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This web page location: GravitationEarly Ideas About Gravitationelectrical force, geodesics, general theory of relativity, time dimension, theory of special relativityIn 1915 Einstein formulated a new theory of gravitation that reconciled the force of gravitation with the requirements of his theory of special relativity. He proposed that gravitational effects move at the speed of c. He called this theory general relativity to distinguish it from special relativity, which only holds when there is no force of gravitation. General relativity produces predictions very close to those of Newton's theory in most familiar situations, such as the moon orbiting the earth. Einstein's theory differed from Newton's theory, however, in that it described gravitation as a curvature of space and time. In Einstein's general theory of relativity, he proposed that space and time may be united into a single, fourdimensional geometry consisting of 3 space dimensions and 1 time dimension. In this geometry, called spacetime, the motions of particles from point to point as time progresses are represented by curves called world lines. If there is no gravity acting, the most natural lines in this geometry are straight lines, and they represent particles that are moving always in the same direction with the same speed—that is, particles that have no force acting on them. If a particle is acted on by a force, then its world line will not be straight. Einstein also proposed that the effect of gravitation should not be represented as the deviation of a world line from straightness, as it would be for an electrical force. If gravitation is present, it should not be considered a force. Rather, gravitation changes the most natural world lines and thereby curves the geometry of spacetime. In a curved geometry, such as the twodimensional surface of the earth, there are no straight lines. Instead, there are special curves called geodesics, an example of which are great circles around the earth. These special curves are at each point as straight as possible, and they are the most natural lines in a curved geometry. The effect of gravitation would be to influence the geodesics in spacetime. Near sources of gravitation the space is strongly curved and the geodesics behave less and less like those in flat, uncurved spacetime. In the solar system, for example, the effect of the sun and the earth is to cause the moon to move on a geodesic that winds around the geodesic of the earth 12 times a year. Article key phrases: 

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