Editing General relativity (section)

{{Short description|Theory of gravitation as curved spacetime}}
{{For|the graduate textbook by Robert Wald|General Relativity (book){{!}}''General Relativity'' (book)}}
{{For introduction}}
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[[File:BBH gravitational lensing of gw150914.webm|266px |thumb|Slow motion computer simulation of the black hole [[binary system]] GW150914 as seen by a nearby observer, during 0.33 s of its final [[Binary black hole#Inspiral|inspiral]], [[Binary black hole#Merger|merge]], and [[Binary black hole#Ringdown|ringdown]]. The star field behind the black holes is being heavily distorted and appears to rotate and move, due to extreme [[gravitational lens]]ing, as [[spacetime]] itself is distorted and dragged around by the rotating [[black hole]]s.<ref name="SXSproject">{{cite web |url= https://www.black-holes.org/2016/02/11/gw150914 |title=GW150914: LIGO Detects Gravitational Waves |website=Black-holes.org |date=11 February 2016 |access-date=18 April 2016}}</ref>]]
{{General relativity sidebar}}

'''General relativity''', also known as the '''general theory of relativity''', and as '''Einstein's theory of gravity''', is the [[differential geometry|geometric]] theory of [[gravitation]] published by [[Albert Einstein]] in 1915 and is the current description of gravitation in [[modern physics]]. General [[theory of relativity|relativity]] generalizes [[special relativity]] and refines [[Newton's law of universal gravitation]], providing a unified description of gravity as a geometric property of [[space]] and [[time in physics|time]], or four-dimensional [[spacetime]]. In particular, the ''[[curvature]] of spacetime'' is directly related to the [[energy]] and [[momentum]] of whatever is 
present, including [[matter]] and [[radiation]]. The relation is specified by the [[Einstein field equations]], a system of second-order [[partial differential equation]]s.

[[Newton's law of universal gravitation]], which describes gravity in classical mechanics, can be seen as a prediction of general relativity for the almost flat spacetime geometry around stationary mass distributions. Some predictions of general relativity, however, are beyond [[Newton's law of universal gravitation]] in [[classical physics]]. These predictions concern the passage of time, the [[geometry]] of space, the motion of bodies in [[free fall]], and the propagation of light, and include [[gravitational time dilation]], [[gravitational lens]]ing, the [[gravitational redshift]] of light, the [[Shapiro time delay]] and [[Gravitational singularity|singularities]]/[[black holes]]. So far, all [[tests of general relativity]] have been shown to be in agreement with the theory. The time-dependent solutions of general relativity enable us to talk about the history of the universe and have provided the modern framework for [[cosmology]], thus leading to the discovery of the [[Big Bang]] and [[cosmic microwave background]] radiation. Despite the introduction of a number of [[Alternatives to general relativity|alternative theories]], general relativity continues to be the simplest theory consistent with [[experimental data]].

Reconciliation of general relativity with the laws of [[quantum mechanics|quantum physics]] remains a problem, however, as there is a lack of a self-consistent theory of [[quantum gravity]]. It is not yet known how gravity can be [[Theory of everything|unified]] with the three non-gravitational forces: [[strong interaction|strong]], [[weak interaction|weak]] and [[electromagnetism|electromagnetic]].

Einstein's theory has [[astrophysical]] implications, including the prediction of [[black holes]]—regions of space in which space and time are distorted in such a way that nothing, not even [[photon|light]], can escape from them. Black holes are the end-state for [[massive star]]s. [[Microquasar]]s and [[active galactic nucleus|active galactic nuclei]] are believed to be [[stellar black hole]]s and [[supermassive black hole]]s. It also predicts [[gravitational lensing]], where the bending of light results in multiple images of the same distant astronomical phenomenon. Other predictions include the existence of [[gravitational wave]]s, which have been [[List of gravitational wave observations|observed directly]] by the physics collaboration [[LIGO]] and other observatories. In addition, general relativity has provided the base of [[Physical cosmology|cosmological]] models of an [[Metric expansion of space|expanding universe]].

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Widely acknowledged as a theory of extraordinary [[Mathematical beauty|beauty]], general relativity has often been described as the most beautiful of all existing physical theories.<ref name=":0" />

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