A test of general relativity using radio links with the Cassini spacecraft

According to general relativity, photons are deflected and delayed by the curvature of space-time produced by any mass(1-3). The bending and delay are proportional to gamma+1, where the parameter gamma is unity in general relativity but zero in the newtonian model of gravity. The quantity gamma-1 measures the degree to which gravity is not a purely geometric effect and is affected by other fields; such fields may have strongly influenced the early Universe, but would have now weakened so as to produce tiny-but still detectable-effects. Several experiments have confirmed to an accuracy of similar to0.1% the predictions for the deflection(4,5) and delay(6) of photons produced by the Sun. Here we report a measurement of the frequency shift of radio photons to and from the Cassini spacecraft as they passed near the Sun. Our result, gamma=1+(2.1+/-2.3)x10(-5), agrees with the predictions of standard general relativity with a sensitivity that approaches the level at which, theoretically, deviations are expected in some cosmological models(7,8).