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Photon wave function and "QED: the strange theory of light "

  1. May 4, 2015 #1
    I read that a wave function of the photon does not exist in coordinate space.

    But, when I read Feynman's "QED: the strange theory of light and Matter", for instance, when the photons travel through the glass, it seems like wave functions of photons. How it is with this?
     
  2. jcsd
  3. May 4, 2015 #2

    zonde

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    If I understand correctly Feynman describes QFT (quantum field theory) which "lives" in spacetime or rather in configuration space that is extension of spacetime with additional dimensions attached to every point in spacetime. I hope it's not totally off.
     
  4. May 4, 2015 #3

    kreil

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  5. May 4, 2015 #4

    zonde

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    In the same book Feynman says (if you haven't read so far):
    "In this example, arrows were multiplied and then added to produce a final arrow (the amplitude for the event), whose square is the probability of the event. It is to be emphasized that no matter how many arrows we draw, add, or multiply, our objective is to calculate a single final arrow for the event. Mistakes are often made by physics students at first because they do not keep this important point in mind. They work for so long analyzing events involving a single photon that they begin to think that the arrow is somehow associated with the photon. But these arrows are probability amplitudes, that give, when squared, the probability of a complete event."
     
  6. May 4, 2015 #5

    bhobba

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    This is an issue with learning QM, and posts I often see on this forum.

    The explanations of beginning texts and popularisations, like Feynman's, are, when viewed from a more advanced standpoint, incorrect. One of these is the concept of the wave-function of a photon. To start with simply accept it has issues and move on. If you don't all you are doing is setting yourself up for a world of difficult to understand advanced explanations that will likely confuse, rather than illuminate. There is a reason Feynman decided not to tell the complete truth to start with.

    Thanks
    Bill
     
  7. May 4, 2015 #6

    zonde

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    Feynman clearly states that he does not tell in this book anything that has to be unlearned. If you are telling it's incorrect point out exact thing.
     
  8. May 4, 2015 #7

    bhobba

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    See our FAQ:
    https://www.physicsforums.com/threads/do-photons-move-slower-in-a-solid-medium.511177/ [Broken]

    Feynman explains it as a photon is emitted then absorbed then emitted. It's wrong.

    But don't get worried - everything is learned in steps.

    Thanks
    Bill
     
    Last edited by a moderator: May 7, 2017
  9. May 5, 2015 #8

    Demystifier

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    Any quantum state of a single photon can be described by a complex function of the form ##\psi(x,t)##. (More precisely, it is a two-component function due to two polarizations.) This function satisfies a linear equation and therefore obeys the superposition principle. For me, these properties are enough to call function ##\psi(x,t)## a wave function.
     
  10. May 5, 2015 #9
    As I understand your above explanations, the problem is at single photon. But, I wish to see:
    1. Feynman's explanation is a pedagogical transition from Maxwell's oscilation of the field to the oscilation of the wave function.
    2. Photons can be used as an example for gauge transformation. Explanation in http://quantummechanics.ucsd.edu/ph130a/130_notes/node296.html Equation before sentence: "There are measurable quantum physics consequences of this symmetry. "

    Here the single photons are not necessary, I suppose?

    Am I correct about these two things?

    Are here still some additional important differences between Feynman's photons and Maxwell wavings?

    Bhobba, what did you think besides the example which have you given, maybe also Fock space?
     
  11. May 5, 2015 #10

    bhobba

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    What's your level of math?

    Unless its advanced you are out of luck.

    If it is then get the following:
    https://www.amazon.com/Quantum-Field-Theory-Gifted-Amateur/dp/019969933X

    Thanks
    Bill
     
  12. May 5, 2015 #11

    zonde

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    I can only offer you to read (reread) Feynman:
    "That's why I said earlier that light appears to go slower through glass (or water) than through air. In reality the "slowing" of the light is extra turning [of the arrow] caused by the atoms in the glass (or water) scattering the light. The degree to which there is extra turning of the final arrow as light goes through a given material is called its "index of refraction"."

    It has very little to do with objections in ZapperZ's FAQ entry.
     
    Last edited by a moderator: May 7, 2017
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