Massenergy

Mass energy, or mass–energy, refers to the concept that mass and energy are related manifestations of the same physical quantity as described by special relativity. The rest energy of a particle with invariant mass m0 is E0 = m0 c^2, where c is the speed of light. The total energy E includes kinetic energy and the momentum contribution, and the mass–energy equivalence is commonly expressed by the equation E = mc^2. More generally, for a particle with rest mass m0 and momentum p, the energy–momentum relation is E^2 = (pc)^2 + (m0 c^2)^2. In the nonrelativistic limit, E ≈ m0 c^2 + p^2/(2m0). The invariant mass remains constant in any closed process; the notion of relativistic mass is rarely used in modern physics.

Mass-energy conservation states that in isolated systems, the total mass-energy E is conserved, though mass can be converted into energy and vice versa. In nuclear and particle physics, binding energy and mass defects mean that bound systems weigh less than the sum of their separate constituents, with the difference appearing as energy released or absorbed. Annihilation of particles with antiparticles converts mass entirely into energy, typically photons, illustrating the practical equivalence of mass and energy.

Historically, Einstein introduced the relation in 1905, linking mass and energy and enabling a quantitative framework for understanding nuclear processes, particle decays, and cosmological phenomena. The concept remains central across physics, from high-energy experiments to astrophysical calculations and models of the universe.