In 1974, prominent physicist Stephen Hawking shocked his colleagues by claiming that black holes emit radiation. Forty years later, a laboratory experiment seems to have confirmed his theory. The study is published in the journal «Nature Physics».
Black holes – points in space where the density of matter approaches infinity – have such a powerful gravitational field that nothing, not even light, can escape. However, in 1974, Hawking came up with a theory that black holes are not completely black, and are constantly emitting a faint light. The so-called Hawking radiation is the result of quantum phenomena. According to quantum theory, the vacuum is never completely empty but is flooded with “virtual particles” of matter and antimatter, which only appear for a moment before annihilating each other.
But if such a pair of particles appears near a black hole, one of them may be trapped in forever, so the second one is free to escape and can continue existing as a normal particle. The Hawking radiation consists of precisely these particles but is too weak to be recorded in real conditions.
Now, James Steinhauer, a researcher at the Technion-Israel Institute of Technology in Haifa, Israel, managed to create a black hole or, more precisely, an analog of a black hole, that traps sound waves instead of light in his lab. Together with his research team, he cooled rubidium atoms to absolute zero to create a so-called Bose-Einstein condensate, a state of matter in which a group of cooled atoms acts as a unit.
Using a laser, researchers accelerated a portion of the condensate over the speed of sound. Thus, any sound wave that traveled through the condensate against the direction of motion would be trapped inside it, just like the light would be trapped inside a black hole.
The condensate is comparable to the so-called event horizon around a black hole, the boundary beyond which there is no possibility of return. As the vacuum is teeming with pairs of virtual particles, pairs of sound waves appeared in the experiment due to quantum phenomena. These waves normally annihilate each other, but sometimes one wave can be trapped in the event horizon while the second one is free to run its course.
The problem is that these waves are very faint and be recorded. To solve this problem, the researchers created a second event horizon which could not be penetrated by the sound waves. This caused the waves to bounce continuously from one horizon to the other, triggering the birth of new wave pairs and increasing the signal to detectable levels.
However, it remains unclear whether the model created by the researchers accurately simulates the behavior of a real black hole. The picture could become clearer if the team had managed to record the analogue of Hawking radiation without using a second event horizon. However, since the Hawking radiation from black holes is considered practically impossible to record with current technologies, the laboratory analogues of black holes will probably remain the main tool for their study for a long time.