black hole

black hole

This entry is compiled and applied by "popular science China" Science Encyclopedia

to examine

.

Black hole is a kind of celestial body existing in space in modern general relativity. The gravity of the black hole is so strong that the escape speed in the event horizon is faster than the speed of light. Therefore, "a black hole is a celestial body whose curvature of space-time is so large that light cannot escape from its event horizon.".

In 1916, German astronomer Karl Schwarzenegger obtained a vacuum solution of Einstein's field equation by calculation. This solution shows that if the actual radius of a static spherically symmetric star is less than a fixed value, there will be a strange phenomenon around it, that is, there is an interface - "Horizon". Once it enters this interface, even light cannot escape. This fixed value is called Schwarzschild radius. This "incredible celestial body" is named "black hole" by American physicist John Archibald Wheeler.

Black hole cannot be observed directly, but its existence and mass can be known indirectly, and its influence on other things can be observed. The existence of black holes can be obtained by the "edge information" of X-rays and gamma rays emitted by the friction caused by the acceleration caused by the gravity of black holes before objects are inhaled. The existence of black holes can also be inferred by indirectly observing the orbits of stars or interstellar clouds, and the position and mass of black holes can also be obtained.

At 21:00 Beijing time on April 10, 2019, the first picture of human black hole was released,

The black hole is located at the center of M87, a giant elliptical galaxy in Virgo, 55 million light-years away from earth, and has a mass about 6.5 billion times that of the sun. There is a shadow in its core area, surrounded by a crescent halo. Einstein's general theory of relativity has been proved to be valid under extreme conditions.

evolutionary process

A black hole consists of a scalar polynomial constructed from the Riemannian curvature tensor at the center, a singular point tending to diverge here and the surrounding space-time, and its boundary is a unidirectional membrane that can only enter but not exit: event horizon, which is invisible within the range of event horizon. According to Einstein's general theory of relativity, when a dying star collapses, it will collapse to the center, where it will become a black hole, devouring all the light and any matter in the nearby universe.

The production process of black hole is similar to that of neutron star: when a star is ready to die, its core shrinks, collapses and explodes rapidly under its own gravity. When all the matter in the core becomes neutrons, the shrinking process stops immediately and is compressed into a dense star, which also compresses the internal space and time. But in the case of black holes, even the repulsive force between neutrons cannot be stopped because the mass of the star's core is so large that the shrinking process goes on endlessly. The neutron itself is crushed into powder under the attraction of squeezing gravity, and the rest is a matter of unimaginable density. Because of the gravity of high mass, any object close to it will be sucked in by it.

It can also be simply understood as: usually stars only contain hydrogen at the beginning, and the hydrogen nuclei in the stars always collide with each other and fuse. Due to the large mass of stars, the energy produced by fusion can compete with the gravity of stars to maintain the stability of star structure. Because of the fusion of hydrogen nuclei, a new element, helium, is produced. Then, helium atoms also participate in the fusion, change the structure, and form lithium. And so on, according to the order of the periodic table of elements, beryllium, boron, carbon, nitrogen, etc. will be generated in turn, until iron is generated, and the star will collapse. This is because the iron element is quite stable, and the energy released when participating in fusion is less than the required energy, so the fusion stops, and the iron element exists in the star, which leads to that the star does not have enough energy to compete with the gravity of the massive star, thus causing the star to collapse and eventually form a black hole. It is said to be "black" because its gravity makes it impossible for the light around it to escape. Like neutron stars, black holes are evolved from stars whose mass is tens or even hundreds of times greater than that of the sun.

When a star ages, its thermonuclear reaction has exhausted the fuel in the center, and the energy produced by the center is not much. In this way, it no longer has enough strength to bear the huge weight of the shell. So under the weight of the shell, the core begins to collapse, and the matter will march toward the center point irresistibly until it finally forms a star with nearly infinitesimal volume and almost infinitesimal density. Once its radius shrinks to a certain extent (must be less than Schwarzschild radius), the distortion of space-time caused by mass makes it impossible for even light to shoot out - "black hole" is born.

accretion

Black holes are usually discovered because they gather gas around them to produce radiation, a process called accretion. The efficiency of high temperature gas radiating heat energy will seriously affect the geometric and dynamic characteristics of the accretion flow. The thin disk with higher radiation efficiency and the thick disk with lower radiation efficiency have been observed. When the accretion gas approaches the central black hole, the rotation of the radiation generated by them to the black hole is the flow of the central extended matter 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 toward the center of a gravitational well created by dark matter. Stars are formed by the collapse and fragmentation of gas clouds under their own gravity, and then by accretion of surrounding gas. Planets (including earth) are also formed around newly formed stars by the accumulation of gas and rock. When the central object is a black hole, accretion will show its most spectacular side. Black holes not only accrete matter, but also radiate particles through Hawking evaporation process.

evaporation

Because the density of black hole is very large, according to the formula, we can know that density = mass / volume. In order to make the density of black hole infinitely large and the mass of black hole unchanged, it means that the volume of black hole must be infinitely small, so that it can become a black hole. Black hole is a kind of dead star formed by the extinction of some stars. Its mass is very large and its volume is very small. According to Hawking's theory, there is a phenomenon called "tunneling effect" in quantum physics, that is, although the distribution of the field strength of a particle is as strong as possible in the place with low energy, even in the place with high energy, the field strength will still be distributed. For the boundary of the black hole, this is a barrier with high energy, but the particle still exists It's possible to get out.

Hawking also proved that every black hole has a certain temperature, and the temperature is inversely proportional to the mass of the black hole. In other words, the large black hole has low temperature and weak evaporation; the small black hole has high temperature and strong evaporation, which is similar to a violent explosion. It takes about 1x10 ^ < I < / i66 years for a black hole equivalent to the mass of the sun to evaporate; it takes 1x10 - < I < / I21 seconds for a black hole equivalent to the mass of an asteroid to evaporate completely.

ruin

Black holes will emit dazzling light, shrink in size, even explode, eject objects, and emit dazzling light. When British physicist Stephen William Hawking made this prediction in 1974, the whole scientific community was shocked.

Hawking's theory is a leap of thinking dominated by inspiration. He combines general relativity and quantum theory. He finds that the gravitational field around the black hole releases energy and consumes the energy and mass of the black hole.

Suppose a pair of particles will be created at any time and any place, and the created particles are positive particles and antiparticles. If the creation process occurs near the black hole, there will be two situations: two particles annihilate and one particle is sucked into the black hole. "A particle is sucked into the black hole": a pair of particles created near the black hole, one of the antiparticles will be sucked into the black hole, and the positive particle will escape. Because energy cannot be created out of thin air, we assume that the antiparticle carries negative energy, and the positive particle carries positive energy, and all the motion processes of the antiparticle can be regarded as the opposite motion process of a positive particle, such as a black hole When an antiparticle is sucked into the black hole, it can be regarded as a positive particle escaping from the black hole. In this case, a particle with positive energy from the black hole escapes, that is, the total energy of the black hole is less. Einstein's mass energy equation E = MC2 shows that the loss of energy will lead to the loss of mass.

As the mass of a black hole gets smaller and smaller, its temperature gets higher and higher. In this way, when a black hole loses mass, its temperature and emissivity increase, so its mass loss is faster. This kind of "Hawking radiation" can be ignored for most black holes, because the radiation of large black holes is relatively slow, while small black holes radiate energy at a very high speed until the explosion of black holes.

Form of expression

According to British media reports, a new theory points out that the way black holes die may be transformed into white holes. Theoretically, the white hole is just the opposite of the black hole in behavior - the black hole is constantly devouring material, while the white hole is constantly ejecting material. The discovery was first reported by a British magazine website, based on the obscure theory of quantum gravity.

The space-time distortion of stars changes the path of light, making it different from the path without stars. Light moves slightly toward the surface of the star

Chinese PinYin : Hei Dong