IN 1974 Stephen Hawking, pictured right, had a startling theoretical insight about black holes—those voracious eaters of matter and energy from whose gravitational clutches not even light can escape. He predicted that black holes should not actually be black. Instead, because of the quirks of quantum mechanics, they should glow ever so faintly, like smouldering embers in a dying fire. The implications were huge. By emitting this so-called Hawking radiation, a black hole would gradually lose energy and mass. If it failed to replenish itself it would eventually evaporate completely, like a puddle of water on a hot summer's day.

Unfortunately for physicists, Dr Hawking also predicted that the typical temperature at which a black hole radiates should be about a billionth of that of the background radiation left over from the Big Bang itself. Proving his theory by observing actual Hawking radiation from a black hole in outer space has therefore remained a practical impossibility.

In a paper just accepted by Physical Review Letters, however, a team of researchers led by Daniele Faccio from the University of Insubria, in Italy, report that they have observed Hawking radiation in the laboratory. They managed this trick not by creating an Earth-gobbling black hole on a benchtop but by firing pulses of laser light into a block of glass. This created a region from which light could not escape (analogous to a black hole) and also its polar opposite, a region which light could not enter. When the team focused a sensitive camera on to the block, they saw the faint glow of Hawking radiation.

Black and light

If a dying star is massive enough, it can collapse to form a region of infinite density, called a singularity. The gravity of such an object is so strong that nothing, not even light itself, can break free if it strays too close. Once something has passed through the so-called event horizon that surrounds this region, it is doomed to a one-way trip.

Dr Hawking's insight came from considering what happens in the empty space just outside the event horizon. According to quantum mechanics, empty space is anything but empty. Rather, it is a roiling, seething cauldron of evanescent particles. For brief periods of time, these particles pop into existence from pure nothingness, leaving behind holes in the nothingness—or antiparticles, as physicists label them. A short time later, particle and hole recombine, and the nothingness resumes.

If, however, the pair appears on the edge of an event horizon, either particle or hole may wander across the horizon, never to return. Deprived of its partner, the particle (or the hole) has no “choice” but to become real. These particles, the bulk of which are photons (the particles of light), make up Hawking radiation—and because photons and antiphotons are identical, the holes contribute equally. The energy that goes into these now-real photons has to come from somewhere; that somewhere is the black hole itself, which thus gradually shrinks. By linking the disparate fields of gravitational science, quantum mechanics and thermodynamics, Hawking radiation has become a crucial concept in theoretical physics over the past quarter of a century.

In 1981, that concept was extended. William Unruh of the University of British Columbia pointed out that black holes are actually extreme examples of a broader class of physical systems that can form event horizons. Consider a river approaching a waterfall. As the water nears the edge, the current moves faster and faster. In theory, it can move so fast that ripples on the surface are no longer able to escape back upstream. In effect, an event horizon has formed in the river, preventing waves from making their way out. Since then, other researchers have come up with other quotidian examples of event horizons.

Dr Faccio and his team were able to create their version because, as the laser pulse moves through the glass block, it changes the glass's refractive index (the speed at which light travels through a material). Light in the vicinity of the pulse is slowed more and more when the refractive index changes as the pulse passes by.

To see how the pulse can act like a black hole, imagine that it is sent chasing after a slower, weaker pulse. It will gradually catch up with this slow pulse, reducing the speed of light in the slow pulse's vicinity. That will slow the slow pulse down still more until eventually it is slowed so much that it is stuck. Essentially, the leading edge of the chasing pulse sucks it in, acting like the event horizon of a black hole.

Now imagine that the chasing pulse is itself being chased, but again by a much weaker pulse. As this third pulse approaches the tail of the second one it will also slow down (because the speed of light in the glass it is passing through has been reduced by the second pulse's passage). The closer it gets, the slower it travels, and it can never quite catch up. The trailing edge of the second pulse, therefore, also acts as an event horizon. This time, though, it stops things getting in rather than stopping them getting out. It resembles the opposite of a black hole—a white hole, if you like.

In the actual experiment, there were no leading and trailing pulses. Instead, their role was played by evanescent photons continually popping into existence around the strong pulse. As the pulse passed through the glass, its event horizons should have swept some of these photons up, producing Hawking radiation from the partners they left behind.

Sure enough, when Dr Faccio and his team focused a suitable camera on the block and fired 3,600 pulses from their laser, they recorded a faint glow at precisely the range of frequencies which the theory of Hawking radiation predicts. After carefully considering and rejecting other possible sources for this light, they conclude that they have indeed observed Hawking radiation for the first time.

Because of its tabletop nature, other groups will certainly attempt to replicate and extend Dr Faccio's experiment. Although such studies cannot prove that real black holes radiate and evaporate, they lend strong support to the ideas that went into Dr Hawking's line of reasoning. Unless a tiny black hole turns up in the collisions of a powerful particle accelerator, that may be the best physicists can hope for. It may even be enough to convince Sweden's Royal Academy of Science to give Dr Hawking the Nobel prize that many think he deserves, but which a lack of experimental evidence has hitherto caused it to withhold.