Testing the Equivalence Principle in Quantum Physics

TL;DR

This paper investigates how nonmetric couplings in gravity theories can cause polarization-dependent light speed variations, leading to potential violations of the Equivalence Principle, and uses pulsar data to set bounds on these effects.

Contribution

It extends previous work by analyzing light propagation near rotating black holes with nonmetric curvature couplings and constrains these couplings using pulsar observations.

Findings

Upper bounds on nonmetric curvature couplings derived from pulsar data

QED-like coupling strength is at least 37 orders of magnitude weaker than the constrained bounds

Demonstrates potential observational tests for nonmetric gravity theories

Abstract

We showed in a previous paper that a wide class of nonmetric theories of gravity encompassed by the $\chi g$ formalism predict that the speed of light rays depends on the their polarization direction relative to directions singled out by the gravitational field. This gravity-induced birefringence of space is a violation of the Equivalence Principle and is due to the nonmetric coupling between gravity and electromagnetism. In this paper we analyze the propagation of light in the gravitational field of a rotating black hole when nonmetric couplings to curvature are included and compute the time delay between rays with orthogonal polarizations. We obtain an upper bound on the strength of QED like and CP violating curvature couplings using time delay data for pulsar PSR 1937+21. By comparison the corresponding coupling strength for QED coupling is 37 orders of magnitude less.