Small-Scale Structure of Spacetime: Bounds and Conjectures

TL;DR

This review discusses astrophysical constraints on spacetime smoothness and explores implications for quantum spacetime structure, including the possibility of a fundamental length scale differing from the Planck length.

Contribution

It synthesizes astrophysical bounds on spacetime smoothness and examines their implications for quantum spacetime theories, proposing a potential deviation from the Planck length.

Findings

Astrophysical bounds limit spacetime smoothness.

Absence of vacuum Cherenkov radiation constrains quantum spacetime models.

Fundamental length scale may differ from the Planck length.

Abstract

This review consists of two parts. The first part establishes certain astrophysical bounds on the smoothness of classical spacetime. Some of the best bounds to date are based on the absence of vacuum Cherenkov radiation in ultrahigh-energy cosmic rays. The second part discusses possible implications of these bounds for the quantum structure of spacetime. One conjecture is that the fundamental length scale of quantum spacetime may be different from the Planck length.