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##### The Collaborative International Dictionary
special relativity

relativity \rel`a*tiv"i*ty\ (-t?v"?-t?), n.

1. The state of being relative; as, the relativity of a subject.
--Coleridge.

2. One of two theories (also called theory of relativity) proposed by Albert Einstein, the special theory of relativity, or the general theory of relativity. The special theory of relativity or special relativity is based on the proposition that the speed of light is a constant no matter how observed, and is independent of the motion of the observer. From this follows several principles, such as the increase of mass with velocity (which has been confirmed: see relativistic mass equation) and the impossibility of acceleration to a speed greater than that of light; the equivalence of mass and energy, expressed by the famous equation E = mc^ 2; and time dilation, which is the apparent slowing of a clock in a system, as observed by an observer in a system moving relative to the clock. The general theory of relativity is based on the proposition that there is no physical difference between gravitational force and the force produced by acceleration. From this follow several results, of which the bending of light rays in a gravitational field and the equivalence of the inertial and gravitational masses have been verified. The possible existence of black holes (believed by many astronomers to have been adequately proven) is another consequence of the theory.

##### Wiktionary
special relativity

n. (context relativity English) A theory that (neglecting the effects of gravity) reconciles the principle of relativity with the observation that the speed of light is constant in all frame of reference.

##### WordNet
special relativity

n. a physical theory of relativity based on the assumption that the speed of light in a vacuum is a constant and the assumption that the laws of physics are invariant in all inertial systems [syn: special theory of relativity, special relativity theory, Einstein's special theory of relativity]

##### Wikipedia
Special relativity

In physics, special relativity (SR, also known as the special theory of relativity or STR) is the generally accepted and experimentally well confirmed physical theory regarding the relationship between space and time. In Albert Einstein's original pedagogical treatment, it is based on two postulates:

1. The laws of physics are invariant (i.e. identical) in all inertial systems (non-accelerating frames of reference).
2. The speed of light in a vacuum is the same for all observers, regardless of the motion of the light source.

It was originally proposed in 1905 by Albert Einstein in the paper " On the Electrodynamics of Moving Bodies". The inconsistency of Newtonian mechanics with Maxwell’s equations of electromagnetism and the lack of experimental confirmation for a hypothesized luminiferous aether led to the development of special relativity, which corrects mechanics to handle situations involving motions nearing the speed of light. As of today, special relativity is the most accurate model of motion at any speed. Even so, the Newtonian mechanics model is still useful (due to its simplicity and high accuracy) as an approximation at small velocities relative to the speed of light.

Special relativity implies a wide range of consequences, which have been experimentally verified, including length contraction, time dilation, relativistic mass, mass–energy equivalence, a universal speed limit and relativity of simultaneity. It has replaced the conventional notion of an absolute universal time with the notion of a time that is dependent on reference frame and spatial position. Rather than an invariant time interval between two events, there is an invariant spacetime interval. 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, where c is the speed of light in a vacuum.

A defining feature of special relativity is the replacement of the Galilean transformations of Newtonian mechanics with the Lorentz transformations. Time and space cannot be defined separately from each other. Rather space and time are interwoven into a single continuum known as spacetime. Events that occur at the same time for one observer can occur at different times for another.

The theory is "special" in that it only applies in the special case where the curvature of spacetime due to gravity is negligible. In order to include gravity, Einstein formulated general relativity in 1915. Special relativity, contrary to some outdated descriptions, is capable of handling accelerated frames of reference.

As Galilean relativity is now considered an approximation of special relativity that is valid for low speeds, special relativity is considered an approximation of general relativity that is valid for weak gravitational fields, i.e. at a sufficiently small scale and in conditions of free fall. Whereas general relativity incorporates noneuclidean geometry in order to represent gravitational effects as the geometric curvature of spacetime, special relativity is restricted to the flat spacetime known as Minkowski space. A locally Lorentz-invariant frame that abides by special relativity can be defined at sufficiently small scales, even in curved spacetime.

Galileo Galilei had already postulated that there is no absolute and well-defined state of rest (no privileged reference frames), a principle now called Galileo's principle of relativity. Einstein extended this principle so that it accounted for the constant speed of light, a phenomenon that had been recently observed in the Michelson–Morley experiment. He also postulated that it holds for all the laws of physics, including both the laws of mechanics and of electrodynamics.

Special relativity (alternative formulations)

As formulated by Albert Einstein in 1905, the theory of special relativity was based on two main postulates:

1. The principle of relativity — The form of a physical law is the same in any inertial frame.
2. The speed of light is constant — In all inertial frames, the speed of light c is the same whether the light is emitted from a source at rest or in motion. (Note this does not apply in non-inertial frames, indeed between accelerating frames the speed of light cannot be constant. Although it can be applied in non-inertial frames if an observer is confined to making local measurements.)

There have been various alternative formulations of special relativity over the years. Some of these formulations are equivalent to the original formulation whereas others result in modifications.