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 comoving 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 comoving 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 comoving frame can have positive phase velocity in certain non-co-moving frames.