Session J4: Precision Tests of Gravity and Relativity

The Experimental Determination of the Newtonian Constant of Gravitation

Just over two hundred years ago in his most celebrated publication Henry Cavendish began the description of his experiment with the words, ``the apparatus is very simple.'' Today the techniques for determining $G$ remain simple in principle. Yet in practice these experiments still present some of the most difficult challenges in precision measurement. I will discuss these challenges as well as two recent $G$ experiments on which I have worked. The first is the BIPM torsion-strip balance and the other is the JILA suspended Fabry-Perot cavity.   

Gravity Probe B

Gravity Probe B is a space-based, controlled physics experiment designed to measure the geodetic and frame-dragging effects predicted by the General Theory of Relativity. The experiment consists of measuring the precession of the spin axis of four redundant, nearly perfect gyroscopes relative to a distant reference star, IM Pegasi. The spacecraft was launched into a 642 km polar orbit on April 20, 2004 from Vandenberg, CA, USA. During the first four months on-orbit the spacecraft was configured to carry out the experiment. Hundreds of operations were performed including using an on-board telescope to lock on to the reference star and spinning up each of the gyroscopes to 60-80 Hz. Since late in August, 2004 the instrument has been collecting the core science experiment data. Instrument calibrations before, during, and after the core science phase allow placing tight limits on systematic errors.   

Theory and Tests of Lorentz Violation

Lorentz invariance is a key feature of our present theories of nature, including General Relativity and the Standard Model. However, this symmetry may be broken in a more fundamental theory that unifies gravity with quantum physics. Whatever the origin of the breaking, the ensuing physical effects at accessible scales are described by the Standard-Model Extension, which is the effective field theory for general Lorentz and CPT violation. Observable signals are potentially within the reach of existing technology, and they are actively being sought in numerous sensitive experiments.