Scientists say they have solved a paradox that has alarmed physicists for 50 years, by showing that black holes are “hairy”.
Black holes are collapsed stars that have become so dense that nothing, not even light, can escape their gravitational pull. As Stephen Hawking once said: “In space, no one can hear you scream. In a black hole, no one can see you disappear.”
It now turns out that anyone who plunges into a black hole may leave behind some trace of their existence after all: an “imprint” in the gravitational field around the black hole, described as “quantum hair” by scientists in Britain, Italy and the US.
The discovery is the fullest outline yet of a mechanism by which black holes could whisper their secrets to the outside universe. It also suggests that the two most fundamental theories of physics may not be as incompatible as once feared — at least as far as black holes are concerned.
Imagine you kick your football over the fence and it lands at the feet of your grumpy neighbour. Instead of throwing it back, he takes it into his house and rips it to tiny shreds. Alas, nothing that enters your neighbour’s house ever comes out again. Your ball has gone forever.
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This is how black holes are perceived under the theory of general relativity. To an observer outside, there is no way to glean any information about what has fallen into the black hole, nor the star that collapsed to form it. This is known as the “no hair” theory: no clues are left straggling outside.
Yet the laws of quantum mechanics suggest the black hole must somehow preserve information about its contents, locked up forever within the “event horizon” at its outer boundary — the equivalent of your neighbour’s back door — because no “quantum information” about subatomic particles can be permanently lost.
Hawking developed a theory in the 1970s that began to challenge this assumption. He suggested that black holes shrank over trillions of years as tiny amounts of radiation eked out, but troublingly, this radiation was thought to contain no information about the lost particles within.
If a black hole erased all information about its contents as it disappeared, it would be as if your football and any trace that it existed vanished completely from our universe.
This was the “Hawking paradox”, and it was seen as a key example of how the two most important theories in physics — general relativity and quantum mechanics — were incompatible.
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General relativity deals with gravity, space and time on the grand scale of stars, galaxies and the fabric of the universe. Quantum mechanics describes physics on the tiny scale of subatomic particles. If black holes could erase information permanently, it might prove that one of these theories has a flaw — a nightmare that plagues scientists looking for a way to unify them.
Now scientists from the University of Sussex, the University of Bologna and Michigan State University have published a study in Physical Review Letters that claims to have disproved the “no hair” theory in a way “very much compatible” with both theories.
They have spent a decade decoding gravitational fields around stars and found that a form of quantum “entanglement” must exist between the particles sucked into a black hole and the gravitational field that remains outside. This leaves an “imprint”, dubbed “quantum hair”.
It had been suggested, including by Hawking in 2016, that the radiation eking out of a black hole may somehow contain encoded information about its interior after all. Researchers now believe they have shown that it does and explained how.
“It turns out that black holes are in fact good children, holding onto the memory of the stars that gave birth to them,” said Professor Xavier Calmet from Sussex, adding that there had been no need to come up with “radical new physics”.
