The Sagnac Effect and General Relativity Foundations

The present note is motivated by an article published in the Annales de la Fondation Louis de Broglie. In that piece, Wolfgang Engelhardt presented an interesting and straightforward argument claiming the application of the Lorentz transformations predicts a null effect for a Sagnac-type experiment, thereby purportedly disproving relativity theory through empirical evidence. Given that outside specialists’ circles, there is widespread confusion over similar arguments involving rotational motion, discussing a correct relativist explanation has didactic relevance. It also allows for revisiting some aspects of the foundations of general relativity.