Elementary Particles, Quarks and Quantum Chromodynamics 245
The observation of meson production in nucleon-nucleon and photon-
nucleon reactions more or less confirmed Yukawa’s hypothesis that
the strong interaction arises from the exchange of virtual mesons.
This hypothesis received additional support by consideration of the
electromagnetic properties of the proton and the neutron, namely
their magnetic moment and charge distributions. The proton and the
neutron are spin one-half particles like the electron and hence their
electromagnetic properties have been described by Dirac’s relativistic
quantum mechanics. According to Dirac’s theory, the magnetic moment
of a spin half particle is related to its mass and charge such that the
magnetic moment μe of the electron is given by the expression, μe =
eh/2πmec, where e is the charge and me the mass of the electron. This
expression provides a fairly accurate description of the electron’s
magnetic moment. Physicists have been able to show that the
discrepancy between Dirac’s prediction and the experimental values of
the electron’s moment can be explained by the effects of the virtual
photons hovering about the electron.
In the case of the two nucleons, Dirac’s theory predicted that the
magnetic moment of the proton and the neutron, μp and μn, respectively,
are given by the following expressions, μp = eh/2πmpc = μN and μn = 0.
The experimental values of these quantities are in fact: μp = 2.79 μN and
μn = –1.91 μN. These discrepancies are quite considerable but can be
understood in terms of the virtual meson hovering about the proton. The
proton sometimes finds itself as a neutron and positive π meson. This
produces a current of positive charge which in turn generates a positive
magnetic moment which, when added to the Dirac magnetic moment,
gives a total value of 2.79 μN. The neutron, on the other hand, finds itself
at times as a proton and a negative π meson. This produces a current of
negative charge, which in turn generates the negative magnetic moment
of the neutron.
In addition to qualitatively explaining the magnetic moments of the
nucleons, Yukawa’s picture of virtual meson exchanges also helps to
explain the charge distribution of the proton and neutron. The charge
distribution of the proton and the neutron is obtained by scattering
electrons off of protons and neutrons. The electrons are assumed to be
point particles without any structure. This is a prediction of Dirac’s
theory. Since the magnetic moment of the electron does not differ very
greatly from that of the Dirac theory, we feel justified in assuming that
the electron is a point of charge. This hypothesis can be tested eventually