Hawking radiation from fluids

The remarkable black hole radiation predicted 40 years ago by Stephen Hawking has never been observed. But an analogous phenomenon has been seen by Jeff Steinhauer (Technion–Israel Institute of Technology) in a Bose–Einstein condensate (BEC) of rubidium-87 atoms. In the analogue fluid system, sound plays the role that light does for a black hole, and a region in which the fluid flow exceeds the speed of sound substitutes for the light-trapping interior of the black hole (see also Physics Today, August 2005, page 19 ). To generate an analogue black hole horizon separating supersonic and subsonic flow, Steinhauer accelerated a portion of the BEC by illuminating it with a half-moon-shaped laser spot. Moreover, the potential Steinhauer used to confine his BEC created a second, inner horizon downstream of the black hole horizon, where the flow again became subsonic. The two horizons are indicated on the figure, which shows time-lapse images of the BEC (the top panel is the earliest). Hawking phonons generated at the black hole horizon carry energy away from the supersonic flow region; to conserve energy, the BEC must also accommodate negative-energy modes. Those modes rattle around between the two horizons. The interference between left-moving and right-moving waves creates the fringes seen in the figure. The increasing intensity and contrast with time reflect the exponential growth of negative energy accompanying the continuing emission of Hawking phonons. (J. Steinhauer, Nat. Phys. 10, 864, 2014, doi:10.1038/nphys3104 .)