Notes for a brief history of quantum gravity

TL;DR

The paper surveys seven decades of quantum gravity research, focusing on three principal directions—covariant perturbative quantization, canonical constraint quantization, and sum-over-histories path integrals—and tracing their development from the 1930s to 2000. It highlights key milestones such as the discovery of nonrenormalizability in perturbative gravity, the Hawking radiation paradigm, the emergence of string theory and loop quantum gravity, and the rise of spin foams and holography, while noting persistent conceptual divides and the lack of experimental confirmation. The analysis underscores how each approach yielded concrete frameworks (e.g., the $Wheeler{-}DeWitt$ equation, area/volume spectra in LQG, and AdS/CFT correspondences) and how cross-fertilization between lines has gradually grown, though a fully background-independent, nonperturbative theory remains elusive. Ultimately, the work clarifies the landscape of quantum gravity research, identifying both the milestones achieved and the major gaps that guide future pursuit toward a unified framework.

Abstract

I sketch the main lines of development of the research in quantum gravity, from the first explorations in the early thirties to nowadays.

Figures (1)

• Figure 1: The weave, in Wheeler's vision.