Role of the electromagnetic momentum in the spin-orbit interaction

The role played by the linear and angular momentum of the electromagnetic fields in the understanding of several aspects of quantum mechanics is discussed. A non-relativistic semi-classical model of the spin-orbit interaction, where the electromagnetic interaction energy U is calculated in the frame of the nucleus, is presented. Taking into account the electron hidden momentum P-h = c(-1) mu x E, the spin-orbit energy splitting turns out to be Delta is an element of(so) = 1/2)U, the factor 1/2 emerging directly by requiring that the energy variation be a minimum. After quantization, the radius of the orbit is found to be spin-dependent, anticipating a feature of the Dirac equation. Finally, a test of the hidden momentum P-h, which may corroborate the approaches based on the hidden momentum and related interpretations of electrodynamics, is proposed and shown to be viable with present technology.