Solar Eclipse of May 29 1919. Sun gravity bends the light: Eddington, the "proof" of GR and the myth of a bias. (please note: not a lecture on GR and Gravitational lensing!)
Bruno Marano (DIFA Univ. di Bologna)
Tuesday 28/05/2019 @ 14:00, Sala IV piano Battiferro
In 1919 Eddington's measurement represented an "experimentum crucis" showing that the deflection of light in the vicinity of the Sun was consistent with the General Relativity space-time distortion (Einstein 1917), and inconsistent with Newtonian gravity. The result was close to the 1".7 displacement, foreseen by the GR theory, ruling out the 0".8 displacement calculated by assigning a mass to the light in a flat, Euclidean space. An enthusiastic reaction of some media, as the London Times, made Einstein, Eddington and the experiment popular in a night. The experiment was "extreme" for the time (and 1919 was really a hard time!). Existing instruments, not necessarily in perfect shape, were shipped and assembled in remote places, and only a few plates could be taken in the few minutes available. Heating and stability of the telescopes were a problem for precise astrometry. At end some plates were discarded, for being considered either of too poor quality, or affected by errors in the reduction. Their inclusion would have eventually led to inconclusive results. This gave birth to the allegation, yet surviving, that Eddington did bias the results. Myth or reality? In the following decades, new optical measurements during eclipses gave confirmation of the 1".7 deviation. Radio interferometry performed after the '70s gave a further, independent, high precision confirmation. Some of Eddington's original plates, and reduction records, were conserved at the Royal Greenwich Observatory. They have been re-analysed with modern techniques in 1978. The outcome, almost unpublished, was that the original selection was based on fair criteria. The original results and the claim of a 0".3 error were confirmed. In conclusion, Dyson, Eddington and Davidson succeeded in performing a real "experimentum crucis". Maybe it did not "prove" the GR, but "falsified" Newton's theory.