Euclidean and Lorentzian Quantum Gravity - Lessons from Two Dimensions
J. Ambjorn, J. L. Nielsen, J. Rolf, R. Loll
TL;DR
Two-dimensional quantum gravity serves as a tractable, background-independent laboratory for studying quantum space-time. The paper shows that Euclidean 2D gravity emerges from a Lorentzian theory only when spatial topology changes are allowed, yielding a fractal space-time with intrinsic dimension $d_H=4$ but spectral dimension $d_s=2$. Using nonperturbative dynamical triangulations, it derives exact amplitudes and demonstrates the fractal structure of quantum geometry, linking diffusion-based probes to dimensionality. These insights illuminate the Euclidean–Lorentzian relationship and provide guidance for exploring higher-dimensional quantum gravity.
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.