Hence, in the zero-order approximation, the set of weights characterizing a Fock state does not change over time, but the corresponding field configuration does. In higher approximations, the weights also change; collider experiments in high-energy physics amount to measurements of the rate of change in these weights (or rather integrals of them over distributions representing uncertainty in the initial and final conditions of a scattering event). The Dyson series captures the effect of the discrepancy between \mathcal{H}_0 and the true Hamiltonian \mathcal{H}, in the form of a power series in the coupling constant g; it is the principal tool for making quantitative predictions from a quantum field theory.
