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The '''General Theory of Relativity''' is an extension of [[Special theory of relativity|special relativity]], dealing with curved coordinate systems, accelerating frames of reference, curvilinear motion, and curvature of spacetime itself.  It could be said that general relativity is to special relativity as vector calculus is to vector algebra.  General relativity is best known for its formulation of gravity as a [[fictitious force]] arising from the curvature of spacetime.  In fact, "general relativity" and "Einstein's formulation of gravity" are nearly synonymous in many people's minds.

The '''General Theory of Relativity''' is an extension of [[Special theory of relativity|special relativity]], dealing with curved coordinate systems, accelerating frames of reference, curvilinear motion, and curvature of spacetime itself.  It could be said that general relativity is to special relativity as vector calculus is to vector algebra.  General relativity is best known for its formulation of gravity as a [[fictitious force]] arising from the curvature of spacetime.  In fact, "general relativity" and "Einstein's formulation of gravity" are nearly synonymous in many people's minds.

The general theory of elativity was first published by [[Albert Einstein]] in 1916.

The general theory of relativity was first published by Marcel Grossman in 1913 and [[David Hilbert]] and [[Albert Einstein]] in 1916.

General relativity, like [[quantum mechanics]] (the other of the two theories comprising "modern physics") both have reputations for being notoriously complicated and difficult to understand.  In fact, in the early decades of the 20^th century, general relativity had a sort of cult status in this regard.  General relativity and quantum mechanics are both advanced college-level and postgraduate level topics.  Hence this article can't possibly give a comprehensive explation of general relativity at the expert level.  But we will attempt to give a rough outline, for lay people, of the general relativistic formulation of gravity.

General relativity, like [[quantum mechanics]] (the other of the two theories comprising "modern physics") both have reputations for being notoriously complicated and difficult to understand.  In fact, in the early decades of the 20^th century, general relativity had a sort of cult status in this regard.  General relativity and quantum mechanics are both advanced college-level and postgraduate level topics.  Hence this article can't possibly give a comprehensive explation of general relativity at the expert level.  But we will attempt to give a rough outline, for lay people, of the general relativistic formulation of gravity.

This is not to say, by any means, that general relativity is the ultimate, perfect theory.  It has never been unified with modern formulations of quantum mechanics, and it is therefore known to be incorrect at extremely small scales.  Just as Newtonian gravity is very nearly correct, and completely correct for its time, general relativity is believed to be very nearly correct, but not completely so.  Contemporary speculation on the next step involves extremely esoteric notions such as string theory, gravitons, and "quantum loop gravity".

This is not to say, by any means, that general relativity is the ultimate, perfect theory.  It has never been unified with modern formulations of quantum mechanics, and it is therefore known to be incorrect at extremely small scales.  Just as Newtonian gravity is very nearly correct, and completely correct for its time, general relativity is believed to be very nearly correct, but not completely so.  Contemporary speculation on the next step involves extremely esoteric notions such as string theory, gravitons, and "quantum loop gravity".

The theory was inspired by a thought experiment developed by Einstein involving two elevators.  The first elevator is stationary on the Earth, while the other is being pulled through space at a constant acceleration of g.  Einstein realized that any physical experiment carried out in the elevators would give the same result.  This realization is known as the [[equivalence principle]] and it states that accelerating frames of reference and gravitational fields are indistinguishable.  General relativity is the theory of gravity that incorporates special relativity and the equivalence principle.

The theory is sometimes explained with a thought experiment developed by Einstein involving two elevators.  The first elevator is stationary on the Earth, while the other is being pulled through space at a constant acceleration of g.  Einstein realized that under Newtonian mechanics, any physical experiment carried out in the elevators would give the same result.  This realization is known as the [[equivalence principle]] and it states that accelerating frames of reference and gravitational fields are indistinguishable.  General relativity is the theory of gravity that incorporates special relativity and the equivalence principle.

The general theory of relativity is a ''metric theory,'' sometimes also called a ''geometric theory.'' Metric theories describe physical phenomena in terms of [[differential geometry]]. This stands in contrast to Isaac Newton's [[Law of Universal Gravitation]], which described gravity in terms of a [[vector field]].  In the case of general relativity, the theory relates ''stress-energy'' — an extension of the concept of [[mass]] — and the [[curvature]] of [[spacetime]]. In the words of physicist John Wheeler, "Space tells matter how to move, matter tells space how to curve."<ref>Misner, Thorne & Wheeler. ''Gravitation.'' (1973)</ref>

The general theory of relativity is a ''metric theory,'' sometimes also called a ''geometric theory.'' Metric theories describe physical phenomena in terms of [[differential geometry]]. This stands in contrast to Isaac Newton's [[Law of Universal Gravitation]], which described gravity in terms of a [[vector field]].  In the case of general relativity, the theory relates ''stress-energy'' — an extension of the concept of [[mass]] — and the [[curvature]] of [[spacetime]]. In the words of physicist John Wheeler, "Space tells matter how to move, matter tells space how to curve."<ref>Misner, Thorne & Wheeler. ''Gravitation.'' (1973)</ref>