DUBLIN, IRELAND--Stephen Hawking today claimed to have solved one of the most important problems in physics: whether black holes destroy information they swallow. At a conference here, the University of Cambridge professor reversed his longstanding position and argued that information survives. As a result, Hawking conceded the most famous wager in physics and handed the winner an encyclopedia.
The question of what happens to information when it falls into a black hole goes to the heart of a central idea in modern physics. Just as scientists in the 19th century figured out that energy can be neither created nor destroyed, many 20th century physicists came to the conclusion that information, too, cannot arise from nothing or be eradicated. If true, information conservation would become one of the most important principles in science. Unfortunately, there was a big obstacle: black holes.
When an object falls into a black hole, the black hole absorbs its mass and energy. But information seemed to be irretrievably lost--there seemed to be no way to tell whether someone had dumped a ton of lead or a ton of feathers into the black hole. In the 1970s, Hawking and colleagues such as Kip Thorne of the California Institute of Technology (Caltech) argued strongly that this necessarily violated the idea of conservation of information. Others, including Caltech's John Preskill, held that some undiscovered loophole would keep that information safe even after the black hole swallowed it. In 1997, Hawking and Thorne made a wager with Preskill; the winner was to get an encyclopedia of his choice, from which information can always be retrieved.
Today, Hawking conceded the bet. Using a mathematical technique known as the Euclidean path integral method, Hawking proved to his own satisfaction that information is not, in fact, destroyed when it falls into a black hole. "It is great to solve a problem that has been troubling me for nearly 30 years, even though the answer is less exciting than the alternative I suggested," said Hawking, who had earlier argued that information might be transported into "baby universes" that spawn inside the black hole. "I'm sorry to disappoint science fiction fans, but if information is preserved, there is no possibility of using black holes to travel to other universes," Hawking said.
Hawking presented Preskill with a sports encyclopedia called Total Baseball. Thorne hasn't yet thrown in the towel but says, "I think that Stephen is very likely right." Others are less certain. John Friedman, a physicist at the University of Wisconsin, Milwaukee, has doubts that the Euclidean method is truly equivalent to the more traditional Lorentzian approach researchers have applied to the problem. Secondly, he says, Hawking's calculation does a sum over all possible idealized black hole locations and all observers in the universe, but the results don't seem to apply to a specific black hole and a specific observer. Even Preskill reserves judgment, although he says he has no qualms about accepting the encyclopedia. "The terms were that the winner would receive the encyclopedia when the other party concedes," he says. "I don't have to agree."