Suspended Fabry-Perot interferometer for determining the Newtonian constant of gravitation

Abstract

Of all the fundamental constants of nature, the Newtonian constant of gravitation, G, has been one of the most difficult to measure. The current CODATA value of G has an uncertainty of 1.5 parts in 1000. Although recent experiments have produced values with uncertainties smaller than this, the adopted CODATA uncertainty reflects the fact that there is still substantial disagreement between the values from these experiments. The majority of previous measurements have used torsion pendulums or balances to convert the small gravitational attraction of a laboratory source mass into a relatively large mechanical displacement. However, our approach is to use simple pendulums, which results in a small displacement that we measure very accurately. This means that the attraction of the source masses is measured against a restoring force provided by earth's gravity rather than the less well-understood torsion of a wire. Also, the shorter period of our pendulums allows us to make measurements much more rapidly than in most other experiments. In our apparatus, two mirrors, each suspended as a simple pendulum, form a Fabry-Perot cavity. A He-Ne laser locked to this cavity monitors the relative displacement of these two pendulums (through changes in its frequency) as laboratory source masses are moved, altering the gravitational pull on the mirrors.