eTheses Repository

# Development of a test of Newton's law of gravitation at micrometer distances using a superconducting spherical torsion balance

Rocco, Emanuele (2008)
Ph.D. thesis, University of Birmingham.

Many theories associated with quantum gravity, such as string theory, predict violations of inverse square law (ISL) of gravity at sub millimetre distances. To search for such experimental signatures we developed a magnetically levitated cryogenic torsion balance, the SSTB, and a set of masses with modulated density across their surfaces. The lateral force has to be measured as one mass is moved in front of the other with a micropositioner without any electrostatic shield between them. The expected Newtonian and electromagnetic forces have been studied to optimise the design of the masses. The torque sensitivity of the SSTB has been modelled and its performances at 4.2 K discussed. A torque sensitivity of 2×10$$^{−10}$$ Nm/√(Hz) at 30 mHz has been measured, most probably due to coupling with ground tilt. Two tests with prototypes of the masses has been performed. During the first test a torque due to electrostatic and magnetic forces of 8.4×10$$^{−11}$$±1.5×10$$^{−11}$$ Nm has been measured at a mass spacing of 42±14μm at the spatial periodicity of the density modulation. During the second test by using improved masses the detected signal was reduced to 1.91×10$$^{−11}$$±4.7×10$$^{−12}$$ Nm at a spacing of 30±10 μm. To look for violations of the ISL of gravity the torque sensitivity of the SSTB has to be further enhanced.