Scientists "take the temperature" of a black hole for the first time

Lab experiments are able to simulate a phenomenon predicted by Stephen Hawking.

"Take the temperature" of a black hole appears as an impossible task. Yet, physicists measured for the first time the temperature of a black hole sonic made in the laboratory, which traps sound instead of light.

A result that confirms Hawking's predictions

If the result is credible, it will confirm a prediction from Stephen Hawking, who had once offered a surprising truth about black holes: they are not really black. Instead, a (relatively small) stream of particles peels off around each black hole at a temperature that depends on its magnitude. Called Hawking radiation, it is too weak to be observed in real black holes, but physicists have spotted evidence of similar radiation emitted by analogs of black holes created in the laboratory.

In the new study, the sonic black hole temperature corresponds to that predicted by the Hawking theory, reports the team in the Nature issue of 30 May: " This is a very important step Said physicist Ulf Leonhardt of the Weizmann Institute of Science in Rehovot, Israel, who did not participate in the study. " It's new in the whole field. Nobody has ever had such an experience before ».

The researchers used rubidium atoms cooled in a state known as Bose-Einstein condensate, and floated. Similar to the light trapping the gravity of a black hole, the atoms that flow prevent sound waves from escaping, like a kayaker rowing against the current of a flow too strong to be controlled. Previous experiments with this configuration showed signs of Hawking radiation, but it was not yet possible to measure its temperature.

Le Hawking radiation comes from pairs of quantum particles that constantly appear everywhere, even in empty space. Normally these particles cancel out immediately. But at the edge of a black hole, if one particle falls inside, the other could escape, resulting in this famous radiation. A similar situation occurs in the black hole: pairs of sound waves called phonons may appear, one falling and the other escaping.

The phonon measurements that escaped and those that fell allowed researchers to estimate the temperature at 0,35 billionth of a Kelvin. " We agreed with the predictions of Hawking's theory Said physicist Jeff Steinhauer of the Technion-Israel Institute of Technology in Haifa.

The study of Hawking radiation

The result is also in agreement with Hawking's prediction that the radiation would be thermal - which means that the energy of the particles would have a distribution similar to that of the glow emitted by a hot object, such as the reddish light of an electric stove when it is heating.

After Hawking proposed his theory, this predicted thermal property of radiation led to an enigma, called black hole information paradox. In quantum mechanics, the information can never be destroyed, but the particles coming out of the black holes would slowly undermine its mass and, over a long period, the black hole would shrink into nothingness.

This means that the information "fallen" into the black hole (in the form of particles, or other) would no longer be contained in it. And if the Hawking radiation is thermal, the information could not have been washed away by the particles. The particles emitted can not be distinguished from those emitted by a trivial object with a given temperature, or even by a black hole different from the same mass. This suggests that information can be lost when a black hole evaporates, which in itself constitutes a "violation" of quantum mechanics.

It is unclear whether the new study could help scientists solve this paradox: it will probably be necessary to find a new theory combining gravitation and quantum mechanics, which is one of the biggest outstanding issues. But this theory would not apply to sonic black holes, because they are not created by gravity. " The solution to this paradox lies in the physics of a real black hole, and not in the physics of an analog black hole Said Steinhauer.

source: Science News

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