I've read the answers (and linked notes) to another question (Tree level QFT and classical fields/particles) and I understand them. They seem to explain how to organise a perturbative calculation of the solution to a classical non-linear ODE by drawing diagrams which look a bit like Feynman diagrams. But it is not clear to me how these diagrams correspond to *actual* Feynman diagrams. In QFT, when Feynman diagrams are used when doing path integrals (to calculate correlation functions etc.) but not to solve for the fields. What does the tree-level expansion of a correlation function mean and what is its classical interpretation?

This post imported from StackExchange Physics at 2016-05-31 07:24 (UTC), posted by SE-user octopus