The famous equation relating energy and mass that Albert Einstein published in 1905 is E=mc². It states that energy E equals mass m multiplied by the speed of light c squared, revealing that mass and energy are different forms of the same physical quantity.
Because the speed of light is extremely large, squaring it produces an enormous conversion factor. Even a tiny amount of mass can therefore correspond to a very large amount of energy. This principle helps explain the energy released in nuclear fission and fusion, as well as the energy produced by stars.
Einstein introduced the relationship in a short 1905 paper connected with his special theory of relativity. The complete relativistic relationship is more general than the familiar formula: E²=(pc)²+(mc²)², where p is momentum. For an object at rest, momentum is zero and the rest-energy equation becomes E=mc².
A common mix-up is confusing this equation with E=hf, which links a photon's energy to its frequency, or F=ma, Newton's relationship between force and acceleration.