Whose theory was E=mc²?

E=mc² was derived by Albert Einstein in his landmark 1905 paper "Does the Inertia of a Body Depend Upon Its Energy Content?" — part of his Special Theory of Relativity. The equation states that energy (E) equals mass (m) multiplied by the speed of light squared (c²). Since c = 299,792,458 m/s, even a small amount of mass converts to a staggering amount of energy. For example, 1 kg of matter fully converted would release about 9 × 10¹⁶ joules — equivalent to 21.5 megatons of TNT. This principle underlies nuclear reactions: in fission (splitting atoms) and fusion (combining atoms), a small fraction of mass is converted to energy. The Sun fuses 620 million tonnes of hydrogen per second, losing 4.3 million tonnes as pure energy via E=mc².