In physics,

**mass–energy equivalence**is the concept that the mass of a body is a measure of its energy content. This means, for example, that the total internal energy*E*of a body at rest is equal to the product of its rest mass*m*and a suitable conversion factor to transform from units of mass to units of energy. Albert Einstein proposed mass–energy equivalence in 1905. The equivalence is described by the famous equation:
where

*E*is energy,*m*is mass, and*c*is the speed of light in a vacuum. The equation*E*=*mc*^{2 }indicates that energy always exhibits relativistic mass in whatever form the energy takes. Mass–energy equivalence does not imply that mass may be "converted" to energy, but it allows for*matter*to be converted to energy. Through all such conversions,*mass*remains conserved, since it is a property of matter and any type of energy. In physics, mass must be differentiated from matter. Matter, when seen as certain types of particles,*can*be created and destroyed (as in particle annihilation or creation), but the system of precursors and products of such reactions, as a whole, retain both the original mass and energy, with each of these system properties remaining unchanged (conserved) throughout the process. Simplified, this means that the total amount of energy (*E*) before the experiment is equal to the amount of energy after the experiment. Letting the*m*in*E*=*mc*^{2}stand for a quantity of "matter" (rather than mass) may lead to incorrect results, depending on which of several varying definitions of "matter" are chosen.
Einstein was not the first to propose a mass–energy relationship. However, Einstein was the first scientist to propose the

*E*=*mc*^{2}formula and the first to interpret mass–energy equivalence as a fundamental principle that follows from the relativistic symmetries of space and time.*adapted from*Wikipedia