Black holes have a closed boundary, and light and any other matter can't jump out of this boundary. This boundary is the horizon of the black hole. According to the general theory of relativity, in the gravitational collapse process of spherical symmetry, as long as the mass of the collapsed core is large enough, it must collapse into a black hole. Once a black hole is formed, it will collapse until the singularity.
Since 1960s, Penrose and others have introduced the method of global differential geometry, which greatly promoted the theoretical study of black holes and gravitational collapse. At the end of 1960s, Penrose put forward the "cosmic information test hypothesis", which holds that singularities can only appear in black holes, or gravitational collapse can't form naked singularity, and people outside black holes can't see it. Although this conjecture is well founded, it has not been strictly proved so far.
After the supernova explosion, if the mass exceeds 2.4 solar masses, the equilibrium state will no longer exist, the star will shrink indefinitely, the radius of the star will become smaller and smaller, and the density will become larger and larger. Finally, it will become an infinitesimal singularity with infinite density and disappear from people's sight. Around this singularity is an area that cannot be returned. The boundary of this area is called "horizon" or "event horizon", and the radius of this area is called "schwarzschild radius". Any matter entering this region, including light, can't escape the great gravity of this singularity. They are like falling into a bottomless pit, just like a dark bottomless pit, so they are called "black holes". When a black hole approaches a celestial body, it will suck away some of the material of the celestial body. The attracted matter rotates in a spiral shape, and atomic particles will fall from the edge of the black hole to the center along the spiral shape, and the speed will be faster and faster until it reaches more than 900 kilometers per second. When an object is swallowed by a black hole, the temperature will rise to several million degrees due to collision, and X-rays and γ-rays will be emitted. In the universe, only black holes can accelerate objects to such a high speed in dense orbits; Only black holes emit χ rays and γ rays in this way. When detected by astronomers, the gravitational field can be outlined and black holes can be found. 1996, astronomers discovered a huge black hole in the center of the Milky Way. It orbits the center of the Milky Way at a speed of 200 kilometers per second. The closer to the center, the faster the speed, and the radio source in the center has great energy, but the volume is small.
In order to understand how black holes are formed, we first need to understand the life cycle of stars. When a star enters old age and runs out of fuel, it begins to cool and contract. 1928, an Indian graduate student, Saramani Ann Chandraseka, came to Cambridge, England by boat, and studied under the British astronomer Sir Arthur Eddington (a general relativist). On his journey from India to Britain, he calculated how capable a star can continue to resist its own gravity and maintain itself after running out of fuel. The idea is that when stars get smaller, matter particles get very close together, and according to Pauli's exclusion principle, they must have very different speeds to disperse them from each other. Reach a balance and keep its radius constant, just as gravity is balanced by heat in the early life. The maximum speed of particles is limited to the speed of light by relativity. This means that when the star becomes dense enough, the repulsive force caused by the incompatibility principle will be less than gravity. Chandraseka calculated that a star about 1.44 times the mass of the sun does not support itself against its own gravity. (This mass is now called the strong Draseka limit. ) The Soviet scientist Lev Davidovic Landau made a similar discovery almost at the same time. Landau pointed out that there is another possible final state of the star. Its final mass is about one or two times that of the sun, but its volume is even much smaller than that of a white dwarf. These stars are supported by the repulsive force of neutron and proton incompatibility principle, not the repulsive force between electrons. So they are called neutron stars. But what if you push it beyond the limit? Will it collapse to infinite density? Eddington was shocked by this and refused to believe Chandraseka's results. Eddington thinks that stars can't collapse into a point. This is the view of most scientists: Einstein himself wrote a paper announcing that the volume of stars will not shrink to zero. Eddington's hostility made Chandraseka give up this job and study other astronomical problems, such as the movement of star clusters. However, he won the 1983 Nobel Prize. According to general relativity, what happens to a star whose mass is 1.44 times that of the sun? This problem was first solved by a young American Robert Oppenheimer in 1939. But the results he got showed that there would be no results when observing with a telescope at that time. Later, due to the interference of World War II, Oppenheimer himself participated in the atomic bomb plan very closely. But in the 1960s, the application of modern technology greatly increased the scope and quantity of astronomical observation, which once again aroused people's interest. Oppenheimer got a picture that the gravitational field of a star changed the path of light, which is different from the path without a star. A cone of light is an orbit that represents the propagation of light in time and space after it is emitted from its top. The cone of light deflects slightly inward near the surface of the star, which can be observed by observing the light emitted by distant stars during the solar eclipse. When a star contracts, the gravitational field on its surface becomes very strong, and the light deflects inward more, which makes it more difficult for the light to escape from the star. For a distant observer, the light becomes darker and redder. Finally, when the star shrinks to a critical radius, the gravitational field on the surface becomes so strong that the light cone deflects inward that light can no longer escape. According to relativity, nothing can travel faster than light. In this way, light can't escape, and other things can't escape, and they will be pulled back by gravity. That is to say, there is a set or space of events-time region, from which it is impossible for light or anything to escape and reach distant observers. Now we call this area a black hole, and its boundary is called the event horizon, which coincides with the trajectory of light escaping from the black hole.
In 197 1 year, john archibald wheeler named such things "black holes" because light could not escape from them. Based on a large amount of evidence, astronomers have many candidate celestial bodies that they think may be black holes (the evidence is that their huge mass can be obtained from their interaction with other celestial bodies; Sometimes they emit X-rays, which are thought to be emitted by substances falling into them.
General relativity predicts that moving heavy objects will lead to the radiation of gravitational waves.
Israel's results only deal with black holes formed by non-rotating objects. 1963, Roy Kerr, a New Zealander, discovered a series of solutions to the general relativity equation describing a rotating black hole. These kerr black holes rotate at a constant speed, and their size and shape only depend on their mass and rotation speed. If the rotation is zero, the black hole is a perfect sphere, and the solution is the same as above. If there is rotation, the black hole bulges near the equator (just like the sun bulges due to rotation), and the faster the rotation, the more. It is speculated that if israel's results are extended to include rotating bodies, then any rotating body will eventually be in the static state described by Kerr solution after it collapses to form a black hole. After gravitational collapse, the black hole will eventually evolve into a state that can rotate but cannot jump. Its size and shape only depend on its mass and rotation speed, and have nothing to do with the nature of the original object that collapsed into a black hole. The result is the well-known proverb: "A black hole has no hair." Hairless theorem is of great practical importance because it greatly limits the possible types of black holes.
Black hole is one of the rarest cases in the history of science. It has developed into a very detailed mathematical model without any observational evidence to prove its correctness. In fact, this is often the main argument against black holes: how can you trust an object based only on calculations based on suspicious general relativity? Astronomers have observed some binary systems in which only one visible star orbits another invisible companion star. Of course, people can't immediately conclude that this companion star is a black hole-it may just be a star that is too dark to see. However, one called Swan X- 1 also happens to be a strong X-ray source. The best explanation for this phenomenon is that when matter is blown up from the surface of a visible star and falls to an invisible companion star, it develops into a spiral orbit (similar to the water flowing out of a bathtub), becomes very hot and emits X-rays. For this mechanism to work, invisible objects must be very small, such as white dwarfs, neutron stars or black holes. By observing the orbits of visible stars, people can calculate the minimum possible mass of invisible objects. Take Swan X- 1 as an example. The mass of this invisible star is about six times that of the sun. According to the results of Chandraseka, it is too massive to be a white dwarf or neutron star, but only a black hole.
Now, in our galaxy and two neighboring galaxies named Magellanic Cloud, there are several evidences of black holes similar to Swan X- 1. However, it is almost certain that the number of black holes is far more than this! In the long history of the universe, many stars should have run out of nuclear fuel and collapsed. The number of black holes is even much larger than the number of visible stars. In our galaxy alone, there are about 1000 billion visible stars. The extra gravity of such a huge number of black holes can explain why our galaxy has such a rotation rate at present, and the mass of visible stars alone is not enough. We also have some evidence that there is a bigger black hole in the center of our galaxy, and its mass is about 654.38+ million times that of the sun. If a star in a galaxy is very close to a black hole, the gravitational difference or tidal force acting on its near and far ends will tear it apart, and their debris and gas thrown by other stars will fall into the black hole. Just like Swan X- 1, the gas will move in a spiral orbit and be heated. Although it is not as hot as Swan X- 1 to emit X-rays, it can be used to describe the very dense radio and infrared source observed in the center of galaxies.
People think that there are similar black holes in the center of quasars, but their mass is about 1 100 million times that of the sun. The matter falling into this overweight black hole can only provide enough energy to explain the huge energy released by these objects. When matter spins into a black hole, it will make the black hole rotate in the same direction, making the black hole produce a magnetic field similar to that on the earth. Falling matter will produce very high-energy particles near the black hole. The magnetic field is so strong that these particles are focused into jets, which are ejected along the rotation axis of the black hole, that is, its north pole and south pole. This jet is indeed observed in many galaxies and quasars.
People can also consider the possibility of black holes with much smaller mass than the sun. Because their mass is below the Chandraseka limit, they can't be caused by gravitational collapse: such a small star can support itself against gravity even after running out of nuclear fuel. Only when matter is compressed into an extremely dense state by very great pressure can this small black hole be formed. A huge hydrogen bomb can provide such conditions. It is more realistic that such a small black hole will be produced under the conditions of high temperature and high pressure in the very early universe. Whether the randomness leading to the formation of stars and galaxies leads to the formation of a considerable number of "primitive" black holes depends on the details of conditions in the early universe. Therefore, if we can determine how many primitive black holes there are now, we can learn a lot about the early universe.
If the light from the event horizon (that is, the boundary of the black hole) can never get close to each other, then the area of the event horizon can remain the same or increase with time, but it will never decrease. In fact, as long as matter or radiation falls into a black hole, this area will increase; Or two black holes collide and merge into a black hole, and the horizon area of the last black hole will be greater than or equal to the sum of the horizon areas of the original black hole. The non-decreasing nature of the event horizon region imposes important restrictions on the possible behavior of black holes.
Near the black hole, there is a very easy way to violate the second law: just throw some objects with high entropy, such as a box of gas, into the black hole. The total entropy of objects outside the black hole will decrease. Of course, people can still say that the total entropy, including the entropy in the black hole, has not decreased-but because there is no way to see the inside of the black hole, we can't know what the entropy of the internal objects is. The discovery of the black hole area theorem (that is, as long as an object falls into a black hole, its horizon area will increase), a graduate student named Jacob Berkenstein of Princeton proposed that the horizon area is a measure of the entropy of a black hole. As the matter with entropy falls into a black hole, its horizon area will increase, so the sum of entropy and horizon area of matter outside the black hole will never decrease.
If a black hole has entropy, it should also have temperature. But an object with a specific temperature must emit radiation at a certain rate. This kind of radiation is necessary in order not to violate the second law of thermodynamics. So black holes are bound to emit radiation. According to the uncertainty principle of quantum mechanics, rotating black holes should produce and radiate particles. The particle spectrum of this radiation is only a spectrum of thermal body radiation. The black hole emits particles and radiation at an accurate rate only to prevent the second law from being violated, and its temperature only depends on the mass of the black hole-the greater the mass, the lower the temperature.
We know that nothing can escape from the horizon of a black hole. Why do black holes emit particles? The answer given by quantum theory is that particles do not come from inside the black hole, but from the "empty" space outside the event near the black hole! We can understand it this way: the space we think is "vacuum" can't be completely empty, because that will mean that all fields such as gravitational field and electromagnetic field must be exactly zero. The numerical value of the field and its time change rate are just like the particle position and velocity expressed by the uncertainty principle. The more accurate you know one quantity, the less accurate you know the other quantity. Therefore, in an empty space, the field cannot be strictly fixed to zero, because then it will have both an accurate value (zero) and an accurate rate of change (also zero). The value of the field must have a certain minimum inaccuracy or quantum fluctuation. People can understand these fluctuations as pairs of particles of light or gravity, which appear at the same time, leave each other, then approach each other and annihilate each other. These particle accelerators detect directly. However, their indirect impact can be measured. For example, small changes in the energy of electrons moving around atoms are so consistent with theoretical predictions that it is surprising. The uncertainty principle also predicts the existence of particle pairs similar to virtual matter, such as electron pairs and quark pairs. However, in this case, one member of a particle pair is a particle and the other member is an antiparticle (antiparticle and particle of light and gravity are exactly the same).
Because energy cannot be created out of nothing, one participant in a particle antiparticle pair has positive energy and the other has negative energy. Because under normal circumstances, real particles always have positive energy, and particles with negative energy are destined to be short-lived virtual particles. It must find its partner and destroy it. However, a real particle close to a massive object has less energy than when it is far away from the object, because it needs energy to push it away against the gravity of the object. Under normal circumstances, the energy of this particle is still positive. But the gravity of a black hole is so strong that even there, the energy of a real particle will be negative. Therefore, if there is a black hole, virtual particles with negative energy may fall into the black hole and become real particles or real antiparticles. In this case, it no longer needs to be annihilated with its partner, and the abandoned partner can also fall into the black hole. It with positive energy can also escape from the vicinity of a black hole as a real particle or a real antiparticle. To a distant observer, it looks like a particle emitted from a black hole. The smaller the black hole is, the shorter the distance that negative energy particles must travel before they become real particles, so the emissivity and apparent temperature of the black hole are also greater. The positive energy of radiation will be balanced by the negative energy particle flow falling into the black hole. According to Einstein's equation E=mc2(E is energy, m is mass and c is the speed of light), when energy and mass are lost, its event horizon area becomes smaller, but the entropy of the radiation it emits overcompensates the entropy reduction of the black hole, so the second law has never been violated.
Similarly, the smaller the mass of a black hole, the higher its temperature. In this way, when a black hole loses its mass, its temperature and emissivity increase, so its mass loses faster. It is not clear what will happen when the mass of the black hole eventually becomes extremely small. But the best guess is that it will eventually disappear in a huge launch explosion equivalent to millions of hydrogen bombs. A black hole with several times the mass of the sun has an absolute temperature of only one thousandth of a degree. This is much lower than the temperature of microwave radiation (about 2.7K) that fills the universe, so the radiation of this black hole is less than it absorbs. If the universe is destined to continue to expand forever, the temperature of microwave radiation will eventually be lower than that of this black hole and begin to lose mass. But even then, its temperature was very low, and it took 1 0 billion years (1followed by 66 O) to completely evaporate. This is much longer than the age of the universe. The age of the universe is only about 100 to 20 billion years (1 or 2 followed by 10 zeros). On the other hand, in the very early stage of the universe, there are very small primitive black holes formed by random collapse. Such a small black hole will have higher temperature and faster radiation speed. The life span of primordial black hole with an initial mass of 65.438 billion tons is roughly the same as the age of the universe. The original black hole with small initial mass should have evaporated, but the black hole with large initial mass is still emitting X-rays and gamma rays. These X-rays and gamma rays are like light waves, but the wavelength is much shorter. Such black holes hardly deserve the nickname of black: they are actually white-hot and release energy at a power of about 1 100 MW. Now we call it a white hole.
Because primitive black holes are very rare, it is unlikely that there is a black hole close enough for us to observe it as a single gamma ray source. But because gravity will pull primitive black holes closer to any matter, their density in and around galaxies should be much greater. Although the gamma ray background tells us that there can't be more than 300 primitive black holes per cubic light year, it doesn't tell us the density of primitive black holes in the Milky Way. For example, if their density is 65.438+billion times higher, then the nearest black hole may be about 65.438+billion kilometers away from us, or about as far as Pluto, the farthest planet known. At this distance, it is very difficult to re-examine the constant radiation of a black hole, even if its power is 1 MW. People must detect several gamma-ray quanta from the same direction within a reasonable time interval, such as one week, in order to observe an initial black hole. Otherwise, they may just be part of the background. Because the frequency of gamma rays is very high, we know from Planck's quantum mass principle that every gamma ray quantum has very high energy, so even if it emits 10,000 MW, it doesn't need many quanta. In order to observe so few particles from Pluto, you need a gamma-ray detector bigger than anything so far. And because gamma rays don't penetrate the atmosphere, this detector must be placed anywhere.
Attachment: wormhole
As a new concept, cavity has been put forward for more than 70 years. Shortly after Einstein put forward the general theory of relativity, physicists became interested in cavities. Large-scale cavity is a solution of Einstein's field equation of general relativity, which marks a geometric structure of space-time. In this structure, two regions of the universe are connected by a short and narrow "throat-like part". 1916 Karl Schwartzchildren solved Eines, but the Schwartzchildren cavity obtained from the field equation of general relativity has a dynamic structure. The cavity expands from zero radius to the maximum radius and then contracts back to zero. This process is very fast, even if it moves at the speed of light, it can't reach from one orifice to another. In addition, the cavity has a strong gravity, and people will be torn to pieces by gravity when they are still far away from it. Of course, such a cavity cannot be used as a passage for travel.
Thorne et al. conceived the geometric structure of a passable wormhole. The throat of the wormhole was kept open, and people only received moderate acceleration and tidal force when they passed. Einstein's field equation shows that any accessible cavity must contain some form of strange matter. This strange substance has a "negative pressure", which is a bit like an elongated spring. At present, no one knows whether it exists. If this substance exists, its interaction with other substances is weak and will not cause harm to travelers, then there is the possibility of the existence of a passable cavity. If we can find the wormhole that Thorne imagined, we can open one side hole near the sun and the other side hole near Vega in Lyra. We can travel along the wormhole by rocket and reach Vega, which is 25 light years away, in a short time. Of course, all this is just an extension of the theory. So far, no one has observed this hole.
Suppose there are two holes in the wormhole, A and B, which make hole B move at an acceleration close to the speed of light, while hole A remains stationary. According to the prediction effect of special relativity, the clock of hole B is slower than that of hole A. At this time, the rocket travels from hole A to hole B at a speed close to the speed of light, and reaches hole B earlier than when it leaves from hole A. At this time, I immediately returned through the wormhole and arrived at the starting point one hole earlier than when I left. That is to say, at 10, you moved from hole A to hole B at a speed close to the speed of light, but it was 9 o'clock when you arrived at hole B. You immediately returned to hole A through the wormhole, which was less than 10, thus completing a reverse time travel through the wormhole. Scientific fantasy can avoid many concrete problems that are difficult to solve and leave them for future generations to study, but scientific inference must face these problems, and the concepts of causality and time evolution in natural phenomena should be re-evaluated when traveling against time. For example, if you meet your parents before you were born while traveling in reverse time, when you try to shoot them, there will be a problem to be solved: if you are shot, how did you come into this world? Scientists believe that in order to make the evolution of natural systems not contradictory, we must adopt some basic principles, that is, the principle of compatibility to supplement the law of causality. In other words, the gun either misfired or missed.
Since Thorne published the new characteristics of large-scale cavity, many physicists began to pay attention to it, and some scholars put forward new hypotheses. Some people have raised many questions and think that the hole theory cannot be established, because it not only destroys a major premise of physics, but also shakes many physical laws. At least from the current human understanding, there is still great uncertainty about the existence of tooth decay.
With the development of science and technology, new research has found that the super-strong force field of wormhole can be neutralized by negative mass to stabilize the energy field of wormhole. Scientists believe that antimatter also has a "negative mass" relative to the "positive matter" that produces energy, and can absorb all the energy around it. Like wormholes, "negative mass" was once thought to exist only in theory. However, many laboratories in the world have successfully proved that "negative mass" can exist in the real world and captured a small amount of "negative mass" in space through spacecraft.
According to the calculation of researchers in the Department of Physics of the University of Washington, the "negative mass" can be used to control the "wormhole". They pointed out that "negative mass" can enlarge the original tiny "wormhole" enough for the spacecraft to pass through. Their research results have aroused great interest from the aerospace departments of various countries, and many countries are considering funding the research of "wormholes", hoping that "wormholes" can be really used for space navigation.
Big bang and black hole
Some people find it hard to understand why the Big Bang was not a black hole. After all, the density of matter in its first few seconds is much higher than that of all known stars, and such a high density of matter should strongly distort space-time. When the density is large enough, there must be an area smaller than schwarzschild radius relative to its internal mass. However, the Big Bang successfully avoided being confined to its own black hole, and the space near the singularity was actually not tightly curled but flat, which was unbelievable. What's going on here?
The simple answer is this: because the Big Bang expanded rapidly at the initial moment, and then the expansion rate gradually decreased, it avoided becoming a black hole. Space can be flattened, but space-time will not. Curling can come from the time part of the space-time scale. This scale determines the deceleration of the expansion of the universe. Therefore, the sum of space-time curl is related to the density of matter, but the expansion and curl of any space have an influence on it. Schwarzschild's solution to the gravitational equation is static, which is the limit before the static sphere collapses into a black hole. Schwarzschild limit does not apply to rapidly expanding substances.
The standard Big Bang model is the solution system of Lederman-Robertson-Walker gravitational field equation of general relativity. These solutions can be used to describe an open or closed universe. All FRW universes have a singularity at the origin of time to represent the Big Bang. Black holes also have singularities. The definition of a closed universe from which no light can escape is exactly the same as that of a black hole. So what's the difference?
The first significant difference is that the singularity in FRW model exists in the past of all events in the universe, while the singularity of black hole exists in the future. So the Big Bang is more like a white hole that turns into a black hole in time. According to the classical general relativity, a white hole cannot exist, just as a black hole cannot be destroyed (time reversal). If they exist, it may not be applicable.
But the standard FRW black hole model is also different from the white hole. The white hole has a horizon as a black hole inversion. Nothing can enter the horizon of the white hole, nor can it escape the horizon of the black hole. Roughly speaking, this is the definition of a white hole. Note that this book can simply be used to compare the differences between FRW model and standard black hole or white hole model (such as static Schwarzschild or rotating Kerr solution). But it is much more difficult than a more general black hole or white hole. The real difference is that the FRW model has different horizons from black holes or white holes. The coordinate axis outside the horizon of the white hole can be traced back to the infinite past without touching the singularity of the white hole. All axes in the FRW universe originate from the singularity.
The real universe may be different from the FRW universe. Can we rule out the possibility of a black hole or a white hole? I don't want to discuss "does singularity really exist?" A kind of problem, but suppose that general relativity is correct within the scope of our discussion.
The previous discussion about denying that the Big Bang is a black hole still applies. The singularity of a black hole is always in the future light cone, and astronomical observations have clearly pointed out that the Big Bang happened in the past. It is possible that the Big Bang is actually the remains of a white hole.
The main assumption (premise) of FRW model is that the universe is homogeneous and isothermal in macroscopic view. That is to say, at any given cosmic time (point), it is the same from any direction. Astronomy has well proved that the distribution of galaxies is quite uniform and isothermal in a large range of millions of light years. This high temperature property of cosmic background radiation (CBR) strongly supports homogeneity. However, the size of hubble volume is limited by the speed of light and the age of the universe. We can only see about 100 to 20 billion light years away, which is about 100 times the distribution structure of known galaxies.
The white hole model that accords with the observation of the universe should be the inversion of the time when stars collapse into black holes. As a good approximation, we can ignore the pressure and regard it as a spherical stardust cloud with no internal force except gravity. Since the pioneering work of Schneider and Harmo in 1939, the collapse of stars has been paid close attention to and studied. This simple situation is easy to understand. It is possible to establish an accurate model of star collapse (regardless of pressure): all FRW solutions are combined with external Schwarzschild solutions outside the sphere.
The next question is: if the stardust ball is much larger than the observable universe, the model that the stardust ball collapses into time reversal will be indistinguishable from the FRW model. In other words, we can't rule out the possibility that the universe is a huge white hole. We won't know until billions of years later when the boundary of the ball comes into our sight.
It must be admitted that there are many other possible models of the universe, some of which are not too complicated, if the hypothesis of homogeneous isothermal is abandoned. But it is difficult to deduce such rigorous things from these theories. The most exciting hypothesis was put forward by C.Hellaby in 1987: He imagined that the beginning of the universe was a string of isolated beads, which came out of the cave generate independently at a certain moment and merged into the universe. All these can be described by the exact single solution of general relativity.