Essential quantum instability of the perturbative Yang-Mills vacuum

It is proved that there exist gauge field configurations whose energy density is lower than that of the perturbative Yang-Mills vacuum by a quantity ΔE∼bΛ4 (Λ is the ultraviolet cut-off) where b remains finite when G→0. This striking physical situation, which prevents the use of perturbation theory even at extremely short distances, is what has been called essential instability