ABSTRACT
The invariance of the Hamiltonian of a physical system under spatial rotation, together with the connection between the rotation operator Rˆn(θ) and the angular momentum operator Jˆ ≡ (Jˆx, Jˆy, Jˆz), leads to the principle of conservation of angular momentum [Chapter 6, Sec. 6.3] and to the status of total angular momentum as a constant of motion. This makes angular momentum a very important quantity in quantum mechanics.
