Negative- and positive-phase-velocity propagation in an isotropic chiral medium moving at constant velocity

Analysis of electromagnetic planewave propagation in a medium which is a spatiotemporally homogeneous, temporally nonlocal, isotropic, chiral medium in a co-moving frame of reference shows that the medium is both spatially and temporally nonlocal with respect to all non-co-moving inertial frames of reference. Using the Lorentz transformations of electric and magnetic fields, we show that plane waves which have positive phase velocity in the co-moving frame of reference can have negative phase velocity in certain non-co-moving frames of reference. Similarly, plane waves which have negative phase velocity in the co-moving frame can have positive phase velocity in certain non-co-moving frames.