Backreaction of stimulated Hawking radiation in an optical analogue

Hawking radiation¹-the emission of quantum particles at the event horizon of a black hole²-connects gravity with quantum mechanics and thermodynamics^3-5. But Hawking radiation has never been observed in astronomy, only in laboratory analogues^6-9, and the chances of ever observing it in space are astronomically small⁹. The energy of Hawking radiation must come from the gravitational field around the black hole², but how field quanta generate Hawking quanta has been unknown. Here we report on experimental and theoretical evidence for the process that generates Hawking radiation in a fibre-optical analogue of the event horizon^10,11. There, as in gravity², it has been believed that Hawking radiation comes from a complicated, cascaded process¹²; here we have identified theoretically a simple, direct process and observed experimentally how this process reacts back onto the field. Our findings suggest an equally direct process for other laboratory analogues^6-8,13-17 and perhaps also for gravitational fields, shedding light on how black holes might radiate.