In general relativity, the space-time metric provides the physical field of gravity and is subject to dynamical laws. For a complete and uniform fundamental description of nature, the gravitational force, and thus space-time, is to be quantized by implementing the usual features of quantum states, endowing it with quantum fluctuations and imposing the superposition principle. Only then do we obtain a fully consistent description of nature, since matter as well as the non-gravitational forces are quantum, described by quantum stress-energy which can couple to gravity only via some quantum version of the Einstein tensor.

An implementation of this program requires a clear distinction of the different concepts used in general relativity. One normally works with the line element for metric purposes, but this is a combination of metric tensor components and coordinate differentials (separating events from each other). Only the geometry is dynamical, not the coordinates. After quantization, we may have a representation for geometrical observables such as the sizes of physically characterized regions, but not for coordinates or distances between mathematical points as mere auxiliary ingredients. Metrics or other tensors may arise in an effective form from quantum gravity, but they are not the basic object. One has to dig deeper, similarly to hydrodynamics where the continuous fluid flow is not suitable for a fundamental quantum theory, which must, rather, be based on an atomic picture.