This award supports an experimental program to probe deviations from Newtonian gravity at distances on the order of 25 microns as predicted by theories of physics beyond the Standard Model of fundamental particle interactions. A low temperature probe has been built to measure tiny forces expected at these distances. The force sensor is a micromachined silicon cantilever. A gold test mass is glued to the end of the cantilever. An alternating pattern of gold and silicon bars is oscillated below the test mass; the coupling between the test mass and the varying gravitational field of the moving drive mass excites the cantilever on resonance. A fiber interferometer measures the cantilever motion and from this measurement, the force between the masses is deduced. Due to the geometry of the drive mass, as the longitudinal equilibrium position of the drive mass is varied, any gravitational force will show a distinct periodicity. Comparison of the measured force to predictions from finite element analysis yields bounds on new gravity-like forces which could signal new physics. While the Standard Model successfully unifies the electromagnetic, the strong and the weak interactions, it ignores gravity. This poses a great mystery in fundamental physics: why is even this most feeble subatomic force, i.e. the weak force, so much stronger than gravity? Modern theories that attempt to answer this question predict modifications to Newton's law at much longer length scales, up to 1 millimeter. The technology used in this experiment is at the cutting edge of micro-fabricated devices.
