Consider the microwave cavity used in a particle accelerator to accelerate particle bunches. Before a bunch of particles enters the microwave cavity can we, if only in principle, quantum mechanically describe the state of the microwave cavity as a proper sum of microwave photon number states, summing over all possible momentum? If yes, after the particle bunch has passed through the cavity and gained energy at the expense of the energy in the cavity can we say that the new state of the microwave cavity has fewer microwave quanta? Can we say that each particle in the accelerated bunch of particles absorbs many microwave quanta while in the cavity? Thanks!