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

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• Iess, L.
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• Tortora, P.

Abstract

According to general relativity, photons are deflected and delayed by the curvature of space-time produced by any mass. The bending and delay are proportional to γ + 1, where the parameter γ is unity in general relativity but zero in the newtonian model of gravity. The quantity γ - 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 ~0.1% the predictions for the deflection and delay 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, γ = 1 + (2.1 +/- 2.3) × 10^-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.