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PULLOUTScientists claim to have solved a prob-
lem 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
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, de-
scribed as “quantum hair” by scientists.
Imagine you kick your football over
‘Hairy’ hypothesis gets to the root of black holes
the 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 for ever.
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 col-
lapsed to form it. This is the “no hair”
theory: no clues are left straggling out-
side. Yet the laws of quantum mechan-
ics suggest the black hole must preserve
information about its contents, locked
up within the “event horizon” at its out-
er boundary — the equivalent of yourneighbour’s back door — because no
“quantum information” about suba-
tomic particles can be permanently lost.
In the 1970s Hawking began to chal-
lenge 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 para-
dox”, and it was seen as a key example
of how the two most important theoriesin physics, general relativity and quan-
tum mechanics, were incompatible.
General relativity deals with gravity,
space and time on the grand scale of
stars, galaxies and the fabric of the uni-
verse. Quantum mechanics describes
physics on the tiny scale of subatomic
particles. If black holes could erase in-
formation 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 pub-
lished a study in Physical Review Letters
that claims to have disproved the “nohair” theory in a way “very much com-
patible” with both of the other theories.
They have spent a decade decoding
gravitational fields around stars and
found that a form of quantum “entan-
glement” must exist between the parti-
cles 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 con-
tain encoded information about its inte-
rior after all. Researchers now believe
they have shown that it does and ex-
plained how. Professor Xavier Calmet,
of Sussex, said there had been no need
to come up with “radical new physics”.Kaya Burgess Science Reporter