Gravitational Redshift, Equivalence Principle, and Matter Waves

TL;DR

This paper reviews tests of gravitational redshift using matter waves and clocks, clarifies their relation to free fall tests, and proposes a space-based atom interferometer to significantly improve redshift measurement precision.

Contribution

It introduces a framework distinguishing redshift violations coupled or independent of UFF violations and proposes the 'Geodesic Explorer' space mission for ultra-precise redshift testing.

Findings

Clock comparisons and atom interferometers are sensitive to similar effects in certain scenarios.

Type III redshift violations are currently poorly constrained.

The 'Geodesic Explorer' could improve redshift test accuracy by up to 12 orders of magnitude.

Abstract

We review matter wave and clock comparison tests of the gravitational redshift. To elucidate their relationship to tests of the universality of free fall (UFF), we define scenarios wherein redshift violations are coupled to violations of UFF ("type II"), or independent of UFF violations ("type III"), respectively. Clock comparisons and atom interferometers are sensitive to similar effects in type II and precisely the same effects in type III scenarios, although type III violations remain poorly constrained. Finally, we describe the "Geodesic Explorer," a conceptual spaceborne atom interferometer that will test the gravitational redshift with an accuracy 5 orders of magnitude better than current terrestrial redshift experiments for type II scenarios and 12 orders of magnitude better for type III.