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

What are the differences between photons and gluons?

  1. Feb 5, 2012 #1
    It is said in particle data sheets that photons and gluons both have 0 mass, 0 electric charge, and a spin of 1. If the two particles both have the same properties, then how is it possible to tell the difference between them? Can a complete table of properties comparing the photon with the gluon be posted in this thread?
  2. jcsd
  3. Feb 5, 2012 #2


    User Avatar
    Science Advisor

    While gluons don't carry electric charge, they do carry color charge, and this makes them very different from photons. The Particle Data Group (PDG) web site has extensive tables of particle properties:

  4. Feb 5, 2012 #3
    You can also go further, due to confinement you will never measure a free gluon in a detector, instead you will see what is known as a jet which is a spray of hadrons.

    You can also differentiate between photons and gluons by what the couple to.

    Photons only couple to particles with electric charge (they don't couple to themselves)
    Gluons only couple to particles with color (other gluons and quarks)
  5. Feb 5, 2012 #4


    User Avatar
    Science Advisor

    Mathematically the electron and the photon field are described by spinors ψ and 4-vectors Aμ. Their coupling is described by one term in the Dirac Lagrangian:

    [tex]\mathcal{L}_\text{int} \sim e\bar{\psi}\gamma^\mu A_\mu\psi[/tex]

    The quark and the gluon fields are described by spinors qi and 4-vectors Gμik carrying additional SU(3) indices ik; the gluon field is a '3*3 color matrix'. Their coupling is described by one term in the QCD Lagrangian:

    [tex]\mathcal{L}_\text{int} \sim g\bar{q}_i\gamma^\mu G_\mu^{ik}q_k[/tex]

    In addition there is a direct self-coupling for gluon fields which does not exist for the photon field
  6. Feb 5, 2012 #5
    So that means that while these two particles possess some similarities in the values of their properties, they both possess properties that the other does not, they are both used for different interactions, and they both behave differently.

    Additionally, the photon has two properties that the gluon does not, which is the parity and C parity, while the gluon possesses a property that the photon does not which is color charge.


    Last edited: Feb 5, 2012
  7. Feb 5, 2012 #6


    User Avatar
    Science Advisor

    yes, exactly
  8. Feb 5, 2012 #7
    Why are gluons always confined? Why can't one exist independent of a quark system?
  9. Feb 5, 2012 #8
    Color confinement is still an unsolved problem. But our best guess so far is that it's a side effect of gluon self-interaction in the strong-interaction limit. Here's a hand-waving, non-rigorous argument as to why it happens.

    When one separates two (anti)quarks/gluons past about 10^(-15) m, the potential energy becomes approximately linearly proportional to the distance, almost like stretching a string. In fact, string theory was first developed to account for the properties of hadron excited states. But if this gluon string gets long enough, it can pull quark-antiquark pairs out of the vacuum and snap.

    This accounts for jets of hadrons produced by high-energy collisions. A speeding quark or gluon stretches a gluon string behind it, which repeatedly snaps and makes hadrons.
  10. Feb 5, 2012 #9
    That would make sense, but I could have sworn scientists knew better than to put real world meanings like "stretching" in quantum mechanics.
    Is there a specific wave function for gluons?
  11. Feb 5, 2012 #10
    Specific wave function? Yes. One can write down the QCD Lagrangian. It closely parallels the QED Lagrangian.

    The main differences are replacement of charge operators with gauge-symmetry-generator operators and addition of self-interaction terms.
  12. Feb 5, 2012 #11
    The latter article states that gluons have negative intrinsic parity. Surely they also have C-parities too, eg (in the Gell-Mann basis)

    [itex]C ((r\bar{b} + \bar{r}b)/\sqrt{2}) = (\bar{r}b + r\bar{b})/\sqrt{2} = (+1) (r\bar{b} + \bar{r}b)/\sqrt{2}[/itex]
  13. Feb 5, 2012 #12


    User Avatar
    Science Advisor

    For teh gluon confinement there are in principle two approaches.

    1) put the QCD lagragian on a lattice and calculate the "effective color potential"; one finds for large radius V(r) ~ r + corrections; this visualizes confinement and provides a very detailed, quantitative description, but unfortunately it does not explain anything

    2) try to find a dynamical explanation for confinement; here a couple of approaches have been studies (color-electric Meissner effect, instantons and merons, center symmetry of SU(3), stochastic scattering in color space ~ anderson localization, IR behaviour of the gluon and ghost propagators, ...) but afaik none of these approaches is really convincing
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook