Bending of Light in the 1919 Eclipse Experiments: Einstein and Eddington

Britain's Royal Astronomical Society organized two expeditions to observe the 1919 eclipse of the sun in order to provide experimental evidence for Einstein's General Theory of Relativity. One of the parties (A.S. Eddington and E. Cottingham) was to observe the eclipse in Principe, an island off the coast of Africa, and the other (A. Crommelin and C. Davidson) in Sobral, Brazil. Complete report of the expeditions was published by Dyson et al. (1920). Nevertheless, a review of the literature shows that not only were the expeditions difficult to conduct, but the data produced also generated considerable amount of controversy even up to the present (Brush, 1989, 1999; Collins & Pinch 1993, 1998; Earman & Glymour, 1980; Hentschel, 1992; Hudson, 2003; Mayo, 2003; Moyer, 1979; Sachs, 1988; Sponsel, 2002; Stanley, 2003; Will, 1990).

Frank Watson Dyson, the Astronomer Royal, and Arthur Stanley Eddington played leading roles in the organization of the expeditions and later in the interpretation of the results, in order to provide support for Einstein's General Theory of Relativity. According to Dyson et al. (1920), “The purpose of the expeditions was to determine what effect, if any, is produced by a gravitational field on the path of a ray of light traversing it” (p. 291), and expected the following possible alternative results: (a) the path of a ray of light is uninfluenced by gravitation; (b) the energy or mass of light is subject to Newton's law of gravitation, which leads to a deflection of 0.87″; (c) the course of a ray of light is in accordance with Einstein's General Theory of Relativity, which leads to a deflection of 1.75″. The predictions of both Newton's and Einstein's values for deflection of light in the sun's gravitational field were well known in the literature before the expeditions were undertaken. Both Dyson and Eddington took particular care to inform the public through the popular press, with respect to how results from the eclipse expeditions would help to corroborate either of the two theories, viz., Newton's or Einstein's (cf. Sponsel, 2002).