the point . The derivative formally represents a variation over that surface given fixed. It is difficult to give a precise mathematical formal interpretation of this equation. This approach is called by Schwinger the differential and field approach opposed to the integral and particle approach of the Feynman diagrams. The core idea is that if the interaction has a small coupling constant (i.e. in the case of electromagnetism of the order of the fine structure constant) successive perturbative terms will be powers of the coupling constant and therefore smaller. Use The purpose of the interaction picture is to shunt all the time dependence due to H0 onto the operators, thus allowing them to evolve freely, and leaving only H1,I to control the time-evolution of the state vectors. The interaction picture is convenient when considering the effect of a small interaction term, H1,S, being added to the Hamiltonian of a solved system, H0,S. By utilizing the interaction picture, one can use time-dependent perturbation theory to find the effect of H1,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. Summary comparison of evolution in all pictures For a time-independent Hamiltonian HS, where H0,S is the free Hamiltonian, References Further reading See also Bra–ket notation Schrödinger equation Haag's theorem Quantum mechanics es:Imagen de evolución temporal