# Mass-energy-equivalence definition

The physical principle that a measured quantity of energy is equivalent to a measured quantity of mass. The equivalence is expressed by Einstein's equation

*E = mc*^{2}, where*E*represents energy,*m*the equivalent mass, and*c*the speed of light.noun

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*, where^{2}*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.
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