New experimental limit on the validity of local position invariance

Local position invariance (LPI) is part of the more general Einstein equivalence principle (EEP) which in turn is a foundation of Einstein's theory of general relativity. The EEP predicts a dependence of clock rates on the local gravitational potential U. LPI predicts that the gravitational shift is independent of the atomic species involved as a reference in the clock. It can thus be tested by comparing two different kinds of atomic frequency standard in the same time-varying gravitational potential $U(t).$ In our experiment we made use of the time dependence of $U(t)$ due to Earth's annual elliptical orbital motion. U(t)/c² varies between ±3.3×10^-10 $(c$ is the speed of light). Comparing a cesium atomic fountain frequency standard with a hydrogen maser for about one year allowed us to set an upper limit on a possible frequency variation of 2.1×10^-5 of this amount. Compared to previous similar experiments the limit of the notional violation of LPI was reduced by a factor of more than 30.