A black hole is a physics playground. This is the place to observe and test the most bizarre and fundamental ideas and concepts from the field of physics. However, today there is no way to directly observe black holes in action; these formations do not emit light or X-rays, which can be detected by modern telescopes. Fortunately, physicists have found ways to simulate the conditions of a black hole in the laboratory, and by creating analogues of black holes, they are beginning to solve the most amazing mysteries of physics.
Jeff Steinhauer, a researcher in the Department of Physics at the Israel Institute of Technology, recently attracted the attention of the entire physics community by announcing that he was using an analogue of a black hole to confirm Stephen Hawking's 1974 theory. This theory states that black holes emit electromagnetic radiation known as Hawking radiation. Hawking suggested that this radiation is caused by the spontaneous appearance of a particle-antiparticle pair at the event horizon, as the point at the edge of a black hole is called, beyond which nothing, not even light, can escape. According to Hawking's theory, when one of the particles crosses the event horizon and is captured by a black hole, the other is thrown into space. Steinhower's experiment was the first demonstration of those spontaneous fluctuations,which confirm Hawking's calculations.
Physicists warn that this experiment still does not confirm the existence of Hawking radiation in astronomical black holes, since the Steinhauer black hole is not exactly what we can observe in space. Physically, it is not yet possible to create powerful gravitational fields that form black holes. Instead, the analogue uses sound to mimic the ability of a black hole to absorb light waves.
“This sound wave is like trying to swim against the current of a river. But the river flows faster than you swim,”says Steinhauer. His team cooled the cloud of atoms to almost absolute zero, creating the so-called Bose-Einstein condensate. By making gas flow faster than the speed of sound, scientists have created a system that sound waves cannot leave.
Steinhauer published his observations in early August in an article in the journal Nature Physics. His experiment is important not only because he made it possible to observe Hawking radiation. Steinhauer claims that he watched the particles emitted by the sonic black hole and the particles inside it "get entangled." This means that two particles at the same time can be in several physical states, such as an energy level, and that knowing the state of one particle, we can immediately know the state of the other.
The concept of a black hole analog was proposed in the 1980s by William Unruh, but it was not created in the laboratory until 2009. Since then, scientists around the world have been creating analogs of the black hole, and many of them are trying to observe Hawking radiation. Although Steinhauer was the first researcher to be successful on this front, analog systems are already helping physicists test the equations and principles long applied to these theoretical systems, but only on paper. In fact, the main hope for black hole analogs is that they can help scientists overcome one of the biggest challenges in physics: to combine gravity with the principles of quantum mechanics that underlie the behavior of subatomic particles, but are not yet compatible with laws. gravity.
Although the methods used are very different, the principle is the same for every analogue of a black hole. Each has a point, which, like the event horizon, cannot be crossed by any wave used in place of light, since the required speed is too high. Here are some of the ways in which scientists simulate black holes in the laboratory.
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Glass
In 2010, a group of physicists from the University of Milan made a splash in the scientific community, claiming that they observed Hawking radiation from a black hole analogue, which was created using high-power laser pulses aimed at silica glass. Although the scientists' assertion was questioned (physicist William Unruh said that the radiation they noticed is much more intense than the calculated Hawking radiation, and that it is going in the wrong direction), the analogue they created is still a very interesting method for modeling the event horizon.
This method works as follows. The first impulse applied to the quartz glass is strong enough to change the refractive index (the rate at which light enters the substance) inside the glass. When the second impulse hits the glass, due to the change in the refractive index, it slows down to a complete stop, creating a "horizon" beyond which light cannot penetrate. This kind of system is the opposite of a black hole, from which no light can escape, and therefore it was called a "white hole". But as Stephen Hawking says, white and black holes are basically the same thing, which means they must exhibit the same quantum properties.
Another research group in 2008 showed that a white hole can be created in a similar way using fiber optics. Further experiments are working to create the same event horizon using diamond, which is less destroyed by laser radiation than silicon.
Polaritons
A team led by Hai Son Nguyen demonstrated in 2015 that a sonic black hole can be created using polaritons - a strange state of matter called a quasiparticle. It is formed when photons interact with elementary excitations of the medium. Nguyen's group created polaritons by focusing a high-power laser on a microscopic cavity of gallium arsenide, which is a good semiconductor. Inside it, scientists deliberately created a small notch that expanded the cavity in one place. When the laser beam hit this microcavity, the emission of polaritons took place, which rushed to the defect in the form of a notch. But as soon as the flux of these excited particles reached the defect, its speed changed. The particles began to move faster than the speed of sound, indicating that there was a horizon,beyond which the sound cannot go.
Using this method, Nguyen's team has not yet detected Hawking radiation, but scientists believe that in the course of further experiments it will be possible to detect oscillations caused by particles leaving the field by measuring changes in the density of their environment. Other experimenters suggest cooling polaritons to a Bose-Einstein condensate, which can then be used to simulate the formation of wormholes.
Water
Watch the water swirling down the drain as you shower. You will be surprised to know that you are looking at something like a black hole. In a laboratory at the University of Nottingham, PhD Silke Weinfurtner simulates black holes in a bathtub, as she calls a 2,000 liter rectangular tank with a beveled funnel in the center. Water is fed into the tank from above and below, which gives it an angular momentum, which creates a whirlpool in the funnel. In this aqueous analogue, light replaces small ripples on the surface of the water. Imagine, for example, that you are throwing a stone into this stream and watch the waves radiate from it in circles. The closer these waves come to the whirlpool, the more difficult it is for them to propagate in the opposite direction from it. At some point, these waves stop spreading altogether,and this point can be considered an analogue of the event horizon. Such an analog is especially useful when simulating strange physical phenomena that occur around rotating black holes. Weinfurtner is currently investigating this issue.
She stresses that this is not a black hole in the quantum sense; this analogue appears at room temperature, and only classical manifestations of mechanics can be observed. “It's a dirty system,” says the researcher. “But we can manipulate it to show that it is resilient to change. We want to make sure that the same phenomena occur in astrophysical systems."