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In a first, scientists took the temperature of a sonic black hole

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Taking a black holes temperature is a seemingly impossible task. But now, physicists report the next best thing. Theyve 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 arent truly black. Instead, a relatively small stream of particles bleeds from each black holes 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 holes temperature agrees with that predicted by Hawkings theory, the team reports in the May 30 Nature.

“Its a very important milestone,” says physicist Ulf Leonhardt of the Weizmann Institute of Science in Rehovot, Israel, who was not involved with the study. “Its 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 holes 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 wasnt yet possible to measure its temperature (SN: 11/15/14, p. 14).

Hawking radiation comes from pairs of quantum particles that constantly pop up everywhere, even in empty space. Normally, those particles immediately annihilate one another. But at a black holes edge, if one particle falls in, the other could escape, resulting in Hawking radiation. In the sonic black hole, a similar situation occurs: Pairs of sound waves known as phonons can appear, with one falling in and the other escaping.

Measurements of the phonons that escaped and those that fell in allowed the researchers to estimate the temperature, 0.35 billionths of a kelvin. “We found very good agreement with the predictions of Hawkings theory,” says physicist Jeff Steinhauer of the Technion-Israel Institute of Technology in Haifa.

The result also agrees with Hawkings prediction that the radiation would be thermal, meaning that the particles energies would have a distribution like that of the glow emitted by a warm object, such as the reddish light of a hot electric stove.

After Hawking proposed his theory, this predicted thermal property of the radiation led to a conundrum known as the black hole information paradox. In quantum mechanics, information can never be destroyed. But particles eRead More – Source