Nature Phys. 10, 864–869 (2014)

Stephen Hawking's name has been associated with black holes — regions of space and time from which no light or matter within the so-called event horizon can escape. Although Hawking predicted in 1974 that black holes emit radiation, astronomers have not been able to observe it as it is expected to be incredibly weak. Now, Jeff Steinhauer from Technion-Israel Institute of Technology in Haifa reports the realization of an analogue event horizon in the lab using the transition from subsonic to supersonic flow in a Bose–Einstein condensate. The flow of the condensate was accelerated to velocities that exceed the speed of sound, trapping sound waves in this supersonic region, just as gravity traps anything within a black hole. Steinhauer observed the appearance and disappearance of pairs of sound waves on this border, with a small part forming across the horizon and resembling Hawking radiation. A secondary potential within the first was used to further trap and amplify this Hawking radiation in a laser-like operation, so that it could be detected experimentally. This result not only provides a way of studying black-hole physics in a laboratory environment but may also prove useful for studying the link between quantum theory and gravity, as Hawking radiation relies on both.