New limits on the violation of local position invariance of gravity

TL;DR

This paper uses observations from the Solar system and pulsars to set new, extremely tight limits on potential violations of local position invariance in gravity, significantly improving previous constraints.

Contribution

It introduces new observational constraints on the Whitehead parameter $\xi$, especially from pulsar data, tightening bounds on local position invariance violation in gravity.

Findings

Most stringent limit: | ext{ extasciitilde}\xi| < 3.9 imes 10^{-9}

Limit is six orders of magnitude tighter than previous gravimeter bounds

Converts to an upper limit of extasciitilde} 4 imes 10^{-16} on gravitational anisotropy

Abstract

Within the parameterized post-Newtonian (PPN) formalism, there could be an anisotropy of local gravity induced by an external matter distribution, even for a fully conservative metric theory of gravity. It reflects the breakdown of the local position invariance of gravity and, within the PPN formalism, is characterized by the Whitehead parameter $\xi$. We present three different kinds of observation, from the Solar system and radio pulsars, to constrain it. The most stringent limit comes from recent results on the extremely stable pulse profiles of solitary millisecond pulsars, that gives $|\hat \xi| < 3.9 \times 10^{-9}$ (95% CL), where the hat denotes the strong-field generalization of $\xi$. This limit is six orders of magnitude more constraining than the current best limit from superconducting gravimeter experiments. It can be converted into an upper limit of $\sim 4 \times 10^{-16}$ on the spatial anisotropy of the gravitational constant.