We introduce a simple quantum theory of the lossy beam splitter. When
applied to describe a Fabry-Perot cavity this leads to apparently anom
alous commutation relations for the intracavity operators. We show tha
t these unfamiliar properties are nevertheless consistent with the fun
damental canonical commutator for the vector potential and electric fi
eld operators. This result is derived as a consequence of causality as
applied to the properties of mirror reflection coefficients.