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Model-Dependence of Shapiro Time Delay and the "Speed of Gravity/Speed of Light" Controversy

S. Carlip
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Abstract

Fomalont and Kopeikin have recently succeeded in measuring the velocity-dependent component of the Shapiro time delay of light from a quasar passing behind Jupiter. While there is general agreement that this observation tests the gravitomagnetic properties of the gravitational field, a controversy has emerged over the question of whether the results depend on the speed of light, $c$, or the speed of gravity, $c_g$. By analyzing the Shapiro time delay in a set of ``preferred frame'' models, I demonstrate that this question is ill-posed: the distinction can only be made in the context of a class of theories in which $c\ne c_g$, and the answer then depends on the specific class of theories one chooses. It remains true, however, that for a large class of theories ``close enough'' to general relativity, the leading contribution to the time delay depends on $c$ and not $c_g$; within this class, observations are thus not yet accurate enough to measure the speed of gravity.

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S. Carlip. 2004-06-02. Model-Dependence of Shapiro Time Delay and the "Speed of Gravity/Speed of Light" Controversy. https://doi.org/10.1088/0264-9381%2F21%2F15%2F011

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