PROPOSED NONCRYOGENIC, NONDRAG-FREE TEST OF THE EQUIVALENCE PRINCIPLEIN SPACE

Ever since Galileo scientists have known that all bodies fall with the same acceleration regardless of their mass and composition. Known as the Universality of Free Fall, this is the most direct experimental ev idence of the Weak Equivalence Principle, a founding pillar of General Relativity according to which the gravitational (passive) mass m(g) a nd the inertial mass m(i) are always in the same positive ratio in all test bodies. A space experiment offers two main advantages: a signal about a factor of a thousand bigger than on Earth and the absence of w eight. A new space mission named GALILEO GALILEI (GG) has been propose d (Nobili et al., 1995 [J. Astronautical Sciences, 43, 219]; GALILEO G ALILEI (GG), PRE PHASE A REPORT, ASI (Agenzia Spaziale Italiana), Sept ember 1996) aimed at testing the weak Equivalence Principle (EP) to 1 part in 10(17) in a rapidly spinning (5 Hz) drag-free spacecraft at ro om temperature, the most recent ground experiments having reached the level of 10(-12) (Adelberger et al., 1990 [PhRvD, 42, 3267]; Su et al. , 1994 [PhRvD, 50, 3614]). Here we present a nondrag-free version of G G which could reach a sensitivity of 1 part in 10(16). The main featur e of GG is that, similarly to the most recent ground experiments, the expected (low frequency) signal is modulated at higher frequency by sp inning the system, in this case by rotating the test bodies (in the sh ape of hollow cylinders) around their symmetry axes, the signal being in the perpendicular plane. They are mechanically suspended inside the spacecraft and have very low frequencies of natural oscillation (due to the weakness of the springs that can be used because of weightlessn ess) so as to allow self-centering of the axes; vibrational noise arou nd the spin/signal frequency is attenuated by means of mechanical susp ensions. The signal of an EP violation would appear at the spin freque ncy as a relative (differential) displacement of the test masses perpe ndicularly to the spin axis, and be detected by capacitance sensors; t hermal stability across the test masses and for the required integrati on time is obtained passively thanks to both the fast spin and the cyl indrical symmetry. In the nondrag-free version the entire effect of at mospheric drag is retained, but a very accurate balancing of the test bodies must be ensured (through a coupled suspension) so as to reach a high level of Common Mode Rejection and reduce the differential effec ts of drag below the target sensitivity. In so doing the complexities of a drag-free spacecraft are avoided by putting more stringent requir ements on the experiment. The spacecraft must have a high area-to-mass ratio in order to reduce the effects of nongravitational forces; it i s therefore a natural choice to have three pairs of test masses (in th ree experimental chambers) rather than one as by Nobili et al. (1995) [J. Astronautical Sciences, 43, 219] and the mission called GALILEO GA LILEI [PRE PHASE A REPORT, ASI (Agenzia Spaziale Italiana), September 1996]. The GG setup is specifically designed for space; however, a sig nificant EP test on the ground is possible-because the signal is in th e transverse plane-by exploiting the horizontal component of the gravi tational and the centrifugal field of the Earth. This ground test is u nderway. (C) 1998 Elsevier Science B. V.