EPSC Abstracts Vol. 13, EPSC-DPS2019-1908-1, 2019 EPSC-DPS Joint Meeting 2019 c Author(s) 2019. CC Attribution 4.0 license.

No Shadow of Doubt: Eddington, Dyson and the Eclipse of 1919

Daniel Kennefick (1)

(1) University of Arkansas, Fayetteville, AR 72701, USA ([email protected]) Abstract to include the lesser-known figures like Dyson and his assistants allows us to properly assess the quality The 1919 eclipse expeditions to test the light-bending of the science performed in 1919. deflection of Einstein’s theory of General Relativity are amongst the best known experiments in the 2. The forgotten people of 1919 history of science. Many modern accounts focus on the celebrated figures of Einstein himself and Arthur Eddington and Dyson led the planning of the Stanley Eddington, leader of the expedition to the expeditions as directors of their respective island of Principe off the coast of Africa. This paper observatories. Eddington personally led the will draw attention to other participants in the Cambridge expedition to Principe. However his plans expeditions, especially England’s Astronomer Royal, were foiled by cloud and the data he did obtain were Frank Watson Dyson, who planned the expeditions of limited value. Dyson sent two of his Greenwich and who was in charge of the expedition to Sobral in assistants, Charles Rundle Davidson and Andrew C. Brazil. It was this expedition, not the one led by D. Crommelin to Sobral, Brazil. They were more Eddington which provided the data which was most fortunate with the weather, but one of their two important in confirming Einstein’s prediction. telescopes, an astrographic, malfunctioned. The other, a four-inch, performed well and its data was mostly relied on in the expeditions’ verdict in favor of 1. Introduction Einstein. The eclipse expeditions of 1919 mounted by the Cambridge and Greenwich observatories play a Examination of the data analysis notes from the central role in the history of 20th century physics. Sobral data made at Greenwich show that the They made Einstein into the celebrity that he remains decision to throw out the data from the astrographic to this day. They caused Newton’s theory of gravity telescope was taken by Dyson after an alternative to be displaced by Einstein’s theory of General data analysis procedure which showed that the Relativity. Despite the enormous reputation of these original data analysis result from that instrument expeditions, in recent decades some controversy has could not be relied upon. Decades later, in the 1970s, surrounded them, especially concerning the role of a team at the Royal Greenwich Observatory (RGO) Arthur Stanley Eddington, It has been alleged that re-analyzed the plates with modern equipment and Eddington was so biased in favor of Einstein and his computers. There results vindicated not only the theory that he overstated the extent to which the result from the four-inch telescope but also the expeditions’ data confirmed relativity’s light alternative data-analysis from the astrographic deflection prediction [1]. However a close examine telescope [2]. of the records demonstrates that the key decisions in the data analysis were taken by Frank Watson Dyson 3. Summary and Conclusions and his assistants at the Royal Observatory, Greenwich. Dyson was skeptical of relativity and did Careful analysis of all available evidence shows that not share Eddington and Dyson’s anti-war views. the controversial decision to throw out the Sobral Moreover modern re-analysis of the plates taken by astrographic data was taken by Dyson, not by the the Greenwich team in Brazil supports his handling allegedly biased Eddington. Modern re-analysis of the data. In short all the evidence is that the verdict supports his decision. The available evidence is that in favor of Einstein’s prediction was taken for sound the data taken in 1919 was capable of determining reasons. Broadening the modern retelling of the story that Einsten’s theory fit the observations better than the existing Newtonian theory. Acknowledgements I would like to thank Raymond Walter for his help in studying the Sobral data analysis notes.

References [1] Earman, John and Glymour, Clark: “Relativity and Eclipses: The British Eclipse Expeditions of 1919 and Their Predecessors.” Historical Studies in the Physical Sciences, Vol. 11, pp. 49–85 (1981).

[2] Harvey, G. M.: “Gravitational Deflection of Light: A Re-examination of the Observations of the Solar Eclipse of 1919.” Observatory Vol. 99, pp. 195–98 (1979).

[3] Kennefick, Daniel: No Shadow of a Doubt: The 1919 Eclipse that Confirmed Einstein’s Theory of Relativity, Princeton University Press, 2019.