  Black holes emit high energy particles which induce a finite density
potential for any scalar field $\phi$ coupling to the emitted quanta. Due to
energetic considerations, $\phi$ evolves locally to minimize the effective
masses of the outgoing states. In theories where $\phi$ resides at a metastable
minimum, this effect can drive $\phi$ over its potential barrier and
classically catalyze the decay of the vacuum. Because this is not a tunneling
process, the decay rate is not exponentially suppressed and a single black hole
in our past light cone may be sufficient to activate the decay. Moreover,
decaying black holes radiate at ever higher temperatures, so they eventually
probe the full spectrum of particles coupling to $\phi$. We present a detailed
analysis of vacuum decay catalyzed by a single particle, as well as by a black
hole. The former is possible provided large couplings or a weak potential
barrier. In contrast, the latter occurs much more easily and places new
stringent limits on theories with hierarchical spectra. Finally, we comment on
how these constraints apply to the standard model and its extensions, e.g.
metastable supersymmetry breaking.
