Israeli physicists indirectly demonstrated the existence of Hawking radiation, a flow of energy that flows from a black hole and causes it to evaporate.

Black holes are strange, in some way “collapsed” areas of space, such as the remains of large stars. The generally accepted concept considers the black hole to be a “vacuum cleaner” that sucks in everything and leaves nothing out, even light. But when quantum mechanics is involved, the whole thing gets remarkably complicated.

As early as 1974, Stephen Hawking showed theoretically that a pair of photons could be formed on the horizon of a black hole event, one with positive energy and one with negative energy. The negative energy photon is drawn into the black hole, while the positive energy is emitted into space. This results in a steady flow of energy out of the black hole, the so-called Hawking radiation. Black holes gradually evaporate. But how to prove this groundbreaking claim?

Israeli physicists led by Jeff Steinhauer tried it with an “artificial” black hole. And they succeeded. They showed that the radiation from the artificial black hole shows a temperature spectrum. This allowed them to assign a temperature to the black hole and indirectly to demonstrate the existence of Hawking radiation. The theoretical impact of this claim is groundbreaking because it means that the black hole has a nonzero temperature.

In statistical mechanics, temperature is defined as the mean energy of a system with a large number of degrees of freedom (similar to a gas containing a large number of molecules). But in Einstein’s general theory of relativity, a black hole is defined only by mass, charge, and spin. Thus, assigning a temperature to a black hole requires either additional degrees of freedom or correction of the temperature definition itself.

Physicists, led by theorist Jeff Steinhauer, have been working for several years on an artificial laboratory black hole, based on Bose-Einstein condensate, a boson-forming substance at a temperature approaching absolute zero (photo: Technion)

Detecting Hawking radiation from a real black hole would be a huge success, but at the same time, it’s an almost impossible task. The problem is that for all known black holes, the predicted temperature is lower than the cosmic microwave background temperature. This means that any radiation from the black hole will be obscured by the radiation absorbed by the black hole.

A possible way to solve this is the fact that the equations of general relativity are mathematically analogous to the equations describing the propagation of waves in a moving medium. As early as 1981, Canadian theorist William Unruh showed that such systems could exhibit Hawking radiation. Many groups have tried to simulate this radiation using waves in water, optical fiber lights, and other systems, but these experiments are very challenging. Some results proved to be wrong, others are still under discussion.

Unruh’s first “artificial” black hole was based on the water that creates the waterfall. At the edge of such a waterfall, Unruh imagined an acoustic phonon, a quasi particle of a sound field. If the flow of water accelerates, the phonon will start to move at supersonic speed, thus trapping it (similar to trapping a photon through an astrophysical black hole). If such phonons were in quantum-linked states, they could escape from the sonic black hole.

Physicists from the University of Haifa, led by theoretician Steinhauer, have been working for several years on an artificial laboratory black hole, based on Bose-Einstein condensate, a boson-forming substance at a temperature close to absolute zero. Under these conditions, a large proportion of the atoms have minimal quantum energy, so it is a collection of cooled captured atoms.

At a higher jump level, the potential energy flows Steinhauer’s condensate slowly, but on the lower energy side, the flow accelerates. Phonon pairs can be created on both sides of this sound horizon. While above the jump the speed of sound is higher than the speed of the condensate itself, and therefore the phonons escape the black hole, at a lower level the speed of sound is lower than the speed of the condensate flow, resulting in sweeping the phonons into the black hole.

In 2014, Steinhauer reported that he was observing the self-amplifying Hawking radiation from an artificial black hole with two horizons. Subsequently, in 2016 he noticed a quantum link between the emitted waves and the waves swept into the black hole. But in either case he wasn’t quite sure. Indeed, another Israeli theorist, Ulf Leonhardt of the Weizman Institute, who himself dealt with artificial black holes in optical fibers, again analyzed the 2016 results and declared them an artifact of statistics.

In order to prove the validity of their results irrevocably, Steinhauer’s team members made 21 different experimental improvements over the next three years. As a result, they were able to measure the energy emitted at the appropriate frequencies and show that the radiation has a black body energy spectrum with a well-defined temperature.

Obviously, such serious results give rise to controversy. While theoretician James Auglin of the University of Kaiserslautern agrees with the results, he believes that the system is not enough to describe a real black hole. All important mysteries about quantum effects in black holes are related to nonlinear dynamics.

But everyone agrees that for real advances in black hole physics, a number of important questions still need to be answered, such as: Does the emission of heat radiation reduce the black hole? If so, does the information captured in the black hole go back during shrinking? Will the shrinkage stop by reaching some residual quantum object that still holds information, or is the information lost? How does the surface of a black hole and entropy relate? Finally, the most important: Is there a strong link between quantum mechanics, thermodynamics, and gravity?

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