ABSTRACT
This chapter discusses spinless, or scalar, particles and then later particles with spin. The reason is that the properties of spin, an angular momentum, are closely subject to the requirements of special relativity and as a consequence the form of the wave equation for a particle depends critically on its spin. The chapter provides a consistent interpretation of the Klein-Gordon equation. The problem can be solved in the same manner as the equivalent nonrelativistic problem by finding the stationary states of the potential. It is easiest to solve the second order Klein-Gordon equation directly. The chapter considers only the coupling of particles to the electromagnetic field. This interaction distinguishes particles from antiparticles.
