Antiparticle

During the late 1920s, the British physicist Paul Dirac attempted to modify the currently accepted model of the atom by including in it the relativistic properties of electrons. As a result of his analysis, Dirac found that electrons should be expected to exist in two energy states, one positive and one negative. The concept of positive energy presents no problems, of course, but Dirac and other physicists were uncertain as to the meaning of a negative energy state. What did it mean to say that an electron had less than zero energy?

Eventually Dirac concluded that the negative energy state for an electron might imply the existence of a kind of electron that no one had yet imagined, one that is identical with the familiar negatively-charged electron in every respect except its charge. It was, Dirac suggested, an electron with a positive charge, or an antielectron.

Within five years, Dirac's hypothesis had been confirmed. In 1932, the American physicist Carl Anderson found in photographs of a cosmic ray shower the tracks of a particle that satisfied all the properties predicted by Dirac for the antielectron. Anderson suggested the name positron for the new particle.