The interaction picture is convenient when considering the effect of a small interaction term, H 1,S, being added to the Hamiltonian of a solved system, H 0,S. By utilizing the interaction picture, one can use time-dependent perturbation theory to find the effect of H 1,I, e.g., in the derivation of Fermi's golden rule, or the Dyson series in quantum field theory : in 1947, Shin'ichirō Tomonaga and Julian Schwinger appreciated that covariant perturbation theory could be formulated elegantly in the interaction picture, since field operators can evolve in time as free fields, even in the presence of interactions, now treated perturbatively in such a Dyson series.