Gravitation

Einstein's general relativity theory predicts special gravitational conditions. The Big Bang theory, which describes the origin and early expansion of the universe, is one conclusion based on Einstein's theory that has been verified in several independent ways.

Another conclusion suggested by general relativity, as well as other relativistic theories of gravitation, is that gravitational effects move in waves. Astronomers have observed a loss of energy in a pair of neutron stars (stars composed of densely packed neutrons) that are orbiting each other. The astronomers theorize that energy-carrying gravitational waves are radiating from the pair, depleting the stars of their energy. Very violent astrophysical events, such as the explosion of stars or the collision of neutron stars, can produce gravitational waves strong enough that they may eventually be directly detectable with extremely precise instruments. Astrophysicists are designing such instruments with the hope that they will be able to detect gravitational waves by the beginning of the 21st century.

Another gravitational effect predicted by general relativity is the existence of black holes. The idea of a star with a gravitational force so strong that light cannot escape from its surface can be traced to Newtonian theory. Einstein modified this idea in his general theory of relativity. Because light cannot escape from a black hole, for any object—a particle, spacecraft, or wave—to escape, it would have to move past light. But light moves outward at the speed c. According to relativity, c is the highest attainable speed, so nothing can pass it. The black holes that Einstein envisioned, then, allow no escape whatsoever. An extension of this argument shows that when gravitation is this strong, nothing can even stay in the same place, but must move inward. Even the surface of a star must move inward, and must continue the collapse that created the strong gravitational force. What remains then is not a star, but a region of space from which emerges a tremendous gravitational force.

Einstein's theory of gravitation revolutionized 20th-century physics. Another important revolution that took place was quantum theory. Quantum theory states that physical interactions, or the exchange of energy, cannot be made arbitrarily small. There is a minimal interaction that comes in a packet called the quantum of an interaction. For electromagnetism the quantum is called the photon. Like the other interactions, gravitation also must be quantized. Physicists call a quantum of gravitational energy a graviton. In principle, gravitational waves arriving at the earth would consist of gravitons. In practice, gravitational waves would consist of apparently continuous streams of gravitons, and individual gravitons could not be detected.

Einstein's theory did not include quantum effects. For most of the 20th century, theoretical physicists have been unsuccessful in their attempts to formulate a theory that resembles Einstein's theory but also includes gravitons. Despite the lack of a complete quantum theory, it is possible to make some partial predictions about quantized gravitation. In the 1970s, British physicist Stephen Hawking showed that quantum mechanical processes in the strong gravitational pull just outside of black holes would create particles and quanta that move away from the black hole, thereby robbing it of energy.

An important trend in modern theoretical physics is to find a theory of everything (TOE), in which all four of the fundamental forces are seen to be really different aspects of the same single universal force. Physicists already have unified electromagnetism and the weak nuclear force and have made progress in joining these two forces with the strong nuclear force (see Grand Unification Theories). However, relativistic gravitation, with its geometric and mathematically complex character, poses the most difficult challenge. Einstein spent most of his later years searching for an all-encompassing theory by trying to make electromagnetism geometrical like gravitation. The modern approach is to try to make gravitation fit the pattern of the other fundamental forces. Much of this work involves looking for mathematical patterns. A TOE is difficult to test using experiments because the effects of a TOE would be important only in the rarest circumstances.