A twist on gravity measurement, hidden in a mysterious envelope, may point to a subtle flaw in our understanding of the universe, raising new questions about its underlying forces.
That envelope held the key to an experiment led by National Institute of Standards and Technology (NIST) physicist Stephan Schlamminger, which attempted to confirm a measurement of the universal gravitational constant made by a French team in 2007.
Working based on the previous team’s processes, Schlamminger made an important discovery that deepens our understanding of the fundamental force of gravity, as revealed in a recent paper published in Metrologia.
The Universal Gravitational Constant
Of the four forces that govern the universe, gravity, electromagnetism, the weak nuclear force, and the strong nuclear force, gravity has remained the most elusive to clearly understand. The problem is that it is incredibly weak compared to the other three, making precise measurements difficult.
An easy example of this disparity is that even a small magnet, small enough to fit in the hand, can overcome the gravitational pull of the entire mass of the Earth, despite the extreme disparity in size. Despite its weakness, gravity is the force that binds our universe together, forming galaxies and holding moons in their orbits around planets, and those planets in orbit around their host stars.
A challenge scientists have pursued for over two centuries is measuring the universal gravitational constant, also known as big G, the fundamental strength of gravity throughout the universe. Schlamminger dedicated a decade to his pursuit of the universal gravity constant problem.
Gravity in the Lab
While we can obviously notice the effect of gravity at the scale of our planet’s effect on our bodies, when considering objects small enough to be manipulated and measured inside a laboratory, the strength of gravity is so faint as to be almost imperceptible.
Scientists have devised various methods using extremely precise equipment to measure the universal gravitational constant, but their results have failed to align. The most intriguing part is that the differences extend beyond the expected room for error in the precision instruments employed, suggesting that physicists’ basic understanding of gravity may be in error.
To investigate these errors, Schlamminger spent a decade leading an effort to recreate a 2007 experiment conducted by the International Bureau of Weights and Measures (BIPM) in France. If Schlamminger could confirm that finding, it would suggest that physicists may finally have a handle on gravity; otherwise, it could indicate some serious fundamental issue in their understanding.
Ensuring Objectivity
The primary concern for Schlamminger was maintaining the work’s integrity, even in the face of any subconscious bias he may hold. To do so, he had a colleague subtract a number from the data and record it in an envelope to be opened later. Only at the end of the project, with all of the work completed, would the figures be adjusted by the mystery number, ensuring that the data would not be forced to fit the previous outcome.
In 2022, Schlamminger came very close to opening the envelope before suddenly identifying one factor that had gone unaccounted for in his experiment, and adding another two years to the work. Finally, in 2024, he spent the envelope and was pleasantly surprised to see a large negative number, something in the ballpark of what would put his work in agreement with the 2007 findings after the adjustments were made.
However, after the adjustments were made, the mystery number was slightly too large, resulting in a 0.0235% difference from the French measurement.
“At face value, we learned that the new measurement at NIST and the previous measurement at BIPM do not agree with each other,” Schlamminger told The Debrief. “That gives us some idea on the reproducibility of the experiment(s). Since this was the very first time that a big G experiment was repeated, that is significant and new information.”
Continuing to Explore Gravity
“While at NIST, we found a brand-new effect that was never described in the literature before. It is a spurious torque that is mediated by a tiny temperature gradient and the residual gas in the vacuum chamber. It is unclear how much that effect may have biased the BIPM result, because we know little about the temperature gradients in that lab or their vacuum pressure,” Schlaminger continued. “Based on some estimates that I made, it seems unlikely that it accounts for the complete difference. But this effect is definitely something that was not accounted for in their uncertainty budget.”
In conversation with The Debrief, Schlamminger noted the bittersweet nature of repeating an existing experiment and ruminated on how he would advise the next generation to pursue the problem. While pointing out that repeating an experiment can be a learning experience, it remains beholden to ideas that may be outdated.
He specifically called attention to the cumbersome coordinate measurement machine used in the work, saying that a pendulum design created by University of Washington researchers in the early 2000s would have been much more practical. His primary advice to future scientists is to scour the literature for anything that may be useful, but also to think outside the box to push the envelope even further.
“Lincoln famously said: Give me six hours to chop down a tree, and I will spend the first four sharpening the axe,” Schlaminger concludes. “So analogous: Give me six years to measure G, and I will spend the first four thinking about the best way.”
The paper, “Redetermination of the Gravitational Constant with the BIPM Torsion Balance at NIST,” appeared in Metrologia on April 16, 2026.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.