Introduction to Modern Canonical Quantum General Relativity

TL;DR

This paper introduces canonical quantum general relativity, also known as loop quantum gravity, emphasizing its background independence, mathematical structures, and implications like spacetime discreteness as a natural UV cutoff.

Contribution

It provides a self-contained, accessible overview of the modern formulation of canonical quantum gravity based on connection variables, highlighting its mathematical innovations and physical predictions.

Findings

Spacetime discreteness predicted by the theory.

Background independence leads to new mathematical structures.

Quantum gravity acts as a natural UV cutoff.

Abstract

This is an introduction to the by now fifteen years old research field of canonical quantum general relativity, sometimes called "loop quantum gravity". The term "modern" in the title refers to the fact that the quantum theory is based on formulating classical general relativity as a theory of connections rather than metrics as compared to in original version due to Arnowitt, Deser and Misner. Canonical quantum general relativity is an attempt to define a mathematically rigorous, non-perturbative, background independent theory of Lorentzian quantum gravity in four spacetime dimensions in the continuum. The approach is minimal in that one simply analyzes the logical consequences of combining the principles of general relativity with the principles of quantum mechanics. The requirement to preserve background independence has lead to new, fascinating mathematical structures which one does not see in perturbative approaches, e.g. a fundamental discreteness of spacetime seems to be a prediction of the theory providing a first substantial evidence for a theory in which the gravitational field acts as a natural UV cut-off. An effort has been made to provide a self-contained exposition of a restricted amount of material at the appropriate level of rigour which at the same time is accessible to graduate students with only basic knowledge of general relativity and quantum field theory on Minkowski space.