# 10 facts about black holes that everyone should know

Black holes are perhaps the most mysterious.objects of the universe. Unless, of course, somewhere in the depths things are not hidden, the existence of which we do not know and cannot know, which is unlikely. Black holes are colossal mass and density, compressed into one point of a small radius. The physical properties of these objects are so strange that they force the most sophisticated physicists and astrophysicists to puzzle over. Sabina Hossfender, a theoretical physicist, made a selection of ten facts about black holes that everyone should know.

What is a black hole?

The defining property of a black hole is itshorizon. This is the border, overcoming which nothing, not even light, can return back. If a separated area becomes separated forever, we are talking about an “event horizon.” If it is only temporarily separated, we are talking about a "visible horizon." But this “temporarily” can also mean that the area will be separated much longer than the present age of the Universe. If the horizon of a black hole is temporary, but long-lived, the difference between the first and the second blurs.

How big are black holes?

You can imagine the horizon of a black hole as a sphereand its diameter will be directly proportional to the mass of the black hole. Therefore, the more mass falls into a black hole, the larger the black hole becomes. Compared to stellar objects, however, black holes are tiny, because mass is compressed into very small volumes under the influence of irresistible gravitational pressure. The black hole's mass radius from planet Earth, for example, is just a few millimeters. This is 10,000,000,000 times smaller than the present radius of the Earth.

The black hole radius is called the Schwarzschild radius in honor of Karl Schwarzschild, who first derived black holes as a solution for Einstein's general theory of relativity.

What is happening on the horizon?

When you cross the horizon, nothing around younothing special happens. All because of the principle of equivalence of Einstein, from which it follows that it is impossible to find the difference between the acceleration in flat space and the gravitational field that creates the curvature of space. Nevertheless, an observer far from a black hole, who is watching someone else falling into it, will notice that a person will move more and more slowly, approaching the horizon. As if the time near the event horizon moves more slowly than far from the horizon. However, some time will pass, and the observer falling into the hole will cross the event horizon and will be inside the Schwarzschild radius.

What you experience on the horizon depends ontidal forces of the gravitational field. Tidal forces on the horizon are inversely proportional to the square of the mass of the black hole. This means that the larger and more massive the black hole, the less power. And if only the black hole is massive enough, you can overcome the horizon before you notice that something is happening. The effect of these tidal forces will stretch you: the technical term that physicists use to do this is called “spaghettification.”

In the early days of the general theory of relativity, it was believed that a singularity existed on the horizon, but this was not the case.

What is inside a black hole?

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No one knows for sure, but definitely not a bookshelf. The general theory of relativity predicts that there is a singularity in a black hole, a place where tidal forces become infinitely large, and as soon as you overcome the event horizon, you can no longer go anywhere except in singularity. Accordingly, it is better not to use GR in these places - it simply does not work. To say what happens inside a black hole, we need a theory of quantum gravity. It is generally recognized that this theory will replace the singularity with something else.

How are black holes formed?

We currently know about four differentways to form black holes. We best understand the stellar collapse. A sufficiently large star forms a black hole after its nuclear synthesis ceases, because everything that could already be synthesized was synthesized. When the pressure created by synthesis ceases, the substance begins to fall to its own gravity center, becoming more dense. In the end, it is so compacted that nothing can overcome the gravitational effect on the surface of the star: this is how a black hole is born. These black holes are called “black holes of the solar mass” and are most common.

The next common type of black holesare “supermassive black holes”, which can be found at the centers of many galaxies and which have masses about a billion times larger than black holes of solar mass. It is not yet known exactly how they are formed. It is believed that once they began as black holes of solar mass, which in densely populated galactic centers absorbed many other stars and grew. Nevertheless, they seem to absorb matter faster than this simple idea suggests, and how they do it is still the subject of research.

More controversial ideas are primary black holes,which could be formed by almost any mass in large density fluctuations in the early Universe. Although it is possible, it is rather difficult to find a model that produces them without creating an excessive amount of them.

Finally, there is a very speculative idea thatAt the Large Hadron Collider, tiny black holes with masses close to the Higgs boson mass can form. This only works if our universe has additional dimensions. While there was no evidence in favor of this theory.

How do we know that black holes exist?

We have a lot of observational evidence.the existence of compact objects with large masses that do not emit light. These objects impose themselves on gravitational attraction, for example, due to the motion of other stars or gas clouds around them. They also create gravitational lensing. We know that these objects do not have a solid surface. This follows from observations, because a substance, falling on an object with a surface, should emit more particles than a substance falling through a horizon.

Why did Hawking say last year that black holes do not exist?

He meant that black holes do not have an eternal horizon of events, but only a temporary seeming horizon (see point one). In the strict sense, only the event horizon is considered a black hole.

How do black holes emit radiation?

Black holes emit radiation due to quantumeffects. It is important to note that these are the quantum effects of matter, and not the quantum effects of gravity. The dynamic space-time of a collapsing black hole changes the very definition of a particle. Like the passage of time, which is distorted next to a black hole, the concept of particles is too dependent on the observer. In particular, when an observer falling into a black hole thinks that he is falling into a vacuum, the observer far from the black hole thinks that this is not a vacuum, but a space full of particles. It is the stretching of space-time that causes this effect.

First discovered by Stephen Hawking,The radiation emitted by a black hole is called Hawking radiation. This radiation has a temperature inversely proportional to the mass of the black hole: the smaller the black hole, the higher the temperature. In the stellar and supermassive black holes that we know, the temperature is well below the microwave background temperature and therefore not observed.