N.28 Adding an­gu­lar mo­men­tum com­po­nents

The fact that net an­gu­lar mo­men­tum com­po­nents can be ob­tained by sum­ming the sin­gle-par­ti­cle an­gu­lar mo­men­tum op­er­a­tors is clearly fol­low­ing the New­ton­ian anal­ogy: in clas­si­cal physics each par­ti­cle has its own in­de­pen­dent an­gu­lar mo­men­tum, and you just add them up,

See also ad­den­dum {A.19}.