  Thermalisation of configurations with initial white noise power spectrum is
studied in numerical simulations of a classical one-component $\Phi^4$ theory
in 2+1 dimensions, coupled to a small amplitude homogenous external field. The
study is performed for energy densities corresponding to the broken symmetry
phase of the system in equilibrium. The effective equation of the order
parameter motion is reconstructed from its trajectory which starts from an
initial value near the metastable point and ends in the stable ground state.
This phenomenological theory quantitatively accounts for the decay of the false
vacuum. The large amplitude transition of the order parameter between the two
minima displays characteristics reflecting dynamical aspects of the Maxwell
construction.
