One of physicist Stephen Hawking’s most famous paradoxes may finally be solved: black holes may actually contain information about the massive stars that created them, according to new research.
This information can hide in the radiation around black holes – colloquially known as “quantum hair” – and could, in theory, be retrieved to retell the origins of these black holes, the research suggests.
These discoveries could finally solve a thorny problem that Hawking had been working on during his last years.
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The black hole problem
According to the work of Stephen Hawking, radiation slowly “leaks” from black holes in the form of thermal energy, known as “Hawking radiation”. But due to its thermal nature, this radiation cannot carry information. This means that as black holes evaporate, they methodically destroy all information about the stars that created them. This is contrary to the laws of quantum mechanics, which say that information cannot be destroyed and that the final state of an object can reveal clues to its initial state. This problem has preoccupied cosmologists for decades and is known as “Hawking’s information paradox”.
“[This research] is the final nail in the coffin of the paradox, as we now understand the exact physical phenomenon by which information escapes from a decaying black hole,” said Xavier Calmet, professor of physics at the University of Sussex and The study’s lead author, to Space.com’s sister publication Live Science via email, suggests a modification of Hawking’s radiation that makes it “non-thermal” and therefore able to carry information about the ultimate fate with it. of the black hole.
Black holes are such massive objects that nothing can escape their gravitational pull, not even light. They form when huge stars run out of fuel and collapse in on themselves.
In classical physics, black holes are “very simple objects,” Calmet said. “So simple that they can be characterized by three numbers: their mass, their angular momentum and their electric charge.”
Famous physicist John Wheeler described this lack of distinguishing features by saying “black holes don’t have hair. (opens in a new tab)“But,” Calmet explained, “while the final black hole is very simple, the original star that spawned it is a complex astrophysical object, made up of a complicated amalgam of protons, electrons, and neutrons coming together to form the elements that build the chemical makeup of that star.
While black holes carry no “memory” of the stars they once were, the rules of quantum physics say the information cannot simply be erased from the universe. In 1976, Hawking introduced a fly to this cosmic ointment by showing that this information also could not reside indefinitely in sealed black holes outside the universe. Applying the rules of quantum mechanics to black holes, Hawking suggested that they emit a type of thermal radiation, later called Hawking radiation. Over immense periods of time, the leakage of this radiation causes the black holes to evaporate completely, leaving behind only a vacuum. In this way, the information is irretrievably lost.
“That, however, is not allowed by quantum physics, which posits that the movie of this black hole’s ‘life’ could be rewound,” Calmet said. “From the radiation, we should be able to reconstruct the original black hole and then possibly the star.”
Find the “hair” of the black hole
Together with his colleague Steve Hsu, professor of theoretical physics at Michigan State University, Calmet has been working since 2021 to decipher Hawking’s paradox. In a previous study, published in March 2022, the team argued that black holes indeed have “quantum hair, (opens in a new tab)” in the form of a unique quantum imprint in the gravitational fields around them
In their new research, the team re-evaluated Hawking’s calculations from 1976, but this time took into account the effects of “quantum gravity” – the description of gravity according to the principles of quantum mechanics – something that Hawking had not done.
“Although these quantum gravitational corrections are tiny, they are crucial for black hole evaporation,” Calmet said. “We were able to show that these effects modify the Hawking radiation in such a way that this radiation becomes non-thermal. In other words, taking quantum gravity into account, the radiation can contain information.”
While the quantum hair suggested in previous work by Calmet and Hsu was an abstract mathematical concept, the team has now identified the exact physical phenomenon by which information escapes from the black hole via Hawking radiation, and how it could be recovered by an outside observer. (opens in a new tab). This is currently not possible, as it would require an instrument sensitive enough to measure Hawking radiation, which is currently purely theoretical.
Currently, there’s no real way for astrophysicists to measure the effect the researchers are proposing because it’s tiny, Calmet acknowledged. Instead, he suggests that one way to advance this theory would be to study black hole simulations in labs on Earth. The team’s mathematical modeling of Hawking radiation and black holes could prove invaluable in these simulations.
The study was published March 6 in the journal Physics Letters B. (opens in a new tab)
The story originally appeared on Live Science.
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