The validity of the classical concepts of absolute and independent time and space was challenged by H. A. Lorentz and others. Since absolute motion cannot be confirmed by objective measurement, Einstein suggested that it be discarded from physical reasoning; he explained the results of the Michelson-Morley experiment by means of the special relativity theory, which he enunciated in 1905. This theory accepts the hypothesis that the laws of nature are the same in different moving systems applies also to the propagation of light, so that the measured speed of light is constant for all observers regardless of the motion of the observer or of the source of the light. Einstein deduced from these hypotheses the full logical consequences and reformulated the mathematical equations of physics, basing them in part on equations of H. A. Lorentz (see Lorentz contraction) by which measurements made in one uniformly moving system can be correlated with measurements in another system if the velocity of one relative to the other is known.

The theory resolves the conflict between Newton's mechanics and Maxwell's electrodynamics by introducing fundamental changes in Newton's theory. In most phenomena of ordinary experience the results obtained from the application of the special theory approximate those based on Newtonian dynamics, but the results deviate greatly for phenomena occurring at velocities approaching the speed of light. In innumerable cases where the results predicted by these theories are incompatible, experimental evidence supports the Einstein theory. Among its assertions and consequences are the propositions that the maximum velocity attainable in the universe is that of light; that mass and energy are equivalent and interchangeable properties (this is spectacularly confirmed by nuclear fission, on which the atomic bomb is based); that objects appear to contract in the direction of motion; that the rate of a moving clock seems to decrease as its velocity increases; that events that appear simultaneous to an observer in one system may not appear simultaneous to an observer in another system; and that, since absolute time is excluded from physical reasoning because it cannot be measured, the results of observers in different systems are equally correct.

- Introduction
- Evolution from Classical Theory
- The Special Theory of Relativity
- The General Theory of Relativity
- Bibliography

*The Columbia Electronic Encyclopedia,* 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.