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- This page is about the scientific concept of relativity, for philosophical or sociological theories about relativity see Relativism.

The theory of relativity , or simply relativity, refers specifically to two theories of Albert Einstein: special relativity and general relativity. However, "relativity" can also refer to Galilean relativity.

The term "theory of relativity" was coined by Max Planck in 1908 to emphasize how special relativity (and later, general relativity) uses the principle of relativity.

Special relativity is a theory of the structure of spacetime. It was introduced in Albert Einstein's 1905 paper "On the Electrodynamics of Moving Bodies". Special relativity is based on two postulates which are contradictory in classical mechanics:

- The laws of physics are the same for all observers in uniform motion relative to one another (Galileo's principle of relativity),
- The speed of light in a vacuum is the same for all observers, regardless of their relative motion or of the motion of the source of the light.

The resultant theory has many surprising consequences. Some of these are:

- Time dilation: Moving clocks are measured to tick more slowly than an observer's "stationary" clock.
- Length contraction: Objects are measured to be shortened in the direction that they are moving with respect to the observer.
- Relativity of simultaneity: two events that appear simultaneous to an observer A will not be simultaneous to an observer B if B is moving with respect to A.
- Mass-energy equivalence: $E=mc^2$, energy and mass are equivalent and transmutable.

The defining feature of special relativity is the replacement of the Galilean transformations of classical mechanics by the Lorentz transformations. (See Maxwell's equations of electromagnetism and introduction to special relativity).

Some of the consequences of general relativity are:

- Time goes more slowly in higher gravitational fields. This is called gravitational time dilation.
- Orbits precess in a way unexpected in Newton's theory of gravity. (This has been observed in the orbit of Mercury and in binary pulsars).
- Even rays of light (which have zero mass) bend in the presence of a gravitational field.
- The Universe is expanding, and the far parts of it are moving away from us faster than the speed of light. This does not contradict the theory of special relativity, since it is space itself that is expanding.
- Frame-dragging, in which a rotating mass "drags along" the space time around it.

Technically, general relativity is a metric theory of gravitation whose defining feature is its use of the Einstein field equations. The solutions of the field equations are metric tensors which define the topology of the spacetime and how objects move inertially.

- Relativity: The Special and General Theory by Publicliterature.org
- Yale University Video Lecture: Special and General Relativity at Google Video
- Wikibooks: Special Relativity
- Relativity Animations
- Relativity tutorial
- Living Reviews in Relativity — An open access, peer-referred, solely online physics journal publishing invited reviews covering all areas of relativity research.
- MathPages - Reflections on Relativity — A complete online course on Relativity.
- Relativity explained in one-syllable words
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- On the Electrodynamics of Moving Bodies
- Special Relativity Simulator
- A Relativity Tutorial at Caltech — A basic introduction to concepts of Special and General Relativity, as well as astrophysics.
- Relativity Gravity and Cosmology — A short course offered at MIT.
- Relativity in film clips and animations from the University of New South Wales.
- Animation easy to understand Animation for Pirelli Ralativity Award.
- What is the experimental basis of Special Relativity?
- Animation clip visualizing the effects of special relativity on fast moving objects.
- Relativity Calculator - Learn Special Relativity Mathematics The mathematics of special relativity presented in as simple and comprehensive manner possible within philosophical and historical contexts.
- Audio clip of Einstein explaining the theory of relativity.

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Last updated on Friday October 10, 2008 at 08:03:00 PDT (GMT -0700)

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This article is licensed under the GNU Free Documentation License.

Last updated on Friday October 10, 2008 at 08:03:00 PDT (GMT -0700)

View this article at Wikipedia.org - Edit this article at Wikipedia.org - Donate to the Wikimedia Foundation

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