What is a black hole? Our definition of a black hole is actually very simple from the beginning, mainly referring to a spatial region from which no matter what matter, even light, can escape. This space region is a black hole, and the edge of this region is called? Event horizon? Any substance or information that falls into the visual interface will fall into the center of the black hole and will never escape, which means that the information inside and outside the visual interface is completely disconnected, so it is impossible for us to know what is going on inside and what is inside will never get out.
However, Hawking put forward the theory of black hole radiation according to the quantum fluctuation in vacuum, arguing that black holes are slowly releasing energy and losing mass, so when a black hole disappears completely, all the information of all the substances that fell into the black hole before will be lost. This is the paradox of black hole information loss, which is inconsistent with the idea of information conservation in quantum mechanics.
Hawking immediately suggested (this is his guess, which has not been confirmed) that the information between the black hole and the outside world is only temporarily disconnected, called? Apparent horizon? This? Apparent horizon? In the future, it will disappear, the information trapped in the black hole will not be lost, and it will return to the universe, but this temporary isolation may be older than the universe, which means that the difference between complete disconnection and temporary disconnection cannot be observed. Personally, I think Hawking is talking nonsense!
How big is a black hole? We can imagine a black hole as a sphere, and its diameter is directly proportional to the mass of the black hole, that is, the more mass that initially forms a black hole or later falls into the black hole, the greater the volume of the black hole. But compared with celestial bodies with the same mass, black holes are still very small. This is because the black hole has compressed its mass to a very small volume under the great gravity. For example, a black hole with the mass equivalent to the Earth has a radius of only a few millimeters, and the radius of the earth is about 654.38+billion times that of a black hole with the same mass.
The radius of a black hole is called schwarzschild radius, which is expressed by Karl? Named after Schwarzenegger. Because of Carl? Schwarzschild first proposed black holes as the solution of Einstein's general theory of relativity. But don't forget that there are tens of billions of times the mass of the sun in the universe, and these black holes are still very big.
What happens when you get stuck in a visual interface? When approaching the black hole, we will see that the background starlight is seriously distorted, but if there is no starlight in the background area where the black hole is located, we will not see any changes in the surrounding environment when approaching the black hole or even crossing the visual interface. You don't even know that you are falling, accelerating or being influenced by gravity. This is because of an inference of Einstein's equivalence principle.
We can't tell the difference between the acceleration of plane space and the gravitational field that causes the space to bend. Because there is no starlight in the background of the black hole, it is all black, and we can't even find a frame of reference to tell us that it is accelerating.
However, if an observer who is far away from the black hole sees someone fall into the black hole, he will notice that the closer he is to the visual interface, the slower his movements will be. Because the time near the black hole horizon is much slower than the time away from the black hole horizon. But for people who fall into a black hole, they will cross the visual interface in a short time.
One more thing, a person's experience of falling into a black hole on the visual interface depends on the size of the gravitational field tidal force. The tidal force on the apparent interface is inversely proportional to the square of the black hole mass. In other words, the greater the mass of a black hole, the smaller the tidal force. If the mass of the black hole is large enough, it will not have any influence on us, and we can safely cross the visual interface. If tidal force is big enough, the head and feet will feel a huge gravity difference, and our bodies will be stretched. What are the technical terms in physics? Pasta? .
What's in a black hole? No one knows this question, but it is almost certain that what is inside a black hole is by no means any material form we have ever seen. In the language of general relativity, there is a singularity inside a black hole, at which gravity becomes infinite. Once any object passes through the visual interface, it will quickly hit the singularity, but general relativity can't tell us what the singularity is, and it also collapses at the singularity, which means that it is not applicable and understandable.
What we need is the theory of quantum gravity. It is generally believed that this theory will replace the singularity with something else.
How are black holes formed? We know that black holes can form in four different ways. The easiest to understand is the stellar collapse type black hole. A star with enough mass will collapse to form a black hole after nuclear fusion stops, because when the radiation pressure generated by fusion stops, the matter will start to fall to its center of gravity and the density will increase. Finally, nothing can overcome the gravity on the surface of the star, so a black hole is produced. These black holes are called. Stellar mass black hole? , is the most common black hole.
What is the next common type of black hole? Supermassive black hole? It is usually located in the center of a galaxy, and its mass is about billions to tens of billions times that of a solar mass black hole. To be precise, the formation of these supermassive black holes is not completely clear at present. However, we generally think that they were originally a stellar-mass black hole. At the center of a galaxy with dense stars and black holes, they merged with each other to devour other stars and grow into supermassive black holes.
The more controversial view is the primitive black hole, which was probably formed under the high-density fluctuation of the early universe. Although this is possible, it is difficult to find a model that can produce primitive black holes without forming too many primitive black holes.
Finally, there is a highly speculative black hole, that is, a tiny black hole with a mass similar to the Higgs boson can be produced in the Large Hadron Collider. This will only happen if there are extra dimensions in our universe. But so far we haven't found any extra dimensions.
How do we know the existence of black holes? From theory to reality, we have a lot of observational evidence about black holes. At first, we discovered black holes through gravity. For example, in the center of the Milky Way, we find a large number of stars moving at high speed around a non-luminous point. According to the law of gravity, we can know that the mass of this central point is millions of times that of the sun.
For example, some black holes with large mass in the center of some galaxies in the universe will also produce some observable optical effects due to their abnormal activities. For example, after accretion, the black holes will radiate X-rays in the accretion disk, and will also form obvious radio sources in the center. It is by using these properties of black holes that we obtained the first black hole photo in history.
Why did Hawking once say that black holes don't exist? This sentence is a bit out of context. In fact, what Hawking wants to express is not here. Black holes are real. What he wants to say is that he thinks that black holes don't have an eternal event horizon, but a short event horizon, so as to solve the problem of information loss of black holes. But this is what he guessed.
How does a black hole emit radiation and lose mass? Black holes emit radiation through quantum effects. It should be noted that this is the quantum effect of matter, not the quantum effect of gravity. Quantum mechanics believes that vacuum is not empty, and there are positive and negative virtual particle pairs in a very short time, and energy returns to the universe. However, if this quantum effect occurs at the edge of the visual interface of a black hole, one of the virtual particles will fall into the black hole, while the other virtual particle will steal energy from the black hole, become a real particle and escape, annihilating nearby anti-real particles and releasing energy.
The radiation emitted by a black hole was originally emitted by Stephen? Hawking proposed that it is called? Hawking radiation? . The radiation temperature is inversely proportional to the mass of the black hole. The smaller the black hole, the higher the radiation temperature. As far as we know, the radiation temperature of stars and supermassive black holes is much lower than that of cosmic microwave background radiation, so it is impossible to detect such an effect. It is no exaggeration to say that we humans may not be able to verify Hawking's statement in the future.
What is the paradox of information loss? The paradox of information loss is caused by Hawking radiation. This radiation is a pure thermal process, which means that the radiation is completely random except for a specific temperature. Moreover, the radiation does not contain any information about the formation of black holes, nor does it contain any information about objects that have fallen into black holes before, but when a black hole emits radiation, it will lose its mass and gradually shrink. Eventually the black hole will be completely transformed into random radiation. That is to say, information about how the black hole was formed before and the objects falling into the black hole will be lost, but quantum mechanics does not allow this to happen.
So the evaporation of black holes is inconsistent with the quantum theory we know, and we must make some concessions. This inconsistency must be eliminated in some way. Most physicists believe that the solution is that Hawking radiation must contain this information in some way, or this information can escape from the black hole in other ways.
How to solve the problem of black hole information loss? This is the frontier problem of physics, which science cannot falsify. Many hypotheses have been put forward, such as: black holes may be connected by white holes in another dimension, and material information entering from black holes will be spit out from white holes; The black hole is actually the entrance of a wormhole, which connects with another time and space, and material information can also come out. These are completely beyond the scope and ability of our human beings to verify and observe. At present, these speculations are meaningless.
Moreover, in order to fill this loophole, Hawking suggested that black holes actually have a way of storing information, which has been neglected before. In fact, material information is stored in the event horizon of a black hole, which can cause tiny motion of particles in Hawking radiation. In these small changes, there may be information about disappearing substances. It's mysterious, but it can't be proved.
These are some questions about black holes. At present, we can be sure that black holes exist. We can find them in the universe and know how they formed and eventually disappeared. But the specific direction of information entering the black hole remains to be studied!