I have now seen it repeated multiple times that a particle (a fermion, perhaps?) moving in a condensate can exchange particles (bosons, most probably) "without effect" -- the version of this that I run into usually goes something like that the energy of the condensate does not change AT ALL when, say, a virtual photon (or z-boson) is absorbed into a BEC. Now, first off, hunh? Does this really mean that the energy of the condensate doesn't change when something is absorbed by it? How can that be? And if so, what aspect of what equation(s) tell us this? One possible way that I can resolve this apparent impossibility is that to require that the emitting particle be a part of the condensate to begin with, so that the absorption is balanced by the emission, but this doesn't seem to be what is being said.