15 July 2004

Hawking changes his mind about black holes
Physicist plans to pay up on long-standing bet.
Mark Peplow

The eminent physicist Stephen Hawking has conceded that information
can escape from black holes after all. The idea has been gaining
popularity with physicists for some time, but the fact that Hawking, a
pioneer of black-hole theory in the 1970s, has finally accepted it is
something of a watershed.

"This will come as a surprise to physicists," says Hawking's Cambridge
University colleague Gary Gibbons. "His style of doing science is
quite dramatic: he will propose a thesis and defend it to the last,
until it is overthrown by better reasoning."

It also means that Hawking loses a long-standing bet with John
Preskill, a theoretical physicist at the California Institute of
Technology, Pasadena.

Hawking had believed that anything swallowed by a black hole was
forever hidden from the outside universe. Preskill bet that the
information carried by an object was not destroyed when it plummeted
into a collapsed star, and could actually be recovered.

"Stephen has changed his position, and I am expecting him to concede
the bet," Preskill says. His prize is to be an encyclopaedia, "from
which information can be recovered at will". Hawking says that he will
indeed honour the wager.

General approach

Hawking's original view follows Einstein's general theory of
relativity, which predicts that, at certain locations in space, matter
collapses into an infinitely small and dense point, called a
singularity. The theory says that the force of gravity at this point
is so great that nothing, not even light itself, can escape, hence the
term 'black hole'.

Because the singularity is infinitely small, it cannot possibly have
any structure and so there is no way that it can hold information. Any
data about particles entering the black hole must be lost forever.

The problem is that quantum theory, which describes space and matter
on very tiny scales, contradicts this. Quantum theory says any process
can be run in reverse, so starting conditions can theoretically be
inferred from the end products alone. This implies that a black hole
must somehow store information about the items that fell into it.

Quantum evolution

Hawking has always stuck resolutely to the idea that once information
goes into a black hole, there is no way out. Until now. When
asked about his change of heart, Hawking smiled and
wrote: "My views have evolved."

The remarkable about-face is the result of Hawking's attempts to
combine quantum theory with general relativity in a powerful new
theory of quantum gravity. Hawking is due to present his latest ideas
at the 17th International Conference on General Relativity and
Gravitation, which runs from 18 July to 23 July in Dublin, Ireland.
But he gave a preview of the talk at his department in Cambridge
University last month.

He has been using a mathematical technique called the "Euclidean path
integral". The technique is extremely complex as it lumps all the
possible histories of a system into one equation. First used by
quantum physicist Richard Feynman, it has generally been applied to
subatomic particles. But Hawking has been working for several years to
apply the idea to black holes.

"The view seems to be forming in his mind that there isn't a black
hole in the absolute sense, there's just a region where things take a
very long time to escape," says Gibbons. This suggests that black
holes do not actually narrow to a singularity at all.

The great escape

So an object falling into a black hole is not completely obliterated.
Instead, the black hole is altered as it absorbs the object. Although
it would certainly be very difficult to retrieve any information about
that object, the data are still there, somewhere inside the black
hole, Gibbons says.

How could that information ever escape? The answer lies in one of
Hawking's greatest discoveries: that black holes slowly evaporate into
space by losing particles from the very edge of the gravitational
precipice at their rim, called Hawking radiation. The black hole
eventually shrinks to a tiny kernel, at which point a growing torrent
of radiation begins to leak out, potentially carrying the lost
information with it.

But Preskill says that Hawking's new take on quantum gravity rests on
shaky mathematical foundations, and is unlikely to be embraced by the
physics community. "I am sceptical about whether he has found a fully
satisfactory resolution to the problem," he says.

Story from :
http://news.nature.com//news/2004/040712/040712-12.html