What happens when an electron and a positron come together under the influence of their opposite electrical charges. Both particles are totally annihilated, their lost mass becoming energy in the form of two gamma ray photons. Since the rest mass of an electron or positron is 0.51 MeV, the resulting two photons must each have an energy of the same amount plus half the kinetic energy of each of the particles relative to their center of mass. The directions of the photons are such as to conserve both linear momentum and energy. Unlike the inverse operation, pair production, this process can occur in empty space. It's E=mc² in its purest form.