Hi All, I am currently reading stuff about the propagator and causality in QFT at the moment and I don't really understand it. Could anyone explain this to me? I understand that a propagator is a function which describes how particles and anti-particles travel from one place to another, but what is it to do with causality? Why is it important if the fields are space-like separated or time-like separated? What does it mean if the propagator does not give zero if it is outside the light-cone? How do we check if Causality is violated or not? Thank you all!