# Strong force

Hi this is a very elementary question. Is the strong force a color force? Or a favor force? Or a combination color and favor?

Also with photons (massless) we have a r^-2 force but with massless gluons the force increase with r (do not know as what function) how can this be?

Thanks.

Meir Achuz
Homework Helper
Gold Member
The strong force depends on both color and flavor.
The increase in V with r for gluon exchanges is because the gluons interac wilt each other, while photons no not.

The strong force depends on both color and flavor.
The increase in V with r for gluon exchanges is because the gluons interac wilt each other, while photons no not.

Thank you this is clear and concise.

May I ask another question? Why do gluons interact with each other while photons do not?

The strong force depends on both color and flavor.
I do not agree with that. The strong force does not know about flavor, gluons only know about color charge. The above statement could be made more precise, if one talks about the effective van der Walls force between hadrons for instance. That may be confusing however.

The reason gluons interact with each other is essentially because SU(3) is non-abelian. Directly in the lagragian, one has self interactions between gluons. Another way to say it, gluons belong the adjoint representation, as such are not color singlet, and therefore carry a non-vanishing color charge, to which other gluons answer.

Meir Achuz
Homework Helper
Gold Member
I do not agree with that. The strong force does not know about flavor, gluons only know about color charge. The above statement could be made more precise, if one talks about the effective van der Walls force between hadrons for instance. That may be confusing however.
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"Strong force" means, for me, the strong force between quarks and between hadrons.
This force depends on more than just the flavor independent gluon exchange.
Your complaint would be appropriate for the "gluon exchange vertex", not the strong force.

"Strong force" means, for me, the strong force between quarks and between hadrons. This force depends on more than just the flavor independent gluon exchange. Your complaint would be appropriate for the "gluon exchange vertex", not the strong force.
It's all a matter of interpretation of the level of the original question. In any case, at the very least your answer should have included "residual strong force", to make it clear that the "gluon vertex" does not depend on flavor. If you look into the literature dating back to when QCD had not been discovered yet, for instance Coleman's first Erice lecture on symmetry, people already knew that the total force between hadrons included "very strong" and "medium strong" forces. This distinction is the origin of the name "strong force".

Otherwise, I can take the opposite illustration : when somebody calls "electroweak interaction between nucleons", it never implicitly includes "gluon exchange as well".