  Larmor's theorem holds for magnetic systems that are invariant under spin
rotation. In the presence of spin-orbit coupling this invariance is lost and
Larmor's theorem is broken: for systems of interacting electrons, this gives
rise to a subtle interplay between the spin-orbit coupling acting on individual
single-particle states and Coulomb many-body effects. We consider a
quasi-two-dimensional, partially spin-polarized electron gas in a semiconductor
quantum well in the presence of Rashba and Dresselhaus spin-orbit coupling.
Using a linear-response approach based on time-dependent density-functional
theory, we calculate the dispersions of spin-flip waves. We obtain analytic
results for small wave vectors and up to second order in the Rashba and
Dresselhaus coupling strengths $\alpha$ and $\beta$. Comparison with
experimental data from inelastic light scattering allows us to extract $\alpha$
and $\beta$ as well as the spin-wave stiffness very accurately. We find
significant deviations from the local density approximation for spin-dependent
electron systems.
