Since total angular momentum is conserved in nature, we will find that eigenstates of the
total angular momentum operator are usually energy eigenstates.
The exceptions will be when we apply external Fields which break the rotational symmetry.
We must therefore learn how to add different components of angular momentum together.
One of our first uses of this will be to add the orbital angular momentum in Hydrogen to the
spin angular momentum of the electron.
Our results can be applied to the addition of all types of angular momentum.
This material is covered in Gasiorowicz Chapter 15, in Cohen-Tannoudji et al. Chapter Xand very briefly in Griffiths Chapter 6.