If the gravitational field of a mass of matter is strong enough to completely bend the space-time around it, so that nothing, even light, can escape, then the mass of matter is called a black hole. Not much matter is compressed to a very high density (for example, the earth is compressed to the size of a pea), or the density of very heavy matter is lower (for example, millions of times the mass of the sun is distributed in a ball with the same diameter as the solar system, and the density is roughly water).

The first person who suggested that there might be a "black hole" whose gravity is too strong to escape was john mitchell, a special member of the Royal Society. He stated this view to the Royal Society in 1783. Mitchell's calculation is based on Newton's gravity theory and particle theory of light. The former was the best theory of gravity at that time. The latter imagines light as a stream of tiny particles (now called photons) like small shells. Mitchell believes that these light particles should be affected by gravity like any other object. Because Ole Romer accurately measured the speed of light as early as 100 years ago, Mitchell was able to calculate how big a celestial body with solar density must be to make its escape speed greater than the speed of light.

If such a celestial body exists, light cannot escape, so it should be black. The escape velocity on the surface of the sun is only 0.2% of the speed of light, but if we imagine a series of larger and larger celestial bodies with the same density as the sun, the escape velocity will increase rapidly. Mitchell pointed out that the escape speed of such a celestial body with a diameter of 500 times that of the sun (similar to the size of the solar system) should exceed the speed of light.

Pierre Laplace independently reached the same conclusion and published it in 1796. Mitchell pointed out in a prescient comment that although such celestial bodies are invisible,' if there happen to be any other luminous celestial bodies orbiting them, it is still possible for us to infer the existence of the central celestial body from the movement of these orbiting celestial bodies. In other words, Mitchell believes that if there is a black hole in a binary star, it will be most easily identified. However, this idea that there are black stars was forgotten in the19th century, and it was not until astronomers realized that black holes could be produced in another way that they were mentioned again when discussing Albert Einstein's general theory of relativity.

Karl schwarzschild, an astronomer who served on the Eastern Front during World War I, was one of the first people to analyze Einstein's theoretical conclusions. General relativity interprets gravity as the result of the bending of space-time near matter. Schwarzschild calculated a strict mathematical model of the space-time geometric characteristics around spherical objects and sent it to Einstein. The latter submitted them to the Prussian Academy of Sciences at the beginning of 19 16. These calculations show that "any" mass has a critical radius, now called schwarzschild radius, which corresponds to an extreme deformation of space-time, so if mass is squeezed within the critical radius, space will bend around the object and cut it off from the rest of the universe. It actually becomes an independent universe, anything (light).

For the sun, schwarzschild radius is one kilometer, and for the earth, it is equal to 0.88 cm. This does not mean that there is something called a black hole in the center of the sun or the earth (this term was first used by john wheeler in 1967). At this distance from the center of the celestial body, there is nothing abnormal in spacetime. Schwarzschild's calculations show that if the sun is squeezed into a sphere with a radius of 2.9 kilometers, or if the earth is squeezed into a sphere with a radius of only 0.88 cm, they will be isolated from the external universe forever in a black hole. Matter can still fall into such a black hole, but nothing can escape.

These conclusions have been regarded as pure mathematical treasures for decades, because no one thinks that real and real objects can collapse to the extreme density needed to form black holes. White dwarfs began to be known in the1920s, but even white dwarfs have roughly the same mass as the sun, but the same size as the earth, and their radius is far greater than 3km. People didn't realize in time that a black hole with the same essence as imagined by Mitchell and Laplace could be created if there were a lot of matter with ordinary density. The schwarzschild radius corresponding to an arbitrary mass m is given by the formula 2GM/c2, where g is the gravitational constant. C is the speed of light.

In the1930s, Braman Chandrasekhar proved that even a white dwarf is stable only when its mass is less than 1.4 times that of the sun, and any death star heavier than this will further collapse. Some researchers think this may lead to the possibility of forming neutron stars. The typical radius of a neutron star is only about 1/700 of that of a white dwarf, which is several kilometers. However, this idea was not widely accepted until1mid-1960s when pulsars were discovered, which proved the existence of neutron stars.

This rekindled people's interest in black hole theory, because neutron stars are almost becoming black holes. Although it is hard to imagine compressing the sun to a radius of less than 2.9 kilometers, it is now known that there are neutron stars with the same mass as the sun and a radius of less than 10 kilometers, and neutron stars are only one step away from black holes.

Theoretical research shows that the behavior of a black hole is only defined by its three characteristics-mass, charge and rotation (angular momentum). A black hole without charge and rotation is described by Schwarzschild solution of Einstein equation. A charged black hole that does not rotate is described by Reisner-Nordstrom solution. A black hole without charge and rotation is described by Kerr solution. The black hole with charge and rotation is described by Kerr-Newman solution. Black holes have no other characteristics, which has been summarized by the famous saying' black holes are hairless'. Real black holes should rotate without charge, so Kerr solution is the most interesting.

It is now believed that both black holes and neutron stars are produced in the dying struggle of supernova explosion of epitaxial stars. The calculation shows that any dense supernova remnant whose mass is less than 3 times that of the sun (Oppenheimer-Vokov limit) can form a stable neutron star, but any dense supernova remnant whose mass is greater than this limit will collapse into a black hole, and its contents will be pressed into the singularity in the center of the black hole, which is the mirror inversion of the singularity of the Big Bang where the universe was born. If such a celestial body happens to be in the orbit around an ordinary star, it will deprive the companion star of its material and form an accretion disk composed of hot matter collected in the black hole. The temperature in the accretion disk can be high enough to radiate X-rays, so that black holes can be detected.

1In the early 1970s, Mitchell's prediction was echoed: such a celestial body was found in a binary system. An X-ray source called Cygnus X- 1 proved to be the star HDE226868. The orbital dynamics characteristics of this system show that the X-rays from this source come from an object smaller than the Earth in the orbit around the visible star, but the mass of the source is greater than the Oppenheimer-volkov limit. This can only be a black hole. Since then, several other black holes have been discovered in the same way. 1994, Cygnus V404 became the best black hole' candidate' so far, which is a system in which a star with 70% mass of the sun moves around an X-ray source with 0/2 times mass of the sun/kloc-. However, these recognized black holes are probably just the tip of the iceberg.

Mitchell realized that such a "stellar mass" black hole can only be detected in a binary system. An isolated black hole is worthy of its name-it is dark and undetectable. However, according to the theory of astrophysics, many stars should eventually be neutron stars or black holes. In fact, observers have detected almost as many suitable black hole candidates in the binary system as they found in the pulsed binary system, which means that the number of isolated stellar mass black holes should be the same as that of isolated pulsars, which is supported by theoretical calculation. There are now about 500 active pulsars in our galaxy. However, the theory shows that pulsars, as radio sources, have a short active period and soon collapse into an undetectable quiet state. Therefore, there should be more' dead' pulsars (quiet neutron stars) around us. Our Milky Way Thumb contains10 billion bright stars, which have existed for billions of years. The best estimate is that our galactic finger contains 400 million dead pulsars today, and even a conservative estimate of the number of stellar mass black holes reaches this number? -1 billion. If there are so many black holes, and the black holes are irregularly scattered in the Milky Way, then the nearest black hole is only 15 light years away. Since there is nothing unique about our galaxy, other galaxies in the universe should contain the same number of black holes. integrated circuit

Galaxies may also contain something very similar to the "black star" originally conceived by Mitchell's Laplace. This kind of celestial body is now called' supermassive black hole' and is thought to exist in the center of active galaxies and quasars. The gravitational energy provided by them may explain the huge energy source of these celestial bodies. A black hole the size of the solar system and millions of times the mass of the sun can devour one or two stars around it every year. In this process, a large part of the star's mass will be converted into energy according to Einstein's division of labor E=mc2. Quiet supermassive black holes may exist in the center of all galaxies, including our Milky Way.

1994, using the Hubble Space Telescope, a hot material disk with the size of 15 million parsec was found in the galaxy M87, which is 0/5 million parsec away from our Milky Way. It moves around the center of the galaxy at a speed of about 2 million kilometers per hour (about 5* 10-7 5 times 15). From the central' engine' of M87, a gas jet with a length exceeding 65,438+0 kiloparsec was ejected. The orbital velocity in the central accretion disk of M87 proves conclusively that it is under the gravitational control of a supermassive black hole with a mass of 3 billion times that of the sun, and the jet can be interpreted as energy gushing from a polar region of the accretion system.

Also in 1994, astronomers from Oxford University and Kiel University discovered a stellar mass black hole in a binary system called Cygnus V404. We have pointed out that the orbital parameters of the system enable them to accurately' weigh' black holes, and come to the conclusion that the mass of black holes is about 12 times that of the sun, while the mass of ordinary stars moving around it is only about 70% of that of the sun. This is by far the most accurate measurement of the mass of black stars, and therefore the best and only proof of the existence of black holes.

Some people speculate that a large number of miniature black holes or primitive black holes may have been produced in the Big Bang, which provided a considerable part of the mass of the universe. The typical size of this miniature black hole is equivalent to an atom, and its mass is about 65438+ billion tons (10-1,10,1kg). There is no evidence that such celestial bodies do exist, but it is difficult to prove that they do not exist.