Astronomer black hole discoverer 6 letters crossword clue History of black holes. Very superficial entropy

Black holes, dark matter, dark matter ... These are undoubtedly the strangest and most mysterious objects in space. Their bizarre properties can challenge the laws of physics of the Universe and even the nature of existing reality. To understand what black holes are, scientists propose to “change landmarks”, learn to think outside the box and apply a little imagination. Black holes are formed from the cores of super massive stars, which can be characterized as a region of space where a huge mass is concentrated in emptiness, and nothing, not even light, can escape gravitational attraction there. This is the area where the second cosmic speed exceeds the speed of light: And the more massive the object of motion, the faster it must move in order to get rid of its gravity. This is known as the second space velocity.

Collier's encyclopedia calls black holes a region in space that has arisen as a result of the complete gravitational collapse of matter, in which the gravitational attraction is so great that neither matter, nor light, nor other information carriers can leave it. Therefore, the interior of the black hole is not causally related to the rest of the universe; the physical processes taking place inside the black hole cannot influence the processes outside it. The black hole is surrounded by a surface with the property of a unidirectional membrane: matter and radiation freely fall through it into the black hole, but nothing can escape from there. This surface is called the “event horizon”.

Discovery history

Black holes predicted by general relativity (the theory of gravity proposed by Einstein in 1915) and other more modern theories of gravitation were mathematically substantiated by R. Oppenheimer and H. Snyder in 1939. But the properties of space and time in the vicinity of these objects turned out to be so unusual, that astronomers and physicists have not taken them seriously for 25 years. However, astronomical discoveries in the mid-1960s made black holes look like a possible physical reality. New discoveries and exploration can fundamentally change our understanding of space and time, shedding light on billions of cosmic secrets.

Formation of black holes

While thermonuclear reactions take place in the interior of the star, they maintain high temperature and pressure, preventing the star from contracting under the influence of its own gravity. Over time, however, the nuclear fuel is depleted and the star begins to shrink. Calculations show that if the mass of a star does not exceed three solar masses, then it will win the “battle with gravity”: its gravitational collapse will be stopped by the pressure of “degenerate” matter, and the star will forever turn into a white dwarf or neutron star. But if the mass of a star is more than three solar masses, then nothing can stop its catastrophic collapse and it will quickly go under the event horizon, becoming a black hole.

Is the black hole a donut hole?

It is not easy to notice that which does not emit light. One way to find a black hole is to look for areas in outer space that are massive and in dark space. While searching for these types of objects, astronomers have found them in two main regions: in the centers of galaxies and in binary star systems in our Galaxy. In total, as scientists suggest, there are tens of millions of such objects.

History of black holes

Alexey Levin

Scientific thinking sometimes constructs objects with such paradoxical properties that even the most astute scientists at first refuse to recognize them. The most graphic example in the history of modern physics is the long-term lack of interest in black holes, extreme states of the gravitational field, predicted almost 90 years ago. For a long time they were considered a purely theoretical abstraction, and only in the 1960s and 70s did they believe in their reality. However, the basic equation of the theory of black holes was derived over two hundred years ago.

John Michell's inspiration

Name of John Michell, physicist, astronomer and geologist, professor University of Cambridge and the pastor of the Church of England, completely undeservedly lost among the stars of English science in the 18th century. Michell laid the foundations of seismology, the science of earthquakes, performed an excellent study of magnetism, and long before Coulomb invented the torsion balance, which he used for gravimetric measurements. In 1783 he tried to combine two of Newton's great creations - mechanics and optics. Newton considered light to be a stream of tiny particles. Michell suggested that light corpuscles, like ordinary matter, obey the laws of mechanics. The consequence of this hypothesis turned out to be very nontrivial - celestial bodies can turn into traps for light.

How did Michell reason? A cannonball fired from the planet's surface will completely overcome its attraction only if its initial velocity exceeds the value now called the second cosmic velocity and escape velocity. If the planet's gravity is so strong that the escape velocity exceeds the speed of light, the light corpuscles released into the zenith cannot go to infinity. The same will happen with reflected light. Consequently, for a very distant observer, the planet will be invisible. Michell calculated the critical value of the radius of such a planet R cr depending on its mass M, reduced to the mass of our Sun M s: R cr = 3 km x M / M s.

John Michell believed his formulas and assumed that the depths of space hide many stars that cannot be seen from Earth in any telescope. Later, the great French mathematician, astronomer and physicist Pierre Simon Laplace came to the same conclusion, including it in both the first (1796) and second (1799) editions of his Exposition of the System of the World. But the third edition was published in 1808, when most physicists already considered light to be oscillations of the ether. The existence of "invisible" stars contradicted the wave theory of light, and Laplace thought it best not to mention them. In subsequent times, this idea was considered a curiosity, worthy of presentation only in works on the history of physics.

Schwarzschild model

In November 1915, Albert Einstein published a theory of gravity, which he called general theory of relativity (GTR). This work immediately found a grateful reader in the person of his colleague at the Berlin Academy of Sciences Karl Schwarzschild. It was Schwarzschild who was the first in the world to use general relativity for solving a specific astrophysical problem, calculating the space-time metric outside and inside a non-rotating spherical body (for the sake of concreteness, we will call it a star).

It follows from Schwarzschild's calculations that the gravity of a star does not overly distort the Newtonian structure of space and time only if its radius is much larger than the same value that John Michell calculated! This parameter was first called the Schwarzschild radius, and is now called the gravitational radius. According to general relativity, gravity does not affect the speed of light, but decreases the frequency of light vibrations in the same proportion as it slows down time. If the radius of a star is 4 times the gravitational radius, then the flow of time on its surface slows down by 15%, and space acquires a tangible curvature. With a two-fold excess, it bends more, and time slows down its run by 41%. When the gravitational radius is reached, time on the surface of the star completely stops (all frequencies are zeroed, the radiation is frozen, and the star goes out), but the curvature of space there is still finite. Far from the star, the geometry still remains Euclidean, and time does not change its speed.

Despite the fact that the values ​​of the gravitational radius for Michell and Schwarzschild are the same, the models themselves have nothing in common. In Michell, space and time do not change, but light slows down. The star, the size of which is less than its gravitational radius, continues to shine, but it is visible only to a not too distant observer. For Schwarzschild, the speed of light is absolute, but the structure of space and time depends on gravity. A star falling under the gravitational radius disappears for any observer, wherever he is (more precisely, it can be detected by gravitational effects, but by no means by radiation).

From disbelief to affirmation

Schwarzschild and his contemporaries believed that such strange space objects did not exist in nature. Einstein himself not only held this point of view, but also mistakenly believed that he had succeeded in substantiating his opinion mathematically.

In the 1930s, the young Indian astrophysicist Chandrasekhar proved that a star that spent nuclear fuel sheds its shell and turns into a slowly cooling white dwarf only if its mass is less than 1.4 times the mass of the Sun. Soon the American Fritz Zwicky realized that supernova explosions produce extremely dense bodies of neutron matter; later Lev Landau came to the same conclusion. After the work of Chandrasekhar, it was obvious that only stars with a mass of more than 1.4 solar masses can undergo such an evolution. Therefore, a natural question arose - is there an upper mass limit for supernovae that leave behind neutron stars?

In the late 1930s, the future father of the American atomic bomb, Robert Oppenheimer, established that such a limit does exist and does not exceed a few solar masses. At that time it was not possible to give a more accurate assessment; it is now known that the masses of neutron stars must be in the range 1.5–3 M s. But even from the approximate calculations of Oppenheimer and his graduate student George Volkov, it followed that the most massive descendants of supernovae do not become neutron stars, but go into some other state. In 1939, Oppenheimer and Hartland Snyder, using an idealized model, proved that a massive collapsing star is contracting to its gravitational radius. From their formulas, it actually follows that the star does not stop there, but the co-authors refrained from such a radical conclusion.

The final answer was found in the second half of the 20th century through the efforts of a whole galaxy of brilliant theoretical physicists, including Soviet ones. It turned out that a similar collapse always compresses the star "all the way", completely destroying its substance. As a result, a singularity arises, a "superconcentrate" of the gravitational field, closed in an infinitely small volume. For a stationary hole, this is a point, for a rotating one, a ring. The curvature of space-time and, consequently, the gravitational force near the singularity tends to infinity. At the end of 1967, American physicist John Archibald Wheeler was the first to call such an end to a stellar collapse a black hole. The new term fell in love with physicists and delighted journalists who spread it around the world (although the French did not like it at first, since the expression trou noir suggested dubious associations).

There, beyond the horizon

A black hole is not matter or radiation. With some degree of figurativeness, we can say that this is a self-sustaining gravitational field, concentrated in a strongly curved region of space-time. Its outer boundary is defined by a closed surface, an event horizon. If the star did not rotate before the collapse, this surface turns out to be a regular sphere, the radius of which coincides with the Schwarzschild radius.

The physical meaning of the horizon is very clear. A light signal sent from its outer surroundings can travel an infinitely far distance. But the signals sent from the inner region, not only will not cross the horizon, but will inevitably "fall" into the singularity. The horizon is the spatial boundary between events that can become known to earthly (and any other) astronomers, and events, information about which will not come out under any circumstances.

As it should be "according to Schwarzschild," far from the horizon, the attraction of a hole is inversely proportional to the square of the distance, so for a distant observer it manifests itself as an ordinary heavy body. In addition to mass, the hole inherits the moment of inertia of the collapsed star and its electric charge. And all other characteristics of the predecessor star (structure, composition, spectral type, etc.) go into oblivion.

Let's send a probe to the hole with a radio station, which sends a signal once a second on board time. For a distant observer, as the probe approaches the horizon, the time intervals between signals will increase - in principle, indefinitely. As soon as the ship crosses the invisible horizon, it will completely shut up for the "supra-hole" world. However, this disappearance will not be without a trace, since the probe will give its mass, charge and torque to the hole.

Black hole radiation

All previous models were built exclusively on the basis of general relativity. However, our world is governed by the laws of quantum mechanics, which also do not ignore black holes. These laws prevent the central singularity from being considered a mathematical point. In the quantum context, its diameter is given by the Planck-Wheeler length, approximately equal to 10 -33 centimeters. In this area, ordinary space ceases to exist. It is generally accepted that the center of the hole is stuffed with a variety of topological structures that appear and die in accordance with quantum probabilistic laws. The properties of such a bubbling quasispace, which Wheeler called quantum foam, are still poorly understood.

The presence of a quantum singularity is directly related to the fate of material bodies falling deep into the black hole. When approaching the center of the hole, any object made from currently known materials will be crushed and torn apart by tidal forces. However, even if future engineers and technologists create some kind of super-strong alloys and composites with unprecedented properties, they are all the same doomed to disappear: after all, in the singularity zone there is neither the usual time, nor the usual space.

Now consider the hole horizon in a quantum mechanical magnifier. Empty space - the physical vacuum - is actually not empty at all. Due to the quantum fluctuations of various fields in a vacuum, many virtual particles are continuously born and destroyed. Since gravity is very strong near the horizon, its fluctuations create extremely strong gravitational bursts. When accelerated in such fields, newborn "virtuals" acquire additional energy and sometimes become normal long-lived particles.

Virtual particles are always born in pairs that move in opposite directions (this is required by the law of conservation of momentum). If the gravitational fluctuation extracts a pair of particles from the vacuum, it may happen that one of them materializes outside the horizon, and the second (the antiparticle of the first) - inside. The “internal” particle will fall into the hole, but the “external” particle can escape under favorable conditions. As a result, the hole turns into a radiation source and therefore loses energy and, consequently, mass. Therefore, black holes are, in principle, unstable.

This phenomenon is called the Hawking effect, after the remarkable English theoretical physicist who discovered it in the mid-1970s. Stephen Hawking, in particular, proved that the horizon of a black hole emits photons in the same way as an absolutely black body heated to a temperature of T = 0.5 x 10 –7 x M s / M. From this it follows that as the hole gets thinner, its temperature rises, and "evaporation" naturally increases. This process is extremely slow, and the lifetime of a hole of mass M is about 10 65 x (M / M s) 3 years. When its size becomes equal to the Planck-Wheeler length, the hole becomes unstable and explodes, releasing the same energy as the simultaneous explosion of a million ten-megaton hydrogen bombs. Curiously, the mass of the hole at the time of its disappearance is still quite large, 22 micrograms. According to some models, the hole does not disappear without a trace, but leaves behind a stable relic of the same mass, the so-called maximon.

Maximon was born 40 years ago - as a term and as a physical idea. In 1965, Academician M.A. Markov suggested that there is an upper limit on the mass of elementary particles. He proposed to consider this limiting value the dimensionality of mass, which can be combined from three fundamental physical constants - Planck's constant h, the speed of light C and the gravitational constant G (for those who like details: to do this, you need to multiply h and C, divide the result by G and extract Square root). These are the same 22 micrograms mentioned in the article, this value is called the Planck mass. The same constants can be used to construct a quantity with the dimension of length (the Planck-Wheeler length, 10 -33 cm, will come out) and with the dimension of time (10 -43 sec).
Markov went further in his reasoning. According to his hypotheses e, the evaporation of a black hole leads to the formation of a "dry residue" - a maximon. Markov called such structures elementary black holes. To what extent this theory corresponds to reality is still an open question. In any case, analogues of Markov maximons have been revived in some black hole models based on superstring theory.

Depths of space

Black holes are not prohibited by the laws of physics, but do they exist in nature? Absolutely rigorous evidence of the presence of at least one such object in space has not yet been found. However, it is highly probable that stellar black holes are sources of X-rays in some binaries. This radiation should arise due to the suction of the atmosphere of an ordinary star by the gravitational field of the neighboring hole. As the gas moves towards the event horizon, it heats up strongly and emits X-ray quanta. No less than two dozen X-ray sources are now considered suitable candidates for the role of black holes. Moreover, the data of stellar statistics suggest that there are about ten million holes of stellar origin in our Galaxy alone.

Black holes can also form in the process of gravitational thickening of matter in galactic nuclei. This is how gigantic holes with a mass of millions and billions of solar masses appear, which, in all likelihood, exist in many galaxies. Apparently, in the center of the Milky Way, covered by dust clouds, there is a hole with a mass of 3-4 million solar masses.

Stephen Hawking came to the conclusion that black holes of arbitrary mass could be born immediately after the Big Bang, which gave rise to our Universe. Primary holes weighing up to a billion tons have already evaporated, but the heavier ones can now hide in the depths of space and, in due time, arrange cosmic fireworks in the form of powerful bursts of gamma radiation. However, such explosions have never been observed so far.

Black hole factory

Is it possible to accelerate the particles in the accelerator to such a high energy and that their collision would give rise to a black hole? At first glance, this idea is simply insane - the explosion of the hole will destroy all life on Earth. Moreover, it is not technically feasible. If the minimum mass of a hole is really equal to 22 micrograms, then in energy units it is 10 28 electron-volts. This threshold is 15 orders of magnitude greater than the capabilities of the world's most powerful accelerator, the Large Hadron Collider (LHC), which will be launched at CERN in 2007.

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However, it is possible that the standard estimate of the minimum hole mass is significantly overestimated. In any case, this is what physicists say, developing superstring theory, which includes the quantum theory of gravity (although far from complete). According to this theory, space has not three dimensions, but at least nine. We do not notice additional dimensions, since they are looped on such a small scale that our instruments cannot perceive them. However, gravity is omnipresent, and it penetrates into the hidden dimensions. In three-dimensional space, the force of gravity is inversely proportional to the square of the distance, and in nine-dimensional space - to the eighth degree. Therefore, in a multidimensional world, the strength of the gravitational field with decreasing distance increases much faster than in a three-dimensional one. In this case, the Planck length increases many times, and the minimum hole mass drops sharply.

String theory predicts that a black hole with a mass of only 10–20 g can be born in nine dimensional space. The calculated relativistic mass of protons accelerated in the CERN super accelerator is approximately the same. According to the most optimistic scenario, it will be able to produce one hole every second, which will live about 10–26 seconds. In the process of its evaporation, all kinds of elementary particles will be born, which will be easy to register. The disappearance of the hole will lead to the release of energy and, which is not enough even to heat one microgram of water per thousandth of a degree. Therefore, there is hope that the LHC will turn into a factory of harmless black holes. If these models are correct, then such holes will be able to register new generation orbital cosmic ray detectors.

All of the above applies to stationary black holes. Meanwhile, there are rotating holes with a bunch of interesting properties. The results of the theoretical analysis of black hole radiation also led to a serious rethinking of the concept of entropy, which also deserves a separate discussion.

Space super flywheels

Static electrically neutral black holes, which we talked about, are not at all typical for the real world... Collapsing stars tend to rotate and can also be electrically charged.

Bald head theorem

Giant holes in galactic cores, in all likelihood, are formed from the primary centers of gravitational condensation - a single "post-stellar" hole or several holes that have merged as a result of collisions. Such germ holes swallow nearby stars and interstellar gas and thus multiply their mass. The matter falling under the horizon, again, has both an electric charge (space gas and dust particles are easily ionized) and a rotational moment (the fall occurs with a twist, in a spiral). In any physical process, the moment of inertia and charge are conserved, and therefore it is natural to assume that the formation of black holes is no exception.

But an even stronger statement is also true, a particular case of which was formulated in the first part of the article (see A. Levin, The Amazing History of Black Holes, "Popular Mechanics" No. 11, 2005). Whatever the ancestors of the macroscopic black hole were, it receives from them only mass, moment of rotation and electric charge. According to John Wheeler, "a black hole has no hair." It would be more correct to say that no more than three "hairs" hang from the horizon of any hole, which was proved by the combined efforts of several theoretical physicists in the 1970s. True, the hole must also retain a magnetic charge, the hypothetical carriers of which, magnetic monopoles, were predicted by Paul Dirac in 1931. However, these particles have not yet been found, and it is too early to talk about the fourth "hair". In principle, there may be additional “hairs” associated with quantum fields, but in a macroscopic hole they are completely invisible.

And yet they are spinning

If a static star is recharged, the spacetime metric will change, but the event horizon will still remain spherical. However, stellar and galactic black holes, for a number of reasons, cannot carry a large charge, therefore, from the point of view of astrophysics, this case is not very interesting. But the rotation of the hole entails more serious consequences. First, the shape of the horizon changes. Centrifugal forces compress it along the axis of rotation and stretch it in the equatorial plane, so that the sphere transforms into something like an ellipsoid. In essence, the same thing happens with the horizon as with any rotating body, in particular, with our planet - after all, the equatorial radius of the Earth is 21.5 km longer than the polar one. Secondly, rotation reduces the linear dimensions of the horizon. Recall that the horizon is the interface between events that may or may not send signals to distant worlds. If the gravity of a hole captivates light quanta, then centrifugal forces, on the contrary, contribute to their escape into outer space. Therefore, the horizon of a rotating hole should be located closer to its center than the horizon of a static star with the same mass.

But that's not all. The hole in its rotation carries away the surrounding space. In the immediate vicinity of the hole, the entrainment is complete; at the periphery, it gradually weakens. Therefore, the hole's horizon is immersed in a special region of space - the ergosphere. The boundary of the ergosphere touches the horizon at the poles and moves farthest from it in the equatorial plane. On this surface, the speed of dragging space is equal to the speed of light; inside it it is greater than the speed of light, and outside it is less. Therefore, any material body, be it a gas molecule, a particle of cosmic dust or a reconnaissance probe, when it enters the ergosphere, inevitably begins to rotate around the hole, and in the same direction as it itself.

Star Generators

The presence of the ergosphere, in principle, allows the hole to be used as a source of energy and. Let some object penetrate into the ergosphere and disintegrate there into two fragments. It may turn out that one of them will fall under the horizon, and the other will leave the ergosphere, and its kinetic energy I will exceed the initial energy of the whole body! The ergosphere also has the ability to amplify electromagnetic radiation that falls on it and scatters back into space (this phenomenon is called superradiation).

However, the law of conservation of energy is also unshakable - perpetual motion machines do not exist. When a hole feeds particle or radiation energy to it, its own rotational energy decreases. The space super flywheel gradually slows down, and in the end it may even stop. It is calculated that in this way it is possible to convert to energy up to 29% of the mass of the hole. Only the annihilation of matter and antimatter is more effective than this process, since in this case the mass is completely converted into radiation. But solar thermonuclear fuel burns out with a much lower efficiency - about 0.6%.

Consequently, a rapidly rotating black hole is almost an ideal generator of energy for cosmic supercivilizations (if, of course, such exist). In any case, nature has been using this resource since time immemorial. Quasars, the most powerful space "radio stations" (sources of electromagnetic waves), feed on the energy of giant rotating holes located in the cores of galaxies. This hypothesis was put forward by Edwin Salpeter and Yakov Zeldovich back in 1964, and since then it has become generally accepted. The material approaching the hole forms a ring-like structure, the so-called accretion disk. Since the space near the hole is strongly twisted by its rotation, the inner zone of the disk is held in the equatorial plane and slowly settles towards the event horizon. The gas in this zone is strongly heated by internal friction and generates infrared, light, ultraviolet and X-rays, and sometimes even gamma quanta. Quasars also emit non-thermal radio emission, which is mainly due to the synchrotron effect.

Very superficial entropy

The bald hole theorem hides a very insidious pitfall. A collapsing star is a blob of superhot gas compressed by gravitational forces. The higher the density and temperature of the stellar plasma, the less order and more chaos in it. The degree of chaos is expressed by a very specific physical quantity - entropy. Over time, the entropy of any isolated object increases - this is the essence of the second law of thermodynamics. The entropy of the star before the onset of the collapse is prohibitively high, and the entropy of the hole seems to be extremely small, since only three parameters are required for an unambiguous description of the hole. Is the second law of thermodynamics violated in the course of gravitational collapse?

Could it be assumed that when a star turns into a supernova, its entropy is carried away along with the ejected shell? Unfortunately no. First, the mass of the envelope cannot be compared with the mass of the star, therefore, the loss of entropy will be small. Second, it is easy to come up with an even more convincing mental "refutation" of the second law of thermodynamics. Let a body of nonzero temperature, possessing some kind of entropy, fall into the zone of attraction of a ready-made hole. Having fallen under the event horizon, it will disappear along with its entropy reserves, and the hole's entropy, most likely, will not increase at all. There is a temptation to argue that the entropy of the alien does not disappear, but is transferred to the interior of the hole, but this is just a verbal trick. The laws of physics are fulfilled in a world accessible to us and our devices, and the area under the event horizon for any outside observer is terra incognita.

This paradox was resolved by Wheeler's graduate student Jacob Bekenstein. Thermodynamics has a very powerful intellectual resource - the theoretical study of ideal heat engines. Bekenstein came up with a mental device that converts heat into useful work using a black hole as a heater. Using this model, he calculated the entropy of the black hole, which turned out to be proportional to the area of ​​the event horizon... This area is proportional to the square of the hole's radius, which, recall, is proportional to its mass. When any external object is captured, the mass of the hole increases, the radius lengthens, the area of ​​the horizon increases and, accordingly, the entropy increases. Calculations have shown that the entropy of a hole that has swallowed a foreign object exceeds the total entropy of this object and the hole before they meet. Similarly, the entropy of the collapsing star is many orders of magnitude less than the entropy of the heir hole. In fact, it follows from Bekenstein's reasoning that the surface of the hole has a nonzero temperature and therefore must simply emit thermal photons (and, if heated enough, other particles as well). However, Bekenstein did not dare to go that far (this step was made by Stephen Hawking).

What have we come to? Reflections on black holes not only leave the second law of thermodynamics unshakable, but also make it possible to enrich the concept of entropy. Entropy of the ordinary physical body is more or less proportional to its volume, and the hole's entropy is proportional to the horizon surface. It can be rigorously proved that it is greater than the entropy of any material object with the same linear dimensions. It means that maximum entropy of a closed area of ​​space is determined exclusively by the area of ​​its outer boundary! As we can see, a theoretical analysis of the properties of black holes allows one to draw very deep conclusions of a general physical nature.

Looking into the depths of the universe

How is the search for black holes in the depths of space carried out? This question was posed by Popular Mechanics to the famous astrophysicist, Harvard University professor Ramesh Narayan.

“The discovery of black holes should be considered one of the greatest achievements of modern astronomy and astrophysics. In recent decades, thousands of X-ray sources have been identified in space, each consisting of a normal star and a very small non-luminous object surrounded by an accretion disk. Dark bodies, with masses ranging from one and a half to three solar masses, are most likely neutron stars. However, among these invisible objects there are at least two dozen practically one hundred percent candidates for the role of a black hole. In addition, scientists have come to a consensus that there are at least two giant black holes lurking in galactic cores. One of them is located in the center of our Galaxy; according to last year's publication by astronomers from the United States and Germany, its mass is 3.7 million solar masses (M s). Several years ago, my colleagues at the Harvard-Smithsonian Astrophysical Center James Moran and Lincoln Greenhill made a major contribution to weighing the hole in the center of the Seyfert galaxy NGC 4258, which pulled 35 million M s. In all likelihood, the cores of many galaxies contain holes with masses from one million to several billion M s.

So far, there is no way to fix a truly unique signature of a black hole from Earth - the presence of an event horizon. However, we already know how to be convinced of its absence. The radius of the neutron star is 10 kilometers; the same order of magnitude and the radius of holes born as a result of stellar collapse. However, a neutron star has a hard surface, while a hole does not. The fall of matter on the surface of a neutron star entails thermonuclear explosions, which generate periodic X-ray bursts of a second duration. And when the gas reaches the horizon of the black hole, it goes under it and does not manifest itself in any radiation. Therefore, the absence of short X-ray flares is a powerful confirmation of the hole-like nature of the object. All two dozen binary systems, presumably containing black holes, do not emit such flares.

It must be admitted that now we are forced to be content with negative evidence of the existence of black holes. The objects that we declare to be holes cannot be anything else from the point of view of generally accepted theoretical models. In other words, we regard them as holes solely because we cannot reasonably consider them to be anything else. Hopefully the next generations of astronomers will be a little more fortunate. ”

To the words of Professor Narayan, we can add that astronomers have long believed in the reality of the existence of black holes. Historically, the first reliable candidate for this position was the dark satellite of the very bright blue supergiant HDE 226868, 6500 light-years distant from us. It was discovered in the early 1970s in the Cygnus X-1 X-ray binary system. According to the latest data, its mass is about 20 M s. It is worth noting that on September 20 of this year, data were published that almost completely dispelled doubts about the reality of another galactic-scale hole, the existence of which astronomers first suspected 17 years ago. It is located in the center of the galaxy M31, better known as the Andromeda Nebula. Galaxy M31 is very old, about 12 billion years old. The hole is also rather big - 140 million solar masses. By the fall of 2005, astronomers and astrophysicists were finally convinced of the existence of three supermassive black holes and a couple of dozen more of their more modest companions.

Theorists' verdict

Popular Mechanics also managed to talk to two of the most authoritative experts on the theory of gravitation, who have devoted decades of research in the field of black holes. We asked them to list the most important achievements in this area. Here's what Kip Thorne, professor of theoretical physics at California Institute of Technology, told us:

“If we talk about macroscopic black holes, which are well described by the equations of general relativity, then in the field of their theory, the main results were obtained back in the 60-80s of the XX century. With regard to recent work, the most interesting of them allowed a better understanding of the processes taking place inside a black hole as it ages. In recent years, considerable attention has been paid to models of black holes in multidimensional spaces, which naturally appear in string theory. But these studies are no longer related to classical, but to quantum holes that have not yet been discovered. The main result of recent years is very convincing astrophysical confirmation of the reality of the existence of holes with a mass of several solar masses, as well as supermassive holes in the centers of galaxies. Today there is no longer any doubt that these holes really exist and that we understand well the processes of their formation. "

Valery Frolov, a student of Academician Markov, a professor at the University of the Canadian Province of Albert, answered the same question:

“First of all, I would call the discovery of a black hole in the center of our Galaxy. Theoretical studies of holes in spaces with additional dimensions are also very interesting, from which it follows the possibility of the creation of minholes in experiments at collider accelerators and in the processes of interaction of cosmic rays with terrestrial matter. Recently, Stephen Hawking sent out a preprint of the work, from which it follows that thermal radiation from a black hole completely returns to the outside world information about the state of objects that have fallen under its horizon. Previously, he believed that this information was irreversibly disappearing, but now he came to the opposite conclusion. Nevertheless, it must be emphasized that this problem can be finally solved only on the basis of the quantum theory of gravity, which has not yet been built. "

Hawking's work deserves a separate comment. From the general principles of quantum mechanics, it follows that no information disappears without a trace, but perhaps passes into a less "readable" form. However, black holes irreversibly destroy matter and, apparently, deal with information just as harshly. In 1976 Hawking published an article where this conclusion was supported by a mathematical apparatus. Some theorists agreed with him, some did not; in particular, string theorists believed that information was indestructible. At a conference in Dublin last summer, Hawking said that the information is still stored and leaves the surface of the evaporating hole along with thermal radiation. At this meeting, Hawking presented only a diagram of his new calculations, promising to publish them in full over time. And now, as Valery Frolov said, this work has become available as a preprint.

Finally, we asked Professor Frolov to explain why he considers black holes to be one of the most fantastic inventions of the human intellect.

“Astronomers have been discovering objects for a long time that did not require substantially new physical ideas to understand. This applies not only to planets, stars and galaxies, but also to exotic bodies such as white dwarfs and neutron stars. But a black hole is something completely different, it is a breakthrough into the unknown. Someone said that her insides best place to accommodate the underworld. The study of holes, especially singularities, simply forces the use of such non-standard concepts and models that, until recently, were practically not discussed in physics - for example, quantum gravity and string theory. Here a lot of problems arise that are unusual for physics, even painful, but, as it is now clear, are absolutely real. Therefore, the study of holes constantly requires fundamentally new theoretical approaches, including those that are on the verge of our knowledge of the physical world. "

Between the French and the British, there is sometimes a half-joking, and sometimes a serious controversy: who should be considered the discoverer of the possibility of the existence of invisible stars - the Frenchman P. Laplace or the Englishman J. Michell? In 1973, the famous English theoretical physicists S. Hawking and G. Ellis, in a book devoted to modern special mathematical problems of the structure of space and time, cited the work of the Frenchman P. Laplace with a proof of the possibility of the existence of black stars; then the work of J. Michell was not yet known. In the fall of 1984, the famous English astrophysicist M Rice, speaking at a conference in Toulouse, said that although it is not very convenient to speak on the territory of France, he must emphasize that the Englishman J. Michell was the first to predict invisible stars, and showed a snapshot of the first page of his corresponding work. This historic remark was greeted with applause and smiles from those present.

How can we not recall the discussions between the French and the British about who predicted the position of the planet Neptune based on the disturbances in the movement of Uranus: the Frenchman W. Le Verrier or the Englishman J. Adams? As you know, both scientists independently correctly indicated the position of the new planet. Then the Frenchman W. Le Verrier was more fortunate. This is the fate of many discoveries. Often they are done almost simultaneously and independently by different people. Usually, priority is given to those who have penetrated deeper into the essence of the problem, but sometimes these are just whims of fortune.

But the prediction of P. Laplace and J. Michill was not yet a real prediction of a black hole. Why?

The fact is that at the time of Laplace it was not yet known that nothing can move faster than light in nature. It is impossible to overtake the light in emptiness! This was established by A Einstein in the special theory of relativity already in our century. Therefore, for P. Laplace, the star he was considering was only black (non-luminous), and he could not know that such a star loses its ability in general to “communicate” with the external world, to “communicate” anything to distant worlds about the events taking place on it. ... In other words, he did not yet know that it was not only a “black”, but also a “hole” into which one could fall, but it was impossible to get out. Now we know that if light cannot come out of a certain area of ​​space, then it means that nothing can come out at all, and we call such an object a black hole.

Another reason why Laplace's reasoning cannot be considered rigorous is that he considered the garvitational fields of enormous strength, in which falling bodies are accelerated to the speed of light, and the outgoing light itself can be delayed, and at the same time he applied the law of gravitation Newton.

A. Einstein showed ”that for such fields Newton's theory of gravitation is inapplicable, and created a new theory that is valid for superstrong, as well as for rapidly changing fields (for which Newton's theory is also inapplicable!), And. called it the general theory of relativity. It is the conclusions of this theory that must be used to prove the possibility of the existence of black holes and to study their properties.

General relativity is an amazing theory. It is so deep and slender that it evokes a sense of aesthetic pleasure in everyone who gets to know it. Soviet physicists L. Landau and E. Lifshits in their textbook "Field Theory" called it "the most beautiful of all existing physical theories." German physicist Max Born said about the discovery of the theory of relativity: "I admire him as a work of art." A Soviet physicist V. Ginzburg wrote that it evokes "... a feeling ... akin to that experienced when looking at the most outstanding masterpieces of painting, sculpture or architecture."

Numerous attempts at a popular exposition of Einstein's theory can, of course, give a general impression of it. But, to be honest, it is just as little like the rapture from the knowledge of the theory itself, as the acquaintance with the reproduction of the "Sistine Madonna" differs from the experience that arises when considering the original created by the genius of Raphael.

And nevertheless, when there is no possibility of admiring the original, it is possible (and necessary!) To get acquainted with the available reproductions, better good ones (and there are all kinds).

Novikov I.D.

On April 10, a group of astrophysicists from the Event Horizon Telescope project released the first ever snapshot of a black hole. These gigantic but invisible space objects are still some of the most mysterious and intriguing in our Universe.

Read below

What is a black hole?

A black hole is an object (region in space-time) whose gravity is so great that it attracts all known objects, including those that move at the speed of light. The quanta of the light itself also cannot leave this region, so the black hole is invisible. You can only observe electromagnetic waves, radiation and distortions of space around the black hole. The published Event Horizon Telescope depicts the black hole's event horizon - the edge of a region of super-gravity framed by an accretion disk - luminous matter that is "sucked in" by the hole.

The term "black hole" appeared in the middle of the XX century, it was introduced by the American theoretical physicist John Archibald Wheeler. He first used the term at a scientific conference in 1967.

However, the assumptions about the existence of objects so massive that even light cannot overcome the force of their attraction, were put forward back in the 18th century. The modern theory of black holes began to form within the framework of general relativity. Interestingly, Albert Einstein himself did not believe in the existence of black holes.

Where do black holes come from?

Scientists believe that black holes are of different origins. Massive stars become a black hole at the end of their life: over billions of years, the composition of gases and temperature change in them, which leads to an imbalance between the gravity of the star and the pressure of hot gases. Then the star collapses: its volume decreases, but since the mass does not change, the density increases. A typical stellar mass black hole has a radius of 30 kilometers and a density of more than 200 million tons per cubic centimeter. For comparison: for the Earth to become a black hole, its radius must be 9 millimeters.

There is another type of black hole - supermassive black holes that form the nuclei of most galaxies. Their mass is a billion times that of stellar black holes. The origin of supermassive black holes is unknown, it is hypothesized that they were once stellar mass black holes that grew, devouring other stars.

There is also a controversial idea of ​​the existence of primordial black holes, which could appear from the compression of any mass at the beginning of the universe. In addition, there is an assumption that very small black holes with a mass close to the mass of elementary particles are formed at the Large Hadron Collider. However, there is no confirmation of this version yet.

Will the black hole swallow our galaxy?

In the center of the Milky Way galaxy there is a black hole - Sagittarius A *. Its mass is four million times the mass of the Sun, and its size - 25 million kilometers - is approximately equal to the diameter of 18 suns. Such a scale makes some wonder: is not a black hole threatening our entire galaxy? Not only science fiction writers have grounds for such assumptions: a few years ago, scientists reported about the galaxy W2246-0526, which is located 12.5 billion light years from our planet. According to the description of astronomers, located in the center of W2246–0526, a supermassive black hole is gradually tearing it apart, and the resulting radiation accelerates in all directions incandescent giant clouds of gas. Torn apart by a black hole, the galaxy glows brighter than 300 trillion suns.

However, our home galaxy is not threatened (at least in the short term). Most objects in the Milky Way, including the solar system, are too far from a black hole to sense its pull. In addition, “our” black hole does not suck in all the material, like a vacuum cleaner, but acts only as a gravitational anchor for a group of stars in orbit around it - like the Sun for planets.

However, even if we ever get beyond the event horizon of a black hole, then, most likely, we will not even notice it.

What happens if you "fall" into a black hole?

An object attracted by a black hole, most likely, will not be able to return from there. To overcome the gravity of a black hole, you need to develop a speed higher than the speed of light, but humanity does not yet know how to do this.

The gravitational field around the black hole is very strong and inhomogeneous, so all objects near it change shape and structure. The side of the object that is closer to the event horizon is attracted with more force and falls with greater acceleration, so the whole object stretches, becoming like macaroni. He described this phenomenon in his book “ Short story time ”the famous theoretical physicist Stephen Hawking. Even before Hawking, astrophysicists called this phenomenon spaghettification.

If you describe spaghettification from the point of view of an astronaut who flew up to a black hole feet first, then the gravitational field will tighten his legs, and then stretch and tear the body, turning it into a stream of subatomic particles.

It is impossible to see a fall into a black hole from the outside, as it absorbs light. An outside observer will only see that an object approaching a black hole gradually slows down, and then stops altogether. After that, the silhouette of the object will become more and more blurred, acquire a red color, and finally just disappear forever.

According to Stephen Hawking's assumption, all objects that are attracted by the black hole remain in the event horizon. From the theory of relativity, it follows that near a black hole, time slows down to a stop, so for someone who falls, the fall into the black hole itself may never occur.

What's inside?

For obvious reasons, there is no reliable answer to this question now. However, scientists agree that inside a black hole the laws of physics we are used to no longer work. According to one of the most exciting and exotic hypotheses, the space-time continuum around a black hole is distorted so much that a hole is formed in reality itself, which could be a portal to another universe - or the so-called wormhole.

Black holes: the most mysterious objects in the universe

Due to the relatively recent increase in interest in making popular science films on the topic of space exploration, the modern viewer has heard a lot about such phenomena as the singularity, or black hole. However, movies, obviously, do not reveal the entire nature of these phenomena, and sometimes even distort the constructed scientific theories for greater effectiveness. For this reason, the representation of many modern people about these phenomena either completely superficially, or completely erroneous. One of the solutions to the problem is this article, in which we will try to understand the existing research results and answer the question - what is a black hole?

In 1784, the English priest and naturalist John Michell first mentioned in a letter to the Royal Society some hypothetical massive body that has such a strong gravitational attraction that the second cosmic speed for it will exceed the speed of light. The second cosmic speed is the speed that a relatively small object will need to overcome the gravitational attraction of a celestial body and go beyond the closed orbit around this body. According to his calculations, a body with the density of the Sun and a radius of 500 solar radii will have on its surface a second cosmic speed equal to the speed of light. In this case, even the light will not leave the surface of such a body, and therefore this body will only absorb the incoming light and remain invisible to the observer - a kind of black spot against the background of dark space.

However, Michell's concept of a supermassive body did not attract much interest, until the work of Einstein. Let us recall that the latter defined the speed of light as the limiting speed of information transmission. In addition, Einstein expanded the theory of gravitation for speeds close to the speed of light (). As a result, it was no longer relevant to apply Newtonian theory to black holes.

Einstein's equation

As a result of the application of general relativity to black holes and the solution of Einstein's equations, the main parameters of a black hole were identified, of which there are only three: mass, electric charge and angular momentum. It should be noted the significant contribution of the Indian astrophysicist Subramanian Chandrasekhar, who created the fundamental monograph: "The Mathematical Theory of Black Holes."

Thus, the solution to Einstein's equations is presented by four options for four possible types of black holes:

• BH without rotation and without charge - Schwarzschild's solution. One of the first descriptions of a black hole (1916) using Einstein's equations, but without taking into account two of the three body parameters. The solution of the German physicist Karl Schwarzschild makes it possible to calculate the external gravitational field of a spherical massive body. The peculiarity of the concept of BH by the German scientist is the presence of an event horizon and the one hidden behind it. Also, Schwarzschild first calculated the gravitational radius, which received his name, which determines the radius of the sphere on which the event horizon for a body with a given mass would be located.
• BH without rotation with charge - Reisner-Nordström solution. A solution put forward in 1916-1918, taking into account the possible electric charge of the black hole. This charge cannot be arbitrarily large and is limited due to the resulting electrical repulsion. The latter should be compensated by gravitational attraction.
• BH with rotation and without charge - Kerr's solution (1963). A rotating Kerr black hole differs from a static one by the presence of the so-called ergosphere (read about this and other components of the black hole below).
• BH with rotation and charge - Kerr - Newman solution. This solution was calculated in 1965 and is currently the most complete, since it takes into account all three BH parameters. However, it is still assumed that in nature black holes have an insignificant charge.

Black hole formation

There are several theories about how a black hole forms and appears, the most famous of which is the formation of a star with sufficient mass as a result of gravitational collapse. This compression can end the evolution of stars with a mass of more than three solar masses. Upon completion of thermonuclear reactions inside such stars, they begin to rapidly collapse into superdense. If the gas pressure of the neutron star cannot compensate for the gravitational forces, that is, the mass of the star overcomes the so-called. the Oppenheimer-Volkov limit, then the collapse continues, with the result that matter is compressed into a black hole.

The second scenario, describing the birth of a black hole, is the compression of protogalactic gas, that is, interstellar gas that is at the stage of transformation into a galaxy or some kind of cluster. If there is insufficient internal pressure to compensate for the same gravitational forces, a black hole can arise.

Two other scenarios remain hypothetical:

• The occurrence of BH as a result - the so-called. primordial black holes.
• Occurrence as a result of nuclear reactions at high energies. An example of such reactions is collider experiments.

Structure and physics of black holes

The Schwarzschild structure of a black hole includes only two elements, which were mentioned earlier: the singularity and the event horizon of the black hole. Briefly speaking about the singularity, it can be noted that it is impossible to draw a straight line through it, and also that within it most of the existing physical theories do not work. Thus, the physics of the singularity remains a mystery to scientists today. a black hole is a kind of border, crossing which, a physical object loses the ability to return back beyond its limits and will definitely "fall" into the singularity of the black hole.

The structure of a black hole becomes somewhat more complicated in the case of the Kerr solution, namely, in the presence of rotation of the BH. Kerr's solution assumes that the hole has an ergosphere. The ergosphere is a certain region outside the event horizon, inside which all bodies move in the direction of rotation of the black hole. This area is not yet exciting and it is possible to leave it, unlike the event horizon. The ergosphere is probably a kind of analogue of the accretion disk, which is rotating matter around massive bodies. If a static Schwarzschild black hole is represented as a black sphere, then the Kerry BH, due to the presence of the ergosphere, has the shape of an oblate ellipsoid, in the form of which we often saw BH in drawings, in old movies or video games.

• How much does a black hole weigh? - The greatest theoretical material on the origin of a black hole is available for the scenario of its appearance as a result of the collapse of a star. In this case, the maximum mass of a neutron star and the minimum mass of a black hole are determined by the Oppenheimer-Volkov limit, according to which the lower limit of the BH mass is 2.5 - 3 solar masses. The heaviest black hole ever discovered (in the galaxy NGC 4889) has a mass of 21 billion solar masses. However, one should not forget about BHs, hypothetically arising as a result of nuclear reactions at high energies, such as those at colliders. The mass of such quantum black holes, in other words, "Planck black holes", has an order of magnitude, namely 2 · 10 −5 g.
• The size of the black hole. The minimum BH radius can be calculated from the minimum mass (2.5 - 3 solar masses). If the gravitational radius of the Sun, that is, the area where the event horizon would be located, is about 2.95 km, then the minimum BH radius of 3 solar masses will be about nine kilometers. Such a relatively small size does not fit into the head when it comes about massive objects that attract everything around. However, for quantum black holes, the radius is - 10 −35 m.
• The average density of a black hole depends on two parameters: mass and radius. The density of a black hole with a mass of the order of three solar masses is about 6 · 10 26 kg / m³, while the density of water is 1000 kg / m³. However, such small black holes have not been found by scientists. Most of the detected BHs have a mass of more than 10 5 solar masses. There is an interesting pattern according to which the more massive a black hole, the lower its density. In this case, a change in mass by 11 orders of magnitude leads to a change in density by 22 orders of magnitude. Thus, a black hole with a mass of 1 · 10 9 solar masses has a density of 18.5 kg / m³, which is one unit less than the density of gold. And BHs with a mass of more than 10 10 solar masses can have an average density less than the density of air. Based on these calculations, it is logical to assume that the formation of a black hole occurs not due to the compression of matter, but as a result of the accumulation of a large amount of matter in a certain volume. In the case of quantum BHs, their density can be about 1094 kg / m³.
• The temperature of a black hole is also inversely proportional to its mass. This temperature is directly related to. The spectrum of this radiation coincides with the spectrum of an absolutely black body, that is, a body that absorbs all incident radiation. The radiation spectrum of an absolutely black body depends only on its temperature, then the BH temperature can be determined from the Hawking radiation spectrum. As mentioned above, the smaller the black hole, the more powerful this radiation is. In this case, Hawking radiation remains hypothetical, since it has not yet been observed by astronomers. It follows from this that if Hawking radiation exists, then the temperature of the observed BHs is so low that it does not allow registering the indicated radiation. According to calculations, even the temperature of a hole with a mass on the order of the mass of the Sun is negligible (1 · 10 -7 K or -272 ° C). The temperature of quantum black holes can reach about 10 12 K, and with their rapid evaporation (about 1.5 minutes), such BHs can emit energy of the order of ten million atomic bombs. But, fortunately, the creation of such hypothetical objects will require an energy 10 14 times greater than that achieved today at the Large Hadron Collider. In addition, such phenomena have never been observed by astronomers.

What does a black hole consist of?

Another question worries, both scientists and those who are simply fond of astrophysics - what does a black hole consist of? There is no unambiguous answer to this question, since it is not possible to look beyond the event horizon surrounding any black hole. In addition, as mentioned earlier, theoretical models of a black hole provide for only 3 of its components: the ergosphere, the event horizon and the singularity. It is logical to assume that in the ergosphere there are only those objects that were attracted by the black hole, and which now revolve around it - various kinds of cosmic bodies and cosmic gas. The event horizon is only a thin implicit border, after falling beyond which, the same cosmic bodies are irretrievably attracted towards the last main component of the BH - the singularity. The nature of the singularity has not been studied today and it is too early to talk about its composition.

According to some assumptions, the black hole may be composed of neutrons. If we follow the scenario of a black hole as a result of the contraction of a star to a neutron star with its subsequent contraction, then, probably, the main part of the black hole consists of neutrons, of which the neutron star itself consists. In simple words: when a star collapses, its atoms contract in such a way that electrons combine with protons, thereby forming neutrons. A similar reaction actually takes place in nature, while neutrino emission occurs with the formation of a neutron. However, these are only assumptions.

What happens if you fall into a black hole?

Falling into an astrophysical black hole stretches the body. Consider a hypothetical suicide astronaut walking into a black hole in nothing but a spacesuit, feet first. Crossing the event horizon, the astronaut will not notice any changes, despite the fact that he no longer has the opportunity to get out. At some point, the astronaut will reach a point (slightly behind the event horizon) at which deformation of his body will begin to occur. Since the gravitational field of a black hole is inhomogeneous and is represented by a force gradient increasing towards the center, the astronaut's legs will be subjected to a noticeably greater gravitational effect than, for example, the head. Then, due to gravity, or rather, tidal forces, the legs will "fall" faster. Thus, the body begins to gradually stretch in length. To describe this phenomenon, astrophysicists have come up with a rather creative term - spaghettification. Further stretching of the body is likely to decompose it into atoms, which, sooner or later, will reach a singularity. What a person will feel in this situation is anyone's guess. It is worth noting that the stretching effect of a body is inversely proportional to the mass of the black hole. That is, if a BH with a mass of three Suns instantly stretches / breaks the body, then the supermassive black hole will have lower tidal forces and, there are suggestions that some physical materials could “endure” such a deformation without losing their structure.

As you know, time flows more slowly near massive objects, which means that time for a suicide astronaut will flow much slower than for earthlings. In this case, perhaps he will outlive not only his friends, but also the Earth itself. Calculations will be required to determine how much time will slow down for the astronaut; however, from the above, it can be assumed that the astronaut will fall into the black hole very slowly and, perhaps, simply will not live to see the moment when his body begins to deform.

It is noteworthy that for an observer outside, all bodies that have flown up to the event horizon will remain at the edge of this horizon until their image disappears. The reason for this is the gravitational redshift. Simplifying somewhat, we can say that the light falling on the body of a suicide cosmonaut “frozen” at the event horizon will change its frequency due to its slowed down time. As time passes more slowly, the frequency of light will decrease and the wavelength will increase. As a result of this phenomenon, at the exit, that is, for an external observer, the light will gradually shift towards the low-frequency - red. A shift of light along the spectrum will take place, as the suicide astronaut moves further and further from the observer, albeit almost imperceptibly, and his time passes more and more slowly. Thus, the light reflected by his body will soon go beyond the visible spectrum (the image will disappear), and in the future the astronaut's body can be caught only in the infrared region, and later in the radio frequency, and as a result, the radiation will be completely elusive.

Despite the above, it is assumed that in very large supermassive black holes, tidal forces do not change so much with distance and act almost uniformly on the falling body. In this case, the falling spaceship would retain its structure. A reasonable question arises - where does the black hole lead? This question can be answered by the work of some scientists, linking two such phenomena as wormholes and black holes.

Back in 1935, Albert Einstein and Nathan Rosen, taking into account, put forward a hypothesis about the existence of so-called wormholes, connecting two points of space-time by a path in places of significant curvature of the latter - the Einstein-Rosen bridge or a wormhole. For such a powerful curvature of space, bodies with a gigantic mass will be required, with the role of which black holes would perfectly cope.

The Einstein-Rosen Bridge is considered an impassable wormhole because it is small and unstable.

A traversable wormhole is possible within the framework of the theory of black and white holes. Where the white hole is the output of information trapped in a black hole. The white hole is described in the framework of general relativity, but today it remains hypothetical and has not been discovered. Another model of a wormhole, proposed by American scientists Kip Thorne and his graduate student, Mike Morris, can be walkable. However, as in the case of the Morris-Thorne wormhole, and in the case of black and white holes, the possibility of travel requires the existence of so-called exotic matter, which has negative energy and also remains hypothetical.

Black holes in the universe

The existence of black holes was confirmed relatively recently (September 2015); however, up to that time, there was already considerable theoretical material on the nature of BHs, as well as many candidate objects for the role of a black hole. First of all, the size of the BH should be taken into account, since the very nature of the phenomenon depends on them:

• Stellar mass black hole... Such objects are formed as a result of the collapse of a star. As mentioned earlier, the minimum mass of a body capable of forming such a black hole is 2.5 - 3 solar masses.
• Medium-mass black holes... A conditional intermediate type of black holes that have increased due to the absorption of nearby objects, such as a gas accumulation, a nearby star (in two-star systems) and other cosmic bodies.
• Supermassive black hole... Compact objects with 10 5 -10 10 solar masses. The distinctive properties of such BHs are the paradoxically low density, as well as the weak tidal forces, which were mentioned earlier. It is such a supermassive black hole at the center of our Milky Way galaxy (Sagittarius A *, Sgr A *), as well as most other galaxies.

Candidates for the Black House

The nearest black hole, or rather a candidate for the role of a BH, is an object (V616 Unicorn), which is located at a distance of 3000 light years from the Sun (in our galaxy). It consists of two components: a star with a mass of half the solar mass, as well as an invisible small body, the mass of which is 3 - 5 solar masses. If this object turns out to be a small black hole of stellar mass, then by right it will be the nearest BH.

Following this object, the second closest black hole is the Cyg X-1 object, which was the first candidate for the role of a BH. The distance to it is approximately 6070 light years. It is well studied: it has a mass of 14.8 solar masses and an event horizon radius of about 26 km.

According to some sources, another closest candidate for the role of a BH may be a body in the star system V4641 Sagittarii (V4641 Sgr), which, according to 1999 estimates, was located at a distance of 1600 light years. However, subsequent studies increased this distance by at least 15 times.

How many black holes are there in our galaxy?

There is no exact answer to this question, since it is rather difficult to observe them, and for the entire time of the study of the sky, scientists have managed to find about a dozen black holes within the Milky Way. Without indulging in calculations, we note that there are about 100 - 400 billion stars in our galaxy, and about every thousandth star has enough mass to form a black hole. It is likely that millions of black holes could have formed during the existence of the Milky Way. Since it is easier to register huge black holes, it is logical to assume that most of the BHs in our galaxy are most likely not supermassive. It is noteworthy that the 2005 NASA studies suggest the presence of a swarm of black holes (10-20 thousand) orbiting the center of the galaxy. In addition, in 2016, Japanese astrophysicists discovered a massive satellite near the object * - a black hole, the core of the Milky Way. Due to the small radius (0.15 light years) of this body, as well as its huge mass (100,000 solar masses), scientists suggest that this object is also a supermassive black hole.

The core of our galaxy, the black hole of the Milky Way (Sagittarius A *, Sgr A * or Sagittarius A *) is supermassive and has a mass of 4.31 10 6 solar masses, and a radius of 0.00071 light years (6.25 light years . or 6.75 billion km). The temperature of Sagittarius A * together with the cluster around it is about 1 · 10 7 K.

The largest black hole

The largest black hole in the Universe that scientists have discovered is a supermassive black hole, FSRQ blazar, in the center of galaxy S5 0014 + 81, at a distance of 1.2 · 10 10 light years from Earth. By preliminary results observations using the Swift space observatory, the mass of the BH was 40 billion (40 · 10 9) solar masses, and the Schwarzschild radius of such a hole was 118.35 billion kilometers (0.013 light years). It is also estimated to have originated 12.1 billion years ago (1.6 billion years after the Big Bang). If this giant black hole does not absorb the surrounding matter, then it will survive to the era of black holes - one of the epochs of the development of the Universe, during which black holes will dominate in it. If the nucleus of the galaxy S5 0014 + 81 continues to grow, then it will become one of the last black holes that will exist in the Universe.

The other two known black holes, although they do not have their own names, are of the greatest importance for the study of black holes, since they confirmed their existence experimentally, and also gave important results for the study of gravity. We are talking about the event GW150914, which is called the collision of two black holes into one. This event made it possible to register.

Detecting black holes

Before considering methods for detecting black holes, one should answer the question - why is a black hole black? - the answer to it does not require deep knowledge in astrophysics and cosmology. The fact is that a black hole absorbs all radiation incident on it and does not emit at all, if we do not take into account the hypothetical. If we consider this phenomenon in more detail, we can assume that processes that lead to the release of energy in the form of electromagnetic radiation do not take place inside black holes. Then, if the BH does radiate, then it is in the Hawking spectrum (which coincides with the spectrum of a heated, absolutely black body). However, as mentioned earlier, this radiation was not detected, which suggests a completely low temperature of black holes.

Another generally accepted theory is that electromagnetic radiation and is not at all capable of leaving the event horizon. It is most likely that photons (light particles) are not attracted by massive objects, since, according to the theory, they themselves have no mass. However, the black hole still "attracts" the photons of light by distorting space-time. If we imagine a black hole in space as a kind of depression on the smooth surface of space-time, then there is a certain distance from the center of the black hole, approaching to which the light will no longer be able to move away. That is, roughly speaking, the light begins to “fall” into the “pit”, which does not even have a “bottom”.

In addition to this, if we take into account the effect of gravitational redshift, then it is possible that light in a black hole loses its frequency, shifting along the spectrum to the region of low-frequency long-wavelength radiation, until it loses energy at all.

So, a black hole is black and therefore difficult to detect in space.

Detection methods

Consider the methods astronomers use to detect a black hole:

In addition to the methods mentioned above, scientists often associate objects such as black holes and. Quasars are some kind of clusters of cosmic bodies and gas, which are one of the brightest astronomical objects in the Universe. Since they have a high intensity of luminescence at relatively small sizes, there is reason to believe that the center of these objects is a supermassive black hole, which attracts surrounding matter. Due to such a powerful gravitational attraction, the attracted matter is so hot that it radiates intensely. Finding such objects is usually compared to finding a black hole. Sometimes quasars can radiate in two directions jets of heated plasma - relativistic jets. The reasons for the appearance of such jets (jets) are not completely clear, however, they are probably caused by the interaction of the magnetic fields of the BH and the accretion disk, and are not emitted by the direct black hole.

Jet in the galaxy M87 striking from the center of the BH

Summing up the above, one can imagine, up close: it is a spherical black object, around which strongly heated matter revolves, forming a luminous accretion disk.

Merging and colliding black holes

One of the most interesting phenomena in astrophysics is the collision of black holes, which also makes it possible to detect such massive astronomical bodies. Such processes are of interest not only to astrophysicists, since phenomena poorly studied by physicists become their consequence. The most striking example is the previously mentioned event called GW150914, when two black holes approached so much that they merged into one as a result of mutual gravitational attraction. An important consequence of this collision was the emergence of gravitational waves.

According to the definition of gravitational waves, these are changes in the gravitational field that propagate in a wave-like manner from massive moving objects. When two such objects approach each other, they begin to revolve around a common center of gravity. As they approach each other, their rotation around their own axis increases. Such variable fluctuations of the gravitational field at some point can form one powerful gravitational wave, which can propagate in space for millions of light years. So at a distance of 1.3 billion light years, two black holes collided, forming a powerful gravitational wave, which reached the Earth on September 14, 2015 and was recorded by the LIGO and VIRGO detectors.

How do black holes die?

Obviously, for a black hole to cease to exist, it will need to lose all of its mass. However, according to its definition, nothing can leave the limits of a black hole if it has crossed its event horizon. It is known that the Soviet theoretical physicist Vladimir Gribov was the first to mention the possibility of a black hole emitting particles in his discussion with another Soviet scientist Yakov Zeldovich. He argued that from the point of view of quantum mechanics, a black hole is capable of emitting particles through the tunneling effect. Later, with the help of quantum mechanics, the English theoretical physicist Stephen Hawking built his own, somewhat different theory. More about this phenomenon You can read it. In short, in a vacuum there are so-called virtual particles that are constantly born in pairs and annihilate with each other, while not interacting with the outside world. But if such pairs appear on the event horizon of a black hole, then strong gravity is hypothetically capable of separating them, with one particle falling inside the BH, and the other going away from the black hole. And since the particle escaping from the hole can be observed, and therefore has positive energies, the particle falling into the hole must have negative energies. Thus, the black hole will lose its energy and there will be an effect called the evaporation of the black hole.

According to the available models of a black hole, as mentioned earlier, as its mass decreases, its radiation becomes more intense. Then, at the final stage of the existence of a BH, when it may decrease to the size of a quantum black hole, it will release a huge amount of energy in the form of radiation, which may be equivalent to thousands or even millions of atomic bombs. This event is somewhat reminiscent of the explosion of a black hole, like the same bomb. According to calculations, as a result of the Big Bang, primordial black holes could have arisen, and those of them, whose mass is about 10 12 kg, should have evaporated and exploded around our time. Be that as it may, such explosions have never been noticed by astronomers.

Despite Hawking's proposed mechanism for destroying black holes, the properties of Hawking's radiation cause a paradox in the framework of quantum mechanics. If a black hole absorbs a body, and then loses the mass resulting from the absorption of this body, then regardless of the nature of the body, the black hole will not differ from what it was before the absorption of the body. In this case, information about the body is forever lost. From the point of view of theoretical calculations, the transformation of the initial pure state into the obtained mixed ("thermal") state does not correspond to the current theory of quantum mechanics. This paradox is sometimes called the disappearance of information in a black hole. A definitive solution to this paradox has not been found. Known options for solving the paradox:

• Inconsistency of Hawking's theory. This entails the impossibility of the destruction of the black hole and its constant growth.
• The presence of white holes. In this case, the absorbed information does not disappear, but is simply thrown out into another Universe.
• Inconsistency of the generally accepted theory of quantum mechanics.

Unsolved problems of black hole physics

Apparently, what was described earlier, although black holes have been studied for a relatively long time, they still have many features, the mechanisms of which are still unknown to scientists.

• In 1970, an English scientist formulated the so-called. "The principle of cosmic censorship" - "Nature abhors a naked singularity." This means that the singularity is formed only in places hidden from view, like the center of a black hole. However, this principle has not yet been proven. There are also theoretical calculations according to which a "naked" singularity can occur.
• Nor has the “no hair theorem” been proven, according to which black holes have only three parameters.
• A complete theory of the black hole magnetosphere has not been developed.
• The nature and physics of gravitational singularity have not been studied.
• It is not known for certain what happens at the final stage of the existence of a black hole, and what remains after its quantum decay.

Interesting facts about black holes

Summing up the above, there are several interesting and unusual features of the nature of black holes:

• BHs have only three parameters: mass, electric charge, and angular momentum. As a result of such a small number of characteristics of this body, the theorem that asserts this is called the "no-hair theorem". This also gave rise to the phrase "a black hole has no hair", which means that two black holes are absolutely identical, their three parameters mentioned are the same.
• The density of BH can be less than the density of air, and the temperature is close to absolute zero... From this, it can be assumed that the formation of a black hole occurs not due to the compression of matter, but as a result of the accumulation of a large amount of matter in a certain volume.
• Time for bodies absorbed by BH runs much slower than for an external observer. In addition, the absorbed bodies are significantly stretched inside the black hole, which was called by scientists - spaghettification.
• There may be about a million black holes in our galaxy.
• There is probably a supermassive black hole at the center of every galaxy.
• In the future, according to the theoretical model, the universe will reach the so-called era of black holes, when black holes will become the dominant bodies in the universe.