Special relativity (SR, also known as the special theory of relativity or STR) is the physical theory of measurement in an inertial frame of reference proposed in 1905 by Albert Einstein (after the considerable and independent contributions of Hendrik Lorentz, Henri Poincaré and others) in the paper "On the Electrodynamics of Moving Bodies".
It generalizes Galileo's principle of relativitythat all uniform motion is relative, and that there is no absolute and well-defined state of rest (no privileged reference frames)from mechanics to all the laws of physics, including both the laws of mechanics and of electrodynamics, whatever they may be. Special relativity incorporates the principle that the speed of light is the same for all inertial observers regardless of the state of motion of the source.
This theory has a wide range of consequences which have been experimentally verified, including counter- intuitive ones such as length contraction, time dilation and relativity of simultaneity, contradicting the classical notion that the duration of the time interval between two events is equal for all observers. (On the other hand, it introduces the space-time interval, which is invariant.) Combined with other laws of physics, the two postulates of special relativity predict the equivalence of mass and energy, as expressed in the mass–energy equivalence formula E = mc 2, where c is the speed of light in vacuum.
The predictions of special relativity agree well with Newtonian mechanics in their common realm of applicability, specifically in experiments in which all velocities are small compared with the speed of light. Special relativity reveals that c is not just the velocity of a certain phenomenonnamely the propagation of electromagnetic radiation (light)but rather a fundamental feature of the way space and time are unified as spacetime. One of the consequences of the theory is that it is impossible for any particle that has rest mass to be accelerated to the speed of light.
The theory was originally termed "special" because it applied the principle of relativity only to the special case of inertial reference frames, i.e. frames of reference in uniform relative motion with respect to each other. Einstein developed general relativity to apply the principle in the more general case, that is, to any frame so as to handle general coordinate transformations, and that theory includes the effects of gravity.
The term is currently used more generally to refer to any case in which gravitation is not significant. General relativity is the generalization of special relativity to include gravitation. In general relativity, gravity is described using noneuclidean geometry, so that gravitational effects are represented by curvature of spacetime; special relativity is restricted to flat spacetime. Just as the curvature of the earth's surface is not noticeable in everyday life, the curvature of spacetime can be neglected on small scales, so that locally, special relativity is a valid approximation to general relativity. The presence of gravity becomes undetectable in a sufficiently small, free-falling laboratory.