The general theory of relativity, which Albert Einstein completed in 1915, is a theory of gravity and motion. His special theory of relativity challenged the classical ideas of space, time, and simultaneity, and the general theory went further to declare that the properties of space-time are neither homogeneous (the same at all locations) nor isotropic (the same in every direction regardless of orientation), and do not necessarily remain the same as time passes. Whereas special relativity deals with frames of reference in relative motion at constant velocity, general relativity also considers accelerated frames of reference and gravity. Measurements of space and time intervals depend not only on the relative motion between observers, but also on the strength of the local gravitational fields.
The central concept in general relativity is the equivalence principle, which states that the effects of gravity and acceleration are indistinguishable and equivalent. Using this principle, Einstein described gravity as a geometric effect rather than as a force acting at a distance. The presence of matter or energy curves space-time, redefining the way objects move in the vicinity. This contrasts with special relativity, which assumes space-time to be flat. General relativity predicts that the universe as a whole may have curvature. If this is the case, space, though of a higher dimension, would be analogous to the surface of Earth, which is finite in area, but because of its curvature has no boundaries. On a spherical surface such as Earth's, the shortest route between two points is not a straight line but a great circle or geodesic. The equator is one example of a geodesic. In curved space, geometry differs from that in flat space, and the curvature effects are interpreted as gravitational fields. Like the spherical surface, curved space is in general non-Euclidean, and for example, parallel lines may cross, and the sum of the angles in a triangle may be more than 180ยบ. By picturing space-time and matter as interacting with each other Einstein radically departed from previous theories that had viewed space and time as a fixed stage on which physical objects moved.
Cosmological effects are predicted by the theory. Prior to any observational evidence, Einstein believed the universe to be static, neither expanding nor contracting, and introduced an arbitrary element into his equations, the cosmological term, to force the model to be stable. After expansion of the universe was experimentally confirmed by Edwin Hubble in 1929, Einstein withdrew the extra term and called it "the greatest blunder of his life." Extrapolating backwards in time, the expansion suggests that the universe was much smaller in the past, and may have begun with a dense hot explosion, the big bang.
Gravitational systems such as binary stars are predicted to emit energy in the form of gravitational radiation. Such radiation would, like the ordinary effects of gravity, propagate at the speed of light, but would be far more feeble than electromagnetic radiation. Gravitational waves are thus exceedingly difficult to observe, and despite attempts at detection, convincing evidence of their interaction with an experimental apparatus has yet to be obtained.
Another astronomical prediction of general relativity is the possibility of black holes, collapsed stars with gravitational fields of such intensity, light cannot escape their surfaces. Stars that can no longer resist gravitational collapse with nuclear fusion become white dwarfs or neutron stars, and if sufficiently massive, the Pauli exclusion principle will be unable to prevent further compression, allowing the star to shrink to its Schwarzschild radius and become a black hole. Worm holes, or Einstein-Rosen bridges, are possible in general relativity, though they have not yet been observed. Worm holes are somewhat like tunnels connecting different regions of space, and if real, would act like shortcuts between points that might be separated by long distances in ordinary space.
The differences between the predictions of Newtonian gravity and general relativity tend to be most pronounced with very large masses, and so it is in astronomical phenomena that the results of the two theories diverge. The first confirmation of general relativity came soon after its publication. The point on Mercury's orbit nearest the Sun, its perihelion, was known to move slowly around the Sun. Newtonian gravity predicted such a precession but the magnitude was smaller than the measured value. Einstein's theory accurately predicted the measured value.
A novel phenomenon was the bending of light by gravity. Measurements during an eclipse in 1919 showed that the Sun's gravity changed the apparent positions of stars and that the amount of shift was consistent with general relativity. Similar effects are seen in gravitational lensing, which is found in various astronomical contexts. In gravitational lensing light is bent by a concentration of mass, such as a cluster of galaxies, so that light from a single object is bent around both sides of the mass concentration, allowing observers on Earth to see multiple images of the object, or arcs of light. The gravity of massive objects such as stars will also extract energy from radiation emitted from them by the process of gravitational red shift, which reduces the frequency of photons leaving the star and so lowers their energy.
Einstein and others attempted to broaden the scope of the general theory of relativity to include a description of electromagnetic interactions as manifestations of the geometry of space-time, but a satisfactory unified field theory was not found. Theories unifying the nuclear forces with electromagnetism have been produced, but general relativity continues to defy merger with the other forces.
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