Stephen Hawking black hole information paradox has bedevilled scientists for half a century and led some to question the fundamental laws of physics. Now scientists say they may have resolved the infamous problem by showing that black holes have a property known as “quantum hair”.
One of physicist Stephen Hawking most famous paradoxes may finally be solved: Black holes may in fact hang onto information about the massive stars that created them, new research indicates.
This information may lurk in the radiation around black holes — colloquially known as “quantum hair” — and could, in theory, be retrieved to retell the origins of those black holes, the research suggests.
Hawking’s paradox boils down to the following: the rules of quantum physics state that information is conserved. Black holes pose a challenge to this law because once an object enters a black hole, it is essentially gone for good – along with any information encoded in it. Hawking identified this paradox and for decades it has continued to confound scientists.
According to Stephen Hawking work, radiation slowly “leaks” out of black holes in the form of thermal energy, which has come to be known as “Stephen Hawking radiation.” But because of its thermal nature, this radiation can’t carry information.
That 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 an object’s final state can reveal clues about its initial state. This problem has troubled cosmologists for decades and is known as the “Stephen Hawking information paradox.”
There have been innumerable proposed solutions, including a “firewall theory” in which information was assumed to burn up before entering the black hole, the “fuzzball theory” in which black holes were thought to have indistinct boundaries, and various exotic branches of string theory. But most of these proposals required rewriting of the laws of quantum mechanics or Einstein’s theory of gravity, the two pillars of modern physics.
“This research is the final nail in the coffin for the paradox because we now understand the exact physical phenomenon by which information escapes a decaying black hole,” Xavier Calmet, a professor of physics at the University of Sussex and lead study author, told Space.com’s sister publication Live Science via email. He suggests a modification to Hawking radiation that makes it “non-thermal” and thus capable of carrying information with it away from the final fate of the black hole.
Calmet and his collaborators think the black hole is more complex – or hairy. As matter collapses into a black hole, they suggest, it leaves a faint imprint in its gravitational field. This imprint is referred to as “quantum hair” and, the authors say, would provide the mechanism by which information is preserved during the collapse of a black hole.
Under this theory, two black holes with identical masses and radii, but with different internal composition, would have very subtle differences in their gravitational fields.
Black holes are objects so massive that nothing can escape the pull of their gravity, not even light. They form when enormous 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, angular momentum, and electric charge.”
“Our solution doesn’t require any speculative idea; instead our research demonstrates that the two theories can be used to make consistent calculations for black holes and explain how information is stored without the need for radical new physics,” said Calmet.
While black holes carry no “memory” of the stars they once were, the rules of quantum physics say that information can’t simply be erased from the universe. In 1976, Hawking introduced a fly to this cosmic ointment by showing this information couldn’t dwell indefinitely within black holes sealed away from the outside universe either.
Applying the rules of quantum mechanics to black holes, Hawking suggested they emit a type of thermal radiation, later called Hawking radiation. Over immense periods of time, the leaking of this radiation causes black holes to completely evaporate, leaving only a vacuum behind. In this way, information is irretrievably lost.
Crucially, he said, the paper suggested it may be possible to get some additional information about what was inside the black hole – but did not show that the phenomenon could account for the entirety of the information apparently lost. “That they haven’t shown and that’s the crux of the paradox,” he said.
Along with his colleague Steve Hsu, a professor of theoretical physics at Michigan State University, Calmet has been working since 2021 to crack Stephen Hawking paradox. In a previous study, published in March 2022, the team argued that black holes do indeed have “quantum hair,
“While these quantum gravitational corrections are minuscule, they are crucial for black hole evaporation,” Calmet said. “We were able to show that these effects modify Hawking radiation in such a way that this radiation becomes non-thermal. In other words, factoring in quantum gravity, the radiation can contain information.”
Calmet said: “When you have a big claim you have to back it up. It’s going to take some time for people to fully accept this. The paradox has been around for a long time and you’ve got very famous people all over the world who’ve been working on this for years.”
Currently there is no real way for astrophysicists to measure the effect the researchers propose, as it is minuscule, Calmet acknowledged. Instead, he suggests one way to progress this theory would be by studying simulations of black holes in labs on Earth. The team’s mathematical modeling of Stephen Hawking radiation and black holes could prove invaluable in these simulations.