The problem of extending the Quantum Theory of Fields to
include a description of decay processes has provoked a rapid
growth of interest in recent years. This problem was
temporarily by-passed in the early attempts to formulate a
relativistic Quantum Field Theory for the obvious reason
that it was simpler initially to ignore decay phenomena and
to consider only the collision processes of stable particles.
The inadequacy of a field theory of stable particles is
evident from the fact that among the sixteen experimentally
established particles, and of course their sixteen antiparticles
— although not all particles are distinct from
their anti-particles — only four; the proton, electron,
photon and neutrino, are stable
