Within the path integral formalism, in a [itex]\phi^4[/itex] theory, the one-loop self-energy is divergent. Moreover the loop does not depend on external momenta. However, if you normal order the interaction in the operator formalism, then the one-loop self-energy is zero, since it represents a contraction of operators that are in normal order, which is zero. How can the two results be the same? Also is it possible to normal order an interaction in the path integral representation? Also, both the operator formalism and the path integral formalism lead to the same Feynman rules, so shouldn't they lead to the same result? But how then in the operator formalism do we get zero?