Gravitational-constant mystery deepens with new precision measurements

Two extremely precise measurements of the gravitational constant G have yielded significantly different values. The two experiments were done by physicists in China and the results deepen the mystery of why it has so far proven impossible to reach a consensus on the value of G, which is a fundamental physical constant.

According to Newton’s universal law of gravitation, the gravitational force (F) that attracts two objects of mass m1 and m2 separated by a distance d is given by Gm1m2/d2. The first measurement of G was made in 1798 by Henry Cavendish, who used a torsion balance designed by John Michell to measure the constant with 1% uncertainty.

A torsion balance comprises a dumbbell-shaped mass suspended fromitscentre by a thin wire. Two large external masses are positioned on either side of the dumbbell in such a way that their gravitational attraction can exert a torque on the dumbbell, causing it to rotate. As the wire twists, the gravitational torque is countered by torsion in the wire until the dumbbell comes to rest. By analysing this motion, G can be calculated.

Since then, more than 200 experiments have been done to measure G to ever higher precision. Today’s accepted value is a combination of several independent measurements and has a relative uncertainty of 47 parts per million (ppm). However, some individual experiments have much smaller uncertainties – until now, the smallest was 13.7 ppm – and some of these very precise measurements disagree by more than 500 ppm.

This has left physicists puzzled as to why it has not been possible to reach an experimental consensus on the value of G. Now, that mystery has been deepened by Shan-Qing Yang, Cheng-Gang Shao, Jun Luo and colleagues at the Huazhong University of Science and Technology and other institutes in China and Russia. They ran two different variations on the torsion balance experiment in the same lab, only to measure significantly different values of G.