Orbiting test masses for an equivalence principle space experiment

Abstract

Deviations from the equivalence principle can be found by observations of the orbital periods of masses in a two body system, in a drag free satellite in geosynchronous orbit. At this altitude the Earth gravitational field tides allow close stable circular motion, within a 30 cm lobe, of a small test mass around the centre of the primary mass, of 75 kg. The repulsive fifth force ought to slow the orbital Newtonian period. A precise clock and laser interferometry provide the determination ofλ in the range between 1 and 102cm and ofα if larger than 10−4.2 or 10−5, for an uncertainty of the value of the constant of gravitation equal to 10−5 or 10−6, respectively (α andλ are the coupling coefficient to gravity and range of action of the fifth force). In the latter case, forλ around 10 cm, the fifth force may be detected forα larger than 10−5.5. This experiment may verify the principle of equivalence at short range improving the actual limits from one to two and a half orders of magnitude. The range of action,λ, up to several meters can be observed with this “local” experimental concept, but only for large values ofα, which have been excluded recently by ground experiments.