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Photon propagation

  1. Dec 13, 2015 #1
    What is the most accurate way to think of a photon moving through space?

    1) Should we think of it as an entity that travels as an integrated corpuscle that parts the "quark sea" of empty space and maintains it's internal integrity as it moves from point A to point B, or..

    2) Should we think of it more as the propagation of a "quanta" or measured value of energy through an essentially unmoving, rigid medium, kind of like a domino effect where a perturbation in one part of the medium in turn perturbates the adjacent part, and so on down the line?

    In latter instance, the medium doesn't move but for a simple oscillation around a center point, and nothing is "cutting through" the medium as we would think of, say, a thrown baseball cutting through the medium of the atmosphere. Whereas in the former case, we have an integrated, autonomous object cutting through some medium.
     
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  3. Dec 14, 2015 #2

    bhobba

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    Not to ascribe it any properties independent of observation.

    A photon is an excitation of a quantum field - with that being detailed in the theory.

    The closest you can get is it's like the harmonic oscillator:
    https://en.wikipedia.org/wiki/Quantum_harmonic_oscillator

    The ground state represents no photons, the one particle state is the creation operator applied to the ground state, the two particle state the creation operator applied to the one particle state etc etc. Quantum fields are a superposition of such states so even the number of particles is not fixed.

    And this applies to the momentum representation - so thinking they have an actual position is incorrect.

    Thanks
    Bill
     
  4. Dec 14, 2015 #3
    Isn't photon propagation always a counterfactual definiteness?
     
  5. Dec 14, 2015 #4

    bhobba

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    The concepts are not in any way related - at least as far as I can discern.

    Thanks
    Bill
     
  6. Dec 14, 2015 #5
    Wiki says, "...counterfactual definiteness (CFD) is the ability to speak meaningfully of the definiteness of the results of measurements that have not been performed (i.e. the ability to assume the existence of objects, and properties of objects, even when they have not been measured)."
    but,
    StackExchange says, "There is no quantum mechanics of a photon, only a quantum field theory of electromagnetic radiation. The reason is that photons are never non-relativistic and they can be freely emitted and absorbed, hence no photon number conservation."

    Is that how you mean?
     
  7. Dec 14, 2015 #6
    Ok, let me rephrase the question more fundamentally; what is the difference between a photon moving through space and a small massive object moving through space? Or is there a difference? Again, as I stated in my initial post, is it that a massive object "cuts" through space but that a photon doesn't? Or alternatively, do we have to eschew a classical picture altogether in reverence to quantum creation and annihilation operators and quantum "wave packets" with their equally confusing properties of "anomalous dispersion" as they propagate through a medium?
     
  8. Dec 14, 2015 #7

    bhobba

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    Photons do not have the property of moving through space.

    Thanks
    Bill
     
  9. Dec 14, 2015 #8

    bhobba

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    Good enough.

    Words are generally inadequate for this stuff.

    Thanks
    Bill
     
  10. Dec 14, 2015 #9
    Ok, so when Hippolyte Fizeau was shining light through the teeths of a wheel at a mirror on a hill 8 miles away in order to determine how fast the light propagated from his hill to that hill and back, what was he measuring? Was he measuring some property of photons that do not move through space?
     
  11. Dec 14, 2015 #10

    bhobba

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    He was measuring the behaviour of a large number of photons which is described accurately by Maxwell's equations that does have that property.

    Remember Ehrenfest's Theorem:
    http://www.physics.ohio-state.edu/~jay/631/ehrenfest.pdf

    Thanks
    Bill
     
  12. Dec 14, 2015 #11

    Nugatory

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    He was measuring the speed of electromagnetic waves, which do travel through space. Of course he didn't know that's what he was doing, as this was before the discovery that light is electromagnetic radiation.

    If you want to understand what photons are, and why you will only confuse yourself if you imagine that they have positions and velocities, you might give Feynman's (layman-friendly) book "QED: The strange theory of light and matter" a try.

    It's also generally good practice to think of light as an electromagnetic wave except when you're working on a problem that involves the quantized exchange of energy or momentum between the electromagnetic wave and some form of matter. Only then does the concept of photon make sense.
     
  13. Dec 14, 2015 #12
    Hi @Nugatory and @bhobba:

    Is there a meaningful way to describe a relationship between the motion of an electromagnetic wave and the motion of the quantum wave function of amplitudes?

    Regards,
    Buzz
     
  14. Dec 14, 2015 #13

    bhobba

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    Check out:
    http://www.cft.edu.pl/~birula/publ/CQO7.pdf

    Thanks
    Bill
     
  15. Dec 14, 2015 #14
    Hi Bill:

    Thanks for the link. It will take me some time to grasp what I want to learn from it, but I am sure the knowledge is there somewhere.

    Regards,
    Buzz
     
  16. Dec 16, 2015 #15
    How do they get from one location to another? Maybe they dont have a location either?
     
  17. Dec 16, 2015 #16

    Nugatory

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    You're right, they don't. I've already recommended Feynman's book once in this thread.
     
  18. Dec 16, 2015 #17

    bhobba

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    They don't have any property until observed to have it.

    For photons its really bad because even the number of particles is unknown.

    Thanks
    Bill
     
  19. Dec 16, 2015 #18
    @Nugatory You often see people saying things like "photons takes about 8 minutes to get from the sun to the earth" or "photons trapped in a reflective cavity". You say this is wrong? Should this terminology be avoided? How else can a person express these ideas to other people?
     
  20. Dec 16, 2015 #19
    I'm not sure what you mean by this. If someone talks about a 25 femtosecond burst of 1.97 eVolt photons, if you know the total energy, you will know the exact number of photons.

    Also, people talk about sending single photons in a 2-slit experiment where it is unknown which slit it goes through are certainly dealing with an exact number of photons (ie. one at a time) that take specific time to arrive at the detector from the emitter.
     
  21. Dec 17, 2015 #20

    Nugatory

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    The easiest way is to say something like "light takes about eight minutes to get from the sun to the earth" because light does move in the usual sense of the the word.

    Often when people say "photon" when they mean "light" they're thinking that a flash of light is a bunch of photons moving through space together the same way that a splash of water is a bunch of water molecules moving through space together. You can get away with this when there are no quantum mechanical effects involved - we do it all the time over in the relativity forum - but it's something that you have to unlearn before you can take on quantum mechanics.
     
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