Casimir effect -- silly question, but... The summation of the standing waves of virtual particles is calculated between the plates, so that you only have the harmonic series to sum over. Fine. However, what about non-virtual background radiation? Wouldn't only standing waves survive between the plates, thereby again giving you only the harmonic series to calculate over, as opposed to outside? I perceive this as being wrong, because otherwise one could explain the effect classically, which one cannot. So where is the difference?