Thirty-six years ago, Stephen Hawking, the famed British theoretical physicist, predicted that black holes—from which no light should escape—could, paradoxically, emit light. No one has ever observed this "Hawking radiation," but now, a team of physicists may have created something very much like it in the lab.
A black hole is literally a hole in space and time. According to Einstein's theory of gravity, mass and energy warp space and time, or "spacetime." That warping alters the paths of free falling objects, creating the effect we know as gravity. A black hole arises when the warping around a point grows so severe that that spacetime in the area becomes like a funnel so steep that nothing can climb back out, as may happen when a massive star collapses. Anything that ventures closer than a certain distance from the black hole falls in to it, even photons zipping along at light speed. This distance of no return defines the black hole's "event horizon."
However, black holes should not be completely black, as Hawking explained in 1974. Thanks to the uncertainty of quantum mechanics, pairs of quantum-entangled photons--whose properties are tied to each other—or other particles can pop out of empty space. Ordinarily, they don't stick around long enough to be directly observed, but if a pair straddles the event horizon, then one photon can fall into the black hole, while the other escapes, carrying energy away as Hawking radiation. Hawking radiation from black holes would be so weak as to be nearly undetectable, however.
But Franco Belgiorno of the University of Milan and colleagues say they have seen something like this radiation in experiments with a laser and a block of glass. The basic idea is that a black hole event horizon is a line behind which light gets trapped. To generate something similar, the team fired intense pulses of laser light into the glass. Within the material, light moves slower than it does in empty space, and that speed reduction is quantified in the glasses "index of refraction." However, the index of refraction varies with the wavelength of light, too, and can also change with the intensity of the light. As a result, the intense pulse's trailing edge acts like an event horizon that traps light of a narrow range of wavelengths. Such light can catch up to the moving pulse, but once it gets close, the increased index of refraction slows it down and traps it behind the pulse.
And just like a genuine black hole event horizon, the artificial one created by the light pulse can emit radiation. Crucially, since the artificial horizon can only trap photons in a certain range of wavelengths, it can only emit Hawking radiation in that range. And by tuning the intensity of the laser pulses, the physicists could control how fast the pulses moved through the glass and hence the wavelengths of Hawking radiation emitted from the glass. Like turning the knob on a radio, the team adjusted the pulse so that, if the artificial horizon emitted any Hawking radiation, its wavelength would be between 800 and 900 nanometers, a range that could not be confused with other sources such as laser-induced fluorescence. As the researchers report this week in Physical Review Letters, when they observed light in exactly this range, they appeared to have observed Hawking radiation for the first time.
"There are very strong indications" the experimenters observed a form of Hawking radiation, says physicist Ulf Leonhardt of the University of Saint Andrews in the United Kingdom, whose theoretical work led to Belgiorno and colleagues' experiment. He says it's particularly persuasive that the experimenters get the wavelengths and the polarization of the light correct. But William Unruh, a theorist at the University of British Columbia, Vancouver, says he has some concerns. The distortions in the glass change very rapidly, for example, which might produce light that could be confused with Hawking radiation. Rather than more experiments, Unruh says more theoretical work is needed to understand this experiment.