Progress on Short-range Tests of Gravity at Humboldt State University*

Of the four fundamental forces, gravity is the only one that does not have a suitable quantum-mechanical description.  General Relativity, which embodies our current understanding of gravity, is separate from the Standard Model that explains the other three fundamental forces.  Theories which attempt to unify the Standard Model and General Relativity often include features that violate the Weak Equivalence Principle (WEP) and Inverse-Square Law (ISL). A violation of either the WEP or ISL at any length scale would bring into question our fundamental understanding of gravity. Motivated by these considerations, undergraduates and faculty at Humboldt State University are building, and consistently improving upon, an experiment to probe gravitational interactions below the 50-micron length scale.  Under high vacuum, we measure the twist of a torsion pendulum as an attractor mass is oscillated nearby in a parallel-plate configuration, providing a time varying torque on the pendulum. The torque variation provides a means to quantify potential deviation from the WEP or ISL at untested distance scales. Recent improvements in analysis and pendulum control will allow the collection of data with unprecedented sensitivity in the near future.