Black holes are a sandbox for physicists. They can observe and test the most abnormal and fundamental concepts of physics. But so far there is no way to study black holes directly; these objects do not emit radiation, such as visible light or X-rays, that telescopes could catch. Fortunately, physicists have figured out how to simulate the conditions that exist in black holes in the laboratory - and, creating BH analogs, they begin to unwind the most amazing puzzles of physics.

Jeff Steinhauer, a researcher at the Physical Department of the Israel Institute of Technology (Technion), recently attracted attention by announcing the successful use of a black hole analogue to confirm Stephen Hawking’s 1974 theory of emitting BH electromagnetic radiation, known as Hawking radiation. Hawking predicted that this radiation would be due to the spontaneous creation of particle-antiparticle pairs on the event horizon, at a point on the edge of the BH, for which nothing, not even light, can escape. According to the theory, one of the particles gets into the BH, and the other runs into space [in fact, everything is a little more complicated, and explained by the link above - approx. trans.]. The Steinhauer experiment for the first time demonstrated Hawking's supporting calculations of spontaneous fluctuations.

Physicists have warned that such an experiment does not confirm the existence of Hawking radiation in astronomical BH, since Steinhauer did not have BH real, different from that existing in space. It is not yet physically possible to create gravitational fields of such intensity as to form a BH. The analogue only simulates the ability of BH to absorb visible light with the help of sound.
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“Imagine that a sound wave is trying to swim against the current, and the current is moving faster than it can swim,” says Steinhauer. His team cooled the cloud of atoms to almost absolute zero, creating an environment called Bose-Einstein condensate. By making the gas move faster than sound, they created a system from which sound waves cannot escape.

Steinhauer's observations were published in the journal Nature Physics in August . In addition to observing Hawking radiation, his experiment was important because, according to the scientist, it was observed that particles emitted by the “sound BH” were entangled. This means that the two particles could be in different physical states, and that, knowing the state of one particle, it was possible to immediately determine the state of the second.

The concept of BH analogue was proposed by William Unruh in the 1980s, but it was only implemented in the laboratory in 2009. Since then, BH analogues have been adopted by physicists from around the world, many of whom have tried to observe Hawking radiation. And although Steinhauer was the first to succeed in this, analogs have already benefited and helped physicists to verify equations and principles that have existed only on paper for a long time. In fact, the largest of the possibilities of BH analogs can help physicists overcome the greatest obstacle of science: to combine gravity and quantum physics, controlling the behavior of subatomic particles, and so far incompatible with the laws of gravity.

To create BH analogs, different methods are used, but their principles are the same: each of them has a point, an event horizon, which cannot be overcome by the oscillation, which is an analogue of light in the experiment, since the speed required for this is too great. But how scientists tried to imitate BH in the laboratory.

Glass

In 2010, a group of physicists from the University of Milan attracted public attention by telling about the observation of Hawking radiation in a BH analog created with the help of laser pulses in quartz glass. And although their statement was questioned - William Unruh considered that their radiation turned out to be too intense, and even went in the wrong direction - their analogue became an interesting method of modeling the event horizon.

It works like this: the first impulse sent to glass is strong enough to change the index of refraction of the glass - the speed of light in the medium. The second impulse entering the glass slows down to a stop, and creates a “horizon” that the light cannot overcome. The system is the opposite of BH, from which the light cannot escape, and therefore it is called the “white hole”. But according to Stephen Hawking, white and black holes are essentially the same, and their quantum properties should be similar.

Separate research groups showed in 2008 that a white hole could be created using fiber. Experiments continue to create a database with diamonds, which are much harder to damage than quartz glass.

Polaritons

The team of High Son Nguyen [Hai Son Nguyen] in 2015 showed the creation of acoustic BH using polaritons - a strange state of matter, quasiparticles, appearing when photons are mixed with matter excited by light. The team created polaritons by focusing a powerful laser on a microcavity in gallium arsenide, a semiconductor. The notch in one part of the cavity expanded it. When the laser hit the microcavity, it emitted polaritons, rushing in the direction of the notch. But in order to achieve it, they changed speed, moving faster than sound, thereby creating a horizon beyond which sound could not exit.

They did not see the Hawking radiation, but they hope that in future experiments it will be possible to observe the fluctuations of the particles, measuring the changes in density in their flow. In other experiments, it is proposed to cool the polaritons to the Bose-Einstein condensate state, which can be used to emulate the formation of wormholes.

Water

Water flowing through the drainage hole into the sewer can be a good simulation of BH. In the laboratory at the University of Nottingham, Dr. Silke Weinfurtner [Silke Weinfurtner] simulated BH in a “whirlpool of a bathtub”, a 2000-liter rectangular container with a sloped funnel in the center. Water enters the tank from above and below, which gives the water an angular momentum, creating a whirlpool at the point of contact with the funnel. The analogue of light is the ripples on the surface of the water. Imagine that we throw a pebble in a stream, and we will look at the ripples diverging from it. The closer the ripples are to the whirlpool, the less opportunity there is for it to spread away from it. At some point, the ripples will no longer go from the whirlpool - this will be our event horizon. This analogue turned out to be useful for simulating the strange physics of rotating black holes, which Weinfurthner is now studying.

She clarified that this analogue does not demonstrate BH in a quantum sense. It works at room temperature and shows only classical mechanics. “The system is dirty,” she says. “But she can be controlled, and she tolerates change well.” We are convinced that the same phenomena occur in astrophysical systems. ”