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In physics, the energy–momentum relation, or relativistic dispersion relation, is the relativistic equation relating total energy to invariant mass and momentum. It is the extension of mass–energy equivalence for bodies or systems with non-zero momentum. It can be formulated as: This equation holds for a body or system, such as one or more particles, with total energy E, invariant mass m0, and momentum of magnitude p; the constant c is the speed of light. It assumes the special relativity case of flat spacetime and that the particles are free. Total energy is the sum of rest energy E 0 = m 0 c 2 and relativistic kinetic energy: E K = E − E 0 = 2 + 2 − m 0 c 2 Invariant mass is mass measured in a center-of-momentum frame. For bodies or systems with zero momentum, it simplifies to the mass–energy equation E 0 = m 0 c 2, where total energy in this case is equal to rest energy. The Dirac sea model, which was used to predict the existence of antimatter, is closely related to the energy–momentum relation.