One of the many curious properties of Bose-Einstein Condensates (BECs) is that the flow of sound through them is governed by the same equations that describe how light is bent by a gravitational field. That sets up the possibility of all kinds of fun and games: in theory, physicists can reproduce with sound and BECs whatever wicked way gravity has with light.
Today, Oren Lahav and his mates at the Israel Institute of Technology, in Haifa, say that they’ve created the sonic equivalent of a black hole in a BEC. That’s some achievement, given that physicists have wondered about this possibility for some 30 years, and various groups with the ability to create BECs have been racing to create acoustic black holes.
The general idea is to set up a supersonic flow of atoms within the BEC. Sound waves moving against this flow can never make any ground. So the region where the flow changes from subsonic to supersonic is an event horizon. Any sound waves (or phonons) created inside the event horizon can never escape because the flow there is supersonic. That’s the black hole.
Lahav and co set up a supersonic flow by creating a deep potential well in the middle of a BEC that attracts atoms. The atoms stream into it but cannot give up their energy when they arrive (they’re already in their lowest energy state), and so they stream across the well at supersonic speed.
The result is a region within the BEC in which the atoms move at supersonic speed. This is the black hole: any phonon unlucky enough to stray into this region cannot escape.
One reason why sonic black holes are so highly prized is that they ought to produce Hawking radiation. Quantum mechanics predicts that pairs of “virtual” phonons with equal and opposite momentum ought to be constantly springing in and out of existence in BECs.
If one of this pair were to cross the event horizon, it would be sucked into the black hole, never to escape. The other, however, would be free to go on its way. This stream of escapees would be the famous, but as yet unobserved, Hawking radiation.
Lahav and his buddies haven’t gotten this far yet, but they’ve made an important step toward observing Hawking radiation and clearly have their eyes on this goal.
There’s no shortage of competition here, and the creation of the first sonic black hole will be sure to spur the competition. Expect to see somebody claim the first observation of Hawking radiation soon.
Ref: arxiv.org/abs/0906.1337: A Sonic Black Hole in a Density-Inverted Bose-Einstein Condensate
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