  We present a high-precision Monte Carlo study of the O(3) spin theory on the
lattice in four dimensions. This model exhibits interesting dynamical features,
in particular in the broken-symmetry phase, where suitable boundary conditions
can be used to enforce monopole-like topological excitations. We investigate
the Euclidean time propagation and the features of these excitations close to
the critical point, where our numerical results show an excellent quantitative
agreement with analytic predictions derived from purely
quantum-field-theoretical tools by G. Delfino. We conclude by commenting on the
implications of our findings for a conjectured violation of Derrick's theorem
at the quantum level and on the consequences in various areas of physics,
ranging from condensed matter to astro-particle physics.
