246 The Poetry of Physics and The Physics of Poetry
once an experiment can be devised in which we can scatter electrons off
each other. In the meantime, we proceed with the assumption that any
structure that the electron might have is small compared with that of the
proton and neutron and hence can be ignored.
The Dirac theory also predicts that the proton will be a point charge
and that the neutron will have absolutely no charge distribution at all. We
expect major violations of this prediction, however, because of the
presence of virtual mesons, which also carry charge. Probing the proton
and the neutron with electrons is equivalent to taking a picture of the
nucleons. Since we can not see the proton or the neutron with our eyes,
we have devised a method of feeling our way about the nucleons by
bouncing electrons off these particles and determining their shape by
studying the distribution of angles into which the electrons are scattered
as a function of the energy of the incoming electrons. These studies
reveal the charge distribution for the nucleons. The majority of the
proton’s charge is located at its core; however, the charge distribution
extends to a distance of the order of 10-13 cm where it tails off. The edges
of the proton’s charge distribution may be interpreted as arising from the
charge of the positive π mesons hovering about the proton at a distance
of approximately 10-13 cm. The charge distribution of the neutron, on the
other hand, reflects the fact that the neutron spends part of its time as a
proton and a negative π meson. This explains the positive core of charge
of the neutron, which is surrounded by a tail of negative charge that
extends for about 10-13 cm about the neutron.
Yukawa’s hypothesis of virtual meson exchange between nucleons
ties together a great number of phenomena into a neat and tidy package
by providing all at once an explanation of the range of the nuclear force,
the production of mesons in nucleon collisions, the magnetic moments of
the nucleons and their deviations from the Dirac theory, and finally the
charge distribution of the proton and the neutron. Yukawa’s theory,
however, only provides a qualitative understanding of the strong
interaction. It does not provide a quantitative description of the nuclear
force between nucleons, nor does it indicate how many mesons are
exchanged when nucleons interact. The theory of strong interaction is
further complicated, as we shall shortly see, by the existence of other
mesons in addition to the pion. These mesons will also be exchanged
between nucleons and hence one has the additional problem of deciding
which mesons are exchanged in a particular reaction. Finally, with the
discovery of the pion and other mesons, and understanding of the nuclear