In physics, mass–energy equivalence is the concept that mass and energy are the same thing, so that every mass has an energy equivalent, and vice versa. This relationship is expressed using the formula

E = m c2


E = energy m = mass c = the speed of light in a vacuum (celeritas), (about 3×108 m/s) In words: energy equals mass multiplied by the speed of light squared. Because the speed of light is a very large number in everyday units, the formula implies that any small amount of matter contains a very large amount of energy. Some of this energy may be released as heat and light by nuclear transformations.

Mass energy equivalence was proposed by Albert Einstein in 1905, in the paper "Does the inertia of a body depend upon its energy-content?", one of his Annus Mirabilis ("Miraculous Year") Papers.[1] While Einstein was not the first to propose a mass–energy relationship, and various similar formulas appeared before Einstein's theory, Einstein was the first to propose that the equivalence of mass and energy is a general principle and a consequence of the symmetries of space and time.

In the formula, c2 is the conversion factor required to convert from units of mass to units of energy. The formula does not depend on a specific system of units. In the International System of Units, the unit for energy is the joule, for mass the kilogram, and for speed meters per second. Note that 1 joule equals 1 kg·m2/s2. In unit-specific terms, E (in joules) = m (in kilograms) multiplied by (299,792,458 m/s)2.

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