In QED we look at all possible path a photon could go from S to P, and I understand the most significant contributions to the final arrow are the few near straight paths connecting S and P while other crazy curved paths cancel out. But does the initial direction the photon is emitted from the source S come into play at all? I mean, if I point a flash light (that's capable of emitting a single photon) 180 deg away from detector P, surely it's not likely to reach the detector, but I can't see how this is used in the "all path" calculation. If the photon indeed takes all path wouldn't the most probably path the one where photon upon leaving source immediately turns around 180 deg and then goes more or less straight to P?

In QED we look at all possible path a photon could go from S to P, and I understand the most significant contributions to the final arrow are the few near straight paths connecting S and P while other crazy curved paths cancel out. But does the initial direction the photon is emitted from the source S come into play at all? I mean, if I point a flash light (that's capable of emitting a single photon) 180 deg away from detector P, surely it's not likely to reach the detector, but I can't see how this is used in the "all path" calculation. If the photon indeed takes all path wouldn't the most probably path the one where photon upon leaving source immediately turns around 180 deg and then goes more or less straight to P?