Spin-Dependent Generalized Collective Model

In an advanced course on quantum mechanics a student learns that the Dirac equation is constructed from a linearization of the Klein-Gordon equation. Whereas the latter describes a particle with no spin the former includes a spin 1/2 degree of freedom. This appears to be a consequence of the linearization procedure and not of the theory of relativity, because the same result also follows from the linearization of the Schrodinger equation1, which represents the nonrelativistic limit of the Klein-Gordon equation.