  We discuss phase transitions in PT-symmetric optical systems. We show that
due to frequency dispersion of the dielectric permittivity, an optical system
can have PT-symmetry at isolated frequency points only. An assumption of the
existence of a PT-symmetric system in a continuous frequency interval violates
the causality principle. Therefore, the ideal symmetry-breaking transition
cannot be observed by simply varying the frequency.

Optics;Mesoscale and Nanoscale Physics;Quantum Physics