For centuries, one of the most important numbers in physics has evaded accurate identification. The results of a recent experiment by researchers at the National Institute of Standards and Technology in the US could help bring us closer to a satisfying answer.
Gravity is the weakest of nature’s four fundamental forces, which is why it has remained persistently hard to measure.
“Science has been trying to pin down the gravitational constant for over 200 years,” says NIST physicist Stephan Schlamminger, whose team set out to replicate a landmark experiment to settle on a precise figure for this powerful, if subtle, force.
The gravitational constant, or simply ‘Big G’, is a force of attraction between masses. While pinning down the exact value of G won’t affect how we measure objects in our daily lives, settling on a precise value for the constant could bring us a step closer to understanding gravity’s nature.
Yet, despite these repeated efforts, the data remains inconsistent. “There are 16 data points that people have measured, and the data points scatter around a lot, with the margin of error being about 10 parts per million,” Schlamminger told Refractor.
This latest experiment was another attempt to bring order to this chaos, while also allowing the scientists to hone their skills in precision measurements.
“For me, it was also a mechanism to become better at measuring small forces and torques,” says Schlamminger.
There were two options the team could use in this quest to measure big G. One was to carry out a completely new experiment. The other was to replicate one that had been done before in a different environment to see if it would yield a different value.
The team went with the second option, repeating an experiment conducted at the International Bureau of Weights and Measures (BIPM) in France in 2014. The team transported the very same apparatus used at BIPM from France to the NIST laboratory in Gaithersburg, Maryland.
This was a unique project, not only because it was a replication experiment but because the initial experiment conducted in France in 2014 yielded one of the most skewed values of Big G. Replicating the experiment could help identify any hidden systematic effects that influenced its results.
The experiment commenced in 2016 and took the scientists 10 years to complete. They determined G to be 6.67387 ± 0.00038 × 10−11 m3 kg−1 s−2, corresponding to a relative standard uncertainty of 5.7 × 10−5. The result was 0.0235% lower than the value determined by the initial experiment at BIPM — a considerable variation given the remarkable precision of so many other measurements in physics.
In addition to a new value for the gravitational constant, however, the team discovered a factor that had been unaccounted for in the previous study.
For the experiment to work, all the air has to be sucked out of the environment, leaving a pure vacuum. The scientists found that despite all efforts, there remained a small amount of air in the vessel, which could influence the derived value of G.
“We couldn’t pop out all the air from the vessel,” says Schlamminger. “There is always a little bit of air left, called a vacuum pressure.”
The scientists found that this air exerts a small amount of force on the apparatus that had not been described in previous attempts, and it could bring us closer to understanding why the value of Big G continues to remain inconsistent.
However, while this is a possibility, Schlamminger says we can’t yet make that conclusion. “We would have to look at each experiment individually and understand what they have done.”
The new gravitational constant is a little lower than the CODATA 2018-recommended value for Big G, but it is still hard to say why.
“For now, we think it could be due to a multitude of effects, but we can’t identify those differences yet,” says Schlamminger.
This study was published in Metrologia.
Fact-checked by Mike McRae.
