Tests of the Laws of Gravity and Search for New Forces
Newton's inverse-square (1/r^2) law is a cornerstone of General Relativity.  However, this law has been challenged by many modern theories of gravity and particle physics.  The supergravity and unified field theories often run into a new short-range force, with an accompanying new particle, which should appear as a violation of the 1/r^2 law.  A possible violation of the 1/r^2 law in the range below 1 mm was suggested by string theories with extra dimensions.  To search for these hypothetical forces, highly sensitive superconducting gravity gradiometers (SGGs) have been developed at the University of Maryland.  A null test of the 1/r2 law at 1 m was carried out in the 1980’s using a 1.5-ton lead pendulum as the source and a three-axis SGG as the null detector.  This experiment yielded the best limit of Newton’s law at 1 m, 2 parts in 10^4.
	More recently, we have been conducting a null test of the 1/r^2 law at < 0.1 mm.  This experiment employs a Newtonian null source and a single-axis SGG as the detector.  A niobium (Nb) or tantalum (Ta) disk of a large diameter-to-thickness ratio is used as the null source, which produces highly uniform Newtonian fields on its two sides.  The signal is detected by two disk-shaped Nb test masses located on the two sides of the source mass and connected to a SQUID.  As the source mass is displaced with respect to the test masses, only a short-range force would generate a time-varying gravity gradient signal.  The potential resolution of this experiment at 0.1 mm is better than 1 part in 10^2.