Observation of self-amplifying Hawking radiation in an analogue black-hole laser

By a combination of quantum field theory and general relativity, black holes have been predicted to emit Hawking radiation. Observation from an actual black hole is, however, probably extremely difficult, so attention has turned to analogue systems in the search for such radiation. Here, we create a narrow, low density, very low temperature atomic Bose-Einstein condensate, containing an analogue black-hole horizon and an inner horizon, as in a charged black hole. We report the observation of Hawking radiation emitted by this black-hole analogue, which is the output of the black-hole laser formed between the horizons. We also observe the exponential growth of a standing wave between the horizons, which results from interference between the negative-energy partners of the Hawking radiation and the negative-energy particles reflected from the inner horizon. We thus observe self-amplifying Hawking radiation.