Amazing Research: Why Black Holes Are Black

Black Holes: Why They Are Black, How They Are Found, And What Quasars Have To Do With It

 

Black holes are one of the most mysterious objects in the universe. Together with astrophysicist Sergey Popov, we figure out what it is and why telescopes from all over the world are needed to shoot them

What Is A Black Hole?

A black hole is an area inside space with gravity so strong that it sucks in everything around it, including light. RAS professor Sergei Popov explains that black holes do not have one clear definition, and even this is one of the options. If you ask different scientists – astrophysicists and physicists – they will approach the answer from different angles. There are encyclopedic dictionaries that fix definitions and give specific answers, but there is no single correct formulation.

Sergei himself defines black holes as the most compact object that does not exhibit surface properties. And the size of this object corresponds to the Schwarzschild radius – the distance from the center of the body to the event horizon. Where the event horizon is the “point of no return” or the boundary of the black hole.

Each object has its own Schwarzschild radius, which can be calculated. If you compress any object to this radius, it will turn into a black hole. Relatively speaking, if we wanted to compress the Sun and transform it into a black hole, its radius would be only 3 km, while the initial one was about 700 thousand km.

The phrase “black hole” itself is just a well-thought-out designation. Kind of like the Big Bang. The very idea of ​​black holes arose at the end of the 18th century. Then they were called differently: there were variants of “frozen stars” or “collapsars”. But in the end, science journalist Ann Ewing came up with the term.

Visualization of a black hole (Photo: NASA)
Sergei says that some phrase often takes root in science precisely because it is convenient. A hole – because if something gets in, it can’t get back out. And black – because by itself this object emits nothing or practically nothing. If we imagine an empty Universe, black space, and place a black hole there, then it will be impossible to see it. She does not stand out against the background of this blackness.

Black Holes As A Region Of Space-Time

Black holes are also defined as a region of space-time. Sergey Popov explains that all modern theories of gravity are geometric theories. They describe gravity as a property of space and time. It means that between space and time you can make an equation, these are interrelated quantities.

Since the beginning of the 20th century, from the first works of Einstein on the theory of relativity, space and time have been united into a certain entity. Any bodies, not only massive but also the smallest, bend the space around them and at the same time affect the course of time. Modern measurements make it possible to determine that in one place time goes differently than in another. You can experiment and find this difference.

A black hole is an extreme way of influencing space  – when so much matter or energy was collected in one place that space-time collapsed and formed a specific region. You can say that a black hole is an object, but from the everyday point of view, an object is something that has a surface.

If you walk through a completely dark room, you can stumble upon a table, it will be an object with a start at a specific point. If you fall into a black hole in a completely dark room or blindfolded, it is impossible to notice its border. Because there is no solid surface, a person will immediately find himself inside this area.

Sergey compares such a transition with state or regional borders. If you walk through the forest from one country to another, then without signs and maps it is impossible to notice at what point one state ends and another begins. The forest in Finland is no different from the forest in Russia, and there is no clear boundary to stumble upon. And a black hole is a region where the mass has folded space-time, and as a result, no objects can leave it as soon as they cross the boundary. Everything that got there will forever remain beyond the horizon.

Black holes are interesting primarily as extreme objects. This is the most twisted space-time, and many effects become more noticeable near black holes. Fundamentally new physical phenomena begin to appear.

Gravity theory seeks to get as close as possible to these extreme objects. Therefore, Sergei says, studying the behavior of matter in the vicinity of black holes is a very interesting thing.

How To Find A Black Hole

 

At the end of their lives, massive stars can turn into black holes. And at the stage when they were just trying to find the first black holes, the question arose: how can they be detected. The first idea was this: stars, especially massive ones, are often born in pairs. One of these stars turns into a black hole, and we stop seeing it. Yet it continues to exist. We were supposed to be able to see the rotation of a nearby star around this invisible object, use calculations to measure its mass and find that there is a black hole in this place.

Sergey Popov says that historically this was the first proposed search method. Since the 60s, scientists have tried to look for them using this method, but found nothing. Over the past couple of years, possible candidates for the title of black holes have begun to appear, but scientists are not yet sure that they are paired with ordinary stars.

If we again turn to the black hole, which is adjacent to the star, then the matter from an ordinary star can flow into the hole. A black hole will suck in this matter with its gravity. If we imagine that two stones were thrown at it at the same time, they could collide over the horizon at a speed almost equal to the speed of light. In such a collision, a lot of energy will be released, which can be seen.

But the stars are not stones, but gas. When different layers of gas rub against each other, they heat up to millions of degrees, and this heat can be seen. Using this method, in the late 60s and early 70s, when the first X-ray detectors were launched into space, the first black holes were also discovered.

In the early 60s, it became clear that there are bright astronomical objects – quasars. Literally, “a radio source similar to a star.” These are the active nuclei of galaxies at the initial stage of development, in the center of which are supermassive black holes. You can find them even at very remote distances.

During the study of quasars, it became clear that this is a small source that is located in the center of a distant galaxy and at the same time emits a lot of energy. Popov says that when scientists discover a quasar, they are sure that a supermassive black hole “sits” there. Now this is the most massive way to discover black holes.

Almost all massive stars turn into black holes, but not all of them are in binary systems, or they do not have spillover. In this case, holes are searched for in a different way. Sergey says that a black hole strongly distorts the space-time around it, but here it is not so much the mass that is important, but the compactness. It is easy to understand, it is enough to imagine a sharp object.

This is an item with a very small area. If you just poke somewhere with your finger, you cannot pierce the surface, and if you press on the needle with the same force, then the finger that presses on it will pierce. So small objects with the same mass bend the space-time around them more strongly. This effect is called gravitational lensing.

Scientists are watching a star and suddenly notice that its brightness increases, and then falls back in a completely symmetrical way. Nothing happened to the star, but a massive object flew between us and the star. And this massive object, distorting space-time, collected light rays.

Therefore, it seems as if the luminosity of the star is increasing, but in fact, just more of its light was collected and came to us. A star with a mass of ten solar masses would shine very noticeably, scientists would not miss it. And in such observations, an absolutely dark object with a mass of about ten solar masses appears. What could it be? Only a black hole.

If there is a pair of black holes, then, merging, they will generate a gravitational wave surge. And in 2015, such bursts of gravitational radiation were detected for the first time. This is the last good way to search for black holes today.

How To Photograph A Black Hole

Sergei Popov suggests recalling films or books about the invisible man. He is not visible, but if he puts on clothes, we see the clothes. If he tries to hide, then you can sprinkle him with flour or notice traces. Black holes are studied in much the same way. Scientists can’t see the event horizon, and they can’t see the interior of a black hole, because nothing can cross the horizon back to us. But they study the behavior of matter around them.

What is commonly called a photograph of a black hole is actually an image of matter moving around a black hole. But there is indeed a dark region at the center because there is a black hole from which no light can come.

For the most part, black holes are small objects that are very far away from us. It was possible to see blackness inside the bright area only in one case. For a high-quality image, the largest black hole in the center of a relatively nearby galaxy was needed. Then the technical task arose – to get an image with sufficient detail. No telescope by itself can make such an image.

But if you combine several telescopes and spread them over long distances, then in terms of details they will work like one large telescope. It was in this way, with the help of several telescopes scattered almost all over the globe, that they managed to take a picture of what everyone calls a photograph of a black hole in the M87 galaxy. This photo is the only one so far.

To get something like a snapshot from other objects, scientists need new tools. Nevertheless, there are direct observations of the behavior of matter around various black holes, almost all the way to the horizon. Up to a distance only a few times the size of the black hole’s horizon.

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