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

Naturevolume 425pages374376 (2003) | Download Citation



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 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 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, 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 models7,8.

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The work of B.B, L.I. and P.T. has been funded by the Italian Space Agency (ASI). The Jet Propulsion Laboratory Radio Science Systems Group, the engineers and staff of the DSS25 station, and the Cassini Project have been crucial for the success of this experiment. We thank J. W. Armstrong, J. J. Bordi and D. C. Roth for their contributions.

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  1. Dipartimento di Fisica Nucleare e Teorica, Università di Pavia, Via U. Bassi 6, I-27100, Pavia, Italy

    • B. Bertotti
  2. Dipartimento di Ingegneria Aerospaziale ed Astronautica, Università di Roma “La Sapienza”, Via Eudossiana 16, I-00184, Roma, Italy

    • L. Iess
  3. II Facoltà di Ingegneria, Università di Bologna, Via Fontanelle 40, I-47100, Forlì, Italy

    • P. Tortora


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The authors declare that they have no competing financial interests.

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Correspondence to L. Iess.

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