12. Angular momentum

The quantum mechanics of angular momentum is fascinating. It is also very basic to much of quantum mechanics. It is a model for dealing with other systems of particles

In chapter 5.4, it was already mentioned that angular momentum of particles comes in two basic kinds. Orbital angular momentum is a result of the angular motion of particles, while spin is “built-in” angular momentum of the particles.

Orbital angular momentum is usually indicated by ${\skew 4\widehat{\vec L}}$ and spin angular momentum by ${\skew 6\widehat{\vec S}}$. A system of particles will normally involve both orbital and spin angular momentum. The combined angular momentum is typically indicated by

$${\skew 6\widehat{\vec J}}={\skew 4\widehat{\vec L}}+{\skew 6\widehat{\vec S}}$$

However, this chapter will use ${\skew 6\widehat{\vec J}}$ as a generic name for any angular momentum. So in this chapter ${\skew 6\widehat{\vec J}}$ can indicate orbital angular momentum, spin angular momentum, or any combin­ation of the two.