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

Abstract

`Galileo Galilei' (GG) is a proposal for a small, low-orbit satellite devotedto testing the equivalence principle (EP) of Galileo, Newton and Einstein.The GG report on the phase A study recently carried out with funding fromASI (Agenzia Spaziale Italiana) concluded that GG can test the equivalenceprinciple to 1 part in 1017 at room temperature. The main novelty isto modulate the expected differential signal of an EP violation at the spinrate of the spacecraft (2 Hz). Compared with other experiments, themodulation frequency is increased by more than a factor of 104, thusreducing 1/f (low-frequency) electronic and mechanical noise. Thechallenge for an EP test in space is to improve over the sensitivity ofground-based experiments (about 1 part in 1012) by many orders ofmagnitude, so as to deeply probe a so far totally unexplored field; doingthat with more than one pair of bodies is an unnecessary complication. Forthis reason GG is now proposed with a single pair of test masses. Atpresent the best and most reliable laboratory-controlled tests of theequivalence principle have been achieved by the `Eöt-Wash' group withsmall test cylinders arranged on a torsion balance placed on a turntablewhich provides the modulation of the signal (a 1-2 h rotationperiod). The torsion balance is not a suitable instrument in space. We havedesigned and built the GGG (`GG on the Ground') prototype. It is made ofcoaxial test cylinders weakly coupled (via mechanical suspensions) andquickly rotating (6 Hz achieved so far); in addition, it is wellsuited to be flown in space - where the driving signal is about threeorders of magnitude stronger and the absence of weight is veryhelpful - inside the coaxial, co-rotating GG cylindrical spacecraft. TheGGG apparatus is now operational. Preliminary measurement data indicatethat weakly coupled, fast-spinning macroscopic rotors can be a suitableinstrument to detect small differential effects. Rotation (up to 6 Hzso far) is stabilized by a small passive oil damper. A finer activedamper, using small capacitance sensors and actuators as in the design ofthe space experiment, is in preparation. The current sensitivity of the GGG system is of 10-9 m s-2/√Hzat about 300 s,which can be improved because horizontalseismic noise is rejected very well;perturbing effects of terrain tilts(due to microseismicity and tides)will be reduced by adding a passivecardanic suspension. As for the capacitance read-out, thecurrent sensitivity (5 pm displacements in 1 s integrationtime at room temperature) is adequate to make GGG competitivewith the torsion balance. Because of the stronger signal andweaker coupling of the test rotors in space, this sensitivityis also adequate for GG to reach its target accuracy(10-17). Information, references, research papers andphotographs of the apparatus are available on the Web(http://tycho.dm.unipi.it/nobili).