CHAPTER 19

The Dirac Equation

Spin is what makes the world go ’round.

19.1. The Groups *SO*(3) and *SU*(2)

How does SU(2) act on its Lie algebra?

FOR physical and mathematical motivation for this section (which involves nonrelativistic quantum mechanics) we refer the reader to some remarks of Feynman and of Weyl. Specifically, Feynman [FF, pp. 8, 9], in his section entitled “Degeneracy,” shows that a process involving a specific choice of direction in space requires that the process be described not by a single wave function but rather by a multicomponent ...

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