Kinematic negative birefringence in fast-moving dielectrics

The propagation of light in a dielectric moving at relativistic velocities is investigated. The analysis shows that, in that case, the medium may support two original propagative modes, analogously to birefringence in crystal optics. The conditions at which the kinematic birefringence occurs and its main features are exhibited and compared with the usual birefringence. In particular, it is shown that this peculiar effect is of second order in relative velocity β=v/c, and the kinematic ordinary refractive index exhibits angular dependence contrary to what happens for the usual ordinary index. Moreover, the existence of negative values for the ordinary and extraordinary indices is manifested in the ultrarelativistic regime.