Editing Angular momentum coupling (section)

{{Short description|Coupling in quantum physics}}
{{Coupling in molecules}}

In [[quantum mechanics]], '''angular momentum coupling''' is the procedure of constructing [[eigenstates]] of  total angular momentum out of eigenstates of separate angular momenta. For instance, the orbit and spin of a single particle can interact through [[spin–orbit interaction]], in which case the complete physical picture must include spin–orbit coupling. Or two charged particles, each with a well-defined angular momentum, may interact   by [[Electrostatic#Coulomb's law|Coulomb forces]], in which case coupling of the two one-particle angular momenta to a total angular momentum is a useful step in the solution of the two-particle [[Schrödinger equation]].
In both cases the  separate angular momenta are no longer [[constants of motion]], but the sum of the two angular momenta usually still is.  Angular momentum coupling in atoms is of importance in  atomic [[spectroscopy]]. Angular momentum coupling of [[electron spin]]s is of importance in [[quantum chemistry]]. Also in the [[nuclear shell model]] angular momentum coupling is ubiquitous.<ref>{{cite book|title=Quantum Physics of Atoms, Molecules, Solids, Nuclei and Particles|edition=2nd|author=R. Resnick, R. Eisberg|publisher=John Wiley & Sons|year=1985|isbn=978-0-471-87373-0|url=https://archive.org/details/quantumphysicsof00eisb}}</ref><ref>{{cite book|title = Quanta: A handbook of concepts|author=P.W. Atkins|publisher=Oxford University Press|year=1974|isbn=0-19-855493-1}}</ref>

In [[astronomy]], '''spin–orbit coupling''' reflects the general law of [[conservation of angular momentum]], which holds for celestial systems as well. In simple cases, the direction of the [[angular momentum]] [[Euclidean vector|vector]] is neglected, and the spin–orbit coupling is the ratio between the frequency with which a [[planet]] or other [[celestial body]] spins about its own axis to that with which it orbits another body. This is more commonly known as [[orbital resonance]]. Often, the underlying physical effects are [[tide|tidal forces]].