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I Two Questions about the Color Force

  1. Aug 3, 2016 #1
    1. Is the color force the same as the strong nuclear force? I've heard that they're identical but I've also heard that the strong force is a "residual effect" of the color force. Not sure what to make of that

    2. This is the more important question: if Gravity is "monopolar" and EMism is "bipolar," is it correct to call the color force "tripolar?" On account of the three "charges" of the color force, "red," "blue," and "green?"
  2. jcsd
  3. Aug 3, 2016 #2


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    Staff: Mentor

    The strong force (the force binding nuclei to each other) is a residual effect of the color force. You will often see the two names used interchangeably unfortunately. I can't help you with your second question.
  4. Aug 3, 2016 #3
    So the color force holds quarks together to form hadrons, and the strong force holds hadrons together to form nuclei? And the weak force only has to do with radioactivity?
    Last edited: Aug 3, 2016
  5. Aug 3, 2016 #4


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    Staff: Mentor

    That's the basics of it, yes.
  6. Aug 4, 2016 #5
    I am confused about your term color force.
    There is only Electro-Weak, Strong and Gravity forces as far as we know right now.

    Electro-Weak (primarily the weak part) and Strong both play roles in the binding of a nucleus.
    However, the Strong force is what is responsible for holding hadrons together. It is mediated with gluons.
    Color is simply the parallel of charge in EM.

    The main difference between the behavior of EM and strong (besides 3 charges that is :D) lies in the fact that as quarks get separated the force between them increases rather than decreases. This eventually leads to color confinement which is why you can never have an isolated quark or particle with unbalanced color. Color confinement may be what you called color force.
  7. Aug 4, 2016 #6
    EM has one kind of charge (and it can have plus or minus sign, for example positron/electron have e+ e-)

    Color force has three kinds of charges (r,g,b) and each of those can have plus or minus sign (antiquarks have antired, antigreen and antiblue colors).
  8. Aug 4, 2016 #7


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    Staff: Mentor

    As I explained earlier, the color force and the strong force are essentially the same thing. The different terminology stems from the fact that physicists named the force binding nucleons to each other the strong force since it had to be much stronger than the EM force to overcome the latter's repulsive effect. It was only later that the force was investigated more thoroughly and the concept of color charge developed. With color charge came the name "color force".

    I don't believe the weak force plays a role in binding nuclei together.

    Also note that while the EM force and the weak force have been unified, they are still considered different forces in most of physics as far as I know.

    Color confinement is not a force. It's the explanation of why we cannot isolate color charged particles.
  9. Aug 4, 2016 #8


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    It isn't uncommon to use terminology that calls the force between hadrons in a nucleus the "strong nuclear force" or the "nuclear force" and calls what has been called the "color force" above the "strong force."

    Indeed, the force mediated by gluons between quarks is usually called the "strong force" and not the "color force" despite the confusion that this can create between that force and the one between protons and neutrons in a nucleus mediated mostly by pions which is a residual effect of the force between quarks mediated directly by gluons. For example, the coupling constant of the force mediated by gluons is commonly abbreviated alpha with a subscript "S" for "strong" and is called the strong force coupling constant.

    "is it correct to call the color force "tripolar?" On account of the three "charges" of the color force, "red," "blue," and "green?"

    Sort of.

    While quarks can have one of three color charges, antiquarks have one of three anti-color charges, anti-red, anti-blue or anti-green. And, while gluons in a crude sense are composed of a color charge paired with an anti-color charge, there are actually only eight distinct combinations of color charges in gluons, not nine. This is because the crude way of discussing color charge that I have just used isn't really perfectly accurate for describing permutations of color combinations in gluons in the SU(3) group of QCD.

    Of course, it also bears noting that all of the color charges are merely theoretical accounting tools used in QCD calculations and are not themselves ever observables. No scientific instrumentation in existence (or even theoretically imagined) can tell you the particular color charge of a particular quark, anti-quark or gluon. All hadrons are "color neutral" and all quarks and anti-quarks except pairs of top quarks and anti-top quarks (which are assumed to have a corresponding color and anti-color when produced) are confined in hadrons. Also, while "charge" is the usual way to describe a "color charge", there are isolated instances I have seen in publications where color charge is viewed as something more analogous to a polarization or parity than to an electro-magnetic charge (although it isn't any of these things, of course).

    We infer the number of colors mostly from the kinds of combinations of quarks and anti-quarks that are observed (and not observed) in hadrons (i.e. composite particles made up of quarks, antiquarks and gluons), from the branching fractions of different decays of those hadrons that are observed, and from the fact that any given quark is three times as likely as any given lepton to be produced in W and Z boson decays at tree level. Color neutrality of hadrons provides that the three simplest color neutral configurations are three color baryons, three anti-color anti-baryons, and color-anticolor pair mesons, although it also allows for example, for tetraquarks and pentaquarks which are just starting to be observed experimentally.

    In the same vein, it is also worth noting that there are generalized Yukawa interactions similar to QCD in which one can change the formulas involved by varying the number of colors and the normal of fermion flavors (i.e. the equivalent of the number of distinct kinds of quarks). For example, one can easily determine the formulas that would apply in a world with four colors and eight quark flavors instead of six. In Lattice QCD it is common to do calculations with a variety of number of color and number of quark flavor variations to estimate the real world value by showing trend lines as the number of colors and flavors change, since the real world values turn out to be very hard to calculate at. (The mass of a hypothetical pion is also often varied to show trend lines and make estimates, usually using values greater than the real world 140 MeV and working one's way towards lighter more realistic values.) Indeed, in low energy interactions where energy-conservation prohibits the creation of heavy quarks except in highly suppressed virtual loops, it is appropriate to do the equations of real world QCD with fewer than all 6 quark flavors. A 2+1 scenario that assumes an up and down quark of identical negligible mass and a strange quark, is particularly common for many Lattice QCD calculations.
    Last edited: Aug 5, 2016
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