We consider the interaction of an harmonic oscillator with the quantum fiel
d via radiation pressure. We show that a "Schrodinger cat" state decoheres
in a time scale that depends on the degree of ''classicality" of the state
components, and which may be much shorter than the relaxation time scale as
sociated to the dynamical Casimir effect. We also show that decoherence is
a consequence of the entanglement between the quantum states of the oscilla
tor and held two-photon states. With the help of the fluctuation-dissipatio
n theorem, we derive a relation between decoherence and damping rates valid
for arbitrary; values of the temperature of the field. Coherent states are
selected by the interaction as pointer states.