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Consistency of semiclassical gravity

We discuss some subtleties that arise in the semiclassical approximation to quantum gravity. We show that integrability conditions prevent the existence of Tomonaga--Schwinger time functions on the space of three-metrics but admit them on superspace. The concept of semiclassical time is carefully examined. We point out that central charges in the matter sector spoil the consistency of the semiclassical approximation, unless the full quantum theory of gravity and matter is anomaly-free. We finally discuss consequences of these considerations for quantum field theory in flat spacetime, but with arbitrary foliations.