Sonic Black Hole Traps Sound Waves

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 ArXiv.org 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.

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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.