Theory of relativity

2010-08-10T03:08:35Z

Corpseymgee:

The '''theory of relativity''', or simply '''relativity''', encompasses two theories of [[Albert Einstein]]: [[special relativity]] and [[general relativity]].<ref>{{Citation|author=Einstein A.|year=1916 (translation 1920)|title=[[s:Relativity: The Special and General Theory|Relativity: The Special and General Theory]]|publisher=H. Holt and Company
|location=New York}}</ref> However, the word "relativity" is sometimes used in reference to [[Galilean invariance]].

The term "theory of relativity" was based on the expression "relative theory" ({{lang-de|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" ({{lang-de|Relativitätstheorie}}).<ref>{{Citation|author=Planck, Max|year=1906|title=[[s:The Measurements of Kaufmann|The Measurements of Kaufmann on the Deflectability of β-Rays in their Importance for the Dynamics of the Electrons]]|journal=Physikalische Zeitschrift|volume=7|pages=753–761}}</ref><ref>{{Citation|author=Miller, Arthur I.|year=1981|title= Albert Einstein's special theory of relativity. Emergence (1905) and early interpretation (1905–1911)|location= Reading|publisher=Addison–Wesley|isbn=0-201-04679-2}}</ref>

==Scope of Relativity==
The theory of relativity enriched [[physics]] and [[astronomy]] during the 20th century. When first published, relativity supersceded a 200-year-old [[Classical mechanics|theory of mechanics]] elucidated by [[Isaac Newton]]. It changed perceptions.<ref name=relativity/><ref name=spacetime/><ref name=fitz-loren/>

For example, it overturned the concept of [[motion]] from Newton's day, into all motion is relative. [[Time]] was no longer uniform and absolute, as related to everyday experience. Furthermore, no longer could physics be understood as space by itself, and time by itself. Instead, an added dimension had to be taken into account with curved [[space-time]]. Time now depended on [[velocity]], and contraction became a fundamental consequence at appropriate speeds.<ref name=relativity/><ref name=spacetime/><ref name=fitz-loren/>

In the field of microscopic physics, relativity catalyzed an added and essential depth of knowledge to the science of [[elementary particles]] and their fundamental interactions, along with introducing the [[atomic age|nuclear age]]. With relativity,[[cosmology]] and [[astrophysics]] predicted extraordinary [[astronomy|astronomical phenomena]] such as [[neutron stars]], [[black holes]], and [[gravitational waves]].<ref name=relativity>{{Cite encyclopedia
| title =Relativity
| encyclopedia =Grolier Multimedia Encyclopedia
| last = Will, Clifford M
| date =August 1, 2010
| url =http://gme.grolier.com/article?assetid=0244990-0
| accessdate =2010-08-01}}</ref><ref name=spacetime>{{Cite encyclopedia
| title =Space-Time Continuum
| encyclopedia =Grolier Multimedia Encyclopedia
| last = Will, Clifford M
| date =August 1, 2010
| url =http://gme.grolier.com/article?assetid=0272730-0
| accessdate =2010-08-01}}</ref><ref name=fitz-loren>{{Cite encyclopedia
| title =Fitzgerald-Lorentz contraction
| encyclopedia =Grolier Multimedia Encyclopedia
| last = Will, Clifford M
| date =August 1, 2010
| url =http://gme.grolier.com/article?assetid=0107090-0
| accessdate =2010-08-01}}</ref>

===Two theory view===
The theory of relativity was representative of more than a single new [[physical theory]]. It affected the theories and methodologies across all the physical sciences. However, as stated above, this is more likely perceived as two separate theories. There are some related explanations for this. First, [[special relativity]] was published in 1905, and the final form of [[general relativity]] was published in 1916.<ref name=relativity/>

Second, special relativity fits with and solves for elementary particles and their interactions, whereas general relativity solves for the [[cosmological]] and astrophysical realm (including astronomy).<ref name=relativity/>

Third, special relativity was widely accepted in the physics community by 1920. This theory rapidly became a notable and necessary tool for theorists and experimentalists in the new fields of [[atomic physics]], nuclear physics, and [[quantum mechanics]]. Conversely, general relativity did not to appear to be as useful. There appeared to be little applicability for experimentalists as most applications were for astronomical scales. It seemed limited to only making minor corrections to predictions of Newtonian gravitation theory. Its impact was not apparent until the 1930s.<ref name=relativity/>

Finally, the [[calculus|mathematics]] of general relativity appeared to be incomprehensibly dense. Consequently, only a small number of people in the world, at that time, could fully understand the theory in detail. This remained the case for the next 40 years. Then, at around 1960 a critical resurgence in interest occurred which has resulted in making general relativity central to physics and astronomy. New mathematical techniques applicable to the study of general relativity substantially streamlined calculations. From this, physically discernible concepts were isolated from the mathematical complexity. Also, the discovery of exotic astronomical [[phenomena]] in which general relativity was crucially relevant, helped to catalyze this resurgence. The astronomical phenomena included [[quasars]] (1963), the 3-kelvin [[microwave background radiation]] (1965), [[pulsars]] (1967), and the discovery of the first [[black hole]] candidates (1971).<ref name=relativity/>

==Special relativity==
{{Main|Special relativity}}
[[File:Albert Einstein 1979 USSR Stamp.jpg|thumb|250px|USSR stamp dedicated to Albert Einstein]]
Special relativity is a theory of the structure of [[spacetime]]. It was introduced in [[Albert Einstein]]'s 1905 paper "[[Annus Mirabilis Papers#Special relativity|On the Electrodynamics of Moving Bodies]]" (for the contributions of many other physicists see [[History of special relativity]]). Special relativity is based on two postulates which are contradictory in [[classical mechanics]]:
# The laws of [[physics]] are the same for all observers in [[inertial frame of reference|uniform motion]] relative to one another ([[principle of relativity]]),
# The [[speed of light]] in a [[vacuum]] is the same for all observers, regardless of their relative motion or of the motion of the source of the [[light]].

The resultant theory agrees with experiment better than classical mechanics, e.g. in the [[Michelson-Morley experiment]] that supports postulate 2, but also has many surprising consequences. Some of these are:
* [[Relativity of simultaneity]]: Two events, simultaneous for one observer, may not be simultaneous for another observer if the observers are in relative motion.
* [[Time dilation]]: Moving [[clock]]s are measured to tick more slowly than an observer's "stationary" clock.
* [[Length contraction]]: Objects are measured to be shortened in the direction that they are moving with respect to the observer.
* [[Mass-energy equivalence]]: <math>E=mc^2</math>, energy and mass are equivalent and transmutable.
* [[Speed of light#Upper_limit_on_speeds|Maximum speed is finite]]: No physical object or message or field line can travel faster than light.

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==
{{Main|General 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]] (for example when standing on the surface of the Earth) are physically identical. The upshot of this is that [[free fall]] is [[inertia|inertial motion]]; an object in free fall is falling because that is how objects move when there is no [[force]] being exerted on them, instead of this being due to the force of [[gravity]] as is the case in [[classical mechanics]]. This is incompatible with classical mechanics and [[special relativity]] because in those theories inertially moving objects cannot accelerate with respect to each other, but objects in free fall do so. To resolve this difficulty Einstein first proposed that spacetime is [[curvature|curved]]. In 1915, he devised the [[Einstein field equations]] which relate the curvature of spacetime with the mass, energy, and momentum within it.

Some of the consequences of general relativity are:
* Time goes slower in higher gravitational fields. This is called [[gravitational time dilation]].
* 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).
* Rays of [[light]] [[General relativity#Light deflection and gravitational time delay|bend in the presence of a gravitational field]].
* [[Frame-dragging]], in which a rotating mass "drags along" the space time around it.
* [[Metric expansion of space|The Universe is expanding]], and the far parts of it are moving away from us [[Faster-than-light#Universal expansion|faster than the speed of light]].

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 tensors]] which define the [[topology]] of the spacetime and how objects move inertially.

==See also==
*[[Special relativity#References|Special relativity references]]
*[[General relativity#References|General relativity references]]

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Theory of relativity