Testing gravity law in the solar system

The predictions of General relativity (GR) are in good agreement with observations in the solar system. Nevertheless, unexpected anomalies appeared during the last decades, along with the increasing precision of measurements. Those anomalies are present in spacecraft tracking data (Pioneer and flyby anomalies) as well as ephemerides. In addition, the whole theory is challenged at galactic and cosmic scales with the dark matter and dark energy issues. Finally, the unification in the framework of quantum field theories remains an open question, whose solution will certainly lead to modifications of the theory, even at large distances. As long as those "dark sides" of the universe have no universally accepted interpretation nor are they observed through other means than the gravitational anomalies they have been designed to cure, these anomalies may as well be interpreted as deviations from GR. In this context, there is a strong motivation for improved and more systematic tests of GR inside the solar system, with the aim to bridge the gap between gravity experiments in the solar system and observations at much larger scales. We review a family of metric extensions of GR which preserve the equivalence principle but modify the coupling between energy and curvature and provide a phenomenological framework which generalizes the PPN framework and "fifth force" extensions of GR. We briefly discuss some possible observational consequences in connection with highly accurate ephemerides.Comment: Proceedings of Journ\'ees 2010 "Syst\`emes de r\'ef\'erence spatio-temporels", New challenges for reference systems and numerical standards in astronom

Nematic elastomers with aligned carbon nanotubes: new electromechanical actuators

We demonstrate, for the first time, the large electromechanical response in nematic liquid crystalline elastomers filled with a very low (~0.01%) concentration of carbon nanotubes, aligned along the nematic director at preparation. The nanotubes create a very large effective dielectric anisotropy of the composite. Their local field-induced torque is transmitted to the rubber-elastic network and is registered as the exerted uniaxial stress of order ~1kPa in response to a constant field of order ~1MV/m. We investigate the dependence of the effect on field strength, nanotube concentration and reproducibility under multiple field-on and -off cycles. The results indicate the potential of the nanotube-nematic elastomer composites as electrically driven actuators

Back-action cancellation in interferometers by quantum locking

We show that back-action noise in interferometric measurements such as gravitational-waves detectors can be completely suppressed by a local control of mirrors motion. An optomechanical sensor with an optimized measurement strategy is used to monitor mirror displacements. A feedback loop then eliminates radiation-pressure effects without adding noise. This very efficient technique leads to an increased sensitivity for the interferometric measurement, which becomes only limited by phase noise. Back-action cancellation is furthermore insensitive to losses in the interferometer.Comment: 4 pages, 3 figures, RevTe

Beating quantum limits in interferometers with quantum locking of mirrors

The sensitivity in interferometric measurements such as gravitational-wave detectors is ultimately limited by quantum noise of light. We discuss the use of feedback mechanisms to reduce the quantum effects of radiation pressure. Recent experiments have shown that it is possible to reduce the thermal motion of a mirror by cold damping. The mirror motion is measured with an optomechanical sensor based on a high-finesse cavity, and reduced by a feedback loop. We show that this technique can be extended to lock the mirror at the quantum level. In gravitational-waves interferometers with Fabry-Perot cavities in each arms, it is even possible to use a single feedback mechanism to lock one cavity mirror on the other. This quantum locking greatly improves the sensitivity of the interferometric measurement. It is furthermore insensitive to imperfections such as losses in the interferometer