Classical black holes are regions of space where gravity is so strong that nothing, not even light, can escape from them. The region beyond which nothing can escape is known as the event horizon. All the information in the light and matter that falls through the event horizon is lost forever because the black hole can be described by just three numbers: its mass, electric charge and angular momentum.

In the 1970s, however, building on earlier work by Jacob Bekenstein and applying quantum theory to black holes, Hawking showed that these mysterious objects also have a temperature, which means that they give off thermal radiation. The black holes should therefore eventually disappear. The problem is that this thermal radiation does not contain any information, which means that the information that originally fell into the black hole disappears. However, this is not allowed by quantum theory.

In their bet, which was made in 1997, Hawking -- who is based at Cambridge University -- and Thorne argued that information was lost in a black hole, whereas Preskill said that it was not. The winner or winners of the bet had to provide the loser or losers with an encyclopaedia of their choice "from which information can be recovered with ease".

Now Hawking has conceded defeat by saying that information can escape from a black hole and therefore is not lost. If he is right, making a such a significant breakthrough in the search for a quantum theory of gravity should overcome the disappointment of losing the bet and having to hand over an encyclopaedia of baseball to Preskill. "It is great to solve a problem that has been troubling me for 30 years," said Hawking, "even though the answer is less exciting than the alternative I suggested."

Hawking presented his solution to the 17th International Conference on General Relativity and Gravitation in Dublin. His solution relies on a black hole being able to have more than one topology at the same time, and when he performs a quantum mechanical "path integral" over all the topologies, he finds that information is not lost. "The way the information gets out [of a black hole] seems to be that a true event horizon never forms," said Hawking, "just an apparent horizon."

Hawking also dismisses his previous suggestion that the information might have leaked into a different "Baby" universe. "The information remains firmly in our universe," he told the conference. "I am sorry to disappoint science fiction fans, but if information is preserved, there is no possibility of using black holes to travel to other universes. If you jump into a black hole, your mass energy will be returned to our universe, but in a mangled form which contains the information about what you were like, but in an unrecognisable state."