In physics, a reaction in which a particle and its antiparticle (see antimatter) collide and disappear. The annihilation releases energy equal to the original mass math.m multiplied by the square of the speed of light math.c, or math.E = math.mmath.c², in accordance with Albert Einstein's special theory of relativity. The energy can appear directly as gamma rays or can convert back to particles and antiparticles (see pair production).

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