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Xinfan Technology News Leakage from Beijing time on August 27, among all astronomical concepts, black holes might be the most peculiar. The density of a black hole is extremely high, even light cannot escape, like a gigantic ominous net. Since ordinary physical laws are not applicable in black holes, they seem tailor-made for science fiction novels. However, abundant direct and indirect evidence suggests that black holes indeed exist in the universe.

Einstein's Prophecy

Black holes are a necessary result of Einstein's General Relativity.

The German astronomer Karl Schwarzschild first predicted the existence of black holes in 1916, considering it a necessary result of Einstein's General Relativity. In other words, if Einstein's theory is correct (which all evidence points to), then black holes must exist. The research by Roger Penrose and Stephen Hawking further consolidated the theoretical foundation for the existence of black holes. Their studies showed that any celestial body collapsing into a black hole would form a singularity, and all traditional physics laws would fail at this point.

Gamma-Ray Bursts

Ground-based observational equipment has detected some gamma-ray bursts produced during black hole formation.

In the 1930s, Indian physicist Subrahmanyan Chandrasekhar conducted research on the fate of the core fuel exhaustion of stars. He discovered that the final result depends on the star's mass. If the star is massive enough, say its mass reaches 20 times that of the Sun, its dense core (which alone has a mass two to three times the Sun's) will continue to collapse, eventually forming a black hole. The collapse speed of the stellar core is exceedingly rapid, taking only a few seconds, during which it releases tremendous energy in the form of gamma-ray bursts, equivalent to the total energy released by an ordinary star throughout its life. Ground-based telescopes have detected multiple gamma-ray bursts, with some even originating from galaxies billions of light-years away, indicating that we have indeed observed the process of black hole birth.

Gravitational Waves

Pictured is an artist's concept of a gravitational wave. The gravitational interaction between two black holes creates ripples in time and space, radiating outward as gravitational waves.

Black holes aren't always solitary; sometimes they appear in pairs, rotating around each other. The gravitational interaction between two black holes creates ripples in time and space, radiating outward as gravitational waves, which is another prophecy put forward by Einstein's theory of relativity. With the help of observatories like LIGO and Virgo, we now have the ability to detect gravitational waves. In 2016, scientists announced for the first time the discovery of gravitational waves generated by the merger of two black holes. Since then, we've detected multiple gravitational wave events. As the sensitivity of detectors continuously improves, scientists have also detected gravitational waves generated by events other than black hole mergers, such as mergers between black holes and neutron stars.

Hidden Companion Star

Pictured is an imagined diagram of the trajectory of celestial bodies in the triple star system HR6819.

Those gamma-ray bursts or gravitational waves capable of producing are energetic events occurring in short timeframes, visible perhaps halfway across the cosmos. But considering their intrinsic nature, most black holes remain undetected. Black holes don't emit any light or radiation, so they can quietly lurk in the cosmos, with astronomers unaware of their existence. However, there's a way to detect their presence: by using the gravitational effect black holes have on other stars. In 2020, when astronomers observed the seemingly ordinary star system HR6819, they noticed that the orbital paths of the two stars had some peculiarities. Unless there's a completely invisible celestial body in that system, this phenomenon cannot be explained. Upon calculating its mass, the researchers realized the truth can only be one: this celestial body must be a black hole. It is located just a thousand light-years from Earth, within the Milky Way Galaxy, making it the closest black hole discovered to Earth so far. 𝒇𝙧𝙚𝓮𝙬𝙚𝓫𝒏𝓸𝓿𝓮𝒍.𝓬𝙤𝓶

X-ray Emissions

The black hole Cygnus X-1 is currently devouring its massive blue companion star.

In 1971, when scientists were studying a stellar system in the Milky Way called Cygnus X-1, they first observed evidence for the existence of a black hole. The system's X-ray emissions were extraordinarily bright, but these emissions don't come from the black hole itself or its visible companion star, but are generated by the accretion disk formed as the black hole siphons material from the star. Like the star system HR6819 mentioned earlier, astronomers can also estimate the mass of the hidden celestial object in the Cygnus X-1 system using the orbital path of its visible star. The final calculation result is approximately 21 times the mass of the Sun, considering that this celestial object occupies a relatively small space, indicating that it can only be a black hole, without considering other possibilities.

Supermassive Black Holes

There's also a supermassive black hole at the center of the Milky Way Galaxy.

Besides black holes formed from the collapse of stars, evidence shows that there might also be supermassive black holes lurking at the centers of galaxies, with masses reaching millions or even billions of times that of the Sun, possibly existing since the early universe. In so-called "active galaxies," evidence of these supermassive black holes is spectacular. NASA noted that there are accretion disks surrounding black holes in the centers of these galaxies, releasing extremely intense radiation across all wavelengths. Additionally, there's a black hole at the center of the Milky Way Galaxy, as the rotational speeds of stars in that region are so astonishingly fast, reaching 8% of the speed of light, indicating that they must be orbiting around some extremely massive yet compact celestial object. The current estimated result suggests that the black hole at the center of the Milky Way Galaxy is about 4 million times the mass of the Sun.