The question is as follows; 'A free quantum field theory is related to an infinite number of quantum mechanical harmonic oscillators as unit mass particles on springs with spring constants k, where k takes all values. Now imagine the following scenario: these particles also have non-zero charges, and I suspend the system in a constant electric field Eo. Ignoring the effects of any outgoing radiations due to the oscillations of charged particles, show that this system of harmonic oscillators is equivalent to a massive scalar field theory with a classical source term int(d^4x J*(phi)). Where J is the source and phi is the scalar field. Find the connection between Eo and J. I've been working on this all day and I feel like I've gotten no where. It *should* be an easy question. If someone could point me in the right direction, that would be great!