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My Confusion Regarding the Strong Force

  1. Jan 30, 2015 #1
    In an effort to clarify my question, allow me first of all to state what I believe I DO understand:

    All observed elementary particles are either fermions or bosons. Ignoring the Higgs boson (which gives other particles mass via the Higgs mechanism), there are four forces operating in the universe:

    Photons are the force carriers of the electromagnetic field.
    W and Z bosons are the force carriers which mediate the weak force.
    (Hypothetical) gravitons are postulated to mediate gravity.
    Finally, gluons are the fundamental force carriers underlying the strong force.

    However, I am given to understand that the strong interaction is observable in two areas, and therein lies my confusion:

    WITHIN a nucleon (a proton or neutron), the strong force acts between quarks, and is mediated by gluons. Unlike all other forces, this manifestation of the strong force does not diminish in strength with increasing distance. After a limiting distance (less than about 0.8 femtometer, the radius of a nucleon) has been reached, it remains at a strength of about 10,000 newtons, no matter how much farther the distance between the quarks (resulting in quark confinement). THIS expression of the strong force is referred to as, simply, the Strong Force.


    OUTSIDE a nucleon (a proton or neutron), the strong force acts between nucleons, and is mediated by pions. And… just like all other forces, this manifestation of the strong force does diminish in strength with increasing distance. After a limiting distance (about 1 to 3 femtometers) it becomes ineffective, and is overpowered by the electromagnetic force. THIS expression of the strong force is referred to as the Nuclear Force or Residual Nuclear Force.


    The Strong Force acts WITHIN a nucleon (binding quarks together), is mediated by bosons called gluons, and does NOT diminish with distance, while the Nuclear Force acts OUTSIDE a nucleon (binding nucleons together), is mediated by bosons called pions, and DOES diminish with distance!

    Can you see my confusion?

    These two forces act on two different particles (quarks versus nucleons), are mediated by two different bosons (gluons versus pions), and have two different “spheres of influence” (0.8 fm versus 1-3 fm).

    Can it be explained so my humble intellect can understand why two such seemingly disparate forces are both classified under the Strong Force, and why would not the so-called Nuclear Force be a FIFTH universal force?

    Thanks in advance, fellow members, for your time and consideration,

  2. jcsd
  3. Jan 30, 2015 #2


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    While nucleons have no net color charge, they still consist of charged objects. Fundamentally, both effects follow the same rules - gluons interact with quarks and other gluons. The description of the interaction with pions is an effective description - it simplifies the interaction enough to make reasonable predictions.

    Do you know the van der Waals force? It is a related concept in electromagnetism: an attractive force between uncharged objects with internal charges.
  4. Jan 31, 2015 #3

    Thanks for your prompt response to my query, mfb; it is much appreciated.

    I had to reread your answer a number of times, and then review the article on the van der Waals Force, but I believe I understand. Essentially, then, the Strong Force acting between quarks (and the gluons themselves) in a single nucleon, ALSO acts between the quarks (and gluons) of neighboring nucleons a well, overcoming the electrostatic repulsion.

    Thanks again, my friend, and have a great day!

  5. Jan 31, 2015 #4


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  6. Jan 31, 2015 #5


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    The strong Strong force energy does not increase forever by distance.... It will start increasing until sometime at a threshold, it will be favored (energetically) to hadronize [make a pair of quark antiquark]... then you are getting at these "large distances"/"lower energies" the effective mediation of pions instead...
    In this context thinking of quarks bound with "strings" (of energy flux) is giving a better image of what is happening...
  7. Jan 31, 2015 #6
    Ahhhhhh. Now THAT explains it perfectly. The key point that I couldn't get straight in my head was what the gluons WITHIN the nucleon (which bind the quarks inside the proton or neutron) had to do with the pions (quark/antiquark particles) OUTSIDE of the nucleon. I just couldn't grasp where the pions were coming from.

    "Hadronization" is the key, and I appreciate your input.

  8. Jan 31, 2015 #7


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    It's an idea useful to remind you that. However I am not sure if that's the correct idea... In general a gluon itself would hadronize at large distances... It's a simplified view, and so I am not sure if it gives a complete answer... it's however getting a thumb up by me, because it's easily memorable ...
    I think the correct way to put it, is that the strong interactions that are mediated by the gluons, have infrared divergencies, and so the QCD at that point is not perturbatively treatable . In other words there is no a-priori way to speak for gluons/quarks below a scale (QCD-scale) where your initial theory stops working /is ill-defined, since that region is already out of your theory . In that case the quarks form condensates and break the symmetry spontaneously, raising up the Nambu Goldstone Bosons which are the pions in that point. The experimental verification for example of the SU(2) isospin with the pion-triplet and nucleon-doublet is showing this is the case. Advances in Lattice QCD show that an effective field theory mediated by mesons (pions) are the best way to treat strong interactions at low energies/large distance... So your Effective Field Theory of QCD is replaced by the Effective Field Theory for the mesons.

    PS. After some notification , from the above you can at least "get out of it" that QCD (quarks+gluons) don't work in all energy scales, and below some scale, it is replaced by another effective field theory.
    Last edited: Jan 31, 2015
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