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The Quest To Test Stephen Hawking's Black Hole Paradox Using A "Relativistic Flying Mirror"

We might have a viable analog for Hawking radiation at last, meaning we can test his black hole paradox.

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Stephen Luntz

Freelance Writer

clockJun 6 2022, 16:42 UTC
illustration of a black hole
Hawking's black hole paradox says that according to quantum theory, black holes emit small amounts of radiation until they evaporate. However, this radiation holds no information on the black hole, meaning when it disappears, information is destroyed - something quantum theory posits can't happen. Image credit: sakkmesterke/Shutterstock.com

Stephen Hawking's proposal that particles appear around black holes, now known as Hawing Radiation, is among the transformational ideas of modern physics. Yet almost 50 years later, we have yet to observe Hawking Radiation in space, or replicate it in the lab. A project has been formed to seek an analog using a “relativistic flying mirror”, and new calculations suggest this is not an unrealistic goal.

If Hawking is right, black holes should slowly evaporate, with each particle appearing outside the event horizon draining mass from the black hole. This should cause them to shrink to nothing billions or trillions of years after they run out of material to feed them. Indeed, the end-fate of the universe may be nothing but slowly evaporating black holes. However, Hawking Radiation appears to violate the principle that information must be maintained. Resolving this paradox could hold the key to addressing the apparent contradictions between General Relativity and Quantum Field Theory. This may in turn lead to the theory of everything Hawking spent much of his life chasing.

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Given the near impossibility of studying Hawking radiation in reality, physicists have theorized more easily testable analogs. The AnaBHEL (Analog Black Hole Evaporation via Lasers) Collaboration has been established to make one of these a reality. In a pre-print (not yet peer-reviewed) on ArXiv.org AnaBHEL members set out why they think the plan might work

One of the puzzling things about black holes for physicists is the question of whether they involve the loss of information. To most of us, this may not seem like a big deal – we lose information every time we forget to save the document we're working on before a computer crash, but to a physicist, such loss violates unitarity, which says information can never be truly lost.

Sadly we cannot prosecute crashing computers for violation of this law of physics, but physicists have proposed many solutions to explain how Hawking evaporation could avoid a violation. Determining which, if any, of these solutions is correct could answer some of physics' biggest questions. However, as the paper notes, “It is almost impossible to settle this paradox through direct astrophysical observations, as typical stellar-size black holes are cold and young, yet the solution to the paradox depends crucially on the end-stage of the black hole evaporation.”

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Since making black holes in the laboratory is well beyond current capacity – and likely to raise a few health and safety issues – proposals to resolve the question usually involve creating an analog for Hawking radiation, rather than the real thing. We might not be certain the analog and the original behave identically, but it's a good place to start.

One proposal for such an analog involves firing a high-intensity laser on a plasma target of decreasing density. This according to a 2017 paper, would create a “relativistic flying mirror” whose quantum field resembles that around a black hole. The parallels are more direct than previously used alternative methods and therefore arguably better, inspiring the establishment of AnaBHEL.

Even if the theory is sound, however, the experiment might not work if the equipment is insufficient, for example if the laser is not powerful enough or the detectors not sensitive enough to capture all the radiation produced. After all, it took years of searching to find the Higgs Boson because, when the hunt began, the particle accelerators used lacked the necessary oomph.

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The paper makes the case that an accelerating relativistic plasma mirror is indeed a good analog for a black hole, and that the setup being developed should be sufficient for the task. An initial trial is planned for this summer. If that fails, a more powerful laser will be applied to the same design next year.

Even if the experiment works the authors acknowledge it will not settle the underlying question. The map is not the terrain. We know that in the circumstances of the laboratory unitarity will be preserved. Nevertheless, the relativistic mirror could teach us how information preservation occurs in this environment, which may reflect what occurs around a black hole, For a question this big, that is enough of a prize to get physicists excited.


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