# Confused in Chapter 3 of RF's QED

1. Mar 11, 2010

### soup_

Hello,

I'm reading through Richard Feynman's QED (for the first time) and I'm at Chapter 3. At the bottom of page 102 (2006 edition), he says, "So the timer we used for the 'imaginary stopwatch' was the monochromatic source:--in reality, the angle of the amplitude for a given path depends on what time the photon is emitted from the source. Once a photon has been emitted, there is no further turning of the arrow as a photon goes from on point to another in space-time."

From what I read from these sentences, he seems to be saying that in Chapters 1 and 2, all the times where he talked about the "stopwatch hand" spinning as the photon traveled through space-time, that he was using it as a simplification.
Am I reading this wrong?

If not, I understand how you could say that the stopwatch hand moving further clockwise with longer paths for the traveling photon is the same as saying it's moving counter clockwise for leaving later. The part I don't understand is how does a photon interfere with itself in the double slit experiment if the hand isn't moving? I.e. Clearly, no matter which path the photon takes it could have only left at a single point in time, so why would we add the probabilities for different paths if path lengths don't effect arrow turning? I think I may have just answered my own question. Is it because we're actually adding the different probabilities for when the photon left the source?

If this is the case, I'm a little annoyed at all the times I heard someone explain the double slit interference as a photon splitting, going through both slits, and interfering with itself. The analogy makes sense if the stopwatch hand turns as the photon moves but it completely falls apart if the photon starts at two different times.

Thanks in advance.

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