The relation between mass and energy in the Theory of Special Relativity: On the history of the formulation of E = mc²

Abstract

This article demystifies the origin of the equation E=mc2, showing that it was not proposed by Einstein in 1905 in the form we know today. Instead, the mass–energy relationship emerged gradually through conceptual refinements, contributions from several scientists, and significant notational changes. In the 1905 paper, Einstein expressed the idea verbally and with different symbols, such as L=mV2, stating that the mass of a body is a measure of its energy content. Earlier discussions by Henri Poincaré in 1900 and Friedrich Hasenhörl in 1904 had already addressed concepts related to electromagnetic inertia and mass–energy relationship. Einstein initially used L for energy and V for the speed of light, adopting E and c only around 1912 in an unpublished manuscript. The simplified form E=mc2 only gained prominence in the 1940s, driven by the Manhattan Project and popular culture. The article distinguishes between symbolic representation and physical meaning, emphasizing that the conceptual essence of the equation was already present in 1905, while its modern notation emerged only decades later. The epistemological and historiographical implications reveal how scientific knowledge evolves gradually and collectively, highlighting Einstein's unifying role within a broader historical context.