It is easy for people to imagine a "black hole" as a "big black hole", but it is not the case. 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 size of a star is very large, its gravitational field has almost no effect on space-time. The gravitational field emitted from a certain point on the star's surface Light can be emitted in a straight line in any direction. The smaller the radius of a star, the greater its effect on the curvature of the surrounding space and time. Light emitted at certain angles will return to the surface of the star along the curved space.

When the radius of the star reaches a certain value (called the "Schwarzschild radius" in astronomy), even the light emitted by the vertical surface is captured. At this point, the star becomes a black hole. It is said to be "black" , means that it is like a bottomless pit in the universe. Once any matter falls into it, it "seems" that it can no longer escape. In fact, black holes are truly "invisible", which we will talk about in a moment.

So, how are black holes formed? In fact, like white dwarfs and neutron stars, black holes are likely to evolve from stars.

We have introduced the formation process of white dwarfs and neutron stars in more detail. When When a star ages, its thermonuclear reaction has exhausted the fuel (hydrogen) in the center, and the energy generated by the center is no longer much. In this way, it no longer has enough strength to bear the huge weight of the outer shell. So in Under the heavy pressure of the outer shell, the core begins to collapse until it finally forms a small and dense star, which is able to balance with the pressure again.

Stars with smaller masses mainly evolve into white dwarfs, and stars with larger masses Stars may form neutron stars. According to scientists' calculations, the total mass of neutron stars cannot be greater than three times the mass of the sun. If this value is exceeded, there will be no force left to compete with its own gravity, triggering another major earthquake. Collapse.

This time, according to scientists’ conjecture, matter will march unstoppably toward the center point until it becomes a “point” where the volume tends to zero and the density tends to infinity. And when its radius once Shrinking to a certain extent (Schwarzschild radius), as we introduced above, the huge gravity makes it impossible for even light to emit outward, thus cutting off all connections between the star and the outside world - a "black hole" is born. < /p>

Compared with other celestial bodies, black holes are too special. For example, black holes have "invisibility" and people cannot directly observe them. Even scientists can only make various conjectures about its internal structure. So, how do black holes hide themselves? The answer is - curved space. We all know that light propagates along straight lines. This is the most basic common sense. But according to the general theory of relativity, space will act on the gravitational field Bend downward. At this time, although the light still travels along the shortest distance between any two points, it is no longer a straight line, but a curve. Figuratively speaking, it seems that the light was originally going to travel in a straight line, but the strong gravity pulled it Pulled away from the original direction.

On the earth, due to the small gravitational field, this bending is minimal. But around the black hole, the deformation of space is very large. In this way, even if Although part of the light emitted by a star blocked by a black hole will fall into the black hole and disappear, the other part of the light will bypass the black hole in the curved space and reach the earth. Therefore, we can effortlessly observe the light on the back of the black hole. The starry sky is as if the black hole does not exist. This is the invisibility of the black hole.

What’s more interesting is that some stars not only emit light towards the earth to directly reach the earth, but also emit light in other directions. It may also be refracted by the strong gravity of a nearby black hole and reach the earth. In this way, we can not only see the "face" of the star, but also its sides and even its back!

"Black hole" It is undoubtedly one of the most challenging and exciting astronomical theories of this century. Many scientists are working hard to unveil its mystery, and new theories are constantly being proposed. However, these contemporary astrophysics The latest results cannot be explained clearly in a few words here. Friends who are interested can refer to specialized works.

Black holes

Black holes are places with extremely strong gravity, and nothing can Even light escapes from there. Black holes can arise from the "death" of a massive star, when a massive star exhausts its interior

When the star reaches the end state of its evolution with its nuclear fuel, the star becomes unstable and undergoes gravitational collapse. The weight of the dead star's material will violently squeeze inward in all directions. When the gravity is so great that there is no other repulsive force to oppose it. , crushing the star into an isolated point called a "singularity".

Details about the structure of a black hole can be calculated using Einstein's theory of general relativity, which explains how gravity bends space and slows down clocks. Singularity It is the center of the black hole, and the gravity around it is extremely strong. The surface of the black hole is usually called the event horizon, or the event horizon, or the "Schwarzschild radius of the stationary spherical black hole". It is those space-time events that can be connected with distant events. and the boundary between those space-time events that cannot be transmitted because the signal is captured by a strong gravitational field. Under the event horizon, the escape velocity is greater than the speed of light. This is a celestial phenomenon that humans have not yet observed and confirmed, but it has been confirmed by some theoretical astronomy such as Hawking. The research on mathematical models is quite complete.

There is a huge gravitational field hidden in the hole. This gravitational force is so strong that nothing, not even light, can escape the palm of the black hole. The black hole does not let anything Anything within its boundaries is seen by the outside world, which is why this object is called a "black hole". We cannot observe it through the reflection of light, and can only learn about black holes indirectly through the surrounding objects affected by it. It is speculated that , a black hole is the remnant of a dead star or an exploded gas mass, which is produced when a special massive supergiant collapses and contracts.

Because black holes are invisible, some people have always questioned whether black holes really exist. .If they really exist, where are they?

The production process of a black hole is similar to the production process of a neutron star; the core of the star shrinks rapidly under its own weight, and a powerful explosion occurs. When all the particles in the core When all the matter turns into neutrons, the contraction process stops immediately and is compressed into a dense planet. But in the case of a black hole, because the mass of the star's core is so large that the contraction process continues endlessly, and the neutrons themselves are squeezing the gravitational force It is crushed into powder by its own attraction, and what is left is a substance with an unimaginable density. Any object close to it will be sucked in by it, and the black hole becomes like a vacuum cleaner

In order to understand To understand the dynamics of black holes and how they keep everything inside from escaping, we need to discuss general relativity. General relativity is a theory of gravity created by Einstein that applies to planets, stars, and black holes. Einstein This theory, proposed in 1916, explains how space and time are distorted by the presence of massive objects. In short, general relativity says that matter curves space, and the curvature of space in turn affects objects traveling through space

Let's take a look at how Einstein's model works. First, consider that time (the three dimensions of space are length, width, and height) is the fourth dimension in the real world (although It is difficult to draw a direction other than the usual three directions, but we can try our best to imagine). Secondly, consider that space and time are the surface of a huge taut spring bed for gymnastic performances.

Einstein's theory holds that mass bends space-time. We might as well put a big stone on the bed of a spring bed to illustrate this situation: the weight of the stone makes the tightened bed sink slightly, although The spring bed surface is still basically flat, but it is still slightly concave in the center. If more stones are placed in the center of the spring bed, it will have a greater effect and make the bed surface sink more. In fact , the more stones there are, the more the spring bed surface will bend.

In the same way, large-mass objects in the universe will distort the structure of the universe. Just as 10 stones make the spring bed surface bend more than 1 stone. By the same token, an object much more massive than the Sun bends space much more than an object with a mass equal to or less than the Sun.

If a tennis ball is placed on a taut, flat spring If it rolls on the bed, it will move in a straight line. On the contrary, if it passes through a concave place, its path will be arc-shaped. In the same way, celestial bodies continue to move in a straight line when traveling through flat areas of space-time, while those that pass through curved areas It will advance along a curved trajectory.

Now let’s look at the impact of a black hole on the space-time area around it. Imagine placing a very large stone on a spring bed to represent an extremely dense black hole. Naturally, The stone will greatly affect the bed surface. Not only will the surface bend and sink, it may also cause the bed surface to break. A similar situation can also occur in the universe. If the universe

If there is a black hole in the space-time structure, the structure of the universe will be torn apart. This rupture of the space-time structure is called the singularity or singularity of space-time.

Now let’s take a look at why nothing can escape from the black hole Get out. Just like a tennis ball rolling over a spring bed will fall into a deep hole formed by a large rock, an object passing through a black hole will also be captured by its gravitational trap. Moreover, it takes an infinite amount of time to save an unlucky object. Energy.

We have already said that nothing can enter a black hole and escape from it. But scientists believe that black holes will slowly release their energy. The famous British physicist Hawking proved in 1974 that black holes have A non-zero temperature has a temperature that is higher than its surrounding environment. According to the principles of physics, all objects with a higher temperature than their surroundings will release heat, and black holes are no exception. A black hole will last for hundreds of It takes trillions of years to emit energy. The energy released by a black hole is called Hawking radiation. When a black hole dissipates all its energy, it disappears.

The black hole between time and space slows down time and changes space. It must be elastic and swallow everything that passes through it. In 1969, American physicist John Atty Wheeler named this kind of insatiable space "black hole".

We all know that because black holes cannot Reflected light, so invisible. In our minds, black holes may be distant and dark. But the famous British physicist Hawking believes that black holes are not as black as most people imagine. Through scientists' observations, there is radiation around black holes. And it is likely to come from a black hole, which means that the black hole may not be as black as imagined.

Hawking pointed out that the source of radioactive material in black holes is a kind of real particles. These particles are produced in pairs in space. Comply with the usual laws of physics. And after these particles collide, some of them will disappear into the vast space. Generally speaking, we may not have a chance to see these particles until they disappear.

Hawking It was also pointed out that when a black hole is created, real particles will appear in pairs. One of the real particles will be sucked into the black hole, and the other will escape. A bunch of escaping real particles will look like photons. To the observer For example, seeing escaping real particles is like seeing rays from a black hole.

Therefore, to quote Hawking, "the black hole is not as black as imagined", it actually diverges Emit a large number of photons.

According to Einstein's law of conservation of energy and mass. When an object loses energy, it also loses mass. Black holes also obey the law of conservation of energy and mass. When a black hole loses energy, The black hole will cease to exist. Hawking predicted that the moment the black hole disappears, there will be a violent explosion, releasing an energy equivalent to the energy of millions of hydrogen bombs.

But don’t raise your head with high expectations. , I thought I would see a fireworks show. In fact, after a black hole explodes, the energy released is very large, which is very likely to be harmful to the body. Moreover, the energy release time is also very long, and some will exceed 10 billion to 200 billion. Billions of years, longer than the history of our universe, and it will take trillions of years to completely dissipate energy.

Recently, international astronomers passed a latest observation by NASA's Spitzer Space Telescope As a result, for the first time, as many as 21 "quasar" black hole groups in the universe that had been deeply hidden were discovered simultaneously in a narrow area of ??the universe.

This major discovery was seen from the front for the first time. It has confirmed the speculations in the field of astronomy for many years about the widespread existence of a large number of invisible black holes in the universe. Sufficient evidence makes people believe that the vast universe is indeed filled with all kinds of undiscovered black holes

The discovery of the huge source of gravity - the "quasar" black hole group. Regarding the details of this latest discovery, the researchers have officially published an article in the "Nature" magazine published on August 4, 2005.

We know that in reality, black holes in the universe, due to their huge gravity, even light is tightly attracted and bound, so they cannot be directly observed by people. In order to confirm the evidence of the existence of black hole objects, astronomers have discovered through research that the behavior of matter around black holes has its own specific behavior: in the cosmic space around black holes, gaseous materials have ultra-high temperatures and are violently attracted by the strong gravitational field of black holes. After acceleration, these substances will be elevated to

Close to the speed of light. When the gas material is completely swallowed by the black hole, the entire process will release a large amount of X-rays. Usually it is these escaped X-rays that show signs that a black hole does exist here. This is what happened in the past People have discovered the most direct evidence of black holes.

On the other hand, around some extremely active super-large black holes in the universe, due to their violent attraction and devouring of surrounding matter, they will also be produced on the periphery of the black hole star. A thick layer of cosmic gas and dust clouds further increases the difficulty of observing the area near the black hole body and hinders astronomers from discovering the existence of these super-large black holes. Astronomy defines these extremely active black holes as "like "Star". Under normal circumstances, the average mass of material a quasar devours in a year is equivalent to the sum of the masses of 1,000 medium stars. Under normal circumstances, these quasars are very far away from the solar system. When we observe them Now it is hundreds of millions of years later, which shows that the activity of such black holes appeared in the early days of the universe. Scientists speculate that this kind of black hole is the precursor of the growing galaxy in the universe, so they named it "quasar".

So far, only a few "quasar" black holes have been discovered. Whether there are a large number of other quasars in the vast depths of the universe still needs to be further discovered. , and astronomers’ research work in this field relies entirely on comprehensive observations and studies of X-rays inside the universe to confirm it.

A universe “full” of black holes

Recently, When introducing his first discovery of hidden black holes in the universe, Professor Ariel Martinez-Saint Singer from the University of Oxford in the UK said, "From previous observations and studies of cosmic X-rays, we expected to find a large number of black holes in the universe. Hidden evidence for the existence of quasars, but the results are indeed unsatisfactory and disappointing." Recently, according to the latest observations from NASA's Spitzer Space Telescope, astronomers have successfully penetrated The outer cosmic dust cloud that obscures the quasar black hole was captured, and the internal black hole body that had been hidden inside was captured. Because the Spitzer Space Telescope can effectively collect infrared light that can penetrate the cosmic dust layer, the researchers were able to successfully detect an extremely In a narrow area of ??space, as many as 21 already existing but "hidden" quasar black hole groups were discovered at the same time.

A research team from the Spitzer Science Center of the California Institute of Technology in the United States Member Mark Reis also said in an interview with the media, "If we put aside the 21 cosmic quasar black holes discovered this time and look at any other area in the universe, we can boldly predict that there will be a large number of hidden quasar black holes. More and more black holes will be discovered one after another. This means that, as we originally speculated, there must be a large number of super-massive black holes in the unknown depths of the universe, using the cover of interstellar dust to conceal themselves. , is constantly developing and growing in the dark."

Black hole

A mass of matter, if its gravitational field is strong enough to completely bend space-time around itself, then anything, Not even light can escape, which is called a black hole. A small amount of matter compressed to a very high density (such as compressing the earth to the size of a pea), or an extremely large mass of lower density matter (such as millions of This situation can occur if a mass twice as large as the sun is distributed in a ball with the same diameter as the solar system and roughly the density of water).

The first person to propose that there may be a gravitational force so strong that light cannot escape' The person who created the black hole was John Mitchell, a special member of the Royal Society, who stated this insight to the Royal Society in 1783. Mitchell's calculations were based on Newton's theory of gravity and the corpuscular theory of light. The former was the best gravity at the time theory. The latter conceived of light as a stream of tiny particles (now called photons) like small cannonballs. Mitchell postulated that these light particles should be affected by gravity like any other object. Thanks to Ole Romer ) accurately measured the speed of light more than 100 years ago. So Mitchell was able to calculate how big a celestial body with the density of the sun must be to have an escape velocity greater than the speed of light.

If such a celestial body exists, light cannot escape from them, so they should be black. The escape velocity from the sun's surface is only 0.2% of the speed of light, but if you imagine a series of increasingly larger but denser

For a celestial body with the same degree as the sun, the escape velocity increases rapidly. Mitchell pointed out that the escape velocity of such a celestial body with a diameter 500 times the diameter of the sun (similar to the size of the solar system) should exceed the speed of light.

Pierre Laplace independently came to and published the same conclusion in 1796. In a particularly prescient comment, Mitchell noted that although such an object would be invisible, 'if If any other luminous objects happen to orbit them, we may still be able to infer the existence of the central object based on the motion of these orbiting objects. In other words, Mitchell believes that if a black hole exists in a binary star, it will be most easily discovered. However, this idea of ??black stars was forgotten in the 19th century. It was not brought up again until astronomers realized that black holes can be produced through another way and studied 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 to analyze the conclusions of Einstein's theory. General relativity explains gravity as the movement of space and time into matter. as a result of nearby curvature. Schwarzschild calculated a rigorous mathematical model of the geometric properties of space-time around a spherical object and sent his calculations to Einstein, who submitted them to the Prussian Academy of Sciences in early 1916. These calculations showed that for 'any 'There is a critical radius for a mass, now called the Schwarzschild radius, which corresponds to an extreme deformation of space-time such that if the mass is squeezed within the critical radius, space will curve around the object and separate it from the rest of the universe Isolated. It actually becomes an independent universe of its own, and nothing (including light) can escape from it.

For the sun, the Schwarzschild radius is kilometers. For the earth, it is equal to 0.88 centimeters. This does not mean that there is a suitably sized thing now called a black hole (a term first used in this sense by John Wheeler in 1967) at the center of the Sun or the Earth. At this distance from the center of the celestial body, There are no anomalies in space and time. What Schwarzschild’s calculations show is that if the sun is squeezed into a ball with a radius of 2.9 kilometers, or if the earth is squeezed into a ball with a radius of only 0.88 centimeters, they will always be inside a black hole And isolated from the outer universe. Matter could still fall into such a black hole but nothing could escape.

These results were regarded as purely mathematical treasures for decades, because no one thought that the real , real objects can collapse to the extreme densities required to form black holes. White dwarfs began to be understood in the 1920s, but even white dwarfs have roughly the same mass as the sun but about the same size as the Earth, and their radius is much greater than 3 kilometers. People also It was not realized in time that if there is a large amount of matter of general density, a black hole can be created that is essentially the same as what Mitchell and Laplace imagined. The Schwarzschild radius corresponding to any mass M is given by the formula 2GM /c2 is given, where G is the gravitational constant. c is the speed of light.

In the 1930s, Subrahmanyan Chandrasekhar proved that even a white dwarf star only has It is stable only when its mass is less than 1.4 times the mass of the sun. Any dead star heavier than this will inevitably collapse further. Some researchers have thought that this may lead to the possibility of forming a neutron star. The typical radius of a neutron star is only about the diameter of a white dwarf. 1/700 of the size, which is a few kilometers in size. However, this idea was not widely accepted until the discovery of pulsars in the mid-1960s, proving that neutron stars did exist.

This reignited the theory of black holes interest, because a neutron star is about to turn into a black hole. Although it is difficult to imagine compressing the sun to a radius of less than 2.9 kilometers, it is now known that there are neutron stars with a mass equal to that of the sun and a radius of less than 10 kilometers. From neutron stars to black holes, it is only one step Far away.

Theoretical research shows that the behavior of a black hole is determined by only three of its characteristics - its mass, its charge and its rotation (angular momentum). No charge, no rotation Black holes are described by the Schwarzschild solution of Einstein's equations; black holes with charge and no rotation are described by the Reissner-Nordstrom solution; black holes with no charge and rotation are described by the Kerr solution; black holes with charge , a black hole with rotation is described by the Kerr-Newman solution. Black holes have no other characteristics, which has been explained by the phrase "black holes have no hair".

To sum up, a realistic black hole should probably spin and have no charge, so the Kerr solution is the most interesting.

It is now believed that both black holes and neutron stars are ejected when an E-mass star undergoes a supernova explosion. Produced in a death struggle. Calculations show that any dense supernova remnant with a mass less than 3 times the mass of the sun (Oppenheimer-Folkov limit) can form a stable neutron star, but any dense supernova remnant with a mass greater than this limit can form a stable neutron star. The remnant of the advancing and retreating nova will collapse into a black hole, and its contents will be pressed into the singularity at the center of the black hole. This is exactly the mirror inversion of the singularity of the Big Bang from which the universe was born. If such a celestial body happens to be orbiting an ordinary star In orbit, it will strip the companion star of its material, forming an accretion disk made of hot material that converges toward the black hole. The temperature in the accretion disk can rise so high that it can radiate X-rays, making the black hole detectable. .

In the early 1970s, Mitchell's predictions had repercussions: such an object was discovered in a binary star system. An X-ray source called Cygnus X-1 was confirmed to be the star HDE226868. The orbital dynamics of this system indicate that the source's X-rays come from an object smaller than the Earth in orbit around a visible star, but the source's mass is greater than the Oppenheimer-Folkov limit. This could only be a black hole Since then, a few other black holes have been confirmed using the same method. In 1994, the system V404 Cygnus became the best black hole 'candidate' so far. This is a star with a mass of 70% of the sun's mass orbiting about 12 times the sun. A system in which a mass They can only be detected when they are in a binary star system. An isolated black hole lives up to its name - it is dark and undetectable. However, according to astrophysics theory, many stars should have neutron stars or black holes as their source of life End. Observers have actually detected about as many suitable black hole candidates in binary systems as they have found pulsars, which means there should be the same number of isolated stellar-mass black holes as there are isolated pulsars, a conjecture supported by theory Supported by calculations. There are currently about 500 active pulsars known in our galaxy. But theory shows that the active period of a pulsar as a radio source is very short, and it quickly exhausts into an undetectable quiet state. Therefore, accordingly, There should be many more 'dead' pulsars (quiet neutron stars) around us. Our Milky Way galaxy contains 100 billion bright stars and has been around for billions of years. The best estimate is that our Milky Way galaxy There are 400 million dead pulsars today, and even a conservative estimate of the number of stellar-mass black holes reaches this number? - 100 million. If there really are so many black holes, and black holes are scattered randomly in the Milky Way, then The nearest black hole is only 15 light-years away from us. Since there is nothing unique about our Milky Way, every other galaxy in the universe should contain the same number of black holes. Ic

Galaxies may also contain some A celestial body that is very similar to the 'black star' originally envisioned by Mitchell's Laplace. Such celestial bodies are now called 'supermassive black holes' and are thought to exist in the centers of active galaxies and quasars. They provide gravitational energy. It 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 eat the material of one to two stars from the surroundings every year. In this process, a large part of the star's mass will be According to Einstein's division of labor E=mc2, it is converted into energy. Quiet supermassive black holes may exist in the centers of all galaxies, including our own Milky Way.

In 1994, using the Hubble Space Telescope, at distance In the 15 million parsec galaxy M87 in our Milky Way, a hot material disk with a size of about 150,000 parsecs was discovered. It is moving around the center of the galaxy at a speed of about 2 million kilometers per hour (about 5*10-7 5 multiplied by 10 to the 7th power, cm/second, almost 0.2% of the speed of light). A jet of gas with a length of more than 1 kiloparsec is emitted from the central 'engine' of M87. The orbital velocity in the central accretion disk of M87 is decisive It has been proved that it is under the gravitational control of a supermassive black hole with a mass of 3 billion times that of the sun. The jet can be explained as the energy pouring out from a polar region of the accretion system.

Also in 1994 Astronomy at Oxford University and Keele University

Scientists have identified a stellar-mass black hole in a binary star system called V404 Cygnus. We have pointed out that the orbital parameters of the system allowed them to accurately 'weigh' the black hole, arriving at a mass of approximately 12 times that of the Sun. times, while the ordinary stars orbiting it only have about 70% of the mass of the sun. This is the most accurate measurement of the mass of a 'black star' so far, so it is also the best and unique proof of the existence of black holes.

Some people speculate that a large number of micro-black holes or primordial 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 such micro-black holes is about the same as an atom, and the mass is about 100 million tons ( 10-11, 10 to the 11th power kilogram). There is no evidence that such celestial bodies do exist, but it is also difficult to prove that they do not exist.