Construction of states for quantum fields in nonstatic space-times

We set out the problem of constructing states for quantum field theories in nonstationary space-time backgrounds. We stress that this problem is a fundamental one, not to be confused with the inessential question "what is a particle." We present a formalism, related to that of Ashtekhar and Magnon, in which the problem can be attacked more easily than at present. By way of illustration we express the ansatz of "energy minimization" in our formalism and thereby find its general solution for noninteracting scalar fields.