A non-perturbative Lorentzian path integral for gravity

TL;DR

A well-defined regularized path integral is constructed for Lorentzian quantum gravity in terms of dynamically triangulated causal space-times, where the degenerate geometric phases found in dynamicallyTriangulated Euclidean gravity are not present.

Abstract

A well-defined regularized path integral for Lorentzian quantum gravity in three and four dimensions is constructed, given in terms of a sum over dynamically triangulated causal space-times. Each Lorentzian geometry and its associated action have a unique Wick rotation to the Euclidean sector. All space-time histories possess a distinguished notion of a discrete proper time. For finite lattice volume, the associated transfer matrix is self-adjoint and bounded. The reflection positivity of the model ensures the existence of a well-defined Hamiltonian. The degenerate geometric phases found previously in dynamically triangulated Euclidean gravity are not present. The phase structure of the new Lorentzian quantum gravity model can be readily investigated by both analytic and numerical methods.