In this setup, the step marks the boundary between the supersonic and subsonic regions, which acts as the black hole’s event horizon. At this event horizon, the flow velocity of the condensate is exactly equal to the speed of sound. On the supersonic side of the step, the density of the condensate is much lower than that on the subsonic side. As the scientists explained, the low density corresponds to a higher flow velocity due to conservation of mass. In their experiments, they could maintain the black hole event horizon for at least 20 milliseconds before it became unstable.
Similar to how a black hole traps photons, the supersonic region of the sonic black hole can trap phonons and a wide range of other Bogoliubov excitations with a wavelength of between 1.6 and 18 micrometers. Excitations with very short wavelengths can escape, and those with longer wavelengths cannot fit in the supersonic region in the first place.
In the future, the scientists plan to use the sonic black hole to study Hawking radiation. As the physicist Stephen Hawking first predicted, black holes may emit a small amount of thermal radiation due to quantum effects. Losing this radiation can cause black holes to shrink and eventually evaporate completely. But so far, detecting this radiation has been very challenging.
In order to observe Hawking radiation in the case of the sonic black hole, there are a few requirements, such as that the trapped excitations must have negative energy. The researchers verified this in simulations: When focusing two laser beams with slightly different frequencies onto the supersonic region of the condensate, the simulated condensate absorbed a photon from one beam and emitted a photon into the second beam, creating an excitation with negative energy. In the future, the sonic black hole may give scientists a glimpse of Hawking radiation.
http://www.physorg.com/news/2011-01-phys…