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With John Preskill (left) and Kip Thorne (middle), Stephen Hawking (right) wrestled with a black hole paradox. 

Caltech Archives

Hawking’s final quest: saving quantum theory from black holes

When Albert Einstein died in 1955, he had spent decades on a lonely, quixotic quest: to derive a theory of everything that would unify gravity and electromagnetism—even though physicists discovered new nuclear forces as he worked. Stephen Hawking, the great British physicist who died last week at age 76, also worked until the end. But he focused on perhaps the most important problem in his area of physics, one his own work had posed: How do black holes preserve information encoded in the material that falls into them?

“He was clearly working on this big loose end, which really represents a profound crisis for physics,” says Steven Giddings, a quantum physicist at the University of California, Santa Barbara. In a final bid to solve it, Hawking and two colleagues proposed a way for information to end up scribbled on a black hole’s inscrutable verge, although others are skeptical.

A black hole is the gravitational field that remains when a star collapses under its own gravity to an infinitesimal point. Within a certain distance from the point—at the black hole’s event horizon—gravity grows so strong that not even light can escape. Or so theorists once assumed. Thanks to quantum uncertainty, the vacuum roils with particle-antiparticle pairs flitting in and out of existence too fast to detect directly. At the event horizon, Hawking realized in 1974, one particle in a pair can fall into the black hole while the other escapes. As the black hole radiates such particles, it loses energy and mass until it evaporates completely. Such “Hawking radiation” is too feeble to observe, but few scientists doubt its existence.

But Hawking’s signature insight led to a troubling conclusion. Imagine throwing a dictionary into a black hole that then evaporates. Because the emerging Hawking radiation is presumably random, the information in the dictionary shouldn’t come back out with it. Such information loss would wreck quantum mechanics, which requires that the “wave function” that describes any system—be it the dictionary or the universe—evolve in a predictable way. That can’t happen if information is lost. If allowed for black holes, such information loss would spread through quantum physics like a cancer, researchers say, spoiling things like energy conservation.

Hawking thought at first that the problem would be solved by changing quantum theory. In 1997, he and Kip Thorne, a gravitational theorist at the California Institute of Technology (Caltech) in Pasadena, entered a wager with John Preskill, also a Caltech theorist. Hawking and Thorne stuck to their position that black holes destroy information. By 2004, however, Hawking changed his mind and conceded the bet. He gave Preskill a baseball encyclopedia—from which arcane information could be recovered at will.

Hawking spent much of his later years trying to figure out how a black hole could regurgitate information—although he also worked on theories of what triggered the big bang. Three years ago he began his last work on black holes with Malcolm Perry, a theoretical physicist and Hawking’s colleague at the University of Cambridge in the United Kingdom, and Andrew Strominger, a theorist at Harvard University. “It was only 2 weeks ago that I saw him,” Perry says. “He certainly wasn’t in the best shape, but his mind was clearly focused on the problem.”

In a pair of recent papers, the scientists attack a pillar of black hole theory called the no-hair theorem. It is widely interpreted to mean that just three parameters—mass, spin, and electric charge, the last presumably zero—suffice to describe a black hole. Like bald pates, black holes of similar masses and spins then have no details—no “hair”—to distinguish them, as American theorist John Archibald Wheeler quipped. That sameness implies a black hole keeps no record of whether, say, it swallowed the play King Lear or the movie King Kong.

But strictly speaking, Strominger says, the theorem states only that two similar black holes can be “transformed” into each other by a handful of mathematical relations called diffeomorphisms, which relabel the coordinates of space-time. An infinite family of other diffeomorphisms has been neglected for decades, he says. They imply that a black hole’s event horizon might be bedecked with an infinity of charges, a bit like electric charges. The charges could distinguish one black hole from another and encode infalling information, Strominger says. “We’re cautiously optimistic about this idea,” he says. “Stephen was very optimistic.”

However, the charges may not encode enough information or may not do so in a unique way, Giddings cautions. One theorist who requested anonymity out of respect for Hawking says his various solutions for the black hole information problem pale next to his best work. Hawking’s latest work also misses a bigger issue, the theorist says. If a black hole preserves information, he argues, then an unavoidable conclusion of Einstein’s theory of gravity—that there’s no way to tell if you’re falling into a huge black hole—must be wrong.

Others credit Hawking for working on important problems in spite of the degenerative nerve disease, amyotrophic lateral sclerosis, that led to his use of a wheelchair and eventually rendered him able to speak only through a computerized voice synthesizer. Ironically, Hawking’s disability may have helped him avoid the isolation that enveloped Einstein, says Marika Taylor, a theoretical physicist at the University of Southampton in the United Kingdom, who from 1995 to 1998 was Hawking’s graduate student. Hawking had to rely on collaborators to flesh out his ideas, she says, and so remained deeply connected to his peers. “Without stepping on the toes of his actual family,” Taylor says, “his physics family was incredibly important to him.”