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**General Relativity**

General Relativity is a theory which incorporates gravity into Einstein’s Special Theory of Relativity. General Relativity is a generalization of Special Relativity: the latter may be seen as a special case of the former.

The central idea in General Relativity is the equivalence principle. It states that a gravitational field is indistinguishable from an acceleration. That is, an observer on the Earth’s crust feels the same pull downwards as an observer on a ship in space, accelerating at 9.8 m/s2 (which is the value of the gravitational field on the Earth’s crust). Similarly, an observer in free-fall will experience no gravity at all. That’s the reason why there’s no gravity when in orbit around the Earth: the shuttle is actually in free-fall.

The theory of General Relativity also incorporates the principles of Special Relativity: that is, the fact that each observer must experience the same laws of Physics and the constancy of the speed of light. This leads to bizarre effects akin to those experienced with time and space in Special Relativity: in the presence of a gravitational field, space-time itself bends. Straight lines become geodesics, which are curved lines that minimize the distance between two points in the new warped space. The bending results in time passing more slowly and space becoming stretched where gravitational fields are stronger: an extreme case of that are black holes.

Wikipedia’s introductory page on General Relativity.

Video extract from Brian Greene’s Elegant Universe on General Relativity.

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