'Galileo Galilei' (GG) small-satellite project: an alternative to the torsion balance for testing the equivalence principle on Earth and in space

'Galileo Galilei' (GG) is a proposal for a small, low-orbit satellite devoted to testing the equivalence principle (EP) of Galileo, Newton and Einstein. The GG report on the phase A study recently carried out with funding from ASI (Agenzia Spaziale Italiana) concluded that GG can test the equivalence principle to 1 part in 10(17) at room temperature. The main novelty is to modulate the expected differential signal of an EP violation at the spin rate of the spacecraft (2 Hz), Compared with other experiments, the modulation frequency is increased by more than a factor of 10(4), thus reducing 1/f (low-frequency) electronic and mechanical noise. The challenge for an EP test in space is to improve over the sensitivity of ground-based experiments (about 1 part in 10(12)) by many orders of magnitude, so as to deeply probe a so Far totally unexplored field; doing that with more than one pair of bodies is an unnecessary complication. For this reason GG is now proposed with a single pair of test masses. At present the best and most reliable laboratory-controlled tests of the equivalence principle have been achieved by the 'Eot-Wash' group with small test cylinders arranged on a torsion balance placed on a turntable which provides the modulation of the signal (a 1-2 h rotation period). The torsion balance is not a suitable instrument in space. We have designed and built the GGG ('GG on the Ground') prototype. It is made of coaxial test cylinders weakly coupled (via mechanical suspensions) and quickly rotating (6 Hz achieved so far); in addition, it is well suited to be flown in space-where the driving signal is about three orders of magnitude stronger and the absence of weight is very helpful-inside the coaxial, co-rotating GG cylindrical spacecraft. The GGG apparatus is now operational. Preliminary measurement data indicate that weakly coupled, fast-spinning macroscopic rotors can be a suitable instrument to detect small differential effects. Rotation (up to 6 Hz so far) is stabilized by a small passive oil damper. A finer active damper, using small capacitance sensors and actuators as in the design of the space experiment, is in preparation. The current sensitivity of the GGG system is of 10(-9) m s(-2)/root Hz at about 300 s, which can be improved because horizontal seismic noise is rejected very well; perturbing effects of terrain tilts (due to microseismicity and tides) will be reduced by adding a passive cardanic suspension. As for the capacitance read-out, the current sensitivity (5 pm displacements in 1 s integration time at room temperature) is adequate to make GGG competitive with the torsion balance. Because of the stronger signal and weaker coupling of the test rotors in space, this sensitivity is also adequate for GG to reach its target accuracy (10(-17)). Information, references, research papers and photographs of the apparatus are available on the Web (http://tycho.dm.unipi.it/nobili).