Classical and Relativistic Derivation of the Sagnac Effect
  Both the classical and the relativistic composition law for velocities are applied to re-calculate the Sagnac Effect. The ensuing formulae for the fringe shift are found to differ already in first order of v/c. Whilst the classical formula is validated by interferometric measurements and verified by the GPS-system, this is not the case for the relativistic result. 
Wolfgang Engelhardt
physics.gen-ph physics.optics