The first picture of the positron (left) was taken in a Wilson cloud chamber. On the right is C. D. Anderson, the discoverer of the positron.
The proton is not the electron’s antiparticle. Though a proton carries the necessary positive charge that is exactly as large as the negative charge of the electron, the proton has a much larger mass than the electron has. Dirac’s theory required that the antiparticle have the same mass as the particle to which it corresponded.
In 1932 C. D. Anderson was studying the impact of cosmic particles on lead. In the process, he discovered signs of a particle that left tracks exactly like those of an electron, but tracks that curved the wrong way in a magnetic field. This was a sure sign that it had an electric charge opposite to that of the electron. He had, in short, discovered the electron’s antiparticle and this came to be called the “positron”.
Positrons were soon detected elsewhere too. Some radioactive isotopes, formed in the laboratory by the Joliot-Curies and by others, were found to emit positive beta particles—positrons rather than electrons. When an ordinary beta particle, or electron, was emitted from a nucleus, a neutron within the nucleus was converted to a proton. When a positive beta particle, a positron, was emitted, the reverse happened—a proton was converted to a neutron.
A positron, however, does not endure long after formation. All about it were atoms containing electrons. It could not move for more than a millionth of a second or so before it encountered one of those electrons. When it did, there was an attraction between the two, since they were of opposite electric charge. Briefly they might circle each other (to form a combination called “positronium”) but only very briefly. Then they collided and, since they were opposites, each cancelled the other.
The process whereby an electron and a positron met and cancelled is called “mutual annihilation”. Not everything was gone, though. The mass, in disappearing, was converted into the equivalent amount of energy, which made its appearance in the form of one or more gamma rays.
(It works the other way, too. A gamma ray of sufficient energy can be transformed into an electron and a positron. This phenomenon, called “pair production”, was observed as early as 1930 but was only properly understood after the discovery of the positron.)
Of course, the mass of electrons and positrons is very small and the amount of energy released per electron is not enormously high. Still, Dirac’s original theory of antiparticles was not confined to electrons. By his theory, any particle ought to have some corresponding antiparticle. Corresponding to the proton, for instance, there ought to be an “antiproton”. This would be just as massive as the proton and would carry a negative charge just as large as the proton’s positive charge.
An antiproton, however, is 1836 times as massive as a positron. It would take gamma rays or cosmic particles with 1836 times as much energy to form the proton-antiproton pair as would suffice for the electron-positron pair. Cosmic particles of the necessary energies existed but they were rare and the chance of someone being present with a particle detector just as a rare super-energetic cosmic particle happened to form a proton-antiproton pair was very small.
