Euclidean and Lorentzian Quantum Gravity - Lessons from Two Dimensions

TL;DR

This paper reviews two-dimensional quantum gravity, highlighting the relationship between Euclidean and Lorentzian sectors, and shows how allowing topology changes leads to a fractal space-time with a Hausdorff dimension of four.

Contribution

It demonstrates that Euclidean 2D quantum gravity can be derived from Lorentzian quantum gravity through analytic continuation with topology changes, revealing fractal properties of space-time.

Findings

Euclidean and Lorentzian 2D quantum gravity are connected via analytic continuation.

Allowing topology changes results in a fractal space-time with Hausdorff dimension four.

Spectral dimension of quantum space-time remains two, indicating some two-dimensional features.

Abstract

No theory of four-dimensional quantum gravity exists as yet. In this situation the two-dimensional theory, which can be analyzed by conventional field-theoretical methods, can serve as a toy model for studying some aspects of quantum gravity. It represents one of the rare settings in a quantum-gravitational context where one can calculate quantities truly independent of any background geometry. We review recent progress in our understanding of 2d quantum gravity, and in particular the relation between the Euclidean and Lorentzian sectors of the quantum theory. We show that conventional 2d Euclidean quantum gravity can be obtained from Lorentzian quantum gravity by an analytic continuation only if we allow for spatial topology changes in the latter. Once this is done, one obtains a theory of quantum gravity where space-time is fractal: the intrinsic Hausdorff dimension of usual 2d Euclidean quantum gravity is four, and not two. However, certain aspects of quantum space-time remain two-dimensional, exemplified by the fact that its so-called spectral dimension is equal to two.