 Early attempts at quantizing general relativity by Bergmann, Dirac, Peres, Wheeler, DeWitt and others in the 1950s and 1960s worked with a seemingly natural choice for configuration variables, namely geometric variables gij corresponding to the various components of the ‘three-metric’ describing the intrinsic geometry of the given spatial slice of spacetime. One can think about arriving at this via an arbitrary slicing of a 4-dimensional “block” universe by 3-dimensional spacelike hypersurfaces. The conjugate momenta πij then effectively encode the time rate-of-change of the metric, which, from the 4-dimensional perspective, is directly related to the extrinsic curvature of the slice (meaning the curvature relative to the spacetime in which the slice is embedded). This approach is known as ‘geometrodynamics’ since it views general relativity as describing the dynamics of spatial geometry.