One thing is prediction and another is experimental discovery.

Black holes were originally predicted by Karl Schwarzschild in 1916 as a solution to Einstein’s equations of general relativity.

In 1916, Karl Schwarzschild discovered the first exact solution of general relativity that can be used to characterize black holes, but David Finkelstein published the Schwarzschild solution for the first time in 1958. For an explanation of an inescapable space region. Black holes have long been considered a mere mathematical curiosity. In the 1960s, theoretical work showed that this was a general prediction of general relativity. The discovery of neutron stars by Jocelyn Bell Burnell in 1967 sparked interest in the possibility that compact objects formed by gravitational collapse could be entities in astrophysics.

The black hole itself is a celestial body with very strong gravitation. To trace the history of black holes, we must trace back to people’s understanding of gravity.

The discovery of gravitation dates back to the 17th century, when Newton sat under an apple tree and saw apples fall, which inspired him. Not only was he aware of this phenomenon, but he wrote down the formula, which is the formula of universal gravitation for gravity. On this basis, Newton obtained the well-known escape velocity formula.

After that, more scientists made further applications and promotions based on Newton’s theory. In 18th-century France, the mathematician Laplace imagined that there might be a celestial body in the universe that is so dense that no light from itself can escape from its surroundings. Now it seems that this celestial body is the simplest idea of a black hole.

In the 19th century, observations by more scientists posed some challenges to Newton’s theory. At the beginning of the 20th century, the physicist Einstein first proposed the special theory of relativity, and ten years later he proposed the general theory of relativity, which put forward a subversive understanding of gravity, such as thinking that the gravitational field is not actually caused by mass, It is the effect after space-time is bent by mass and objects with mass.

A few months after Einstein proposed the general theory of relativity in 1915, the German physicist Schwarzschild obtained the exact solution of Einstein’s field equations, which is the “Schwarzschild solution without rotating black hole”. In the decades after Schwarzschild got the solution in 1916, the progress of black hole research was actually very slow.

In the late 1930s, Oppenheimer, the father of the atomic bomb in the United States, and his students came to the theory that it is possible for a star to collapse into a dense singularity as it dies and collapses, and deduced the lower limit is about 3.2 solar masses.

When the time entered the 1960s, the research on black holes ushered in two breakthroughs: In 1963, the New Zealand mathematician Roy Kerr obtained the Einstein field equation accurately for the first time by means of mathematical solutions Exact solution with rotating black holes. In 1964, the first stellar-level black hole was discovered by observation. It is the simultaneous breakthrough of theory and observation that brought the field of black hole research into its golden age. In the next 20 to 30 years, a large number of astronomers and physicists devoted themselves to this field. The knowledge about black holes that people now know is basically obtained during this period.

During this period, there was a well-known relativistic physics master—John Wheeler, a professor at Princeton University. He not only had excellent academic research, but also did a lot of work in science communication. The name black hole became known to everyone after his promotion. In addition, the term wormhole was also proposed by him.

After Wheeler, Hawking further discovered the so-called Hawking radiation, which changed the previous understanding of black holes in classical general relativity.

So far, scientists have discovered a lot of black holes, and they can be broken down into three categories by mass:

One category is a stellar black hole, which means its mass can range from 3 times the mass of the sun to 100 times the mass of the sun.

The second type is called supermassive black holes, whose mass starts from hundreds of thousands of times the sun’s mass, or millions of times the sun’s mass, to billions or even tens of billions of times the sun’s mass. This type of black hole in between is called an intermediate-mass black hole. But for medium-mass black holes, there is very little direct evidence from observations, but theoretical research proves that they should exist, so searching for medium-mass black holes is also a hot topic of current research.

For black holes, it can be said to be the most amazing and simplest type of celestial body in the universe. For a black hole, only three physical quantities are needed to describe it, one is its mass, one is its rotation, and the other is its charge.

In the universe, gas almost always exists in a plasma state, and there will be a lot of free charges. If a black hole is charged, it is easy to attract charged particles around it to achieve electrical balance. So in the end there are only two physical quantities left, one mass and one rotation. At this time, the so-called Kerr metric can be used to fully describe black holes in astrophysics. The main task of scientists is to measure these two basic quantities of black holes.

In the Milky Way, according to theory, there should be hundreds of millions of black holes on the order of stars. But it is a pity that humans have only detected dozens of them so far, and only less than 20 stellar-level black holes have very accurate mass measurements. Other nearly hundreds of millions of black holes have not been detected yet.

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