Mysterious and elusive black holes. The laws of physics confirm the possibility of their existence in the universe, but many questions still remain. Numerous observations show that holes exist in the universe and there are more than a million of these objects.

What are black holes?

Back in 1915, when solving Einstein's equations, such a phenomenon as "black holes" was predicted. However, the scientific community became interested in them only in 1967. They were then called "collapsed stars", "frozen stars".

Now a black hole is called a region of time and space that has such gravity that not even a ray of light can get out of it.

How are black holes formed?

There are several theories of the appearance of black holes, which are divided into hypothetical and realistic. The simplest and most widespread realistic theory is the theory of gravitational collapse of large stars.

When a sufficiently massive star before "death" grows in size and becomes unstable, consuming the last fuel. At the same time, the mass of the star remains unchanged, but its size decreases as the so-called compaction occurs. In other words, during compaction, a heavy nucleus "falls" into itself. In parallel with this, the compaction leads to a sharp increase in temperature inside the star and the outer layers of the celestial body are torn off, new stars are formed from them. At the same time, in the center of the star - the core falls into its own "center". As a result of the action of gravitational forces, the center collapses into a point - that is, the gravitational forces are so strong that they absorb the compacted core. This is how a black hole is born, which begins to distort space and time, so that even light cannot escape from it.

At the centers of all galaxies is a supermassive black hole. According to Einstein's theory of relativity:

"Any mass distorts space and time."

Now imagine how much a black hole distorts time and space, because its mass is huge and at the same time squeezed into an ultra-small volume. Because of this ability, the following oddity occurs:

“Black holes have the ability to practically stop time and compress space. Because of this strong distortion, the holes become invisible to us.”

If black holes are not visible, how do we know they exist?

Yes, even though a black hole is invisible, it should be noticeable due to the matter that falls into it. As well as stellar gas, which is attracted by a black hole, when approaching the event horizon, the temperature of the gas begins to rise to ultra-high values, which leads to a glow. This is why black holes glow. Thanks to this, albeit a weak glow, astronomers and astrophysicists explain the presence in the center of the galaxy of an object with a small volume, but a huge mass. At the moment, as a result of observations, about 1000 objects have been discovered that are similar in behavior to black holes.

Black holes and galaxies

How can black holes affect galaxies? This question torments scientists all over the world. There is a hypothesis according to which it is the black holes located in the center of the galaxy that affect its shape and evolution. And that when two galaxies collide, black holes merge and during this process such a huge amount of energy and matter is thrown out that new stars are formed.

Types of black holes

• According to the existing theory, there are three types of black holes: stellar, supermassive, miniature. And each of them was formed in a special way.
• - Black holes of stellar masses, it grows to enormous sizes and collapses.
  - Supermassive black holes, which can have a mass equivalent to millions of suns, are very likely to exist at the centers of almost all galaxies, including our own Milky Way. Scientists still have different hypotheses for the formation of supermassive black holes. So far, only one thing is known - supermassive black holes are a by-product of the formation of galaxies. Supermassive black holes - they differ from ordinary ones in that they have a very large size, but paradoxically low density.
• - No one has yet been able to detect a miniature black hole that would have a mass less than the Sun. It is possible that miniature holes could have formed shortly after the "Big Bang", which is the initial exact existence of our universe (about 13.7 billion years ago).
• - More recently, a new concept has been introduced as "white black holes". This is still a hypothetical black hole, which is the opposite of a black hole. Stephen Hawking actively studied the possibility of the existence of white holes.
• - Quantum black holes - they exist so far only in theory. Quantum black holes can be formed when ultra-small particles collide as a result of a nuclear reaction.
• - Primordial black holes are also a theory. They formed immediately after the occurrence.

At the moment, there are a large number of open questions that have yet to be answered by future generations. For example, can there really be so-called "wormholes" with which you can travel through space and time. What exactly happens inside a black hole and what laws these phenomena obey. And what about the disappearance of information in a black hole?

A black hole in physics is defined as a region in space-time, the gravitational attraction of which is so strong that even objects moving at the speed of light, including quanta of light itself, cannot leave it. The boundary of this region is called the event horizon, and its characteristic size is called the gravitational radius, which is called the Black Forest radius. Black holes are the most mysterious objects in the universe. They owe their unfortunate name to the American astrophysicist John Wheeler. It was he who in the popular lecture "Our Universe: Known and Unknown" in 1967 called these superdense bodies holes. Previously, such objects were called "collapsed stars" or "collapsers". But the term "black hole" has taken root, and it has become simply impossible to change it. There are two types of black holes in the Universe: 1 - supermassive black holes, the mass of which is millions of times greater than the mass of the Sun (it is believed that such objects are located in the centers of galaxies); 2 - less massive black holes that result from the compression of giant dying stars, their mass is more than three solar masses; as the star contracts, the matter becomes more and more compacted, and as a result, the object's gravity increases to such an extent that light cannot overcome it. Neither radiation nor matter can escape a black hole. Black holes are super-powerful gravitators.

The radius to which a star must shrink in order to turn into a black hole is called the gravitational radius. For black holes formed from stars, it is only a few tens of kilometers. In some pairs of binary stars, one of them is invisible to the most powerful telescope, but the mass of the invisible component in such a gravitational system turns out to be extremely large. Most likely, such objects are either neutron stars or black holes. Sometimes invisible components in such pairs rip matter off a normal star. In this case, the gas is separated from the outer layers of the visible star and falls into an unknown where - into an invisible black hole. But before falling into the hole, the gas emits electromagnetic waves of various wavelengths, including very short X-ray waves. Moreover, near a neutron star or a black hole, the gas becomes very hot and becomes a source of powerful high-energy electromagnetic radiation in the X-ray and gamma ranges. Such radiation does not pass through the earth's atmosphere, but it can be observed using space telescopes. One of the likely candidates for black holes is considered to be a powerful source of X-rays in the constellation Cygnus.

Every person who gets acquainted with astronomy sooner or later experiences a strong curiosity about the most mysterious objects in the universe - black holes. These are the real masters of darkness, capable of "swallowing" any atom passing nearby and not letting even light escape - their attraction is so powerful. These objects present a real challenge for physicists and astronomers. The former still cannot understand what happens to the matter that has fallen inside the black hole, and the latter, although they explain the most energy-intensive phenomena of space by the existence of black holes, have never had the opportunity to observe any of them directly. We will talk about these most interesting celestial objects, find out what has already been discovered and what remains to be known in order to lift the veil of secrecy.

What is a black hole?

The name "black hole" (in English - black hole) was proposed in 1967 by the American theoretical physicist John Archibald Wheeler (see photo on the left). It served to designate a celestial body, the attraction of which is so strong that even light does not let go of itself. Therefore, it is "black" because it does not emit light.

indirect observations

This is the reason for such mystery: since black holes do not glow, we cannot see them directly and are forced to look for and study them, using only indirect evidence that their existence leaves in the surrounding space. In other words, if a black hole engulfs a star, we can't see the black hole, but we can observe the devastating effects of its powerful gravitational field.

Laplace's intuition

Despite the fact that the expression "black hole" to refer to the hypothetical final stage of the evolution of a star that collapsed into itself under the influence of gravity appeared relatively recently, the idea of ​​the possibility of the existence of such bodies arose more than two centuries ago. The Englishman John Michell and the Frenchman Pierre-Simon de Laplace independently hypothesized the existence of "invisible stars"; while they were based on the usual laws of dynamics and Newton's law of universal gravitation. Today, black holes have received their correct description based on Einstein's general theory of relativity.

In his work “Statement of the system of the world” (1796), Laplace wrote: “A bright star of the same density as the Earth, with a diameter 250 times greater than the diameter of the Sun, due to its gravitational attraction, would not allow light rays to reach us. Therefore, it is possible that the largest and brightest celestial bodies are invisible for this reason.

Invincible Gravity

Laplace's idea was based on the concept of escape velocity (second cosmic velocity). A black hole is such a dense object that its attraction is able to detain even light, which develops the highest speed in nature (almost 300,000 km / s). In practice, in order to escape from a black hole, you need a speed faster than the speed of light, but this is impossible!

This means that a star of this kind would be invisible, since even light would not be able to overcome its powerful gravity. Einstein explained this fact through the phenomenon of light deflection under the influence of a gravitational field. In reality, near a black hole, space-time is so curved that the paths of light rays also close on themselves. In order to turn the Sun into a black hole, we will have to concentrate all its mass in a ball with a radius of 3 km, and the Earth will have to turn into a ball with a radius of 9 mm!

Types of black holes

About ten years ago, observations suggested the existence of two types of black holes: stellar, whose mass is comparable to the mass of the Sun or slightly exceeds it, and supermassive, whose mass is from several hundred thousand to many millions of solar masses. However, relatively recently, high-resolution X-ray images and spectra obtained from artificial satellites such as Chandra and XMM-Newton brought to the fore the third type of black hole - with an average mass exceeding the mass of the Sun by thousands of times.

stellar black holes

Stellar black holes became known earlier than others. They form when a high-mass star, at the end of its evolutionary path, runs out of nuclear fuel and collapses into itself due to its own gravity. A star-shattering explosion (known as a “supernova explosion”) has catastrophic consequences: if the core of a star is more than 10 times the mass of the Sun, no nuclear force can withstand the gravitational collapse that will result in the appearance of a black hole.

Supermassive black holes

Supermassive black holes, first noted in the nuclei of some active galaxies, have a different origin. There are several hypotheses regarding their birth: a stellar black hole that devours all the stars surrounding it for millions of years; a merged cluster of black holes; a colossal cloud of gas collapsing directly into a black hole. These black holes are among the most energetic objects in space. They are located in the centers of very many galaxies, if not all. Our Galaxy also has such a black hole. Sometimes, due to the presence of such a black hole, the cores of these galaxies become very bright. Galaxies with black holes in the center, surrounded by a large amount of falling matter and, therefore, capable of producing an enormous amount of energy, are called "active", and their nuclei are called "active galactic nuclei" (AGN). For example, quasars (the most distant space objects from us available to our observation) are active galaxies, in which we see only a very bright nucleus.

Medium and "mini"

Another mystery remains the medium-mass black holes, which, according to recent studies, may be at the center of some globular clusters, such as M13 and NCC 6388. Many astronomers are skeptical about these objects, but some recent research suggests the presence of black holes. medium-sized even not far from the center of our galaxy. English physicist Stephen Hawking also put forward a theoretical assumption about the existence of the fourth type of black hole - a "mini-hole" with a mass of only a billion tons (which is approximately equal to the mass of a large mountain). We are talking about primary objects, that is, those that appeared in the first moments of the life of the Universe, when the pressure was still very high. However, no trace of their existence has yet been discovered.

How to find a black hole

Just a few years ago, a light came on over black holes. Thanks to constantly improving instruments and technologies (both terrestrial and space), these objects are becoming less and less mysterious; more precisely, the space surrounding them becomes less mysterious. Indeed, since the black hole itself is invisible, we can only recognize it if it is surrounded by enough matter (stars and hot gas) orbiting it at a small distance.

Watching double systems

Some stellar black holes have been discovered by observing the orbital motion of a star around an invisible binary companion. Close binary systems (that is, consisting of two stars very close to each other), in which one of the companions is invisible, are a favorite object of observation for astrophysicists looking for black holes.

An indication of the presence of a black hole (or neutron star) is the strong emission of X-rays, caused by a complex mechanism, which can be schematically described as follows. Due to its powerful gravity, a black hole can rip matter out of a companion star; this gas is distributed in the form of a flat disk and falls in a spiral into the black hole. Friction resulting from collisions of particles of falling gas heats the inner layers of the disk to several million degrees, which causes powerful X-ray emission.

X-ray observations

Observations in X-rays of objects in our Galaxy and neighboring galaxies that have been carried out for several decades have made it possible to detect compact binary sources, about a dozen of which are systems containing black hole candidates. The main problem is to determine the mass of an invisible celestial body. The value of the mass (albeit not very accurate) can be found by studying the motion of the companion or, which is much more difficult, by measuring the X-ray intensity of the incident matter. This intensity is connected by an equation with the mass of the body on which this substance falls.

Nobel Laureate

Something similar can be said about the supermassive black holes observed in the cores of many galaxies, whose masses are estimated by measuring the orbital velocities of the gas falling into the black hole. In this case, caused by a powerful gravitational field of a very large object, a rapid increase in the speed of gas clouds orbiting in the center of galaxies is revealed by observations in the radio range, as well as in optical beams. Observations in the X-ray range can confirm the increased release of energy caused by the fall of matter into the black hole. Research in X-rays in the early 1960s was started by the Italian Riccardo Giacconi, who worked in the USA. He was awarded the Nobel Prize in 2002 in recognition of his "groundbreaking contributions to astrophysics that led to the discovery of X-ray sources in space."

Cygnus X-1: the first candidate

Our Galaxy is not immune from the presence of black hole candidate objects. Fortunately, none of these objects are close enough to us to pose a danger to the existence of the Earth or the solar system. Despite the large number of noted compact X-ray sources (and these are the most likely candidates for finding black holes there), we are not sure that they actually contain black holes. The only one among these sources that does not have an alternative version is the close binary Cygnus X-1, that is, the brightest X-ray source in the constellation Cygnus.

massive stars

This system, with an orbital period of 5.6 days, consists of a very bright blue star of large size (its diameter is 20 times that of the sun, and its mass is about 30 times), easily distinguishable even in your telescope, and an invisible second star, the mass which is estimated at several solar masses (up to 10). Located at a distance of 6500 light years from us, the second star would be perfectly visible if it were an ordinary star. Its invisibility, the system's powerful X-rays, and finally its mass estimate lead most astronomers to believe that this is the first confirmed discovery of a stellar black hole.

Doubts

However, there are also skeptics. Among them is one of the largest researchers of black holes, physicist Stephen Hawking. He even made a bet with his American colleague Keel Thorne, a strong supporter of the classification of Cygnus X-1 as a black hole.

The dispute over the nature of the Cygnus X-1 object is not Hawking's only bet. Having devoted several decades to theoretical studies of black holes, he became convinced of the fallacy of his previous ideas about these mysterious objects. In particular, Hawking assumed that matter after falling into a black hole disappears forever, and with it all its informational baggage disappears. He was so sure of this that he made a bet on this subject in 1997 with his American colleague John Preskill.

Admitting a mistake

On July 21, 2004, in his speech at the Relativity Congress in Dublin, Hawking admitted that Preskill was right. Black holes do not lead to the complete disappearance of matter. Moreover, they have a certain kind of "memory". Inside them may well be stored traces of what they absorbed. Thus, by “evaporating” (that is, slowly emitting radiation due to the quantum effect), they can return this information to our Universe.

Black holes in the galaxy

Astronomers still have many doubts about the presence of stellar black holes in our Galaxy (like the one that belongs to the Cygnus X-1 binary system); but there is much less doubt about supermassive black holes.

In the center

There is at least one supermassive black hole in our galaxy. Its source, known as Sagittarius A*, is precisely located in the center of the plane of the Milky Way. Its name is explained by the fact that it is the most powerful radio source in the constellation Sagittarius. It is in this direction that both the geometric and physical centers of our galactic system are located. Located at a distance of about 26,000 light-years from us, a supermassive black hole associated with the source of radio waves, Sagittarius A *, has a mass estimated at about 4 million solar masses, enclosed in a space whose volume is comparable to the volume of the solar system. Its relative proximity to us (this supermassive black hole is without a doubt the closest to Earth) has caused the object to come under particularly deep scrutiny by the Chandra space observatory in recent years. It turned out, in particular, that it is also a powerful source of X-rays (but not as powerful as sources in active galactic nuclei). Sagittarius A* may be the dormant remnant of what was the active core of our Galaxy millions or billions of years ago.

Second black hole?

However, some astronomers believe that there is another surprise in our Galaxy. We are talking about a second black hole of average mass, holding together a cluster of young stars and not allowing them to fall into a supermassive black hole located in the center of the Galaxy itself. How can it be that at a distance of less than one light year from it there could be a star cluster with an age that has barely reached 10 million years, that is, by astronomical standards, very young? According to the researchers, the answer lies in the fact that the cluster was not born there (the environment around the central black hole is too hostile for star formation), but was “drawn” there due to the existence of a second black hole inside it, which has a mass of average values.

In orbit

The individual stars of the cluster, attracted by the supermassive black hole, began to shift towards the galactic center. However, instead of being dispersed into space, they remain together due to the attraction of a second black hole located at the center of the cluster. The mass of this black hole can be estimated from its ability to hold an entire star cluster "on a leash". A medium-sized black hole appears to revolve around the central black hole in about 100 years. This means that long-term observations over many years will allow us to "see" it.

Black holes are perhaps the most mysterious objects in the universe. Unless, of course, somewhere in the depths there are things that we do not know and cannot know about, which is unlikely. Black holes are a colossal mass and density, compressed into one point of a small radius. The physical properties of these objects are so strange that they make the most sophisticated physicists and astrophysicists rack their brains. Sabine Hossfender, a theoretical physicist, has compiled a list of ten facts about black holes that everyone should know.

What is a black hole?

The defining property of a black hole is its horizon. This is the border, overcoming which nothing, not even light, can return back. If a separated area becomes permanently separated, we speak of an "event horizon". If it is only temporarily separated, we speak of a "visible horizon". But this "temporary" could also mean that the region will be separated much longer than the current age of the universe. If the horizon of a black hole is temporary but long-lived, the difference between the first and second blurs.

How big are black holes?

You can imagine the horizon of a black hole as a sphere, and its diameter will be directly proportional to the mass of the black hole. Therefore, the more mass falls into the 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 by overwhelming gravitational pressure. The radius of a black hole with the mass of the planet Earth, for example, is only a few millimeters. This is 10,000,000,000 times less than the actual radius of the Earth.

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

What's happening on the horizon?

When you cross the horizon, nothing much happens around you. All because of Einstein's principle of equivalence, from which it follows that it is impossible to find the difference between acceleration in flat space and the gravitational field that creates the curvature of space. However, an observer away from the black hole who is watching someone else fall into it will notice that the person will move slower and slower as they approach the horizon. As if time near the event horizon moves more slowly than away from the horizon. However, some time will pass, and the observer falling into the hole will cross the event horizon and find himself inside the Schwarzschild radius.

What you experience on the horizon depends on the tidal forces of the gravitational field. Tidal forces at the horizon are inversely proportional to the square of the black hole's mass. This means that the larger and more massive the black hole, the less force. And if only the black hole is massive enough, you will be able to cross the horizon even before you notice that anything is happening. The effect of these tidal forces will stretch you out: the technical term physicists use for this is "spaghettification."

In the early days of general relativity, it was believed that there was a singularity on the horizon, but this turned out not to be the case.

What's inside a black hole?

No one knows for sure, but definitely not a bookshelf. predicts that a black hole has a singularity, a place where tidal forces become infinitely large, and once you cross the event horizon, you can't go anywhere else but into a singularity. Accordingly, it is better not to use general relativity in these places - it simply does not work. To tell what happens inside a black hole, we need a theory of quantum gravity. It is generally accepted that this theory will replace the singularity with something else.

How are black holes formed?

We currently know of four different ways that black holes form. Best understood associated with stellar collapse. A sufficiently large star forms a black hole after its nuclear fusion stops, because everything that could already be synthesized has been synthesized. When the pressure created by the fusion stops, the matter begins to fall towards its own center of gravity, becoming more and 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 "solar-mass black holes" and are the most common.

The next common type of black hole are the "supermassive black holes" found at the centers of many galaxies, which have masses about a billion times greater than solar-mass black holes. It is not yet known exactly how they are formed. It is believed that they once began as solar-mass black holes that, in densely populated galactic centers, swallowed many other stars and grew. However, they seem to absorb matter faster than this simple idea suggests, and how exactly they do so is still a matter of research.

A more controversial idea has been primordial black holes, which could have been formed by almost any mass in large density fluctuations in the early universe. While it's possible, it's hard enough to find a model that makes them without making too many of them.

Finally, there is the very speculative idea that tiny black holes with masses close to that of the Higgs boson can form at the Large Hadron Collider. This only works if our universe has extra dimensions. So far, there has been no evidence to support this theory.

How do we know that black holes exist?

We have a lot of observational evidence for the existence of compact objects with large masses that do not emit light. These objects give themselves away by gravitational attraction, such as the movement 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 matter, falling on an object with a surface, should cause the emission of a larger number of particles than matter falling through the horizon.

Why did Hawking say last year that black holes don't exist?

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

How do black holes emit radiation?

Black holes emit radiation due to quantum effects. It is important to note that these are quantum effects of matter, not 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 near 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, an 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 that is inversely proportional to the mass of the black hole: the smaller the black hole, the higher the temperature. The stellar and supermassive black holes that we know have temperatures well below the microwave background temperature and are therefore not observed.

What is an information paradox?

The information loss paradox is due to Hawking radiation. This radiation is purely thermal, that is, random and of certain properties it has only temperature. The radiation itself does not contain any information about how the black hole formed. But when a black hole emits radiation, it loses mass and shrinks. All this is completely independent of the matter that became part of the black hole or from which it was formed. It turns out that knowing only the final state of evaporation, it is impossible to say from what the black hole was formed. This process is "irreversible" - and the catch is that there is no such process in quantum mechanics.

It turns out that the evaporation of a black hole is incompatible with quantum theory as we know it, and something needs to be done about it. Fix the inconsistency somehow. Most physicists believe that the solution is that Hawking radiation must contain information in some way.

What does Hawking propose to solve the black hole information paradox?

The idea is that black holes must have a way to store information, which has not yet been accepted. The information is stored at the black hole's horizon and can cause tiny displacements of particles in the Hawking radiation. In these tiny displacements there may be information about the matter that got inside. The exact details of this process are not currently determined. The scientists are awaiting a more detailed technical paper from Stephen Hawking, Malcolm Perry and Andrew Strominger. They say it will appear at the end of September.

At the moment, we are sure that black holes exist, we know where they are, how they are formed and what they will become in the end. But the details of where the information goes into them is still one of the biggest mysteries in the universe.

S. TRANKOVSKY

Among the most important and interesting problems of modern physics and astrophysics, Academician VL Ginzburg named questions related to black holes (see Science and Life, Nos. 11, 12, 1999). The existence of these strange objects was predicted more than two hundred years ago, the conditions leading to their formation were precisely calculated in the late 30s of the XX century, and astrophysics came to grips with them less than forty years ago. Today, scientific journals around the world publish thousands of articles on black holes every year.

The formation of a black hole can occur in three ways.

This is how it is customary to depict the processes taking place in the vicinity of a collapsing black hole. As time passes (Y), the space (X) around it (shaded area) shrinks towards the singularity.

The gravitational field of a black hole introduces strong distortions into the geometry of space.

A black hole, invisible through a telescope, reveals itself only by its gravitational influence.

In the powerful gravitational field of a black hole, particle-antiparticle pairs are born.

The birth of a particle-antiparticle pair in the laboratory.

HOW THEY APPEAR

A luminous celestial body with a density equal to that of the Earth and a diameter two hundred and fifty times greater than the diameter of the Sun, due to the force of its attraction, will not allow its light to reach us. Thus, it is possible that the largest luminous bodies in the universe, precisely because of their size, remain invisible.
Pierre Simon Laplace.
Presentation of the system of the world. 1796

In 1783, the English mathematician John Mitchell, and thirteen years later independently of him, the French astronomer and mathematician Pierre Simon Laplace conducted a very strange study. They considered the conditions under which light would not be able to leave a star.

The scientists' logic was simple. For any astronomical object (planet or star), you can calculate the so-called escape velocity, or the second cosmic velocity, which allows any body or particle to leave it forever. And in the physics of that time, the Newtonian theory reigned supreme, according to which light is a stream of particles (almost a hundred and fifty years remained before the theory of electromagnetic waves and quanta). The escape velocity of particles can be calculated on the basis of the equality of the potential energy on the surface of the planet and the kinetic energy of the body "escaping" to an infinitely large distance. This speed is determined by the formula #1#

where M is the mass of the space object, R is its radius, G is the gravitational constant.

From here, the radius of a body of a given mass is easily obtained (later called the "gravitational radius r g "), at which the escape velocity is equal to the speed of light:

This means that a star compressed into a sphere with radius r g< 2GM/c 2 will stop emitting - the light will not be able to leave it. A black hole will appear in the universe.

It is easy to calculate that the Sun (its mass is 2.1033 g) will turn into a black hole if it shrinks to a radius of about 3 kilometers. The density of its substance in this case will reach 10 16 g/cm 3 . The radius of the Earth, compressed to the state of a black hole, would decrease to about one centimeter.

It seemed incredible that forces could be found in nature that could compress a star to such an insignificant size. Therefore, the conclusions from the work of Mitchell and Laplace for more than a hundred years were considered something like a mathematical paradox that has no physical meaning.

A rigorous mathematical proof that such an exotic object in space is possible was obtained only in 1916. The German astronomer Karl Schwarzschild, having analyzed the equations of the general theory of relativity of Albert Einstein, received an interesting result. Having studied the motion of a particle in the gravitational field of a massive body, he came to the conclusion that the equation loses its physical meaning (its solution goes to infinity) when r= 0 and r = r g.

The points at which the characteristics of the field lose their meaning are called singular, that is, special. The singularity at the zero point reflects a point, or, what is the same, a centrally symmetric field structure (after all, any spherical body - a star or a planet - can be represented as a material point). And the points located on a spherical surface with a radius r g , form the very surface from which the escape velocity is equal to the speed of light. In the general theory of relativity, it is called the Schwarzschild singular sphere or the event horizon (why - it will become clear later).

Already on the example of objects familiar to us - the Earth and the Sun - it is clear that black holes are very strange objects. Even astronomers dealing with matter at extreme temperatures, density and pressure consider them to be very exotic, and until recently not everyone believed in their existence. However, the first indications of the possibility of the formation of black holes were already contained in A. Einstein's general theory of relativity, created in 1915. The English astronomer Arthur Eddington, one of the first interpreters and popularizers of the theory of relativity, in the 1930s derived a system of equations describing the internal structure of stars. It follows from them that the star is in equilibrium under the action of oppositely directed gravitational forces and internal pressure created by the motion of hot plasma particles inside the luminary and by the pressure of radiation generated in its depths. And this means that the star is a gas ball, in the center of which there is a high temperature, gradually decreasing towards the periphery. From the equations, in particular, it followed that the surface temperature of the Sun is about 5500 degrees (which is quite consistent with the data of astronomical measurements), and in its center there should be about 10 million degrees. This allowed Eddington to make a prophetic conclusion: at such a temperature, a thermonuclear reaction is “ignited”, sufficient to ensure the glow of the Sun. Atomic physicists of that time did not agree with this. It seemed to them that it was too "cold" in the bowels of the star: the temperature there was insufficient for the reaction to "go". To this the enraged theorist replied: "Look for a hotter place!"

And in the end, he turned out to be right: there really is a thermonuclear reaction in the center of the star (another thing is that the so-called "standard solar model", based on ideas about thermonuclear fusion, apparently turned out to be incorrect - see, for example, "Science and life" No. 2, 3, 2000). Nevertheless, the reaction in the center of the star takes place, the star shines, and the radiation that occurs in this case keeps it in a stable state. But now the nuclear "fuel" in the star burns out. The release of energy stops, the radiation goes out, and the force holding back the gravitational attraction disappears. There is a limit on the mass of a star, after which the star begins to irreversibly shrink. Calculations show that this happens if the mass of the star exceeds two or three solar masses.

GRAVITATIONAL COLLAPSE

At first, the rate of contraction of the star is small, but its rate continuously increases, since the force of attraction is inversely proportional to the square of the distance. Compression becomes irreversible, there are no forces capable of counteracting self-gravity. This process is called gravitational collapse. The speed of the shell of the star towards its center increases, approaching the speed of light. And here the effects of the theory of relativity begin to play a role.

The escape velocity was calculated based on Newtonian ideas about the nature of light. From the point of view of general relativity, phenomena in the vicinity of a collapsing star occur somewhat differently. In its powerful gravitational field, the so-called gravitational redshift occurs. This means that the frequency of radiation coming from a massive object is shifted towards low frequencies. In the limit, at the boundary of the Schwarzschild sphere, the radiation frequency becomes equal to zero. That is, an observer who is outside of it will not be able to find out anything about what is happening inside. That is why the Schwarzschild sphere is called the event horizon.

But reducing the frequency is tantamount to slowing down time, and when the frequency becomes zero, time stops. This means that an outside observer will see a very strange picture: the shell of a star falling with increasing acceleration, instead of reaching the speed of light, stops. From his point of view, the contraction will stop as soon as the size of the star approaches the gravitational radius
mustache. He will never see even one particle "diving" under the Schwarzschild sphere. But for a hypothetical observer falling into a black hole, everything will end in a matter of moments according to his watch. Thus, the gravitational collapse time of a star the size of the Sun will be 29 minutes, and a much denser and more compact neutron star - only 1/20,000 of a second. And here he is in trouble, connected with the geometry of space-time near a black hole.

The observer enters a curved space. Near the gravitational radius, the gravitational forces become infinitely large; they stretch the rocket with the astronaut-observer into an infinitely thin thread of infinite length. But he himself will not notice this: all his deformations will correspond to the distortions of space-time coordinates. These considerations, of course, refer to the ideal, hypothetical case. Any real body will be torn apart by tidal forces long before approaching the Schwarzschild sphere.

BLACK HOLES DIMENSIONS

The size of a black hole, or rather, the radius of the Schwarzschild sphere is proportional to the mass of the star. And since astrophysics does not impose any restrictions on the size of a star, a black hole can be arbitrarily large. If, for example, it arose during the collapse of a star with a mass of 10 8 solar masses (or due to the merger of hundreds of thousands, or even millions of relatively small stars), its radius would be about 300 million kilometers, twice the Earth's orbit. And the average density of the substance of such a giant is close to the density of water.

Apparently, it is precisely such black holes that are found in the centers of galaxies. In any case, astronomers today count about fifty galaxies, in the center of which, judging by indirect signs (we will talk about them below), there are black holes with a mass of about a billion (10 9) solar ones. Apparently, our Galaxy also has its own black hole; its mass was estimated quite accurately - 2.4. 10 6 ±10% of the mass of the Sun.

The theory assumes that, along with such supergiants, black mini-holes with a mass of about 10 14 g and a radius of about 10 -12 cm (the size of the atomic nucleus) should have arisen. They could appear in the first moments of the existence of the Universe as a manifestation of a very strong inhomogeneity of space-time with a colossal energy density. The conditions that existed then in the Universe are now realized by researchers at powerful colliders (accelerators on colliding beams). Experiments at CERN earlier this year made it possible to obtain quark-gluon plasma - matter that existed before the appearance of elementary particles. Research into this state of matter continues at Brookhaven, the American accelerator center. It is capable of accelerating particles to energies one and a half to two orders of magnitude higher than an accelerator in
CERN. The upcoming experiment caused serious anxiety: will a black mini-hole arise during its implementation, which will bend our space and destroy the Earth?

This fear caused such a strong response that the US government was forced to convene an authoritative commission to test this possibility. The commission, which consisted of prominent researchers, concluded that the energy of the accelerator is too low for a black hole to form (this experiment is described in the journal Nauka i Zhizn, No. 3, 2000).

HOW TO SEE THE INVISIBLE

Black holes emit nothing, not even light. However, astronomers have learned to see them, or rather, to find "candidates" for this role. There are three ways to detect a black hole.

1. It is necessary to follow the circulation of stars in clusters around a certain center of gravity. If it turns out that there is nothing in this center, and the stars revolve, as it were, around an empty place, we can say quite confidently: there is a black hole in this "emptiness". It was on this basis that the presence of a black hole in the center of our Galaxy was assumed and its mass was estimated.

2. A black hole actively sucks matter into itself from the surrounding space. Interstellar dust, gas, matter of nearby stars fall on it in a spiral, forming the so-called accretion disk, similar to the ring of Saturn. (This is exactly what was frightening in the Brookhaven experiment: a black mini-hole that arose in the accelerator will begin to suck the Earth into itself, and this process could not be stopped by any forces.) Approaching the Schwarzschild sphere, particles experience acceleration and begin to radiate in the X-ray range. This radiation has a characteristic spectrum similar to the well-studied radiation of particles accelerated in a synchrotron. And if such radiation comes from some region of the Universe, we can say with certainty that there must be a black hole there.

3. When two black holes merge, gravitational radiation occurs. It is calculated that if the mass of each is about ten solar masses, then when they merge in a matter of hours, energy equivalent to 1% of their total mass will be released in the form of gravitational waves. This is a thousand times more than the light, heat and other energy that the Sun has emitted over the entire period of its existence - five billion years. They hope to detect gravitational radiation with the help of gravitational-wave observatories LIGO and others, which are now being built in America and Europe with the participation of Russian researchers (see "Science and Life" No. 5, 2000).

And yet, although astronomers have no doubts about the existence of black holes, no one can categorically state that exactly one of them is located at a given point in space. Scientific ethics, the conscientiousness of the researcher require an unambiguous answer to the question posed, which does not tolerate discrepancies. It is not enough to estimate the mass of an invisible object, you need to measure its radius and show that it does not exceed the Schwarzschild one. And even within our Galaxy, this problem is not yet solved. That is why scientists show a certain restraint in reporting their discovery, and scientific journals are literally full of reports of theoretical work and observations of effects that can shed light on their mystery.

True, black holes also have one more property, predicted theoretically, which, perhaps, would make it possible to see them. But, however, under one condition: the mass of the black hole must be much less than the mass of the Sun.

A BLACK HOLE MAY BE "WHITE"

For a long time, black holes were considered the embodiment of darkness, objects that in a vacuum, in the absence of absorption of matter, do not radiate anything. However, in 1974, the famous English theorist Stephen Hawking showed that black holes can be assigned a temperature and therefore must radiate.

According to the concepts of quantum mechanics, vacuum is not a void, but a kind of "foam of space-time", a hodgepodge of virtual (unobservable in our world) particles. However, quantum energy fluctuations are capable of "thrown" a particle-antiparticle pair out of vacuum. For example, when two or three gamma quanta collide, an electron and a positron appear as if from nothing. This and similar phenomena have been repeatedly observed in laboratories.

It is quantum fluctuations that determine the processes of radiation from black holes. If a pair of particles with energies E and -E(the total energy of the pair is zero), arises in the vicinity of the Schwarzschild sphere, the further fate of the particles will be different. They can annihilate almost immediately or go under the event horizon together. In this case, the state of the black hole will not change. But if only one particle goes under the horizon, the observer will register another, and it will seem to him that it was generated by a black hole. In this case, a black hole that has absorbed a particle with energy -E, will reduce its energy, and with energy E- increase.

Hawking calculated the rates at which all these processes go, and came to the conclusion that the probability of absorption of particles with negative energy is higher. This means that the black hole loses energy and mass - it evaporates. In addition, it radiates as a completely black body with a temperature T = 6 . 10 -8 M With / M kelvins, where M c is the mass of the Sun (2.1033 g), M is the mass of the black hole. This simple relationship shows that the temperature of a black hole with a mass six times the Sun's is one hundred millionth of a degree. It is clear that such a cold body radiates practically nothing, and all the above arguments remain valid. Another thing - mini-holes. It is easy to see that with a mass of 10 14 -10 30 grams, they are heated to tens of thousands of degrees and are white hot! However, it should be immediately noted that there are no contradictions with the properties of black holes: this radiation is emitted by a layer above the Schwarzschild sphere, and not below it.

So, the black hole, which seemed to be forever frozen object, sooner or later disappears, evaporating. Moreover, as it "loses weight", the rate of evaporation increases, but it still takes an extremely long time. It is estimated that mini-holes weighing 10 14 grams, which appeared immediately after the Big Bang 10-15 billion years ago, should evaporate completely by our time. At the last stage of their life, their temperature reaches a colossal value, so the products of evaporation must be particles of extremely high energy. It is possible that they are the ones that generate wide atmospheric showers - EASs in the Earth's atmosphere. In any case, the origin of anomalously high-energy particles is another important and interesting problem, which can be closely related to no less exciting questions in black hole physics.