‘Hairy’ hypothesis gets to the root of black holes
Scientists claim to have solved a problem that has troubled physicists for 50 years, by showing that black holes are ‘‘hairy’’.
Black holes are collapsed stars so dense that nothing, not even light, can escape their gravitational pull. As renowned physicist Stephen Hawking once said: ‘‘In a black hole, no-one can see you disappear.’’
It now turns out that anyone who falls into a black hole would leave a trace of their existence after all: an ‘‘imprint’’ in the gravitational field around it, described as ‘‘quantum hair’’ by scientists.
Imagine kicking a football over a fence. Instead of throwing it back, your neighbour takes it into his house and rips it to shreds. Alas, nothing that enters your neighbour’s house ever comes out. Your ball has gone forever.
This is how black holes are perceived under the theory of general relativity. To an observer, 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 called the ‘‘no hair’’ theory, because no clues are left straggling outside.
Yet the laws of quantum mechanics suggest that the black hole must preserve information about its contents, locked up 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.
In the 1970s, Hawking began to challenge this assumption. He suggested that black holes shrank over trillions of years as tiny amounts of radiation emerged – 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.
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 the journal Physical Review Letters that claims to have disproved the ‘‘no hair’’ theory in a way ‘‘very much compatible’’ with both of the other theories.
The scientists spent a decade decoding the 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.
It had been suggested, including by Hawking in 2016, that the radiation from a black hole may somehow contain encoded information about its interior after all.
The researchers now believe they have shown that it does, and have explained how. Professor Xavier Calmet, of Sussex, said there had been no need to come up with ‘‘radical new physics’’.