Comment on 'Model-dependence of Shapiro time delay and the "speed of gravity/speed of light" controversy'

TL;DR

This paper critiques Carlip's calculation of the Shapiro time delay, clarifying that the correction depends on the speed of gravity when properly interpreted within the SI metric system, impacting the interpretation of gravitational experiments.

Contribution

It demonstrates that Carlip's vector-tensor theory calculation implies a non-metric system of units disconnected from practical measurement, and reinterprets the correction as dependent on the speed of gravity within the SI system.

Findings

Carlip's calculation implies a non-metric unit system.

Re-adjustment to SI units shows the correction depends on the speed of gravity.

Emphasizes the importance of metric units for interpreting gravitational experiments.

Abstract

In a recent paper published in Classical and Quantum Gravity, 2004, vol. 21, p. 3803 Carlip used a vector-tensor theory of gravity to calculate the Shapiro time delay by a moving gravitational lens. He claimed that the relativistic correction of the order of v/c beyond the static part of the Shapiro delay depends on the speed of light c and, hence, the Fomalont-Kopeikin experiment is not sensitive to the speed of gravity c_g. In this letter we analyze Carlip's calculation and demonstrate that it implies a gravitodynamic (non-metric) system of units based on the principle of the constancy of the speed of gravity but it is disconnected from the practical method of measurement of astronomical distances based on the principle of the constancy of the speed of light and the SI metric (electrodynamic) system of units. Re-adjustment of theoretically-admissible but practically unmeasurable Carlip's coordinates to the SI metric system of units used in JPL ephemeris, reveals that the velocity-dependent correction to the static part of the Shapiro time delay does depend on the speed of gravity c_g as shown by Kopeikin in Classical and Quantum Gravity, 2004, vol. 21, p. 1. This analysis elucidates the importance of employing the metric system of units for physically meaningful interpretation of gravitational experiments.