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Unlike most advances in physics, the theory of relativity was proposed based on mathematical theory rather than observation.  The theory rests on two postulates that are difficult to test, and then derives mathematically what the physical consequences should be.  Those two postulates are that the speed of light never changes, and that all laws of physics are the same in every (inertial) frame of reference no matter where it is or how fast it is traveling.  This theory rejects [[Isaac Newton]]'s God-given theory of gravitation and replaces it with a concept that there is a continuum of space and time, and that large masses (like the sun) bend space in a manner similar to how a finger can depress an area of a balloon.  From this proposed bending of space the expression arose that "space is curved." But experiments later proved that space is flat overall.

[[Image:spacetime_curvature.png|thumb|right|300px|Two-dimensional analogy of space-time distortion described in General Relativity.]]

Nothing useful has even been built based on the theory of relativity.  Scientists claim that this is because relativity only applies to extremely heavy or fast objects and rely on future scientists to finally come up with the proof that will vindicate their life's work.  Most conservatives are skeptical since science is supposed to be about finding proof before a theory becomes a fact, not after.  [[Albert Einstein]]'s work had nothing to do with the development of the [[atomic bomb]], contrary to popular opinion.  Only one Nobel Prize has ever been given (in 1993) that relates to relativity, and the validity of that particular award is questionable.  Many things predicted by the theory of relativity, such as gravitons, have never been found despite much searching for them.  Many observed phenomenon, such as the bending of light passing near the sun or the advance of the perihelion in the orbit of Mercury, can be also predicted by Newton's theory.

The '''theory of relativity''', or simply '''relativity''', refers specifically to two theories: [[Albert Einstein]]'s [[special relativity]] and [[general relativity]].   

British Historian Paul Johnson declares the turning point in 20th century to have been when fellow Brit Sir [[Arthur Eddington]], the top English astronomer, ventured out on a boat off Africa in 1919 to observe the bending of starlight around the sun during a total eclipse.  The theory of relativity predicts twice the bending of light around massive objects compared to Newton's theory, and an eclipse is required to darken the sun so that the starlight may be seen in proximity to the sun.  Eddington liked publicity and probably dreamed of winning a Nobel Prize, and upon his return to England declared that his observations proven the theory of relativity.  That was good enough for reporters and historians, but the Nobel committee was not impressed and declined to give him an award.  Recent analysis of Eddington's work revealed that he was biased in selecting his data, and that overall his data was inconclusive about the theory of relativity.

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

Eddington next promoted the theory of relativity to the English-speaking world in his Mathematical Theory of Relativity (1923).  As the title suggests, this theory was more a mathematical vision of how the universe should be, rather than what it actually was.  When a reporter asked Eddington whether only three people even understood the theory, Eddington supposedly retorted, "Who's the third?"

== Special relativity ==

Just as “social Darwinism” arose from Darwinism, many seized upon the theory of relativity to apply it in a vague way to morality and social issues. “All things are relative” became popular as atheists and others used relativity to attack Christian values. There remains enormous political support for the theory of relativity that has nothing to do with physics, and Congress continues to spend billions of dollars unsuccessfully searching for particles predicted by the theory of relativity.

Special relativity is a theory of the structure of [[spacetime]]. The reason we have three space dimensions and one time dimension, or better, space-time, could lie in the way gravity works. One of the foundations of Poincaré’s Theory is the fact that gravity can affect time. It was introduced in [[Albert Einstein]]'s [[1905]] paper "[[Annus Mirabilis Papers#Papers|On the Electrodynamics of Moving Bodies]]". Special relativity is based on two postulates which are contradictory in [[classical mechanics]]:

#  That observers, when in [[inertial reference frame|uniform motion relative to one another]], have no way of determining whether one of them is "stationary" ([[Galileo Galilei|Galileo]]'s [[principle of relativity]]),

# That the [[speed of light]] in a [[vacuum]] is the same for all observers, regardless of their motion or of the motion of the source of the [[light]].

 

* [[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.

* [[E=mc²]]: energy and mass are equivalent and interchangeable.

 

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

 

General relativity is a theory of gravitation developed by Einstein in the years [[1907]] - [[1915]].

 

The development of general relativity began with the [[equivalence principle]], under which the states of [[acceleration|accelerated motion]] and being at rest in a [[gravity|gravitational field]] are physically identical.  The upshot of this is that [[free fall]] is [[inertia|inertial motion]].  This is incompatible with [[classical mechanics]] and [[special relativity]] because in those theories intertially moving objects cannot accelerate with respect to each other, but objects in free fall do so.  To resolve this difficulty Einstein first proposed the spacetime is [[curvature|curved]], and in [[1915]] devised the [[Einstein field equations]] which relate the curvature of spacetime with the mass, energy, and momentum within it.

 

Technically, general relativity is a [[metric (mathematics)|metric]] theory of [[gravitation]] whose defining feature is its use of the Einstein field equations.  The solutions of the field equations are [[metric tensor (general relativity)|metric tensor]]s which define the topology of the spacetime and how objects move intertially.

 

Some of the consequences of general relativity are:

* Orbits [[precession|precess]] in a way unexpected in Newton's theory of gravity.  (This has been observed in the orbit of [[Mercury (planet)|Mercury]] and in [[binary pulsar]]s).

* Even rays of [[light]] (which are weightless) [[General relativity#Bending of light|bend in the presence of a gravitational field]].

*  [[Frame-dragging|A rotating mass "drags along" the space time continuum]] around it.