  We show that single-mode nonclassicality of the output of an optomechanical
cavity and the noncausal linear optical response of this cavity emerge at the
same critical cavity-mechanical coupling. In other words, single-mode
nonclassicality emerges when the barrier (in electromagnetism) avoiding
faster-than-light communication is lifted off. The nature of the emergence of
noncausal behavior does not depend on the length (boundary conditions) and the
type of the cavity. Origin of the noncausal behavior is the temporal/frequency
relations between the incident and reflected waves at the outer surface of the
cavity. We further discuss the relations with the recent studies; (i)
equivalence of the entanglement among identical particles to the
nonclassicality of their quasiparticle excitations, (ii) necessity of
superfluid behavior of vacuum, and (iii) entanglement-wormhole equivalence.
