One prerequisite for a theory of quantum gravity is that it reproduce General Relativity in some domain. Clearly this cannot be done using the techniques of General Relativity, which discuss fundamental properties of classical manifolds. Recall that a manifold is based on a numerical continuum, such as the real or complex numbers. A complete derivation of GR is therefore required (yes, required) to reconstruct these numerical continua from the axioms of quantum gravity.
Numbers aren't free, but perhaps they do grow on the trees of measurement.
6 years ago