Equations expressing the relationship between mass and energy of a body.

The equation from Einstein’s theory of special relativity expressing that matter can change into energy and vice versa. The energy of anybody at rest is equal to its rest mass multiplied by the speed of light squared. Theoretically, it is possible to convert a small amount of mass into a large amount of energy. In nuclear reactions (fusion or fission), some of the rest mass of the nuclei of atoms is indeed converted into energy. The sum of the masses of the particles entering the reaction is greater than the sum of the masses of the particles leaving the reaction. Part of their mass has been converted into energy. The complete conversion of mass to energy is annihilation, the reaction of a particle with its antiparticle.