Paper detail

Expanded solar-system limits on violations of the equivalence principle

Most attempts to unify general relativity with the standard model of particle physics predict violations of the equivalence principle associated in some way with the composition of the test masses. We test this idea by using observational uncertainties in the positions and motions of solar-system bodies to set upper limits on possible differences $Δ$ between the gravitational and inertial mass of each body. For suitable pairs of objects, it is possible to constrain three different linear combinations of $Δ$ using Kepler's third law, the migration of stable Lagrange points, and orbital polarization (the Nordtvedt effect). Limits of order $10^{-10}-10^{-6}$ on $Δ$ for individual bodies can then be derived from planetary and lunar ephemerides, Cassini observations of the Saturn system, and observations of Jupiter's Trojan asteroids as well as recently discovered Trojan companions around the Earth, Mars, Neptune, and Saturnian moons. These results can be combined with models for elemental abundances in each body to test for composition-dependent violations of the universality of free fall in the solar system. The resulting limits are weaker than those from laboratory experiments, but span a larger volume in composition space.