  Vacuum consists of a bath of balanced and symmetric positive and negative
frequency fluctuations. Media in relative motion or accelerated observers can
break this symmetry and preferentially amplify negative frequency modes as in
Quantum Cherenkov radiation and Unruh radiation. Here, we show the existence of
a universal negative frequency-momentum mirror symmetry in the relativistic
Lorentzian transformation for electromagnetic waves. We show the connection of
our discovered symmetry to parity-time (PT) symmetry in moving media and the
resulting spectral singularity in vacuum fluctuation related effects. We prove
that this spectral singularity can occur in the case of two metallic plates in
relative motion interacting through positive and negative frequency plasmonic
fluctuations (negative frequency resonance). Our work paves the way for
understanding the role of PT-symmetric spectral singularities in amplifying
fluctuations and motivates the search for PT-symmetry in novel photonic
systems.
