As always I will preface this question with the fact that I only have a high school education, so I may be overlooking something, or unaware of something that is inherent to the question.

That being said, as I understand it, in QCD color represents the strength of the strong force on a given quark. Additionally a baryon must be color neutral in composition. However, quarks of different color behave identically. Why do these quarks behave the same if they are affected by the strong force differently?

I've been thinking about this a bit, and the only thing that I can think of is that as gluons are particles that carry the strong force the difference is what gluons the quarks are able to interact with, and since color is conserved the baryon is able to persist through these color changes.

Am I on the right track with this or is there something else going on here?


1 Answer 1


Quarks can be RGB color, but color charges are paired (color with anticolor) but there is no gauge invariant meaning of the color. Now gluons do not have a definite color, they are in a superposition of colors and quarks have temporarily a color for a given feynman diagram, which is the only way we know how to display their interactions.


Now when one quark interacts with another (different color), the mediator is a gluon so that the quarks swap color.

There are basically two reasons why the quarks color swap is confusing:

  1. even three quarks (which is not the real picture of a proton or neutron) are constantly exchanging gluons and so swapping their color

  2. the three valence quark model is not realistic, because in reality, the protons and neutrons consist of a sea of quarks, antiquarks and gluons constantly interacting, appearing, disappearing, and only if you take the remaining valence quarks will you see theoretically three quarks.

enter image description here

The quarks and antiquarks are constantly interacting, swapping color, and it is the proton or neutron itself that needs to be color neutral.

You say that the swapping of color does not have any consequences on the quark itself, which is correct, since they are in confinement. We have never experimentally seen quarks outside of confinement, so how would we be able to tell if they (different color) behave identically.

You are saying that the difference is, that the different color quarks are interacting via different color gluons, correct, but these are all in superpositions of states.

So the answer to your question is that the different color quarks might behave differently, but we would need to experimentally see that outside confinement.

  • $\begingroup$ "Now quarks do not have a definite color, they are in a superposition of colors just like gluons." this is not correct, they do have temporarily a color for a given feynman diagram,which is the only way we know how to display their interactions ,see hyperphysics.phy-astr.gsu.edu/hbase/Forces/feyns.html $\endgroup$
    – anna v
    Oct 13, 2019 at 3:53
  • $\begingroup$ But because the quarks are confined within the baryon, these temporary color charges are inconsequential? $\endgroup$
    – RudyJD
    Oct 13, 2019 at 4:02
  • $\begingroup$ They have the consequence of the algebra, what exchanges are allowed affects how the attractive power of the gluons is expressed. You can see in the complicated proton depiction above that this becomes a large statistical problem. That is why QCD on the lattice has been developed, in order to sort of "integrate" over this infinity of exchanges . They are still working but have some success in describing mass differences between hadrons. $\endgroup$
    – anna v
    Oct 13, 2019 at 4:15
  • $\begingroup$ @annav thank you so much, I edited. $\endgroup$ Oct 13, 2019 at 15:47

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