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Photon questions

  1. Dec 24, 2009 #1
    1) Is a photon elastic?
    2)Is it possible to prove gravity of photon along its line of motion?
    IMO no, because this would causse the photon to travel at either more or less than c, or change its frequency.
    Perhaps diffraction can be taken as a cause of gravity of photon perpendicular to its motion, it is observed "when" the photon passes through the slit, not before or after.
    We have also learnt the force required to circulate a photon in a grav.field.
  2. jcsd
  3. Dec 25, 2009 #2
    on your second question if you are asking if a photon has a graviational field i can tell you yes it does . thats about all i can add
  4. Dec 25, 2009 #3


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    Since a photon does not have "size" or "shape" in the usual sense, it makes no sense to talk about "elasticity" of a photon.
  5. Dec 25, 2009 #4
    The photon cannot be elastic because it would then be possible to manipulate its energy by means of forces(gravity, collision etc.) causing a change of frequency, which is not observed.
    A similar argument would not be in favour of gravitational effects of the photon along or opposite to its motion and so the force "mustn't work".Any instance of this kindhttp://www.adamauton.com/warp/emc2.html"only mentions recoil of the surface when the photon approaches and hits a reflective surface
    Last edited by a moderator: Apr 24, 2017
  6. Dec 25, 2009 #5
    There is a not overly popular idea called the tired light mechanisms, one of the versions of which states that over long periods of time, photons loose energy slowly and their frequency decreases. No mechanism however has been proposed for this effect, but it does offer an alternative explanaiton for certain redshifts we observe.

    Personally, I'm a little skeptical, but with science one must always keep an open mind.
  7. Dec 25, 2009 #6

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    Nonsense. An open mind does not mean "believing in things that have been excluded by data".

    Tired light can't be correct - among other problems, it requires a degree of blurring of distant objects that is unobserved, and it makes the wrong prediction about distant supernova light curves.
    Last edited: Dec 26, 2009
  8. Dec 25, 2009 #7
    Hmmm. I didn't happen to know that information about it. Now I'm a lot more skeptical.
  9. Dec 25, 2009 #8
    http://books.google.co.in/books?id=...=onepage&q=lev okun concepts of mass&f=false"
    Can anyone cite me a derivation of the formula of grav. force on pg.14? The force acts on the photon, but if it doesn't accelerate, then what happens?
    Could it be that the gravitons emitted by the photon keep travelling with the photon at its own speed so information transfer is not possible and all the energy is absorbed at once during collision?
    Last edited by a moderator: Apr 24, 2017
  10. Dec 25, 2009 #9
    The Compton effect is a case where an electron and a proton collide and then move away from each other with conservation of energy as well as conservation of momentum. The term "perfectly elastic collision" is usually applied to the event. The photon's frequency is indeed changed.
  11. Dec 26, 2009 #10
    The experiment in post#4 is also an example of perfectly elastic collision, but the trick is that during collision the velocity of the photon is not c, and something which does not move at c is not a photon.So the "photon" does not change frequency on collision.
    The compton effect is a case of absorption and reemission, not of interest here.
    the term "point particle" has the clue.
  12. Dec 26, 2009 #11

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    Are you here to ask questions or to lecture us about how photons behave? Your first post suggests the former, but your later posts suggest the latter. They also include some iconoclastic views - I would recommend you take a look at the PF Rules on overly speculative posts.
  13. Dec 26, 2009 #12


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    The change in frequency (gravitational redshift) for a vertically-travelling photon was clearly measured in the Pound-Rebka experiment.
  14. Dec 26, 2009 #13
    I made a serious typo. I meant to write that an electron and a photon collide.
  15. Dec 27, 2009 #14
  16. Dec 27, 2009 #15
    Which part are you talking about?The gravitons part, most probably.How do gravitons emitted by the photon reach the heavier mass when both travel at the same speed?
  17. Dec 27, 2009 #16
    That's because first these questions came to my mind and posted them, then I kept thinking about them and posted my thoughts
  18. Dec 27, 2009 #17
    I found this thesis from arXiv.
    Detection of elastic photon-photon scattering through four-wave coupling
  19. Dec 28, 2009 #18
    ya i'm also confused here... photons have a gravitational field? that means they exchange gravitons? so the photon loses energy as it travels? (since gravitons can't catch up to the photon)
    can someone point me to an article to explain how photons have a gravitational field?
    Last edited: Dec 28, 2009
  20. Dec 28, 2009 #19
    I don’t know if this is the explanation you are looking for but we can show that photons have a gravitational field by imagining a bar of matter and a bar of anti-matter and they both have a gravitational field associated with them and when they collide they will annihilate and photons will be released and the gravitational field will still be there it wont go away. And another way if light can be bent in a gravitational field then based on action -reaction it must also have a force .
  21. Dec 28, 2009 #20
    with the matter and anti matter example, your saying that gravity IS different from EM? if they can become 1 force at high enough energy (as the 4 forces unite) they they should be the same thing right? then your example wouldn't work right?

    about bending light, then that implies the picture about gravitational field bending space-time itself is wrong and the gravitons are actually interacting with the photon(s)?
    or they can co-exist and we can abandon the bending space-time itself picture?
    and in the action reaction picture the photons can pull on say a marble in space if there are enough of them? since photons are bosons the number of photons in the same spot can be limitless? so if we sum their gravitational effects it should be able to pull on a marble?
    or am i missing something again?
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