168
recognizable at both receiving stations in California and Massachusetts. Then, a delay ∆ could then be measured between the two receiving stations located in the plane perpendicular to the initial source direction (see figure 3). However, since that extragalactic radiation grazes the Sun, it also passes through the solar corona, which is a plasma surrounding the Sun. In order to correct for the change of velocity of the radiation due to the solar plasma, the radio signal received at each station is passed through narrow band filters selecting three different frequencies. In the case of a plasma (here the solar plasma) , the velocity of transmission of radiation is different at different frequencies. These pairs of signals are recorded as a function of the time given by a local atomic clock. Technically, it is reported that the correlation between the same pattern of a radio fluctuation recorded at each station, is detected using a “filter estimator” combined with a “parameterized theoretical model of the delays” developed for that experiment. The aim of Lebach’s et al.(7) paper is to determine the parameter γ, defined in the parameterized post-Newtonian (PPN) formalism (7) (equation #2). This parameter gives a relationship between the deflection predicted by Einstein and the delay of the same pattern between each antenna. For a perfect agreement with Einstein’s general relativity, the parameter γ must equal unity. In order to get the picosecond accuracy claimed in the article, the two local clocks, located in California and in Massachusetts, must be synchronized at the picosecond (or 0.3 millimeter) accuracy claimed in the paper. It is not clear how such a limit can be achieved. 8 - Origin of the Fluctuations of the Radio Source. General relativity predicts that the velocity of light is reduced in the solar gravitational potential. Furthermore, the solar plasma adds a supplementary delay to the transmission of radiation, but more importantly, it adds important fluctuations to the original extragalactic signal. Unfortunately, the density of that plasma as well as its short time fluctuations are totally unpredictable. Signal fluctuations have been observed lasting a few minutes. They were measured (7) to vary by as much as 500%. Since random variations of the intensity of that plasma are taking place very rapidly and are totally unpredictable, a parameterized correction cannot give an accurate prediction. The fluctuations in the plasma density even vary within one period of accumulation of data. Consequently, even if three different frequency components are measured, the theoretical inverse quadratic correction (as a function of frequency) introduced to determine the density of the plasma cannot provide the picosecond accuracy stated in the paper. One must recall that the change of distance corresponding to one picosecond is equal to only a difference of 0.3 millimeter (at light velocity) between the receiving antennas on Earth.