Hawking answers his own paradox of the black holes

The physicist Stephen Hawking has made a discovery that contradicts some of his famous work on black holes that made his name 30 years ago. Professor Hawking thinks his finding so important he has requested time to present a paper at an international conference on relativity and gravitation in Dublin next week, New Scientist magazine reports.

The physicist Stephen Hawking has made a discovery that contradicts some of his famous work on black holes that made his name 30 years ago. Professor Hawking thinks his finding so important he has requested time to present a paper at an international conference on relativity and gravitation in Dublin next week, New Scientist magazine reports.

The work covers a peculiarity of black holes that Professor Hawking thought he had solved in 1976. Then, the scientist, who holds the Lucasian chair of mathematics at Cambridge, revealed that black holes, previously thought to be one-way siphons for everything, could actually emit radiation.

Black holes are the remains of huge stars that have collapsed in on themselves, creating a region where gravity is so intense not even light can escape its pull, giving rise to an "event horizon" around the black hole beyond which nothing can be seen. He showed that some heat would leak, then named "Hawking radiation".

But Professor Malcolm MacCallum of the University of London, who has spoken to Professor Hawking about his new work, said a paradox remained. "When you create a black hole, a lot of information - ordered matter - goes into it," he said. "The thermal radiation doesn't carry any information, but it leaks, and means the black hole dissipates into nothing. So where does the information go? It can't be destroyed." That paradox, set up by Professor Hawking's 1976 findings, has puzzled scientists since. But in a five-line proposal for a paper hastily scheduled for the last session of next week's conference, Professor Hawking will argue that black holes never quite shut themselves off from the outside universe.

Instead, as the universe cools, they emit more of their heat in Hawking radiation, and eventually open up to reveal again the "information" that they originally sucked in when they formed. In his five-line brief to the conference organisers, Professor Hawking said that "the way the information gets out seems to be that a true event horizon never forms, just an apparent horizon". Professor MacCallum said: "My attitude was that if he's saying, 'Look at this' then it's not done lightly."

Curt Cutler, a member of the conference's scientific committee, said: "To be quite honest, I went on Hawking's reputation" in agreeing to let him speak.

If other scientists agree that Professor Hawking has solved his own long-standing paradox, he stands to lose a bet. In 1997, he and the physicist Kip Thorne of the California Institute of Technology (Caltech) made a wager, arguing with John Preskill, also of Caltech, that "information swallowed by a black hole is forever hidden, and can never be revealed".

The price of losing will be an encyclopaedia of Professor Preskill's choice, but Professor Hawking will probably feel the price is worthwhile to solve a paradox that has haunted his work for nearly three decades.