One scientist’s 10-year quest to calculate the strength of gravity
Earth’s gravitational force, g, has been known for centuries. But the exact value of G, the universal gravitational constant, is elusive
NIST scientists Stephan Schlamminger (left) and Vincent Lee examine the torsion balance they used to measure the gravitational constant, big G, a decade-long undertaking.
After a decade of meticulous research, physicist Stephan Schlamminger awaited a pivotal moment in a hotel water park. That afternoon, he was set to present his latest measurement of the gravitational constant, G, to his colleagues. In the hours leading up to the presentation, he sought solace in the chlorinated surroundings.
“I was so stressed out,” he recalls. “I almost wanted to cancel it.”
While Earth’s gravity easily pulls objects like baseballs to the ground, measuring the constant that governs gravitational force between masses is a complex task. On April 16, Schlamminger published a new measurement of G, contributing another piece to the puzzle in determining its precise value.
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According to Isaac Newton’s law of universal gravitation, the force between two objects is the gravitational constant, G, multiplied by the product of their masses, divided by the square of the distance between them. This is expressed as F = G(m1m2)/ r2.
The gravitational pull of Earth, known as “little g,” is measured with high precision: g = 9.80665 m/s2 at Earth’s surface. However, “big G,” the universal gravitational constant, remains less certain. Previous measurements of G vary significantly, creating a scatter plot of results. Schlamminger notes that isolating this weak force is challenging, even with the most advanced instruments.
“G is kind of special,” Schlamminger remarks. “It’s like the lady clad in red velvet, it’s always wrapped in scandal.”
Schlamminger’s team used methods from a 2014 study by the International Bureau of Weights and Measurements (BIPM), aiming for consistent results. The study employed a torsion balance, an updated version of a centuries-old method from the Cavendish experiment. This technique, initially developed to measure Earth’s density, involved a beam with lead balls that twisted due to mutual gravitational attraction. The angle of the twist allowed calculation of G.
Schlamminger’s experiment at the National Institute of Standards and Technology (NIST) in Gaithersburg, Md., replicated the BIPM setup from 2014, which was sent to NIST in 2016. Researchers placed masses on torsion disks, with lighter masses inside and heavier ones outside, all within a vacuum chamber. Despite using the same methods, the team made updates, such as using both copper and sapphire masses to eliminate material effects, ensuring parallel torsion disks, and enhancing device control software.
Setup at NIST for measuring the strength of gravity.
The final measurement for G, 6.67387 × 10–11m3kg–1s–2, was lower than both the BIPM measurement and the standard set by the Committee on Data of the International Science Council (CODATA), derived from top measurements. This indicates that the precision of G is not yet as refined as desired. Terry Quinn, who led the 2014 study, believes additional measurements are valuable, though the CODATA standard is sufficient for now.
Measuring G is significant for testing precision instruments, and minor discrepancies might hint at unknown physics, according to Schlamminger. Yet, he acknowledges the value of G itself has limited practical application, finding excitement in the measurement process itself.
“I love taking measurements. Measurement science is my passion,” Schlamminger says. “I know it’s difficult to understand for many people, but it is. It can be exciting and very fulfilling.”
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