How to understand wormholes and their weird quantum effects

Classical relativity suggests that nothing could pass through a wormhole and exit, but quantum effects change that, says space reporter Leah Crane.

NASA Goddard Space Flight Center; background, ESA/Gaia/DPAC

Over the past month, I’ve been doing a lot of thinking about wormholes, mostly because there’s been some drama in the quantum computing community around an attempt to simulate a wormhole on a quantum computer, and some analysis indicating that the attempt may not have actually been successful. While those stories don’t really have much to do with space, one thing I’ve realised is that, despite the many sci-fi movie scenes of a nerdy science teacher poking a pencil through a piece of paper, understanding wormholes is really hard. So, I thought I’d take you through the basics of wormholes and some of the latest research on them.

In classical (meaning not quantum) relativity, a wormhole is a tunnel through space with a black hole at either end. It can connect any two regions of space to one another, but you can’t go through it because nothing can escape a black hole. “You can’t ever travel through the wormhole. You fall in, and you get stuck in the interior and you can never go out the other side,” says Don Marolf, an astrophysicist at the University of California, Santa Barbara.

From the outside, a classical wormhole would just look like two black holes at separate locations in space-time. And even its inside, called the throat, would look pretty much like the inside of a black hole. That means they’d be essentially impossible to spot. “In classical relativity, wormholes are extremely hard to observe in any context even if you’re willing to jump into a black hole,” says Marolf. A simulated image of a black hole – or the opening to a classical wormhole – is pictured above.

But classical relativity isn’t quite right. The universe is, in fact, quantum, and quantum effects change things. When it comes to wormholes, that change comes in the form of negative energy – which has all the opposite properties of regular energy. It’s been demonstrated that very small amounts of this negative energy can be formed through quantum effects, and if there’s enough of it in a wormhole, it could in theory prop the throat open to make the wormhole traversable. This sort of wormhole wouldn’t have a black hole at either end – each opening to the throat would probably look a lot like the surrounding space.

“If [traversable wormholes] were to be macroscopic, they would only exist in fairly exotic theories of physics – they require all kinds of novel fundamental fields to exist that we haven’t detected in our universe,” says Marolf. “We haven’t ruled them out, but we certainly don’t have any evidence that they’re there.”

The most common question that I get asked about wormholes is whether they would enable faster-than-light travel – after all, they do provide a shortcut between two spots in space. Unfortunately, the answer is no. This is because of a relativistic effect called gravitational time dilation. If you’ve seen the movie Interstellar, you’ve seen how this might play out: close to an object with an extreme gravitational field, time passes slower.

So if you jumped into a wormhole, you would appear to an outside observer to move through it in slow motion, and you wouldn’t be able to get to the other side any faster than a beam of light travelling outside the wormhole from end to end. From your perspective, it would seem like you were accelerating rapidly towards the centre of the wormhole’s throat, but no matter how fast you felt like you were moving, you’d still lose the race every time.

The powerful gravity within a wormhole would be a problem in a few other ways, too. For one, anything else that fell into the wormhole – even a photon of light – would quickly get boosted to such high energies that if it hit you on your way through it would probably kill you immediately.

For another, traversable wormholes are expected to be extremely unstable, and even something as small as those high-energy photons could ruin the whole thing. “If this is even a sizeable wormhole, and some itty-bitty photon wanders into it, the photon gains more energy as it falls in and speeds up, and by the time it gets to the middle this photon has this enormous energy, and it overwhelms the negative energy holding the wormhole open and it collapses,” says Marolf. “That’s kind of a problem if you want to see the wormhole – even the act of trying to illuminate the wormhole with a flashlight destroys it.”

There are some ideas for how we might be able to detect a traversable wormhole, if they exist. They could change the polarisation of the light around them, although we’d have to be up close or have extremely powerful telescopes to detect that. They could also stretch the light coming from objects behind them in ways that are subtly different from regular black holes, but there would have to be a fairly high number of them for us to be able to detect that. There are two sides to that coin: we probably won’t actually detect any wormholes anytime soon… but our lack of a detection also doesn’t disprove their existence. They could be out there.

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