The fundamental constants and their variation: observational and theoretical status

TLDR

Investigating variations of fundamental constants is crucial for reconciling disparate results and interpreting recent quasar absorption claims of changes in the fine‑structure constant and electron‑to‑proton mass ratio. The review aims to describe experimental bounds on fundamental‑constant variation, explain how each measurement constrains a constant, and highlight underlying assumptions. It reviews metrological definitions, experimental constraints on the fine‑structure constant, gravitational, weak, strong couplings, and electron‑to‑proton mass ratio, and discusses theoretical models such as Kaluza‑Klein and string theories, cosmological implications, and links to tests of general relativity.

Abstract

This article describes the various experimental bounds on the variation of the fundamental constants of nature. After a discussion of the role of fundamental constants, their definition and link with metrology, it reviews the various constraints on the variation of the fine-structure constant, the gravitational, weak- and strong-interaction couplings and the electron-to-proton mass ratio. The review aims (1) to provide the basics of each measurement, (2) to show as clearly as possible why it constrains a given constant, and (3) to point out the underlying hypotheses. Such an investigation is of importance in comparing the different results and in understanding the recent claims of the detection of a variation of the fine-structure constant and of the electron-to-proton mass ratio in quasar absorption spectra. The theoretical models leading to the prediction of such variation are also reviewed, including Kaluza-Klein theories, string theories, and other alternative theories. Cosmological implications of these results are also discussed. The links with the tests of general relativity are emphasized.