In 1974 Stephen Hawking postulated that black holes should give off a trickle of particles, or radiation, from their outer boundaries. The finding established Hawking’s reputation as a brilliant scientist and set the stage for his highly visible public profile, which includes provocative best-selling books and guest appearances on The Simpsons. In the midst of all the celebrity, the original theory of Hawking radiation, as the black hole phenomenon is known, has almost been forgotten, at least by the general public. The faint emission has never been detected from a real black hole, and researchers have not been able to produce the effect in the lab.
A few years ago a group of scientists in Italy decided to try a new approach to test Hawking’s thesis. They used a piece of glass to re-create a black hole’s “event horizon”—the point of no return beyond which even light is too slow to escape, where Hawking believes the radiation would arise. Alongside ordinary matter and light falling into a black hole, he reasoned, ought to be particles popping in and out of existence. Quantum mechanics dictates that such short-lived particle pairs arise even from empty space; in most corners of the cosmos, those pairs quickly disappear together back into the vacuum. But at an event horizon, one particle may be captured by the black hole, leaving the other free to escape as radiation.
Daniele Faccio of the University of Insubria and his colleagues created the event horizon in a section of fused silica glass, a medium in which intense laser pulses can locally perturb the speed at which light passes through the glass. That perturbation forms a moving event horizon, blocking photons from overtaking it. If a pair of photons is produced close enough to that event horizon, they will become separated and will be unable to return to the vacuum. The researchers recorded photons streaking outward from the glass, about one photon per 100 laser pulses, with all the traits they had predicted for Hawking radiation. They recently published their results in Physical Review Letters.
Physicists disagree about exactly what the observation means. Ulf Leonhardt of the University of St. Andrews in Scotland says the new research indeed represents the first observation of Hawking radiation. Others are not as sure. Theodore A. Jacobson of the University of Maryland says he is more convinced by another group’s recent paper on a nonquantum analogue of Hawking radiation in flowing water. He points out that Faccio’s group cannot verify that photons appear in pairs at the event horizon. “In our big piece of glass we have no way of saying where the other photon will end up,” Faccio notes. But Leonhardt, who proposed the artificial event horizon scheme and is investigating the phenomenon in optical fibers, could detect both photons and show their common origin. “Once he does that, I think it will close all the discussions,” Faccio says.