Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Time lag before the photon exhibits its new energy level?

  1. Nov 18, 2003 #1

    wolram

    User Avatar
    Gold Member

    when a photon aquiers more energy is the transfer instant
    or is there a time lag before the photon exhibits its
    new energy level? sorry if this is old question but i
    cant find aswer.
     
  2. jcsd
  3. Nov 18, 2003 #2

    chroot

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    How would a photon acquire more energy?

    - Warren
     
  4. Nov 18, 2003 #3

    jcsd

    User Avatar
    Science Advisor
    Gold Member

    The inverse Compton effect (photon is scattered in a collison with a high enenrgy free electron and gains some of the electons energy) can impart more energy to a photon.

    Wolram - I don't see why you'd think it wouldn't be instaneous, it must because of the conservation of energy (ignoring HUP).
     
    Last edited: Nov 18, 2003
  5. Nov 18, 2003 #4

    wolram

    User Avatar
    Gold Member

    this may seem very basic to most posters, but i am thinking
    about how the energy is transfered to the photon from the
    electron, my image is of two fields coming into contact
    with each other and one loosing energy to the other, but
    i dont know how or why or at what point the exchange takes place
    it may sound daft but i cant find the answer.
     
  6. Nov 18, 2003 #5
    Well I may sound even more daft, but maybe my lines of thinking help you a bit, wolram.

    I think a photon, by definition, is a quantum of the electromagnetic field and thus has sharp energy. This means it cannot 'aquire more energy'. However, a photon can be absorbed, and then another photon can be emitted. We thus have a photon in the initial state, and a photon in the final state, and thus, we can (loosely) speak of a photon 'having aquired energy'.
    Now let's consider a photon-electron interaction. If we draw the Feynman graph, it shows that the photon and electron collide at a vertex, forming a new particle. Of course (from classical collision physics), such a process cannot conserve both energy and momentum. Thus, the intermediate particle is a virtual particle which can only be there because of the Q.M. 'tunnel effect'. Which means it is unstable, and will decay with a certain half-life into two real particles, which may be another electron and photon.
    Now, we may identify the half-life of the virtual particle with the 'time lag to aquire energy' that you asked about, wolram.
    I think the clue to this is Heisenberg's uncertainty relation which says
    ΔEΔt>hbar.
     
  7. Nov 18, 2003 #6

    jcsd

    User Avatar
    Science Advisor
    Gold Member

    arcnets as I mentioned above a phton can acquire more enrgy via the inverse Compton effect, the electron in the collison is a free electron and does NOT absorb the photon. As I only have technical knowledge up to the rare basics of quantum field theory, I don't know how this interaction is described in QED though racking my memory I think I have seen the Feynman diagram for it and IIRC it is mediated by a virtual photon.
     
  8. Nov 18, 2003 #7

    chroot

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    The Feynman diagram is easy -- an electron moves along, and absorbs a photon of one frequency. The electrons momentum is changed. A while later, the electron emits another photon of a possibly different frequency, and again its momentum is changed.

    (I'm looking into the feasability and utility of including the feynmf package for drawing Feynman diagrams here on physicsforums.)

    - Warren
     
  9. Nov 18, 2003 #8
  10. Nov 18, 2003 #9

    jcsd

    User Avatar
    Science Advisor
    Gold Member

    So free electrons absorb all the enrgy of a photon in QED? I didn't know that and that's why Chroots a mentor and I'm not :)
     
  11. Nov 18, 2003 #10

    chroot

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    jcsd,

    Yep. There's no way for an electron to absorb "part" of a photon any more than an electron can emit "part" of a photon. It's an all or nothing deal.

    An electron can absorb one photon and almost immediately afterward emit another, however -- the net effect is Compton scattering.

    - Warren
     
  12. Nov 18, 2003 #11

    jcsd

    User Avatar
    Science Advisor
    Gold Member

    It's just that sometimes Compton Scattering is explained as the inabilty of a free electron to absorb all the enrgy of a photon.
     
  13. Nov 18, 2003 #12

    chroot

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Hmm.. reference?

    There's nothing stopping the electron from absorbing a photon, changing its momentum accordingly, and flying off in that new direction forever, never once emitting any more photons.

    It just wouldn't be called Compton scattering then -- it'd be called free-free absorption.

    - Warren
     
  14. Nov 18, 2003 #13

    jcsd

    User Avatar
    Science Advisor
    Gold Member

  15. Nov 18, 2003 #14

    chroot

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    This is sounding quite fishy to me. First, there is no conservation law for spin. Second, the photon is spin-one. Third, bremsstrahlung (free-free) emission is well known. Any process is reversible microscopically. If a free electron can emit a photon, then a free electron can also absorb a photon.

    I strongly question the validity of your link, jcsd, because it seems to be a snippet taken out of context... I'm still trying to figure out which of us is wrong. :smile:

    - Warren

    edit: two typos
     
    Last edited: Nov 18, 2003
  16. Nov 19, 2003 #15

    jcsd

    User Avatar
    Science Advisor
    Gold Member

    A free electron's quantum number J will be it's spin, for the most part, it just works out that you can't satisfy all the conservation laws and have the electron absorb the entire photon.

    I've had a look at free-free emission and it's a relativistic effect caused by electrons being accelerated.
     
  17. Nov 19, 2003 #16
    Since the photon has a spin one and an electron has spin half, their total angular momentum is one half or three half. So that the absorption of a photon by an electron is in no violation of angular momentum conservation.

    However, the absorption or emmission of a photon by a single free electron is in violation of 4-momentum conservation.
     
    Last edited: Nov 19, 2003
  18. Nov 19, 2003 #17

    jcsd

    User Avatar
    Science Advisor
    Gold Member

    yes, but the way it works *i think* in the situations where a whole photon is absorbed you find you can't conserve angular momentum and energy.
     
  19. Nov 19, 2003 #18

    wolram

    User Avatar
    Gold Member

    thankyou all, i have learnt something and am now able
    to find related info on the web.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?