Hamiltonian of a many-electron atom in an external magnetic field and classical electrodynamics

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Abstract
The increasing precision of experiments on the Zeeman effect of many electron atoms requires an ever increasing knowledge of relativistic and radiative effects. The generalization of the Breit equation as proposed by Hegstrom includes all these effects up to order c— 3. In the non relativistic limit the resulting Hamiltonian is quite intricate, but we show in this paper that it is possible to explain almost all of its terms within the frame of classical electrodynamics. In particular, picturing the atoms as point-like particles with intrinsic magnetic moments, we show that the physical phenomena underlying the Breit equation and its recent extension are mainly two well-known classical phenomena : i) the interaction of the magnetic moment of the constituent particles with the magnetic field they experience in their rest frame; ii) the Thomas precession of the spins of the accelerated particles