ABSTRACT
We have seen in the previous chapter that the Klein-Gordon equation can describe the motion of a particle with spin 0. In this chapter, we will develop the Dirac equation which can be used to describe the motion of a particle with spin 1/2. It is conceptually easier to investigate the properties of the Dirac equation in its single-particle wave equation form, as we will do here. A characteristic feature of the Dirac relativistic wave equation1 is that the spin of the particle is built into the theory from the beginning, and is not added afterwards as Pauli added the electron spin to the nonrelativistic Schro¨dinger equation. Built-in features, such as spin, provide a useful measure of the applicability of a particular wave equation to the description of a particular spin of a particle.
