Stephen Hawking liked to claim that, if his most famous prediction had been verified experimentally, he would have won a Nobel prize. The prediction was that, as he once put it, “black holes ain’t so black”. These stars, which collapse to an infinitely dense singularity, can emit intense radiation from just outside their event horizon – the point of no return beyond which even light can’t escape from the intense gravity.
Few doubt that this Hawking radiation, predicted in 1974, is a real phenomenon – but no-one has ever seen it. Direct astronomical observations are very challenging because the radiation is too feeble; the X-rays streaming from suspected black holes are instead emitted by incredibly hot gas as it spirals inwards. But researchers believe that the equivalent of Hawking radiation might be seen emerging from laboratory experiments that mimic black holes in other media, such as light, acoustic or water waves. Now, a team at the Weizmann Institute of Science in Rehovot, Israel, has reported experiments that they say come one step closer to producing Hawking radiation in an “optical black hole”.
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