Published online 15 July 2004 | Nature | doi:10.1038/news040712-12


Hawking changes his mind about black holes

Physicist plans to pay up on long-standing bet.

Stephen Hawking has admitted he was wrong.Stephen Hawking has admitted he was wrong.© SPL

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'.

“My views have evolved”

Stephen Hawking
Cambridge University, UK

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. 

Cambridge University, UK