Scientists have created a "black hole" of light

Physicists have taken another step towards verifying one of Stephen Hawking's most famous ideas. According to the journal Live Science, an international group of researchers created an optical analogue of a black hole in the laboratory and used it to study the processes that, according to theory, should occur at the boundary of real black holes.

The work does not mean that scientists have learned how to create space objects in the laboratory, but it allows them to experimentally verify predictions that are almost impossible to confirm with astronomical observations. About what exactly the researchers managed to do and why this experiment is called important for modern physics, see the Izvestia article.

How physicists created a "black hole" out of light

The researchers did not create a real black hole. Instead, they built an analog system that reproduces some of the properties of these space objects. For the experiment, laser pulses and a nonlinear optical material were used, in which light began to propagate in such a way that for some of the waves an analog of the event horizon appeared — a boundary, after crossing which the signal can no longer return. Although there is no gravity in such an installation, the mathematical equations describing the behavior of light largely coincide with those used to describe processes near real black holes.

The main purpose of the experiment was to test one of Hawking's key predictions. In 1974, a British physicist suggested that black holes are not completely "black" due to quantum effects: they should emit extremely weak thermal radiation and gradually lose energy over time. Today, this phenomenon is known as Hawking radiation, but it is still impossible to register it in real astrophysical black holes because the signal is too weak compared to the surrounding cosmic radiation. That is why physicists create laboratory analogues where processes can be observed directly.

The peculiarity of the new work lies in the fact that scientists have managed to study not only the very appearance of the Hawking radiation analogue, but also the so—called backreaction, the process by which the emitted radiation begins to change the state of the system that generated it. In theory, this effect should play an important role in the evolution of real black holes, but it is still impossible to observe it in space. The authors of the study believe that such experiments help to test the fundamental predictions of quantum theory and better understand how the laws of quantum mechanics and general relativity can interact.

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Why Hawking radiation is one of the most important ideas of modern physics

Until the mid-1970s, it was believed that black holes fully justified their name: if an object crossed the event horizon, it was impossible to leave it. It followed from this that black holes can only absorb matter and energy, but emit nothing.

In 1974, British physicist Stephen Hawking proposed an unexpected idea: if we take into account the laws of quantum mechanics, the situation turns out to be much more complicated. According to his calculations, pairs of virtual particles constantly appear near the event horizon. They usually instantly destroy each other, but near a black hole, one of the particles may end up beyond the event horizon, and the other may leave the vicinity of the object. To an outside observer, it looks as if the black hole is emitting its own thermal radiation and is gradually losing mass.

The paradox lies in the fact that for half a century of the existence of this theory, astronomers have not been able to directly detect Hawking radiation from real black holes. The reason is simple: the more massive a black hole is, the weaker its radiation should be. For known astrophysical objects, it is so insignificant that it is completely lost against the background of cosmic microwave radiation and other signal sources. Therefore, scientists are looking for workarounds — they create laboratory systems in which the same mathematical principles operate, but quantum effects become available for observation.

Why do physicists create "toy" black holes?

At first glance, it may seem that laboratory analogues have little in common with real space objects. However, in modern physics, this approach has long become a familiar research tool. If it is impossible to conduct an experiment with a real black hole, scientists create a system that obeys the same equations. Such models are called analog gravity systems, and they help test hypotheses that cannot be tested directly.

In recent years, researchers have already created analogues of black holes in liquid streams, supercooled Bose—Einstein condensates, acoustic systems, and optical media. Each such experiment allows us to test individual elements of Hawking theory or other models of quantum gravity. The new work was another step in this direction, as for the first time it allowed us to study the effect of radiation itself on the system that generates it.

The authors of the study emphasize that such installations will not replace observations of real black holes. However, they are the ones that allow us to experimentally test ideas that have existed for decades exclusively in the form of mathematical models. If such research continues to develop successfully, it can bring physicists closer to one of the main goals of modern science — the creation of a unified theory that combines quantum mechanics and Einstein's general theory of relativity and explains how the universe works at the most fundamental levels.