Physicist Stephan Schlamminger opened a sealed envelope in Aurora, Colorado, last July and confronted a puzzle that has vexed scientists for more than two centuries: the universe still won't give up gravity's exact strength.
For nearly a decade, Schlamminger and his team at the National Institute of Standards and Technology in Gaithersburg, Maryland, had been measuring big G—the universal gravitational constant that governs everything from how objects fall on Earth to the motion of distant galaxies. Yet this seemingly fundamental number remains stubbornly difficult to pin down. Despite more than 225 years of increasingly precise experiments since Isaac Newton first described gravity mathematically, modern measurements still disagree with each other by about one part in 10,000, a gap far larger than expected.
To avoid bias, Schlamminger made an unusual choice: he asked colleague Patrick Abbott to scramble the data by secretly subtracting a hidden value from measurements. For nearly ten years, Schlamminger deliberately kept himself from knowing what his own experiment had found. When he finally opened the envelope on July 11, 2024, at the annual Conference on Precision Electromagnetic Measurements, he discovered that his measurement of big G was 6.67387×10⁻¹¹ meters³/kilogram/second²—remarkably precise, yet mysteriously different from previous results.
The challenge of measuring gravity lies in its weakness. While electromagnetism can be so strong that a tiny magnet lifts a paper clip against Earth's entire gravitational pull, the gravitational force between laboratory objects is almost impossibly faint. The masses used in experiments are roughly 500 billion trillion times smaller than Earth itself, making their mutual gravitational attraction vanishingly small and incredibly difficult to detect accurately.
Both Schlamminger's team and the International Bureau of Weights and Measures in Sèvres, France—whose landmark 2007 measurement they were attempting to replicate—used a torsion balance, a device that detects these minuscule forces by measuring how much a thin fiber twists. It's a technique dating back to English physicist Henry Cavendish, but modern versions incorporate sophisticated controls and measurements of effects Cavendish never imagined.
Schlamminger's result differed from the French measurement by 0.0235 percent—a gap that sounds trivial until you consider that most other fundamental constants are known to six or more significant digits with much greater agreement. The discrepancy is too small to change how much you weigh on a bathroom scale or affect how manufacturers measure a 16-ounce jar of peanut butter. Yet throughout scientific history, such tiny inconsistencies have sometimes revealed hidden gaps in existing theories and pointed toward major discoveries.
The question now hanging over both research teams is whether they're overlooking subtle flaws in their experiments—maybe unaccounted effects of air pressure, temperature, or other environmental factors—or whether gravity itself is more complicated than our current understanding suggests. Schlamminger has already refined his analysis twice, discovering potential sources of error even after opening the envelope. The next chapter of this patient, meticulous hunt for one of nature's most elusive numbers continues to unfold.