New Measurements of G Deepen Uncertainties About its Value

New measurements of the Newtonian gravitational constant - the number that describes the strength of gravity - depart significantly from the accepted value established in the 1980s, deepening uncertainties as to its actual value. Affectionately dubbed "Big G", it is the least accurately known of all the fundamental constants; the accepted value of 6.6726 x 10-11 m3/kg-s2 is known with a relatively high uncertainty of 0.01 percent.

Big G is arguably the most difficult constant to measure because, among other reasons, gravity is the weakest of all forces and it is impossible to shield delicate measurements from the gravity influences of buildings and other nearby objects. Underscoring this difficulty, three scientists from international labs - the German Bureau of Standards, the Measurement Standards Laboratory of New Zealand, and the University of Wuppertal in Germany - reported new measurements of G which disagreed widely with one another and with the standard value in an invited session of the Precision Measurement and Fundamental Constants Topical Group at the April meeting.

The Wuppertal value was 0.07 percent below the currently accepted value (corresponding to 7 standard deviations), the New Zealand measurements were 0.07-0.08 percent below (7-8 standard deviations) and the German Bureau of Standards value was a whopping 0.6 percent above (60 standard deviations). "There hasn't been this big a difference in the value of G for many years," said Eric Adelberger (University of Washington) of the results.

Although the techniques differed, the groups all essentially determined G by measuring the gravitational effects of cylindrical masses acting on objects suspended above the ground. For example, the New Zealand group used a compensated torsion balance in which the gravitational torque was balanced by an electrostatic torque from an electrometer, which was calibrated by accelerating the entire apparatus.

Researchers at Los Alamos, which helped set the 1980s standard, are undertaking a new measurement of G which may be five times as precise as current measurements, and may shed light on these puzzling results. A firmly established value of G is important to delineate between grand unified theories that attempt to relate G to fundamental constants associated with the other three physical forces (see figure on page 7).