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.