relativity a term applied to Einstein’s theories of electrodynamics (special relativity, 1905) and gravitation (general relativity, 1916) because both hold that certain physical quantities, formerly considered objective, are actually ‘relative to’ the state of motion of the observer. They are called ‘special’ and ‘general’ because, in special relativity, electrodynamical laws determine a restricted class of kinematical reference frames, the ‘inertial frames’; in general relativity, the very distinction between inertial frames and others becomes a relative distinction.
Special relativity. Classical mechanics makes no distinction between uniform motion and rest: not velocity, but acceleration is physically detectable, and so different states of uniform motion are physically equivalent. But classical electrodynamics describes light as wave motion with a constant velocity through a medium, the ‘ether.’ It follows that the measured velocity of light should depend on the motion of the observer relative to the medium. When interferometer experiments suggested that the velocity of light is independent of the motion of the source, H. A. Lorentz proposed that objects in motion contract in the direction of motion through the ether (while their local time ‘dilates’), and that this effect masks the difference in the velocity of light. Einstein, however, associated the interferometry results with many other indications that the theoretical distinction between uniform motion and rest in the ether lacks empirical content. He therefore postulated that, in electrodynamics as in mechanics, all states of uniform motion are equivalent. To explain the apparent paradox that observers with different velocities can agree on the velocity of light, he criticized the idea of an ‘absolute’ or frame-independent measure of simultaneity: simultaneity of distant events can only be established by some kind of signaling, but experiment suggested that light is the only signal with an invariant velocity, and observers in relative motion who determine simultaneity with light signals obtain different results. Furthermore, since objective measurement of time and length presupposes absolute simultaneity, observers in relative motion will also disagree on time and length. So Lorentz’s contraction and dilatation are not physical effects, but consequences of the relativity of simultaneity, length, and time, to the motion of the observer. But this relativity follows from the invariance of the laws of electrodynamics, and the invariant content of the theory is expressed geometrically in Minkowski spacetime. Logical empiricists took the theory as an illustration of how epistemological analysis of a concept (time) could eliminate empirically superfluous notions (absolute simultaneity). General relativity. Special relativity made the velocity of light a limit for all causal processes and required revision of Newton’s theory of gravity as an instantaneous action at a distance. General relativity incorporates gravity into the geometry of space-time: instead of acting directly on one another, masses induce curvature in space-time. Thus the paths of falling bodies represent not forced deviations from the straight paths of a flat space-time, but ‘straightest’ paths in a curved space-time. While space-time is ‘locally’ Minkowskian, its global structure depends on mass-energy distribution. The insight behind this theory is the equivalence of gravitational and inertial mass: since a given gravitational field affects all bodies equally, weight is indistinguishable from the inertial force of acceleration; freefall motion is indistinguishable from inertial motion. This suggests that the Newtonian decomposition of free fall into inertial and accelerated components is arbitrary, and that the freefall path itself is the invariant basis for the structure of space-time. A philosophical motive for the general theory was to extend the relativity of motion. Einstein saw special relativity’s restricted class of equivalent reference frames as an ‘epistemological defect,’ and he sought laws that would apply to any frame. His inspiration was Mach’s criticism of the Newtonian distinction between ‘absolute’ rotation and rotation relative to observable bodies like the ‘fixed stars.’ Einstein formulated Mach’s criticism as a fundamental principle: since only relative motions are observable, local inertial effects should be explained by the cosmic distribution of masses and by motion relative to them. Thus not only velocity and rest, but motion in general would be relative. Einstein hoped to effect this generalization by eliminating the distinction between inertial frames and freely falling frames. Because free fall remains a privileged state of motion, however, non-gravitational acceleration remains detectable, and absolute rotation remains distinct from relative rotation. Einstein also thought that relativity of motion would result from the general covariance (coordinate-independence) of his theory – i.e., that general equivalence of coordinate systems meant general equivalence of states of motion. It is now clear, however, that general covariance is a mathematical property of physical theories without direct implications about motion. So general relativity does not ‘generalize’ the relativity of motion as Einstein intended. Its great accomplishments are the unification of gravity and geometry and the generalization of special relativity to space-times of arbitrary curvature, which has made possible the modern investigation of cosmological structure.
See also EINSTEIN , FIELD THEORY, PHILOS- OPHY OF SCIENCE , SPACE -TIM. R.D.