Letters to Nature

Nature 425, 374-376 (25 September 2003) | doi:10.1038/nature01997; Received 25 February 2003; Accepted 15 August 2003

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

B. Bertotti1, L. Iess2 & P. Tortora3

  1. Dipartimento di Fisica Nucleare e Teorica, Università di Pavia, Via U. Bassi 6, I-27100, Pavia, Italy
  2. Dipartimento di Ingegneria Aerospaziale ed Astronautica, Università di Roma "La Sapienza", Via Eudossiana 16, I-00184, Roma, Italy
  3. II Facoltà di Ingegneria, Università di Bologna, Via Fontanelle 40, I-47100, Forlì, Italy

Correspondence to: L. Iess2 Email: iess@hermes.ing.uniroma1.it

According to general relativity, photons are deflected and delayed by the curvature of space-time produced by any mass1, 2, 3. The bending and delay are proportional to italic gamma + 1, where the parameter italic gamma is unity in general relativity but zero in the newtonian model of gravity. The quantity italic 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 approx0.1% the predictions for the deflection4, 5 and delay6 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, italic gamma = 1 + (2.1 plusminus 2.3) times 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 models7, 8.