Black hole "black hole" can easily be imagined as "big black hole", but it is not. The so-called "black hole" is such a celestial body: its gravitational field is so strong that even light cannot escape. According to the general theory of relativity, the gravitational field will bend space-time. When the star is large, its gravitational field has little influence on time and space, and the light emitted from a certain point on the surface of the star can be emitted in any direction in a straight line. The smaller the radius of the star, the greater the bending effect on the surrounding space-time, and the light emitted at some angles will return to the surface of the star along the curved space. When the radius of a star is as small as a certain value (called "schwarzschild radius" in astronomy), even the light emitted from the vertical plane is captured. At this time, the star becomes a black hole. To call it "black" means that it is like a bottomless pit in the universe. Once anything falls in, it seems that it can't escape. In fact, black holes are really invisible, which we will talk about later. So, how are black holes formed? In fact, like white dwarfs and neutron stars, black holes probably evolved from stars. We have introduced the formation process of white dwarfs and neutron stars in detail. When a star ages, its thermonuclear reaction has exhausted the fuel (hydrogen) in the center, and the energy generated by the center is running out. In this way, it no longer has enough strength to bear the huge weight of the shell. Therefore, under the heavy pressure of the shell, the core began to collapse, until finally a small and dense star was formed, which could balance the pressure again. Stars with smaller mass mainly evolve into white dwarfs, while stars with larger mass may form neutron stars. According to scientists' calculations, the total mass of neutron stars cannot be more than three times that of the sun. If it exceeds this value, there will be no force to compete with its own gravity, which will lead to another big collapse. This time, according to scientists' guesses, matter will move relentlessly to the center point until it becomes a "point" with zero volume and infinite density. And once its radius shrinks to a certain extent (schwarzschild radius), as we mentioned above, the huge gravity makes it impossible to shoot out even light, thus cutting off all the connection between the star and the outside world-a "black hole" is born. Compared with other celestial bodies, black holes are too special. For example, a black hole is invisible, so people can't directly observe it, and even scientists can only make various guesses about its internal structure. So how does a black hole hide itself? The answer is-bending space. As we all know, light travels in a straight line. This is a basic common sense. But according to the general theory of relativity, space will bend under the action of gravitational field. At this time, although the light still propagates along the shortest distance between any two points, it is not a straight line, but a curve. Figuratively speaking, it seems that light should go straight ahead, but strong gravity pulls it away from its original direction. On earth, because the gravitational field is very small, this bending is very small. Around the black hole, this space deformation is very large. In this way, even if the light emitted by the star is blocked by the black hole, although part of it will fall into the black hole and disappear, the other part will bypass the black hole in the curved space and reach the earth. So we can easily observe the starry sky on the back of the black hole, just as the black hole does not exist. This is the invisibility of black holes. More interestingly, some stars not only send light energy directly to the earth, but also send light in other directions, which may be refracted by the strong gravity of nearby black holes and reach the earth. In this way, we can see not only the "face" of this star, but also its side and even its back! Black holes with white holes are like bottomless abyss in the universe. Once a substance falls in, it can never escape. According to the familiar viewpoint of "contradiction theory", scientists boldly speculate whether there will be a "spring" in the universe in which matter can only enter. And gave it a name relative to the black hole, called "white hole". Scientists suspect that the white hole also has a closed boundary similar to a black hole, but unlike a black hole, the matter and radiation inside the white hole can only move outside the boundary, while the matter and radiation outside the white hole cannot enter its interior. Figuratively speaking, the white hole seems to be a source of constantly spraying matter and energy. It provides matter and energy to the outside world, but does not absorb it. So far, the white hole is only a guess of scientists, and no evidence of its possible existence has been observed. There is no major breakthrough in theoretical research. However, the latest research may come to an exciting conclusion that the "white hole" is probably the "black hole" itself! In other words, a black hole absorbs matter at this end and ejects matter at the other end, just like a huge time tunnel. Scientists have recently proved that it is actually possible for black holes to emit energy. According to the theory of modern physics, energy and mass can be transformed into each other. This theoretically predicts the possibility of "black hole and white hole fusion". It is too early to fully understand the mysteries of black holes and white holes. However, every time scientists make a little progress, their achievements are exciting. We believe that the key to the mystery of the universe lies behind the mystery of black holes and white holes. Wormholes pass through imaginary channels of space-time structures. Wormholes can be imagined as a shortcut through time and space, that is, a cosmic tunnel connecting two black holes or (more speculatively) a black hole and a white hole. The' other end' of the wormhole can be anywhere in space or at any time, so that any object passing through the wormhole can appear in other parts of the universe in an instant-not only in another place, but also at another moment. The solution of the general relativity equation describing the wormhole was actually discovered in 19 16 years shortly after the theory was put forward, but it was not explained at that time. Albert Einstein himself cooperated with nathan rosen in Princeton in 1930, and found that the black hole represented by Schwarzschild's solution is actually a bridge between two flat space-time regions (now called Einstein-Rosen Bridge). Although these equations are studied as mathematical masterpieces (especially the work of john wheeler and his colleagues), no one regards them as the real characteristics of the universe before 1985, because all the examples studied in mathematics can only be opened for a short moment, and then slammed again before anything including light passes through the tunnel (according to the equations). Although this idea is loved by science fiction writers, scientists generally believe that there must be some natural law that prevents the existence of wormholes. However, when the relevant scholars of California Institute of Technology tried to prove this in the1980s, they found it impossible. There is nothing in general relativity (this is the best theory of gravity and space-time that we have, and it has passed all the tests on it) that prohibits the existence of wormholes. Not only that, Kip Sonny and his colleagues also found that Einstein's equation even has a solution that allows long-lived wormholes. The mouth of this wormhole should look like the horizon of a spherical black hole, but there is an important difference. The horizon is a one-way surface, from which nothing can be seen. But the surface of the wormhole allows two-way traffic. If we look at a wormhole near Vega at the other end, we will see Vega's light coming out of the tunnel-observers near Vega will also see sunlight when they look at the same wormhole from the other end. But it is still extremely difficult to build a large wormhole that humans can cross (see time travel), and it may be impossible for all practical purposes. But physicists are interested in the possibility of natural wormholes with Planck length. This wormhole provides a basic foam-like space-time structure, and the space-time fabric itself is woven from wormhole fibers (using a mixed metaphor similar to poetic rhetoric). If so, there will be many strange possibilities. For example, this tiny (ultra-submicroscopic) wormhole can connect distant regions in the universe, so that information can be leaked, thus ensuring that the physical laws on the earth are the same as those on distant quasars. Or a small wormhole is disconnected from our universe and begins to grow into an independent wormhole through skyrocketing. It is also called Einstein-Rosen Bridge. The explanation of "A Brief History of Time" is: "A wormhole is a thin tube of time and space connecting distant regions of the universe. It can also connect parallel universes or baby universes and provide the possibility of time travel. " . Theoretically, it can be inferred that it exists, but the solution of the equation is very vague, so it is impossible to infer a large wormhole. Because wormholes need a lot of negative energy to maintain, the only thing in the universe that can hopefully produce wormholes is Hawking radiation from black holes (see the link I gave). Therefore, wormholes are considered as the link between black holes and white holes.
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