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Black Holes

               I'm kinda sure you've all heard about black holes on tv, internet, about emerging a black hole at Large Hadron Collider in Geneva etc. But do we fully understand what a BH is? Do we know how it works? Is there an extraterrestrial threat from a black-hole to enocunter our planet? Hm... i dunno if i really have all the answers, but let me see what i know so far.

"by environment clean generations"

               A black hole is a star that has collapsed into a tiny point known as a singularity. It is so dense that it sucks in everything near it, including light. Black holes can be seen via the death throes of the matter being sucked in. Although it becomes invisible past a certain point, an accretion disk, which is visible, develops as the matter swirls toward the black hole. The collapsed star is so dense that nothing can escape its gravitational pull, neither light nor time. 

               A black hole is a concentration of mass great enough that the force of gravity prevents anything from escaping from it except through quantum tunneling behavior. The gravitational field is so strong that the escape velocity near it exceeds the speed of light. This implies that nothing, not even light, can escape its gravity, hence the word "black." The term "black hole" is widespread, even though it does not refer to a hole in the usual sense, but rather a region of space from which nothing can return. Theoretically, black holes can have any size, from microscopic to near the size of the observable universe.

              Black holes are predicted by general relativity. According to classical general relativity, neither matter nor information can flow from the interior of a black hole to an outside observer. For example, one cannot bring out any of its mass, or receive a reflection back by shining a light source such as a flashlight, or retrieve any information about the material that has entered the black hole. Quantum mechanical effects may allow matter and energy to radiate from black holes; however, it is thought that the nature of the radiation does not depend on what has fallen into the black hole in the past.
             The existence of black holes in the universe is well supported by astronomical observation, particularly from studying supernovae and X-ray emissions from active galactic nuclei.

                           Event horizon 

                  The "surface" of a black hole is the so-called event horizon, an imaginary surface surrounding the mass of the black hole. Using the Gauss-Bonnet theorem, Stephen Hawking proved that the topology of the event horizon of a (four dimensional) black hole is a 2-sphere. At the event horizon, the escape velocity is equal to the speed of light. Thus, anything inside the event horizon, including a photon, is prevented from escaping across the event horizon by the extremely strong gravitational field. Particles from outside this region can fall in, cross the event horizon, and will never be able to leave. 
                    According to classical general relativity, black holes can be entirely characterized according to three parameters: mass, angular momentum, and electric charge. This principle is summarized by the saying, coined by John Wheeler, "black holes have no hair." 

                   Objects in a gravitational field experience a slowing down of time, called time dilation. This phenomenon has been verified experimentally in the Scout rocket experiment of 1976 [2], and is, for example, taken into account in the GPS system. Near the event horizon, the time dilation increases rapidly. From the point of view of an external observer, it takes an infinite amount of time for an object to approach the event horizon, at which point the light coming from it is infinitely red-shifted. To the distant observer, the object, falling slower and slower, approaches but never reaches the event horizon. The object itself might not even notice the point at which it crosses the event horizon, and will do so in a finite amount of proper time. 

                    At the center of the black hole, well inside the event horizon, general relativity predicts a singularity, a place where the curvature of spacetime becomes infinite and gravitational forces become infinitely strong. Spacetime inside the event horizon is peculiar in that the singularity is in every observer's future, so all particles within the event horizon move inexorably towards it. This means that there is a conceptual inaccuracy in the nonrelativistic concept of a black hole as originally proposed by John Michell in 1783. 

                   It is expected that future refinements or generalizations of general relativity (in particular quantum gravity) will change what is thought about the nature of black hole interiors. Most theorists interpret the mathematical singularity of the equations as indicating that the current theory is not complete, and that new phenomena must come into play as one approaches the singularity. The question may be largely academic, as the cosmic censorship hypothesis asserts that there are no naked singularities in general relativity: Every singularity is hidden behind an event horizon and cannot be probed. 

                  Another school of thought holds that no singularity occurs, because of a bubble-like local inflation in the interior of the collapsing star. Radii stop converging as they approach the event horizon, are parallel at the horizon, and begin diverging in the interior. The solution resembles a wormhole (from the exterior to the interior) in a neighborhood of the horizon, with the horizon as the neck.In Michell's theory, the escape velocity equals the speed of light, but it would still, for example, be theoretically possible to hoist an object out of a black hole using a rope. General relativity eliminates such loopholes, because once an object is inside the event horizon, its time-line contains an end-point to time itself, and no possible world-lines come back out through the event horizon. 



                            Entering a black hole
                      The effects of a black hole's gravity as decribed by the Theory of Relativity cause a number of peculiar effects. An object approaching simple Schwarzschild-type (non-rotating) black hole's center will appear to distant observers as having an increasingly slow descent as the object approaches the event horizon. This is because a photon takes an increasingly long time to escape from the pull of the black hole to allow the distant observer to gain information on the object's fate.

                    From the object's frame of reference, it will cross the event horizon and reach the singularity, or center of the black hole, all within a finite amount of time. Once the object crosses over the event horizon, light will no longer escape the black hole, and the object can no longer be observed outside of the black hole. As the object continues to approach the singularity, it will elongate, and the parts closest to the singularity will begin to red shift, until they finally become invisible. Nearing the singularity, the gradient of the gravitational field from head to foot will become considerable, will stretch and tear because of tidal forces: the parts closest to the singularity feel disproportionatly stronger gravitational force than those parts farther away. This process is known as spaghettification.

                      Entropy and Hawking Radiation 

                 In 1971, Stephen Hawking showed that the total area of the event horizons of any collection of classical black holes can never decrease. This sounded remarkably similar to the Second Law of Thermodynamics, with area playing the role of entropy. Classically, one could violate the second law of thermodynamics by material entering a black hole disappearing from our universe and resulting in a decrease of the total entropy of the universe. 

               Therefore, Jacob Bekenstein proposed that a black hole should have an entropy and that it should be proportional to its horizon area. Since black holes do not classically emit radiation, the thermodynamic viewpoint was simply an analogy. However, in 1974, Hawking applied quantum field theory to the curved spacetime around the event horizon and discovered that black holes can emit thermal radiation, known as Hawking radiation. 

               Using the first law of black hole mechanics, it follows that the entropy of a black hole is one quarter of the area of the horizon. This is a universal result and can be extended to apply to cosmological horizons such as in de Sitter spacetime. It was later suggested that black holes are maximum-entropy objects, meaning that the maximum entropy of a region of space is the entropy of the largest black hole that can fit into it. This led to the holographic principle.
               Hawking radiation originates just outside the event horizon and, so far as it is understood, does not carry information from its interior since it is thermal. However, this means that black holes are not completely black: the effect implies that the mass of a black hole slowly evaporates with time. Although these effects are negligible for astronomical black holes, they are significant for hypothetical very small black holes where quantum-mechanical effects dominate. Indeed, small black holes are predicted to undergo runaway evaporation and eventually vanish in a burst of radiation. Hence, every black hole that cannot consume new mass has a finite life that is directly related to its mass. 

               On 21 July 2004 Stephen Hawking presented a new argument that black holes do eventually emit information about what they swallow, reversing his previous position on information loss. He proposed that quantum perturbations of the event horizon could allow information to escape from a black hole, where it can influence subsequent Hawking radiation [5]. The theory has not yet been reviewed by the scientific community and if it is accepted it is likely to resolve the black hole information paradox. In the meantime, the announcement has attracted a lot of attention in the media.


Reality of Black Holes

                  General relativity (as well as most other metric theories of gravity) not only says that black holes can exist, but in fact predicts that they will be formed in nature whenever a sufficient amount of mass gets packed in a given region of space, through a process called gravitational collapse. As the mass inside that region increases, its gravity becomes stronger - or, in the language of relativity, the space around it becomes increasingly deformed. When the escape velocity at a certain distance from the center reaches the speed of light, an event horizon is formed within which matter must inevitably collapse onto a single point, forming a singularity. 
                      A quantitative analysis of this idea led to the prediction that a star remaining about three times the mass of the Sun at the end of its evolution (usually as a neutron star), will almost inevitably shrink to the critical size needed to undergo a gravitational collapse. Once it starts, the collapse cannot be stopped by any physical force, and a black hole is created. 

                     Stellar collapse will generate black holes containing at least three solar masses. Black holes smaller than this limit can only be created if their matter is subjected to sufficient pressure from some source other than self-gravitation. The enormous pressures needed for this are thought to have existed in the very early stages of the universe, possibly creating primordial black holes which could have masses smaller than that of the Sun.

                    Supermassive black holes containing millions to billions of solar masses could also form wherever a large number of stars are packed in a relatively small region of space, or by large amounts of mass falling into a "seed" black hole, or by repeated fusion of smaller black holes. The necessary conditions are believed to exist in the centers of some (if not most) galaxies, including our own Milky Way . 


                   Theory says that we cannot detect black holes by light that is emitted or reflected by the matter inside them. However, those objects can be inductively detected from observation of phenomena near them, such as gravitational lensing and stars that appear to be in orbit around space where there is no visible matter.


                        The most conspicuous effects are believed to come from matter falling into a black hole, which (like water flowing into a drain) is predicted to collect into an extremely hot and fast-spinning accretion disk around the object before being swallowed by it. Friction between adjacent zones of the disk causes it to become extremely hot and emit large amounts of X-rays. This heating is extremely efficient and can convert about 50% of the mass energy of an object into radiation, as opposed to nuclear fusion which can only convert a few percent of the mass to energy. Other predicted effects are narrow jets of particles at relativistic speeds squirting off along the disk's axis. 
                        However, accretion disks, jets, and orbiting objects are found not only around black holes, but also around other objects such as neutron stars; and the dynamics of bodies near these non-black hole attractors is largely similar to the dynamics of bodies around black holes, and is currently a very complex and active field of research involving magnetic fields and plasma physics. 

                      Hence, for the most part, observations of accretion disks and orbital motions merely indicate that there is a compact object of a certain mass, and says very little about the nature of that object. The identification of an object as a black hole requires the further assumption that no other object (or bound system of objects) could be so massive and compact. Most astrophysicists accept that this is the case, since according to general relativity, any concentration of matter of sufficient density must necessarily collapse into a black hole.
                    One important observable difference between black holes and other compact massive objects is that any infalling matter will eventually collide with the latter, at relativistic speeds, leading to irregular intense flares of X-rays and other hard radiation. Thus the lack of such flare-ups around a compact concentration of mass is taken as evidence that the object is a black hole, with no surface onto which matter can be suddenly dumped.

                     Have we found them?
                   There is now a great deal of indirect astronomical observational evidence for black holes in two mass ranges:
  • Stellar mass black holes with masses of a typical star (4­15 times the mass of our Sun)
  • supermassive black holes with masses perhaps 1% that of a typical galaxy
Additionally, there is some evidence for intermediate-mass black holes (IMBHs), those with masses of a few thousand times that of the Sun. These black holes may be responsible for the formation of supermassive black holes. 

                        Candidates for stellar-mass black holes were identified mainly by the presence of accretion disks of the right size and speed, without the irregular flare-ups that are expected from disks around other compact objects. Stellar-mass black holes may be involved in gamma ray bursts (GRBs), although observations of GRBs in association with supernovae or other objects that are not black holes have reduced the possibility of a link. 

                     Candidates for more massive black holes were first provided by the active galactic nuclei and quasars, discovered by radioastronomers in the 1960s. The efficient conversion of mass into energy by friction in the accretion disk of a black hole seems to be the only explanation for the copious amounts of energy generated by such objects. Indeed the introduction of this theory in the 1970s removed a major objection to the belief that quasars were distant galaxies - namely, that no physical mechanism could generate that much energy. 

                     From observations in the 1980s of motions of stars around the galactic center, it is now believed that such supermassive black holes exist in the center of most galaxies, including our own Milky Way. Sagittarius A is now agreed to be the most plausible candidate for the location of a supermassive black hole at the center of the Milky Way galaxy.

                    The current picture is that all galaxies may have a supermassive black hole in their center, and that this black hole swallows gas and dust in the middle of the galaxies generating huge amounts of radiation - until all the nearby mass has been swallowed and the process shuts off. 

                   This explains why there are no nearby quasars. Though the details are still not clear, it seems that the growth of the black hole is intimately related to the growth of the spheroidal component - an elliptical galaxy, or the bulge of a spiral galaxy - in which it lives. Interestingly, there is no evidence for massive black holes in the center of globular clusters, suggesting that these are fundamentally different from galaxies.

                   The formation of micro black holes on Earth in particle accelerators have been tentatively reported, but not yet confirmed. So far there are no observed candidates for primordial black holes. 
                     .... and here is a funny one....

"by environment clean generations"

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