Spontaneous parity-time-symmetry breaking in moving media

Optical instabilities in moving media are linked to a spontaneous parity-time-symmetry breaking of the system. It is shown that in general the time evolution of the electromagnetic waves in moving media is determined by a non-Hermitian parity-time-symmetric operator. For lossless systems the frequency spectrum of the time evolution operator may be complex valued, and has a mirror symmetry with respect to the real-frequency axis. The possibility of optical amplification of a light pulse in the broken parity-time-symmetry regime is demonstrated.