Dirac, P.A.M.; A Theory of Electrons and Protons
Proc. Roy. Soc. A126 (1930) 360;
The difficulty of the negative energy electron is discussed and a Solution is proposed. Such electrons move in an external field as though carrying a positive charge. The most stable states for an electron - the states of lowest energy -are those with negative energy and very high velocity. All the electrons in the world will tend to fall into these states with emission of radiation. Assume there are so many electrons in the world that all the states of negative energy are occupied except perhaps
a few of small velocity. Any electrons with positive energy will now have very little chance of jumping into negative-energy states and their behaviour will be as usually observed. There will be an infinite number of electrons in negative-energy states, but if their distribution is exactly uniform they will be completely unobservable. Only the small departures from exact uniformity, brought about by some of the negative-energy states being unoccupied, can be observed. The properties of the vacant
states or "holes'' are discussed. These holes will be things of positive energy and in this respect like ordinary particles. The motion of one of these holes in an external electromagnetic field will be the same as that of the negative-energy electron that would fill it, and will thus correspond to its possessing a charge +e. This leads to the assumption that the holes in the distribution of negative-energy electrons are the protons. When an electron of positive energy drops into a hole and fills
it up an electron and proton disappear together with emission of radiation. This theory is applied to the problem of the scattering of radiation by an electron. (Science Abstracts. 1930. 2416. J.J.S.).
Introduction of the negative energy electron sea with holes treated as positive electrons. Attempt to identify these holes with protons.