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# Black Holes on the Edge

16 March 2022

Nothing can escape a black hole. General relativity is very clear on this point. Cross a black hole’s event horizon, and you are forever lost to the universe. Except that’s not entirely true. It’s true according to Einstein’s theory, but general relativity is a classical model. It doesn’t take into account the quantum aspects of nature. For that, you’d need a quantum theory of gravity, which we don’t have. But we do have some ideas about some of the effects of quantum gravity, and one of the most interesting is Hawking radiation.

One way to study quantum gravity is to look at how quantum objects might behave in curved space. Typically in quantum theory, we assume space is a fixed and flat background. Special relativity still applies, but general relativity doesn’t. Basically, we just ignore gravity since its effects are so teeny. This works great for things like atoms in Earth’s gravity. But quantum mechanics around the event horizon of a black hole is very different.

Hawking wasn’t the first to study the quantum effects of black holes, but he did show that event horizons aren’t immutable. If a quantum object was forever bound by a black hole, we would know with absolute certainty where the object is. But quantum systems are fuzzy, and there is always an uncertainty to their location. We could say the quantum object is *probably* within the black hole, there is a small chance it isn’t. This means that over time objects can quantum tunnel past the event horizon and escape. This causes the black hole to lose a bit of mass, and the less mass a black hole has, the more easily quantum objects can escape.

So black holes can emit faint energy thanks to Hawking radiation. What’s interesting about this is that the effects connect black holes to thermodynamics. Since black holes emit some light, they, therefore, have a temperature. From this simple fact, physicists have developed the theory of black hole thermodynamics, which helps us understand what happens when black holes merge, among other things.

It’s brilliant stuff, but the problem is we have never observed Hawking radiation. Most physicists think it does occur, but we can’t prove it. And given (theoretically) how faint Hawking radiation is, and how far away even the closest black holes are, we aren’t likely to detect Hawking radiation in the foreseeable future. So instead, scientists look at analog systems such as water vortices or optical systems that have horizon-like properties.

A recent study in *Physical Review Letters* looks at optical black hole analogs, and found an interesting effect of Hawking radiation.^{1} One way to simulate black holes is to create a constrained packet of light in a non-linear optical material. The material acts as a kind of one-way gate, so photons can enter the packet in only one direction (like the one-way nature of a black hole event horizon). At the other side of the packet, photons can only leave, which is similar to a hypothetical white hole. So the optical system models a black-hole/white-hole pair.

The team used computer simulations to study what would happen when a quantum system passes through the simulated pair. They found that the pair could be used to create a quantum effect known as entanglement. When two particles are created as a quantum pair, they are entangled, which means an interaction with one particle affects the other as well. We think that when particles escape a black hole via Hawking radiation, they do so as entangled pairs. According to this latest work, the simulated black-hole/white-hole pair can be used to change the entanglement of a system passing through it. The system can even be tuned so that the entanglement is strengthened or weakened.

This work supports the idea that Hawking radiation occurs in entangled pairs, but it also shows how entanglement could be tweaked experimentally, which would be very useful to other research, such as information theory and quantum computing. The next step is to actually perform this kind of experiment in the lab. If it works as predicted, we could have a powerful new way to study quantum systems.

Agullo, Ivan, Anthony J. Brady, and Dimitrios Kranas. “Quantum Aspects of Stimulated Hawking Radiation in an Optical Analog White-Black Hole Pair.”

*Physical Review Letters*128.9 (2022): 091301. ↩︎