Testing local position invariance with four cesium-fountain primary frequency standards and four NIST hydrogen masers

We report the most sensitive tests to date of the assumption of local position invariance (LPI) underlying general relativity, based on a 7 yr comparison of cesium and hydrogen atomic clocks (frequency standards). The latest results place an upper limit that is over 20 times smaller than the previous most sensitive tests; this is consistent with the null shift predicted by LPI. The result is based on precise comparisons of frequencies of four hydrogen masers maintained by NIST, with four independent Cs fountain clocks-one at NIST and three in Europe-as the Sun's gravitational potential at Earth's surface varies due to Earth's orbital eccentricity.