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Electron recoil

  1. Jun 12, 2015 #1
    Has electron recoil due to photon emission ever been confirmed by experiment? cause I can't find any reference to electron recoil being measured anywhere I look. If it has been measured, what methods do they use?
     
    Last edited: Jun 12, 2015
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  3. Jun 12, 2015 #2

    Orodruin

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    What do you mean by photon emission by an electron? A free electron cannot emitt a photon so what kind of process are you imagining?
     
  4. Jun 12, 2015 #3
    Any situation where an electron emits a photon, say in a free electron laser, or maybe after an electron spin flip. my question is, how can the recoil be measured?
     
  5. Jun 12, 2015 #4

    bhobba

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    QED does not have a picture of emission like that. Even ordinary QM doesn't - there is no such thing as recoil - the electron changes state - and a photon is detected - pictures like emitted, recoil etc are classical - QM is not classical eg electrons do not have a well defined position and momentum for the idea of recoil to even make sense.

    Thanks
    Bill
     
  6. Jun 12, 2015 #5
    Alright, is there any way to detect the electrons state change? surely if the energy of the light emitted is high enough the recoil would be measurable .
     
  7. Jun 12, 2015 #6

    bhobba

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    Recoil is change in momentum. Electrons in atoms do not have a definite momentum for the concept to be applicable.

    Thanks
    Bill
     
  8. Jun 12, 2015 #7
    So an object emitting light won't experience recoil? What about in a free electron laser?
     
  9. Jun 12, 2015 #8

    bhobba

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    Just to elaborate a bit further recoil is change in momentum. For that to be meaningful the electron would need to be in a state of definite momentum which only happens for a free electron - which cant emit a photon. Its not a concept applicable to photon emission.

    Thanks
    Bill
     
  10. Jun 12, 2015 #9

    bhobba

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    What about it? Do all electrons in a free electron laser gain and loose the same energy to make such a concept as individual electron recoil meaningful? As far as I know its a phenomena of a beam of electrons going through a magnetic field.

    Before going any further define the exact set-up you are considering, what you are defining as recoil, and exactly how you will measure it.

    Thanks
    Bill
     
    Last edited: Jun 12, 2015
  11. Jun 12, 2015 #10
    When an electron emits a photon, conservation of momentum implies that the electron will recoil in response, this should happen in any situation where light is emitted, as for how I measure it, well that's the question I asked, I don't know how they measure electron recoil from photon emission
     
  12. Jun 12, 2015 #11
    when I shine light on my crookes radiometer it starts spinning, isn't this because the electrons in the atoms absorb the lights momentum?
     
  13. Jun 12, 2015 #12

    bhobba

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    That's false in QM. For it to even be meaningful the electron needs to have a definite momentum. It is a basic fact that only free electrons have a definite momentum.

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    Bill
     
    Last edited: Jun 12, 2015
  14. Jun 12, 2015 #13

    bhobba

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    So? Photons have momentum - that does not mean electrons recoil when they absorb it.

    Thanks
    Bill
     
  15. Jun 12, 2015 #14
    Actually they would have to recoil or else the crookes radiometer wouldn't spin.
     
  16. Jun 12, 2015 #15

    bhobba

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    That's not true. I suggest you look up something called Ehrenfest's Theorem. Its also relevant to a free electron laser.

    Can I ask where you learnt QM from? What I am saying is very very basic to QM.
    .
    Thanks
    Bill
     
  17. Jun 12, 2015 #16
    Ehrenfest's theorem states that the expectation values of quantum mechanics (QM) obey the classical laws of physics. This in turn means that the vast experimental evidence in support of classical mechanics do not contradict QM.

    Regardless of that, the momentum that is absorbed by electrons in the atoms of the crookes radiometer does result in a recoil that is measurable, I think this means that the electrons do in fact recoil when they absorb the light, why not recoil when emitting light?
     
  18. Jun 12, 2015 #17

    jtbell

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  19. Jun 12, 2015 #18

    bhobba

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    Your thinking is incorrect. That momentum change is a valid concept for a large ensemble of electrons does not imply it is a meaningful concept for individual electrons. That is very very basic foundational QM.

    Again I ask you - where have you learned QM from? If you haven't I suggest you go and do some study on it.

    Thanks
    Bill
     
  20. Jun 12, 2015 #19

    bhobba

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  21. Jun 12, 2015 #20
    I should change my original question, I actually wanted to know if it was possible to measure the recoil of large number of electrons when they emitted light, say by measuring the recoil of a laser, i couldn't find any information online about that.
     
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