BLACK HOLE A black hole is a theorized celestial body whose surface gravity is so strong that nothing can escape from within it. Although black holes have been of intense scientific interest only in the later 20th century, the concept goes back to the French mathematician Pierre Simon de LAPLACE. In a 1798 treatise Laplace agreed with Newton that light is composed of particles. He reasoned that if enough mass were added to a star like the Sun, the gravitational force of the star eventually would become so great that its escape velocity would equal the velocity of light. At that point, light particles would not be able to leave the surface of the star, and it would "blink out" and become an invisible black star. More than a century later, Einstein, developing his special theory of relativity, argued that nothing can move faster than light. This means that Laplace's black stars are also black holes, because if light cannot escape, all other matter must be trapped as well. The surface of a black hole thus acts like a one-way membrane: material may fall into a black hole, but no information or energy can come out of a black hole. The detailed properties of black holes are studied by using Einstein's general theory of relativity and gravitation. In 1917 a German astrophysicist, Karl SCHWARZSCHILD, used Einstein's theory to calculate that if a star of any given mass were compressed to a size smaller than a critical radius, now called the SCHWARZSCHILD RADIUS in his honor, the density would become so high and the gravitational force so great that the star would become a black hole. The spherical surface about the star at the Schwarzschild radius is called the "event horizon" and marks the outer surface of the black hole at which the escape velocity just equals the velocity of light. Further calculations revealed to Schwarzschild that the critical radius of a black hole is proportional to its mass. For the Sun, this radius is about 3 km (2 mi). To find the Schwarzschild radius of any other object, one need only divide the mass of that object by the Sun's mass and then multiply by 3 km. If a star more massive than the Sun undergoes gravitational collapse at the end of its evolution, it will form either a WHITE DWARF, a NEUTRON STAR, or a black hole, depending primarily on its mass (see STELLAR EVOLUTION). If this collapse process is nonspherical, perhaps because the star is rotating and flattened at the poles, then theoretical GRAVITATIONAL WAVES could be given off just before the black hole is formed. Attempts are being made to detect such waves. The only other way to identify a black hole would be through its interactions with other matter. For example, if the black hole is formed in a BINARY STAR system, gas from the normal star may later flow toward the black hole. As the gas falls toward the hole, its molecules increase in speed and approach the speed of light. The molecules begin to bunch up and collide, heating them to temperatures at which X rays are emitted. Such X rays have been detected in eclipsing binary star systems in which the X-ray source is not visible. The binary star Cygnus X1 may include such a companion; other candidates are an X-ray source in the nearby Large Magellanic Cloud galaxy, and X-ray novas in the constellations Monoceros and Vulpecula. No black hole has yet been positively identified. Black holes, if they exist, could come in an extreme range of sizes. The English physicist Stephen HAWKING has speculated that tiny black holes with masses no larger than that of a large mountain are possible. Such black holes, in the size range of elementary particles, would have been formed only under the extreme conditions that COSMOLOGY theories indicate existed in the very first moments of the universe (see INFLATIONARY THEORY). On the other hand, gigantic black holes may lie at the center of galaxies. Some astronomers suggest that such black holes may be linked to the differences that exist between galaxies, ranging from normal ones such as our own to the highly active galaxies called radio galaxies and quasars (see EXTRAGALACTIC SYSTEMS).
Various speculations have been made as to the permanence or instability of black holes themselves. Laws of physics suggest that black holes would emit particles and shrink with time. According to Hawking, their temperatures would rise as they shrank, and they might fully evaporate with an enormous burst of energy. Other theorists suggest that certain factors would halt this process along the way. Some theorists have even suggested the existence of so-called white holes in which matter would flow through a completely collapsed black hole--a singularity, or body of zero radius and infinite density--into another universe. This notion, however, is extremely speculative. A popular misconception is that black holes act like cosmic vacuum cleaners, sucking up everything within reach. In fact the gravitational attraction of a black hole would be no stronger than that of a normal star of the same mass. For instance, if the Sun could collapse to form a black hole (it cannot, however, and will instead end its life as a white dwarf), the Earth would continue to orbit just as it does now. Bibliography: Chaisson, Eric, Relatively Speaking (1988); Greenstein, George, Frozen Star (1984); Hawking, Stephen, A Brief History of Time (1988); McClintock, Jeffrey, "Do Black Holes Exist?" Sky and Telescope, January 1988; Novikov, Igor, Black Holes and the Universe, translated by Vitaly Kisin (1991); Parker, Barry, "In and Around Black Holes," Astronomy, October 1986; Price, R. H., and Thorne, K. S., "The Membrane Paradigm for Black Holes," Scientific American, April 1988; Sullivan, Walter, Black Holes: The Edge of Space, the End of Time (1980).