A most mysterious place, but also a place you can never know, at the same time!

Einstein's general theory of relativity is like a magnificent palace that looks imposing. He summarized the universal laws of the universe with only a few simple assumptions. For example, the bending of light has been measured and is almost identical to the calculations given by general relativity. The cosmic microwave background radiation also verifies the Big Bang. With the intervention of the cosmological constant, the accelerated expansion of the universe has also been rationalized. Einstein's theory of relativity has a nearly perfect explanation for the macroscopic universe. But as we mentioned earlier, general relativity explains the general laws of the universe, but the universe is not only general, it has many dualities. Today, let's talk about the ultimate mystery that haunted Einstein's like - black holes.

Before talking about black holes, I would like to introduce some basic concepts. First of all, the first cosmic speed is the minimum speed required to make a uniform circular motion around the Earth near the ground, which is 7.9 kilometers per second. This is the case, for example, for those satellites that fly around the Earth. The second cosmic speed is the minimum speed required to break away from the Earth's gravity, which is 11.2 kilometers per second. This is the case, for example, of those probes that land on the moon. The third cosmic speed is the minimum speed required to escape from the sun's gravitational bindings, 16.7 kilometers per second, such as Voyager II, which is escaping the solar system. The fourth cosmic speed is the minimum speed needed to escape the gravitational bind of the galaxy, 525 kilometers per second. No human vehicle has yet been able to reach this speed.

Except for the first cosmic speed, the other three are to escape from gravitational binding. This speed is called escape velocity. The greater the gravitational pull of the object, the greater the required escape velocity. 1783, the British astronomer John Michell introduced the concept: He said that if a star could become large enough that its escape velocity was equal to the speed of light, then even light could not escape. If this is the case, the object comes from outside and should be unobservable. This was the prototype of the original black hole concept. Then the question arises, what kind of celestial body can do it?

In 1916, the German astronomer Carl Swasey answered. He first introduced the concept of the Swasey radius. The Swasey radius is a very important concept in both gravity and general relativity. Simply put, any body with mass has a critical radius. If this matter is compressed within this radius value, it will compress itself to a starting point by its gravity, and this critical radius value is the Swartz radius. The Sun's Swartz radius is three kilometers. Does that mean the sun will eventually become a black hole with a radius of three kilometers? Unfortunately, it can't be done. Why would that be the case? Because the Sun is not massive enough. When it burns out, its gravity cannot collapse it to within the Swartz radius because it is not massive enough, so it cannot eventually form a black hole. Stars that are less than 1.4 times the mass of the Sun will end up as white dwarfs, just like the Sun. Stars with 1.4 to 3 times the mass of the Sun will eventually become neutron stars, and stars with more than 3 times the mass of the Sun will eventually become black holes.

As I said earlier, the Swastika radius is proportional to the mass of the star. The Sun's Swastika radius is three thousand meters, so a star three times the mass of the Sun will eventually become a black hole with a radius of nine thousand meters. According to relativity, light bends severely as it approaches such a massive object. When it is 1.5 times the Swastika radius from the black hole, the light will orbit the black hole in a circular orbit. That is, once the light reaches this distance, it cannot escape. This light will circle the black hole, so the light circling the black hole will not end up in your eyes, so the black hole is not directly observable by the naked eye, but only indirectly. Astronomers have found that the accretion disk around a black hole emits intense radiation and heat. And the light that passes through a black hole but is not bound by too close a distance will be severely bent so that we can indirectly know the existence of a black hole.

Above is the first photo of a black hole taken by humans on April 10, 2019. This black hole is located at the center of a giant elliptical galaxy in the constellation Virgo. It is 55 million light-years from Earth and 6.5 billion times the mass of the Sun. This photo is significant. It is the first real confirmation of the existence of a black hole.

There are currently two types of black holes known to mankind. The first one is the stellar black hole we introduced earlier, i.e. a star with more than three times the mass of the Sun, which burns up and collapses into a black hole under its enormous gravitational force. Before I introduce the second type of black hole, let me ask you a question. Why are our galaxies not falling apart? Because there is a huge black hole at the center of our galaxy, and our sun is leading the entire solar system around the black hole at the center of our galaxy at a super speed of 240 kilometers per second. This black hole is called a galactic black hole. These black holes are generally very massive, with masses typically between a million and several billion times that of the Sun. If you want to see what the center of the galaxy looks like? On this subject, scientists have long had an idea, but it's impossible to see. That's because the center of the galaxy has a thick cloud of dust that keeps it taut.

Okay, now that you know a lot about black holes, let me show you how it works. First, the mass of a black hole is enormous. According to relativity, the enormous mass distorts the space-time around it, potentially sending you instantly to the home of the Grays. If so, you can talk about cooperation in the morning and come back for tea in the afternoon. Second, time travel. We say that black holes can warp space-time, which means we can also use them for time travel. Speaking of time travel, he travels to the future and back to the past. Going to the future is quite simple. You can rely on the huge gravitational force around a black hole to get yourself infinitely close to the speed of light. At this time, your time is relatively stationary. Although your time is relatively stationary, the time of people living on Earth still flows 1 minute and 1 second. When you get tired of playing in it, and then escape faster than the speed of light, you return to Earth. My God, the Earth has passed 100,000 years.

Going back to the past is much more difficult than traveling to the future. First, you have to solve a paradox - the grandfather paradox, which I believe we should all be familiar with. That is, if you turn back time and kill your grandfather, you will never be born in the future. Since you don't even exist in the future, how can you go back in time and kill your grandfather? This is the grandfather paradox, and the logic is not self-consistent. Is it impossible to turn back the clock? Not necessarily. If there is a mysterious force in the universe that can prevent you from performing any action that can change the future at any time, then it is still possible for you to travel back in time to the past in the same timeline.