The equivalence of mass and energy, also called the conservation of mass-energy, states that in any closed system of interacting bodies the total mass (m) plus the total energy (E) is constant. The mass-energy equivalence is expressed by the equation E=mc2 with the constant c2 being the square of the speed of light.

The law of conservation of mass-energy is a generalization of the classical laws of conservation of energy and conservation of mass. Under the classical (or Newtonian) laws of physics, mass and energy are two separate entities, both with separate conservation laws. The conservation law of energy stated by Prussian physicist Hermann von Helmholtz (1821-1894) asserts that in a closed system energy is conserved; that is, the total energy of the system remains constant over time. Energy could be transformed from one form to another (for example...