The Constancy of the Speed of Light: From Maxwell's Equations to the Michelson-Morley Experiment and Beyond

This paper investigates one of the most profound and enigmatic features of modern physics: the invariance of the speed of light. Unlike ordinary velocities in classical mechanics, the speed of light remains the same for all observers, regardless of the motion of the source or the observer. We review the historical path that led to this realization, beginning with Maxwell’s theoretical prediction of electromagnetic wave propagation and culminating in the Michelson–Morley experiment, which decisively challenged the notion of a luminiferous ether. We emphasize that the constancy of the speed of light is not something derived from deeper mechanical principles, but rather a fundamental postulate of nature that motivated the formulation of Special Relativity. The paper examines the experimental foundations that confirm this invariance, the theoretical framework that incorporates it, and the profound conceptual consequences of accepting a universal speed limit. While the invariance of the speed of light can be verified with extraordinary precision, we highlight that its ultimate origin remains unexplained, standing as an empirical fact about how nature operates rather than a phenomenon reducible to more basic assumptions.