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Particle disturbance by passing EM wave reveals momentum of EM wave?

  1. Aug 31, 2009 #1

    jaketodd

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    Hey guys and gals,

    Here are two of a vast sea of sources that say a passing EM wave will disturb charged particles:

    http://books.google.com/books?id=jC4...age&q=&f=false [Broken]
    "...a wave passing over a row of electrons; the arrows indicate the magnitude and direction of the electromagnetic force the wave exerts on the charges."

    http://webpages.ursinus.edu/dnagy/ph...cRadiation.pdf [Broken]
    "As an electromagnetic wave passes a charged particle, the Electromagnetic Waves Electric field produces a strong upward force on a
    electric field at the particle varies with time, dragging the particle back and forth along the direction of the field."

    So we know that momentum can be transferred to particles without the EM wave collapsing to a particle and a collision happening.

    My question: If you know how far away the EM wave is from the particle and you measured how much the particle is disturbed by it, would that reveal the momentum of the whole EM wave/photon?

    Thanks in advance,

    Jake
     
    Last edited by a moderator: May 4, 2017
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  3. Sep 6, 2009 #2

    jaketodd

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    Anyone?
     
  4. Sep 6, 2009 #3

    ZapperZ

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    I'm not sure I understand your question, why it is puzzling, and why it is in the quantum physics forum.

    EM wave is NOT the "wavefunction" that is the solution to the Schrodinger equation/Hamiltonian. So there is no "collapse". Secondly, RF signal is used in ALL particle accelerating structure as the mechanism to accelerate charged particles. So this isn't anything new, or surprising.

    Thirdly, do you know about Compton scattering?

    And finally, what does "how far the EM wave is from the particle" has anything to do with this? If the EM wave is NOT interacting with the particle, nothing will happen to that particle. If you are talking about single-photon interaction, then refer to the Compton scattering I mentioned earlier.

    Zz.
     
  5. Sep 6, 2009 #4

    jaketodd

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    I guess you didn't look at the sources I cited. They clearly say that an EM wave can be a distance away from a charged particle and effect it.
     
  6. Sep 6, 2009 #5

    Doc Al

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    I can't get those links to work, but are you sure they didn't say that the source of the EM wave can be a distance away?
     
  7. Sep 6, 2009 #6

    jaketodd

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    Ya, now I can't get the links to work either. They used to work. I assure you they are from credible sources. Do a google search for yourself if you don't believe me. And yes, the plethora of sources I found talk about an EM wave passing close to a charged particle but not touching it.
     
  8. Sep 6, 2009 #7

    Doc Al

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    That makes no sense to me, so I'd like to see a reference.

    Edit: FYI, you posted the links incorrectly. Try again. (They are incomplete.)
     
    Last edited: Sep 6, 2009
  9. Sep 6, 2009 #8

    jtbell

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    I don't read the second quote as saying that. I think you're misinterpreting the word "passes" in this context. It's not supposed to imply that the wave "misses" the particle.

    The first quote is probably the same sort of thing, but the phrasing "passing over" is a bit strange. I think it's just sloppy language, but I'd have to see it in the context of the entire page, to be sure.

    I think you may have been misled by the very common diagrams such as this one into thinking that an electromagnetic wave is confined to a line, sort of like a wave along a stretched string. It actually fills a volume of space. See this post and the attached diagram for a more realistic picture.
     
    Last edited: Sep 6, 2009
  10. Sep 6, 2009 #9

    jaketodd

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    Well I guess those links only last awhile. I found the source again, copied the link to my clipboard, cleared my cookies, shut down all IE windows, opened a new one, pasted in the link and it worked. So here it is again: http://books.google.com/books?id=jC...wave passing over a row of electrons"&f=false

    Please examine it and if I have misinterpreted it, let me change my question: Can you know the momentum of an EM wave by how much it disturbs charged particles without the EM wave collapsing to a photon?

    And no, I don't think an EM wave travels in a line, I know it has a curved wavefront that expands laterally as it propagates.

    Thanks guys.

    Jake
     
  11. Sep 6, 2009 #10

    jtbell

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    To repeat what ZapperZ wrote in post #3, classical electromagnetic waves are not the same thing as the photon's QM wave function. Classical electromagnetic waves correspond to the net effect of bazillions of photons, and do not "collapse." If you're in a situation where there is just a single photon, I don't think you can meaningfully use the classical electromagnetic field.
     
  12. Sep 6, 2009 #11

    jaketodd

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    So you're saying an EM wave has two parts: A physical wavefront that oscillates and a non-physical wave function that gives a probability of where the photon will manifest? I thought that the two were one in the same. And why wouldn't one EM wave (the wave nature of one photon) have electromagnetic properties? Please explain.

    And please let's get back to the question: Can the momentum of an EM wave, touching or just near a charged particle, be determined by the movement of the charged particle?

    Thank you!
     
  13. Sep 7, 2009 #12

    ZapperZ

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    This is the fatal flaw in your argument. A wavefunction that is the solution to Schrodinger equation is not physical. It "lives" in configuration space. It is not the "EM wave" that is the solution of Maxwell equation.

    Zz.
     
  14. Sep 7, 2009 #13

    jaketodd

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    So you're saying that the momentum of an EM wave, touching or just near a charged particle, can't be determined by the movement of the charged particle?
     
  15. Sep 7, 2009 #14

    ZapperZ

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    What does that have anything to do with :

    ????

    This is a weird thread going in many directions at once.

    Zz.
     
  16. Sep 7, 2009 #15

    jaketodd

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    Re: the momentum of an EM wave, touching or just near a charged particle, be determin

    Ok, I wasn't sure what you were saying. You said "fatal flaw in your argument" so I thought that you were saying that the answer is no. But I wasn't sure so I asked. All I want to know is, which is the title of this thread, "Particle disturbance by passing EM wave reveals momentum of EM wave?"
     
  17. Sep 7, 2009 #16

    ZapperZ

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    Re: the momentum of an EM wave, touching or just near a charged particle, be determin

    .. and like I asked earlier, I have no idea why this in the quantum physics forum, since this is basically a classical E&M question, since you keep insisting on "EM wave" picture.

    So read this about momentum of a classical EM wave, since that is what you're asking.

    http://electron9.phys.utk.edu/phys135d/modules/m10/emwaves.htm [Broken]

    Now, from there, try to justify if, without the presence of such wave, you will be able to transfer momentum to a charge particle. We're not talking about some exotic QM phenomenon like the Aharonov-Bohm effect here, are we?

    Zz.
     
    Last edited by a moderator: May 4, 2017
  18. Sep 7, 2009 #17

    jaketodd

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    Re: the momentum of an EM wave, touching or just near a charged particle, be determin

    So it's a yes?
     
    Last edited by a moderator: May 4, 2017
  19. Sep 7, 2009 #18

    ZapperZ

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    No.

    Zz.
     
  20. Sep 7, 2009 #19

    jaketodd

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    EM waves don't disturb charged particles? Didn't we agree that if the wave touches them then they will be disturbed? Oh you're saying you can't tell how much momentum the whole wave has from that disturbance even if you know how much of the wave touched the charged particle and the width of the wavefront?
     
  21. Sep 7, 2009 #20

    ZapperZ

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    Who said anything about that?

    YOUR SCENARIO that you are asking has the EM wave somewhere else! What is "just near"? Next door? In the next cavity?

    If you are explicit in what you are asking, I will be explicit in my answers. If you simply type 4 words, I'll response in kind.

    I work with particle accelerators in which the RF field IS the accelerating field. It would be silly of me to not know that EM wave can impart energy and momentum to charged particles.

    BTW, the link that I gave you tells you EXACTLY how to calculate the momentum of a EM wave! So what is the problem now?

    Zz.
     
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