  We study the contribution to vacuum decay in field theory due to the
interaction between the long and short-wavelength modes of the field. The field
model considered consists of a scalar field of mass $M$ with a cubic term in
the potential. The dynamics of the long-wavelength modes becomes diffusive in
this interaction. The diffusive behaviour is described by the reduced Wigner
function that characterizes the state of the long-wavelength modes. This
function is obtained from the whole Wigner function by integration of the
degrees of freedom of the short-wavelength modes. The dynamical equation for
the reduced Wigner function becomes a kind of Fokker-Planck equation which is
solved with suitable boundary conditions enforcing an initial metastable vacuum
state trapped in the potential well. As a result a finite activation rate is
found, even at zero temperature, for the formation of true vacuum bubbles of
size $M^{-1}$. This effect makes a substantial contribution to the total decay
rate.
