is there a discussion somewhere on the notion of an advanced or retarded feynman propagator.(adsbygoogle = window.adsbygoogle || []).push({});

i don't mean the advanced or retarded propagator juxtaposed against the feynman propagator. I mean the feynman propagator itself with a theta step function multiplied by it to effectively give an advanced/retardedfeynmanpropagator.

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# Advanced/retarded feynman propagator?

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