Testing the Einstein Equivalence Principle with two Earth-orbiting clocks

TL;DR

This paper proposes an optimal space experiment using Earth-orbiting atomic clocks to test the Einstein Equivalence Principle via gravitational redshift, achieving significantly improved accuracy over previous experiments.

Contribution

It identifies optimal orbit configurations and estimates achievable measurement accuracy for current and future space clocks in testing EEP.

Findings

Optimal orbit parameters for maximum measurement accuracy.

Current space clocks can improve EEP test precision by over 100 times.

Future optical clocks could reach an accuracy of 3×10⁻¹⁰ in EEP testing.

Abstract

We consider the problem of testing the Einstein Equivalence Principle (EEP) by measuring the gravitational redshift with two Earth-orbiting stable atomic clocks. For a reasonably restricted class of orbits we find an optimal experiment configuration that provides for the maximum accuracy of measuring the relevant EEP violation parameter. The perigee height of such orbits is $\sim$~1,000~km and the period is 3--5~hr, depending on the clock type. For the two of the current best space-qualified clocks, the VCH-1010 hydrogen maser and the PHARAO cesium fountain clock, the achievable experiment accuracy is, respectively, $1\times10^{-7}$ and $5\times10^{-8}$ after 3 years of data accumulation. This is more than 2 orders of magnitude better than achieved in Gravity Probe A and GREAT missions as well as expected for the RadioAstron gravitational redshift experiment. Using an anticipated future space-qualified clock with a performance of the current laboratory optical clocks, an accuracy of $3\times10^{-10}$ is reachable.