© David Kaplan.
Just like in QED, the coupling of the strong force changes its measured value as one gets closer or farther from a quark. This is again due to the effect of virtual particles in the field. The difference, due to the fact that the gluons carry color and anti-color, is that the QCD coupling strength gets smaller as one probes it closer. This effect, called asymptotic freedom, means that the higher in energy you go, the weaker the force gets, up to a point where in the scattering, the quarks act as if they are free instead of bound. (Unit: 2)