‘Galileo Galilei’ (GG): space test of the weak equivalence principle to 10 ⁻¹⁷ and laboratory demonstrations

Abstract

The small satellite ‘Galileo Galilei’ (GG) will test the universality of free fall and hence the weak equivalence principle which is the founding pillar of general relativity to 1 part in 1017. It will use proof masses whose atoms differ substantially from one another in their mass energy content, so as to maximize the chance of violation. GG will improve by four orders of magnitude the current best ‘Eöt-Wash’ tests based on slowly rotating torsion balances, which have been able to reach their thermal noise level. In GG, the expected violation signal is a relative displacement between the proof masses of ≃ 0.6 pm caused by a differential acceleration aGG ≃ 8 × 10−17 ms−2 pointing to the center of mass of the Earth as the satellite orbits around it at νGG ≃ 1.7 × 10−4 Hz. GG will fly an innovative acceleration sensor based on rapidly rotating macroscopic test masses weakly coupled in 2D which up-converts the signal to νspin ≃ 1 Hz, a value well above the frequency of natural oscillations of the masses relative to each other νd = 1/Td ≃ 1/(540 s). The sensor is unique in that it ensures high rotation frequency, low thermal noise and no attenuation of the signal strength (Pegna et al 2011 Phys. Rev. Lett. 107 200801). A readout based on a very low noise laser interferometry gauge developed at Jet Propulsion Laboratory (≃ 1 pm Hz−1/2 at 1 Hz demonstrated) allows the short integration time to be fully exploited. A full scale sensor with the same degrees of freedom and the same dynamical features as the one to fly in GG has been setup on ground (GGG). The proof masses of GGG are affected by acceleration and tilt noise acting on the rotating shaft because of ball bearings and terrain microseismicity (both absent in space). Overall, by means of appropriate 2D flexure joints, these noise sources have been reduced by a factor almost 105 down to a differential acceleration between the proof masses of ≃ 7 × 10−11 m s−2 (at 1.7 × 10−4 Hz up-converted by rotation to ≃ 0.2 Hz). The corresponding noise in the relative displacements of the proof masses, read by co-rotating capacitance bridges, is ≃ 180 pm, which is 300 times larger than the target in space. GGG error budget shows that it can reach a differential acceleration sensitivity aGGGgoal ≃ 8 × 10−16 m s−2, not limited by thermal noise. This value is only a factor 10 larger than what GG must reach in space to meet its target, and slightly smaller than the acceleration noise of the torsion balance. It can be achieved partly by means of weaker joints and an optimized mechanical design—so as to improve the attenuation factor—and partly by replacing the current ball bearings with much less noisy air bearings (also used in torsion balance tests) so as to reduce input noise. A laser gauge readout with noise level rlaser-ro ≃ 30 pm Hz−1/2 at 0.2÷3 Hz will be implemented.

Correlative methods for dual-species quantum tests of the weak equivalence principle

“…The most precise tests of the WEP have been carried out with lunar laser ranging techniques [8], or using a rotating torsion balance [9,10], which have both measured η at the level of a few parts in 10 13 . Various Space missions to test the WEP at improved levels (10 −15 or better) using other classical devices are presently in progress [11][12][13]. On a separate frontier, a number of groups have carried out tests between cold atoms [14][15][16][17][18] in an effort to probe the WEP at the quantum level.…”

A low noise laser interferometry readout for challenging acceleration measurements in space

“…Preliminary tests carried on by using off the shelf components, show the feasibility of the interferometer with the required resolution and noise target and the possibility of realizing a compact and lightweight measurement gauge. Furthermore, the interferometer developed in this work goes in the direction of the realization of an laser interferometer gauge for the measurement of the test masses of the Galieo Galilei (GG) mission [6] requiring a noise floor of 1 pm/√Hz at 1 Hz.…”

Testing general relativity

“…To day measurements done on earth by means of torsion balances tell us that the inertial and the gravitational mass do coincide up to a few parts in 10 14 . Tighter constraints can be pursued by a number of experiments in space that have been proposed, like STEP (Satellite Test of the Equivalence Principle) [14], and GG (Galileo Galilei) [13]), aiming respectively to an accuracy of 10 −18 and 10 −17 . Under development is MICROSCOPE (MICROSatellite à trainée Compensée pour l'Observation du Principe d'Equivalence) [15], whose objective is also an accuracy of 10 −18 ; the mission could fly in 2015-16.…”