The Shapiro Delay: A Frequency Dependent Transit-Time Effect
Edward Henry Dowdye
First noticed by Irvin L. Shapiro in 1964, the transit time required for a microwave signal to propagate through space, arrive at a satellite orbiting Venus or Mercury, required a measurable time delay for the reply signal to propagate back to the earth to be received at the antenna of the observatory. The time delay was noticeably affected by the relative position of the sun. Controlled measurements conducted by Shapiro determined that the time-tagged microwave signals had measurable effects that varied as a function of the impact parameter of the microwave beam relative to the sun. The delays were observed to be in the 100's of microseconds when the impact parameter of the microwave beam was at a minimum. After repeated measurements, varying time delays were recorded and were referred to as the Shapiro delay. These measurements permitted a precise determination of the electron density profile of the solar wind as a function of the radial distance r from the sun. The electron density profile of solar wind is found to behave very nearly as an inverse square of r, namely as r-2, with electron density profile models ranging from r-2.05 to r-2.08, and with effects that engulf the outmost planets of the solar system. The bulk of all the Shapiro delay measurements were done using microwave frequencies from 500 MHz to 8.8MHz (with wavelengths from 80cm to 3.5cm). Significant findings of this research reveal that, for all microwave signals propagating in the solar wind atmosphere of the solar system, the waves are subjected to a frequency dependent plasma index of refraction n that exceeds unity, i.e., n > 1.0000000000. For optical, IR and UV wavelengths, the plasma index of refraction is practically n = 1.0000000000 and these wavelengths are virtually unaffected by the widespread atmosphere of the expanding solar wind described by the electron density profile. As a consequence, the Shapiro delay is a very good measurement of a frequency dependent transit-time effect and can not be a space-time effect of General Relativity that is independent of frequency.
