General Relativity

The German astronomer Karl Schwarzschild (1873–1916) first used general relativity to predict the existence of black holes, which are stars that are so dense that not even light can escape from their gravitational field. Because the gravitational field around a black hole is so strong, we must use general relativity to understand the properties of black holes; indeed, most of what we know about black holes comes from theoretical studies based on general relativity. Ordinarily we think of black holes as having been formed from the collapse of a massive star, but U.S. physicist Stephen Hawking (1942–) has combined general relativity with quantum mechanics to predict the existence of primordial quantum black holes. These primordial black holes (if they exist) were formed by the extreme turbulence of the big bang during the formation of the Universe. Hawking predicts that over sufficiently long time these small, quantum black holes—and larger black holes, too—can evaporate, that is, lose their mass to surrounding space despite their intense gravity, like drops of water evaporating into dry air. This view has replaced the earlier, too-simple belief that nothing can escape from a black hole.

General relativity also has important implications for cosmology, the study of the structure of the Universe. The equations of general relativity state not only that the Universe is finite but that it may be contracting or expanding. Einstein noticed this result of his theory, but assumed that the Universe must be stable in size, neither contracting nor expanding, and therefore added to his equations a numerical term called the "cosmological constant." This constant was basically a fudge factor that Einstein used to adjust his equations so that they predicted a static universe. Later, U.S. astronomer Edwin Hubble (1889–1953), after whom the Hubble Space Telescope is named, discovered that the Universe is expanding. Einstein visited Hubble, examined his data, and admitted that Hubble was right. Einstein later called his cosmological constant the biggest blunder of his life; however, modern cosmologists have found that Einstein may have been right after all about the need for a cosmological constant in the equations of general relativity. Recent observations show that the Universe's rate of expansion is probably accelerating.This means that some force resembling negative gravity—a "force" that originates in matter but that pushes other matter away rather than attracting it—may exist. If it does, a nonzero value for Einstein's cosmological constant may be required to describe the structure of the Universe. Astronomers are debating and researching this question intensively.

Albert Einstein's general theory of relativity fundamentally changed the way we understand gravity and the Universe in general. So far, it has passed all experimental tests. This, however, does not mean that Newton's law of gravity is wrong. Newton's law is an approximation of general relativity; that is, in the approximation of small gravitational fields, general relativity reduces to Newton's law of gravity. General relativity, too, is only an approximate description of certain aspects of Nature: this is known because general relativity does not agree with the predictions of quantum mechanics (the other great organizing idea of modern physics) in describing extremely small phenomena. Quantum mechanics, similarly, makes erroneous predictions at the cosmic scale. Physicists are striving to discover an even more general or unified theory that will yield both general relativity and quantum mechanics as special cases.