Speaking of black holes, I believe that everyone is not unfamiliar, it should be the most mysterious celestial body in the universe, scientists know that it is there, but they only listen to its sound, do not see its shape.
At first, scientists didn't know that there were extreme objects like black holes in the universe. Prophecies about the existence of such celestial bodies are derived from Einstein's general theory of relativity.
When a large amount of mass is concentrated in a very small range of space, under the action of strong gravity, space-time will be highly curved, and a closed space-time with infinite curvature will be formed at a certain radius, which will form a black hole.
Numerous astronomical observations have shown that there is a supermassive black hole in the center of almost every galaxy, such as a supermassive black hole in the center of the Milky Way, Sagittarius A, which has a mass about 4 million times that of the Sun.
Are black holes scary? Can I get close to a black hole?
In the understanding of many people, a black hole is a bottomless pit that can swallow everything, even light can swallow. Some even think that walking through a black hole can instantly reach another parallel universe, or another distant place in the universe.
Indeed, the gravitational pull around the black hole is very strong, and once any object is captured by the gravitational pull of the black hole, it can no longer come out. Because the escape velocity within the event horizon of a black hole has exceeded the speed of light, it is impossible for any massive object to reach or exceed the speed of light.
However, black holes are not as dangerous as everyone thinks, as long as the laws of physics are used reasonably, they will not fall into black holes. The Earth did not fall to the Sun because the Earth was orbiting the Sun at a speed of 30 kilometers per second. If future humans want to fly a spaceship to see a black hole up close without falling into it, they will have to orbit the black hole in a very precise orbit at a suitable distance at tens of thousands of kilometers per second.
Only when you are too close to the black hole will it be torn apart or even swallowed up by the black hole. That is, although there is a large black hole at the center of each galaxy, it will not eventually swallow up the entire galaxy.
Although it is possible to avoid falling into the black hole, the tidal force generated by the strong gravitational pull of the black hole should not be underestimated, and if it is not done well, the spacecraft will disintegrate.
What happens if we stay near a black hole?
In 2019, scientists took a picture of the supermassive black hole at the center of the M87 galaxy more than 50 million light-years from Earth through the Event Horizon Telescope (EHT), the first photo of a black hole taken by humans.
In fact, this photo does not capture the black hole itself. Because in theory, it is impossible to photograph the black hole body through electromagnetic detection, because the black hole body does not radiate electromagnetic waves to the outside world.
According to physicists, the black hole itself is composed of singularities and the event horizons that envelop the singularities. A singularity is a point of infinite small size and infinite density. The event horizon is the outermost layer of the black hole, which is the sub-interface between space-time inside the black hole and the outside world. Once you pass through this visual interface, even the light cannot come out.
The bright halo in the first photo of the black hole is actually an accretion disk around the black hole. The accretion disk is made up of a large amount of material torn apart by the black hole, and before falling into the black hole, it will rotate around the black hole body at high speed, releasing a large amount of light and heat in the process of friction. Some of the material also ejects radiation from the poles of the black hole in the form of radiation in the process.
It can be seen that staying near the black hole, the most fatal thing is not only gravity, but also radiation, which is not only harmful to organisms, but also damages the spacecraft.
But there are some surprises relative to these risks.
According to Einstein's general theory of relativity, in a strong gravitational field, the passage of time slows down relative to a weak gravitational field. This conclusion has long been confirmed by experiments and has practical applications. For example, the height difference will bring about a small change in the intensity of the gravitational field, and the Global Positioning System (GPS) must be designed to take into account the resulting problem that the ground equipment is out of sync with the satellite time, and the solution to this problem is to calibrate the atomic clock on the satellite every once in a while to synchronize it with the time of the ground atomic clock.
Near black holes have the strongest known gravitational field in the universe. When we are in the strong gravitational field of a black hole, because the gravitational field of the Earth is much weaker than that of the black hole, then the time we experience is slower than the time experienced by people on Earth.
This is not only reflected in the results of the measurement of clocks and watches, but we can also experience this wonderful feeling ourselves. If we could return to Earth in an instant, we would find that we had only been around the black hole for a few days, and that people on Earth might have been tens or hundreds of years old.
Theoretically, as long as we are close enough to the black hole and stay long enough, we can travel to the distant future in this way. As the so-called "one day in the sky, one year in the ground", it seems that there are really such magical places in the universe.
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