ABSTRACT
Spin is an internal angular momentum, it must have the transformation properties of an angular momentum. The smallest dimension that the matrices representing α β and σ can have is four. That means relativistically a spin ½ particles must have no fewer than four internal states – twice as many as non-relativistically. This doubling of the number of internal states is similar to the phenomenon that occurred in the Klein-Gordon equation for a spin zero particles, and as there, the doubling means that the theory will encompass both particles and antiparticles at once. A spinning particle at rest has only a magnetic moment. But a magnetic moment seen from a moving frame appears to be a combination of a magnetic moment and an electric moment.
