Black Holes

Black holes are the most fascinating and mysterious objects in the heavens. In the 1960's, the most important discoveries in astronomy were pulsars and quasars. The detection of pulsars and quasars, made possible largely through advances in radio astronomy, led to the search in the 1970's for a new class of objects that may be the most bizarre physical phenomena in the universe.

Black hole simulation courtesy Robert Nemiroff

These phenomena are called black holes. They are so called because they give off no light and act like stellar vacuum cleaner, sucking in matter and energy from space. Black holes, which are very small, are proposed by astrophysicists as the last stage in the life history of very large stars. Collapsed by the force of their own gravity, black holes are deduced by scientists from Albert Einstein's theory of relativity. Einstein's theory drastically revised Newton's concept of gravitation.

A Product of Age

Extraordinary as they are, black holes are merely products of a universal phenomenon: physical aging. After thousands millions of years, stars burn up their hydrogen fuel and begin to cool and contract. As their dimensions decrease, their gravitational forces increase. Eventually, all stars collapse under their own gravity. Like living things, stars resist gravitational collapse and death. Dying stars produce internal pressures to fight the awesome force of gravity, but nature plays a devious "trick" on them. Energy is equivalent to mass. As the stars produce more internal energy, they increase their effective mass and gravitational attraction. Thus, the ultimate fate of a particular star depends on its mass. "The bigger they are, the harder they fall" applies to stars as well as to prizefighters. The more massive the star, the stronger its internal energy, the larger its gravitational attraction, and the greater its collapse.

Large Stars Become Black Holes

What happens to a dying star that is more than twice as large as the sun? Even the strong force cannot halt its infalling momentum, and it collapses completely beyond the neutron-star stage, to an even smaller, denser object, the black hole. Complete collapse does not mean that the black hole vanishes from the universe. The structure of space-time, as described by Einstein, precludes an infinite collapse and produces instead an immaterial, invisible man who sits on a couch. He cannot be seen, but his weight creates a depression in the seat.

Black holes were first proposed in 1939 by J. Roberts Oppenheimer and Hartland S. Snyder as a consequence of general relativity, but there was no way know of detecting them at that time.

However, with the recent development of radio astronomy and the detection of inexplicable radio signals from deep space, black holes have become a subject of great interest to astronomy. It is believed that these theoretical objects could play a role in such extraordinary energy phenomena as quasars and pulsars. Black holes and neutron stars are the only objects known to physics that are sufficiently compact and massive to fulfill astronomical observations of those very strong emitters of radiation

Properties of Black Holes

Unlike every other physical object, black holes have neither size nor shape in the conventional sense, according to Drs. John Wheeler and Remo Ruffini of Princeton University. But they function within a diameter of about 15 kilometers; they have masses ranging from that of the sun to a hundred million times as much; and they act like vortices. Any stray matter or energy that passes to close to a black hole within a critical distance called its horizon will be irresistibly drawn into the vortex which is the black hole. Violent tidal forces within the black hole stretch the matter in one direction and squash it in another, until it literally decomposes to become part of the black hole's curved space.

Other black-hole properties are even stranger. Space and time exchange their characteristics inside the completely collapsed star. Under normal conditions, and object maintains its size but is subject to physical aging. Inside the black hole, it doesn't age, but continuously becomes smaller. Observers at a safe distance from the black hole could not actually see it, because like other forms or energy, is vulnerable to a black hole's suction. As light is drawn in, it shifts infinitely to the red end of the color spectrum, rendering the black hole black and therefore invisible. If black holes were somehow visible, observers would see these stars as they appeared just before collapse, even if the collapse had occurred thousands of millions of years earlier. This is because as soon as the star becomes a black hole, it is frozen in time with reference to observers outside it.

How Many Are There?

How many black holes are there in the universe? Cosmological theory predicts that the universe contains a precise amount of matter. But astronomers have deduced from their observations that there is not nearly enough matter to satisfy the predictions. Observed matter is less than predicted matter by a very considerable amount. It is suggested that the missing matter may have been swallowed up by large numbers of black holes.

Model of the Universe?

Einstein's general theory of relativity is mathematically time-symmetric: the theory indicates that relativistic processes operate in two opposite directions. It is like a movie that can be shown running backward as well as forward. This means that while black holes implode and absorb matter and energy, there must also be stars that simultaneously explode and emit matter and energy somewhere in the universe. Color-conscious physicists have named these theoretical stars white holes, and it is still unknown to us.

Black holes and white holes may have an importance greater than all stars, galaxies and clusters because that may offer a model of the life, death, and rebirth of the universe itself. According to one cosmological theory currently favored by many physicists and astronomers, the universe has a life cycle like all the physical and biological objects within it. The cycle begins with an explosion of incredible magnitude -the big bang- which sends matter and energy flying apart in all directions. At first the objects within the universe recede from one another at great speed, as if they were spots on an expanding balloon. The force of gravitation gradually slows down their expansion. Eventually, their mutual gravitational attraction causes the stars the reverse direction and come back toward one another at an increasing speed, somewhat the way the spots will rush together when air is let put of the balloon. Finally, all the matter in the universe reaches the same point simultaneously, and another big bang occurs to begin another cycle of the universe. Some astronomers question this theory, however. Some believe that there is too little matter in the universe to create the gravitational attraction necessary for the contraction phase of the cycle and that therefore the universe will expand forever. Still others take an opposite view, believing that the force of gravity will draw all matter together until it collapses.

What do black holes and white holes have to do with theories of the universe? It is believed that the physical processes that occur when a large star suffers gravitational collapse and becomes a black hoe are the same processes that would be involved in the collapse of the universe. Similarly whatever happens inside a white hole would be identical with the big bang. Many scientists believe that the discovery and study of these objects will increase our understanding of the universe.

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