  Spin-orbit coupling characterizes quantum systems such as atoms, nuclei,
hypernuclei, quarkonia, etc., and is essential for understanding their
spectroscopic properties. Depending on the system, the effect of spin-orbit
coupling on shell structure is large in nuclei, small in quarkonia,
perturbative in atoms. In the standard non-relativistic reduction of the
single-particle Dirac equation, we derive a universal rule for the relative
magnitude of the spin-orbit effect that applies to very different quantum
systems, regardless of whether the spin-orbit coupling originates from the
strong or electromagnetic interaction. It is shown that in nuclei the near
equality of the mass of the nucleon and the difference between the large
repulsive and attractive potentials explains the fact that spin-orbit
splittings are comparable to the energy spacing between major shells. For a
specific ratio between the particle mass and the effective potential whose
gradient determines the spin-orbit force, we predict the occurrence of giant
spin-orbit energy splittings that dominate the single-particle excitation
spectrum.
