 Thomas precession

Thomas precession is the kinematic rotation arising when the successive instantaneous rest frames of an accelerated particle are connected by noncollinear Lorentz boosts. Its contribution is needed in deriving the leading atomic spin-orbit coupling: for an electron in central potential energy $U(r)$, the corrected term is $H_{\mathrm{so}}=[2m_e^2{c}^{2}r{]}^{-1}(dU/dr)\mathbf{L}\cdot \mathbf{S}$. A rest-field argument alone gives twice this coefficient.^[ref]

Thomas precession is a relativistic effect that describes the precession of the spin of a particle, such as an electron, due to its motion in a gravitational or electromagnetic field. It arises in the context of relativistic quantum mechanics and is named after the physicist Llewellyn Thomas who first described it in 1927.