240 The Poetry of Physics and The Physics of Poetry
speaking the class of elementary particles consists of the quarks, the
leptons and the bosons. The hadrons like the proton (p) and the neutron
(n) are composites of quarks and strictly speaking are no longer
elementary but they are the subject of elementary particle physics and are
regarded by some as elementary particles.
Elementary particle physics comprises the study of leptons and
hadrons composed of quarks and their strong, electromagnetic and weak
interactions mediated by the bosons. The only other force in nature is
gravity, which is studied in the domain of general relativity and
cosmology. Attempts to integrate gravity with the three basic forces of
the strong, electromagnetic and weal interactions have met with little or
no success.
The electric and magnetic forces, as well as the interactions of light,
were shown by the work of Maxwell and Einstein to be the result of a
single electromagnetic interaction. They arise as a result of the exchange
of virtual photons. The nuclear force due the exchange of mesons has a
range of 10-13 cm, is the same for neutrons and protons and is
approximately 100 times the strength of the electric force.
The Nuclear Force or the Strong Interaction
The first attempts to explain the nuclear force took their inspiration from
the fact that the electromagnetic interaction between charged particles
arises from the exchange of a virtual photon. Because the photon is
massless, the range of the electromagnetic force is infinite since it is
always possible to choose a photon with an energy small enough such
that the product of its energy with time for its exchange is less than
Planck’s constant, h, and hence unobservable. With a massive particle,
however, the minimum energy of the exchanged particle is at least equal
to its rest mass energy, mc^2. The minimum time for transit of the
exchanged particle is t = R/c where R is the distance between the
interacting particles and c is the maximum allowed velocity for the
exchanged particle. If the exchanged particle is to remain unobservable
within the context of the uncertainty principle, then the product of its
energy, mc^2 , with its transit time, R/c must be less than or equal to h, or
mc^2 × R/c < h so that the range of the force is approximately R = h/mc.
The range of the interaction is inversely proportional to the rest mass
of the exchanged particle. We, therefore, see immediately why the range
of the electromagnetic force is infinite since the rest mass of the