  Motivated by recent interest in TeV-scale gravity and especially by the
possibility of fast baryon decay mediated by virtual black holes, we study
another dangerous aspect of spacetime foam interactions: lepton flavor
violation. We correlate existing limits on gravity-induced decoherence in the
neutrino sector with a lower bound on the scale of quantum gravity, and find
that if spacetime foam interactions do not allow an S-matrix description the UV
cutoff is well beyond the electroweak scale. This suggests that string theory
provides the appropriate framework for description of quantum gravity at the
TeV-scale.
