(Physics) The theory that space and time are relative concepts rather than absolute concepts; SYN. theory of relativity, relativity theory.
Phyics, principle, formulated by Einstein, and based primarily on the constancy of the velocity of light, denying the absoluteness of space and time and establishing time as “fourth dimension”. special theory of relativity, as above. general theory of relativity, conclusions from special theory as it affects gravitation, which is identified with inertia and interpreted by the varying geometrical structure of the space in which masses are moving. relativity of knowledge, Philosophy, doctrine that knowledge is limited by nature of mind, which is unable to perceive the reality of, but only the relations between, objects.
In physics, the theory of the relative rather than absolute character of motion and mass, and the interdependence of matter, time, and space, as developed by German-born us physicist Albert Einstein in two phases:
Special theory of relativity (1905) Starting with the premises that (1) the laws of nature are the same for all observers in unaccelerated motion, and (2) the speed of light is independent of the motion of its source, Einstein arrived at some rather unexpected consequences. Intuitively familiar concepts, like mass, length, and time, had to be modified. For example, an object moving rapidly past the observer will appear to him to be both shorter and heavier than when it is at rest (i.e. at rest relative to the observer), and a clock moving rapidly past him will appear to be running slower than when it is at rest. These predictions of relativity theory seem to be foreign to everyday experience merely because the changes are quite negligible at speeds less than about 1500 km s-1, and they only become appreciable at speeds approaching the speed of light.
General theory of relativity (1915) The geometrical properties of space-time were to be conceived as modified locally by the presence of a body with mass. A planet’s orbit around the Sun (as observed in three-dimensional space) arises from its natural trajectory in modified space-time; there is no need to invoke, as Isaac Newton did, a force of gravity coming from the Sun and acting on the planet. Einstein’s general theory accounts for a peculiarity in the behavior of the motion of the perihelion of the orbit of the planet Mercury that cannot be explained on Newton’s theory. The new theory also said that light rays should bend when they pass by a massive object. The predicted bending of starlight was observed during the eclipse of the Sun 1919. A third corroboration is found in the shift toward the red in the spectra of the Sun and, in particular, of stars of great density—white dwarfs such as the companion of Sirius.
Einstein showed that, for consistency with premises (1) and (2), the principles of dynamics as established by Newton needed modification; the most celebrated new result was the equation E = mc2, which expresses an equivalence between mass (m) and energy (e), c being the speed of light in a vacuum. In “relativistic mechanics”, conservation of mass is replaced by the new concept of conservation of “mass-energy”.
Although since modified in detail, general relativity remains central to modern astrophysics and cosmology; it predicts, for example, the possibility of black holes. General relativity theory was inspired by the simple idea that it is impossible in a small region to distinguish between acceleration and gravitation effects (as in a lift one feels heavier when the lift accelerates upward), but the mathematical development of the idea is formidable. Such is not the case for the special theory, which a nonexpert can follow up to E = mc2 and beyond.
A famous relativity problem is the “twin paradox”. If one of two twins, o’, sets off at high speed in a space ship, travels for some years, and then turns around and returns at high speed to rejoin the other twin O, who stayed at home, he will have aged less than O, because his clocks have been running slower. If his speed is v = 0.995c in each direction, he will have aged only one year for every 10 years that O has aged. This conclusion has been hotly disputed, because it appears to contradict the symmetry that ought to exist between O and o’. But now there is no such symmetry: O has remained in an unaccelerated state, whereas o’ has suffered three very large changes in velocity, and both will in fact agree that O has aged more than o’. The observed slowing down of the decay of mesons when moving conclusively shows the existence of time dilation.