The Sagnac Effect : a historical study of the discovery and of its earlier interpretations

In 1913 the French physicist Georges M. M. Sagnac discovered an effect now called after him: a fringe shift in a rotational interferometer. This effect was verified many times and is nowadays used in aerospace navigation. The discovery was connected to previous researches of a proof of the relative motion between Ether and Earth. In fact, a similar experiment was already well suggested by Sir O. Lodge in 1893. Sagnac himself asserted that his experiment was the proof of ether’s existence and a rebuttal of the postulate of the constancy of the velocity of light. In the development of relativity this effect was included in the studies on rotational frames and there were many derivations of it in both GRT and SRT frameworks. There is a strong debate on the relativistic derivation of the Sagnac effect today. In this debate, there are some unthorough historical reviews on the Sagnac effect, which furthermore are, at the best of my knowledge, all part of a theoretical derivation of the effect itself. I want to fill this lack with a pure historical analysis and by asking some questions on the relations between a theory and an observational proof in the historical process of Physics.