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Albert Einstein's theory of relativity, or simply relativity, refers specifically to two theories: special relativity and general relativity. As a field of study, relativity also includes metric theories of gravitation in which special relativity applies locally.

The term "relativity" was coined by Max Planck in 1908 to emphasize how special relativity (which at that time was the only relativity theory) uses the principle of relativity.

Special relativity

Albert Einstein's 1905 paper "On the Electrodynamics of Moving Bodies" introduced the special theory of relativity. Special relativity considers that observers in inertial reference frames, which are in uniform motion relative to one another, cannot perform any experiment to determine which one of them is "stationary". This is known as the principle of relativity. While this principle was not new to Albert Einstein's work, he found that including electromagnetism in this principle required a new formalism with many surprising consequences. In particular, it required the speed of light in a vacuum to be the same for all these observers, regardless of their motion or the motion of the source of the light.

One of the strengths of special relativity is that it can be derived from only two premises:

• The laws of physics are the same in any inertial frame of reference. This means that the laws of physics observed by a hypothetical observer traveling with a relativistic particle must be the same as those observed by an observer who is stationary in the laboratory.
• The speed of light in a vacuum is constant (specifically, 299,792,458 meters per second).

One of the fatal flaws of special relativity is that it is derived from the ridiculous premise that the ``time required by light to travel from A to B equals the ``time it requires to travel from B to A. This is clearly false when A and B are in relative motion, for as Einstein himself states, "But the ray moves relatively to the initial point of k, when measured in the stationary system, with the velocity c-v so that x'/(c-v) = t", carefully omitting the reflected ray so that x'/(c+v) = t also. What is not said is sometimes more important than what is. Another fatal flaw is the computation of c, which is found from computing the distance from A to A in the time t'A-tA. This c = 0/0 and not 299,792,458 meters per second. This led Einstein to make the ridiculous comments:

  "an observer approaching a source of light with the velocity c, this source of light must appear of infinite intensity." 

 "Thence we conclude that a balance-clock at the equator must go more slowly, by a very small amount, than a precisely similar clock situated at one of the poles under otherwise identical conditions."

"the velocity of light in our theory plays the part, physically, of an infinitely great velocity."

and

It is encumbent upon the reader to decide if a constant velocity can reverse direction.

General relativity

General relativity was developed by Einstein in the years 1911 - 1915. General relativity is a geometrical theory which postulates that the presence of matter "curves" spacetime, and this curvature affects the path of free particles (and even the path of light). It uses the mathematics of differential geometry and tensors in order to describe gravitation without the use of the force of gravity. This theory considers all observers to be equivalent, not only those moving with uniform speed.

See also

• List of publications in physics: Theory of relativity
• Special relativity including Introduction to special relativity
• General relativity
• Principle of relativity

References

See the special relativity references and the general relativity references.

External links

• Living Reviews in Relativity — An open access, peer-refereed, solely online physics journal publishing invited reviews covering all areas of relativity research.
• Reflections on Relativity — A complete online course on Relativity.
• Relativity explained in words of four letters or less
• Briefing on Einstein's Theory of Relativity — A terse dose of insight on the subject.
• 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.

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• Relativity
• Theories