The equation expressing mass–energy equivalence in special relativity is E=mc².
In this formula, E is rest energy, m is an object’s rest mass, and c is the speed of light in vacuum. Because c is approximately 300,000 kilometres per second and is squared, even a small amount of mass corresponds to an enormous amount of energy.
Albert Einstein published the key result in 1905 in his paper “Does the Inertia of a Body Depend Upon Its Energy-Content?” The compact modern notation became famous as a consequence of special relativity, which unifies measurements of space and time for observers moving at constant velocity.
A frequent mistake is treating E=mc² as the full energy formula for every moving object. For a system with momentum, the broader relation is E²=(pc)²+(mc²)²; E=mc² is the zero-momentum, rest-energy case. Nuclear reactions release energy because the products have slightly less rest mass than the starting particles.