Black Holes
The modern theoretical prediction of black holes came as a spectacular consequence of German-American physicist Albert Einstein's general theory of relativity. Einstein's theory predicted that massive stars ultimately collapse into black holes with gravitational fields so intense that not even light can escape. In 1969, American physicist John A. Wheeler popularized the term black hole. During the later half of the twentieth century the discovery of, and physics associated with, black holes became one of the preeminent quests of modern astronomy.
In the 1930s Indian-born American astrophysicist Subrahmanyan Chandrasekhar mathematically proved that black holes were the remains of massive stars and fully articulated the evolution of stars into supernova, white dwarfs, neutron stars or black holes. Before the intervention of WWII, American physicist J. Robert Oppenheimer, who ultimately supervised project Trinity (the making of the first atomic bombs), made detailed calculations reconciling Chandrasekhar's predictions with general relativity theory. In the late 1960s English mathematician and physicist Stephen Hawking along with English mathematician Roger Penrose drew from both quantum and relativity theory to show that within a black hole there must exist a singularity (a geometric point without space) of infinite density. Penrose advanced a scientific and philosophic concept known as the Law of Cosmic Censorship, an assertion that regions around the singularity (i.e., black holes) are regions of space cut off from direct human observation. In 1963 New Zealand astrophysicist, Roy Patrick Kerr (1934-) used Einstein's field equations to predict the existence of rotating black holes.
Essentially, throughout the life of a star a tug-of-war exists between the compressing force of the star's own gravity and the expanding pressures generated by nuclear reactions at its core. After cycles of swelling and contraction associated with the burning of progressively heavier nuclear fuels, the star eventually runs out of useable nuclear fuel. The spent star then contracts under the pull of it own gravity. Modern understandings of astrophysics allow three possible fates for such a collapsing star.
The particular fate for any star determined by the mass of the star left after blowing away its outer layers during its paroxysmal death spasms. A star less than 1.44 times the mass of the Sun (termed the Chandrasekhar limit) collapses until the pressure in the increasing compacted electron clouds exerts enough pressure to balance the collapsing gravitational force. Such stars become white dwarfs contracted to the a radius of only a few thousand miles (roughly the size of a planet). Because most stars in the visible universe are low-mass stars, this is the fate of most stars. If the star retains between 1.4 and roughly three times the mass of the Sun, the pressure of the electron clouds is insufficient to stop the gravitational collapse. In such stars contraction continues to produce a neutron star. Only a few miles in radius, within a neutron star the nuclear forces and the repulsion of the compressed atomic nuclei balance the crushing force of gravity. With more massive stars, however, there is no known force in the universe that can withstand the gravitational collapse. Such extraordinary stars will continue their collapse to form a singularity--a star collapsed to a point of infinite density. As such a star collapses its gravitational field warps space-time so intensely that not even light can escape and a black hole forms around the singularity.
The event horizon is the boundary of a black hole. The size of the event horizon is termed the Schwarzschild radius (named after the German astronomer Karl Schwarzschild. Inside the event horizon the gravitational attraction of the singularity is so strong that the required escape velocity is greater than the speed of light. As a consequence, because no object can exceed the speed of light, not even light itself can escape from the region of space within the event horizon.
Because no information generated within the black hole can escape to us, the event horizon is an important observational boundary. Essentially all we can know with any certainty regarding the processes of the singularity are the external gravitational effects exerted by its tremendous mass. Astronomers consider systems good candidates to contain a black hole wherever a star orbits around an unseen companion and there is strong electromagnetic radiation from an unidentified source near the center of rotation. In binary star systems (two stars orbiting each other), for example, although the black hole can not be directly observed its gravitational strength by the orbit its observable companion star. In the last two decades of the twentieth century, astronomers have found a half-dozen or so binary star systems where black holes may exist.
In 1994 the Hubble Space Telescope provided arguably conclusive evidence for the existence of a supermassive black hole located at the center of the M87 galaxy. Similar evidence indicates that a black hole also lies at the center of our Milky Way galaxy.
An accretion disk forms as matter accelerates toward the event horizon of the black hole. As the matter in the accretion disk spirals toward the black hole it is heated to very high temperatures and emits strong highly energetic electromagnetic radiation (e.g., x rays and gamma rays). Some astronomers assert that such a mechanism, working on a galactic scale may account for the phenomena associated with quasars.
Although the events that occur inside a black hole remain an enigma, some physicist have attempted to speculate about the nature of time dilations and contractions near the event horizon. Hawking radiation, involving massless virtual particles and particle-antiparticle pairs, for example, may explain mass and radiation leakage from blackholes. Astronomers also suggest that in the early Universe, black holes could have formed as a consequence of the collection and collapse of large volumes of interstellar gas.
The existence of black-holes opens the possibility for the existence of another stunning concept known as worm-holes (also termed Einstein-Rosen bridges). A wormhole is a theoretical opening in space-time that connects two black holes.
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