I have started to do literature search on "Angular Momentum ". This often leads back to spin , which is inevitable. However often circular orbits, having set circumference as a constrain for standing waves and hence leading to quantum mechanical ideas as supported by Bhor, are quoted as the classical angular momentum from orbitals. The angular momentum for electrons and other particles is said to be an additional element to the overall angular momentum of the system or atom. I am currently reading the Cambridge University Press book THE NEW QUANTUM UNIVERSE by Tony Hey and Patrick Walters which has some good explanations. The fact that the term angular momentum is applied directly to spin, still seems to infer that something ( even if it is ill defined by locality and non locality issues ) is generating a "force" be it by a principle of equivalence or some other mathematical transform. Can anyone share more ideas on the difference between classical angular rotation giving rise to standing waves, and Spin ( angular momentum) again in quantum terms of up and down spin?