Editing Gravity (section)

===Einstein's general relativity===
{{main| History of general relativity}}
{{General relativity sidebar}}
Eventually, astronomers noticed an eccentricity in the orbit of the planet [[Mercury (planet)|Mercury]] which could not be explained by Newton's theory: the [[perihelion]] of the orbit was increasing by about 42.98 [[arcseconds]] per century. The most obvious explanation for this discrepancy was an as-yet-undiscovered celestial body, such as a planet orbiting the Sun even closer than Mercury, but all efforts to find such a body turned out to be fruitless. In 1915, [[Albert Einstein]] developed a theory of [[general relativity]] which was able to accurately model Mercury's orbit.<ref>{{Cite journal |last=Nobil |first=Anna M. |date=March 1986 |title=The real value of Mercury's perihelion advance |journal=Nature |volume=320 |issue=6057 |pages=39–41 |bibcode=1986Natur.320...39N |doi=10.1038/320039a0 |s2cid=4325839 | issn=0028-0836}}</ref>

Einstein's theory brought two other ideas with independent histories into the physical theories of gravity: the [[principle of relativity]] and [[non-Euclidean geometry]]

The principle of relativity, introduced by Galileo and used as a foundational principle by Newton, lead to a long and fruitless search for a [[luminiferous aether]] after [[Maxwell's equations]] demonstrated that light propagated at a fixed speed independent of reference frame. In Newton's mechanics, velocities add: a cannon ball shot from a moving ship would travel with a trajectory which included the motion of the ship. Since light speed was fixed, it was assumed to travel in a fixed, absolute medium. Many experiments sought to reveal this medium but failed and in 1905 Einstein's [[special relativity]] theory showed the aether was not needed. Special relativity proposed that mechanics be reformulated to use the Lorentz transformation already applicable to light rather than the Galilean transformation adopted by Newton. Special relativity, as in [[special case]], specifically did not cover gravity.<ref name=Weinberg-1972/>{{rp|4}}

While relativity was associated with mechanics and thus gravity, the idea of altering geometry only joined the story of gravity once mechanics required the Lorentz transformations. Geometry was an [[history of geometry|ancient science]] that gradually broke free of Euclidean limitations when [[Carl Gauss]] discovered in the 1800s that [[hypersurface|surfaces in any number of dimensions]] could be characterized by a [[metric space|metric]], a distance measurement along the shortest path between two points that reduces to Euclidean distance at infinitesimal separation. Gauss' student [[Bernhard Riemann]] developed this into a complete geometry by 1854. These geometries are locally flat but have global [[curvature]].<ref name=Weinberg-1972/>{{rp|4}}

In 1907, Einstein took his first step by using special relativity to create a new form of the [[equivalence principle]]. The equivalence of inertial mass and gravitational mass was a known empirical law. The {{mvar|m}} in Newton's first law, <math>F=ma</math>, has the same value as the {{mvar|m}} in Newton's law of gravity on Earth, <math>F=GMm/r^2</math>.  
In what he later described as "the happiest thought of my life" Einstein realized this meant that in free-fall, an accelerated coordinate system exists with no local gravitational field.<ref>{{Cite web |last1=Webb |first1=Joh |last2=Dougan |first2=Darren |date=23 November 2015 |title=Without Einstein it would have taken decades longer to understand gravity |url=https://phys.org/news/2015-11-einstein-decades-longer-gravity.html#:~:text=In%201907%2C%20Einstein%20had%20the,not%20feel%20his%20own%20weight. |access-date=21 May 2022 |archive-date=21 May 2022 |archive-url=https://web.archive.org/web/20220521182328/https://phys.org/news/2015-11-einstein-decades-longer-gravity.html#:~:text=In%201907%2C%20Einstein%20had%20the,not%20feel%20his%20own%20weight. |url-status=live }}</ref> Every description of gravity in any other coordinate system must transform to give no field in the free-fall case, a powerful [[invariance]] constraint on all theories of gravity.<ref name=Weinberg-1972/>{{rp|20}}

Einstein's description of gravity was accepted by the majority of physicists for two reasons. First, by 1910 his special relativity was accepted in Germany physics and was spreading to other countries. Second, his theory explained experimental results like the perihelion of Mercury and the bending of light around the Sun better than Newton's theory.<ref>{{Cite journal |last=Brush |first=S. G. |date=1 January 1999 |title=Why was Relativity Accepted? |url=https://ui.adsabs.harvard.edu/abs/1999PhP.....1..184B |journal=Physics in Perspective |volume=1 |issue=2 |pages=184–214 |doi=10.1007/s000160050015 |bibcode=1999PhP.....1..184B |s2cid=51825180 |issn=1422-6944 |access-date=22 May 2022 |archive-date=8 April 2023 |archive-url=https://web.archive.org/web/20230408021700/https://ui.adsabs.harvard.edu/abs/1999PhP.....1..184B |url-status=live }}</ref> 

In 1919, the British astrophysicist [[Arthur Eddington]] was able to confirm the predicted the deflection of light during [[Solar eclipse of May 29, 1919|that year's solar eclipse]].<ref>{{cite journal |last1=Dyson |first1=F.W. |author-link1=Frank Watson Dyson |last2=Eddington |first2=A.S. |author-link2=Arthur Eddington |last3=Davidson |first3=C.R. |date=1920 |title=A Determination of the Deflection of Light by the Sun's Gravitational Field, from Observations Made at the Total Eclipse of May 29, 1919 |url=https://zenodo.org/record/1432106 |url-status=live |journal=[[Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences|Phil. Trans. Roy. Soc. A]] |volume=220 |issue=571–581 |pages=291–333 |bibcode=1920RSPTA.220..291D |doi=10.1098/rsta.1920.0009 |archive-url=https://web.archive.org/web/20200515065314/https://zenodo.org/record/1432106 |archive-date=15 May 2020 |access-date=1 July 2019 |doi-access=free}}. Quote, p. 332: "Thus the results of the expeditions to Sobral and Principe can leave little doubt that a deflection of light takes place in the neighbourhood of the sun and that it is of the amount demanded by Einstein's generalised theory of relativity, as attributable to the sun's gravitational field."</ref><ref>{{cite book |last=Weinberg |first=Steven |url=https://archive.org/details/gravitationcosmo00stev_0 |title=Gravitation and cosmology |date=1972 |publisher=John Wiley & Sons |isbn=9780471925675 |author-link=Steven Weinberg |url-access=registration}}. Quote, p. 192: "About a dozen stars in all were studied, and yielded values 1.98 ± 0.11" and 1.61 ± 0.31", in substantial agreement with Einstein's prediction θ<sub>☉</sub> = 1.75"."</ref> Eddington measured starlight deflections twice those predicted by Newtonian corpuscular theory, in accordance with the predictions of general relativity. Although Eddington's analysis was later disputed, this experiment made Einstein famous almost overnight and caused general relativity to become widely accepted in the scientific community.<ref>{{Cite journal |last1=Gilmore |first1=Gerard |last2=Tausch-Pebody |first2=Gudrun |date=20 March 2022 |title=The 1919 eclipse results that verified general relativity and their later detractors: a story re-told |journal=Notes and Records: The Royal Society Journal of the History of Science |volume=76 |issue=1 |pages=155–180 |doi=10.1098/rsnr.2020.0040|s2cid=225075861 |doi-access=free |arxiv=2010.13744 }}</ref>

In 1959, American physicists [[Robert Pound]] and [[Glen Rebka]] performed [[Pound–Rebka experiment|an experiment]] in which they used [[gamma ray]]s to confirm the prediction of [[gravitational time dilation]]. By sending the rays down a 74-foot tower and measuring their frequency at the bottom, the scientists confirmed that light is [[Doppler shift]]ed as it moves towards a source of gravity. The observed shift also supports the idea that time runs more slowly in the presence of a gravitational field (many more wave crests pass in a given interval). If light moves outward from a strong source of gravity it will be observed with a [[redshift]].<ref>{{Cite web |title=General Astronomy Addendum 10: Graviational Redshift and time dilation |url=https://homepage.physics.uiowa.edu/~rlm/mathcad/addendum%2010%20gravitational%20redshift%20and%20time%20dilation.htm |access-date=29 May 2022 |website=homepage.physics.uiowa.edu |archive-date=14 May 2022 |archive-url=https://web.archive.org/web/20220514063358/https://homepage.physics.uiowa.edu/~rlm/mathcad/addendum%2010%20gravitational%20redshift%20and%20time%20dilation.htm |url-status=live }}</ref> The [[time delay of light]] passing close to a massive object was first identified by [[Irwin I. Shapiro]] in 1964 in interplanetary spacecraft signals.<ref>{{Cite journal |last=Asada |first=Hideki |date=20 March 2008 |title=Gravitational time delay of light for various models of modified gravity |url=https://www.sciencedirect.com/science/article/pii/S0370269308001810 |journal=Physics Letters B |volume=661 |issue=2–3 |pages=78–81 |doi=10.1016/j.physletb.2008.02.006 |arxiv=0710.0477 |bibcode=2008PhLB..661...78A |s2cid=118365884 |language=en |access-date=29 May 2022 |archive-date=29 May 2022 |archive-url=https://web.archive.org/web/20220529140019/https://www.sciencedirect.com/science/article/pii/S0370269308001810 |url-status=live }}</ref>

In 1971, scientists discovered the first-ever black hole in the galaxy [[Cygnus A|Cygnus]]. The black hole was detected because it was emitting bursts of [[x-rays]] as it consumed a smaller star, and it came to be known as [[Cygnus X-1]].<ref>{{Cite web |title=The Fate of the First Black Hole |url=https://www.science.org/content/article/fate-first-black-hole |access-date=30 May 2022 |website=www.science.org |language=en |archive-date=31 May 2022 |archive-url=https://web.archive.org/web/20220531125138/https://www.science.org/content/article/fate-first-black-hole |url-status=live }}</ref> This discovery confirmed yet another prediction of general relativity, because Einstein's equations implied that light could not escape from a sufficiently large and compact object.<ref>{{Cite web |title=Black Holes Science Mission Directorate |url=https://webarchive.library.unt.edu/web/20170124200640/https://science.nasa.gov/astrophysics/focus-areas/black-holes |access-date=30 May 2022 |website=webarchive.library.unt.edu |archive-date=8 April 2023 |archive-url=https://web.archive.org/web/20230408021657/https://webarchive.library.unt.edu/web/20170124200640/https://science.nasa.gov/astrophysics/focus-areas/black-holes |url-status=live }}</ref>

[[Frame dragging]], the idea that a rotating massive object should twist spacetime around it, was confirmed by [[Gravity Probe B]] results in 2011.<ref>{{cite web |url=http://www.nasa.gov/home/hqnews/2011/may/HQ_11-134_Gravity_Probe_B.html |title=NASA's Gravity Probe B Confirms Two Einstein Space-Time Theories |publisher=Nasa.gov |access-date=23 July 2013 |archive-date=22 May 2013 |archive-url=https://web.archive.org/web/20130522024606/http://www.nasa.gov/home/hqnews/2011/may/HQ_11-134_Gravity_Probe_B.html |url-status=live }}</ref><ref>{{Cite web |title="Frame-Dragging" in Local Spacetime |url=https://einstein.stanford.edu/content/education/lithos/litho-fd.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://einstein.stanford.edu/content/education/lithos/litho-fd.pdf |archive-date=9 October 2022 |url-status=live |website=Stanford University}}</ref> In 2015, the [[LIGO]] observatory detected faint [[gravitational waves]], the existence of which had been predicted by general relativity. Scientists believe that the waves emanated from a [[black hole merger]] that occurred 1.5 billion [[light-years]] away.<ref>{{Cite news |title=Gravitational Waves Detected 100 Years After Einstein's Prediction |url=https://www.ligo.caltech.edu/news/ligo20160211 |access-date=30 May 2022 |newspaper=Ligo Lab &#124; Caltech |archive-date=27 May 2019 |archive-url=https://web.archive.org/web/20190527101043/https://www.ligo.caltech.edu/news/ligo20160211 |url-status=live }}</ref>