  A simple, general discussion of the problem of inertia is provided both in
classical physics and in the quantum world. After briefly reviewing the
classical principles of equivalence (weak (WEP), Einstein (EEP), strong (SEP)),
I pass to a presentation of several equivalence statements in nonrelativistic
quantum mechanics and for quantum field vacuum states. It is suggested that a
reasonable type of preferred quantum field vacua may be considered: those
possessing stationary spectra of their vacuum fluctuations with respect to
accelerated classical trajectories
