Clarifying Einstein's First Postulate of Special Relativity, post BIPM 2019

Since 1905, all aspects of the Theory of Special Relativity (SR) have been confirmed experimentally. Today, the knowledge derived from SR is central to well-known technologies such as GPS, and the BIPM publishes the speed of light in vacuum as 299 792 458 m/s. We may therefore conclude that SR works well within its tested domain. That said, certain conceptual aspects of the theory still benefit from clarification. This article focuses on clarifying the empirical content implicitly assumed in Einstein’s first postulate, in a way that aligns with measurable electromagnetic properties of the vacuum and with the modern understanding of symmetry principles. The implications of the 2019 SI redefinition for ε0 and µ0 are examined, followed by an analysis of the epistemological ambiguity in the traditional wording of the first postulate. Three mathematical formulations of the postulate are presented, leading to a clarified empirical formulation. The compatibility of this clarification with Maxwell’s equations and with Lorentz transformations is then demonstrated. The classical textbooks of French (1968) and Morin (2017), separated by nearly fifty years, present the first postulate in essentially identical form, showing that SR is a mature and well-established theory. However, this stability also reveals that the traditional formulation has never been conceptually refined despite its ambiguity. The goal of this article is not to contradict Einstein or standard textbooks, but to clarify the empirical content of the first postulate in a manner consistent with classical electromagnetism, Lorentz symmetry, and modern metrological conventions.