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Last Updated: Thursday, 22 July, 2004, 10:23 GMT 11:23 UK
Black holes turned 'inside out'
By Dr David Whitehouse
BBC News Online science editor

Hawking presents his position in Dublin, PA
Hawking presented his new position to a packed lecture hall
Stephen Hawking has put forward a new theory that changes the way scientists view black holes, saying he was wrong about them in the past.

The physicist told a conference on gravitation in Dublin that he has revised his belief that black holes destroy everything that falls on them.

He now believes that black holes may allow information to get out.

His new research could even help solve the "black hole information paradox", a crucial puzzle for modern physics.

He spoke to a packed lecture hall at the 17th International Conference on General Relativity and Gravitation, giving his new views in a presentation entitled The Information Paradox For Black Holes.

He is revising his 1975 ideas that are regarded as the most astonishing breakthrough in black hole studies.

A black hole is an object from which once inside it is not possible to escape. Its boundary is called its "event horizon".

But now Hawking believes that it might not be a one-way trip after all.

I have been thinking about this problem for 30 years, but I now have an answer to it
Stephen Hawking

Gary Gibbons, a physicist at Cambridge University, said Hawking's newly defined black holes did not have a well-delineated event horizon that hid everything in them from the outside Universe.

Kip Thorne, a leading cosmologist from the California Institute of Technology said of Hawking's new idea: "This looks to me on the face of it to be a lovely argument, but I haven't yet seen all the details".

No escape, no hair

In 1975, Hawking calculated that once a black hole forms, it radiates energy and starts losing mass by giving off "Hawking radiation".

Scientists were astounded because Hawking's work on a mathematical description of the ever-shrinking black hole forged a link between gravity and entropy - a measure of how energy changes from one form to another.

It was said that black holes had no hair, meaning that it did not matter what came together to make them.

Black hole representation, BBC
Black holes in space: Hawking says all is not lost
All a black hole had was mass, charge, and spin. There was no information about matter inside the black hole, and once the hole disappeared, all the information went with it.

"It used to be thought that once something had fallen into a black hole it was gone and lost forever and the only information that remained was its mass and spin," the Lucasian Professor of Mathematics at Cambridge University told the BBC.

"The Hawking radiation seemed to be random and featureless so it appeared that all information about what fell into a black hole was lost."

But this runs contrary to the laws of quantum physics, which describe the behaviour of the Universe at the smallest scales. These dictate that information can never be completely lost.

Whether information is or is not lost has practical and philosophical consequences.

"We could never be certain of the past or predict the future precisely. A lot of people therefore wanted to believe that information could escape from a black hole but they didn't know how it could get out," he said.

Losing bet

For years, the physicist argued that the extreme gravitational fields of black holes somehow overturned the quantum laws. Now, he has dropped this idea.

Professor Hawking's new black holes never completely destroy everything that falls in. Instead, they continue to emit radiation for extended periods, and eventually open up to reveal the information within them.

"I have been thinking about this problem for 30 years, but I now have an answer to it," he explained.

"The black hole only appears to form but later opens up and releases information about what fell in, so we can be sure of the past and we can predict the future."

Preskill holds Total Baseball book aloft, PA
The Hawking U-turn won John Preskill a book on baseball
Hawking's latest theory seems to rule out using black holes as time machines or as gateways to other universes.

"I am sorry to disappoint science fiction fans. But if you jump into a black hole, your mass energy will be returned to our Universe but in a mangled form," he said.

The U-turn cost Professor Hawking a reference book called Total Baseball. He and Kip Thorne made a bet on the subject with an opponent of the idea, John Preskill, also of Caltech.

Just before he handed over the book Hawking said: "I had great difficulty in finding one over here, so I offered him an encyclopaedia of cricket as an alternative, but John wouldn't be persuaded of the superiority of cricket."

Later, Preskill said he was very pleased to have won the bet but added: "I'll be honest, I didn't understand the talk." He said he was looking forward to reading the detailed paper that Hawking is expected to publish next month.

The BBC's Pallab Ghosh
"Today the scientist who developed the theory of black holes will announce that he has changed his mind"

Q&A: Hawking and black holes
21 Jul 04  |  Science/Nature
New support for black hole theory
15 Apr 04  |  Science/Nature
'No evidence' of Hawking assault
29 Mar 04  |  Cambridgeshire
Hawking takes top book prize
25 Jun 02  |  Science/Nature


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