Confused in Chapter 3 of RF's QED

[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.

 

[eljose79]

Thank you for sharing your thoughts and questions on Richard Feynman's QED. It's always exciting to see someone diving into such a complex and fascinating topic. From your understanding, it seems like you have grasped the concept of the stopwatch hand moving as a simplification for the photon's path in Chapters 1 and 2. However, you are correct in your interpretation that Feynman is saying this is not entirely accurate and that the angle of the amplitude for a given path actually depends on the time the photon is emitted from the source.

In regards to your question about the double slit experiment, the concept of the stopwatch hand moving is still applicable. Although the photon may have only left the source at a single point in time, the probability of it being emitted at any other point in time also needs to be taken into account. This is why we add the probabilities for different paths, as you mentioned. It's important to remember that in quantum mechanics, particles can exist in multiple states simultaneously, which is why the analogy of the photon splitting and interfering with itself is often used.

It's understandable that this may be confusing and even frustrating at times, but it's important to keep in mind that quantum mechanics is a complex and often counterintuitive field. It's a constant process of learning and understanding, and it's always good to question and critically think about these concepts. Keep on reading and exploring, and don't hesitate to ask for clarification or further explanation when needed. Good luck on your journey through QED!