Semiclassical stability of supergravity vacua

We discuss the existence of instantonic decay modes which would indicate a semiclassical instability of the vacua of 10- and 11-dimensional supergravity theories. Decay modes whose spin structures are incompatible with those of supersymmetric vacua have previously been constructed, and we present generalizations including those involving nontrivial dilaton and antisymmetric tensor fields. We then show that the requirement that any instanton describing supersymmetric vacuum decay should admit both a zero momentum hypersurface from which we describe the subsequent Lorentzian evolution and a spin structure at infinity compatible with the putative vacuum excludes all such decay modes, except those with unphysical energy-momentum tensors which violate the dominant energy condition.