  Perturbative estimates suggest that extended topological defects such as
cosmic strings emit few particles, but numerical simulations of the fields from
which they are constructed suggest the opposite. In this paper we study the
decay of the two-dimensional prototype of strings, domain walls in a simple
scalar theory, solving the underlying quantum field theory in the Hartree
approximation. We conclude that including the quantum effects makes the picture
clear: the defects do not directly transform into particles, but there is a
non-perturbative channel to microscopic classical structures in the form of
propagating waves and persistent localised oscillations, which operates over a
huge separation of scales. When quantum effects are included, the microscopic
classical structures can decay into particles.
