Artist illustration of a spacecraft passing through a wormhole to a distant galaxy. Image credit: NASA. Click to enlarge.
Listen to the interview: Unlikely Wormholes (4.5 mb)
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Fraser Cain: Now, I’ve watched my share of Star Trek episodes. How well has this prepared me for the actual scientific understanding of a wormhole?
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Dr. Stephen Hsu: In Star Trek they don’t really use wormholes, but maybe the best treatment in sci-fi for wormholes was in the movie Contact, which is based on a book by Carl Sagan. And actually historically, when Sagan was writing the novel – Sagan was an astronomy professor – he contacted an expert in General Relativity, a guy named Kip Thorne, at Caltech, and wanted to make sure that the way wormholes were treated in Contact was actually as close to being scientifically correct as possible. And that actually stimulated Thorne to do a lot of research on wormholes. Our work is actually an extension of things that he did.
Fraser: So if you wanted to build a wormhole, theoretically, what would you do?
Hsu: You need to have a very weird or exotic kind of matter and that matter has to have highly negative pressure. It turns out that to stabilize the throat or the tube of the wormhole you need very strange matter and our work has to do with how possible that kind of matter would be in models of particle physics.
Fraser: Let’s say you build a tear in spacetime and you fill it with exotic matter to keep it open, and then you could move the two end points of the wormhole around the Universe and they would connect both in space and in time.
Hsu: But in some science fiction stories they postulate that there are just some wormholes left over from the Big Bang, and we would just discover one and start using it. But the constructive model is that humans, or some alien civilization, actually build their own, and in that case the two ends of the wormhole probably are pretty close together at the beginning but then you pull them apart.
Fraser: Where has your research led you to look at wormholes?
Hsu: We were studying fundamental constraints on something called the “equation of state of matter” – what properties, like pressure or energy density can matter have. We found some very strong constraints, and it turns out those constraints are very negative for the possibility of building a wormhole.
Fraser: What effect will they have on the wormhole?
Hsu: To get the very weird exotic matter that I mentioned before with very negative pressure, it turns out the equations show that when you force the pressure to be that negative, there always some unstable mode in the matter, which means that if you were to bump your apparatus, you might find the exotic matter – which is stabilizing the wormhole – just collapses into a bunch of photos or something.
Fraser: Is it a matter of not bumping your apparatus, or is it theoretically impossible to reach a stable point?
Hsu: I would say it’s theoretically impossible to build classical matter which is stable and can stabilize a wormhole. You might ask, well maybe I’ll just avoid bumping the thing, but if you were to send a person through the wormhole, that itself would provide a bump and would very likely cause the whole thing to fall apart.
Fraser: Let’s say you didn’t want to send people, you just wanted some way of sending information – talking back in time.
Hsu: That’s not excluded. It turns out the constraints we derive have to do with matter in which quantum effects are relatively small. If you have matter in which quantum effects are very big, then you could still have a stable wormhole. The wormhole itself would be fuzzy in a quantum way. The tube of the wormhole would be fluctuating like a quantum state. Now, that doesn’t prevent you from sending a message back in time; you might have to try to send the message many times to get it to go where you want it to go. But, perhaps you could still send a message. Sending a person might be dangerous if the wormhole is fluctuating because the person might end up in the wrong place or the wrong time.
Fraser: I’d heard estimations that building a wormhole would require more energy than the entire Universe. Have you got some kind of calculations to that effect?
Hsu: Our calculations don’t necessarily show that. It does take a tremendous amount of energy density to create a wormhole which is big enough for a human to fit through. But, usually considering this kind of problem, you assume that whatever civilization is trying to do this has arbitrarily advanced technology. What we’re trying to understand is whether there’s a limitation not coming from technology but really coming from the fundamental laws of physics.
Fraser: And where will your research lead you from this point on? Is there something that you’re still a little unsure about?
Hsu: Our result mainly has to deal with the classical wormholes, or wormholes whose spacetime is not very quantum mechanical, and we’re still interested to see if we can extend our results to cover wormholes in which spacetime is fuzzy.
Fraser: There’s some new work on dark energy where they’re saying that the dark energy effect seems to be happening in the Universe, that it’s accelerating. Either there’s a new form of energy that’s not been seen before, or maybe it’s a breakdown in Einstein’s theories at a large level. If some of that work starts to show that maybe Einstein’s relativity isn’t able to explain it at the larger level, will it have an implication on the classical understanding of what a wormhole is?
Hsu: In the context of dark energy, since it’s something that affects the large scale structure of the Universe, the behaviour of the Universe on length scales of megaparsecs, it’s always possible that General Relativity as a theory is modified at very large distances and because we haven’t been able to test it on those distances. So it’s always possible that conclusions you get from Relativity are just not applicable. In our case, the length scale over which we’re using General Relativity is on the size of a human. So, it would be somewhat surprising if General Relativity were to break down already at those length scales, though it’s possible.
Fraser: So it’s more on the small side what you’re looking at. It still explains things quite nicely at this scale.
Hsu: Right, there are stronger experimental tests of General Relativity, or at least Newtonian gravity, on length scales of metres than on megaparsecs. So we’re a little more confident that the mathematical formulation of gravity that we’re using is correct.
Fraser: If I wanted to get across the Universe quite rapidly, I should look perhaps to the warp drive instead, or maybe just plain old moving in regular space.
Hsu: I’m a huge science fiction fan, and have been since I was a kid, but as a scientist, I’d have to say it’s looking like our Universe seems to not be constructed in a very convenient way for humans to get from star to star. And the sci-fi which we end up staying close to our Sun, but we do amazing things with bioengineering or information technology or A.I. seem more likely to be realizable with our physical laws, than Star Trek.