A Sonic Black Hole

18 June 2009—In a first step toward observing the faint radiation that a black hole is supposed to emit, an Israeli team of physicists led by Jeff Steinhauer at Technion-Israel Institute of Technology has created an analogue of a black hole in the lab using sound waves and a frigid state of matter called a Bose-Einstein condensate. The theorist Stephen Hawking has predicted that black holes are not totally black but instead give off a mysterious and faint glow, known as Hawking radiation. Steinhauer says that his lab-created black hole could ultimately lead to the discovery of this radiation.

William Unruh of the University of British Columbia, the theorist who in 1981 first outlined the principles of creating an acoustic analogue of a black hole, calls the work ”very exciting.”

”This is clearly just the first step in being able to measure the analogue of the Hawking radiation,” Unruh says. ”Detecting the radiation is really very difficult, but as always, I am astonished by the ability of experimentalists to overcome what for me as a theorist would be absolutely impossible difficulties.”

Black holes are among the most exotic objects in the universe. Astronomers think that black holes form when huge stars die in spectacular collapses, forming incredibly dense objects. There’s evidence that black holes exist at the centers of galaxies. Their powerful gravitational fields are so intense that nothing, not even light, can escape. A black hole is characterized by its ”event horizon”—a boundary that defines the region from which nothing can escape.

In the 1970s, Hawking applied quantum mechanics to classical black hole theory and found that the objects should give off a ghostly glow. The reason is that, according to quantum mechanics, empty space is not empty at all; it is actually teeming with short-lived pairs of virtual particles and their antiparticles that,are constantly emerging and then annhilating each other. Around the event horizon of a black hole, however, things go awry, and one particle of a virtual pair will occasionally get captured by the black hole. Once one particle enters the event horizon, its partner virtual particle becomes a real particle. This happens fairly frequently around a black hole, and the net result, Hawking theorized, was that black holes would glow. However, his prediction is untested, since the radiation is too faint to be seen from Earth.

Steinhauer and his colleagues created a cigar-shaped Bose-Einstein condensate out of a gas of rubidium atoms inside a magnetic trap with two compartments. In the condensate, the waves of the rubidium atoms overlapped, so that quantum effects ordinarily seen only at the atomic scale appeared at the scale of the entire condensate.

The Israeli team then compressed and decompressed the trap, setting off sound waves within the condensate. The team took photographs of the shaken condensate and calculated from the snapshots the speed at which the condensate flowed as well as the speed of sound within. The data showed that the condensate flowed faster than the speed of sound, thereby resulting in an acoustic event horizon. Normally, a quantum fluid disintegrates when it moves faster than the speed of sound, but Steinhauer’s technique kept the condensate intact.

This is the first time an acoustic black hole like the one suggested by Unruh has been observed. As for Hawking radiation, the team has not yet seen it but is trying to adapt the experiment in the hopes of doing so.

”Hawking radiation is sound waves emitted by the sonic black hole,” says Steinhauer. ”Observing Hawking radiation is so difficult because the wavelength of the sound waves is very long, which creates a variety of technical difficulties. For example, it would be difficult to distinguish the Hawking radiation from background noise. We would need to decrease the wavelength [of the sound waves] by an order of magnitude before we could even hope to see Hawking radiation.”

There are other groups racing to prove Hawking right. Last year, Ulf Leonhardt at the University of St. Andrews, in Scotland, and his colleagues created an analogue of a black hole in an optical fiber. But Leonhardt hasn’t managed to see Hawking radiation yet either, and he has found the expense of building a sensitive enough experiment a problem. At this point, it’s impossible to say which technique will allow scientists to see the radiation first. ”The jury really is out on which system is best,” he says. ”Probably Hawking radiation will eventually be observed in several [systems].”