Pay attention to the development of black holes

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 hole" is undoubtedly one of the most challenging and exciting astronomical theories in this century. Many scientists are trying to uncover its mystery, and new theories are constantly put forward. However, these latest achievements in contemporary astrophysics cannot be explained clearly here in a few words. Interested friends can refer to special works.

The splitting of black holes

Black holes can be divided into two categories according to their composition. One is a dark energy black hole and the other is a physical black hole.

■ Dark energy black hole

It is mainly composed of huge dark energy rotating at high speed, and there is no huge mass inside. Huge dark energy rotates at a speed close to the speed of light, and a huge negative pressure is generated inside to devour objects, thus forming a black hole. See "Cosmic Black Hole Theory" for details. Dark energy black holes are the basis of galaxy formation, as well as galaxy clusters and galaxy clusters. Physical black holes are formed by the collapse of one or more celestial bodies, and their mass is huge. When the mass of a physical black hole is equal to or greater than that of a galaxy, we call it a strange black hole. Dark energy black holes are very big, and can be as big as the solar system.

■ Physical black hole

Compared with the dark energy black hole, its volume is very small, and it can even be reduced to a singularity.

Accretion of black holes

Black holes are usually found because they gather around gas to produce radiation. This process is called accretion. The efficiency of high temperature gas radiating heat energy will seriously affect the geometric and dynamic characteristics of accretion flow. At present, thin disks with high radiation efficiency and thick disks with low radiation efficiency have been observed When accretion gases approach the central black hole, their radiation is extremely sensitive to the rotation of the black hole and the existence of the horizon. The photometric and spectral analysis of accretion black holes provides strong evidence for the existence of rotating black holes and horizons. The numerical simulation also shows that relativistic jets often appear in accretion black holes, some of which are driven by the rotation of black holes.

Astrophysicists use the word "accretion" to describe the flow of matter to a central gravitational body or a central expanding material system. Accretion is one of the most common processes in astrophysics, and it is precisely because of accretion that many common structures around us are formed. In the early universe, galaxies were formed when gas flowed to the center of gravitational potential well caused by dark matter. Even today, stars are still formed by the collapse and fragmentation of gas clouds under their own gravity, and then accreted by the surrounding gas. Planets, including the earth, are also formed by the accumulation of gas and rocks around newly formed stars. But when the central celestial body is a black hole, accretion will show its most spectacular side.

However, black holes do not absorb everything. They also emit protons outward.

The destruction of black holes

■ Shrinkage until failure

Black holes will glow, shrink in size and even explode. When the British physicist Steven. When Hawking made this language in 1974, the whole scientific community was shocked.

Black holes were once considered as the last deposit in the universe: nothing can escape from black holes. They devour gas and stars, and their mass increases, so the volume of holes will only increase.

Hawking's theory is an inspiration-led thinking leap. He combined general relativity with quantum theory. He found that the gravitational field around the black hole releases energy and consumes the energy and mass of the black hole at the same time (when a particle escapes from the black hole without repaying the borrowed energy, the black hole will lose the same amount of energy from its gravitational field, and Einstein's formula E = MC 2 shows that the loss of energy will lead to the loss of mass). When the mass of a black hole is getting smaller and smaller, its temperature will get higher and higher. In this way, when a black hole loses its mass, its temperature and emissivity increase, so its mass loses faster. This "Hawking radiation" is negligible for most black holes, while small black holes radiate energy at a very high speed until the black hole explodes.

■ Boil until it is destroyed.

All black holes will evaporate, but big black holes boil very slowly, and their radiation is very weak, so it is difficult to be detected. But as the black hole becomes smaller, this process will accelerate and eventually get out of control. When the black hole contracts, the gravity will become steeper, producing more escaping particles, and the more energy and mass will be plundered from the black hole. Black holes shrink faster and faster, which makes the evaporation speed faster and faster, and the surrounding gas field becomes brighter and hotter. When the temperature reaches 10 15℃, the black hole will be destroyed in the explosion.

Black holes and the earth

A black hole has no specific shape, so you can't see it. You can only judge the existence of the surrounding planets by their directions. Maybe you will cry out in horror because of its mystery, but there is no need to worry too much. Although it is very attractive, it is also an important evidence to judge its status. Even if it had an impact on the material very close to the earth, we still had enough time to save it, because its "official boundary" was still far away from us at that time. And most stars will become neutron stars or white dwarfs when they collapse. But that doesn't mean we can relax our vigilance (who knows if we will be inhaled next moment? ), which is one of the reasons why humans study it.

Stars, white dwarfs, neutron stars, quarks and black holes are five kinds of stars with equal density in turn. Of course, stars are the lowest density, and black holes are the ultimate form of matter. BIGBANG will happen after the black hole, and the energy will enter a new cycle after it is released.

Black hole proposal

1967, graduate student Lin of Cambridge University? Bell discovered objects that emit regular radio waves in the sky, which further encouraged the prediction of the existence of black holes. At first, Bell and her mentor Anthony? Hurvis thinks they may have made contact with the alien civilization in our galaxy! I do remember that at the seminar where they announced their discovery, they called the first four sources LGM 1-4, and LGM stands for "little green men". However, in the end, they and everyone else came to a less romantic conclusion. These objects, called pulsars, are actually rotating neutron stars. Because of the complex interaction between their magnetic fields and the surrounding materials, they will emit radio waves. This is bad news for the author of Space Exploration, but it is a great hope for those of us who believed in black holes at that time-this is the evidence of the existence of the first neutron star. The radius of a neutron star is about 10 mile, which is only several times the critical radius of a star becoming a black hole. If a star can collapse to such a small scale, other stars will naturally be expected to collapse to a smaller scale and become black holes.

When the concept of black hole was first put forward, there were two theories of light: one was Newton's theory of light particles; The other is the wave theory of light. We now know that, in fact, both are correct. Due to the wave-particle duality of quantum mechanics, light can be considered as both a wave and a particle. In the wave theory of light, it is not clear how light reacts to gravity. But if light is composed of particles, people can expect them to be affected by gravity like shells, rockets and planets. At first, people thought that light particles move infinitely fast, so gravity can't slow them down, but Luo Mai's discovery about the limited speed of light shows that gravity can have an important influence on it.

1783, John, President of Cambridge University? On the basis of this assumption, Michelle published an article in the Journal of Philosophy of the Royal Society of London. He pointed out that a star with large enough mass and compact enough will have such a strong gravitational field that even light can't escape-any light emitted from the surface of the star will be attracted back by the gravity of the star before reaching the distance. Michelle hinted that there may be a large number of such stars. Although we can't see them because the light they emit won't reach us, we can still feel their gravity. This is what we now call a black hole. It is worthy of the name-a space black hole. A few years later, the French scientist Marquis Laplace apparently put forward an idea similar to Michelle alone. Interestingly, Laplace only included this idea in the first and second editions of his book The World System, and deleted it in later editions. Maybe he thinks it's a stupid idea. (In addition, the particle theory of light became out of fashion in19th century; It seems that everything can be explained by wave theory, and according to wave theory, it is not clear whether light is affected by gravity. )

In fact, because the speed of light is fixed, it is really uncoordinated to treat light as a cannonball in Newton's theory of universal gravitation. (The shells fired from the ground decelerated due to gravity, and finally stopped rising and turned back to the ground; However, a photon must continue upward at a constant speed, so how does Newton's gravity affect light? Until Einstein put forward the general theory of relativity in 19 15, there was no theory about how gravity affected the coordination of light. Even after a long time, the significance of this theory to massive stars was understood.

Explore black holes

1928, Indian graduate student-Salamanian? Chandraseka and British astronomer Arthur came to Cambridge by boat? Sir Eddington (a general relativist) studied it. According to records, in the early 1920s, a reporter told Eddington that he had heard that only three people in the world could understand the general theory of relativity. Eddington paused and then replied, "I wonder who this third person is." On the journey from India to Britain, Chandraseka worked out how big a star can continue to fight against its own gravity and maintain its own operation after running out of fuel. The idea is that when stars get smaller, matter particles get close together, and according to the Pauli exclusion principle, their speeds must be very different. This causes them to spread out and try to expand the star. The reason why a star can keep its radius constant is because of the balance of attraction and repulsion caused by the principle of incompatibility, just as gravity is balanced by heat in the early life.

However, Chandraseka realized that the repulsive force provided by the principle of incompatibility was limited. The maximum velocity difference of particles in a star is limited by relativity to the speed of light. This means that when the star becomes dense enough, the repulsive force caused by the incompatibility principle will be smaller than that caused by gravity. Strong draseka calculation; A cold star whose mass is about 0/.5 times that of the sun/kloc-0 cannot support itself by its own gravity. (This mass is now called the strong Draseka limit. ) Soviet scientist Lev? Davidovic? Landau made a similar discovery almost at the same time.

This is of great significance to the ultimate destination of massive stars. If the mass of a star is less than the Chandraseka limit, it will eventually stop shrinking and eventually become a "white dwarf" with a radius of several thousand miles and a density of several hundred tons per cubic inch. White dwarfs are supported by the repulsion of electrons in their matter. We have observed a large number of such white dwarfs. The first planet observed revolved around Sirius, the brightest star in the night sky.

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. Their radius is only about 10 mile, and their density is several hundred million tons per cubic inch. When the neutron star was first predicted, there was no way to observe it. In fact, it was a long time before people observed them.

On the other hand, when the stars whose mass is greater than the strong Draseka limit run out of fuel, there will be a big problem: in some cases, they will explode or throw enough substances to reduce their mass below the limit to avoid catastrophic gravitational collapse. But it is hard to believe that this will happen no matter how big the star is. How do you know it must lose weight? Even if every star tries to lose enough weight to avoid collapse, what will happen if you add more mass to a white dwarf or neutron star and make it exceed 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. The hostility of other scientists, especially his former teacher Eddington, the main authority of star structure, made Chandraseka give up this work and study other astronomical problems such as the movement of star clusters. However, he won the Nobel Prize in 1983, at least in part because of his early work on the mass limit of cold stars.

Strong Draseka pointed out that the principle of incompatibility cannot prevent the collapse of stars whose mass is greater than the limit of strong Draseka. However, according to general relativity, what will happen to such a star? This question was put forward by a young American, Robert? Oppenheimer solved this problem for the first time 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. After the war, the problem of gravitational collapse was forgotten by most people, because most scientists were attracted by physics at atomic and nuclear scales.

Now, we get an image from Oppenheimer's work: the gravitational field of a star changes the path of light, which is different from that 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, if light can't escape, other things are even less likely to escape and 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.

When you watch a star collapse to form a black hole, in order to understand what you see, remember that there is no absolute time in relativity. Every observer has his own time measurement method. Because of the gravitational field of the star, some people will spend different time on the star than others in far places. Suppose there is a fearless astronaut on the surface of collapsed star, who collapses inward with a star. According to his watch, a signal is sent to a spaceship orbiting the star every second. At some point in his watch, such as 1 1, the star just contracted to the critical radius. At this time, the gravity field is so strong that nothing can escape, and his signal will never reach the spacecraft. When 1 1 arrived, his partner in the spaceship found that the interval between astronauts sending out a series of signals was getting longer and longer. But this effect is very small before 10: 59: 59. They only need to wait a little more than one second between receiving the two signals from 10: 59: 58 and 10: 59: 59, but they have to wait for the signal from 1 1 indefinitely. According to the astronaut's watch, between 10: 59: 59 and 1 1: 00, the surface of the star emits light waves. Seen from the spaceship, light waves are scattered into infinite time intervals. The time interval between receiving this series of light waves on the spacecraft is getting longer and longer, so the light emitted by the stars is getting redder and weaker. Finally, the star became so blurred that it could no longer be seen from the spaceship, leaving only a black hole in space. But the stars continue to act on the spacecraft with the same gravity, making the spacecraft continue to rotate around the formed black hole.

However, due to the following problems, the above scenario is not completely true. The farther away from the star, the weaker the gravity, so the gravity acting on the fearless astronaut's feet is always greater than the gravity acting on his head. Before the star has shrunk to the critical radius and formed the horizon, this force difference has already pulled our astronaut into spaghetti and even torn him! However, we believe that there are much more massive celestial bodies in the universe, such as the central region of galaxies, which suffer gravitational collapse and produce black holes; The astronauts on this object will not be torn to pieces before the black hole is formed. In fact, when he reached the critical radius, he didn't feel anything strange, even when he passed the point of never looking back, he didn't notice it. However, as the area continues to collapse, within a few hours, the gravity difference acting on his head and feet will become so big that it will be torn again.

Roger? Penrose and I studied from 1965 to 1970, and pointed out that according to general relativity, there must be infinite density and curvature of spacetime singularity in black holes. This is very similar to the Big Bang at the beginning of time, but for a collapsed object and astronauts, it is the end of time. At this singularity, the laws of science and our ability to predict the future fail. However, any observer who stays outside the black hole will not be affected by the failure of predictability, because light and any other signal from the singularity cannot reach him. This amazing fact led Roger? Penrose put forward the cosmic censorship conjecture, which can be translated as: "God hates naked singularities." In other words, the singularity caused by gravitational collapse can only occur in places like black holes, covered by the event horizon and not seen by the outside world. Strictly speaking, this is the so-called weak universe censorship conjecture: it protects the observers who stay outside the black hole from the failure of predictability at the singularity, but is helpless to the poor astronauts who unfortunately fall into the black hole.

The equation of general relativity has some solutions, which makes it possible for our astronauts to see naked singularity. He may be able to avoid hitting a singularity and reach another part of the universe through a wormhole. It seems that this provides great possibilities for time travel. Unfortunately, all these solutions seem to be unstable; The smallest disturbance, such as the existence of an astronaut, will change it, so that he can't see this singularity, so he crashes into it and ends his time. In other words, the singularity always happens in his future, not in the past. The strong cosmic censorship conjecture means that in real solutions, singularities always exist in the future (such as gravitational collapse singularities) or in the past (such as BIGBANG). Because it is possible to travel to the past near naked singularity, some kind of conjecture of cosmic censorship is promising. This is good for science fiction writers. It shows that no one's life is safe: someone can go back in time and kill your father or mother before you are reborn!

Event horizon, the boundary of the unavoidable region in the sky, is like a one-way film around a black hole: objects, such as unsuspecting astronauts, can fall into the black hole through the event horizon, but nothing can escape from the black hole through the event horizon. Remember that the event horizon is the space where light tries to escape from the black hole-time orbit, and nothing can move faster than light. People can appropriately apply the poet Dante's words about the entrance to hell to the event horizon: "Those who enter from here must abandon all hope." Once anything or anyone enters the event horizon, it will quickly reach the infinite dense area and the end of time.

The general theory of relativity predicts that moving heavy objects will lead to the radiation of gravitational waves, which are ripples of curvature of spacetime and travel at the speed of light. Gravitational waves are similar to ripples in electromagnetic fields, but they are much more difficult to detect. Just like light, it will take away the energy of luminous objects. Because any energy in motion will be taken away by the radiation of gravitational waves, it can be expected that the system of a massive object will eventually tend to a constant state. This is very similar to throwing a cork into the water. At first, it turned upside down for a long time, but when the ripples took away its energy, it finally calmed down. For example, the earth's rotation around the sun will produce gravitational waves. The effect of its energy loss will change the earth's orbit, make it closer and closer to the sun, and finally hit the sun, and return to the final state in this way. Take the earth and the sun as an example, the energy loss rate is very small-it only needs a small electric heater to ignite, which means that the collision between the earth and the sun will take about 654.38 billion years, so don't worry about it immediately! The process of changing the earth's orbit is too slow to be observed at all. But a few years ago, this effect was observed in a system called PSR1913+16 (PSR stands for "pulsar" and is a special neutron star that emits regular radio waves). This system consists of two neutron stars moving around each other. Due to gravitational wave radiation, their energy loss makes them approach each other in a spiral orbit.

When a star collapses to form a black hole, it moves much faster, so the energy is taken away at a much higher speed. So it won't take long to reach the same state. What will this final state be like? People will think that this will depend on all the complex characteristics of the star that forms a black hole-not only its mass and rotation speed, but also the different densities of different parts of the star and the complex movement of gas in the star. If black holes are as changeable as their original objects, it will be very difficult to make any prediction about them.

However, Canadian scientist Nai Nai? Israel (he was born in Berlin, grew up in South Africa and received his doctorate in Ireland) completely changed the study of black holes in 1967. He pointed out that according to general relativity, a black hole that does not rotate must be a very simple and perfect sphere; Its size depends only on their mass, and any two such black holes with the same mass must be equal. In fact, they can be described by Einstein's special solution, which was 19 17 years after the discovery of general relativity. Schwartz Schild found it At first, many people (including israel himself) thought that since black holes must be perfectly spherical, black holes can only be formed by the collapse of perfectly spherical objects. Therefore, any actual star-never a perfect sphere-will only collapse to form a bare singularity.

However, for Israel's achievements, some people, especially Roger? Penrose and John? Wheeler advocates a different explanation. They believe that the rapid movement involving the collapse of a star shows that the gravitational wave it releases makes it closer and closer to a sphere, and when it finally comes to rest, it becomes an accurate sphere. According to this view, any non-rotating star, no matter how complicated its shape and internal structure, will eventually become a perfect spherical black hole after gravitational collapse, and its size depends only on its mass. This view was further supported by calculation and was quickly accepted by everyone.

Israel's results only deal with black holes formed by non-rotating objects. 1963, New Zealander Roy? Kerr discovered a family 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 this solution is the same as that of Schwartz Schild. If there is rotation, the black hole protrudes outward near the equator (just like the earth or the sun protrudes outward due to rotation). The faster the rotation, the more it protrudes outward. 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.

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? However, in 1963, astronomer Martin? Schmidt measured the redshift of a faint quasar in the direction of a radio source named 3C273 (No.273, Category III, Cambridge Radio Source Catalogue). He found that the gravitational field could not cause such a big redshift-if it is a gravitational redshift, then such a star must have such a large mass and be so close to us to interfere with the orbits of planets in the solar system. This implies that this redshift is caused by the expansion of the universe, which further shows that this celestial body is very far away from us. Since it can be observed at such a long distance, it must be very bright, which means it must radiate a lot of energy. People will think that the only mechanism that produces such a large amount of energy seems to be not just a star, but the gravitational collapse of the entire central region of a galaxy. Many other quasars have also been found, all of which have great red shifts. But they are all too far away from us, and it is too difficult to observe them to provide conclusive evidence for black holes.