Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute, host of "Ask a Spaceman" and "Space Radio," and author of "How to Die in Space." Sutter contributed this article to Space.com's Expert Voices: Op-Ed & Insights.
It may be possible to build a real, traversable wormhole, but only if our universe has extra dimensions, a team of physicists has found.
To make a wormhole, you need to glue together different parts of the universe, connecting them by a bridge or a tunnel, usually called a "throat." This throat can be as big or as long as you want, but typically, you want it to be shorter than the normal distance to your destination. In Einstein's theory of general relativity, making a wormhole is pretty straightforward: You just build a black hole and connect it to a white hole (which is the exact opposite of a black hole), and boom, there you have it: a tunnel through space-time.
Unfortunately, the biggest problem with wormholes is that they are fantastically unstable. As soon as they form, their enormous gravitational strengths (they are literally made of black holes, after all) rip them apart faster than the speed of light, which makes them rather useless as actual shortcuts through the universe.
The only known way to stabilize a wormhole is to use some form of exotic matter. Exotic matter can take the form of matter with negative mass, which doesn't appear to exist in the universe, or some other scenario that violates what are known as the energy conditions of general relativity. The energy conditions simply state that everybody should experience positive energy, on average, pretty much everywhere they go. To stabilize a wormhole, however, a traveler would have to experience a region of negative energy. This negative energy would balance out the positive energy of the mass of the traveler, keeping the wormhole open as they passed through it.
There are some physical scenarios that lead to violations of some of the energy conditions some of the time. However, physicists do not know of a single instance in which all of the energy conditions are violated, on average, over long periods of time — which is exactly what you need to do to build a wormhole.
Your "brane" on physics
Gravity is extremely weak; it's billions upon billions of times weaker than any other force of nature. This fact troubles many physicists, because when something is so strikingly different from the rest of the universe, there's usually some interesting physical explanation behind it.
But we have no physical explanation for why gravity is so weak. One idea among theoretical physicists is that there's more to the universe than meets the eye. Inspired by string theory's concept of many extra spatial dimensions all wrapped up on themselves and compressed to submicroscopic scales, some theories propose that there are additional spatial dimensions to reality, besides the usual three.
In these theories, our three dimensions are just a "brane," a relatively thin membrane that exists within a higher-dimensional "bulk." Those extra dimensions aren't necessarily huge; if they were, we would've noticed particles or planets appearing and disappearing from the extra dimension. But the extra dimensions might be larger than the minuscule dimensions of string theory — perhaps as big as a millimeter.
In this scenario, all the forces and particles of nature are then confined to the three-dimensional brane, while gravity alone has the privilege of traveling through the bulk. Thus, gravity could be just as strong as every other force, but it's so heavily diluted among all the extra dimensions that it appears weak to our three-dimensional experience.
Through the wormhole
Because these brane-based ideas are attempts to understand gravity, they open up new opportunities to explore the nature of wormholes. Our knowledge of wormholes is governed by general relativity, but perhaps the presence of extra dimensions changes how general relativity operates, thus making wormholes possible, an Indian research team proposes in a new paper posted to the preprint database arXiv.
In the paper, the physicists explored whether it would be possible to build a wormhole in the "braneworld" model first proposed by physicists Lisa Randall and Raman Sundrum in 1999.
The authors of the new paper found that they could indeed build a stable, traversable wormhole in this brane-based model of gravity. Even better, they didn't need any exotic matter to do it.
Although the team did find that this situation still violated the energy conditions of general relativity, they argued that this violation was a feature, not a bug. Instead of requiring some weird and exotic (and probably impossible) ingredient to build a wormhole, the nature of gravity in the extra spatial dimensions naturally gave rise to a violation of the energy conditions. Once those conditions were broken, wormholes became a natural consequence, they said.
The researchers even went so far as to suggest that if we were to ever directly observe or create a wormhole, this might indicate that the universe has more spatial dimensions than the usual three.
As with all theoretical work on the subject of wormholes, this is not the final word. Nobody knows if the Randall-Sundrum theory, or any other theory based on branes and extra dimensions, is correct. And nobody has a quantum theory of gravity — a theory of strong gravity at small scales — which would almost certainly change the calculations, perhaps to the point of once again eliminating the possibility of wormholes.
But this result is still interesting, as it joins a number of efforts to explore the edges of our understanding of gravity, taking general relativity to the absolute limits. Wormholes may or may not exist, but attempting to understand them will definitely increase our knowledge of the universe.