Introduction
A black hole is a region in space-time, the gravitational attraction which is so large that it can not leave even objects moving at the speed of light, including photons of light itself. The boundary of this region is called the event horizon, and its characteristic dimension - the gravitational radius. In the simplest case of a spherically symmetric black hole Schwarzschild radius is equal to it.
Theoretically, the possible existence of such regions of space-time should be of some exact solutions of Einstein's equations, the first of which was obtained by Karl Schwarzschild in 1915. The exact inventor of the term is unknown, but the designation was popularized by John Archibald Wheeler and the first public lecture is used in the popular "Our Universe: The known and the unknown," December 29, 1967. Previously, such astrophysical objects called "collapsed star" or "collapsars" (from the English. Collapsed stars), as well as "frozen star" (Eng. frozen stars).
The question of existence of black holes is closely related to how true theory of gravity, which implies their existence. In modern physics, the standard theory of gravity, it is best confirmed experimentally, is the general theory of relativity (GTR), confidently predicts the possibility of the formation of black holes (but their existence is possible and in other (not all) models, see: Alternative theories of gravity). Therefore, the observational data are analyzed and interpreted, especially in the context of general relativity, although, strictly speaking, this theory is not confirmed experimentally for conditions corresponding to a region of space-time in the vicinity of the stellar mass black holes (but well documented in conditions appropriate supermassive black holes). Therefore, allegations of direct evidence of the existence of black holes, including in this article below, strictly speaking, should be understood in the sense of confirmation of the existence of astronomical objects such dense and massive, as well as having some other observable properties that can be interpreted as black holes general theory of relativity.
In addition, black holes are often called objects that are not strictly relevant to the above definition, but only approaching its properties to a black hole - for example, it may be collapsing star in the late stages of collapse. In modern astrophysics, this distinction is not given much importance, as the observational manifestations of "almost the collapsed" ("frozen") stars and the "real" ("eternal") black hole almost identical. This is because the differences of physical fields around collapsar from those for "eternal" black hole is reduced by power laws with a characteristic time of the order of the gravitational radius divided by the speed of light.
Distinguish 4 scenarios of black holes, two realistic: gravitational collapse (compression) of a sufficiently massive, Collapse central part of the galaxy or protogalactic gas and two hypothetical: the formation of black holes immediately after the Big Bang (primordial black holes), the emergence of a high-energy nuclear reactions.
Black Holes in the Universe
Since the theoretical prediction of black holes left open the question of their existence, since the presence of solutions of the " black hole" does not guarantee that there are mechanisms for the formation of such objects in the universe. From a mathematical point of view, it is known that at least the collapse of gravitational waves in general relativity stable leads to the formation of trapped surfaces, and hence the black hole, as evidenced by Demetris Christodoulou in the 2000s (Shao Award for the year 2011).
Collapse of a star. Metric inside a shaded area is unknown to us (or uninteresting)
Depicted darker area is filled with the substance of its star, and the metric is determined by the properties of the substance. But the light gray area coincides with the corresponding region of space Schwarzschild see Fig. above. It is about these situations in astrophysics is spoken of as the formation of black holes, from the formal point of view is some freedom of speech. Outside, however, very soon this place will be virtually indistinguishable from the black hole in all of its properties, so this term can be applied to the resulting configuration with very high accuracy.
In reality, due to the accretion of matter on the one hand, and (possibly) of the Hawking radiation, on the other hand, the space - time around the collapsar deviates from the above exact solutions of Einstein's equations. And although in any small area (except in the neighborhood of the singularity), the metric is distorted slightly, the global causal structure of space-time can differ dramatically. In particular, the space - time may, according to some theories, no longer possess the event horizon. This is due to the fact that the presence or absence of an event horizon is determined by, among other things, and events in the infinitely distant future observer.
According to modern concepts, there are four scenarios for the formation of a black hole:
Gravitational collapse (catastrophic compression) of a sufficiently massive star at the final stage of its evolution.
The collapse of the central part of the galaxy or protogalactic gas. Modern ideas put enormous () black hole at the center of many, if not all, spiral and elliptical galaxies. For example, in the center of our galaxy is a black hole Sagittarius A * mass.
Formation of black holes at the moment just after the Big Bang as a result of fluctuations of the gravitational field and / or matter. Such black holes are called primary.
Occurrence of black holes in high-energy nuclear reactions - quantum black holes.
Black Holes of Stellar Masses
Black holes of stellar masses formed as the final stage of a star's life, after complete combustion of fuel and thermonuclear reaction terminated star theoretically should begin to cool, thereby reducing the internal pressure and compression of the star under the action of gravity. Compression can stay at a certain stage, and can go into rapid gravitational collapse. Depending on the star's mass and angular momentum, the following final states:
- Extinguished very dense star consisting mostly depending on the mass of helium, carbon, oxygen, neon, magnesium, silicon or iron (the main elements are listed in order of increasing mass balance star). These residues are called white dwarfs, their mass is limited from above the Chandrasekhar limit.
- Neutron star whose mass is bounded limit Oppenheimer - Volkov.
- Black hole.
Increasing mass balance star equilibrium configuration is a movement down the above sequence. Torque limiting mass increases at each stage, but not qualitatively and quantitatively (maximum 2-3 times).
Conditions (mainly weight) in which the final state of evolution of the star is a black hole, are not well understood, since it is necessary to know the behavior and state of matter at extremely high densities that are inaccessible to experimental study. Additional complexity is modeling stars in the late stages of their evolution due to the complexity arising chemical composition and a sharp decrease in the characteristic time of processes. Suffice it to mention that one of the biggest cosmic catastrophes, supernova explosions, occur precisely at these stages of stellar evolution. Different models give a lower estimate of the mass of the black hole, the resulting gravitational collapse, from 2.5 to 5.6 solar masses. Radius of the black hole at the same time is very small - a few tens of kilometers.
Subsequently, the black hole can grow by absorbing substance - usually a gas neighboring stars in binary star systems (black hole collision with any other astronomical object is very unlikely due to its small diameter). The process of falling gas on any compact astrophysical objects, including a black hole, called accretion. At the same time due to the rotation of gas accretion disk is formed in which the substance is accelerated to relativistic velocities, heated and as a result strongly radiates, including X-rays, which, in principle possible to detect such accretion disks (and hence black holes) by means of ultraviolet and X-ray telescopes. The main problem is the small size and the difficulty of registering differences accretion disks of neutron stars and black holes, which leads to uncertainty in the identification of astronomical objects with black holes. The main difference is that the gas falling onto objects, sooner or later meets a hard surface, which leads to intense radiation during braking, but a cloud of gas falling into a black hole because of growing indefinitely gravitational time dilation (redshift) just fades quickly when approaching the event horizon, as observed by the Hubble telescope in the case of Cygnus X-1.
Collision of black holes with other stars, as well as the collision of neutron stars, causing the formation of a black hole, resulting in a powerful gravitational radiation, which is expected, it will be possible to detect in the coming years with the help of gravitational telescopes. Currently, there are reports of the observation of collisions in X-rays. August 25, 2011 it was reported that for the first time in the history of science, a group of Japanese and American experts could in March 2011 to fix the time of death of the star, which absorbs the black hole
Supermassive black holes
Overgrown very large black holes, according to modern ideas, form the core of most galaxies. Their number includes the massive black hole at the core of our galaxy is Sagittarius A*.
At present the existence of black holes of stellar and galactic scales is considered by most scientists reliably proven by astronomical observations.
American astronomers found that the mass of supermassive black holes may be significantly underestimated. Researchers have found that in order to move the stars in the galaxy M87 (which is located at a distance of 50 million light years from Earth) as it is the case now, the mass of the central black hole should be at least 6.4 billion solar masses, i.e. twice the current estimates of the nucleus of M87, which make up three billion solar masses.
Considered the most reliable evidence of the existence of supermassive black holes in the central regions of galaxies. Today, the resolution of the telescope is not sufficient to distinguish the region of space dimensions of the order of the gravitational radius of the black hole (in addition to the black hole at the center of our galaxy, which is observed by long baseline radio interferometry at the limit of resolution). Therefore, the identification of the central objects like black holes, galaxies, there is a certain degree of assumptions (except the center of our galaxy). It is believed that the upper limit the size of these objects is not sufficient to consider them as clusters of white or brown dwarfs, neutron stars, black holes, or even normal weight.
There are many ways to determine the mass and supermassive approximate dimensions of the body, but most of them are based on the measurement of the characteristics of orbits around rotating objects (stars, radio, gas disks). In the simplest and quite frequent case handling occurs in Keplerian orbits, as evidenced by the proportionality of the rotation speed of the satellite to the square root of the semi-major axis:
In this case, the mass of the central body is well-known formula
In some cases, when the objects are satellites in a continuum (gas disk, a dense star cluster) that its gravity affects the characteristics of the orbit, the radial distribution of mass in the galaxy's core is obtained by solving the so-called. collisionless Bernoulli equation.
Trends in Research in the Physics of Black Holes
The event horizon of the future is a necessary feature of a black hole as a theoretical object. The event horizon of a spherically symmetric black hole is called the Schwarzschild sphere and has a characteristic dimension, called gravitational radius.
Energy may possibly leave the black hole through the so-called. Hawking radiation, which is a quantum effect. If so, the true event horizons in the strict sense, the collapsed objects in our universe formed. Nevertheless, as astrophysical objects collapsed - it's very classic system, the accuracy of their descriptions classical model of a black hole is sufficient for all conceivable astrophysical applications.
It is known that the black hole horizon behaves like a membrane: horizon perturbations caused by external bodies and fields, turning off interactions begin to fluctuate and partly radiated outward in the form of gravitational waves, and partially absorbed most hole. Horizon then calms down, and the black hole reaches equilibrium Kerr black hole - Newman. Features of this process are interesting from the viewpoint of the generation of gravitational waves, which may be registered by gravitational wave observatories in the near future.
Hawking radiation emission process called hypothetical variety of elementary particles, mainly photons, black hole. Temperature known to astronomers of black holes are too small to Hawking radiation from them could be recorded - mass holes are too large. Therefore, until the effect is not yet confirmed the observations.
According to general relativity, the formation of the universe could be produced primordial black holes, some of which (with the initial mass of 1012 kg) would have to finish evaporate in our time. Since the rate of evaporation increases with decreasing size of the black hole, the last step should be essentially the explosion of a black hole. While such explosions have been recorded.
Known about trying to study "Hawking radiation" based on the model an analog of the event horizon of a white hole, in the physical experiment, conducted by researchers from the University of Milan (English).
Disappearance of Information in a Black Hole
The disappearance of the information in the black hole is the most serious problem facing quantum gravity, since it is incompatible with the general principles of quantum mechanics.
In the framework of classical (non-quantum) theory of gravity a black hole - an object indestructible. It can only continue to grow, but can neither be reduced or to disappear altogether. This means that in principle it is possible that she was in a black hole of information is not really gone, it continues to be inside a black hole, but just is not observable from the outside. Another variation of this same thought: if a black hole serves as a bridge between our universe and any other universe, the information may have just flipped into another universe.
However, given the quantum phenomena, the hypothetical result will contain contradictions. The main result of the application of quantum theory to the black hole is that it gradually evaporates due to Hawking radiation. This means that there will be a time when the black hole mass is reduced again to its initial value (before throwing it in the body). Thus, as a result it becomes obvious that the black hole has transformed the original body in the stream of various radiations, but she is not changed (since it returned to the original weight). Radiation emitted by it is completely independent of the nature caught in her body. That is a black hole destroyed lodged in her information that is expressed mathematically as a non-unitary evolution of the quantum state of the hole and its surrounding fields.
In this situation, it becomes apparent paradox. If we consider the same for fall and subsequent evaporation of a quantum system in any pure state, then - because the black hole itself has not changed - we obtain the transformation of the original pure state into a "thermal " (mixed) state. Such a transformation, as already mentioned, non-unitary, and the whole quantum mechanics is based on unitary transformations. Thus, this situation contradicts the original postulates of quantum mechanics.
Conclusion
Black holes are very unusual in its properties to objects. Despite all the progress made in their study, the nature of space and time, black holes largely remains a mystery. Some aspects of this problem still look like science fun, interesting only to specialists.
With regard to the practical implementation of new ideas, I would like to remind that in the middle of the XIX century, even a practical (now) a thing as electricity, seemed scientific abstraction. When the British Prime Minister at the time asked Faraday practical value of electricity, Faraday replied: "Someday your government will introduce a tax on it.
What would happen if mankind will be able to create an artificial black hole? It turns out that black holes are not really "black", they emit so-called " Hawking radiation " that causes them to lose energy, and hence the weight over time. For large black holes, the amount of radiation is very small, but small black holes can quickly turn their mass into a huge amount of energy.
Louis Crane and Westmoreland Swan tried to calculate that it would take to create a small black hole, so you can use its energy. They believe that there is a "golden mean" for artificial black holes, which are small enough to create a huge amount of energy, but large enough that they could not immediately gave all my energy. Scientists have calculated the ideal artificial black hole must have a mass of about one million metric tons, and its size is about one- thousandth the size of a proton. Black hole will start momentarily give energy, which has been compressed.
Black holes are predicted by general relativity (theory of gravitation proposed by Einstein in 1915) and other, more modern theories of gravity, were mathematically justified and in 1939. But the properties of space and time in the vicinity of these objects were so unusual that astronomers and physicists for 25 years did not take them seriously. However, the astronomical discoveries of the mid -1960s forced to look at black holes as a possible physical reality. Their discovery and study may fundamentally change our understanding of space and time.
Works Cited
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"Ripped Apart by a Black Hole". ESO Press Release. Retrieved 19 July 2013.
Nitta, Daisuke; Chiba, Takeshi; Sugiyama, Naoshi (September 2011). "Shadows of colliding black holes". Physical Review D 84 (6). arXiv:1106.242. Bibcode:2011PhRvD..84f3008N.
Cavaglià, M. (2010). "Particle accelerators as black hole factories?". Einstein-Online (Max Planck Institute for Gravitational Physics (Albert Einstein Institute)) 4: 1010.
Melia, Fulvio (2012). The Galactic Supermassive Black Hole. Princeton U Press. ISBN 978-0-691-13129-0.
