The discovery of black holes is brief. Black holes: the story of the discovery of the most mysterious objects in the Universe that we will never see. Discovery Channel Black Holes Video
A black hole is a region of space in which the gravitational attraction is so strong that neither matter nor radiation can leave this region. For bodies located there, the second cosmic velocity (escape velocity) would have to exceed the speed of light, which is impossible, since neither matter nor radiation can move faster than light... Therefore, nothing can fly out of a black hole. The border of the area, beyond which the light does not go out, is called the "event horizon", or simply the "horizon" of the black hole.
The essence of the hypothesis of the formation of black holes is as follows: if a certain mass of matter turns out to be in a relatively small volume, critical for it, then under the action of the forces of its own gravity, such matter begins to contract uncontrollably. There is a kind of gravitational catastrophe - gravitational collapse. As a result of compression, the concentration of the substance increases. Finally, a moment comes when the force of gravity on its surface becomes so great that to overcome it, it is necessary to develop a speed exceeding the speed of light. Such speeds are practically unattainable, and neither rays of light nor particles of matter can escape from the closed space of a black hole. The radiation from the black hole turns out to be "locked" by gravity. Black holes can only absorb radiation
For the gravitational field to be able to "lock" the radiation that creates this field, the mass (M) must be compressed to a volume with a radius less than the "gravitational radius" r g = 2GM / c 2. For this reason, it is practically impossible to create and study a black hole in a laboratory: for a body of any reasonable mass (even millions of tons) to become a black hole, it must be compressed to a size smaller than the size of a proton or neutron, so the properties of black holes are still being studied only theoretically. ...
However, calculations show that bodies of an astronomical scale (for example, massive stars), after depletion of thermonuclear fuel in them, can, under the action of their own gravity, contract to the size of their gravitational radius. The search for such objects has been going on for more than 40 years, and now it is possible with great confidence to indicate several highly probable candidates for black holes with masses from a few to billions of solar masses. However, their study is hindered by the huge distances from the Earth. And although the very fact of the existence of black holes is already difficult to question, the practical study of their properties is still ahead.
1. The history of the idea of black holes.
The English geophysicist and astronomer John Michell suggested that stars so massive can exist in nature that not even a ray of light is able to leave their surface. Using Newton's laws, Michell calculated that if a star with the mass of the Sun had a radius of no more than 3 km, then even the particles of light (which he, following Newton, considered corpuscles) could not fly away far from such a star. Therefore, such a star would appear completely dark from afar. Michell presented this idea at a meeting of the Royal Society of London on November 27, 1783. Thus, the concept of a "Newtonian" black hole was born.
The same idea was expressed in his book System of the World (1796) by the French mathematician and astronomer Pierre Simon Laplace. A simple calculation allowed him to write: "A luminous star with a density equal to that of the Earth, and a diameter 250 times the diameter of the Sun, does not allow any light ray to reach us due to its gravity; therefore, it is possible that the brightest celestial bodies in the Universe turn out to be invisible for this reason. " However, the mass of such a star would have to be tens of millions of times that of the sun. And since further astronomical measurements showed that the masses real stars not very different from solar, Mitchell and Laplace's idea of black holes was forgotten.
Throughout the 19th century, the idea of bodies invisible due to their massiveness did not arouse much interest among scientists. This was due to the fact that, in the framework of classical physics, the speed of light is not of fundamental importance. However, in late XIX- at the beginning of the 20th century, it was established that the laws of electrodynamics formulated by J. Maxwell, on the one hand, are satisfied in all inertial reference frames, and on the other hand, do not have invariance under Galileo transformations. This meant that the concepts of the nature of the transition from one inertial reference system to another, which had developed in physics, needed a significant correction.
In the course of further development of electrodynamics, G. Lorentz proposed a new system of transformations of space-time coordinates (known today as Lorentz transformations), with respect to which Maxwell's equations remained invariant. Developing the ideas of Lorentz, Poincaré suggested that all other physical laws are also invariant under these transformations.
In 1905, A. Einstein used the concepts of Lorentz and Poincaré in his special theory of relativity (SRT), in which the role of the law of transformation of inertial reference frames finally passed from Galileo's transformations to Lorentz's transformations. Classical (Galilean-invariant) mechanics was then replaced by a new, Lorentz-invariant relativistic mechanics. Within the framework of the latter, the speed of light turned out to be the limiting speed that a physical body can develop, which radically changed the meaning of black holes in theoretical physics.
However, Newton's theory of gravitation (on which the original theory of black holes was based) is not Lorentz invariant. Therefore, it cannot be applied to bodies moving at near-light and light speeds. Devoid of this drawback, the relativistic theory of gravitation was created mainly by Einstein (who formulated it finally by the end of 1915) and received the name general theory of relativity (GR).
For the second time, scientists "collided" with black holes in 1916, when the German astronomer Karl Schwarzschild obtained the first exact solution of the equations of general relativity. It turned out that the empty space around a massive point has a singularity at a distance r g from it; that is why the quantity r g is often called the "Schwarzschild radius", and the corresponding surface (event horizon) is called the Schwarzschild surface. In the next half century, the efforts of theorists have clarified many amazing features of the Schwarzschild solution, but black holes have not yet been considered as a real object of study.
True, in the 1930s, after the creation of quantum mechanics and the discovery of the neutron, physicists investigated the possibility of the formation of compact objects (white dwarfs and neutron stars) as products of the evolution of normal stars. Estimates have shown that after depletion of nuclear fuel in the interior of a star, its core can shrink to turn into a small and very dense white dwarf, or into an even denser and very tiny neutron star.
In 1934, the European astronomers Fritz Zwicky and Walter Baade, who worked in the United States, put forward a hypothesis that supernova explosions are a very special type of stellar explosions caused by the catastrophic compression of the star's core. This is how the idea of the possibility of observing the collapse of a star was born for the first time. Baade and Zwicky hypothesized that a super-dense degenerate star consisting of neutrons is formed as a result of a supernova explosion. Calculations have shown that such objects can indeed be born and be stable, but only with a moderate initial star mass. But if the mass of a star exceeds three solar masses, then nothing can stop its catastrophic collapse.
In 1939, American physicists Robert Oppenheimer and Hartland Snyder substantiated the conclusion that the core of a massive star should non-stop collapse into an extremely small object, the properties of space around which (if it does not rotate) are described by Schwarzschild's solution. In other words, the core of a massive star at the end of its evolution should rapidly contract and go under the event horizon, becoming a black hole. But since such an object (as they said then, a "collapsar", or "frozen star") does not emit electromagnetic waves, astronomers understood that it would be incredibly difficult to detect it in space and therefore did not start searching for a long time.
Since no medium of information is able to get out of the event horizon, interior the black hole is not causally connected with the rest of the Universe, physical processes taking place inside the black hole cannot influence the processes outside it. At the same time, matter and radiation falling from the outside onto the black hole freely penetrate inward through the horizon. We can say that the black hole absorbs everything and does not release anything. For this reason, the term "black hole" was born, proposed in 1967 by the American physicist John Archibald Wheeler.
2. Formation of black holes
The most obvious way for a black hole to form is through the collapse of the core of a massive star. Until the supply of nuclear fuel is depleted in the interior of a star, its equilibrium is maintained due to thermonuclear reactions (conversion of hydrogen into helium, then into carbon, etc., up to iron in the most massive stars). The heat released in this case compensates for the loss of energy leaving the star with its radiation and stellar wind. Thermonuclear reactions support high pressure in the interior of a star, preventing its compression under the influence of its own gravity. However, over time, nuclear fuel is depleted and the star begins to shrink.
The core of the star shrinks most rapidly, while it heats up strongly (its gravitational energy turns into heat) and heats the envelope surrounding it. As a result, the star loses its outer layers in the form of a slowly expanding planetary nebula or a catastrophically discarded supernova envelope. And the fate of a collapsing nucleus depends on its mass. Calculations show that if the mass of the star's core does not exceed three solar masses, then it "wins the battle with gravity": its compression will be stopped by the pressure of degenerate matter, and the star will turn into a white dwarf or neutron star. But if the mass of the star's core is more than three solar ones, then nothing can stop its catastrophic collapse, and it will quickly go under the event horizon, becoming a black hole. As follows from the formula for r g, a black hole with a mass of 3 solar has a gravitational radius of 8.8 km.
Astronomical observations are in good agreement with these calculations: all components of binary stellar systems exhibiting the properties of black holes (in 2005 there were about 20 known), have masses from 4 to 16 solar masses. The theory of stellar evolution indicates that over 12 billion years of existence of our Galaxy, containing about 100 billion stars, as a result of the collapse of the most massive of them, several tens of millions of black holes should have been formed. In addition, black holes of very large masses (from millions to billions of solar masses) can be found in the cores of large galaxies, including ours. This is evidenced by astronomical observations, although the pathways of the formation of these giant black holes are not entirely clear.
If in our era the high density of matter necessary for the birth of a black hole can arise only in the collapsing cores of massive stars, then in the distant past, immediately after the Big Bang, from which the expansion of the Universe began about 14 billion years ago, a high density of matter was everywhere ... Therefore, small fluctuations of density in that era could lead to the birth of black holes of any mass, including small ones. But the smallest of them, due to quantum effects, had to evaporate, losing their mass in the form of radiation and particle flows. "Primordial black holes" with a mass of more than 10 12 kg could survive to this day. The smallest of them, weighing 10 12 kg (like a small asteroid), should have a size of about 10 -15 m (like a proton or neutron).
Finally, there is a hypothetical possibility of the birth of microscopic black holes in mutual collisions of fast elementary particles. This is one of the predictions of string theory - one of the currently competing physical theories structure of matter. String theory predicts that space has more than three dimensions. Gravity, unlike other forces, must propagate through all of these dimensions and therefore increase significantly over short distances. In a powerful collision of two particles (for example, protons), they can contract hard enough to create a microscopic black hole. After that, it will almost instantly collapse ("evaporate"), but observation of this process is of great interest for physics, since, evaporating, the hole will emit all types of particles existing in nature. If the hypothesis of string theory is correct, then the creation of such black holes can occur during collisions of energetic particles of cosmic rays with atoms of the earth's atmosphere, as well as in the most powerful accelerators of elementary particles.
3. Properties of black holes
Tension near a black hole gravitational field so great that physical processes there can be described only with the help of the relativistic theory of gravitation. According to general relativity, space and time are curved by the gravitational field of massive bodies, with the greatest curvature occurring near black holes. When physicists talk about intervals of time and space, they mean numbers counted from any physical clock and rulers. For example, a molecule with a certain vibration frequency can play the role of a clock, the number of which between two events can be called a "time interval".
It is important that gravity acts on all physical systems in the same way: all clocks show that time is slowing down, and all rulers show that space is stretching near a black hole. This means that the black hole bends the geometry of space and time around itself. Away from the black hole, this curvature is small, and near it is so great that the rays of light can move around it in a circle. Away from a black hole, its gravitational field is exactly described by Newton's theory for a body of the same mass, but near the black hole, gravity becomes much stronger than Newton's theory predicts.
If it were possible to observe a star through a telescope at the moment of its transformation into a black hole, then at first it would be seen how the star is contracting faster and faster, but as its surface approaches the gravitational radius, the compression will begin to slow down until it stops altogether. In this case, the light coming from the star will fade and turn red until it finally goes out. This is because, overcoming the force of gravity, photons lose energy and it takes more and more time for them to reach us. When the surface of the star reaches the gravitational radius, the light that leaves it will take an infinite time to reach any observer, even those located relatively close to the star (and in this case the photons will completely lose their energy). Consequently, we will never wait for this moment and, moreover, we will not see what is happening with the star under the event horizon, but theoretically this process can be investigated.
Calculation of the idealized spherical collapse shows that a short time matter under the event horizon contracts to a point where they reach infinitely large values density and gravity. This point is called "singularity". Moreover, mathematical analysis shows that if an event horizon has arisen, then even a non-spherical collapse leads to a singularity. However, all this is true only if the general theory of relativity is applicable down to very small spatial scales, which is not yet certain. In the microcosm, quantum laws operate, and the quantum theory of gravity has not yet been created. It is clear that quantum effects cannot stop a star from collapsing into a black hole, but they could prevent the appearance of a singularity.
Studying the fundamental properties of matter and space-time, physicists consider the study of black holes to be one of the most important directions, since the hidden properties of gravity are manifested near black holes. For the behavior of matter and radiation in weak gravitational fields, various theories of gravitation give almost indistinguishable predictions, but in strong fields characteristic of black holes, the predictions of various theories differ significantly, which provides a key to identifying the best one among them. Within the framework of the most popular theory of gravity - Einstein's general relativity - the properties of black holes have been studied in great detail. Some of the most important are:
1) Time flows more slowly near a black hole than far from it. If a distant observer throws a lighted lantern towards the black hole, he will see how the lantern will fall faster and faster, but then, approaching the Schwarzschild surface, it will begin to slow down, and its light will dim and redden (since the rate of vibration of all its atoms and molecules). From the point of view of a distant observer, the lantern will practically stop and become invisible, never being able to cross the surface of the black hole. But if the observer himself jumped there with the lantern, then in a short time he would cross the Schwarzschild surface and fall to the center of the black hole, being torn apart by its powerful tidal gravitational forces arising from the difference in attraction at different distances from the center.
2) No matter how complex the original body may be, after its compression into a black hole, an external observer can determine only three of its parameters: total mass, angular momentum (associated with rotation) and electric charge. All other features of the body (shape, density distribution, chemical composition etc.) are "erased" during the collapse. The fact that for an outside observer the structure of a black hole looks extremely simple, John Wheeler expressed a joking statement: "A black hole has no hair."
During the collapse of a star into a black hole in a small fraction of a second (according to the clock of a distant observer), all its external features associated with the initial inhomogeneity are emitted in the form of gravitational and electromagnetic waves. The resulting stationary black hole "forgets" all information about the original star, except for three quantities: total mass, angular momentum (associated with rotation) and electric charge. Studying a black hole, it is no longer possible to know whether the original star consisted of matter or antimatter, whether it was elongated or flattened, etc. In real astrophysical conditions, a charged black hole will attract particles of the opposite sign from the interstellar medium, and its charge will quickly become zero. The remaining stationary object will either be a non-rotating "Schwarzschild black hole", which is characterized only by mass, or a rotating "Kerr black hole", which is characterized by mass and angular momentum.
3) If the original body rotated, then a "vortex" gravitational field is preserved around the black hole, dragging all neighboring bodies into rotational motion around it. The gravitational field of a rotating black hole is called the Kerr field (mathematician Roy Kerr found a solution to the corresponding equations in 1963). This effect is typical not only for a black hole, but for any rotating body, even for the Earth. For this reason, a freely rotating gyroscope placed on an artificial satellite of the Earth experiences a slow precession relative to distant stars. Near the Earth, this effect is barely noticeable, but near a black hole it is much more pronounced: the speed of the gyroscope precession can be used to measure the angular momentum of the black hole, although it is not visible itself.
The closer we get to the horizon of the black hole, the stronger the drag effect becomes. " vortex field"Before reaching the horizon, we find ourselves on the surface, where the entrainment becomes so strong that no observer can remain stationary (ie, be" static ") relative to distant stars. On this surface (called the static limit) and within all objects must orbit around the black hole in the same direction as the hole itself rotates in. No matter what power its jet engines develop, an observer within the static limit can never stop his rotational motion relative to distant stars.
The static limit everywhere lies outside the horizon and touches it only at two points, where they both intersect with the axis of rotation of the black hole. The region of space-time located between the horizon and the limit of static is called the ergosphere. An object caught in the ergosphere can still escape outward. Therefore, although the black hole "eats everything up and does not let go of anything," nevertheless, an exchange of energy between it and outer space is possible. For example, particles or quanta flying through the ergosphere can carry away the energy of its rotation.
4) All matter inside the event horizon of a black hole certainly falls to its center and forms a singularity with an infinitely high density. English physicist Stephen Hawking defines the singularity as "the place where the classical concept of space and time is destroyed, just like all the known laws of physics, since they are all formulated on the basis of classical space-time."
5) In addition, S. Hawking discovered the possibility of very slow spontaneous quantum "evaporation" of black holes. In 1974, he proved that black holes (not only rotating, but any) can emit matter and radiation, but this will be noticeable only if the mass of the hole itself is relatively small. A powerful gravitational field near a black hole should generate particle-antiparticle pairs. One of the particles of each pair is absorbed by the hole, and the second is emitted outside. For example, a black hole with a mass of 10 12 kg should behave like a body with a temperature of 10 11 K, emitting very hard gamma quanta and particles. The idea of "evaporation" of black holes completely contradicts the classical idea of them as bodies incapable of radiating.
4. Search for black holes
Calculations within the framework of general relativity indicate only the possibility of the existence of black holes, but by no means prove their presence in the real world, the discovery of a black hole would be an important step in the development of physics. Finding isolated black holes in space is incredibly difficult: you need to spot a small dark object against a backdrop of cosmic blackness. But there is hope to detect a black hole by its interaction with the surrounding astronomical bodies, by its characteristic influence on them.
Considering the most important properties of black holes (massiveness, compactness and invisibility), astronomers gradually developed a strategy for their search. The easiest way to detect a black hole is by its gravitational interaction with the surrounding matter, for example, with nearby stars. Attempts to detect invisible massive satellites in binary stars have been unsuccessful. But after launching X-ray telescopes into orbit, it turned out that black holes actively manifest themselves in close binary systems, where they take matter from a nearby star and absorb it, while heating it to a temperature of millions of degrees and making it a source of X-ray radiation for a short time.
Since in a binary system, a black hole paired with a normal star revolves around a common center of mass, using the Doppler effect, it is possible to measure the speed of the star and determine the mass of its invisible companion. Astronomers have already identified several dozen binary systems, where the mass of an invisible companion exceeds 3 times the mass of the Sun and characteristic manifestations of the activity of matter moving around a compact object are noticeable, for example, very rapid fluctuations in the brightness of streams of hot gas rapidly rotating around an invisible body.
The X-ray binary V404 Cygnus is considered especially promising, the mass of the invisible component of which is estimated to be no less than 6 solar masses. Other black hole candidates are found in the binary systems Cygnus X-1, LMC X-3, V616 Unicorn, QZ Chanterelles, as well as the X-ray novae Ophiuchus 1977, Fly 1981 and Scorpio 1994. Almost all of them are located within our Galaxy, and the system LMC X-3 is in the nearby Large Magellanic Cloud galaxy.
Another direction in the search for black holes is the study of galactic nuclei. In them, huge masses of matter accumulate and condense, stars collide and merge, so supermassive black holes can form there, which are millions of times larger than the Sun. They attract the surrounding stars, creating a peak in brightness in the center of the galaxy. They destroy stars flying close to them, the matter of which forms an accretion disk around the black hole and is partially ejected along the axis of the disk in the form of fast jets and streams of particles. This is not a speculative theory, but processes actually observed in the nuclei of some galaxies and indicating the presence in them of black holes with masses of up to several billion solar masses. Recently, very convincing evidence has been obtained that there is a black hole with a mass of about 2.5 million solar masses in the center of our Galaxy.
It is likely that the most powerful processes of energy release in the Universe occur with the participation of black holes. They are considered the source of activity in the cores of quasars - young massive galaxies. It is their birth, as astrophysicists believe, is marked by the most powerful explosions in the Universe, manifested as gamma-ray bursts.
5. Thermodynamics and evaporation of black holes
The concept of a black hole as an absolutely absorbing object was corrected by S. Hawking in 1975. Studying the behavior of quantum fields near a black hole, he predicted that a black hole necessarily radiates particles into outer space and thereby loses mass. This effect is called Hawking radiation (evaporation). To put it simply, the gravitational field polarizes the vacuum, as a result of which the formation of not only virtual, but also real particle-antiparticle pairs is possible. One of the particles, which turned out to be just below the event horizon, falls into the black hole, and the other, which turned out to be just above the horizon, flies away, carrying away the energy (that is, part of the mass) of the black hole. The radiation power of the black hole is
The composition of the radiation depends on the size of the black hole: for large black holes, these are mainly photons and neutrinos, and heavy particles begin to be present in the spectrum of light black holes. The spectrum of Hawking radiation for massless fields turned out to strictly coincide with the radiation of an absolutely black body, which made it possible to ascribe the temperature to the black hole
,
where is the reduced Planck's constant, c is the speed of light, k is the Boltzmann constant, G is the gravitational constant, M is the mass of the black hole.
On this basis, the thermodynamics of black holes was built, including the key concept of the entropy of a black hole, which turned out to be proportional to the area of its event horizon:
where A is the area of the event horizon.
The evaporation rate of a black hole is the greater, the smaller its size. The evaporation of black holes of stellar (and even more so galactic) scales can be neglected, but for primary and especially for quantum black holes, the evaporation processes become central.
Due to evaporation, all black holes lose mass and their lifetime is finite:
.
At the same time, the evaporation rate grows like an avalanche, and the final stage of evolution is in the nature of an explosion, for example, a black hole with a mass of 1000 tons will evaporate in about 84 seconds, releasing energy equal to the explosion of about ten million atomic bombs of average power.
At the same time, large black holes, the temperature of which is lower than the temperature of the relict radiation of the Universe (2.7 K), at the present stage of the development of the Universe can only grow, since the radiation emitted by them has less energy than the absorbed one. This process will last until the photon gas of the CMB cools down as a result of the expansion of the Universe.
Without the quantum theory of gravity, it is impossible to describe the final stage of evaporation, when black holes become microscopic (quantum). According to some theories, after evaporation there should be a "cinder" - a minimal Planck black hole.
6. Falling into a black hole
Imagine what a fall into a Schwarzschild black hole should look like. A body freely falling under the influence of gravitational forces is in a state of weightlessness. The falling body will experience the action of tidal forces that stretch the body in the radial direction and compress in the tangential direction. The magnitude of these forces grows and tends to infinity at. At some point in its own time, the body will cross the event horizon. From the point of view of the observer falling with the body, this moment is not highlighted by anything, but now there is no return. The body finds itself in a throat (its radius is at the point where the body is located), which is contracting so quickly that it is no longer possible to fly away from it until the moment of the final collapse (this is the singularity), even moving at the speed of light.
Let us now consider the process of a body falling into a black hole from the point of view of a distant observer. For example, let the body be luminous and, in addition, send signals back with a certain frequency. At first, a distant observer will see that the body, being in the process of free fall, is gradually accelerating under the action of gravity towards the center. The color of the body does not change, the frequency of the detected signals is almost constant. However, when the body begins to approach the event horizon, the photons coming from the body will experience more and more gravitational redshift. In addition, due to the gravitational field, all physical processes from the point of view of a distant observer will go slower and slower than the gravitational time dilation): a clock fixed on the radial coordinate r without rotation () will go slower than an infinitely distant one in 
once. It will appear that the body - in an extremely flattened form - will slow down as it approaches the event horizon and, in the end, will practically stop. The signal frequency will drop sharply. The wavelength of the light emitted by the body will grow rapidly, so that the light will quickly turn into radio waves and then into low-frequency electromagnetic oscillations, which it will no longer be possible to fix. The observer will never see the crossing of the event horizon by the body, and in this sense, the fall into the black hole will last indefinitely. However, there is a moment from which the remote observer will no longer be able to influence the falling body. A ray of light sent after this body will either never catch up with it at all, or it will catch up already beyond the horizon. In addition, the distance between the body and the event horizon, as well as the "thickness" of the flattened (from the point of view of an outside observer) body, will quickly reach the Planck length and (from a mathematical point of view) will decrease further. For a real physical observer (making measurements with a Planck error), this is equivalent to the fact that the mass of the black hole will increase by the mass of the falling body, which means that the radius of the event horizon will increase and the falling body will be "inside" the event horizon in a finite time.
The process of gravitational collapse will look similar for a distant observer. At first, the matter will rush to the center, but near the event horizon it will begin to slow down sharply, its radiation will go into the radio range, and as a result, a distant observer will see that the star has gone out.
7. Types of black holes
A) Supermassive black holes
Overgrown very massive black holes, according to modern concepts, form the nuclei of most galaxies. Among them is the massive black hole in the core of our galaxy - Sagittarius A *.
Currently, the existence of black holes of stellar and galactic scales is considered by most scientists to be reliably proven by astronomical observations.
American astronomers have found that the masses of supermassive black holes can be significantly underestimated. The researchers found that in order for the stars to move in the galaxy M87 (which is located 50 million light years from Earth) as it is now, the mass of the central black hole must be at least 6.4 billion solar masses, that is, twice the current estimates of the core of M87, which are 3 billion solar masses.
B) Primary black holes
Primary black holes are currently hypothesized. If at the initial moments of the life of the Universe there were sufficient deviations from the homogeneity of the gravitational field and the density of matter, then black holes could form from them through collapse. Moreover, their mass is not limited from below, as in a stellar collapse - their mass, probably, could be quite small. The discovery of primordial black holes is of particular interest in connection with the possibilities of studying the phenomenon of evaporation of black holes.
C) Quantum black holes
It is assumed that as a result of nuclear reactions, stable microscopic black holes, the so-called quantum black holes, can arise. For a mathematical description of such objects, a quantum theory of gravity is needed. However, from general considerations, it is very likely that the mass spectrum of black holes is discrete and there is a minimal black hole - the Planck black hole. Its mass is about 10 −5 g, and its radius is 10 −35 m. The Compton wavelength of a Planck black hole is, in order of magnitude, equal to its gravitational radius.
Conclusion
Thus, all "elementary objects" can be divided into elementary particles(their wavelength is greater than their gravitational radius) and black holes (the wavelength is less than their gravitational radius). The Planck black hole is a boundary object, for which you can find the name maximon, indicating that it is the heaviest possible elementary particle. Another term sometimes used to refer to it is plankeon.
Even if quantum black holes exist, their lifetime is extremely short, making them very problematic to detect directly.
Recently, experiments have been proposed to find evidence of the appearance of black holes in nuclear reactions. However, for the direct synthesis of a black hole in an accelerator, an unattainable energy of 10 26 eV is required. Apparently, virtual intermediate black holes can arise in ultrahigh-energy reactions.
Bibliography
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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 the general theory of 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 occur in the interior of the star, they maintain high fever and pressure, preventing the star from contracting under 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 the binary star systems of our Galaxy. All in all, as scientists suggest, there are tens of millions of such objects.
Between the French and the British, there is sometimes a half-joking, and sometimes a serious polemic: 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 British theoretical physicists S. Hawking and G. Ellis, in a book devoted to modern special mathematical questions 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 in 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 the audience.
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 motion 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. They are often done almost simultaneously and independently. different people Usually priority is given to the one who penetrates deeper into the essence of the problem, but sometimes it is just the whims of fortune.
But the foresight 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 outside 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." And the 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.
The concept of a black hole is known to everyone - from schoolchildren to old people, it is used in science and science fiction literature, in the yellow media and at scientific conferences. But what exactly such holes are is not known to everyone.
From the history of black holes
1783 g. The first hypothesis of the existence of such a phenomenon as a black hole was put forward in 1783 by the English scientist John Michell. In his theory, he combined two of Newton's creations - optics and mechanics. Michell's idea was this: if light is a stream of the smallest particles, then, like all other bodies, the particles should experience the attraction of the gravitational field. It turns out that the more massive the star, the more difficult it is for the light to resist its attraction. Thirteen years after Michell, the French astronomer and mathematician Laplace put forward (most likely, independently of his British colleague) a similar theory.
1915 g. However, all their works remained unclaimed until the beginning of the 20th century. In 1915, Albert Einstein published General Relativity and showed that gravity is the curvature of space-time caused by matter, and a few months later the German astronomer and theoretical physicist Karl Schwarzschild used it to solve a specific astronomical problem. He investigated the structure of curved spacetime around the Sun and rediscovered the phenomenon of black holes.
(John Wheeler introduced the term "Black Holes" into scientific use)
1967 year American physicist John Wheeler outlined a space that can be crumpled, like a piece of paper, into an infinitesimal point and designated the term "black hole".
1974 year British physicist Stephen Hawking proved that black holes, while absorbing meteria without return, can emit radiation and eventually evaporate. This phenomenon is called Hawking radiation.
2013 g. Latest research pulsars and quasars, as well as the discovery of relic radiation, finally made it possible to describe the very concept of black holes. In 2013, the gas cloud G2 approached a very close distance to a black hole and is likely to be absorbed by it, observing the unique process provides tremendous opportunities for new discoveries of the features of black holes.
(Massive object Sagittarius A *, its mass is 4 million times greater than the Sun, which implies a cluster of stars and the formation of a black hole)
2017 year... A group of scientists from the Event Horizon Telescope collaboration of several countries, having linked eight telescopes from different points of the Earth's continents, carried out observations of the black hole, which is a supermassive object and is located in the M87 galaxy, the constellation Virgo. The mass of the object is 6.5 billion (!) Solar masses, gigantic times more massive object Sagittarius A *, for comparison, with a diameter slightly less than the distance from the Sun to Pluto.
The observations were carried out in several stages, starting from the spring of 2017 and during the periods of 2018. The amount of information was calculated in petabytes, which then had to be deciphered and a true snapshot of a very distant object was obtained. Therefore, it took another two whole years to thoroughly process all the data and combine them into one whole.
Dec 2019 The data was successfully decoded and rendered into the first ever image of a black hole.
(The first ever snapshot of a black hole in the M87 galaxy in the constellation Virgo)
Image resolution allows you to see the shadow of the point of no return at the center of the object. The image was obtained as a result of interferometric observations with a very long baseline. These are the so-called synchronous observations of one object from several radio telescopes, interconnected by a network and located in different parts of the globe, pointing in the same direction.
What black holes really are
A laconic explanation of the phenomenon sounds like this.
A black hole is a space-time region, whose gravitational attraction is so great that no object, including light quanta, can leave it.
The black hole was once a massive star. As long as thermonuclear reactions maintain high pressure in its bowels, everything remains normal. But over time, the supply of energy is depleted and the celestial body, under the influence of its own gravity, begins to shrink. The final stage of this process is the collapse of the stellar core and the formation of a black hole.
- 1. Ejection of a jet by a black hole at high speed
- 2. The disk of matter grows into a black hole
- 3. Black hole
- 4. Detailed diagram of the black hole region
- 5. Size of found new observations
The most widespread theory says that similar phenomena exist in every galaxy, including in the center of our Milky Way. The huge force of gravity of the hole is capable of holding several galaxies around itself, preventing them from moving away from each other. The "coverage area" can be different, it all depends on the mass of the star, which turned into a black hole, and can be thousands of light years.
Schwarzschild radius
The main property of a black hole is that any substance that gets into it can never return. The same goes for light. At their core, holes are bodies that completely absorb all light that falls on them and do not emit their own. Such objects can visually appear as lumps of absolute darkness.
- 1. Matter in motion at half the speed of light
- 2. Photon ring
- 3. Inner photonic ring
- 4. Event horizon in a black hole
Based on Einstein's General Theory of Relativity, if a body has approached the critical distance to the center of the hole, it will no longer be able to return. This distance is called the Schwarzschild radius. What exactly happens within this radius is not known for certain, but there is the most common theory. It is believed that all the substance of a black hole is concentrated in an infinitely small point, and in its center is an object with infinite density, which scientists call a singular perturbation.
How does a fall into a black hole happen?
(In the picture, the black hole of Sagittarius A * looks like an extremely bright cluster of light)
Not so long ago, in 2011, scientists discovered a gas cloud, giving it the uncomplicated name G2, which emits unusual light. Such a glow can give rise to friction in gas and dust caused by the action of the Sagittarius A * black hole and which revolve around it in the form of an accretion disk. Thus, we become observers of the amazing phenomenon of absorption of a gas cloud by a supermassive black hole.
According to the latest studies, the closest approach to a black hole will occur in March 2014. We can recreate a picture of how this spectacular spectacle will take place.
- 1. When it first appears in the data, the gas cloud resembles a huge ball of gas and dust.
- 2. Now, as of June 2013, the cloud is tens of billions of kilometers from the black hole. It falls into it at a speed of 2500 km / s.
- 3. It is expected that the cloud will pass by the black hole, but tidal forces caused by the difference in attraction acting on the leading and trailing edges of the cloud will cause it to take an increasingly elongated shape.
- 4. After the cloud is broken apart, most of it is likely to flow into the accretion disk around Sagittarius A *, generating shock waves in it. At the same time, the temperature will jump to several million degrees.
- 5. Part of the cloud will fall directly into the black hole. No one knows exactly what will happen to this substance later, but it is expected that in the process of falling, it will emit powerful beams of X-rays, and no one else will see it.
Video: a black hole engulfs a gas cloud
(Computer simulation of how most of the G2 gas cloud will be destroyed and absorbed by the black hole Sagittarius A *)
What's inside the black hole
There is a theory that claims that a black hole is practically empty inside, and all its mass is concentrated in an incredibly small point located in its very center - a singularity.
According to another theory, which has existed for half a century, everything that falls into a black hole goes into another universe, located in the black hole itself. Now this theory is not the main one.
And there is a third, most modern and tenacious theory, according to which everything that falls into a black hole dissolves in vibrations of strings on its surface, which is designated as the event horizon.
So what is an event horizon? It is impossible to look inside a black hole even with a super-powerful telescope, since even light, getting inside a giant cosmic funnel, has no chance to emerge back. Everything that can be seen at least somehow is in its immediate vicinity.
The event horizon is a conventional surface line, from under which nothing (neither gas, nor dust, nor stars, nor light) can no longer escape. And this is the very same mysterious point of no return in the black holes of the Universe.
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 vivid 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
The name of John Michell, physicist, astronomer and geologist, professor at Cambridge University and pastor of the Church of England, was completely undeservedly lost among the stars of 18th century English science. 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 shot 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 critical importance 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 hid many stars that cannot be seen from Earth through any telescope. Later, the great French mathematician, astronomer and physicist Pierre Simon Laplace came to the same conclusion, including him in 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 had spent its 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 greater than 1.4 times the mass of the Sun 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 of 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, the American physicist John Archibald Wheeler was the first to call such a final 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
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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 ultra-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, while the "external" particle can escape under favorable conditions. As a result, the hole turns into a radiation source and therefore loses its energy and, consequently, mass. Therefore, black holes are basically 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. It follows from this 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: for this you need to multiply h and C, divide the result by G and extract Square root). This is 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 create a black hole? At first glance, this idea is simply crazy - 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 the physicists who are developing the superstring theory, which includes the quantum theory of gravity (although far from complete), say so. According to this theory, space has not three dimensions, but at least nine. We do not notice the extra dimensions, since they are looped back 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 also be able to register orbital detectors of cosmic rays of the new generation.
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, special case 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 they are completely invisible in a macroscopic hole.
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, it will certainly begin 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 needed to unambiguously describe the hole. Is the second law of thermodynamics violated in the course of gravitational collapse?
Could it not 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 and 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 instruments, 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 capturing any external object 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, from Bekenstein's reasoning it follows 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. The entropy of an ordinary physical body is more or less proportional to its volume, and the entropy of a hole is proportional to the surface of the horizon. 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 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 can be added 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 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 in 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 classical, but quantum holes that have not yet been discovered. The main result recent years- 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 birth 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 paper, from which it follows that thermal radiation from a black hole completely returns to external 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. Hawking said at a conference in Dublin last summer 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 many 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. "
