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Apologies if this has been asked before. After watching Feynman’s QED lectures on the probabilities of different paths that photons can take towards a photomultiplier, a few questions came up in me.

Let’s take the simple scenario in which a photon emitter is faced straight towards a photomultiplier, without any mirrors or glass whatsoever. The photon emitter sends just one photon out. As Feynman stated, that photon can take different paths until it reaches the photomultiplier and you can calculate the probabilities of these paths that the photon would take.

Also as stated, the probability that the photon would take a certain range of more or less straight paths is higher than the probability of it taking a certain range of bent paths.

My questions:

1. Do the probabilities of all possible paths that a photon could take on its way to the photomultiplier together equal 100%?

2. If someone puts a photon detector directed at a certain range of paths, will the probability of the photon taking that range of certain paths be 100%minusthe probability of all the possible paths other than that certain range?

3. If the answer to question 2 is yes, does this mean that statistically, if one repeats the experiment from question 2 a 100 times, the detector would go off a number of times according to that probability?

Another question came up when, after watching the lecture, I read further that this one photon actually takes all these possible pathssimultaneously. This got me confused because if that’s true, how then can a photon take certain paths based on probabilities while it actually doesn’t have any preference and takes all these paths simultaneously?

4. Does this probability scenario merely arise when one repeats the experiment while using photon detectors directed at a certain range of possible paths?

5. Or is probability merely another term for intensity?

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# I A few questions about Feynman's QED Lecture

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