Propagation Speed of Gravity and the Relativistic Time Delay

Abstract

We calculate the delay in the propagation of a light signal past a massive body that moves with speed v, under the assumption that the speed of propagation of the gravitational interaction c_g differs from that of light. Using the post-Newtonian approximation, we consider an expansion in powers of v/c beyond the leading ``Shapiro'' time delay effect, while working to first order only in Gm/c^2, and show that the altered propagation speed of the gravitational signal has no effect whatsoever on the time delay to first order in v/c beyond the leading term, although it will have an effect to second and higher order. We show that the only other possible effects of an altered speed c_g at this order arise from a modification of the parametrized post-Newtonian (PPN) coefficient \alpha_1 of the metric from the value zero predicted by general relativity. Current solar-system measurements already provide tight bounds on such a modification. We conclude that recent measurements of the propagation of radio signals past Jupiter are sensitive to \alpha_1, but are not directly sensitive to the speed of propagation of gravity.