It is a well-established idea in physics that for every kind of subatomic particle that exists, there is also a corresponding antiparticle. This idea was proposed by Paul Dirac in 1928. He worked out a version of quantum mechanics that was consistent with the Special Theory of Relativity and found that it had solutions both for electrons and for particles of the opposite electric charge. These were later observed and dubbed positrons, confirming Dirac's prediction.