4 minute read
‘No particle without inter
According to quantum mechanics a particle is no longer in one single place, and entangled particles seem to know what state the other particle is in. Incomprehensible? Not if you accept that such particles are entirely defined by their interactions, argues Italian physicist Carlo Rovelli.
Text: Martijn van Calmthout Illustration: Pepijn Barnard
In his latest book Helgoland, Italian physicist Carlo Rovelli takes the reader back to the eponymous birthplace of quantum physics: a treeless German North Sea island where young physics student Werner Heisenberg took refuge from hay fever in 1925. He came back with a theory of the atom that we still enjoy today.
But that pleasure comes at a price, Rovelli says from his study in Toronto, Canada.
‘Quantum physics has not only brought us microelectronics, computers, molecular biology, nanochemistry and much more. It has also brought the realisation that our intuition does not work at all in the world of the very small. Quantum reality is incomprehensible. In my book I try to show that we are beginning to understand why.’
Carlo Rovelli. Italian by birth. Theoretical physicist. A friendly sixty-something with twinkling eyes and curls. About five years ago, he became world famous with his bestseller Seven Brief Lessons on Physics.
A reassuringly thin, somewhat philosophical work on what we know about the foundations of reality. Heligoland is thicker. Even more philosophical. But it is also meant to be reassuring, Rovelli says. ‘The key message is: the particle world is no crazier than the everyday world and we can deal with it like adults. It is like the modern solar system. We see the sun moving across the sky every day, rising and setting. Yet we know: it is not the sun that moves, it is us. And no one is upset about that.’
What makes quantum physics so elusive?
‘It is that we think classically about the world of particles and atoms. You cannot do that. It goes back to Heisenberg on Helgoland. He tried to find order in the energy jumps of electrons in atoms, which we see thanks to the spectral lines in their light. How could electrons make only those steps? Heisenberg found the formulas that precisely describe the process. Provided, and there the incomprehensible begins, that numbers are replaced by matrices – by grids of numbers.’
What did that insight change?
‘Everything. Quantities are no longer fixed, as with Newton; there are only transitions between states and probabilities. This leads mathematically to particles that are no longer in one place, but everywhere. To entangled particles that seem to know what state the other is in. To a cat in a box that is both awake and asleep – I don’t like to kill Schrödinger’s cat. In short, the famous quantum magic. And it’s very different from what objects do in everyday reality.’
Particles that are also waves.
‘It is often put that way, yes. But an electron is not one or the other. It manifests itself as one or the other, depending on the perspective; in the interaction with the researcher.’
There are quite a lot of interpretations in physics that aim to contain the craziness of the quantum world. You don’t like most of them. Why?
‘Take the popular many-worlds interpretation, in which reality continually splits
HOW MANY
DIMENSIONS ARE THERE?
when a particle jumps to a particular state. The idea is that in a parallel reality it ends up in a different state. But, of course, the idea of infinite realities is at least as crazy as the indeterminacy of states. If you subscribe to it, you are anxiously trying to hold on to your classical idea about particles having properties – at all costs.’
In Helgoland, you make a case for what is called the relational interpretation of quantum mechanics. What does that mean?
‘The main point is that objects like particles are entirely defined by their interactions; by their relationships with other objects. A particle without interactions is invisible and untraceable. It is not there.’
That is a pretty radical position. Are you saying that objects in themselves do not exist?
‘Of course objects do exist. I am a physicist, a realist; I investigate the material world, stones, stars, particles. But if you are trying to understand why the particle world is so foreign to us, it is a useful idea.’
IS THERE A THEORY
OF EVERYTHING?
Does it help physicists?
‘Certainly. The interpretation you use is not just a philosophy; it gives new form to intuition. I research the quantum nature of gravity, one of the great questions of physics. Of all science, I would even say. For this, a good perspective helps to ask the right questions.’
Will ordinary people ever get used to this idea?
‘Quantum effects rarely play a role in everyday life. But in the everyday world, it is not at all surprising that entities are defined by their interactions. This applies to people and their relationships, to organisms in ecosystems, to social structures and even to politics. I do like the idea that the particle world is in fact structured in the same way. It is not a totally different reality. We know it very well.’
Provided you find the right perspective.
‘You may ask yourself why a physicist should be concerned with philosophy. Apart from the fact that it fascinates me as an inquisitive person, it has always been the case that important scientists thought deeply about their basic assumptions. Einstein arrived at his relativity when he realised that time and distance are fundamentally dependent on the motion of the observer. Insight changes everything; that is the way it has always been.’
