Taking a black hole’s temperature is a seemingly impossible task. But now, physicists report the next best thing. They’ve measured the temperature of a lab-made sonic black hole, which traps sound instead of light.

If the result holds up, it will confirm a prediction of cosmologist Stephen Hawking, who first proposed a surprising truth about black holes: They aren’t truly black. Instead, a relatively small stream of particles bleeds from each black hole’s margin at a temperature that depends on how massive the black hole is. Such Hawking radiation is too faint to observe in true black holes. But physicists have spotted hints of similar radiation from analogs of black holes created in the lab (SN: 12/18/10, p. 28). In the new study, the sonic black hole’s temperature agrees with that predicted by Hawking’s theory, the team reports in the May 30 Nature.

“It’s a very important milestone,” says physicist Ulf Leonhardt of the Weizmann Institute of Science in Rehovot, Israel, who was not involved with the study. “It’s new in the entire field. Nobody has done such an experiment before.”

To produce the sonic black hole, the researchers used ultracold atoms of rubidium, chilled to a state known as a Bose-Einstein condensate, and set them flowing. Analogous to a black hole’s gravity trapping light, the flowing atoms prevent sound waves from escaping, like a kayaker rowing against a current too strong to overcome. Previous experiments with this setup have shown signs of Hawking radiation, but it wasn’t yet possible to measure its temperature (SN: 11/15/14, p. 14).