The behaviour of rods and clocks in general relativity, and the meaning of the metric field

The notion that the metric field in general relativity can be understood as a property of space-time rests on a feature of the theory sometimes called universal coupling -- the claim that rods and clocks "measure" the metric in a way that is independent of their constitution. It is pointed out that this feature is not strictly a consequence of the central dynamical tenets of the theory, and argued that the metric field would better be regarded as a (possibly emergent) field in space-time, rather than as the very fabric of space-time itself.