mass-energy equivalence An equation derived from Einstein's theory of Special Relativity expressing the relationship between the mass and energy of objects with mass. The equation is E = mc^{2}, where E is the energy of the object in joules, m is its relativistic mass in kilograms, and c is the speed of light (approximately 3 × 10^{8} meters per second). Mass-energy equivalence entails that the total mass of a system may change, although the total energy and momentum remain constant; for example, the collision of an electron and a proton annihilates the mass of both particles, but creates energy in the form of photons. The discovery of mass-energy equivalence was essential to the development of theories of atomic fission and fusion reactions.