Spontaneous Dimensional Reduction in Quantum Gravity

TL;DR

This paper reviews evidence and discusses the concept of spontaneous dimensional reduction to two dimensions near the Planck scale in quantum gravity, exploring its physical implications and potential explanations.

Contribution

It clarifies the meaning of 'dimension' in quantum gravity and summarizes evidence for dimensional reduction across various approaches.

Findings

Multiple quantum gravity approaches suggest dimensional reduction near the Planck scale.

The phenomenon indicates a transition to two-dimensional spacetime at high energies.

Physical explanations for dimensional reduction remain an active area of research.

Abstract

Hints from a number of different approaches to quantum gravity point to a phenomenon of "spontaneous dimensional reduction" to two spacetime dimensions near the Planck scale. I examine the physical meaning of the term "dimension" in this context, summarize the evidence for dimensional reduction, and discuss possible physical explanations.