  Quantum fields in time-dependent backgrounds generally lead to particle
production. Here we consider "unexciting" backgrounds for which the net
particle production vanishes. We start by considering the simple harmonic
oscillator and explicitly construct all unexciting time-dependent frequencies.
This allows us to construct homogeneous backgrounds in field theory for which
there is no particle production in any given mode, though we are able to show
that there are no homogeneous backgrounds for which the particle production
vanishes in every mode. We then construct general inhomogeneous unexciting
field theory backgrounds. The set of all unexciting field theory backgrounds
will be further restricted by the choice of physical interactions and this
leads to an interesting open problem.
