  In view of the enormous difficulties we seem to face in quantizing general
relativity, we should perhaps consider the possibility that gravity is a
fundamentally classical interaction. Theoretical arguments against such mixed
classical-quantum models are strong, but not conclusive, and the question is
ultimately one for experiment. I review some work in progress on the
possibility of experimental tests, exploiting the nonlinearity of the
classical-quantum coupling, that could help settle this question.
