# Relativity

**Relativity**, theory of the nature of space, time, and matter. Albert Einstein's special theory of relativity (1905) is based on the premise that different observers moving at a constant speed with respect to each other find the laws of physics to be identical, and, in particular, find the speed of light waves to be the same (the principle of relativity). Among its consequences are (1) that events occurring simultaneously according to one observer may happen at different times according to an observer moving relative to the first (although the order of two causally related events is never reversed), (2) that a moving object is shortened in the direction of its motion, (3) that time runs more slowly for a moving object, (4) that the velocity of a projectile emitted from a moving body is less than the sum of the relative ejection velocity and the velocity of the body, (5) that a body has a greater mass when moving than when at rest, and (6) that no massive body can travel as fast as, or faster than, the speed of light. These effects are too small to be noticed at normal velocities; they have nevertheless found ample experimental verification and are common considerations in many physical calculations. The relationship between the position and time of a given event according to different observers is known (for H.A. Lorentz) as the Lorentz transformation. In this, time mixes on a similar footing with the three spatial dimensions, and it is in this sense that time has been called the fourth dimension. The greater mass of a moving body implies a relationship between kinetic energy and mass; Einstein made the bold additional hypothesis that *all* energy is equivalent to mass, according to the famous equation *E = mc*^{2}. The conversion of mass to energy is now the basis of nuclear reactors and is indeed the source of the energy of the sun itself.

Einstein's general theory (1916) is of importance chiefly to cosmologists. It asserts the equivalence of the effects of acceleration and gravitational fields and that gravitational fields cause space to become “curved,” so that light no longer travels in straight lines, while the wavelength of light falls as the light falls through a gravitational field. The direct verification of these last two predictions, among others, has helped deeply to entrench the theory of relativity in the language of physics.

*See also:* Einstein, Albert.

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