NIST physicist Stephan Schlamminger spent a decade measuring big G, the gravitational constant. His blinded experiment reveals why gravity remains physics' most elusive number.
By Adam Makins·
For a decade, physicist Stephan Schlamminger chased one of science’s most frustratingly elusive numbers. After countless experiments, meticulous corrections, and painstaking analysis, the answer finally arrived in a sealed envelope.
Schlamminger, working at the National Institute of Standards and Technology (NIST), had devoted years to measuring big G, the universal gravitational constant. This fundamental number determines how strongly gravity pulls throughout the entire cosmos. It governs everything from keeping people anchored to Earth to shaping the cosmic web of galaxy clusters stretching across the universe.
Yet despite more than 225 years of attempts, scientists still cannot measure big G with the precision they’ve achieved for other fundamental forces of nature.
The problem is deceptively simple: gravity is extraordinarily weak. A tiny magnet the size of a pinhead can lift a paper clip against the gravitational pull of the entire Earth. In that contest, electromagnetism crushes gravity without breaking a sweat.
In the laboratory, the challenge becomes even more daunting. Scientists cannot move planets around to perform controlled experiments. Instead, they measure gravitational attraction between much smaller objects, roughly 500 billion trillion times smaller than Earth. The forces they’re trying to detect are incredibly faint, and even with modern instruments of extraordinary sensitivity, measurements of big G keep producing slightly different answers.
These disagreements hover around one part in 10,000. That might sound trivial, but for fundamental physics, it’s deeply troubling. Other fundamental constants are known to six or more significant digits. Big G stubbornly refuses such precision.
Schlamminger’s team decided to replicate an experiment conducted by France’s International Bureau of Weights and Measures in 2007. The strategy was straightforward: if an independent team could reproduce the French measurement using the same approach, it might help resolve the disagreement.
But Schlamminger worried about introducing his own bias. Scientists can unknowingly influence how they interpret measurements when they know what answer they expect. So he asked colleague Patrick Abbott to blind the experiment by scrambling the data.
Abbott subtracted a secret number from the carefully measured weights of experimental masses. Only Abbott knew this number, allowing Schlamminger to analyze the data without knowing the true value of big G his team was measuring. The correction sat sealed inside an envelope, waiting to reveal the truth.
Then, at the last moment, Schlamminger realized the team hadn’t fully accounted for a subtle effect involving air pressure. In an experiment this sensitive, even tiny disturbances matter. He postponed the reveal and returned to analysis.
On July 11, 2024, Schlamminger presented his results at the Conference on Precision Electromagnetic Measurements in Colorado. When he finally opened the envelope and revealed Abbott’s secret correction, he immediately sensed trouble. The correction was too large. Once the data were restored, NIST’s measurement didn’t match the French result.
After two more years of detailed analysis, Schlamminger’s team reported their value for G: 6.67387 x 10-11 meters3/kilogram/second2. This result is 0.0235% lower than the French experiment.
In everyday life, such a difference means nothing. It won’t change your bathroom scale or affect how much peanut butter fills a 16-ounce container. For fundamental science, though, the discrepancy is significant.
History teaches physicists to pay attention to tiny mismatches. Several times before, small discrepancies between measurements and expectations revealed that scientists were missing something crucial about how nature works. This doesn’t mean big G’s mystery points to new physics. Experimental error remains the more likely culprit. But the continued inability of precision experiments to converge keeps the puzzle alive.
Schlamminger’s decade-long struggle ultimately produced another important data point in science’s continuing quest to determine big G’s true value. And after all that work, he’s ready to pass the torch to younger scientists.
“Every measurement is important, because the truth matters,” Schlamminger said. Perhaps the real question isn’t whether we’ll ever know big G precisely, but what we might discover about the universe itself while chasing it.
Source: National Institute of Standards and Technology (NIST)
