Editing Arthur Eddington (section)

==Relativity==

During [[World War I]], Eddington was secretary of the [[Royal Astronomical Society]], which meant he was the first to receive a series of letters and papers from [[Willem de Sitter]] regarding Einstein's theory of general relativity. Eddington was fortunate in being not only one of the few astronomers with the mathematical skills to understand general relativity, but owing to his internationalist and pacifist views inspired by his Quaker religious beliefs,<ref name=Douglas1956>{{cite book |title=The Life of Arthur Eddington |first=A. Vibert |last=Douglas |pages=92–95 |publisher=Thomas Nelson and Sons |year=1956 |url=https://books.google.com/books?id=Bw0XAQAAMAAJ&pg=PA92 }}</ref><ref name=Chandrasekhar1983>{{cite book|title=Eddington: The Most Distinguished Astrophysicist of His Time |first=Subrahmanyan|last=Chandrasekhar|pages=25–26|publisher=Cambridge University Press|date=1983|isbn=978-0521257466}}</ref> one of the few at the time who was still interested in pursuing a theory developed by a German physicist. He quickly became the chief supporter and expositor of relativity in Britain. He and [[Astronomer Royal]] [[Frank Watson Dyson]] organized [[Eddington experiment|two expeditions]] to observe a [[Solar eclipse of May 29, 1919|solar eclipse in 1919]] to make the first empirical test of Einstein's theory: the measurement of the deflection of light by the Sun's gravitational field. In fact, Dyson's argument for the indispensability of Eddington's expertise in this test was what prevented Eddington from eventually having to enter military service.<ref name=Douglas1956 /><ref name=Chandrasekhar1983 />

When conscription was introduced in Britain on 2 March 1916, Eddington intended to apply for an exemption as a [[conscientious objector]].<ref name=Douglas1956 /> Cambridge University authorities instead requested and were granted an exemption on the ground of Eddington's work being of national interest. In 1918, this was appealed against by the [[Ministry of National Service]]. Before the appeal tribunal in June, Eddington claimed conscientious objector status, which was not recognized and would have ended his exemption in August 1918. A further two hearings took place in June and July, respectively. Eddington's personal statement at the June hearing about his objection to war based on religious grounds is on record.<ref name=Douglas1956 /> The [[Astronomer Royal]], [[Sir Frank Dyson]], supported Eddington at the July hearing with a written statement, emphasising Eddington's essential role in the [[Solar eclipse of May 29, 1919|solar eclipse]] expedition to [[Príncipe]] in May 1919. Eddington made clear his willingness to serve in the [[Friends' Ambulance Unit]], under the jurisdiction of the British [[Red Cross]], or as a harvest labourer. However, the tribunal's decision to grant a further twelve months' exemption from military service was on condition of Eddington continuing his astronomy work, in particular in preparation for the Príncipe expedition.<ref name=Douglas1956 /><ref name=Chandrasekhar1983 /> The war ended before the end of his exemption.

[[File:1919 eclipse positive.jpg|260px|thumb|right|One of Eddington's photographs of the total [[solar eclipse of 29 May 1919]], presented in his 1920 paper announcing its success, confirming [[Albert Einstein|Einstein]]'s theory that light "bends"]]
After the war, Eddington travelled to the island of Príncipe off the west coast of Africa to watch the [[Solar eclipse of May 29, 1919|solar eclipse of 29 May 1919]]. During the eclipse, he took pictures of the stars (several stars in the [[Hyades (star cluster)|Hyades cluster]], including [[Kappa Tauri]] of the constellation [[Taurus (constellation)|Taurus]]) whose line of sight from the Earth happened to be near the Sun's location in the sky at that time of year.<ref name="Eddington1920"/> This effect is noticeable only during a total solar eclipse when the sky is dark enough to see stars which are normally obscured by the Sun's brightness. According to the theory of [[general relativity]], stars with light rays that passed near the Sun would appear to have been slightly shifted because their light had been curved by its gravitational field. Eddington showed that Newtonian gravitation could be interpreted to predict half the shift predicted by Einstein.

Eddington's observations published the next year<ref name="Eddington1920">{{cite journal|last=Dyson|author2=Eddington, A.S.|author3=Davidson, C.R. |date=1920 |title=A Determination of the Deflection of Light by the Sun's Gravitational Field, from Observations Made at the Solar eclipse of May 29, 1919|journal= [[Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences|Philosophical Transactions of the Royal Society A]]|volume=220|issue=571–581|pages= 291–333|bibcode=1920RSPTA.220..291D|doi=10.1098/rsta.1920.0009|first =F.W.|url=https://zenodo.org/record/1432106|doi-access=free}}</ref> allegedly confirmed Einstein's theory, and were hailed at the time as evidence of general relativity over the Newtonian model. The news was reported in newspapers all over the world as a major story. Afterward, Eddington embarked on a campaign to popularize relativity and the expedition as landmarks both in scientific development and international scientific relations.<ref>{{Cite journal|last=Sponsel|first=Alistair|date=2002|title=Constructing a 'Revolution in Science': The Campaign to Promote a Favourable Reception for the 1919 Solar Eclipse Experiments|url=https://www.jstor.org/stable/4028276|journal=The British Journal for the History of Science|volume=35|issue=4|pages=439–467|doi=10.1017/S0007087402004818|jstor=4028276|s2cid=145254889|issn=0007-0874}}</ref>

It has been claimed that Eddington's observations were of poor quality, and he had unjustly discounted simultaneous observations at [[Sobral, Brazil]], which appeared closer to the Newtonian model, but a 1979 re-analysis with modern measuring equipment and contemporary software validated Eddington's results and conclusions.<ref>{{Cite journal|title=Not Only Because of Theory: Dyson, Eddington and the Competing Myths of the 1919 Eclipse Expedition|first=Daniel|last=Kennefick|journal=Studies in History and Philosophy of Science Part A |date=5 September 2007|doi=10.1016/j.shpsa.2012.07.010|arxiv=0709.0685|bibcode = 2007arXiv0709.0685K|s2cid=119203172}}</ref> The quality of the 1919 results was indeed poor compared to later observations, but was sufficient to persuade contemporary astronomers. The rejection of the results from the expedition to Brazil was due to a defect in the telescopes used which, again, was completely accepted and well understood by contemporary astronomers.<ref name="PhysToday">{{Cite journal|title=Testing relativity from the 1919 eclipse – a question of bias|first=Daniel|last=Kennefick|date=1 March 2009|journal=Physics Today|volume=62|issue=3|pages=37–42|doi=10.1063/1.3099578|bibcode = 2009PhT....62c..37K|doi-access=free}}</ref>

[[File:Minutes of Eddington talk to the Del-Squared V Club.jpg|thumb|The minute book of Cambridge ∇<sup>2</sup>V Club for the meeting where Eddington presented his observations of the curvature of light around the Sun, confirming Einstein's theory of general relativity. They include the line "A general discussion followed. The President remarked that the 83rd meeting was historic".]]

Throughout this period, Eddington lectured on relativity, and was particularly well known for his ability to explain the concepts in lay terms as well as scientific. He collected many of these into the ''Mathematical Theory of Relativity'' in 1923, which [[Albert Einstein]] suggested was "the finest presentation of the subject in any language." He was an early advocate of Einstein's general relativity, and an interesting anecdote well illustrates his humour and personal intellectual investment: [[Ludwik Silberstein]], a physicist who thought of himself as an expert on relativity, approached Eddington at the [[Royal Society]]'s (6 November) 1919 meeting where he had defended Einstein's relativity with his Brazil-Príncipe solar eclipse calculations with some degree of scepticism, and ruefully charged Arthur as one who claimed to be one of three men who actually understood the theory (Silberstein, of course, was including himself and Einstein as the other). When Eddington refrained from replying, he insisted Arthur not be "so shy", whereupon Eddington replied, "Oh, no! I was wondering who the third one might be!"<ref name="autogenerated1">As related by Eddington to Chandrasekhar and quoted in Walter Isaacson "Einstein: His Life and Universe", p. 262</ref>