Mass–energy equivalence

Mass–energy equivalence is the principle that a body's rest mass corresponds to energy according to E = mc², and that mass and energy can be converted into one another.

Rest energy: The energy associated with an object's invariant mass, equal to its mass multiplied by the square of the speed of light. E = mc² gives the energy associated with mass even when the object is at rest.
Nuclear fission: A nuclear reaction in which a heavy atomic nucleus splits into lighter nuclei, releasing energy and usually neutrons. The products have slightly less mass than the initial nucleus; the difference emerges as energy.
Albert Einstein: A theoretical physicist who developed special and general relativity and made major contributions to quantum theory. Einstein derived the mass–energy relation in work published in 1905.
Classical mechanics: The framework describing motion and forces in which space and time are treated as absolute and relativistic effects are neglected. Its ordinary treatment keeps mass separate from energy rather than relating them through relativistic dynamics.
Invariant mass: The mass of a system measured in its center-of-momentum frame, independent of the observer's inertial frame. It is the mass in the relation between a system's rest energy and its energy.
Nuclear fusion: A nuclear reaction in which light nuclei combine to form a heavier nucleus, often releasing energy. Fusion releases energy when the final nucleus has less mass than its ingredients.
Special relativity: A theory of space, time, and motion in inertial frames, founded on the constancy of light speed and the relativity principle. Mass–energy equivalence follows from the relativistic structure of energy and momentum.
Relativistic mass: An older convention defining an object's mass as its total energy divided by c², making it velocity-dependent. It can obscure that E = mc² specifically uses invariant mass for rest energy.
Relativistic energy: The total energy of an object in special relativity, including rest energy and kinetic energy. This broader energy includes both rest energy and motion-dependent contributions.
Nuclear binding energy: The energy required to separate a nucleus into its constituent protons and neutrons. A nucleus's mass is lower than the separated particles' total mass by its binding energy divided by c².

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