Varying Constants

TL;DR

This paper reviews string-inspired models of varying fundamental constants, highlighting the tension between observed variations and experimental constraints, and emphasizing the importance of high-precision equivalence principle tests.

Contribution

It critically analyzes string-inspired models for varying constants and demonstrates the incompatibility of large variations with phenomenological constraints.

Findings

Large variations of the fine-structure constant are incompatible with experimental constraints.

High-precision tests of the universality of free fall are the best probes of varying constants.

Claims that fine-structure-constant variability does not violate the equivalence principle are refuted.

Abstract

We review some string-inspired theoretical models which incorporate a correlated spacetime variation of coupling constants while remaining naturally compatible both with phenomenological constraints coming from geochemical data (Oklo; Rhenium decay) and with present equivalence principle tests. Barring unnatural fine-tunings of parameters, a variation of the fine-structure constant as large as that recently ``observed'' by Webb et al. in quasar absorption spectra appears to be incompatible with these phenomenological constraints. Independently of any model, it is emphasized that the best experimental probe of varying constants are high-precision tests of the universality of free fall, such as MICROSCOPE and STEP. Recent claims by Bekenstein that fine-structure-constant variability does not imply detectable violations of the equivalence principle are shown to be untenable.