ABSTRACT
The theory of the free field describes an unchangeable world in which the energy and momentum of every particle of the system is conserved separately.
The variety of phenomena which we observe requires instead some form of interaction between the fields. In this case, as we saw in the classical limit, Section 5.3, particles can exchange energy and momentum giving rise to scattering processes or to the emission and absorption of light; the Sun can shine, the sky can be blue and our eyes can perceive the external world via photons absorbed by the retina.
