We analyze the problem of the Nielsen-Olesen unstable modes in the SU(2)
lattice gauge theory by means of a recently introduced gauge-invariant
effective action. We perform numerical simulations in the case of a constant
Abelian chromomagnetic field. We find that for lattice sizes above a certain
critical length the density of effective action shows a behaviour compatible
with the presence of the unstable modes. We put out a possible relation between
the dynamics of the unstable modes and the confinement.Comment: 15 pages, LaTeX2e file, 5 figure