In 1974, Stephen Hawking made one of his most famous predictions: that black holes will eventually evaporate completely.
According to Hawking’s theory, black holes are not completely “black” but actually emit particles. This radiation, Hawking believed, could extract enough energy and mass from black holes to make them disappear. This theory was assumed to be correct, but it was thought to be almost impossible to prove.
Fortunately and for the first time in history, a team of scientists from the Technion Institute of Technology in Israel, led by physicist Jeff Steinhauer, has shown this elusive radiation (which, in his honor, was dubbed Hawking radiation), at least in a laboratory. Although Hawking radiation is too faint to be detected in space by our current instruments, physicists have seen this radiation in a black hole analog created with sound waves and some of the coolest and strangest matter in the universe. In the experiment with this virtual black hole, detailed in revist to Nature, they have managed to “see” Hawking radiation in action.
Particle pairs
Black holes exert such an incredibly powerful gravitational force that even a photon, traveling at the speed of light, could not escape. While the vacuum of space is generally considered a vacuum, the uncertainty of quantum mechanics dictates that a vacuum is filled with virtual particles that flow in and out of existence in matter-antimatter pairs. (Antimatter particles have the same mass as their matter counterparts, but the electrical charge is opposite.)
Normally, after a pair of virtual particles emerge, they immediately annihilate each other. However, next to a black hole, the extreme forces of gravity pull the particles apart, with one particle absorbed by the black hole while the other is launched into space. The absorbed particle has negative energy , which reduces the energy and mass of the black hole. If the black hole swallows enough particles, it will eventually evaporate. Yes, this fugitive particle is what we call Hawking radiation.
How to Measure Hawking Radiation in a Laboratory
Physicist Jeff Steinhauer and his colleagues used an extremely cold gas called Bose-Einstein condensate to model the event horizon of a black hole, the invisible boundary from which nothing can escape. In a flowing stream of this gas, they placed a cliff, creating a “waterfall” of gas; When the gas flowed over the waterfall, it converted enough potential energy into kinetic energy to flow faster than the speed of sound.
Instead of particles of matter and antimatter, the researchers used pairs of phonons, or quantum sound waves, in the gas flow . The phonon on the slow side could travel against the flow of the gas, away from the waterfall, while the phonon on the fast side could not, trapped by the “black hole” of the supersonic gas.
For us to understand each other, it would be like trying to swim upstream in a fast river. It would seem that you are moving forward but all you do is feel dragged. This is how photons in a black hole feel.
Stephen Hawking predicted that the radiation from the emitted particles would be in a continuous spectrum of wavelengths and energies. He also said that it could be described by a single temperature that depended solely on the mass of the black hole . This experiment in Israel has confirmed both predictions of the British physicist on the sonic virual black hole.
Referencia: Observation of thermal Hawking radiation and its temperature in an analogue black hole. Juan Ramón Muñoz de Nova, Katrine Golubkov, Victor I. Kolobov & Jeff Steinhauer. Nature 569, 688–691 (2019) DOI: https://doi.org/10.1038/s41586-019-1241-0
