Comment on 'Model-dependence of Shapiro time delay and the "speed of gravity/speed of light" controversy'
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In a recent paper published in Class. Quant. Grav., 2004, 21, 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 cg. 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 cg as shown by Kopeikin in Class. Quant. Grav., 2004, 21, 1. This analysis elucidates the importance of employing the metric system of units for physically meaningful interpretation of gravitational experiments.