A non-perturbative Lorentzian path integral for gravity

TL;DR

This paper introduces a well-defined Lorentzian path integral for quantum gravity in three and four dimensions, utilizing causal triangulations, with a unique Wick rotation and a well-behaved transfer matrix, enabling further analysis.

Contribution

It constructs a non-perturbative Lorentzian quantum gravity model with a regularized path integral based on causal triangulations, ensuring mathematical consistency and physical interpretability.

Findings

Unique Wick rotation for Lorentzian geometries

Self-adjoint, bounded transfer matrix in finite volume

Absence of degenerate geometric phases seen in Euclidean models

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.