by s e p a r a t i n g science from faith a n d opting for a p a r t i c u l a r system of belief, w h i c h offers theiji solace in places w h e r e science can't g o . T h e y c l a i m that their science., illustrates even m o r e clearly the beauty of the C r e a t i o n a n d the wonderful spirit of God (or gods) that p e r m e a t e s all things. Powerful a n d inspiring as this c o m p r o m i s e is, I still believe that science can do better than just offer a rational e x p l a n a tion of the w o r l d , w h i c h is m e r e l y reconciled w i t h religion in the privacy of people's m i n d s . I believe that science, in an effort to u n d e r s t a n d 248

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the u n k n o w n , transcends its more i m m e d i a t e role of q u a n t i f y i n g the w o r k i n g s of n a t u r e . Perhaps this is w h a t Einstein referred to as his "cosmic religious feeling," the essentially r e l i g i o u s inspiration behind the act of rationally u n d e r s t a n d i n g the w o r l d . Science a n d religion spring from the s a m e anxieties that baffle the h u m a n spirit. A n d the one c o m m o n t h r e a d t y i n g them together is our finite existence in an a p p a r e n t l y infinite cosmos. It is in the passage of cosmic t i m e , the s a m e for a b a c t e r i u m , a person, a star, a n d the u n i v e r s e , that we find the true unity of all things. T i m e connects our existence to e v e r y t h i n g else in the cosmos, i n c l u d i n g the cosmos itself. T h i n k that every t i m e your heart beats, countless insects hatch out of their e g g s w h i l e countless others die, w a v e s break on all the beaches of the w o r l d , g a l a x i e s move farther a w a y from each other, a n d stars a r e born a n d die s o m e w h e r e in the universe. T i m e sets the r h y t h m of existence. It is w h e n we focus e x c l u sively on t r y i n g to m a k e sense of the b e g i n n i n g and end of time, forgetting our l i n k s to existence now, that we lose our w a y a n d fall prey to either r e l i g i o u s fervor, existentialist despair, or scientific pontification. As I have a r g u e d throughout this book, we all have a d e e p need to u n d e r s t a n d our o r i g i n a n d d e m i s e , a n d this need is expressed in m a n y different w a y s . T h e m i s t a k e we m a k e , w h i c h , I believe, is in l a r g e part responsible for the " w a r s " between r e l i g i o u s belief a n d science a n d b e t w e e n h u m a n i s t s a n d scientists, is to forget that these different narratives inspire a n d c o m p l e m e n t each other a n d a r e not m u t u a l l y e x c l u sive. T i m e should not be divisive but inclusive. T h i s c o m p l e m e n t a r y approach to the question of t i m e could be called chronosophy: we should certainly w o r r y about the b e g i n n i n g a n d the end, a n d try as m u c h as possible to u n d e r s t a n d these events t h r o u g h science, faith, a n d art. But we should never forget that we live in b e t w e e n these t w o a l l important events, a n d that our very existence is the product of m a n y creations a n d destructions, cosmic a n d emotional. To p a r a p h r a s e John L e n n o n , life is w h a t h a p p e n s w h i l e you are w o r r y i n g about death. We m a y all be inspired by death in different d e g r e e s , but we should never be p a r a l y z e d by it. 249

CHAPTER

Time Regained

Worlds on

worlds are rolling ever

From creation to decay, Like the bubbles on a river Sparkling, —

bursting, P E R C Y

borne away. B Y S S H E

S H E L L E Y , L I N E S

Hellas,

I 9 7 - 2 0 0 '

T h e third m a i n stage of d e v e l o p m e n t of m o d e r n cosmology is best c h a r a c t e r i z e d by a double explosion—of theory a n d observation. On the theoretical side, the t r i u m p h of the big b a n g model led to an increased effort to u n d e r s t a n d the earliest stages of the universe's history, m a d e possible t h r o u g h an influx of ideas from h i g h - e n e r g y particle physics. If n u c l e o s y n t h e s i s — t h e synthesis of the lightest nuclei between one second a n d three m i n u t e s after the b a n g — w a s understood t h r o u g h the application of nuclear physics to cosmology, for e a r l i e r times, we m u s t use a description of m a t t e r at energies h i g h e r than nuclear, w h e n it is broken into its most fundamental constituents. T h i s is w h e r e my research comes into the g a m e , in t r y i n g to m a k e sense of the cosmos in its earliest infancy. On the observational side, the results of P e n z i a s a n d W i l s o n w e r e further confirmed a n d refined to an a m a z i n g d e g r e e by a series of spect a c u l a r m e a s u r e m e n t s of the properties of the cosmic m i c r o w a v e b a c k g r o u n d . T h e s e w e r e achieved t h r o u g h Earth-based e x p e r i m e n t s , such 251

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as m i c r o w a v e detectors flown over the South Pole on h i g h - a l t i t u d e balloons, a n d t h r o u g h the C o s m i c B a c k g r o u n d Explorer ( C O B E ) satellite, w h i c h m e a s u r e d the m i c r o w a v e b a c k g r o u n d over the w h o l e sky and constructed a m a p w i t h an a n g u l a r accuracy of about ten d e g r e e s . T h i s m a p is a snapshot of the universe w h e n it w a s only 300,000 years old. L i k e any m a p , it invites us to travel to its l a n d s w i t h their promise of w o n d e r s a n d riches. A n d m a n y riches w e r e indeed found, and remain to be found in the next few y e a r s , as n e w satellite missions produce m u c h m o r e detailed m a p s . For a l t h o u g h the C O B E m a p confirmed the a m a z i n g t e m p e r a t u r e homogeneity of the cosmic m i c r o w a v e backg r o u n d , it also revealed w h a t lies beneath this c a l m ocean of radiation: small t e m p e r a t u r e fluctuations that c a r r y information from the very e a r l y m o m e n t s of cosmic history (see figure 35 in insert). But the m i c r o w a v e b a c k g r o u n d is only one aspect of a vast observational effort, w h i c h involves, a m o n g other things, telescopes scanning all w a v e l e n g t h s , from radio w a v e s to g a m m a r a y s , a l l o w i n g us to study objects that are several billion l i g h t - y e a r s a w a y , and thus older than the solar system itself. T h e s e observations have revealed a very complex universe, w h e r e g a l a x i e s cluster in long three-dimensional filamentary webs that can cross tens of millions of l i g h t - y e a r s , w h i l e at other spots there are e n o r m o u s regions practically without any matter, the cosmic voids (see figure 36 in insert). An i m a g e often used to help visualize the large-scale structure of the cosmos is that of a bubble bath, w i t h soapbubble w a l l s touching a n d coalescing to form a rich froth. Now, sprinkle black pepper on the froth (without popping the bubbles), a n d observe the distribution of the pepper g r a i n s . If you i m a g i n e each pepper g r a i n as a g a l a x y , y o u get a rough picture of the universe at very l a r g e scales: g r a i n s cohering a r o u n d ÂŤJTipty space. As w e will see, modern cosmological theory t r i e s to w e a v e these two key observational discoveries together, s h o w i n g that the t e m p e r a t u r e fluctuations of the m i c r o w a v e b a c k g r o u n d are tied in w i t h the fluctuations that g e n e r a t e d , through gravitational instabilities, the rich a n d complex large-scale structure of the observable universe. W h a t ' s more, the theory predicts

252

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that

these

fluctuations

were

generated

the

R E G A I N E D

universe's

earliest

moments of existence, the physics of the very small, e l e m e n t a r y particles and their interactions, influencing the physics of the very l a r g e , the past tied to the present and to the future.

The

Cosmos,

Building

circa

1998:

Blocks

I will start by briefly r e v i e w i n g some of the i d e a s of particle physics from the last three decades. T h e goal of h i g h - e n e r g y particle physics is to find the smallest constituents of matter, the basic b u i l d i n g blocks of e v e r y t h i n g in the cosmos. T h i s mission, of course, a s s u m e s that these constituents exist; if you k e e p d i v i d i n g matter into s m a l l e r c h u n k s , you will e v e n t u a l l y reach the smallest c h u n k s , the e l e m e n t a r y particles. D u r i n g the 1940s a n d 1950s, e x p e r i m e n t s involving high-speed collisions between heavy nuclei and s m a l l e r particles, such as electrons a n d protons, revealed an e x t r e m e l y rich subatomic w o r l d , populated by hundreds of other particles of matter. T h i s proliferation of particles w a s a direct consequence of Einstein's prediction that matter and e n e r g y are interconvertible, given the proper conditions. I m a g i n e colliding an electron and its antiparticle, the positron, w h i c h we encountered in chapter 5. T h e i r identical masses contribute to the total e n e r g y of the electron-positron system before they collide. But they also have k i n e t i c energy, e n e r g y of motion, w h i c h g r o w s w i t h their velocity. T h u s , the total e n e r g y of the electron-positron pair before they collide is m u c h h i g h e r than their rest masses, because of the contribution from the e n e r g y of their motion. W h e n they collide, as we have seen, they d i s i n tegrate into photons, w h i c h c a r r y all the e n e r g y of the electronpositron pair, their rest mass and their k i n e t i c energy. T h e photons m a y then create other particles, w h i c h can be m u c h more massive than the original electron-positron pair. T h e only r e q u i r e m e n t is that the

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total e n e r g y before a n d after is the s a m e . T h e r e is thus a wonderful t r a n s m u t a t i o n of e n e r g y into matter, following r o u g h l y a relation like this: electron + positron + lots of kinetic e n e r g y â&#x20AC;&#x201D;> h i g h l y energetic photons â&#x20AC;&#x201D;> heavier particles + some kinetic energy.- A physicist once r e m a r k e d that an e q u i v a l e n t phenomenon at h u m a n scales w o u l d be to collide two tennis balls a n d get t w o elephants! T h i s proliferation of particles w a s c l e a r l y a m a j o r c h a l l e n g e for those w h o searched for the basic constituents of m a t t e r ; after a l l , w h a t is the m e a n i n g of e l e m e n t a r y b u i l d i n g blocks if y o u have h u n d r e d s of t h e m ? T h e resolution of this d i l e m m a w a s proposed in 1963 by the C a l t e c h physicist M u r r a y G e l l - M a n n , a n d i t bears some resemblance to h o w we e x p l a i n the periodic table of e l e m e n t s . T h e r e are n i n e t y t w o n a t u r a l l y o c c u r r i n g c h e m i c a l e l e m e n t s , all m a d e o f the s a m e three basic constituents, electrons, protons, a n d n e u t r o n s . By c o m b i n i n g these three particles in different n u m b e r s , we recover all the n i n e t y t w o c h e m i c a l e l e m e n t s . G e l l - M a n n proposed that a s i m i l a r idea could e x p l a i n the h u n d r e d s of particles that a p p e a r e d in the h i g h - e n e r g y collision e x p e r i m e n t s . T h e y all h a d one very i m p o r t a n t property in c o m m o n ; they interacted via the strong n u c l e a r force, the s a m e force that binds n e u t r o n s a n d protons together in the a t o m i c n u c l e u s . P a r t i cles that interact via the strong force a r e called hadrons. G e l l - M a n n s h o w e d that all h a d r o n s can be described as c o m b i n a t i o n s of m o r e f u n d a m e n t a l particles, w h i c h he n a m e d q u a r k s , t a k i n g a w o r d from J a m e s Joyce's Finnegans Wake. We n o w k n o w that there are six fundam e n t a l q u a r k s i n n a t u r e , w h i c h g o b y the n a m e s u p , d o w n , c h a r m , s t r a n g e , beauty, a n d top. T h e proton, for e x a m p l e , is m a d e of three q u a r k s , t w o ups a n d one d o w n ( u u d ) , w h i l e the n e u t r o n i s m a d e o f t w o d o w n s a n d one u p ( u d d ) . Present particle e x p e r i m e n t s g i v e n o indication that a n y a d d i t i o n a l q u a r k s exist. A p a r t from the six q u a r k s , there is another g r o u p of six particles known

leptons,

from

the

Greek

word

for

"lightweight."

encountered t w o of t h e m , the electron a n d the electron neutrino, w h i c h can in fact be thought of as forming a pair. T h e other four lep-

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tons a r e also g r o u p e d in pairs, the m u o n a n d its n e u t r i n o , a n d the tau and its neutrino. T h e m u o n a n d the tau a r e pretty m u c h l i k e heavy electrons, the m u o n being about two h u n d r e d s t i m e s as massive, w h i l e the tau is almost four thousand times as m a s s i v e . Indeed, the tau is almost t w i c e as heavy as the proton, m a k i n g the n a m e lepton, " l i g h t w e i g h t , " a n interesting misnomer. T h e s e t w e l v e e l e m e n t a r y particles, the six q u a r k s a n d the six leptons, a r e e l e g a n t l y g r o u p e d into three " f a m i l i e s " of four m e m b e r s each. (To each particle we can a d d its antiparticle as w e l l . )

THE THREE F A M I L I E S OF ELEMENTARY P A R T I C L E S electron

muon

e-neutrino

tau

m-neutrino

charm

down

strange

t-neutrino bottom top

T A B L E 2 : The three families of elementary particles.

The

Cosmos,

circa

The

Quartet

1998:

Fundamental

Forces

T h e m a t t e r w e a r e f a m i l i a r w i t h â&#x20AC;&#x201D; p r o t o n s , neutrons, a n d e l e c t r o n s â&#x20AC;&#x201D; belongs exclusively to the first family. M e m b e r s of the other families appear only in h i g h l y energetic astrophysical p h e n o m e n a or in particle collisions on Earth. We thus a r r i v e at a classification of m a t t e r in t e r m s of only t w e l v e e l e m e n t a r y particles, six q u a r k s a n d six leptons (and their antiparticles). But this is only h a l f of the story. In order to describe the physics of matter, we must u n d e r s t a n d h o w those b u i l d i n g blocks interact w i t h each other. After a l l , we cannot build a sturdy house s i m ply by piling up b r i c k s on top of each other; we need mortar. H e r e

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enters the concept of the fundamental forces of n a t u r e , w h i c h we believe are four in all. We have encountered three of t h e m so far: the gravitational force, with

the

electric

electromagnetic force, charge,

and

which the strong

acts

nuclear force,

any which

particle binds

q u a r k s together into h a d r o n s , such as protons a n d neutrons, a n d also binds protons a n d neutrons into atomic nuclei. T h e last m e m b e r of the q u a r t e t of f u n d a m e n t a l

forces is the feeble weak^ nuclear force,

whose

most important effect is to promote the radioactive decay of unstable nuclei, such as u r a n i u m or p l u t o n i u m . L i k e the strong nuclear force, the w e a k force acts only w i t h i n nuclear distances; in our e v e r y d a y lives we experience only the g r a v i t a t i o n a l a n d e l e c t r o m a g n e t i c forces, the ones capable of being felt at long r a n g e . So, the subatomic w o r l d is described in t e r m s of t w e l v e particles of m a t t e r a n d four f u n d a m e n t a l forces. T h e s e f u n d a m e n t a l forces are also described in t e r m s of particles, m e s s e n g e r s c a r r y i n g the information about the interactions between the different m a t t e r particles. Each force has its o w n set of c a r r i e r s , w h i c h we call the particles of force. For e x a m p l e , e l e c t r o m a g n e t i c interactions, such as the electric repulsion between two electrons, a r e represented by an e x c h a n g e of photons. A suggestive i m a g e often used is that of t w o ice-skaters t h r o w i n g baseballs at each other; the e n e r g y a n d m o m e n t u m of the baseball is exchanged

between

the

two

skaters,

establishing

interaction

between t h e m . T h e photons a r e the carriers of the e l e c t r o m a g n e t i c interactions. Note that photons are massless particles, their e n e r g y b e i n g m e a s u r e d only by their frequency. T h i s "masslessness" is no coincidence, but a consequence of the fact that the e l e c t r o m a g n e t i c force is l o n g - r a n g e , d e c a y i n g w i t h the s q u a r e of the distance from its source, v a n i s h i n g only w h e n the source is infinitely distant. If the photon had a mass, the e l e c t r o m a g n e t i c force w o u l d be short-range, l i k e the w e a k a n d strong n u c l e a r forces*. T h e s a m e is t r u e of g r a v i t y ; since the g r a v i t a t i o n a l attraction between t w o bodies also decays w i t h the s q u a r e of their separation, the graviton, the particle conjectured (it has not been observed y e t ) to t r a n s m i t the g r a v i t a t i o n a l force, is also massless. 256

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Since the strong a n d w e a k forces a r e s h o r t - r a n g e , their description in terms of c a r r i e r s must be different. T h e w e a k n u c l e a r force has not one but three force c a r r i e r s , w h i c h w e r e in the 1960s predicted to exist b y S h e l d o n G l a s h o w , A b d u s S a l a m , a n d Steven W e i n b e r g , all w i t h masses w i t h i n e i g h t y to ninety times that of a proton. T h e s e three particles, w h i c h go by the u n i n s p i r e d n a m e s of W% W% a n d Z째 (the superscripts denote their electric c h a r g e s ) , w e r e found in the early 1980s by the Italian physicist C a r l o Rubbia a n d his team at the European C e n t e r for Particle Physics ( C E R N ) . T h i s r e m a r k a b l e t r i u m p h of theoretical and e x p e r i m e n t a l particle physics w a s r e c o g n i z e d w i t h w e l l - d e s e r v e d Nobel P r i z e s a n d shaped the d r e a m s of a w h o l e g e n e r a t i o n of y o u n g physicists g r o w i n g up in the 1970s a n d 1980s, i n c l u d i n g this one. W h a t could be m o r e e x c i t i n g than contributing to the g r a n d mission of u n v e i l i n g the innermost constitution of the m a t e r i a l w o r l d ? I have met the three theorists at different stages of my career. W e i n berg's The First Three Minutes, an e a r l y popular account of big b a n g cosmology a n d the discovery of the cosmic m i c r o w a v e b a c k g r o u n d , g r e a t l y inspired me to become a cosmologist, especially one w h o applied particle physics to the early universe, as he had masterfully done. I met h i m in the m i d - 1 9 8 0 s , d u r i n g a m e e t i n g of the Royal Society in L o n d o n , w h e n I w a s a g r a d u a t e student at K i n g ' s C o l l e g e . He certainly does not r e m e m b e r this, but I sat by his side, completely transfixed by being close to such a g r e a t m a n ; W e i n b e r g ' s textbook on g e n e r a l relativity a n d cosmology w a s (and still is) a k i n d of bible to m e . W h a t I most r e m e m b e r is the profusion of nervous e n e r g y c o m i n g out of h i m ; his leg bounced nonstop, he dropped his n e w s p a p e r q u i t e a few times ( w h i c h I offered to pick up w i t h a pathetic reverent look in my eyes), a n d c o n t i n u a l l y looked a r o u n d as if s e a r c h i n g for someone or something. P e r h a p s he w a s just c h e c k i n g his a u d i e n c e . A l t h o u g h I really w a n t e d to tell him about my thesis w o r k on cosmology in h i g h e r d i m e n s i o n s (a popular topic then a n d , curiously, now a g a i n ) , his attitude, m i x e d w i t h my o w n shyness, w a s not very inviting. I sat in silence and paid attention to w h a t he had to say d u r i n g his talk. A b d u s S a l a m k n e w of my w o r k w h e n I w a s still a t h i r d - y e a r g r a d 257

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uate student, because he w a s also interested in the idea that the u n i verse perhaps has m o r e spatial d i m e n s i o n s than the f a m i l i a r three. T h e key question at the t i m e w a s to try to m a k e sense of how this could be; that is, how could a universe, w h i c h started w i t h ten or eleven d i m e n sions, be so obviously four-dimensional today. ( T h e s e n u m b e r s i n c l u d e one dimension for time.) My w o r k , together w i t h my Ph.D. adviser, John G. Taylor, w a s to d r e a m up scenarios w h e r e the " e x t r a " spatial dimensions got curled up into a ball, w h i c h s o m e h o w did not e x p a n d w i t h the rest of the universe. T h u s , as t i m e w e n t by, there e m e r g e d a clear separation b e t w e e n the t h r e e - d i m e n s i o n a l space we live in a n d the extra, "compactified" d i m e n s i o n s , w h i c h r e m a i n e d m u c h , m u c h smaller. W h y w e r e we interested in a universe w i t h so m a n y d i m e n sions? T h e idea w a s that in a h i g h e r - d i m e n s i o n a l universe it m i g h t be possible to interpret all four forces of n a t u r e as b e i n g initially one force, the "unified field." As the universe e x p a n d e d , its g e o m e t r y split, c a u s ing the unified force to differentiate into the four forces we see today. T h i s apparently strange approach w a s first put forward in 1919 by the Polish m a t h e m a t i c i a n T h e o d o r K a l u z a , w h o showed that in a fivedimensional universe, e l e c t r o m a g n e t i s m , the only other force k n o w n then, could, l i k e gravity, be interpreted geometrically. W h a t we perceive as a four-dimensional w o r l d w i t h g r a v i t y a n d e l e c t r o m a g n e t i s m can be explained as a five-dimensional w o r l d w i t h only one force. T h e distinction between the t w o forces comes from a peculiar choice for the geometry of this five-dimensional w o r l d , w h i c h can be best visualized as cylindrical, like a stretched g a r d e n hose; i m a g i n e that the four usual dimensions we live in (three space a n d one t i m e ) a r e represented by the (infinitely) long axis of a "cylinder." Each point along the axis has a cir1

cle "attached" to it, the cross sectipnV )! the cylinder, as illustrated in figu r e 22. T h i s circle is associated w i t h the extra, fifth d i m e n s i o n , w h i c h is thus perpendicular to our four-dimensional reality. Just as a hose, w h e n seen from afar, looks l i k e a one-dimensional line, the extra circular dimension can be "seen" only at e x t r e m e l y small distances. In 1926, the S w e d i s h m a t h e m a t i c i a n O s k a r Klein applied q u a n t u m mechanics to K a l u z a ' s five-dimensional unification scheme to show that the r a d i u s of 258

TIME

REGAINED

F I G U R E 2 2 : Schematic diagram of Kaluza's five-dimensional "cylindrical" universe: a "circle" is attached to each point in our four-dimensional space-time.

the extra dimension could be as small as the smallest length scale w h e r e our description of gravity by m e a n s of g e n e r a l relativity still m a k e s sense, called the Planck length, equal to 1 0

- 4 3

centimeters. It is no w o n -

der that, if the universe does indeed have extra spatial dimensions, they have so far e l u d e d direct detection; present h i g h - e n e r g y experiments probe distances of about 1 0

1 6

centimeters. According to the K a l u z a -

Klein theory, w h a t we perceive as electromagnetism and gravity in our w o r l d are but c h a n n e l e d - d o w n vibrations of a peculiar five-dimensional w o r l d w h e r e g r a v i t y rules alone. D u r i n g the 1980s, it w a s shown that, in principle, the K a l u z a - K l e i n approach could be extended to include the strong and w e a k interactions. T h i s being the case, the four forces of nature all originated from a single force living in a higherdimensional space. It w a s an exciting time to be doing a Ph.D. in h i g h e n e r g y physics. For my thesis, I had obtained several solutions of Einstein's e q u a tions in these h i g h e r - d i m e n s i o n a l w o r l d s , investigating in p a r t i c u l a r their cosmology, that is, how these g e o m e t r i e s w o u l d evolve in time. T h i s approach w a s reminiscent of the d e s k t o p universes of the first half of the twentieth century, for we had no observational evidence (and still don't) to g u i d e our m a t h e m a t i c a l models, only physical intuition and consistency; we k n o w w h a t properties these theories must 259

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have w h e n reduced to four d i m e n s i o n s , since they m u s t conform to the universe w e live in. T h e s e w e r e h i g h e r - d i m e n s i o n a l desktop universes, g e n e r a l i z i n g K a l u z a ' s old idea to include all four forces of n a t u r e into a single g e o m e t r i c a l scheme, a g r a n d realization of Plato's d r e a m of an u n d e r l y i n g reality of perfect g e o m e t r i c a l forms translated into m o d e r n physical thought. H a d w e succeeded, w e could have proudly stated, "In the b e g i n n i n g all w a s g e o m e t r y . " But we haven't yet.

S a l a m w a s e x t r e m e l y g e n e r o u s to m e , q u o t i n g my w o r k in his t a l k s a n d papers, m u c h t o m y a m a z e m e n t ; i m a g i n e t h a t â&#x20AC;&#x201D; a Nobel P r i z e w i n n e r citing the w o r k of a B r a z i l i a n g r a d u a t e student. T h i s attention to my w o r k , s i m p l e as it w a s , g a v e me t r e m e n d o u s self-confidence. I think S a l a m k n e w that a n d , as head of the International C e n t e r for T h e o r e t i c a l Physics in Trieste, Italy, w a n t e d to help y o u n g physicists from T h i r d W o r l d a n d d e v e l o p i n g countries to succeed. My adviser invited h i m to be the external e x a m i n e r ( w h o at first sounds m o r e l i k e the chief i n q u i s i t o r ) at my Ph.D. thesis defense, w h i c h , in E n g l a n d , but not in the U n i t e d States, is a private affair, involving just the student, the adviser, a n d the external e x a m i n e r between four w a l l s . For about an hour, S a l a m a s k e d me the usual questions about my w o r k , mostly focused on the motivations for a s s u m i n g this or that behavior, a n d all w e n t well e n o u g h . Inspired by all this, I a s k e d h i m to w r i t e me a reco m m e n d a t i o n letter for a postdoctoral research position, a t w o - to t h r e e - y e a r a p p o i n t m e n t that is a m u c h needed stepping-stone t o w a r d the elusive university professorship. I e n d e d up accepting a postdoctoral position w i t h the astrophysics g r o u p at F e r m i N a t i o n a l A c c e l e r a tor Laboratory ( F e r m i l a b , for short), near C h i c a g o , w h i c h w a s then, a n d still is, a powerhouse of ideas in cosmology a n d astrophysics. It w a s at F e r m i l a b that I met my longtime' collaborator a n d friend R o c k y Kolb, w h o b e c a m e my true rrfentor d u r i n g my transition to A m e r i c a n academia. My encounter w i t h S h e l d o n G l a s h o w w a s of a m o r e " d o m e s t i c " n a t u r e . T e r r y W a l k e r , m y F e r m i l a b c o l l e a g u e n o w a t Ohio State U n i versity, had gotten a postdoctoral position at Boston University a n d invited me to g i v e a s e m i n a r there. T h e a r r a n g e m e n t s surprised me a 260

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bit; instead of the usual motel room, I w a s to spend a w e e k as a guest at Glashow's h o m e . It w a s q u i t e a w e e k , also t h a n k s to my B r a z i l i a n friend A n g e l a Olinto, w h o w a s finishing her Ph.D. at M I T , and k n e w w h e r e to go and w h a t to do in Boston. I w a s invited to Shelly's b i r t h d a y celebration, w h i c h w a s clearly not his favorite t i m e of the year, a small affair i n c l u d i n g t w o other professors from B U , a n i m a l m a s k s brought by one of t h e m , and a string quartet. T h e h i g h l i g h t of the w e e k , apart from m y s e m i n a r and m y outings w i t h A n g e l a and her friends, c a m e o n S a t u r d a y m o r n i n g , w h e n S h e l l y offered to m a k e me cappuccino in his kitchen, and we spent q u i t e some time t a l k i n g about physics and movies. We both had w a t c h e d Dar\ Eyes the n i g h t before, the romantic I t a l i a n - R u s s i a n movie starring M a r c e l l o M a s t r o i a n n i , w h i c h S h e l l y clearly enjoyed, j u d g i n g by his big smile on the w a y out from the theater. " W a s n ' t that w o n d e r f u l ? " he said, his eyes g l o w i n g . It m a y have been the e l e g a n c e and poetry of the photography, or the impossibility of the love affair between the m a r r i e d M a s t r o i a n n i a n d an elusive Russian w o m a n . In a n y case, it w a s obvious that we felt s o m e t h i n g in c o m m o n , perhaps a l o n g i n g for the i n t a n g i b l e , hard to describe but easy to recognize.

The The

Quest

for

Unification:

Inner-Space/Outer-Space

Connection

Back to the four fundamental forces. T h e w e a k force is thus c a r r i e d by the three heavy particles postulated by the G l a s h o w - S a l a m - W e i n b e r g m o d e l s and found at C E R N . W h a t about the strong nuclear force? We have seen that protons, neutrons, a n d all the hadrons, the particles that interact via the strong force, are m a d e of q u a r k s a n d their antiparticles, the a n t i q u a r k s . T h e particles that k e e p the q u a r k s together in the h a d r o n s , acting as a k i n d of nuclear g l u e , a r e called, properly e n o u g h , g l u o n s . T h e r e are eight g l u o n s , all massless l i k e the photon. T h a t the 261

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strong force has massless carriers a n d still is a s h o r t - r a n g e force, an a p p a r e n t contradiction, is d u e to a s o m e w h a t m y s t e r i o u s property k n o w n as confinement; the best w a y to v i s u a l i z e w h a t goes on inside a proton is to t h i n k of a b a g w i t h three q u a r k s s o m e h o w stuck inside, as if the g l u o n s w e r e elastic strings connecting them. T h e a m a z i n g property of these strings is that the farther you try to separate the q u a r k s , the h a r d e r the strings pull t h e m back together; in fact, to pull the q u a r k s outside a proton, you have to spend so m u c h e n e r g y that the string snaps a n d two n e w q u a r k s a p p e a r at the b r o k e n ends. In other w o r d s , the q u a r k s are confined inside the h a d r o n s in such a w a y as to m a k e it impossible to liberate them. We cannot see free q u a r k s r o a m ing a r o u n d the w o r l d . Only t h r o u g h h i g h l y energetic particle i n t e r a c tions can we conclude that protons, neutrons, a n d other h a d r o n s have an inner structure represented by three point-like particles inside a small region p e r m e a t e d by a g l u o n i c sea, the q u a r k s in the bag. We can t h i n k of n u c l e a r m a t t e r as h a v i n g t w o distinct phases, just as l i q u i d w a t e r a n d ice a r e t w o distinct phases of w a t e r ; at low e n e r g i e s , n u c l e a r m a t t e r a p p e a r s as protons, neutrons, a n d other h a d r o n i c particles, w i t h no sign of its inner q u a r k - g l u o n structure. But at h i g h e n o u g h t e m p e r a t u r e s (and thus e n e r g i e s ) , e q u i v a l e n t to about one trillion d e g r e e s Kelvin ( h u n d r e d s of t i m e s h i g h e r than at the core of a s u p e r n o v a ) , the h a d r o n s melt, so to speak, into a q u a r k - g l u o n p l a s m a . At these incredibly h i g h t e m p e r a t u r e s a n d densities the reverse of confinement happens; w h e n s q u e e z e d close together, the q u a r k s behave l i k e free particles, a property aptly n a m e d asymptotic freedom. T h e r e a r e t w o places w h e r e a q u a r k - g l u o n p l a s m a can exist; before one h u n d r e d thousandth of a second after the big bang, w h e n t e m p e r a t u r e s w e r e sufficiently h i g h , or at h i g h l y e n e r g e t i c nuclear collisions, such as those now u n d e r w a y (2002) at t h e - R e l a t i v i s t i c H e a v y Ion C o l l i d e r ( R H I C ) at the B r o o k h a v e n National Laboratory, on L o n g Island. T h i s transition of the properties of nuclear m a t t e r is crucial to our u n d e r s t a n d i n g of the early history of the universe. Recall that G a m o w started

his

primordial

soup w i t h

protons a n d

neutrons, that is,

hadrons. If we w a n t to go further back in t i m e , t o w a r d the initial cos262

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mological s i n g u l a r i t y , we should really start the soup w i t h q u a r k s a n d leptons, interacting via the four fundamental forces, as d e t e r m i n e d by m o d e r n particle physics. T h i s recipe w o u l d be correct at least a r o u n d one m i l l i o n t h of a second after the bang. As we w i l l see, at earlier times other c h a n g e s occur. T h e lesson here is clear: we cannot study the h i s tory of the very e a r l y universe w i t h o u t incorporating particle physics into cosmology. A n d since, as we w i l l also see, the " e n d " d e p e n d s on the b e g i n n i n g , we cannot investigate the future evolution of the u n i verse w i t h o u t k n o w i n g its history; in m a n y w a y s , the universe's c h i l d hood d e t e r m i n e s m u c h of its m a t u r e life, a notion that echoes our o w n private histories. T h e universe cannot run from its past.

EVENT Decoupling Nucleosynthesis Quark-hadron

TIME

TEMPERATURE

300,000 y e a r s

1000 Kelvin

1 sec lO-'sec

Kelvin

T A B L E 3 : Important cosmological events discussed so far.

We w a n t to k e e p g o i n g b a c k w a r d in t i m e , g e t t i n g as close as we can to the initial event. H o w e v e r , as we go b a c k w a r d , we must deal w i t h physics at h i g h e r a n d h i g h e r e n e r g i e s ; we have seen this g o i n g from G a m o w ' s proton-dominated p r i m o r d i a l soup, fine for describing the synthesis of light nuclei at t i m e s of r o u g h l y one second after the bang, to the q u a r k - a n d l e p t o n - d o m i n a t e d soup at t i m e s of a m i l l i o n t h of a second after the bang. T h e next step b a c k w a r d initiates a trend that w i l l repeat itself all the w a y to the b e g i n n i n g , or pretty close to it. T h e trend has to do w i t h a deep c h a n g e in the w a y particles interact at e n e r g i e s h i g h e r than the ones c a u s i n g the h a d r o n s to melt into q u a r k s ; the four f u n d a m e n t a l forces, w h i c h r e g u l a t e how particles interact, start to shed their u n i q u e behaviors a n d , in pairs, act m o r e as a single force. T h e h i g h e r the energy, the more the forces approach one another, one by one, until, at the earliest of t i m e s , there is only one force. We see that there a r e , in fact, t w o a v e n u e s t o w a r d unification. One 263

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could be called the geometrical approach, w h i c h uses extra spatial dimensions

describe

all

forces

equal

footing,

top-down

approach. T h i s is the case of the K a l u z a - K l e i n unification, w h i c h , for all its attractiveness, has problems i n c l u d i n g o r d i n a r y q u a r k s a n d leptons; it w o r k s well for the particles of force but not for the particles of matter. A very i m p o r t a n t variation on K a l u z a - K l e i n h i g h e r - d i m e n sional unification involves superstring theories, w h e r e the notion of an e l e m e n t a r y particle is replaced by the notion of fundamental strings. A c c o r d i n g to these theories, w h a t we call e l e m e n t a r y particles in our l o w - e n e r g y reality are really different vibrational modes of the fundamental strings, a truly e l e g a n t approach to unification. Unfortunately, because of their m a t h e m a t i c a l complexity, superstring theories a r e still very m u c h a w o r k in progress, a l t h o u g h a p r o m i s i n g one. I will restrict m y s e l f to the second a v e n u e t o w a r d unification of forces, w h i c h r e m a i n s in four d i m e n s i o n s , the bottom-up approach. It leaves g r a v i t y aside but attempts to bring the electromagnetic, w e a k , and strong forces together as e n e r g i e s increase. Irrespective of the role superstring theories m a y play at the earliest stages of the universe's history, given our focus on questions related to the universe's future, we should be able to m a k e progress w i t h o u t t r a v e l i n g d e e p into extra d i m e n s i o n s . In fact, we will adopt w h a t cosmologists call the " l o w - e n e r g y effective theory" approach, in w h i c h the information from the universe's hazy b e g i n n i n g s (superstrings or not) is encoded into a s i m p l e model that supposedly encapsulates most of w h a t we need to carry on.

\ The

Quest

for

Unification:

v,r , * Toward

the

Beginning M

T h e f i r s t unification b r i n g s e l e c t r o m a g n e t i s m and the w e a k n u c l e a r 15

force together at t e m p e r a t u r e s of one thousand trillion (lO ) d e g r e e s Kelvin. At t e m p e r a t u r e s h i g h e r than this, the w e a k nuclear force, w h i c h we perceive from our m u n d a n e l o w e r - e n e r g y reality as a short264

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range force, becomes l o n g - r a n g e , just l i k e e l e c t r o m a g n e t i s m . In practice, the three force c a r r i e r s of the w e a k nuclear force become as m a s s less as the photon above this t e m p e r a t u r e ; the t w o forces behave as one w i t h four massless c a r r i e r s . T h i s deep c h a n g e in the behavior of these two f u n d a m e n t a l forces is at the borderline of w h a t we can study w i t h particle accelerators here on Earth, in p a r t i c u l a r at F e r m i l a b a n d C E R N . We h a v e g r e a t confidence that our description, based on the G l a s h o w - S a l a m - W e i n b e r g model, is correct, a l t h o u g h one key i n g r e dient is missing, the elusive particle k n o w n as the H i g g s boson. T h e H i g g s , n a m e d after the Scottish physicist Peter H i g g s , plays a crucial role in e l e c t r o w e a k unification, as the particle responsible for g i v i n g mass to all t w e l v e (or nine, if the neutrinos a r e massless) massive q u a r k s and leptons. We can think of the H i g g s as a sticky fellow that h u g s every particle w i t h a g i v e n strength, k n o w n as a c o u p l i n g constant. It h u g s the electron w i t h a certain strength and the up q u a r k w i t h another strength. Once the H i g g s h u g s a particle, the particle is doomed to c a r r y it a r o u n d l i k e a p e r m a n e n t b a c k p a c k ; the net result is that the strengths of these h u g s , or, if you w a n t , the masses of these b a c k p a c k s , d e t e r m i n e the masses of the particles on a one-by-one basis. W h a t controls the h u g g i n g behavior of the H i g g s is the a m b i e n t t e m p e r a t u r e . For t e m p e r a t u r e s above the scale of e l e c t r o w e a k unification, a r o u n d 1 0 ^ d e g r e e s K e l v i n , the H i g g s i s pretty m u c h innocuous, for the heat agitation interferes w i t h a n y h u g g i n g attempts. But as the t e m p e r a t u r e drops, w h i c h , as we have seen, necessarily happens as the universe e x p a n d s , the H i g g s begins to stick to the q u a r k s and leptons w i t h different strengths, g i v i n g them the masses m e a s u r e d at low energies. At this point, it is worth i n t r o d u c i n g the concept of a field. Every particle in n a t u r e , be it a particle of matter, l i k e an electron or a q u a r k , or a particle of force, l i k e a photon or a g l u o n , has a field associated w i t h it. In fact, particle physicists prefer to describe particles as being excitations of their fields, s o m e w h a t the w a y a sound note on a g u i t a r string is an excitation of the string. T h e string can be excited in m a n y different w a y s , p r o d u c i n g notes of a different pitch (frequency) and 265

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loudness ( a m p l i t u d e ) . L i k e w i s e , the field can be excited in m a n y different w a y s , p r o d u c i n g particles of different e n e r g y a n d m o m e n t u m . W h e n we apply particle physics to the early universe, it is often more convenient to describe matter in t e r m s of continuous fields, as opposed to their associated discrete particles. T h e c h a n g e from a h i g h - e n e r g y w o r l d , w h e r e the electromagnetic and w e a k interactions are unified into a single e l e c t r o w e a k interaction a n d all particles a r e massless, to a l o w - e n e r g y w o r l d , w h e r e the two forces act separately a n d the q u a r k s and leptons are massive, is also a phase transition controlled by the v a l u e of the H i g g s field. Just as w a t e r has different properties at different t e m p e r a t u r e s , so do the f u n d a m e n tal forces. T h e net result of e l e c t r o w e a k unification is that the universe, at t e m p e r a t u r e s above lO

Kelvin, is a s i m p l e r place, w h e r e all parti-

cles but the H i g g s are massless a n d their interactions are described by three, rather than four, f u n d a m e n t a l forces. T h i s simplicity is accomp a n i e d by a h i g h e r s y m m e t r y as w e l l ; just as in l i q u i d w a t e r molecules can be found e q u a l l y in every position and direction (a h i g h l y s y m m e t ric state) and in ice they are lodged in r i g i d crystalline lattices (a less s y m m e t r i c state), the unified universe has a h i g h e r d e g r e e of s y m m e t r y than the l o w - e n e r g y universe. T h i s s y m m e t r y is of a more abstract n a t u r e than the positional s y m m e t r y of w a t e r versus ice, h a v i n g to do w i t h m a t h e m a t i c a l properties of the particle interactions themselves, whose details a r e of no concern to us here. But it is s y m m e t r y nevertheless, l e n d i n g e l e g a n c e to a unified universe, a step closer to the P l a tonic ideal of m a t h e m a t i c a l perfection as nature's most fundamental property. T h e e l e c t r o w e a k unification corresponds to times of less than r o u g h l y one trillionth of a second, after the b a n g ( 1 0 ~

seconds). T h i s

m a y seem l i k e an outrageously short timescale to us, but it is e x t r e m e l y long for a photon, whose cruising time across a proton is a m e r e 1 0

- 2 4

secondsâ&#x20AC;&#x201D;that is, a trillion times s m a l l e r ! So, don't be a l a r m e d as we continue to m a r c h on to even e a r l i e r times and h i g h e r t e m p e r a t u r e s , a l t h o u g h , as we do so, we leave behind the safety of observations and m o v e into the r e a l m of sound theoretical speculation. As we w i l l see, it 266

TIME

REGAINED

is presently e x t r e m e l y h a r d , but not impossible, to test physics at the very h i g h e n e r g i e s prevalent in the e a r l y universe. T h e next step t o w a r d bottom-up unification of forces is the incorporation of the strong force into the e l e c t r o w e a k unification. T h i s theory, k n o w n as g r a n d unified theory ( G U T for short), w a s initially developed d u r i n g the 1970s by G l a s h o w a n d H o w a r d Georgi of H a r vard University. It r e m a i n s u n c o n f i r m e d , a l t h o u g h m a n y believe its confirmation is just a question of t i m e . Quite a few variations on the G U T t h e m e have a p p e a r e d , different schemes to b r i n g the strong a n d e l e c t r o w e a k forces together. But they all h a v e one property in c o m mon; since the strong interaction rules the behavior of q u a r k s , w h i l e the leptons interact via the w e a k force (and both via e l e c t r o m a g n e t i s m ) , a unified theory of all three interactions should a l l o w for q u a r k s to t r a n s m u t e into leptons a n d vice versa. Leptons a n d q u a r k s a r e part

F I G U R E 2 3 : Unification of the four fundamental forces. The diagram shows the times when unification of the different forces is expected to occur during the history of the universe. TOE stands for "Theory of Everything." (Times are approximate.)

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of our l o w - e n e r g y , a s y m m e t r i c reality; in the unified w o r l d of G U T s , they can easily become each other. An i m m e d i a t e consequence of this prediction is that the proton, being m a d e of q u a r k s , should not be a stable particle but should decay as its q u a r k s m u t a t e into leptons; the m a t ter we are m a d e of is not eternal. T h e predicted lifetime of protons has c h a n g e d over the y e a r s , but it started close to 1 0

i 0

y e a r s , apparently a

ridiculously long span, c o m p a r e d w i t h the universe's a g e of about fourteen billion y e a r s (1.4 X 10'째 y e a r s ) . H o w e v e r , if we fill a big e n o u g h tank w i t h water, there w i l l be plenty of protons for us to w i t n e s s the occasional decay. T h i s e x p e r i m e n t has been a t t e m p t e d at different u n d e r g r o u n d t a n k s ( u n d e r g r o u n d to avoid spurious decay s i g n a l s ) across the w o r l d , but the proton has stubbornly refused to decay. L e a r n i n g from these e x p e r i m e n t s , theorists rushed to modify their G U T models, a l l o w i n g for longer-lived protons. So, even though the proton's longevity ruled out the simpler m o d e l s of G U T s , there are m a n y variants that have protons sufficiently long-lived to e l u d e the w a t e r tank m e a s u r e m e n t s . G r a n d unification w o r k s in s o m e w h a t the w a y electroweak unification does; at t e m p e r a t u r e s above a certain value (the enormous 1 0

d e g r e e s K e l v i n ) , the universe is best described as h a v i n g two forces: g r a v i t y and the g r a n d unified force. T h i s G U T force is also carried by particles, at least twenty-four of them (in its simplest version). T h e r e is also a h u g g i n g H i g g s , w h i c h d e t e r m i n e s the t w o phases of the theory; at h i g h temperatures every particle is massless and the G U T force is longr a n g e , w h i l e at low t e m p e r a t u r e s this G U T - H i g g s gives masses to every particle that needs one. As the universe e x p a n d s and cools below the critical t e m p e r a t u r e of G U T unification, the g r a n d unified force splits into the strong and electroweak force*'and the initial l a r g e s y m m e t r y is reduced. T h e actual m e c h a n i s m by w h i c h this reduction (or b r e a k i n g ) of s y m m e t r y occurs is still very m u c h a matter of contention, not just for g r a n d unification but also for electroweak unification. But this is the subject of a w h o l e other book.

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1998:

Challenges

It does seem that we have it all figured out, a hot big b a n g model m a r ried to the unification of the forces, an e l e g a n t description of the u n i verse consistent

with

the

motto

"The earlier

the

simpler."

But

problems a n d open questions abound. On the particle physics side, we don't k n o w , for e x a m p l e , w h y protons a n d electrons have the s a m e (and opposite) electric c h a r g e , or w h y their masses are w h a t they a r e . As we build our m o d e l s to describe their interactions, we use the values m e a s u r e d in the laboratory as input p a r a m e t e r s . C l e a r l y , a m u c h m o r e satisfying theory w o u l d be able to predict these values from a fundamental level of description. We have but a h a z y u n d e r s t a n d i n g of n a t u r e , as if its true reality, in all its stark a n d profound beauty, w e r e covered by a veil. T h i s is w h a t physicists w o r k i n g on superstring m o d els are t r y i n g to do, lift the veil a n d pierce right t h r o u g h the essence of physical reality. Only t i m e will tell w h e t h e r reality is indeed fundam e n t a l l y described by v i b r a t i n g strings. Even w i t h the influx of ideas from particle physics, big b a n g cosmology is far from complete. H e r e , too, we must use several input p a r a m e t e r s , w h i c h we obtain from observations a n d feed into our m o d e l s to obtain a consistent picture of the universe, but w o u l d l i k e to u n d e r s t a n d from more f u n d a m e n t a l physical processes. One of them is the c u r v a t u r e of the universe. As we have seen, w h e t h e r the g e o m e t r y of the cosmos is flat, open, or closed d e p e n d s on t w o k e y contributions to Einstein's e q u a t i o n s , the total e n e r g y density of matter a n d radiation a n d the cosmological constant or some s i m i l a r sort of " a n t i g r a v i t y " component. Since the fate of the universe is d e t e r m i n e d by its g e o m e try, u n d e r s t a n d i n g w h a t m a k e s the cosmos bend is deeply related to u n d e r s t a n d i n g its destiny. T h e r e a r e t w o schools of thought a m o n g cosmologists. T h e purists believe that the c u r v a t u r e of the universe w a s d e t e r m i n e d d u r i n g its early m o m e n t s of existence. F u r t h e r m o r e , their calculations a n d m o d e l s predict that the universe is flat. To the purists, it 269

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is just a m a t t e r of t i m e before the observers a m a s s e n o u g h evidence to confirm their prediction. T h e gazers believe that we should m e a s u r e a n d find out, that the m o d e l s upon w h i c h the purists base their prediction for a flat universe a r e too speculative to be t a k e n as the final call. Let us delve deeper into this controversy. A l a n G u t h , now at M I T , o r i g i n a l l y developed the theory that predicts the flatness of the universe a n d goes by the n a m e of inflation. Ideas h i n t i n g at Guth's solution w e r e floating in the air by the late 1970s, but no one had applied t h e m yet to the relevant context or w i t h Guth's d i s a r m i n g l y s i m p l e clarity a n d elegance. H i s paper a p p e a r e d in 1981 a n d w a s q u i c k l y followed by variations by A n d r e i L i n d e (now at Stanford) a n d i n d e p e n d e n t l y by A n d r e a s A l b r e c h t (now at the U n i v e r sity of C a l i f o r n i a at D a v i s ) a n d Paul S t e i n h a r d t (now at Princeton). Since the a p p e a r a n c e of Guth's seminal paper, there have been dozens of s c e n a r i o s — s o m e a d m i t t e d l y by this a u t h o r — a s s u m i n g different recipes for the p r i m o r d i a l soup of particles, but a c h i e v i n g essentially the same final results, w i t h better or worse t w e a k i n g of the p a r a m e t e r s . It is only fair to say that inflation is still very m u c h an idea in search of a theory; m u c h of the present debate as to w h e t h e r this or that model is the best or "most n a t u r a l " boils d o w n l a r g e l y to aesthetical considerations, a d a n g e r o u s criterion w h e n applied almost by itself. After a l l , beauty is in the eye of the beholder. In a n y case, the g e n e r a l idea of inflation is so simple a n d e l e g a n t that most of us expect it to survive in some form or another w h e n e v e r our i m a g i n a t i o n g r a c e s us w i t h a c o m pelling theory w e l l motivated by fundamental physics a n d consistent w i t h observations. Or, at least, a theory that can be clearly disting u i s h e d from other c o m p e t i n g theories t h r o u g h observations. Before we discuss how inflatio4'piv?dicts the flatness of the universe (in most of its versions anyv»*ay—.some a r e consistent w i t h an open g e o m e t r y ) , it is w o r t h i n v e s t i g a t i n g another of its t r i u m p h s , the resolution of the horizon problem. One of the most g l a r i n g limitations of the standard model of cosmology, the hot big b a n g model, is its failure to e x p l a i n its best-measured property, the a m a z i n g h o m o g e n e i t y of the m i c r o w a v e b a c k g r o u n d t e m p e r a t u r e . T h a t an observer can point her 270

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m i c r o w a v e a n t e n n a a n y w h e r e in the sky, Northern or Southern H e m i sphere, and m e a s u r e the s a m e t e m p e r a t u r e to an accuracy of one part in one h u n d r e d thousand is q u i t e r e m a r k a b l e . But this t e m p e r a t u r e d e m o c r a c y becomes even more r e m a r k a b l e , actually p u z z l i n g , w h e n we try to u n d e r s t a n d h o w that can be so. As we have seen, Einstein's special theory of relativity relies on the principle that the fastest speed w i t h w h i c h information can travel is the speed of light. T h i s includes how fast different regions of the universe can c o m m u n i c a t e â&#x20AC;&#x201D; t h r o u g h interactions of particles w i t h photonsâ&#x20AC;&#x201D;to regulate their t e m p e r a t u r e . Because of the finiteness of the speed of light, a region w h e r e causal contact is a l l o w e d , the causal horizon, surrounds each point in the u n i verse (say, Earth). As the universe becomes older, the b o u n d a r y of the horizon distance a r o u n d each point g r o w s at the speed of light. T h u s , any region beyond the horizon is also beyond reach; we cannot possibly k n o w w h a t goes on there. For e x a m p l e , the Sun has existed for about five billion y e a r s . An observer on a planet six billion l i g h t - y e a r s a w a y from the S u n w i l l k n o w of its existence only in one billion y e a r s . C l e a r l y , particles b e l o n g i n g to regions outside each other's causal horizons cannot e x c h a n g e information. Let's illustrate this point w i t h a m o r e prosaic e x a m p l e . P r e p a r e a very hot bath and m e a s u r e its t e m p e r a t u r e at different spots. After being satisfied that the w a t e r t e m p e r a t u r e is fairly h o m o g e n e o u s , t h r o w in a bucket of very cold water at one end of the bathtub; clearly, it w i l l be some time before the t e m p e r a t u r e at the other e x t r e m e of the b a t h t u b will c h a n g e as a result of the i n c o m i n g cold water. Microscopically, the hot (fast) w a t e r molecules w i l l collide and e x c h a n g e e n e r g y and m o m e n t u m w i t h the cold (slow) molecules, until t e m p e r a t u r e i n e q u a l i t i e s a r e mostly evened out. Of course, collisions between molecules do not stop once the t e m p e r a t u r e is e v e r y w h e r e the s a m e , but their a v e r a g e velocity will r e m a i n nearly constant. T h e point is that it t a k e s t i m e for differences in t e m p e r a t u r e to be e q u a l i z e d . You can also test this, but carefully, by placing a metal poker into a hot fireplace. Heat w i l l propagate u p w a r d t o w a r d the end of the poker until it reaches your h a n d . T h i s e q u a l i z a t i o n of t e m p e r a t u r e , be it in the bath27\

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tub, the hot poker, or the universe, happens because of interactions b e t w e e n the m a t e r i a l components in each of these m e d i u m s . W a t e r m o l e c u l e s collide more or less in response to t e m p e r a t u r e c h a n g e s , as do atomic a n d structural vibrations of the metal poker, a n d as do photons a n d other particles in the e a r l y universe. T h e p u z z l i n g fact for the big b a n g model is this: as we have seen, the last t i m e photons interacted w i t h matter a n d could thus adjust their t e m p e r a t u r e s w a s at d e c o u p l i n g , w h e n the universe w a s 300,000 y e a r s old. T h e causal horizon at that t i m e â&#x20AC;&#x201D; t h e region w i t h i n w h i c h causal processes m o v i n g at most w i t h the speed of light could have smoothed out t e m p e r a t u r e fluctuationsâ&#x20AC;&#x201D;corresponds to an a r e a in today's sky that is s m a l l e r than one d e g r e e (about twice the size of a full Moon). T h i s being the case, how can photons across distant regions of the universe " k n o w " they should all be at the s a m e t e m p e r a t u r e ? H e r e is another w a y of v i s u a l i z i n g this: i m a g i n e that you cover a l a r g e t r a n s parent globe w i t h q u a r t e r s . N o w transport yourself to the center of the globe. Each q u a r t e r represents (not to scale!) the size of the causal horizon at decoupling, w h i l e the w h o l e globe represents the causal horizon today. You could also i m a g i n e pointing an (infrared, if q u a r t e r s are at a m b i e n t t e m p e r a t u r e ) a n t e n n a at different q u a r t e r s a n d m e a s u r i n g their " t e m p e r a t u r e . " T h e r e is no a priori reason for all the q u a r t e r s to have the same t e m p e r a t u r e , unless they w e r e all heated in the same w a y . T h i s , in a nutshell, is the horizon problem. Guth proposed the solution. Suppose that very early on in the history of the universe, at a r o u n d the same t i m e a G U T is plausible (at 10~ 36

seconds), a very abrupt c h a n g e occurred to the expansion rate, causing

a tiny, causally connected region to be violently stretched to an enorm o u s size, so h u g e as to comprise all of our observable universe today, as is illustrated in figure 24. After a very brief period, the ultrafast e x p a n sion rate slowed d o w n a n d the now "inflated" universe recovered the usual expansion rate of the standard big bang model. (Recall that in the usual big bang model g r a v i t y g r a d u a l l y , but consistently, slows d o w n the expansion.) Because of this extremely short-lived a n d fast-paced period of expansion (for the m a t h e m a t i c a l l y savvy reader, exponentially 272

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fast), Guth's proposal and all its variations b e c a m e k n o w n as inflationary cosmologies.

The

Cosmos,

Inflating

the

circa Big

1998: Bang

H o w does this short period of accelerated expansion solve the horizon problem of the standard big bang m o d e l ? C o n s i d e r a g a i n the globe covered w i t h q u a r t e r s . A c c o r d i n g to the s t a n d a r d scenario, the photons in each of the q u a r t e r s w o u l d have nothing to do w i t h the photons in the other q u a r t e r s ; the fact that their overall t e m p e r a t u r e is so r e m a r k ably h o m o g e n e o u s w o u l d be a mystery. W i t h i n the inflationary scenario, there

w o u l d n ' t be

separate causally

disconnected q u a r t e r s

patching up our observed universe, but the w h o l e sky w o u l d have o r i g inated from a single connected region, as if one coin covered the entire globe. T h e d i a g r a m s in figure 24 a t t e m p t to illustrate the differences b e t w e e n an inflating a n d a noninflating cosmos. T h e k e y point is that, in an inflating cosmos, the photons that we m e a s u r e today all come from (or a r e the progeny of processes w i t h i n ) the s a m e causally connected region, thus e n s u r i n g their e q u a l t h e r m a l properties. A noninflating cosmos is l i k e a causally disconnected q u i l t , each patch h a v i n g its share of photons w i t h properties u n r e l a t e d to those in n e i g h b o r i n g patches. A p a r t from offering a plausible solution to the horizon problem, inflationary cosmologies also solve a host of other shortcomings of the big b a n g model. One that we briefly m e n t i o n e d above is the flatness of the universe. At the b e g i n n i n g of this chapter, we s a w h o w the global g e o m e t r y of the universe is d e t e r m i n e d by the contributions to its e n e r g y density from matter, radiation, a n d a possible cosmological constant or s o m e t h i n g that m i m i c s its effects. D u r i n g the early 1990s, a host of different observations placed the e n e r g y density of the universe 273

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24:

AND

THE

ASTRONOMER

How inflation solves the "horizon problem." In an inflating cosmology

(top) the whole observable universe fits well within one causally connected patch; in a standard Friedmann cosmology, the observable universe today contains many regions that, at decoupling, were causally disconnected. (Diagram not to scale.)

to w i t h i n 30 percent a n d t w i c e the critical density, that is, close to flatness but still u n d e t e r m i n e d . T h e observational data at the t i m e provided no c o m p e l l i n g reason to i n c l u d e a cosmological constant in the g a m e , a l t h o u g h some m o d e l s w i t h it w e r e proposed to stave off a reincarnation of Hubble's a g e problem: m e a s u r e m e n t s of the oldest stars in small g a l a x i e s called g l o b u l a r clustery indicated they w e r e older than the universe, a c c o r d i n g to some m e a s u r e m e n t s of its a g e . But neither the stellar a g e m e a s u r e m e n t s nor the" a g e of the universe m e a s u r e m e n t s w e r e conclusive e n o u g h to justify the inclusion of a cosmological constant; there w a s room to have a cosmology w i t h o u t it. (One of the key p l a y e r s w h o helped dispel this " a g e p r o b l e m " w a s my D a r t m o u t h coll e a g u e Brian C h a b o y e r . H e w a s one o f the good g u y s w h o m a d e the 274

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stars y o u n g e r than the universe.) T h e (fairly b r o a d ) consensus by late 1997 w a s that, w i t h no cosmological constant, the g e o m e t r y a n d d e s tiny of the universe should be completely d e t e r m i n e d by the a m o u n t of m a t t e r in it. Easy, r i g h t ? All w e had t o d o w a s m e a s u r e how m u c h matter w a s out there, c o m p a r e it w i t h the critical density, a n d get the answer. But here trouble b e g i n s . First, theory alone indicated that an old universe such as ours must be pretty m u c h flat or it w o u l d not have a g e d as it has. If it had been overdense, g r a v i t y w o u l d have caused it to collapse a long t i m e a g o into a big crunch; if it had been u n d e r d e n s e , it w o u l d have e x p a n d e d too fast for g a l a x i e s a n d thus stars a n d life to develop. T h e m e r e existence of an old universe therefore points t o w a r d its flatness. T h i s is k n o w n as the flatness problem of the s t a n d a r d big bang cosmology. S o m e t h i n g must have fine-tuned the initial balance b e t w e e n g r a v i t y a n d expansion w i t h incredible precision in order for the u n i verse to still be here, filled w i t h g a l a x i e s a n d people. T h i n k of how precise a target shooter must be to hit the bull's-eye; a bit up she overshoots, a bit d o w n she undershoots. So it is w i t h the universe. But the observations w e r e telling s o m e t h i n g q u i t e differentâ&#x20AC;&#x201D;that there w a s not e n o u g h m a t t e r out there to m a k e the universe flat. T h e m a t t e r that m a k e s up t h i n g s that shine, such as stars, w a s estimated to compose only about 1 percent of the critical a m o u n t for flatness. To that, we should a d d m a t t e r that doesn't shine by itself, l i k e stars too light to ignite

full-blown

thermonuclear

reactions (called b r o w n

d w a r f s ) , planets, asteroids, a n d cool g a s clouds. T h i s m a t t e r is part of w h a t is called the d a r k - m a t t e r component of the universe: we can infer its existence only by the g r a v i t a t i o n a l effect it has on m a t t e r that shines. Still, o r d i n a r y d a r k m a t t e r â&#x20AC;&#x201D; m a d e u p o f protons, neutrons, a n d elect r o n s â&#x20AC;&#x201D; d i d not help m u c h , a d d i n g up to at most 5 percent of the critical density. But t h i n g s w e r e not so bad. Since the 1930s, F r i t z Z w i c k y ' s pioneering observations of the motion of g a l a x i e s in clusters showed that there is m u c h more m a t t e r in g a l a x i e s a n d in clusters of g a l a x i e s than meets the e y e . M o r e accurate m e a s u r e m e n t s d u r i n g the last t w o d e c a d e s of the twentieth century led astronomers to conclude that d a r k 275

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matter can a d d up to about 30 percent of the critical density, closer to flatness but not there yet. Of the several p u z z l i n g facts about d a r k matter, none is m o r e p u z z l i n g than its very n a t u r e . W h a t is it, e x a c t l y ? We still don't k n o w ! We k n o w it is out there, a n d we also k n o w that most of it is not m a d e up of o r d i n a r y stuff such as protons, neutrons, a n d electrons. It is q u i t e i r o n i c â&#x20AC;&#x201D; a n d h u m b l i n g â&#x20AC;&#x201D; t h a t m u c h of the m a t t e r that fills the universe is u n r e l a t e d to the m a t t e r we a r e m a d e of. T h i s is exciting n e w s for particle p h y s i c i s t s â&#x20AC;&#x201D; a l l this m y s t e r i o u s m a t t e r out there, w a i t i n g to be discovered. Quite possibly, this exotic d a r k matter, w h i c h interacts w i t h o r d i n a r y m a t t e r only g r a v i t a t i o n a l l y (or very w e a k l y o t h e r w i s e ) , is part of a unified theory, be it at the G U T level or even at the superstring level. Indeed, several unified theories predict the existence of particles w i t h the right properties to behave l i k e d a r k matter, such as massive neutrinos. But the case for the composition of d a r k m a t t e r is still w i d e open.

It w a s at this j u n c t u r e that the t w o schools of t h o u g h t m e n t i o n e d above, the purists a n d the g a z e r s , clearly d i s a g r e e d . T h e purists a r g u e d that the flatness problem, the existence of an old universe, w a s c o m pelling e n o u g h for t h e m to stick w i t h a flat u n i v e r s e . T h e g a z e r s a r g u e d instead that the observations supported an open u n i v e r s e , mostly at 40 or so percent of the critical density. T h i s v i e w w a s s u m m a r i z e d by Peter Coles, of Queen M a r y C o l l e g e , a n d G e o r g e F. R. Ellis, of C a p e T o w n University, in their book Is the Universe Open or Closed?, published in 1997: "Please be a w a r e at the outset of our v i e w that, ultimately, the question of [the universe's flatness] is an observational question a n d our theoretical prejudices must bow to e m p i r i c a l evi5

d e n c e . " T h i s is, to say the least, a y&ry sound scientific position. We m a y be inspired by our theoretical l o n g i n g s , but never to the point of blindness. As my g r a n d f a t h e r used to-say, a hat b i g g e r than y o u r head covers y o u r eyes. T h e purists m a i n t a i n e d that inflation predicts a flat g e o m e t r y for the universe. As the universe w e n t t h r o u g h a brief period of accelerated expansion, w h a t e v e r c u r v a t u r e it m a y have h a d w a s flattened 276

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a w a y . T h e s a m e h a p p e n s to a balloon as it gets inflated. Focus on a patch on its surface; as the balloon g r o w s , the initially curved patch becomes progressively flattened, until it is i n d i s t i n g u i s h a b l e from a flat tabletop. Of course, the balloon as a w h o l e is still c u r v e d , but the local c u r v a t u r e of the small patch is n e g l i g i b l e . Inflation m a k e s sure that the observable part of our universe fits well w i t h i n a patch that w a s flattened almost to perfection; for all practical purposes, we cannot m e a s ure a n y c u r v a t u r e . (Note that the universe m a y still be c u r v e d at distances beyond w h a t we could ever m e a s u r e , that is, beyond our causal horizon.) It is easy to u n d e r s t a n d w h y m a n y cosmologists are so attracted to the inflationary universe idea; with one single s w e e p , it solves two k e y riddles of the standard big b a n g scenarioâ&#x20AC;&#x201D;the horizon a n d the flatness problems. To see how inflation accomplishes this, we must investigate the mysterious m e c h a n i s m that drives it: W h a t can possibly m a k e the early universe accelerate so substantially? First, recall that the e x p a n sion of the universe is an expansion of its g e o m e t r y ; picture two points on the surface of a balloon drifting a w a y from each other as the balloon g r o w s . W h a t e v e r drives inflation must stretch the g e o m e t r y quite d r a matically. We saw before that Einstein had proposed a sort of " a n t i g r a v ity" t e r m in his equations, the cosmological constant, to counterbalance the implosive tendencies of his static universe. In other w o r d s , the balance between the attractive matter a n d the repulsive cosmological constant stabilized his static universe. Now, i m a g i n e a hypothetical universe devoid of any matter, whose evolution is completely determ i n e d by a cosmological constant. F r o m the a r g u m e n t above, it is clear w h a t will happen to its geometry: the repulsive cosmological constant, acting alone, w i l l drive an accelerated expansion! T h i s w a s a l r e a d y k n o w n in 1917, t h a n k s to the w o r k of W i l l e m de Sitter. Of course, this universe is not our o w n , since we k n o w that for most of its history the universe w a s not accelerating (moreover, it is certainly not e m p t y ) . But, if we could a r r a n g e things so that something l i k e a cosmological constant d o m i n a t e s the e n e r g y density for a short w h i l e a n d then goes a w a y (by relaxing to zero), the universe w o u l d accelerate d u r i n g this short 277

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phase a n d then go back to its normal decelerated expansion. T h i s is precisely w h a t happens in an inflationary cosmology.

The

Cosmos,

The

Quest

circa

for

New

1998: Cosmic

Origins

T h e period of inflation w a s d r i v e n by w h a t is called the potential energy of a hypothetical field, inspired o r i g i n a l l y by the H i g g s field of the G U T model. In the dozens of versions of the inflationary idea proposed d u r i n g the last t w o decades of the t w e n t i e t h century, the d r i v i n g m e c h a n i s m s have c h a n g e d , but the core idea of m i m i c k i n g a cosmological constant for a brief period r e m a i n e d . We saw e a r l i e r that there are t w o k i n d s of e n e r g i e s in physics: e n e r g y of motion, the kinetic e n e r g y ; a n d stored energy, the potential energy. An u n r e s t r a i n e d system w i t h potential e n e r g y w i l l move, as potential e n e r g y is transformed into kinetic energy. If y o u raise a rock above the g r o u n d , it will have stored g r a v i t a t i o n a l potential e n e r g y ; if you let go of the rock, it will fall until it hits the zero of energy, in this case the g r o u n d . If you bring t w o electrically c h a r g e d particles to w i t h i n a certain distance from each other, they will have stored potential e n e r g y ; if they have opposite c h a r g e s a n d you let go, they w i l l move t o w a r d each other, a n d if they have the s a m e c h a r g e , they w i l l move a w a y from each other. T h e point is that if there

interaction

between

objects

(gravitational

attraction

b e t w e e n the rock a n d Earth, electrical force b e t w e e n t w o c h a r g e d particles), there is the possibility of storing different a m o u n t s of potential energy. W h i l e there is a stored a m o u n t of potential energy, the system is not at its zero of e n e r g y artd it w i l l move (or try to) until it gets there, l i k e the rock that w i l l stop only onceSt hits the g r o u n d . T h e field d r i v i n g i n f l a t i o n â&#x20AC;&#x201D; c a l l e d

the inflatonâ&#x20AC;&#x201D;can have t w o

k i n d s of interactions: it can interact w i t h itself via some force, as t w o electrons do via e l e c t r o m a g n e t i s m , a n d , since it has a m a s s (and e n e r g y ) , via gravity. (In some m o d e l s , it can also interact w i t h other 278

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k i n d s of particles, but the net result is essentially the same.) Different inflationary m o d e l s a s s u m e different k i n d s of interactions, but they all w o r k to provide the inflaton w i t h some potential energy. T h e k e y point is that this potential e n e r g y has the same effect on the g e o m e t r y of the universe as does a cosmological constant; it w i l l drive an accelerated expansion, acting as a repulsive source of gravity. "Now, how can that be s o ? " you m a y justifiably ask. T h e explanation has t w o steps. First, in g e n e r a l relativity not only the e n e r g y density but also the pressure of matter contribute to the b e n d i n g of the g e o m e t r y . We can think of it as the e n e r g y stored in a tensed m a t e r i a l such as a stretched rubber sheet. (For g e n e r a l relativity, things a r e m o r e subtle, though. A stretched rubber sheet tends to go back to its o r i g i n a l shape and not get stretched farther.) Second, the pressure of the inflaton's potential e n e r g y is n e g a t i v e . T h i s negativity of the pressure will d r i v e the accelerated expansion. If you bear w i t h me for the next few lines, I w i l l explain w h y . An e x p a n d i n g normal g a s has positive pressure; as the g a s e x p a n d s , it cools a n d its pressure drops. L i k e w i s e , a universe filled w i t h hot g a s decelerates a n d cools as it e x p a n d s , since the g a s pressure drops. In the case of the inflaton, it w i l l be its stored potential e n e r g y a n d its associated n e g a t i v e pressure that will m a k e the g e o m e t r y accelerate. Just as the a m o u n t of g r a v i t a t i o n a l potential e n e r g y stored in a rock g r o w s as we increase its height, the a m o u n t of potential e n e r g y stored in the inflaton also g r o w s as it takes values a w a y from zero. ( T h i n k of the inflaton's v a l u e a w a y from zero as its "height," l i k e the rock's height from the g r o u n d . ) Suppose the inflaton p e r m e a t e s the early universe w i t h a fairly smooth v a l u e . T h i s w i l l set its initial potential energy. N o w i m a g i n e a tiny bubble w i t h i n this "inflaton sea," w i t h i n w h i c h the inflaton actually has a s m a l l e r value than outside, as is indicated in figure 25. T h i s m e a n s that w i t h i n this bubble the potential e n e r g y is s m a l l e r than outside. N o w , in n a t u r e , systems left alone a l w a y s tend to the point of l o w est possible energy, just l i k e the rock that falls to the g r o u n d . T h i s m e a n s that the small bubble w i l l tend to g r o w , l o w e r i n g the total 279

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25:

Left,

AND

THE

ASTRONOMER

inflaton rolling down its potential energy.

Right,

regions where the

inflaton has smaller values (denoted by 2) than its surroundings (denoted by I) grow because of pressure difference: the pressure outside is smaller than inside the region.

potential e n e r g y of the system. In order for the bubble to g r o w , it must exert pressure on its s u r r o u n d i n g s â&#x20AC;&#x201D; t h a t is, it must have h i g h e r pressure than its s u r r o u n d i n g s . So, we established that a bubble w i t h l o w e r potential e n e r g y has h i g h e r pressure than the s u r r o u n d i n g inflaton sea. T a k i n g this a r g u m e n t to the e x t r e m e , i m a g i n e a bubble w i t h zero potential e n e r g y inside, indicated by the n u m b e r 3 in the figure. Since this bubble is at the absolute m i n i m u m of the potential energy, it w i l l also certainly g r o w . But since the inflaton w i t h i n this bubble has zero v a l u e , the bubble has zero pressure! T h e bubble can g r o w only if its s u r r o u n d i n g s , the inflaton sea, have s m a l l e r pressure than zero, that is, n e g a t i v e pressure. T h u s , any nonzero potential e n e r g y leads to n e g a tive pressure a n d to accelerated expansion. Inflation ends w h e n the inflaton K i l l s d o w n to the zero of potential energy. T h e universe is a cold and e m p t y place at this point, because any k i n d of m a t t e r or g a s that m a y have been there before inflation has been d i l u t e d to almost total oblivion as a result of the rapid expansion. (If you " e x p a n d " a glass full of w a t e r to the size of a bathtub, the bathtub will be pretty empty.) W h e r e , then, is all the matter that fills u p the universe after inflation? T h i s question is at the forefront of cosmologi280

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cal research. We believe that the inflaton doesn't simply freeze at the lowest point of potential e n e r g y but bounces a r o u n d l i k e a yo-yo ( i n d i cated by the double a r r o w in figure 25). As it does so, a n d because the inflaton interacts possibly w i t h itself a n d other fields, a fantastic t r a n s m u t a t i o n of potential a n d kinetic e n e r g y into m a t e r i a l particles occurs, liberating an e n o r m o u s a m o u n t of heat. In fact, w i t h i n the inflationary scenario, we m a y l e g i t i m a t e l y call this p e r i o d â&#x20AC;&#x201D; k n o w n as r e h e a t i n g â&#x20AC;&#x201D; the o r i g i n of the hot big b a n g model, an explosive creation of m a t t e r 6

a n d heat. Inflation reinvented the big bang. It is no coincidence that A l a n Guth's p o p u l a r i z a t i o n of inflationary cosmology is subtitled, The Quest for a

The

New

Theory

Cosmos,

Fluctuations

of Cosmic

circa

Origins.

2002:

Remembrance

Past

Inflation is a very c o m p e l l i n g idea. All we have to do is assume there w a s an inflaton field early on in the universe slightly displaced from the m i n i m u m of its potential energy, a n d e v e r y t h i n g else follows: the horizon a n d flatness problems are solved, a n d even the matter that fills the universe is created d u r i n g the d r a m a t i c finale to the accelerated expansion, as the inflaton yo-yos about the zero of potential energy. But this is not all inflation does. It also helps us u n d e r s t a n d a crucial aspect of the observations of the m i c r o w a v e b a c k g r o u n d obtained by the C O B E satellite and, more recently, by a host of other m e a s u r e m e n t s . Recall that w h e n I mentioned how e x t r a o r d i n a r i l y smooth the m i c r o w a v e backg r o u n d w a s , I quoted its temperature as being homogeneous to w i t h i n one part in a h u n d r e d thousand. T h i s m e a n s that, in spite of its incredible smoothness, there are small fluctuations on the t e m p e r a t u r e of little over a millionth of a d e g r e e . To v i s u a l i z e h o w small these fluctuations are, i m a g i n e tiny ripples on the surface of a l a k e gently caressed by a breeze; if the l a k e is one h u n d r e d meters deep, the ripples are on the order of a tenth of a millimeter. T h e r e are high spots and low spots,

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w h i c h , for the m i c r o w a v e b a c k g r o u n d , translate into hotter and colder spots in the m i c r o w a v e m a p . T w o m a p s are shown in figure 37 in the insert, one by C O B E , w i t h an accuracy of about ten d e g r e e s , and one by the B O O M E R A N G e x p e r i m e n t , w i t h an accuracy of about one degree. T w o n e w satellite missions will be collecting a n d a n a l y z i n g high-precision data in the near future, the A m e r i c a n M A P mission, launched d u r ing the s u m m e r of 2001, a n d the European Space A g e n c y P L A N C K mission, planned for 2007. W h a t m e c h a n i s m caused these tiny t e m p e r a t u r e fluctuations in the m i c r o w a v e b a c k g r o u n d ? W h a t e v e r it w a s , it h a d to create inequalities in the gravitational pull acting on the photons; recall that strong g r a v i tational fields cause photons to redshift, that is, become less energetic (colder) as they try to escape. We can picture the universe at about 300,000 years of age, w h e n the photons decoupled from matter, as being permeated by overdense regions, w h e r e g r a v i t y w a s stronger than avera g e . T h e s e regions w o u l d pull on the photons, c a u s i n g the cold a n d the hot spots we see in the m i c r o w a v e b a c k g r o u n d today; roughly, the cold spots are related to photons t r y i n g to escape the gravitational pull, w h i l e the hot spots are caused by photons falling into the overdense regions. T h e s e regions are the predecessors of the g a l a x i e s a n d the clusters of g a l a x i e s we see today, the seeds of the large-scale structure of the universe; once an overdense region forms, its g r a v i t y will induce o r d i n a r y and d a r k matter to c l u m p a r o u n d it. An e x t r e m e e x a m p l e is the socalled Great Attractor, a concentration of over 1 0

solar masses, discov-

ered in 1985 by a g r o u p of astronomers k n o w n as the seven s a m u r a i . G a r y W e g n e r , one of my colleagues at D a r t m o u t h (and one of the s a m u r a i ) , convened a conference here to discuss the observational results that led to the conclusion th'at such a concentration of mass exists.

W^v

vÂť T h e standard big bang model does not provide any mechanism to

create these overdense regions, h a v i n g to assume a distribution of fluctuation sizes that matched astronomical observations, l i k e a g a r d e n e r w h o guesses w h a t seeds w e r e used in a g a r d e n by looking at the various blossoms. T h e inflationary universe, on the other hand, not only predicted 282

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the sizes of these fluctuations but also predicted them correctly! According to inflation, the shapes and forms of today's universe are direct descendants of tiny q u a n t u m fluctuations of the inflaton. It is a t r i u m p h of the inner space—outer space connection that the d y n a m i c s of the u n i verse w h e n it w a s only l O

3 6

seconds old can actually d e t e r m i n e its l a r g e -

scale properties today, the very small deeply interwoven w i t h the very large. T h e overdense regions that will affect the photons at d e c o u p l i n g are a m p l i f i e d — o r better, i n f l a t e d — q u a n t u m fluctuations of the inflaton field. A c c o r d i n g to q u a n t u m physics, e v e r y t h i n g fluctuates; y o u cannot pinpoint the exact position of an electron w i t h o u t i n c u r r i n g l a r g e errors in the m e a s u r e m e n t of its velocity. T h i s s o m e w h a t p a r a doxical behavior is k n o w n as the u n c e r t a i n t y principle, proposed by the G e r m a n physicist W e r n e r H e i s e n b e r g in 1926. In other w o r d s , in the q u a n t u m w o r l d , nothing stands still. You can p r e p a r e a system at the m i n i m u m of its energy, sometimes called the v a c u u m of the system, 8

but there will a l w a y s be small fluctuations about it. T h i s is also true of the inflaton field; w h a t e v e r its v a l u e a n d , thus, potential energy, there

FIGURE

26:

After inflation, fluctuations ( d a s h e d l i n e ) are stretched to sizes larger than

the observable universe (left). However, as the universe expands, these fluctuations reenter the observable universe ( r i g h t ) , causing the overdense regions seen as temperature fluctuations in the cosmic microwave background. These regions are the seeds for the formation of large-scale structures in the universe.

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w i l l a l w a y s be fluctuations about it, little bubbles of v a r y i n g sizes w h e r e i n the inflaton has a different value (see figure 25). D u r i n g inflation, these tiny q u a n t u m fluctuations are stretched to e n o r m o u s sizes, s o m e w h a t l i k e the stretched bellows of a g i a n t accordion. S o m e of them w i l l be so h u g e that they will c u r v e on length scales l a r g e r than the size of our observable universe for a long time, until we can catch up w i t h t h e m , so to speak (see figure 26). A c c o r d i n g to the inflationary cosmology, these a m p l i f i e d — o r i n f l a t e d — q u a n t u m fluctuations of the inflaton field will become, together w i t h accreted matter a n d d a r k matter, the overdense regions that promote the hot a n d the cold spots in the m i c r o w a v e b a c k g r o u n d observed by C O B E a n d others. T h e d i s covery of these t e m p e r a t u r e fluctuations a n d the a g r e e m e n t w i t h the prediction from inflationary cosmologies lent t r e m e n d o u s support to these theories. It is h a r d to think of alternative m e c h a n i s m s that can do so m u c h w i t h so little.

The

Cosmos,

The

Return

circa of the

2002: Ether?

T h e r e w a s , however, a very n a g g i n g problem. By e a r l y 1998, inflation predicted a flat universe a n d C O B E seemed to strongly support that. But astronomical observations kept m e a s u r i n g the total a m o u n t of s h i n i n g a n d d a r k m a t t e r to no m o r e than 40 percent of the r e q u i r e d critical density to e x p l a i n the flatness. H o w could inflation be right in a universe so e m p t y ? S o m e t h i n g w a s m i s s i n g from this picture. In 1998, t w o independent g r o u p s of a s t r o n o m e r s — o n e led by S a u l P e r l m u t t e r at the L a w r e n c e B e r k e l e y N a t i o h l f L a b o r a t o r y , in C a l i f o r n i a , a n d the other led by Brian S c h m i d t of M o u n t S t r o m l o a n d S i d i n g S p r i n g Observatories, in A u s t r a l i a , a n d Robert K i r c h n e r of the H a r v a r d S m i t h s o n i a n C e n t e r for A s t r o p h y s i c s — a n n o u n c e d an a m a z i n g discovery: their observations of distant T y p e I supernovae indicated that the universe is e x p a n d i n g faster now than in the past. T h e y found that these very distant objects, about h a l f the a g e of the universe a w a y in 284

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l i g h t - y e a r s , w e r e receding slower than nearby objects. T h e y concluded that w h e n the u n i v e r s e w a s y o u n g e r it e x p a n d e d slower than it does now. F u r t h e r m o r e , a n d here is the clincher, their results i m p l i e d that the universe is accelerating n o w ! T h i s n e w s rocked the astronomical c o m m u n i t y w o r l d w i d e . But there w a s more: not only is the universe a c c e l e r a t i n g n o w ; it is d o i n g so at a rate d e t e r m i n e d by a cosmological constant—or s o m e t h i n g m i m i c k i n g i t — w h i c h comprises 60 to 70 percent of the r e q u i r e d critical e n e r g y density for flatness! T h e m a t h is easy; a d d s h i n i n g a n d d a r k m a t t e r at 30 or so percent of the critical density to this n e w repulsive g r a v i t y at 70 percent, a n d you obtain the needed critical density, reconciling inflationary theory a n d the C O B E data w i t h m e a s u r e m e n t s of the e n e r g y density. Because of their fundamental importance to cosmology, the observations of distant T y p e I supernovae have been subjected to intense scrutiny by the t w o g r o u p s and m a n y others. So far, the results have survived. Effects that could m i m i c the extra d i m m i n g of the distant supernovae that led astronomers to their surprising conclusion, such as obscuring dust a n d variations on h o w early supernovae detonate, have not yet caused any major revisions in the data. H o w e v e r , g i v e n that we do not k n o w m u c h about h o w earlier T y p e I supernovae detonate, I believe we should t a k e these observations seriously but w i t h a g r a i n of salt; it is a l w a y s possible that supernova detonation in a y o u n g e r u n i verse w a s d i m m e r than in more recent times. F u r t h e r studies, c o m b i n ing a satellite that will probe even deeper into the supernova population (called S N A P , S u p e r n o v a Acceleration Project) w i t h g r o u n d - b a s e d observations, will help clarify these issues. N e v e r t h e l e s s , g i v e n the present robustness of these observations, cosmologists i m m e d i a t e l y responded by t r y i n g to m a k e sense of this new and

m y s t e r i o u s l y convenient

form

of energy—dubbed

dark

e n e r g y — w h i c h d o m i n a t e s the expansion of the universe in an a m o u n t consistent w i t h observations of the m i c r o w a v e b a c k g r o u n d a n d the flatness predicted by inflation. T w o questions spring to m i n d . W h a t is this d a r k e n e r g y ? A n d w h y has it started to d o m i n a t e cosmic e x p a n sion only recently? We don't k n o w the a n s w e r s to either of these q u e s 285

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tions. As we have seen, the existence of a cosmological constant has been a very controversial topic in cosmology; it has been i n v o k e d in t i m e s of conflict b e t w e e n observations a n d theory, only to be s u m m a r ily dismissed once these conflicts have been resolved. F r o m the point of v i e w of particle physics, the existence of a cosmological constant is a real problem, tangled up w i t h the e n e r g y of the v a c u u m fluctuations m e n t i o n e d above. Every m a t t e r or radiation field in n a t u r e should have fluctuations about its a v e r a g e v a l u e that, w h e n a d d e d up, come to a truly e n o r m o u s result, 1 0

1 2 0

t i m e s l a r g e r than w h a t is observed.

C l e a r l y , we don't k n o w how to h a n d l e these v a c u u m fluctuations properly. T h e usual operational procedure is to s w e e p the dust u n d e r the carpet by s a y i n g that some as yet u n k n o w n m e c h a n i s m o p e r a t i n g at very small scales e n s u r e s that the v a c u u m fluctuations all a d d up to zero. M a n y proposals have been a d v a n c e d to deal w i t h this e m b a r r a s s ing issue, a n d some, such as superstring theories, a r e q u i t e p r o m i s i n g . But it is fair to say that the problem of v a c u u m fluctuations is one of the key open questions in h i g h - e n e r g y physics. T h e existence of a very s m a l l , but nonzero, cosmological t e r m , as r e q u i r e d by present observations, m a k e s life even harder. It is m u c h m o r e satisfying to suppose that there must be a m e c h a n i s m to exactly cancel a q u a n t i t y than one that almost exactly cancels a quantity. A very e l e g a n t resolution of this issue, even if m e r e l y tentative at this point, is to suppose that the d a r k e n e r g y is d u e not to a cosmological constant t e r m , the s a m e across the w h o l e universe, but to a field s o m e w h a t l i k e the inflaton, displaced from its m i n i m u m of potential energy. Just as in inflation, if there is a bit of potential e n e r g y in this field, it w i l l e v e n t u a l l y d r i v e an accelerated expansion of the cosmic g e o m e t r y . (Recall that d u r i n g the cosmic expansion, as m a t t e r gets m o r e d i l u t e d , its contribution t o the e n e r g y density g i t s progressively w e a k e r . T h u s , even though m a t t e r a n d radiation m a y d o m i n a t e the e a r l y d e c e l e r a t i n g expansion of the universe, at some point the potential e n e r g y of this field will t a k e over, starting the accelerated era.) T h e key difference between the inflaton a n d this n e w proposed field, called quintessence, lies in their relevant e n e r g y scales; w h i l e the inflaton d o m i n a t e d the 286

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expansion of the universe at its earliest m o m e n t s of existence, at very high e n e r g i e s , the quintessence field started to d o m i n a t e cosmic e x p a n sion only recently. T h i s idea w a s refined in 1998 by my D a r t m o u t h coll e a g u e Robert C a l d w e l l , together w i t h R a h u l Dave a n d S t e i n h a r d t , a n d since then by m a n y others. T h e k e y assumption is that the universe is p e r m e a t e d by an ethereal m a t t e r field (hence the quintessence d e n o m i n a t i o n â&#x20AC;&#x201D; t h e fifth essence of the Aristotelian cosmos) w i t h a nonzero potential energy, whose m a i n role is to d r i v e its present accelerated expansion. In spite of its attractiveness, the quintessence idea does not i m m e d i ately a n s w e r the t w o questions raised by the o b s e r v a t i o n s â&#x20AC;&#x201D; n a m e l y , w h a t is the d a r k energy, a n d w h y is it important now. In r e g a r d to the first question, e q u a t i n g the d a r k e n e r g y w i t h a d y n a m i c a l field certainly addresses the issue of w h y there should be a small nonzero cosmological constant, as opposed to none at a l l . But it doesn't e x p l a i n w h e r e the quintessence field comes from, that is, from w h a t k i n d of particle physics model. T h i s question, as I r e m a r k e d above, is also u n a n s w e r e d for the inflaton field of inflationary m o d e l s . T h e g e n e r a l expectation is that both fields (or could they be one field at different periods of a c t i v i t y ? ) , if they indeed exist, are probably part of a m o r e profound theory u n i f y i n g the four f u n d a m e n t a l forces. W h a t e v e r this theory m a y be, its predictions w i l l have to a g r e e w i t h astronomical o b s e r v a t i o n s â&#x20AC;&#x201D; p r o d u c i n g , we hope, the t w o fields cosmology so b a d l y needs. T h e second question, w h y the d a r k e n e r g y started to d o m i n a t e cosm i c expansion only recently, is also a difficult one. Quite possibly, the resolution of this question is tied to the resolution of the first one; once we have a f u n d a m e n t a l theory d e s c r i b i n g the properties of the inflaton a n d quintessence fields, we will k n o w at w h a t e n e r g y scales these fields d o m i n a t e the cosmic d y n a m i c s . An a l t e r n a t i v e a r g u m e n t to e x p l a i n the " W h y n o w ? " question, k n o w n a s the a n t h r o p i c principle, has the astronomical c o m m u n i t y deeply d i v i d e d . T h e anthropic a r g u m e n t c o m e s i n t w o versions: w e a k a n d strong.

In its strong version, the p r i n c i p l e

states that the universe must have the right properties to p e r m i t intelli287

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gent life to d e v e l o p at some stage in its history. Put another a w a y , if there is a universe, there must be intelligent life. As the astrophysicist Mario

Livio

remarked

his

book

The Accelerating

Universe,

"The

strong principle really crosses the borders of physics into t e l e o l o g y , "

by stating that the universe has as its purpose the creation of life. In d o i n g so, the strong anthropic principle reverses the obvious historical trend of cosmology, w h i c h is to show how insignificant our existence is in the g r a n d scheme of t h i n g s ; the strong principle puts us right back at the center. F u r t h e r m o r e , the principle is scientifically void, for it does not offer any m e c h a n i s m to prove or disprove its foundational statement. T h e w e a k version states that the observed values of physical a n d cosmological q u a n t i t i e s , such as the mass of the electron or the cosmological constant, are not accidental, but a r e d e t e r m i n e d by the r e q u i r e m e n t that carbon-based life forms (or possibly others) can exist a n d have a l r e a d y developed. In other w o r d s , the w e a k principle states that of all possible universes, y o u n g a n d old, and w i t h different values of the f u n d a m e n t a l p a r a m e t e r s , this one surely m a t c h e s the r e q u i r e m e n t s for life; the universe is this w a y because we a r e here. Let's investigate this in more detail. A s s u m e that there is a m u l t i t u d e of universes out t h e r e â&#x20AC;&#x201D; t h a t we are part of a " m u l t i v e r s e " â&#x20AC;&#x201D; w h e r e different universes (henceforth cosmoids) bubble in a n d out of existence for all eternity, some lasting for a very long t i m e and others d i s a p p e a r i n g as fast as they appear. We can see how this idea e m e r g e s from inflationary cosmologies; as its m a i n proponent, the A m e r i c a n - b a s e d Russian physicist A n d r e i L i n d e , reasons, it is q u i t e possible that different cosmoids w i t h i n the m u l t i v e r s e w i l l have different values of fundamental physical p a r a m e t e r s , such as the cosmological constant or thefijicrSs of the electron. T h i s being the case, some cosmoids will expand torever and produce life, others w i l l q u i c k l y collapse, others m a y expand for a w h i l e , produce life, a n d then collapse, a n d so on. Of course, the m u l t i v e r s e hypothesis suffers from a serious problem: because these cosmoids are not causally connected to ours, we have not been a n d never w i l l be able to c o m m u n i c a t e w i t h t h e m or even to test their existence. ( T h e r e have been some speculations 288

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as to the possibility that information can be sent between cosmoids. But given the present uncertainties in the formulation of these ideas, it is wise to let them m a t u r e s o m e w h a t before p u b l i c i z i n g them.) In any case, for a cosmoid b r a n c h i n g out of the m u l t i v e r s e to be "successful"â&#x20AC;&#x201D; that is, to live long e n o u g h to be able to support l i f e â&#x20AC;&#x201D; t h e fundamental physical and cosmological p a r a m e t e r s must assume the values we m e a s u r e in our universe. For e x a m p l e , a cosmoid with l a r g e values of the cosmological constant w o u l d e x p a n d too fast to form g a l a x i e s and thus stars and life, w h i l e cosmoids w i t h a v a l u e even s m a l l e r than the current r e q u i r e d value (a " v a c u u m " e n e r g y ten thousand times s m a l l e r than the e n e r g y b i n d i n g a proton to an electron in a h y d r o g e n a t o m ) w o u l d be even more improbable than we are. We live in a very i m p r o b able cosmoid, but a successful one at that, capable of g e n e r a t i n g g a l a x i e s a n d intelligent life to ask questions about it. A c c o r d i n g to the w e a k anthropic principle, it is "clear" w h y the quintessence field started to accelerate the cosmic expansion only recently: had it been otherwise, the universe w o u l d have e x p a n d e d too fast for g a l a x i e s to have formed, a n d we w o u l d n ' t be here. I fall w a y to the e x t r e m e end of the g r o u p that deeply d i s l i k e s the anthropic principle, strong or w e a k . In my opinion, the principle l a c k s e x p l a n a t o r y and predictive power, only justifying our presence in this u n i v e r s e a posteriori; it is the scientific e q u i v a l e n t of t h r o w i n g in the towel, of g i v i n g up. T h e goal of science is not to justify w h y t h i n g s are the w a y they a r e but to build testable e x p l a n a t i o n s as to w h y they are the w a y they are. To say that we u n d e r s t a n d our presence in the u n i verse by stating that we could live only in a universe old e n o u g h , and w h e r e the physical p a r a m e t e r s have the values we m e a s u r e , does not e x p l a i n w h a t physical m e c h a n i s m s d e t e r m i n e d the universe's a g e and values of the physical p a r a m e t e r s in it, but simply confirms that it could not have been otherwise. P e r h a p s the w e a k anthropic principle should be r e n a m e d the cosmological consistency principle, since it basically confirms that this universe is consistent w i t h carbon-based life. W h y ? W e still don't k n o w . I prefer to believe that our i m a g i n a t i o n and observations w i l l k e e p 289

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on revealing the nature of the universe we live in. O u r scientific explanations constitute a narrative, a representation of the physical w o r l d , as we are able to u n d e r s t a n d it through the use of tools, logic, a n d intuition. To m a r v e l at cosmic coincidences, such as " i f the electric c h a r g e of the electron w a s not exactly equal to that of the proton, then atoms w o u l d not be neutral a n d matter w o u l d not be stable," is to confuse our description of n a t u r e w i t h nature itself. An electron exists only w i t h i n the science we create; it is not a universal entity that we discovered. W h a t we d i d do w a s interpret several observations through the creation of an entity to w h i c h we assigned the properties of the "thing" we call an electron. To put it differently, if there is another intelligent c i v i l i z a tion s o m e w h e r e in the universe, its m e m b e r s w i l l also describe natural phenomena by m e a n s of their tools, logic, and intuition. T h e y w i l l create their o w n entities to m a k e sense of their observations. T h e i r science w i l l be completely different from ours, even though both should be based on the same u l t i m a t e l a w s . We could compare their l a w s to ours a n d should a g r e e to a proper translation. But they w o u l d not, in all l i k e lihood, have created an "entity" such as our electron. W h e n faced w i t h the enormity of the cosmos, we should marvel not at how "finely t u n e d " it is for the existence of intelligent life, w h i c h implicitly a s k s for a teleological explanation, but at the fact that we have the i m a g i n a t i o n to comprehend a n d represent so m u c h of it. We m a y be small players in the big scheme of things, isolated on a small planet, orbiting a small star a m o n g billions of others in an a v e r a g e g a l a x y , floating alongside billions of others in this vast universe. But our creativity spreads its w i n g s across the cosmos, revealing w o r l d s w e m a y never touch. T h e h u m b l e r w e are a s we contemplate the u n k n o w n , the farther our flight will take us.

Short

Discourse

Time:

Part

T h e possibility that we a r e living in an accelerated universe, d r i v e n by some cosmological constant or an ethereal quintessence field, forces us to revise the s t a n d a r d relationship between g e o m e t r y a n d destiny. A 290

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universe w i t h a cosmological constant does not obey the simple rules of F r i e d m a n n ' s cosmology we discussed in chapter 7, w h e r e the fate of the universe is u n i q u e l y d e t e r m i n e d by its e n e r g y density (see figure 19). It is s o m e w h a t paradoxical that the m o r e we k n o w about the universe, the m o r e we seem not to k n o w . We do k n o w that we live in a flat universe, and we k n o w that its a g e is very close to fourteen billion y e a r s . For the universe to be flat, it must have three forms of contributions to its e n e r g y density: o r d i n a r y matter, l i k e protons and electrons, w h i c h m a k e up no m o r e than 5 percent of the total; an u n k n o w n form (or forms) of d a r k matter, m a k i n g up about 30 percent of the total; and an u n k n o w n form (or forms) of d a r k energy, m a k i n g up the rest. We k n o w the a m o u n t s that go into the cosmic recipe, but not most of the ingredients. W i t h a cosmological constant, it is possible for a closed universe to k e e p e x p a n d i n g forever, thus a v o i d i n g the "death by fire" of the big crunch predicted by F r i e d m a n n ' s cosmology. Of course, if we accept the present d e t e r m i n a t i o n of the flatness of the universe, this exercise is s o m e w h a t a c a d e m i c ; w e " k n o w " h o w l a r g e the cosmological constant must be. But, as we have l e a r n e d from cosmology's history, it is a l w a y s very wise to k e e p an open m i n d . T h e contribution from matter decreases

with

the expansion, w h e r e a s

the cosmological constant

r e m a i n s , w e l l , constant. Hence, in a closed universe, if the cosmological constant t e r m becomes l a r g e r than the matter contribution before the t u r n a r o u n d point (see figure 19), w h e n collapse begins, its repulsive g r a v i t y w i l l d r i v e an accelerated phase, a v o i d i n g the big crunch: in the presence of a cosmological constant, a closed g e o m e t r y does not necessarily i m p l y a big crunch. If a quintessence field permeates a closed universe, the situation is even m o r e delicate. T h e quintessence field, acting just l i k e a cosmological constant, m a y also come to d o m i n a t e the expansion rate before collapse b e g i n s , thus a v o i d i n g the big crunch. But only temporarily. W h e n the quintessence field relaxes to the zero of its potential energy, it will no longer d r i v e an accelerated expansion. T h e matter density, albeit very d i l u t e d , w i l l still have a chance to catch up a n d , eventually, reverse the trend c a u s i n g the universe to collapse 291

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upon itself. T h u s , it is really the n a t u r e of the d a r k e n e r g y that determ i n e s the fate of the universe. H e r e things get s o m e w h a t complicated. W e can a l w a y s i m a g i n e , from the depths of our present i g n o r a n c e , that the behavior of this d a r k e n e r g y m a y c h a n g e in the future, say, by h a v i n g a s o m e w h a t b i z a r r e potential e n e r g y for the quintessence field, or different types of cosmological constants d o m i n a t i n g at different times. In that case, unless we have complete k n o w l e d g e of the properties of the cosmological constant or quintessence from a f u n d a m e n t a l theory of the forces of n a t u r e , we cannot confidently predict the future behavior of the u n i verse by s i m p l y m e a s u r i n g the present values of these p a r a m e t e r s . As K r a u s s a n d T u r n e r g l o o m i l y r e m a r k e d , " W e m a y never b e confident that any presently inferred d y n a m i c a l evolution can be extrapolated indefinitely into the f u t u r e . " " If the existence of a cosmological constant or quintessence field is confirmed by future observations, we m u s t abandon all hope of pred i c t i n g the l o n g - t e r m behavior of the cosmos. Unless, of course, we do come across a f u n d a m e n t a l theory of matter a n d forces, w h i c h w i l l explain it all to us, from the n a t u r e of the inflaton field to the s i n g u l a r ity inside black holes, a n d from the origin of the universe to the n a t u r e of quintessence. A n d w h y not call this theory G O D (Geometry of Dest i n y ) , as opposed to the c u r r e n t T O E ( T h e o r y of E v e r y t h i n g ) ? After all, w h a t separates us from the d i v i n e is our finite life span, the fact that our bodies perish in t i m e , even though our ideas m a y r e m a i n . We exist w i t h i n time, w h e r e a s God exists without. It is only t h r o u g h our creativity that we can transcend our corporeal b o u n d a r i e s a n d join the eternal. O u r search for a l l - e m b r a c i n g m e a n i n g is no less passionate than that of Aristotle, N e w t o n , or^Einstein. A theory that can determ i n e the o r i g i n a n d destiny of the

universe represents our final

rational e m b r a c e w i t h the abstract concept of the d i v i n e , a c u l m i n a t i o n of a d r e a m started w i t h the P y t h a g o r e a n s , w h o first believed that n u m bers described the essential n a t u r e of all things. Our y e a r n i n g for a u n i fied theory could not have evolved outside a r e l i g i o u s culture deeply influenced by m o n o t h e i s m . T h i s c l a i m m a y cause some atheistic h i g h 292

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e n e r g y physicists to c r i n g e , since they v i e w the pursuit of k n o w l e d g e as completely devoid of theological influences. In response, I ask t h e m to consider the notion that we live w i t h i n time a n d y e a r n to transcend it. T h e pursuit of an a l l - e n c o m p a s s i n g theory, rational and technical as it is, is also the passionate pursuit of s o m e t h i n g m u c h l a r g e r than ourselves, something timeless, universal, a l l - d e t e r m i n i n g . F r o m it, we w i l l obtain the "initial conditions" that set our classical universe in motion; from it, we w i l l u n d e r s t a n d the behavior of matter a n d the f u n d a m e n tal forces at the smallest of scales, w h i c h we can then use to interpret the conditions inside a black hole or close to the big b a n g s i n g u l a r i t y ; from it, we w i l l uncover the fields that inflate the universe to flatness and that fill it up w i t h ethereal e n e r g y ; a n d , from it, we w i l l d e t e r m i n e the fate of the cosmos a n d , in d o i n g so, our o w n . Even if we never achieve this lofty goal, we will live m o r e fully for h a v i n g tried. O u r search is our redemption.

293

Epilogue: Celestial Wisdom

When one tugs at a single thing in Nature, He

finds

it hitched to the rest of the Universe. â&#x20AC;&#x201D;

JOHN

M U I R

T h e skies a r e full of m a g i c . A n d this m a g i c compels us to look u p , to e x p l a i n , somehow, our place in this vast cosmos. After a l l , we a r e stardust, the c h e m i s t r y of our bodies d e r i v e d from stellar explosions that occurred w e l l before the formation of the solar system. If, d u r i n g the history of h u m a n k i n d , our interpretations of celestial p h e n o m e n a w e r e o r i g i n a l l y put forward by the v a r i o u s religions, today they are d e r i v e d from science. H o w e v e r , as I have tried to a r g u e in this book, there isn't an abrupt r u p t u r e b e t w e e n the r e l i g i o u s a n d the scientific discourses. T h e a w e of a n d fascination w i t h the skies a n d its mysteries, w h i c h are an integral part of most religions, influenced a n d still influence the d e v e l o p m e n t of the scientific theories created to explain the motions a n d properties of the celestial bodies. W h a t w a s once unexpected a n d terrifying, so often interpreted as a m e s s a g e from the g o d s or even as a portent of i m p e n d i n g doom, is n o w incorporated in our cosmic theories, w h i c h strive to e x p l a i n the m a n y celestial p h e n o m e n a as n a t u r a l consequences of causal relations b e t w e e n m a t e r i a l objects. But the 295

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m a g i c , even if n o w part of the scientific discourse, persists. It is difficult to accept the idea that we are insignificant w i t h i n the cosmos, that our existence, as i n d i v i d u a l s or as a species, has scant influence on the u n f o l d i n g of the m y r i a d creations a n d destructions o c c u r r i n g across the u n i v e r s e . Is it a perverse twist of creation that we can w o n d e r a n d question only to k n o w we w i l l never have all the a n s w e r s ? H o w can we reconcile our ability to reflect about the w o r l d a n d ourselves w i t h the fact that we a r e transient beings of l i m i t e d k n o w l e d g e a n d w i s d o m ? Perhaps an a n s w e r lies in the reality that our lives are l i m i t e d by space a n d by time. W i t h o u t limits there is no desire. A n d w i t h o u t desire there is no creation. L i k e stars, w h i c h g e n e r a t e pressure to survive the crush of g r a v i t y , we create to survive the crush of time. A life spent w o r r y i n g about w h a t or w h o m we won't have a chance to love or u n d e r s t a n d w o u l d be w a s t e d ; it w o u l d be a life focused on loss a n d not on the balance dictated by the celestial w i s d o m . We saw h o w regeneration springs forth from destruction; asteroids a n d comets have fallen on Earth, e x t i n g u i s h i n g countless species, but a l l o w i n g for the a p p e a r a n c e of countless others; stars a r e created from the r e m a i n s of others, recycling matter across each of the billions of g a l a x i e s ; even our u n i v e r s e â&#x20AC;&#x201D; p o s s i b l y one a m o n g m a n y c o s m o i d s â&#x20AC;&#x201D; h a s a history, a l t h o u g h we still don't k n o w h o w it began a n d possibly w i l l never k n o w h o w it ends. T h e s e processes of regeneration do not belong exclusively to the skies; they happen every d a y all a r o u n d us. Every tree that falls feeds the g r o u n d from w h i c h m a n y others w i l l g r o w ; each h u m a n life m a y g i v e birth to several others a n d inspire countless more. We a r e c o m p l e x beings, capable of the most beautiful creations a n d the most h o r r e n d o u s c r i m e s . Perhaps, by l e a r n i n g m o r e about the w o r l d a r o u n d us, we w i l l be able to see,bey,pnd our o w n differences a n d to w o r k together for the preservation of our planet a n d species. T h e r e is m u c h w i s d o m to be found in the skies; but we must be e a g e r to learn. T h e first step is easy; it r e q u i r e s only that we look a r o u n d w i t h respect, curiosity, h u m i l i t y , a n d a d m i r a t i o n .

296

Notes

CHAPTER

THE S K I E S

ARE FALLING

1. Quoted in Carl Sagan and Ann Druyan, Comet (New York: Random House, 1985), p. 15. 2. Ibid. 3. Ibid., p. 20. 4. Peter B. Ellis, The Druids (Grand Rapids, Mich.: Eerdmans, 1994), p. 53. 5. Ibid., p. 56. 6. Ibid. Readers who are familiar with the brilliant comic-book series Asterix the Gaul would know of the confrontations between Celts and Romans and the roles of the Druids as shamans and makers of "magic potions" using the mistletoe. When I was a teenager, I too wondered what Getafix, the Druid, put in that superstrength potion he made; I can attest that several home-brewed experiments were all failures. 7. Ibid., p. 130. 8. John B. Noss, Man's Religions, 4th ed. (New York: Macmillan, 1969), p. 45. 9. I thank Susan Ackerman, of the Department of Religion at Dartmouth, for mentioning this connection. It should be noted that only chapters 7-12 in the Book of Daniel are dated at approximately 165 earlier, probably between 350 and 250

C H A P T E R I:

B.C.E.

Chapters 1-6 were written

B.C.E.

H E A V E N ' S A L A R M TO THE WORLD

1. Increase Mather, Heavens Alarm to the World. A Sermon (Boston, 1681). 2. Increase Mather, Cometography: A Discourse concerning Comets (Boston, 1683), p. 2. 3. Ibid., p. 62.

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