  We show that if a massive body is put in a quantum superposition of spatially
separated states, the mere presence of a black hole in the vicinity of the body
will eventually destroy the coherence of the superposition. This occurs
because, in effect, the gravitational field of the body radiates soft gravitons
into the black hole, allowing the black hole to acquire "which path"
information about the superposition. A similar effect occurs for quantum
superpositions of electrically charged bodies. We provide estimates of the
decoherence time for such quantum superpositions. We believe that the fact that
a black hole will eventually decohere any quantum superposition may be of
fundamental significance for our understanding of the nature of black holes in
a quantum theory of gravity.
