Editing Theory of relativity (section)

== General relativity ==
{{Main|General relativity|Introduction to 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 [[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 [[Curved space|spacetime is curved]]. Einstein discussed his idea with mathematician [[Marcel Grossmann]] and they concluded that general relativity could be formulated in the context of [[Riemannian geometry]] which had been developed in the 1800s.<ref>{{cite journal | last1 = Einstein | first1 = A. | author-link2 = Marcel Grossmann | last2 = Grossmann | first2 = M. |date= 1913 | title = Entwurf einer verallgemeinerten Relativitätstheorie und einer Theorie der Gravitation |trans-title= Outline of a Generalized Theory of Relativity and of a Theory of Gravitation | journal = Zeitschrift für Mathematik und Physik | volume = 62 | pages = 225–261 }}</ref>
In 1915, he devised the [[Einstein field equations]] which relate the curvature of spacetime with the mass, energy, and any momentum within it.

Some of the consequences of general relativity are:
* [[Gravitational time dilation]]: Clocks run slower in deeper gravitational wells.<ref>
{{cite book
 |title=Feynman Lectures on Gravitation
 |first1=Richard Phillips |last1=Feynman
 |first2=Fernando B. |last2=Morínigo
 |first3=William |last3=Wagner
 |first4=David |last4=Pines
 |first5=Brian |last5=Hatfield
 |publisher=West view Press
 |date=2002
 |isbn=978-0-8133-4038-8
 |page=68
 |url=https://books.google.com/books?id=jL9reHGIcMgC
}}{{Dead link|date=January 2023 |bot=InternetArchiveBot |fix-attempted=yes }}, Lecture 5</ref>
* [[precession#Relativistic (Einsteinian)|Precession]]: Orbits 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).
* [[General relativity#Light deflection and gravitational time delay|Light deflection]]: Rays of [[light]] bend in the presence of a gravitational field.
* [[Frame-dragging]]: Rotating masses "drag along" the [[spacetime]] around them.
* [[Expansion of the universe]]: The universe is expanding, and certain components within the universe can [[accelerated expansion|accelerate the expansion]].

Technically, general relativity is a 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.