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The special theory of relativity was based on two main postulates: first, that the speed of light is constant for all observers; and second, that observers moving at constant speeds should be subject to the same physical laws. Following this logic, Einstein theorized that time must change according to the speed of a moving object relative to the frame of reference of an observer. Scientists have tested this theory through experimentation - proving, for example, that an atomic clock ticks more slowly when traveling at a high speed than it does when it is not moving. The essence of Einstein's paper was that both space and time are relative (rather than absolute), which was said to hold true in a special case, the absence of a gravitational field. Relativity was a stunning concept at the time; scientists all over the world debated the veracity of Einstein's famous equation, E=mc2, which implied that matter and energy were equivalent and, more specifically, that a single particle of matter could be converted into a huge quantity of energy. However, since the special theory of relativity only held true in the absence of a gravitational field, Einstein strove for 11 more years to work gravity into his equations and discover how relativity might work generally as well. According to the Theory of General Relativity, matter causes space to curve. It is posited that gravitation is not a force, as understood by Newtonian physics, but a curved field (an area of space under the influence of a force) in the space-time continuum that is actually created by the presence of mass. According to Einstein, that theory could be tested by measuring the deflection of starlight traveling near the sun; he correctly asserted that light deflection would be twice that expected by Newton's laws. This theory also explained why the light from stars in a strong gravitational field was closer to the red end of the spectrum than those in a weaker one. For the final thirty years of his life, Einstein attempted to find a unified field theory , in which the properties of all matter and energy could be expressed in a single equation. His search was confounded by quantum theory 's uncertainty principle , which stated that the movement of a single particle could never be accurately measured, because speed and position could not be simultaneously assessed with any degree of assurance. Although he was unable to find the comprehensive theory that he sought, Einstein's pioneering work has allowed countless other scientists to carry on the quest for what some have called "the holy grail of physicists."
This article is about the scientific concept. For philosophical or ontological theories about relativity, see Relativism. For the silent film, see The Einstein Theory of Relativity. Two-dimensional projection of a three-dimensional analogy of spacetime curvature described in general relativity The theory of relativity, or simply relativity, generally encompasses two theories by Albert Einstein: special relativity and general relativity. (The word relativity can also be used in the context of an older theory, that of Galilean invariance.) Concepts introduced by the theories of relativity include: Measurements of various quantities are relative to the velocities of observers. In particular, space and time can dilate. Spacetime: space and time should be considered together and in relation to each other. The speed of light is nonetheless invariant, the same for all observers. The term "theory of relativity" was based on the expression "relative theory" (German: Relativtheorie) used by Max Planck in 1906, who emphasized how the theory uses the principle of relativity. In the discussion section of the same paper Alfred Bucherer used for the first time the expression "theory of relativity" (German: Relativitätstheorie).