  In this paper, we study the interactions of electromagnetic waves with a
non-dispersive dynamic medium that is temporally dependent. Electromagnetic
fields under material time-modulation conserve their momentum but not their
energy. We assume a time-variation of the permittivity, permeability and
conductivity and derive the appropriate time-domain solutions based on the
causality state at a past observation time. We formulate a time-transitioning
state matrix and connect the unusual energy transitions of electromagnetic
fields in time-varying media with the exceptional point theory. This
state-matrix approach allows us to analyze further the electromagnetic waves in
terms of parity and time-reversal symmetries and signify parity-time symmetric
wave-states without the presence of a spatially symmetric distribution of gain
and loss, or any inhomogeneities and material periodicity. This paper provides
a useful arsenal to study electromagnetic wave phenomena under time-varying
media and points out novel physical insights connecting the resulting energy
transitions and electromagnetic modes with exceptional point physics and
operator symmetries.
