The famous physicist admits he was wrong about black holes — and pays
off a long-standing wager
By MICHAEL D. LEMONICK
For days the scientific grapevine had been buzzing with the news that Stephen
Hawking, the brilliant physicist whose disease has put him in a wheelchair,
heir to the revered Cambridge professorship once held by Isaac Newton, would
be making a big announcement at a conference in Dublin, Ireland. Sure enough,
last week before an array of TV cameras and hundreds of colleagues at the ordinarily
obscure International Conference of General Relativity and Gravitation, Hawking
declared that he had solved what he called "a major problem in theoretical
physics." Black holes, he said, do not forever annihilate all traces of
what falls into them.
In making that announcement, Hawking recanted a position he had held for nearly 30 years. He also pulled the rug out from under a generation of science-fiction fans, declaring dead a favorite plot device. "There is no possibility of using black holes to travel to other universes," he said, with evident regret. And, finally, he conceded defeat in a long-standing bet with Caltech astrophysicist John Preskill, who thought there wasn't a problem in the first place.
The issue arose back in the 1960s, when physicists first described black holes, objects whose gravity is so powerful that nothing can escape, not even light. Under such crushing force, all information about what formed the hole would be forever locked away: you would have no way of distinguishing between a black hole made from a dying star and one formed from a quadrillion tons of Gorgonzola cheese.
But in the mid-'70s, Hawking threw a theoretical monkey wrench into the works. It was known that empty space is bubbling with "virtual particles"--pairs of particles and antiparticles that spontaneously appear out of nowhere, then annihilate each other and vanish. That happens so fast the particles can hardly be said to exist. But if it happens right at the edge of a black hole, Hawking realized, one particle might fall in before the annihilation, leaving its twin to escape in the form of radiation. Because the books of the cosmos must balance, the particle that escaped would have to be subtracted from the mass of the black hole. And since virtual particles are appearing everywhere all the time, every black hole must constantly radiate energy and will eventually disappear.
That was the problem. According to basic quantum theory, information can't be destroyed. Hiding it away forever inside a black hole is bad enough, but hiding it in a hole that eventually evaporates is unthinkable.
Others have tackled that conundrum and solved it to their satisfaction using various mathematical stratagems. But, says Juan Maldacena, a string theorist at the Institute for Advanced Study in Princeton, N.J., who has worked on the question, "what wasn't done was to explain precisely what was wrong with Hawking's original calculations in the '70s." Now Hawking claims to have done so. By doing the calculations a different way, he has convinced himself that the radiation flowing from a black hole actually carries information about the hole back out into the universe, although in a mangled form.
Whether he's right is another question. Many of the scientists present in Dublin last week were skeptical of his solution and said they would need to study it much more carefully before passing judgment. Says Preskill: "The mechanism he has in mind may not be so revolutionary." Nevertheless, Hawking acknowledged that Preskill had been right all along and paid off with the agreed-on prize: an encyclopedia. (The joke is that an encyclopedia, unlike a black hole, yields information easily. Hawking offered one on cricket, but Preskill held out for Total Baseball: The Ultimate Baseball Encyclopedia, which had to be flown in.)
The bottom line is that Hawking may or may not have solved a problem that may or may not have been a problem to begin with. You might well wonder why his announcement generated so much excitement. But when the subject is black holes and you're the world's most famous physicist — Hawking is not only a best-selling author but has also guest-starred on both Star Trek and The Simpsons — the usual rules don't apply.