In 1974 Stephen Hawking argued that black holes are not black. Due to a still poorly understood quantum instability near the horizon, he calculated that a black hole will continuously emit radiation with a thermal spectrum. The temperature is inversely proportional to the mass of the black hole (about 10-7 K for a solar-mass black hole).
Hawking’s idea will not soon be testable on astrophysical black holes. But in a 1981 paper, I suggested that analogs to the horizon and to the Hawking process could exist for other waves. Irrotational flow of a fluid, such that a surface exists where the velocity of the fluid equals that of sound, would act as a (sonic) horizon analog. Hawking’s argument would predict that an analog black hole would also emit quantum thermal radiation at the horizon. Such “dumb” (not able to speak) holes could thus be a test of the assumptions that went into Hawking’s black hole calculations.
Since then, many other systems with horizons have been shown, theoretically, to display thermal emission. But over the past several years, scientists have also carried out horizon-temperature experiments. In 2011 Silke Weinfurtner and colleagues at the University of British Columbia (including myself) measured the surface waves in flowing water near a horizon. Using stimulated emission experiments, we showed, via Einstein's relation between stimulated and spontaneous emission, that the spontaneous quantum emission from that horizon would be thermal. The latest and most impressive work comes from Jeff Steinhauer at the Technion–Israel Institute of Technology, whose 15 August paper reports detecting the spontaneous emission of the sonic analog of quantum-entangled Hawking radiation in a Bose–Einstein condensate (BEC). Although Steinhauer’s claim needs to be verified, his dumb-hole horizon could be the first direct experimental evidence that horizons have a quantum-induced temperature.
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