ABSTRACT
In what is undoubtedly one of the great papers in physics of this century, Dirac set up a relativistic wave equation which avoids the difficulties of negative probability density of the Klein-Gordon equation, and describes naturally the spin of the electron. Until Pauli and Weisskopf reinterpreted the Klein-Gordon equation, it was believed that this Dirac equation was the only valid relativistic equation. It is now recognized that the Dirac equation and the Klein-Gordon equation are equally valid; the Dirac equation governs particles of spin ½, the Klein-Gordon equation those of spin zero. Between them they describe most of the known elementary particles (although the proper definition of “elementary particle” is unclear). Formally one can extend the ideas of the Dirac theory to particles with nonzero rest mass with higher spin, but these theories have not proved to be successful, in that their interaction with the electromagnetic field leads to uncorrectable divergences. We shall not discuss these extensions, nor shall we discuss the successful Weyl equations, which describe relativistic massless particles of spin ½ and 1. The former, which describes the neutrino, can be considered a natural simplification of the Dirac equation.
