GRAVITATION Gravitation, or gravity, is a force that attracts all objects in the universe; it is the most familiar of the four FUNDAMENTAL INTERACTIONS of matter. Gravitation has several basic characteristics that distinguish it from the other fundamental interactions. First, it is universal, affecting all forms of matter and energy in essentially the same way, whereas all the other interactions directly affect only certain types of particles. The electromagnetic force, for example, affects only charged particles. Second, gravity is always attractive, since it interacts with mass-energy, which is always positive. In electromagnetism, on the other hand, charges can either attract or repel. Third, gravitation is a long-range interaction. Electromagnetism is also long-range, but the strong and weak nuclear forces generally operate only within a distance the size of an atomic nucleus. Fourth, gravity is the weakest of the four fundamental forces. It has a negligible effect on elementary particles. The electromagnetic attractive force between a proton and an electron is nearly 10 to the 40th power times greater than the gravitational force at the same separation. Because gravity is a long-range attractive force affecting all matter, however, it is the dominant force in the universe. HISTORICAL THEORIES Throughout history there have been many attempts to describe or explain gravitation. About 330 BC, Aristotle claimed that the four elements--earth, water, air, and fire--have their natural places, toward which they tend to travel. He argued that objects containing greater amounts of earth than others would fall toward the Earth faster and that their speed would increase as they neared their natural place. GALILEO GALILEI deduced (1604) that gravity imparts a definite acceleration, rather than a velocity, and that this acceleration is the same for all objects traveling in a vacuum. The universality of gravitational acceleration is known as the weak equivalence principle. Sir Isaac NEWTON made the most significant contribution to gravitational theory when he perceived (1606) that the orbit of the Moon depended on the same type of force that causes an apple to fall to Earth. This proposition required that the magnitude of the force decrease in inverse proportion to the square of the distance from the Earth's center. Newton combined the inverse square law with his three LAWS OF MOTION to formulate a theory of universal gravitation, which stated that there is a gravitational attraction between every pair of objects, inversely proportional to the square of the distance between them. Rene DESCARTES (1596-1650) had earlier proposed a nonquantitative theory of gravitation based on the inward pressure of vortices on planets (see PHYSICS, HISTORY OF), but Newton did not offer a cause for the attraction. In fact, he avoided even calling it an attraction, speaking instead of "bodies gravitating towards one another." This description was sufficient to deduce KEPLER'S LAWS of planetary motion, the oceanic tides (see TIDE), and the PRECESSION OF THE EQUINOXES. In 1846 it was used to predict and discover a new planet, Neptune. Thus Newton's theory of gravitation stands as one of the greatest advancements of scientific knowledge. Expressed mathematically, Newton's theory states that there is an attractive force F, given by F = Gm(1)m(2)/r squared, between two particles having masses m(1) and m(2) and separation distance r. G is the gravitational constant of proportionality, an unknown quantity that could not be determined by solar-system observations, which give only ratios of masses and thus the product of G and some mass. The value of G was first determined in 1798 by Henry Cavendish, who measured the force of gravitational attraction between two spheres of known mass. This experiment has come to be known as "weighing the Earth," because once G was determined, the Earth's mass, m(e), could be determined from the astronomically known value for Gm(e). The experiment has been repeated many times with increasing precision. The currently accepted value of G is 6.67259 times 10 to the minus 8 power cm cubed /g sec squared. MODERN THEORIES In 1905, Albert EINSTEIN developed his theory of special RELATIVITY, which modified Newton's theory of gravitation. Einstein sought to describe gravitation in a way that was independent of the motion of observers and of the coordinates chosen to label events. His work led to a geometrical theory that described gravity purely by the structure of the SPACE-TIME CONTINUUM. According to this geometrical theory, gravity affects all forms of matter and energy, all of which move in space-time. Thus Einstein's theory obeys the weak equivalence principle and gives the same gravitational acceleration for all freely falling objects. In addition to describing the effect of gravity on matter, Einstein described the effect of matter on gravity. This theory, which Einstein completed in 1915, is called general relativity. Although Einstein's theory is much different from Newton's, it predicts nearly the same effects in systems in which gravitational fields are weak and velocities are slow compared to the velocity of light. Planetary motion had provided a particularly accurate verification of Newton's theory, but Einstein's accounted for some phenomena in the solar system not considered by Newton. One such phenomenon was the perihelion precession of Mercury. In the 19th century, it was observed that the rate of precession differed by 43'' per century from what Newton's theory predicted; Einstein's theory predicted precisely such a difference in the precession rate. Another such phenomenon is the bending of light rays by the Sun's gravitational field, which Newton's theory did not predict at all. Einstein's prediction was confirmed by Arthur S. Eddington during a total eclipse in 1919, and later by others to 1% accuracy. Einstein also predicted the gravitational RED SHIFT, a change in the frequency of electromagnetic waves escaping from a strong gravitational field, which was confirmed by Robert Pound and Glen Rebka in 1960. In 1964, Irving Shapiro used general relativity to predict a time delay of signals passing near the Sun, an effect since confirmed. In addition, Einstein's theory of general relativity predicts several qualitatively new effects in other systems and is especially useful in dealing with COSMOLOGY. Relativity asserts that the universe must be either expanding or contracting. Einstein was not sufficiently bold to believe this prediction, however, so he modified his equations to allow for a static universe. In 1929, though, Edwin Hubble discovered that the universe is expanding. Whether gravity will eventually cause it to collapse is a subject of current investigation and debate. General relativity also predicts GRAVITATIONAL WAVES from masses in nonuniform motion, but these waves are so weak that they have not yet been definitely detected. Finally, Einstein's theory predicts GRAVITATIONAL COLLAPSE of sufficiently massive objects into BLACK HOLES. Today there is mounting evidence that several astronomical systems may contain black holes. Einstein's general relativity is not the only 20th-century theory of gravity, though it is perhaps the simplest and most elegant. All viable theories of gravity must, like Einstein's, be complete, self-consistent, and relativistic. They must also have the correct Newtonian limit, uphold the weak equivalence principle, and predict the same Einstein shift as measured by all ideal clocks at the same position. There is strong experimental evidence for accepting these criteria as fundamental, and it has been conjectured by L. I. Schiff that they can be satisfied only by geometrical, or metric, theories. The strongest rival theory has been the Brans-Dicke theory. Like general relativity, it is a geometrical theory that satisfies the fundamental criteria. Its field equations are different, however, and it claims that the geometry of space-time is affected not only by matter but also by an additional scalar field. Unlike Einstein's calculations, the Brans-Dicke theory cannot predict the perihelion shift of Mercury. Some recent theories attempt to explain gravitation non-geometrically, proposing instead that particles called gravitons are responsible. These so-called supersymmetry theories place gravitational phenomena within the realm of quantum physics. They are part of an attempt to show that the four fundamental interactions of nature are related, and that they were a single, united force at the birth of the universe (see GRAND UNIFICATION THEORIES; UNIFIED FIELD THEORY). Multidimensional analysis has also raised the issue of the constancy of the gravitational constant G. This idea was proposed earlier by British physicist Paul Dirac in his so-called "large numbers" hypothesis. Dirac noted that the ratio of the strength of the electromagnetic force to that of the gravitational force (approximately 10 to the 40th power) is roughly equivalent to the age of the universe in atomic terms. He wondered whether there might be a deep physical connection requiring this similarity and proposed that this would be the case if G slowly decreased as the age of the universe increased. If G were decreasing, however, gravitational time would change with respect to atomic time; thus far, experiments have not shown this to be happening. Don N. Page Bibliography: Bergmann, P. G., The Riddle of Gravitation (1968); Hawking, S. W., and Israel, W., Three Hundred Years of Gravitation (1987; repr. 1989); Mathews, P. M., et al., eds., Gravitation, Quantum Fields and Superstrings (1988); Misner, Charles, et al., Gravitation (1973); National Research Council, Gravitation, Cosmology, and Cosmic-Ray Physics (1986); Thorne, K. S., Gravitational Radiation (1989); Zee, Anthony An Old Man's Toy (1989).