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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.
The popular conception of black holes reflects the behavior of the massive black holes found by astronomers and described by classical general relativity. These objects swallow up whatever comes near and emit nothing. Physicists who have tried to understand the behavior of black holes from a quantum mechanical point of view, however, have arrived at quite a different picture. The difference is analogous to the difference between thermodynamics and statistical mechanics. The thermodynamic description is a good approximation for a macroscopic system, but statistical mechanics describes what one will see if one looks more closely.