On April 10, a team of researchers at the Telescope Event Horizon project showed the first ever black hole image. Indirectly, the existence of these objects, which absorb even the light, was confirmed before, but now they were first shown clearly. https://meduza.io/feature/2019/04/10/astrofiziki-vpervye-pokazali-izobrazhenie-chernoy-dyry
I asked Visiting Professor of the Laboratory of Fundamental and Applied Research of Relativistic Objects of the MIPT Universe Stanislav Babak and Head of the Laboratory Yuri Kovalev to answer common questions about black holes that were asked by young people from Europe (Germany, Czech Republic, Bulgaria).
What are black holes? What is the event horizon?
Black holes are the most compact and simplest objects in the Universe. They are described by only two parameters - mass and speed of rotation. From the point of view of astrophysics, black holes are the final stage of the evolution of stars. For example, a heavy star evolves, explodes, and its center shrinks into a black hole. The event horizon of a black hole is its conditional surface, the shell, there is no physical surface, it is just some distance from the center. This is the place where the object or the light can no longer be able to get back, because here begins a very strong gravity, which does not release objects that fall into it. This can be explained mathematically: going out of the event horizon means moving in the opposite direction in time, which is impossible from the point of view of physics.
Why are black holes so called? Can they be of some color?
One of the first names of black holes - collapsars. The name “black holes” was coined in the 20th century by journalists and taken up by one of the great scientists of the time, the American theoretical physicist John Wheeler. Why blacks? This is an object that can not emit anything, the light does not come out from there. Although this is not entirely true: black holes can emit the so-called Hawking radiation (the evaporation of black holes), but at the classical level, if the light enters them, then nothing comes out, which is why they are called “black holes” - they can not be seen . We can observe a black hole only if there is some matter around it: a gas or a nearby star, from which a black hole pulls off the shell. Thanks to gravitational waves, we can “see” two merging black holes. In fact, black holes have no color, this is a conventional name, meaning that everything falls into it and nothing comes out.
Where do black holes come from?
It depends on their mass. Black holes are small (several masses of our Sun), and can be very massive (millions of masses of the Sun). The size of black holes is proportional to their mass.
Black holes with near-solar mass could originate in the early Universe. Such a black hole can be formed by squeezing a star 20–60 times the size of our Sun, which explodes at the end of life, and what remains in the center will shrink to the size of a black hole, collapse. The masses of such black holes are limited — more than 5 solar masses, but less than 50–60 solar masses.
An example of a massive black hole was shown on April 10 thanks to the "Telescope Event Horizon", which photographed an object in the center of the galaxy M87 with a mass of billions of solar. In the center of our galaxy, too, there is a similar black hole, its mass is 4 million solar. This black hole was formed together with the Galaxy - either from a giant gas cloud that contracted and formed a black hole, or it was the first generation of heavy stars that formed the first black holes, which, in turn, merged and formed black holes the size of a thousand solar masses The main mechanism for the growth of the mass of black holes in the center of galaxies is the ingestion of gas from the environment. The more you throw into a black hole, the more it grows. Moreover, if you “throw” gas or particles into a black hole in one direction, it will still unwind.
What form are they? If everything falls into a black hole, does it have a bottom?
Black holes can be of two forms: if it almost does not rotate - spherical, and if it rotates faster - spheroidal (flattened sphere). As a rule, all black holes rotate - some faster, some slower. There is no bottom in a black hole. We do not fully understand what exactly happens in the very center where the laws of physics stop working. The general theory of relativity tells us that the curvature of space at the center of a black hole is infinite. But this only means that the theory in the center of the black hole does not work. Therefore, for example, attempts to create a quantum theory of gravity are attempts to answer what could be in the very center of a black hole. While the simplest answer is infinity, singularity. But there is nothing infinite in nature, which means that so far there is something wrong in our equation.
Why study black holes?
Quite a lot of questions about the nature of the Universe are based on the concept of black holes, so it is important for scientists to confirm experimentally this idea in order to say: “Yes, we don’t deal with nonsense, these theories are valid, our constructions are correct, and we can move on such a theoretical concept as a black hole. ” We can recall the discovery of gravitational waves - the merging of two black holes with a mass of 20-30 masses of the sun. This was also indirect evidence of the existence of black holes. The discovery of the shadow of a black hole announced on April 10 is very important because it confirms the self-consistency of our theories. The prediction was this: if in the center of the Galaxy Deva A (or Messier 87) there is a black hole weighing about 6 billion solar masses, then we will see a photon ring of light just as large as we managed to measure.
Everyone noticed that the group “Event Horizon Telescope” practically did not say a word about the center of our Galaxy. The fact is that Sagittarius A * is rather unfriendly towards us for objective physical reasons. Our central black hole has a rather small event horizon. Accordingly, the explosive processes there occur very quickly - within or less than one hour. “Telescope event horizon” is difficult to restore the picture when the black hole is changing so quickly. This can be compared with the attempts of parents to photograph their constantly spinning child - the picture is always blurry. Scientists need to learn how to restore the image in the center of our galaxy in a very short time. There are no such tools yet.
How many black holes are already open?
There are a lot of objects that behave like single black holes (they pull matter and gas on themselves). As for double black holes, according to the LIGO and VIRGO catalog, there are only 10 pairs.
Do black holes really expand endlessly, absorbing everything in their path?
Not. If you specifically throw objects into a black hole, the mass will increase, but it is very difficult to get into it. The black hole at the same time will remain very small. Stars and gas that revolve around a black hole do not threaten anything - you really need to be very close to the black hole in order for it to start absorbing them.
What happens if you get into a black hole?
When you fall into a black hole, on the one hand, you begin to flatten, on the other - very stretch. There is a well-known task: what happens to an astronaut who falls into a black hole - will he tear his head first or flatten it? The correct answer is to tear off your head at the beginning. When passing through the event horizon, space is bent, and the curvature of space quickly grows until the cosmonaut himself becomes a singularity (disappears).
Can two black holes collide? What will happen then?
Two black holes are very hard to push. To do this, they must be at a very close distance to each other. One of the ways - double stars, which both as a result of "aging" turned into black holes, then they will be close to each other from the very beginning.
For two massive black holes in colliding galaxies to be close, billions of years are needed. And they must be very close for the gravitational radiation to become such that they merge over several decades. As a result of the merging of black holes, it turns out again a black hole with a larger mass and larger in size.
I think the topic of black holes is very interesting. But young people in Europe are not particularly interested in astronomy, and in particular these issues. After all, the unknown is very interesting. For my country, a very big pride that it was Yuri Gagarin on April 12, 1961 that first flew into outer space. In Russia, it is a big holiday celebrated by youth organizations throughout Russia. Volunteers organize various events and visit the planetarium on this day. Mom told me that when she was little she wanted to become an astronaut, and this went on for almost 10 years. Now if young people in Europe want to become spontaneously, we are unlikely to hear the answer by astronauts or scientists exploring black holes. But who knows, maybe when humanity will move to Mars, thanks to Illon Mask, we can be volunteers there too).