Modern theories attempt to bridge the gap between the scale of the accepted model for particle physics (the 'Standard Model'), and the scale at which the force of gravity becomes as strong as all other forces in nature (about 23 orders of magnitudes smaller than the size of an atom), predicting modifications to Newton's law at much longer length scales, up to 1mm. Recently, there have been several experiments designed to detect or constrain deviations from Newton's law at this macroscopic length scale. At Stanford we have built such an experiment based on a low temperature probe to measure forces as low as attoNewton between masses separated by distances on the order of 20 micrometers. In our experiment 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.
The holy grail of theoretical physics is to provide a compact and elegant theory that unifies all forces of nature. While the Electromagnetic (force between charged particles), Strong (force that holds the nucleus together), and Weak (force associated with certain decays of nuclei) forces are well understood, the force of gravity, the first force that human beings probably experienced, is poorly understood, and unifying it with the other forces poses an enormous intellectual challenge. The new theoretical insight in this field gave researchers an opportunity to try and contribute to this field of research through precision, 'table-top' experiments. At Stanford University we utilized advanced techniques used in physics, chemistry, materials science, and engineering to build an apparatus that can measure forces that are more than twenty orders of magnitudes smaller than the force measured by a normal scale. The challenge to design, to construct, to test, and to use in a real experiment such an apparatus forced us to come up with many elegant solutions to engineering problems. This benefitted not only the students working on the project, but also other researchers working in different fields. For example, the miniature cantilevers that we developed for the project are now being used as torque magnetometers for measuring magnetic properties of nano-samples. In another example, a solution we found for controlling the speed of a spinning rotor in our experiment bears on a fundamental physics law of pressure exerted by radiation.
