Theoretical Aspects of the Equivalence Principle

TL;DR

This paper reviews the theoretical foundations of the Equivalence Principle, discusses potential violations in advanced theories, and emphasizes the importance of improved experimental tests to explore fundamental physics questions.

Contribution

It highlights the tension between the EP's absolute coupling constants and modern theories suggesting dynamical structures, and discusses phenomenology of EP violations in dilaton-like models.

Findings

EP violation signals linked to nuclear Coulomb energy and quark mass variations.

Theoretical arguments suggest small but detectable EP violations.

Motivates improved experimental tests to explore fundamental physics.

Abstract

We review several theoretical aspects of the Equivalence Principle (EP). We emphasize the unsatisfactory fact that the EP maintains the absolute character of the coupling constants of physics while General Relativity, and its generalizations (Kaluza-Klein,..., String Theory), suggest that all absolute structures should be replaced by dynamical entities. We discuss the EP-violation phenomenology of dilaton-like models, which is likely to be dominated by the linear superposition of two effects: a signal proportional to the nuclear Coulomb energy, related to the variation of the fine-structure constant, and a signal proportional to the surface nuclear binding energy, related to the variation of the light quark masses. We recall the various theoretical arguments (including a recently proposed anthropic argument) suggesting that the EP be violated at a small, but not unmeasurably small level. This motivates the need for improved tests of the EP. These tests are probing new territories in physics that are related to deep, and mysterious, issues in fundamental physics.