Authors: John A. Gowan
The exact origin of the strong force (holding compound atomic nuclei together) is not yet a completely settled matter. Some authors (Robert Oerter) attribute this force to the exchange of virtual mesons between protons and neutrons (as in the original theory of Yukawa), while others (Frank Close) claim this old model has been superseded by the modern theory of quantum chromodynamics (QCD), and attribute the binding of nucleons to a magnetic analog of the color charge, originating in the exchange of gluons between quarks (theory of Gell-Mann). Still others (Nicholas Mee), think both gluons and mesons play a role. My own view is that the original Yukawa model is correct, but the reader will have to make his own choice, and realize that not all experts would agree with me (or each other). My reasons for preferring the original Yukawa model are several: 1) Yukawa's mathematics work, correctly predicting the mass of the exchanged mesons. If we deny the validity of this model, what are we to do with this mathematical structure and these mesons? 2) If the color-magnetism theory is correct, then all proton-neutron combinations should be equivalent, whereas we know that some are favored - the alpha particle, for example, and all combinations of even numbers of nucleons. There are also "magic numbers" of nucleons, combinations of special stability among the heavier nuclei. Finally, why do we not find isolated neutron-neutron pairings? The pion exchange model answers all these questions. 3) Because mesons carry both flavor and color charges, it is also possible that both effects are at work simultaneously (after all, gluons do attract each other, so if nucleons are sufficiently closely packed, there might be gluon-gluon attraction between nucleons as well as within nucleons). Mesons carry color-anticolor charges (always of the same color), so they can neatly substitute themselves for the color charge of a baryon's quark. Because they also carry flavor/anti-flavor charges (in this case not necessarily of the same flavor: d and anti- u, for example), they can just as neatly change a baryon's "u" quark into a "d" quark (and hence a proton into a neutron), or vice versa. A "magnetic" color effect, however, could not by itself change a quark's flavor. The exchange of mesons allows the neutron to satisfy its natural tendency to undergo beta decay via a virtual reaction rather than an actual decay. 4) A true magnetic analog of the color charge is expressed as "asymptotic freedom" - the increasing freedom of movement of the quarks as they approach each other at the center of the baryon. Hence this is an inwardly directed "magnetic" effect, typical of the strong force, not a likely source of binding energy beyond the confines of the baryon. The symmetry-keeping role of the color charge is to permanently confine the fractional charges of the quarks to whole quantum charge units. While "asymptotic freedom" is completely understandable within this conservation context as a "local gauge symmetry" effect, the external binding of other baryons is not.
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