The Sagnac Effect as Hydrodynamic Drift: Empirical Evidence for the Substratum and Violation of Light Speed Constancy in Rotating Frames

The central postulate of Special Relativity (SR)—the invariance of the speed of light (c) in all reference frames—faces contradictions with optical phenomena observed in rotating systems. The Global Positioning System (GPS) and Ring Laser Gyroscopes (RLG) operate based on the Sagnac effect, which mathematically necessitates anisotropy in light propagation speed (c ± v). This paper proposes a physical interpretation of this phenomenon within the framework of Substratum Hydrodynamics [4, 7]. Through the analysis of historical experiments and modern satellite data, we demonstrate that the Sagnac effect represents a classical “Ether Drift.” This confirms that light propagates relative to a fundamental physical medium (the Substratum), locally associated with the Earth’s gravitational center, rather than relative to the source or receiver. This empirical fact validates the variable light speed mechanism and the material nature of the vacuum.