The eerie glow is called Hawking radiation, and physicists have been hunting it for decades. In 1974, Stephen Hawking calculated that, rather than gobbling up everything in their path and giving nothing back, black holes can radiate like the heating element in a toaster.

But astrophysical black holes, the ultradense gobs of mass that lurk at the centers of galaxies and are left behind when stars collapse, radiate too dimly to be seen. So instead of looking at real black holes, a group of physicists led by Francesco Belgiorno of the University of Milan, Italy, created a miniature analog by shooting short pulses of intense laser light into a chip of glass. The results will appear in Physical Review Letters.

“This is an extremely important paper,” said physicist Ulf Leonhardt of the University of St Andrews in Scotland, who built an artificial black hole in a phone line in 2008. “The experiment confirms that Hawking radiation can exist in principle.”

The basic idea behind Hawking radiation is that the quantum vacuum is not actually empty. Instead, it is a roiling mess of virtual particles and anti-particles that constantly pop into existence and eliminate each other when they meet. If one member of the particle/anti-particle pair is created on the wrong side of an event horizon — the edge of a black hole beyond which not even light can escape — the particles can never meet to destroy each other. An observer outside the black hole would see a perpetual stream of real particles.

But until now, no one had seen any evidence of these particles. Radiation from a black hole the mass of our sun would be 10 million times colder than the cosmic microwave background radiation, the ambient temperature of the universe left over from the Big Bang, which itself is only a few degrees warmer than absolute zero. Larger black holes would be colder still.

Luckily, conceptual counterparts to black holes and their event horizons are not hard to come by. In the 1980s, two physicists independently suggested this thought experiment: Picture a black hole as a river that flows faster and faster as it approaches a waterfall. Fleet-finned fish headed upstream can escape the falls, but at a certain point the water flows faster than the fish can swim. Any hapless fish caught behind that point are doomed to flop backwards over the falls. Replacing fish with light and the river with gravity yields a good simulation of a black hole.

Replace the fish with any other wave and the river with any fluid moving faster than that wave, and the likeness goes deeper. Physicists have found that the math describing light moving in the warped space-time geometry around a black hole is exactly the same as the math describing waves flowing through moving fluids. The analogy works for white holes, theoretical objects where nothing can get in rather than out, as well. And mathematically, Hawking radiation doesn’t need gravity or curved space-time at all. It just needs an event horizon.

In the new study, Belgiorno and colleagues created an event horizon with two quick pulses of laser light inside a piece of glass.

“Your piece of glass, which is equivalent to the river, you can’t think of making this travel at velocities that are faster than the speed of light,” said laser physicist Daniele Faccio of Heriot-Watt University in Scotland, a member of Belgiorno’s team. “But you can create a perturbation inside it.”

Light always moves through a vacuum at the same speed, but it gets slowed down by a factor called the refractive index in a medium like water or glass. A pulse of laser light traveling through the glass can change the refractive index, slowing light down even further.

The physicists sent two pulses of infrared laser light into a small rod of silica glass. The first pulse warped the glass, and the second pulse bumped up against this warp, eventually slowing to a standstill. This is exactly what happens to light trying to enter a white hole, Leonhardt says.

A light detector perpendicular to the laser beam picked up one extra photon for every 100 laser pulses on average, Faccio said. The light was extremely dim, invisible to human eyes, but it was there.

“It was pretty amazing,” Faccio said. “My first reaction was, it has to be something else, it can’t be so easy.”

To make sure the photons weren’t coming from somewhere else — particularly the fluorescent glow of the glass itself — Faccio and colleagues changed the velocity at which the warp moved through the glass. Theory predicted that changing the warp velocity should alter the wavelength, and therefore the color, of the extra photons.

“We changed the velocity and saw that the color was changing, and then we changed it again and saw it was still changing, and the original colors disappeared and it had shifted to this new wavelength,” Faccio said. “There’s no other physical mechanism out there that can give the same effect. Hawking radiation is the only physical model known which can give rise to something like this.”

Understanding Hawking radiation could help physicists toward a unified theory of physics that works on the scales of stars and galaxies, which are described by Einstein’s general relativity, and on the scales of electrons and quarks, described by quantum mechanics.

“These laboratory analogues are important, because they literally shed light on a mysterious phenomenon that seems to connect three areas of physics: gravity, quantum physics and thermodynamics,” Leonhardt said. “They show first of all that Hawking radiation is not a mere theoretical dream, but something real.”

“While this measurement can’t actually tell you anything about quantum gravity,” Faccio said, “it does tell you that some of the simple approaches in this direction do work, and they do give you correct predictions. This means that if you develop a quantum theory of gravity, you have something to test this theory on.”

There are a few problems with this particular model black hole, Faccio points out. The biggest is that physicists can see only one photon of the pair supposedly created at the event horizon. That means there’s no way to tell whether the two photons are quantum entangled, a key feature of Hawking radiation. Leonhardt and his colleagues are working on making a radiating black hole in an optical fiber that would show whether or not the photons are entangled.

Physicist Dentcho Genov of Louisiana Tech University, who also makes lab-bench-scale black holes using a class of materials called metamaterials, points out that this is only an indirect proof of Hawking radiation. A direct proof would have to come from observing a tiny black hole radiating away in space.

“To have a direct proof is very difficult. I don’t know if in my lifetime or in my kids or grandkids lifetime that’s going to happen,” Genov said. “The actual full-scale experimental validation of Hawking radiation is still far away in the future. But this one I think is sufficient.”

Image: 1) An artist’s conception of the black hole at the center of the Milky Way. Gallery of Space Time Travel. 2) The laser setup at Belgiorno’s lab. Reproduced courtesy of Daniele Faccio.

See Also:

• Scientists Make Desktop Black Hole
• Scientists Make Fake Black Hole in a Phone Line
• Scientists Make Blackest Material Ever
• A Solution to the Black Hole Information Paradox

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Authors: Lisa Grossman

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