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Xinfan Technology reported on August 27th, Beijing time, among all astronomical concepts, black holes may be the most peculiar. The density of black holes is extreme, even light cannot escape, like a terrifying absorber of darkness. Because ordinary physical laws do not apply in black holes, they seem almost made for science fiction. However, numerous direct and indirect evidence suggests that black holes truly exist in the universe.

Einstein's Prophecy

Black holes are a necessary result of Einstein's general relativity.

German astronomer Karl Schwarzschild was the first to predict the existence of black holes in 1916, believing it to be a necessary result of Einstein's general relativity. In other words, if Einstein's theory is correct (all evidence points to this), then black holes must exist. The research by Roger Penrose and Stephen Hawking further solidified the theoretical foundation for the existence of black holes. Their research showed that any star collapsing into a black hole will form a singularity, and traditional laws of physics completely break down at this point.

Gamma-ray Burst

Earth's observation equipment has already detected some gamma-ray bursts generated during the birth of black holes.

In the 1930s, Indian astrophysicist Subrahmanyan Chandrasekhar studied the end of stellar nuclear fuel. He found that the ultimate result depends on the star's mass. If the star is massive, say 20 times the mass of the sun, the dense core of the star (whose mass alone can be two to three times that of the sun) will continue to collapse until it becomes a pure black hole. The collapse speed of the star's core is extremely fast, taking only a few seconds, during which an immense amount of energy is released as gamma-ray bursts, equivalent to the total energy released by an ordinary star over its lifetime. Earth's telescopes have detected multiple gamma-ray bursts, some even from galaxies billions of light years away, indicating that we are truly observing the process of black hole formation.

Gravitational Waves

The image shows an artist's impression of gravitational waves. The gravitational interactions between two black holes create ripples in space-time, expanding outward as gravitational waves.

Black holes are not always solitary; sometimes they appear in pairs, orbiting each other. The gravitational interactions between two black holes create ripples in space-time, expanding outward as gravitational waves, another prophecy made by Einstein's theory of relativity. With the help of observatories like LIGO and Virgo, we now have the capability to detect gravitational waves. In 2016, scientists announced the first discovery of gravitational waves produced by the merger of two black holes. Since then, we have detected multiple gravitational wave events. With the increasing sensitivity of detectors, scientists have also detected gravitational waves produced by events other than black hole mergers, such as the interaction between black holes and neutron stars.

Invisible Companion Star

The image shows an artist's rendering of several celestial bodies in the HR6819 star system.

Events capable of producing gamma-ray bursts or gravitational waves occur over a short time frame and can be seen from half a universe away. However, considering their nature, most black holes go undetected. Black holes do not emit any light or radiation, allowing them to quietly lurk in the universe, unbeknownst to astronomers. However, there is a method to detect their presence: using the gravitational effects caused by black holes on other stars. In 2020, while observing the seemingly ordinary star system HR6819, astronomers noticed unusual orbital trajectories of the two stars, leading them to conclude that there must be an entirely invisible celestial body in the system. After calculating its mass, researchers realized there could only be one truth: this celestial body must be a black hole, located just a thousand light-years away from Earth, within the Milky Way Galaxy, making it the closest black hole discovered so far.

X-ray

The black hole Cygnus X-1 is devouring the giant blue companion star next to it.

In 1971, while studying a stellar system in the Milky Way Galaxy named Cygnus X-1, scientists observed the first evidence of a black hole's existence. The X-rays emitted by the system are extremely bright, but these rays do not originate from the black hole or its visible companion star; instead, they come from the accretion disk formed as the black hole devours stellar material. Like the previously mentioned HR6819 star system, astronomers can also use the orbital trajectory of the visible star to estimate the mass of the invisible celestial body in the Cygnus X-1 system. The final calculated result is about 21 times the mass of the sun, and considering the small space it occupies, it can only be a black hole, with no need to consider other possibilities.

Supermassive Black Holes

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

Aside from black holes formed by stellar collapse, there is evidence indicating that the centers of galaxies may harbor supermassive black holes, ranging from millions to even billions of times the mass of the sun. They may have existed since the early universe. In so-called "active galaxies," the evidence of these supermassive black holes is nothing short of spectacular. NASA notes that these black holes are surrounded by accretion disks emitting extremely intense radiation across various spectra. The center of the Milky Way Galaxy also contains a black hole because the rotation speeds of stars in that region are found to be astonishing, reaching up to 8% of the speed of light, suggesting that they are orbiting a very small, but extraordinarily massive celestial body. Current estimates suggest the central black hole in the Milky Way Galaxy has a mass approximately 4 million times that of the sun.