  A theory governing the metric and matter fields in spacetime is {\it locally
causal} if the probability distribution for the fields in any region is
determined solely by physical data in the region's past, i.e. it is independent
of events at space-like separated points. General relativity is manifestly
locally causal, since the fields in a region are completely determined by
physical data in its past. It is natural to ask whether other possible theories
in which the fundamental description of space-time is classical and geometric
-- for instance, hypothetical theories which stochastically couple a classical
spacetime geometry to a quantum field theory of matter -- might also be locally
causal.
  A quantum theory of gravity, on the other hand, should allow the creation of
spacetimes which violate local causality at the macroscopic level. This paper
describes an experiment to test the local causality of spacetime, and hence to
test whether or not gravity behaves as quantum theories of gravity suggest, in
this respect. The experiment will either produce direct evidence that the
gravitational field is not locally causal, and thus weak confirmation of
quantum gravity, or else identify a definite limit to the domain of validity of
quantum theory.
