Gravity and Gravitation

German physicist Albert Einstein (1879–1955) realized that Newton's theory of gravity had problems. He knew, for example, that Mercury's orbit showed unexplained deviations from that predicted by Newton's laws. However, he was worried about a much more serious problem. As the force between two objects depends on the distance between them, if one object moves closer, the other object will feel a change in the gravitational force. According to Newton, this change would be immediate, or instantaneous, even if the objects were millions of miles apart. Einstein saw this as a serious flaw in Newtonian gravity. Einstein assumed that nothing could travel instantaneously, not even a change in force. Specifically, nothing can travel faster than light in a vacuum, which has a speed of approximately 186,000 mi/s (300,000 km/s). In order to fix this problem, Einstein had not only to revise Newtonian gravity, but to change the way we think about space, time, and the structure of the Universe. He stated this new way of thinking mathematically in his general theory of relativity.

Einstein said that a mass bends space, like a heavy ball making a dent on a rubber sheet. Further, Einstein contended that space and time are intimately related to each other, and that we do not live in three spatial dimensions and time (all four quite independent of each other), but rather in a four-dimensional space-time continuum, a seamless blending of the four. It is thus not "space," naively conceived, but space-time that warps in reaction to a mass. This, in turn, explains why objects attract each other. Consider the Sun sitting in space-time, imagined as a ball sitting on a rubber sheet. It curves the spacetime around it into a bowl shape. The planets orbit around the Sun because they are rolling across through this distorted space-time, which curves their motions like those of a ball rolling around inside a shallow bowl. (These images are intended as analogies, not as precise explanations.) Gravity, from this point of view, is the way objects affect the motions of other objects by affecting the shape of space-time.

Einstein's general relativity makes predictions that Newton's theory of gravitation does not. Since particles of light (photons) have no mass, Newtonian theory predicts that they will not be affected by gravity. However, if gravity is due to the curvature of space-time, then light should be affected in the same way as matter. This proposition was tested as follows: During the day, the Sun is too bright to see any stars. However, during a total solar eclipse the Sun's disk is blocked by the Moon, and it is possible to see stars that appear in the sky near to the Sun. During the total solar eclipse of 1919, astronomers measured the positions of several stars that were close to the Sun in the sky. It was determined that the measured positions were altered as predicted by general relativity; the Sun's gravity bent the starlight so that the stars appeared to shift their locations when they were near the Sun in the sky. The detection of the bending of starlight by the Sun was one of the great early experimental verifications of general relativity; many others have been conducted since.

Another surprising prediction made by general relativity is that waves can travel in gravitational forces just as waves travel through air or other media. These gravitational waves are formed when masses move back and forth in space-time, much as sound waves are created by the oscillations of a speaker cone. In 1974, two stars were discovered orbiting around each other, and scientists found out that the stars were losing energy at the exact rate required to generate the predicted gravity waves; that is, they were steadily radiating energy away in the form gravitational waves. So far, gravitational waves have not been detected directly, but new detectors will be completed in the U.S., Japan, and Europe in 2003 and it is expected that these devices will detect gravitational waves produced by violent cosmic events such as supernovae. Scientists have already verified that changes in gravitation do propagate at the speed of light, as predicted by Einstein's theory but not by Newton's.

Of all the predictions of general relativity, the strangest is the existence of black holes. When a very massive star runs out of fuel, the gravitational self-attraction of the star makes it shrink. If the star is massive enough, it will collapse it to a point having finite mass but infinite density. Space-time will be so distorted in the vicinity of this "singularity," as it is termed, that not even light will be able to escape; hence the term "black hole." Astronomers have been searching for objects in the sky that might be black holes, but since they do not give off light directly, they must be detected indirectly. When material falls into a black hole, it must heat up so much that it glows in x rays. Astronomers look for strong x-ray sources in the sky because these sources may be likely candidates to be black holes. Numerous black holes have been detected by these means, and it is now believed that many or most galaxies contain a supermassive black hole at their center, having a mass millions or billions of times greater than that of the Sun.

The greatest remaining challenge for gravity theory is unification with quantum mechanics. Quantum theory describes the physics of phenomena at the atomic and subatomic scale, but does not account for gravitation. General relativity, which employs continuous variables, does not describe the behavior of objects at the quantum scale. Physicists therefore seek a theory of "quantum gravity," a unified set of equations that will describe the whole range of known phenomena.