10
Angular Momentum and Rotation Symmetry
In Chapter 7, we studied the orbital angular momentum L = r × p by considering its differential operators in coordinate representation. Here, we discuss one more kind of angular momentum called spin angular momentum, which does not emerge from r × p. It arises from the study of angular momentum, using the abstract operator formalism. However, there is another motivation to discuss angular momentum operators in terms of abstract operators, which come from the consideration of rotational symmetry.
We have already seen that under rotational symmetry
r → Rr|α〉 → Û|α〉 = e−iθ·Jħ|α〉
where 
. The angular momentum ...
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