Through the looking glass: Theories of stable and safe wormhole travel
Envisioning a traversable wormhole in Interstellar
Kip Thorne included black holes, wormholes, higher dimensions, and more in his original screenplay for a film about warped space-time. The end result that became the film Interstellar was rather unlike the idea Thorne first concocted, but the concept of warped space-time remained. To keep Interstellar as accurate as possible, Thorne applied his expertise in the field of black holes, wormholes, and warped space-time. His years of research and theorizing benefitted the film greatly. Thorne was one of the first to research the possibility of a traversable wormhole, where a traveler might be able to use one to reach other parts of the Universe.
Why “Wormhole”? Before discussing the specifics of Thorne’s theories into wormholes, the question that must be answered first is, ‘What exactly is a wormhole?’ The simplest answer is that a wormhole is—hypothetically speaking—a shortcut that connects two points in spacetime and could link extreme distances together. A wormhole is also known as an Einstein-Rosen bridge. Because of the colloquial imagery of a two-dimensional hole connected by a three-dimensional tube or tunnel to another two-dimensional hole, the name wormhole became attached to the concept. The most common representation looks like two funnels fused together at the narrow end (see right).
Challenges and Solutions The original concepts of wormholes use a proof called the Schwarzschild metric. Through study over the years, this metric has shown that these types of wormholes are not actually traversable. The main reason is that the ‘throat’ of a wormhole—the ‘tunnel’ through which light and matter would travel—is under the same sort of gravitational stresses that would be occurring at the horizon (the escape boundary) of a Schwarzschild black hole. This type of black hole does not have an electric charge and does not rotate, as most black holes do. Put simply, the tidal (gravitational) forces would not permit any traveler to survive the trip through a Schwarzschild wormhole, because they would be pulled apart. Thorne, along with his graduate student Michael Morris, theorized that a wormhole could become traversable if the throat were to be held open using exotic matter. What is exotic matter, though? Aside from the obvious given its name, exotic matter has properties unusual or ‘exotic’ to normal matter, in that it behaves differently than the normal matter forming the Universe. Some examples of exotic matter include dark matter, the hypothetical substance that cannot be seen but which makes up most of the matter in the universe. For the purposes of their theory, Thorne and Morris refer to exotic matter as that with negative mass. Exotic matter with negative mass would allow for the forces exerted by the tidal stresses of a wormhole to be negated, and therefore make travel through the wormhole possible. In their 1987 paper, Wormholes in spacetime and their use for interstellar travel: A tool for teaching general relativity, Thorne and Morris produce the following nine conditions, or properties, in which a wormhole would be traversable:
1. The metric should be spherically symmetric and static, independent of time. 2. The solution must everywhere obey the Einstein field equations. 3. To be a wormhole, the solution must have a throat that connects two asymptomatically flat regions of space-time. 4. There should be no horizon, since a horizon, if present, would prevent two-way travel through the wormhole. 5. The tidal gravitational forces experienced by a traveler must be bearably small. 6. A traveler must be able to cross through the wormhole in a finite and reasonably small proper time (e.g. less than a year) as measured by herself, but also by observers who remain behind or who await her outside the wormhole. 7. The matter and fields that generate the wormhole’s spacetime curvature must have a physically reasonable stress-energy tensor. 8. The solution should be perturbatively stable (especially as a spaceship passes through). 9. It should be possible to assemble the wormhole.
This image of a simulated traversable wormhole uses the Morris-Thorne metric to connect the square at University of Tübingen in Baden-Württemberg, Germany with sand dunes near Boulogne sur Mer in the north of France.
These nine conditions, or properties, would allow for the creation of a wormhole permitting travel for an adventurer in both directions. This hypothetical set of properties posited by Thorne and Morris also forms the basis for the wormhole featured in Interstellar. Following their paper’s publication, the theory became known as the Morris-Thorne wormhole metric. It uses a spherical shell of exotic matter to hold the wormhole open and accepts that Einstein’s theory of general relativity is correct.
The Problem with Theory... There are unanswered questions and challenges beyond the current understanding of theoretical physics. The creation of exotic matter, in this case negative mass-energy, is something that is still a hypothetical. It is, as science is currently understood, only a possibility, but then so are wormholes. Both traversable wormholes and exotic matter, as Thorne and Morris classify it for the purposes of their theory, are feasible. The concept of wormholes is allowable in general relativity, and negative mass is both mathematically sound and does not violate the laws of the conservation of momentum or energy. They could exist, but there is no current proof of either, as of yet. One challenge is maintaining the structure of a stable wormhole against someone or something traversing through it. Thorne and Morris suggest that an advanced civilization might be able to monitor any ‘perturbations’ of a spaceship passing through a wormhole and maintain the structural stability using feedback forces (of negative mass-energy). The most challenging aspect of Thorne and Morris’ theory is the assembly of a traversable wormhole. They state, after listing the ninth property for a traversable wormhole, that assembling one should take less than the mass of the Universe and less than the age of the Universe, but they only imagine the ways in which it could be done. They also state that assembling a wormhole would require a change in the topology of space. This is a feat that would be incredibly complex and have all sorts of reactionary effects, but it could be imagined to be possible by some extremely advanced civilization. In any case, while there is not yet concrete evidence that proves the existence of wormholes, there is certainly a sound principle for their theoretical existence. Looking at the advances in science throughout the last century, it is entirely possible to see that the next century could bring evidence proving Thorne’s theories correct, as greater scientific knowledge is unlocked and greater understanding achieved.