A New Test of the Einstein Equivalence Principle and the Isotropy of Space

TL;DR

This paper tests the Einstein Equivalence Principle by examining gravity-induced birefringence of light, using polarization data from extragalactic sources to set a significantly improved constraint on violations.

Contribution

It demonstrates that a broad class of nonmetric gravity theories predict polarization-dependent light propagation, and provides the strongest observational constraint to date.

Findings

New constraint on birefringence is 10^8 times sharper than previous limits.

Gravity-induced birefringence can alter light polarization in gravitational fields.

Data from extragalactic sources effectively tests fundamental physics principles.

Abstract

Recent research has established that nonsymmetric gravitation theories like Moffat's NGT predict that a gravitational field singles out an orthogonal pair of polarization states of light that propagate with different phase velocities. We show that a much wider class of nonmetric theories encompassed by the $\chi g$ formalism predict such violations of the Einstein equivalence principle. This gravity-induced birefringence of space implies that propagation through a gravitational field can alter the polarization of light. We use data from polarization measurements of extragalactic sources to constrain birefringence induced by the field of the Galaxy. Our new constraint is $10^8$ times sharper than previous ones.