# Retarded and advanced Green's function and Feynman Proagator

1. Is there a use of advanced Green's functions? If yes then when or in which context?

2. Why in quantum field theory, we always use Feynman's prescription for finding the propagator and not the retarded one?

EDIT: 3. Why the Feynman's prescription of propagator does not make sense in a classical field theory?

• @DanielSank: The "full" Green's function is the actual Green's function (i.e. the function which gives the Dirac delta when plugged into the equation). The advanced and retarded Green's functions are just the "positive" and "negative" parts of it (the parts that depends on $t >0$ and $t <0$, in a sense). – ACuriousMind May 19 '15 at 16:28