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Are photons massless or practically massless

  1. Oct 5, 2011 #1
    Title says it all. Also, do photons in a vacuum travel in a straight line? I find that kind of strange. Also, are gluons massless or practically massless? How are gluons detected?
  2. jcsd
  3. Oct 5, 2011 #2


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    Photons are mass-less. "Straight line" kind of depends on what geometry. Photons that travel through a region of space that is being influenced by a high gravitational force (like near a black hole or at the edge of a galaxy) follow a "straight line" that is not what Euclid had in mind.
  4. Oct 5, 2011 #3


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    Please start by reading the https://www.physicsforums.com/forumdisplay.php?f=210" [Broken] in the Relativity forum.

    Last edited by a moderator: May 5, 2017
  5. Oct 5, 2011 #4
    OP, do you understand what he said there at the end? If you don't need it explained, then I won't. If you do, lemme know :)
  6. Oct 5, 2011 #5
    The experimental limit on the photon mass is pretty good : a billionth of a billionth of an eV, from the magnetic field of the Sun. Now mind you, there is an excellent discussion in Feynman's lectures on gravitation. One thing he says is that it is pretty futile to ask whether something is so small that it is impossible to observe.

    The experimental limit on the gluon mass is much poorer. From the Millikan experiment, and the failure to observe free fractional charges, we can set a limit on the gluon mass at about a tenth of thousandth of eV.

    Please let me know if you want further details or reference.
  7. Oct 5, 2011 #6

    photons do not travel in a straight line in the Euclidean sense - I guess I don't understand that. Please illuminate.


    I think you've given me enough information for now. I'm not going to get anywhere unless I open the textbook, so for now I'm good.
  8. Oct 5, 2011 #7
    Photons travel a "straight" line in 4 dimensions, called a geodisk. Because time and space are connected, and we cannot see this time dimension, in 3d it appears that photons are curving in space.
  9. Oct 5, 2011 #8


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  10. Oct 5, 2011 #9
    I was wondering why firefox was correcting my spelling. Thanks :P
  11. Oct 5, 2011 #10
    And there you have it :)
  12. Oct 6, 2011 #11


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    Afaik a tiny photon mass (which would turn Maxwell into Proca theory) could be formulated as a renormalizable QFT as well, whereas a gluon mass would spoil renormalizability of (non-abelian) QCD. That's a strong indication that gluons must be exactly massless.
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