244 The Poetry of Physics and The Physics of Poetry
there is no reason why the meson would have to be returned. On the
basis of Yukawa’s theory, therefore, one would expect to produce
mesons by providing a nucleon with enough energy to provide for the
rest mass energy of the meson. One mechanism for providing this energy
is to knock one nucleon against another hard enough so that the kinetic
energy of the incoming nucleon can be transformed into the rest mass of
the π meson, allowing a virtual meson to escape. Yukawa’s theory,
therefore, predicted that once protons could be accelerated to kinetic
energies of 140 MeV or more that the following reactions would be
observed: p + p → p + p + πo or p + p → p + n + π+ or p + n → p + p + π.
Once accelerators were build that could achieve these energies, the
above reactions were indeed observed lending greater credence to
Yukawa’s theory. Mesons were also released from nucleons by
bombarding the nucleons with high-energy photons so that reactions of
the following type were observed: γ + N → π + N. In these reactions, the
energy of the photons are directly absorbed by the nucleon and converted
into the rest mass and kinetic energy of the meson.
As the bombarding energies of the nucleons and photons were
increased, it was discovered that two or three or more π mesons could
be released from a single nucleon to produce reactions of the form
N + N → N + N + π + π + ... or γ + N → N + π + π + ....
The number of mesons produced seems to be limited only by the
amount of kinetic energy available that may be transformed into the rest
mass energy of the mesons. The probability for a particular multiple
meson production reaction decreases as the number of mesons increase,
nevertheless reactions in which a large number of mesons are produced
are still observed. The number of nucleons in production reactions
always remained the same, however. But it is possible to have a reaction
in which a nucleon-antinucleon pair is created such as p + p → p +
p +p + p. In this reaction the number of nucleons remains the same
since an antinucleon is counted as negative nucleon. The conservation of
the number of nucleons in production reactions leads to the concept of
assigning a quantum number to nucleons and pions called the baryon
number. The proton and neutron each have baryon number 1, the
antiproton and antineutron each have baryon number –1, and the pions
and leptons have baryon number zero. The number of baryons is always
conserved in all reactions including the decay of the neutron,
n → p + e– +ν.