New limits on the violation of local position invariance of gravity

Understand

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 $\xi$.