A flying mirror made by blasting a gas of electrons and ions with a powerful laser could help solve a long-standing mystery known as the black hole information paradox.
This paradox arose in 1974 when Stephen Hawking calculated that, according to quantum theory, black holes should emit small amounts of electromagnetic radiation until they eventually evaporate. This so-called Hawking radiation doesn’t hold any information about the contents of the black hole, which means that once a black hole disappears, information is destroyed. But quantum theory asserts that information can never truly vanish.
However, the paradox could be resolved if each particle that falls into the black hole, such as a particle of light, or photon, shares information with a partner particle outside the black hole through so-called quantum entanglement. This would mean that even if one particle is destroyed, the information it carried isn’t fully lost.
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Hawking radiation has never been observed from a black hole, but scientists have been trying to mimic it in the lab for years. In some cases, researchers have produced analogues of Hawking radiation, but they have never analysed each particle comprising it in detail. Now, at National Taiwan University and his colleagues have used mathematical models and simulations to show that Hawking radiation can be created in the lab in a way that will allow them to detect each of its photons – and confirm whether they can be entangled with a partner particle.
To that end, they devised an experiment involving an accelerating mirror that serves as an analogue black hole. They plan to test it at a powerful laser facility in Osaka, Japan, in a few months. Chen says their simulations suggest that each day of the experiment, they should detect about two photons that represent Hawking radiation.
The black hole’s event horizon – the point beyond which nothing can escape its gravitational pull – will be represented by a wave of plasma created by shooting a laser into a cloud of ionised gas of increasing density. The laser’s energy will spread through the unevenly dense gas and result in an accelerating tsunami-like wave of electrons that acts like a permeable mirror.
One detector will catch photons emanating from the “mirror” – these are equivalent to Hawking radiation. Another detector will collect photons that penetrate the “mirror”– these represent particles that enter a black hole. Detecting both photons will let researchers check whether they really come in entangled pairs. If the answer is yes, the information paradox may finally be resolved.
To make sure they don’t miss any of these rare escaping photons, the researchers fabricated sensitive detectors using wires a billionth of a metre wide. The way these nanowires carry electric currents changes when they are hit by even a single photon. “I think the theoretical challenges have mostly been taken care of,” says Chen. “The major challenge now is in finalising hardware developments.”
He says that only the world’s most powerful lasers can make these plasma mirrors. In the next few months, the researchers will use a laser that packs 10 trillion times more power than a 100-watt light bulb. Next year, they will up the ante by going to Saclay, France, to use a laser five times as powerful than that.
“If the team succeeds, this will open up a new frontier in exploring the connection between the quantum world, information and [event] horizons,” says at the University of California, Berkeley. Researchers haven’t figured out how to experiment on black holes directly, so being able to demystify the connection between information and these extreme features of space-time is an exciting prospect, he says.
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