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Friday, June 19, 2009

Sonic Black Hole Traps Sound Waves

Eric Bland, Discovery News

Sound Vacuum
Sound Vacuum | Discovery News Video

June 17, 2009 -- A black hole created by Israeli scientists won't destroy Earth, but it could make our planet just a little bit less noisy.

Using Bose-Einstein condensates, the scientists created a black hole for sound. The new research could help scientists learn more about true black holes and help confirm the existence of as-yet to be discovered Hawking radiation.

"It's like a black hole because waves get sucked in and can't escape," said Jeff Steinhauer, a scientist at the Israel Institute of Technology and the corresponding author of the article recently posted on the pre-print Web page. "But in this case we use sound waves instead of light."

To create the sonic black hole, the scientists first had to create the Bose-Einstein condensate, a cloud of atoms cooled to almost absolute zero that acts like a light wave. The Israeli scientists actually created two clouds of rubidium 87 atoms cooled to 50 nano Kelvins and separated by a small gap.

The gap is key. Known as a "density inversion," the gap creates a region of space with a very low density, allowing atoms to flow between the two clouds virtually unimpeded at nearly three millimeters per second. That's more than four times the speed of sound.

It's an inversion because unlike Earth's atmosphere, where the clouds are lighter than the air underneath, the Bose-Einstein condensate clouds are denser than the space below them.

Since atoms move between the clouds faster than sound, any sound wave trying to escape will fall farther and farther behind, never able to escape the sonic event horizon.

"It's like trying to swim slowly against a fast current," said Steinhauer. "The sound waves fall behind because the current is moving faster than the waves."

Scientists observed the sonic black hole for a total of eight milliseconds using lasers. Since it's a sonic black hole, not a true black hole, light waves, which travel much faster than sound waves, can still escape.

According to James Anglin, a professor at the Technische Universitat Kaiserslautern, scientists in Germany, the United States, and Austria and elsewhere have tried to create a sonic black hole since they were first theorized back in the early 1980s by the Canadian physicist Bill Unruh. But Steinhauer and his colleagues have been the first to actually create one.

"Jeff's experiment confirms that it really is possible to create a reasonably stable supersonic flow in a superfluid gas," said Anglin. The research "will give us a new perspective on some really deep issues involving quantum mechanics, thermodynamics and gravity."

Specifically, Steinhauser wants to use his sonic black hole to help confirm the existence of the predicted, but as yet unobserved, Hawking radiation.

First predicted by the physicist Steven Hawking in 1974, Hawking radiation is radiation is theoretically emitted from just outside the event horizon of a black hole. Its existence would mean matter can escape from a true black hole.

If the amount of matter being ejected from the black hole is greater than the amount of incoming mass, the black hole will eventually evaporate.

Despite numerous ongoing efforts, definitive proof of Hawking radiation has eluded scientists. According to the theory, Hawking radiation is just slightly below the temperature of the surrounding area, making it very difficult to detect.

The sonic black hole equivalent of Hawking radiation would be a a cloud of phonons, basically vibrating packets of energy that behave like particles. Phonons escaping from a black hole could be found easily, as a small cloud between the two large clouds of Bose-Einstein condensates, or with more difficulty, as one cloud sitting on top of another.

Finding such a cloud of phonons wouldn't definitively confirm Hawking radiation, but it would lend experimental evidence to its existence, says Steinhauer.

"This is about understanding the basic laws of physics," said Steinhauer. "What this research is good for in day to day life I'm not sure, but we as humans want to understand how the universe works."