NIST Researchers Complete 10-Year Effort to Measure Gravitational Constant

Measuring the Gravitational Constant

Physicists at the U.S. National Institute of Standards and Technology (NIST) have concluded a 10-year project to determine the gravitational constant, known as big G. The team, led by Stephan Schlamminger, arrived at a value of 6.67387 x 10-11 cubic meters per kilogram per square second. This measurement contributes to ongoing efforts to refine the accepted value of big G, which currently stands at 6.67430 × 10-11 m3/(kg s2) with a notable uncertainty.

Methodology and Challenges

The NIST team employed a torsion balance with a fourfold geometry to conduct their measurements. This apparatus is designed to isolate horizontal gravitational forces from the Earth's vertical gravity, making it sensitive to the gravitational attraction between small masses. The setup involved four small cylinders arranged in a plus sign within a vacuum, with four larger cylinders outside gravitationally attracting them. The rotation angle caused by moving the outer masses was measured to determine the gravitational torque.

Measuring big G is challenging due to the extreme weakness of gravity compared to other fundamental forces. The force between everyday objects, such as two coffee cups, is imperceptible, requiring highly sensitive instruments and controlled environments to detect and quantify.

Addressing Inconsistencies in Prior Measurements

A primary motivation for the NIST project was to replicate and shed light on inconsistencies found in previous measurements of big G, particularly an earlier result from the International Bureau of Weights and Measures (BIPM) that was notably higher than most other findings. The NIST result is lower than the BIPM value by 0.0235 percent. While the experiment did not identify a single cause for the discrepancies, it provides a new data point in the ongoing scientific endeavor to precisely define this fundamental constant.