Massimo Bassan

Strong theoretical arguments suggest that a breakdown of Lorentz Invariance could arise under some very particular conditions. From an experimental point of view, it is important to test the local Lorentz invariance with ever greater precision and in all contexts, regardless of the theoretical motivation for the possible violation. In this paper we discuss some aspects of the gravitational sector. Tests of Lorentz Invariance in the context of gravity are difficult and rare in the literature. Possible violations could arise from quantum physics applied to gravity or the presence of vector and tensor fields mediating the gravitational interaction together with the metric tensor of general relativity. We present our results in the latter case. We analyzed the orbit of the LAGEOS and LAGEOS II satellites over a period of almost three decades. The effects of the possible preferred frame represented by the cosmic microwave background radiation on the mean argument of latitude of the satellites orbit were considered. These effects would manifest themselves mainly through the post-Newtonian parameter α 1 , a parameter that has a null value in general relativity. We constrain this parametrized post-Newtonian parameter down to the level of ∼ 2 × 10 − 5 , improving a previous limit obtained through the lunar laser ranging technique.

We report on measurements of the mechanical vibrations of the fundamental mode of resonant capacitive transducers, consisting of parallel plates at different gaps (10 - 26 $μ$m). Resonant frequency data were taken vs the electric field at constant charge, at various temperatures and different configurations. All measurements exhibited unexpectedly large tuning, caused by an additional, unpredicted attractive force. Such force has a divergent behaviour characterized by a critical electrical field and start to diverge long before the critical point. Many known physical mechanisms have been considered to explain this force, but all of them had to be discarded. Thus, we turn to non-conventional physical mechanisms and suggest that vacuum fluctuations inside the gap could be a possible cause of this anomalous force.

We describe a proof-of-principle experiment aiming to investigate the inverse-square law of gravitation at the centimeter scale. The sensor is a two-stage torsion pendulum, while actuation is accomplished by a variable liquid mass. The time-varying gravitational force is related to the level of the circulating fluid in one or two containers at a short distance from the test mass, with all moving mechanical parts positioned at a large distance. We provide a description of the apparatus and present the first results. We identified a systematic effect of thermal origin, producing offsets of few fNm in torque and of about 10 pN in force. When this effect is neutralized, the measurements agree well with the predictions of simulations. We also discuss the upcoming instrument upgradations and the expected sensitivity improvement that will allow us to perform measurements with adequate accuracy to investigate the unexplored regions of the α−λ parameter space of a Yukawa-like deviation from the Newtonian potential.

We discuss the idea of a measurement that, making use of data from space interferometers, could detect the gravito-magnetic field generated by the rotation of the Milky Way, including the possible contribution of the dark matter halo. The galactic signal would be superposed to the gravito-magnetic field of the Sun. The proposed technique is based on the asymmetric propagation of light along the closed contour of the space interferometer, in a Sagnac-like approach. We discuss the principles of detection as well as some practical aspects of the proposed experiment using, as a case study, LISA, the most mature project to date. Both gravito-magnetic signals will be modulated thanks to the annual oscillation of the plane of the interferometer with respect to the galactic plane and to the spin axis of the Sun. Although larger than the detector intrinsic noise, these signals will be superposed to a much larger kinematic modulation due to orbital motion, making them very hard to be observed. We also mention a second phenomenon, where the gravito-magnetic field rotates the polarization of the propagating electromagnetic beams: the effect will be present in LISA, although exceedingly small and out of reach of present technology.

A new satellite-based experiment in the field of gravitation, SaToR-G, is presented. It aims to compare the predictions of different theories of gravitation in the limit of weak-field and slow-motion. The ultimate goal of SaToR-G is searching for possible “new physics” beyond General Relativity, which represent the state-of-the-art of our current knowledge of gravitational physics. A key role in the above perspective is the theoretical and experimental framework that confines our work. To this end, we will exploit as much as possible the classical framework suggested by R.H. Dicke over fifty years ago.

A new experiment in the field of gravitation, SaToR-G, is presented. The experiment aims to compare the predictions of different theories of gravitation in the limit of weak field and slow motion. The ultimate goal of the experiment is to look for possible "new physics" beyond the current standard model of gravitation based on the predictions of general relativity. A key role in the above perspective is the theoretical and experimental framework within which to confine our work. To this end, we make our best efforts to exploit the framework suggested by Dicke over 50 years ago.

We report on a simple, inexpensive readout for torsion pendulums, suitable for robust applications like teaching-lab equipment or monitoring of large amplitude oscillations. A short light pulse is recorded every time a reflective band on the pendulum inertial member passes in front of the sensor, an infrared LED pair (emitter–receiver). Simple algebraic manipulations on the time series of these pulses arrival times allow one to extract the resonant frequency as well as the decay time of the pendulum. This readout is insensitive to the amplitude of oscillation and is therefore suitable for monitoring torsional oscillations of large amplitude, where traditonal readouts like optical levers or autocollimators encounter dynamic range limitations.

In this paper the enhancement of the sensitivity and of the duty-cycle of a double stage suspended torsion pendulum are described. The increase in sensitivity, both for force and torque measurements, was obtained rearranging the pendulum mechanics, reducing the value of the suspended masses and moments of inertia and by adopting additional read-outs based on optical levers. The improvement of the duty-cycle was obtained by controlling the suspension point of the pendulum to reduce the effect of seismic noise on the translation resonances of the system. The reduction of these motions allowed continuous operation for three weeks, in high sensitivity regime of the read-out electronics, resulting in a force sensitivity of and a torque sensitivity of around 10 mHz.