This is the master equation for the evolution of the P representation in the Schrodinger picture. If, as an example, one applies Eq. (17) to the simple caseone has i-which is the well-known equation 4 for the Hamiltonian described by Eq. (18).New radar observations yield a more stringent test of the predicted relativistic increase in echo times of radio signals sent from Earth and reflected from Mercury and Venus. These "extra" delays may be characterized by a parameter X which is unity according to general relativity and 0.93 according to recent predictions based on a scalartensor theory of gravitation. We find that X = 1.02. The formal standard error is 0.02, but because of the possible presence of systematic errors we consider 0.05 to be a more reliable estimate of the uncertainty in the result.General relativity predicts that the round-trip time delay of an electromagnetic wave is influenced by the gravitational potential along the path of the radiation. A test of this prediction involving the transmission of radar signals from Earth to either Mercury or Venus and the detection of the echoes was suggested in 1964.* These echoes are expected on the basis of general relativity to be retarded by solar gravity by an amount 2 A* <=* (4r 0 /c) ln[(r, + r p +R)/(r e + r p -where At, expressed in harmonic coordinates, is the coordinate-time retardation, r 0~ 1.5 km is the gravitational radius of the sun, c is the speed of light far from the sun, r e is the Earthsun distance, r p is the planet-sun distance, and R is the Earth-planet distance. The quantity At is not an observable but is indicative of the magnitude and behavior of the measurable effect as predicted by general relativity. The operational interpretation of the effect has been discussed in detail elsewhere. 3 To test whether or not the echo time-delay data are in agreement with this theory, we may insert an ad hoc multiplicative parameter X on the right side of Eq. (1) and estimate it along with the other unknown parameters that affect the data. 4 This experiment was first performed in 1967 and yielded the result 5 X =0.09 ±0.2 which corre-1132

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Measurements of echo delays of radar signals transmitted from Earth to Mercury have yielded an accurate value for the advance of the latter's perihelion position. Given that the Sun's gravitational quadrupole moment is negligible, the result in terms of the Eddihgton-Robertson parameters is (2 + 2y-iS)/3-1.005 ±0.007, where 7=)S = 1 in general relativity, and where 0.007 represents the statistical standard error. Inclusion of the probable contribution of systematic errors raises the uncertainty to about 0.02.