  We show that for an observer in translational, rotational or gravitational
motion, a linearly polarized plane wave has two modes of propagation in a
stationary, homogeneous and isotropic medium according to Hertz's version of
Maxwell's theory. The first mode is characterized by polarization at right
angles to the direction of propagation and has a phase velocity that is
controlled by the material constants of the medium. The second mode is
characterized by polarization along the propagation direction and has a phase
velocity that is controlled by the motion of the observer. We outline some
applications of the second mode in emerging technologies.
