[Revised entry by Francisco Fernflores on October 1, 2026.
Changes to: Main text, Bibliography]
Einstein correctly described the equivalence of mass and energy as “the most important upshot of the special theory of relativity” (Einstein 1919). Many commentators have observed that he did not write the famous equation (E = mc^2)in his first derivation (Einstein 1905b). Instead, Einstein concluded that if an object, which is at rest relative to an inertial frame, either absorbs or emits an amount of energy (L), its inertial mass will correspondingly either increase or decrease by an amount (L/c^2). In…

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