  By using perturbation theory, we show that a hydrogen atom with magnetic
moment due to the orbital angular momentum of the electron has "hidden
momentum" in the presence of an external electric field. This means that the
atomic electronic cloud has a nonzero linear momentum in its center-of-mass
rest frame due to a relativistic effect. This is completely analogous to the
hidden momentum that a classical current loop has in the presence of an
external electric field. We discuss that this effect is essential for the
validity of the Lorentz force law in quantum systems. We also connect our
results to the long-standing Abraham-Minkowski debate about the momentum of
light in material media.
