Search for Deviations from Newtonian Gravity at Micron Scale (a continuation proposal)

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This award supports an experimental program to probe deviations from Newtonian gravity at distances on the order of 25 micrometers as predicted by theories of physics beyond the Standard Model of fundamental particle interactions. A cryogenic helium gas bearing is used to rotate a disc containing a drive mass pattern of alternating density under a small test mass mounted on a micromachined cantilever. Any mass-dependent force between the two will produce a time-varying force on the test mass, and consequently a time-varying displacement of the cantilever. This displacement is read out with a laser interferometer, and the position of the drive mass is simultaneously recorded using an optical encoder. Comparison of the measured force to predictions from finite element analysis yields bounds on new gravity-like forces which could signal new physics. The experiment uses several novel techniques to overcome some of the challenges associated with the necessary precision measurements that could prove to be useful in similar experiments or other areas. These include radiation pressure to cool a micromachined cantilever, the combination of encapsulated Micro-Electrical-Mechanical-System (MEMS) sensors with an integrated cryopump, the use of a macroscopic gas bearing as a general-purpose low-temperature motor, the methods of simple and precise fabrication of intermediate-scale metallic structures, and some of the techniques of dimensionally stable planarization. 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.