A wormhole that you can actually fly through is indeed possible. At least: if we assume that our universe has an extra dimension, say Indian scientists.

In science fiction stories it is always nice if you have a quick way to cover the enormous distances in our universe in a short time. One popular way to do that – see the movies for example Contact and Interstellar – is a wormhole: a shortcut between two points in space.

Only problem is: to actually travel through such a wormhole, exotic matter is needed. And that matter does not exist as far as we know. Rikpratik Sengupta from Aliah University in India and colleagues now have a devised a way to make that kind of matter superfluous† But: they have to assume that there is an extra dimension.

Tiny effect

In themselves, wormholes are allowed by Albert Einstein’s general theory of relativity, our current theory of gravity. What’s called, Einstein himself wrote about it, together with his assistant Nathan Rosen, in 1935. However, it later turned out that such a wormhole ‘clamps’ before anything can travel through it. That seemed to strip the phenomenon—which it’s already unclear whether it actually exists—of any practical use for space travelers.

fast forward to the eighties, when the famous astronomer Carl Sagan for his novel Contact needed a way to quickly transport his protagonist to another place in the universe. He asked physicist – and later Nobel laureate – Kip Thorne for help. Thorne then thought that you can keep a wormhole open if you have access to exotic matter. That is, matter with negative mass or energy.

That solution was good enough for Sagan’s story, but not very satisfying for physicists. In principle, there is no matter at all with such properties. Yes, thanks to quantum mechanics it is possible to to get the energy negative on a very small scale† But how you should ‘blow up’ that tiny effect to build an entire wormhole is completely unclear.

Higher Dimensional Space

Sengupta and colleagues therefore do not assume the existence of exotic matter in their article. They do, however, use a theory that sounds at least as exotic: the so-called Randall-Sundrum II model† That assumes that our universe, which has three spatial dimensions plus time, is not the whole story. Instead, it floats around in a space with one more dimension.

Now that’s not just a crazy idea. In any case, string theory – in which nature consists of strings on the smallest scale – requires more dimensions than the dimensions we experience. Moreover, it could explain why gravity is so much weaker than other forces. This force is the only one not bound to our universe, as can be deduced from string theory, but it can also travel through higher dimensional space. As a result, he becomes, as it were, ‘diluted’.

Not pulled apart

Are you willing to assume that the Randall-Sundrum II model is correct? Then you can suddenly build a wormhole without exotic matter, Sengupta and his team show in their article. In fact, that higher dimension provides the necessary negative energy, they write. So you no longer need crazy stuff that can’t be found anywhere and probably can’t be made.

Now such a wormhole is of no use if you are immediately torn to pieces when you try to travel through it. But that is not the case here, write the Indian physicists. “The acceleration due to tidal forces is small enough in the neck of the wormhole to allow a traveler to travel through it without being pulled apart.” Fine.

Other recipes

Now, Sengupta and colleagues aren’t the first to resort to the Randall-Sundrum II model to create a realistic wormhole. Previously, the physicists Juan Maldacena and Alexey Milekhin the same† About that study, Maldacena says: “There is no good reason to assume that this model describes nature. We only used it because it a) allows wormholes and b) has not been ruled out by experiments.” He therefore calls his earlier article “a theoretical exercise”. The same could be said about Sengupta’s new article.

Yet Indian physicists seem to take the possibility of a universe floating around in a “space” with an extra dimension more seriously than Maldacena. “If a wormhole is ever found,” they even write, “there is a good chance that our universe is a three-dimensional object, embedded in higher dimensions.”

In fact, they say: if wormholes actually exist, they must have been created according to our recipe. But the many physicists who have come up with other recipes for wormholes will no doubt agree.