NIST's Decade-Long Gravitational Constant Experiment Deepens
A ten-year NIST experiment has produced a new measurement of the gravitational constant, big G, that differs from a leading 2007 result, maintaining a
Physicists at the National Institute of Standards and Technology (NIST) have reported a new measurement of the universal gravitational constant after a decade of work. Their result deepens a 225-year-old puzzle by disagreeing with another high-precision experiment.
Stephan Schlamminger, a NIST physicist, led the effort to measure big G, the fundamental number that sets the strength of gravity. To avoid bias, part of the experimental data was blinded for ten years using a secret correction factor sealed in an envelope.
Gravity's Most Elusive Number
Gravity shapes planets, stars, and galaxies, yet its fundamental strength is known with far less precision than other natural forces. The force's extreme weakness complicates laboratory experiments, as researchers must detect the tiny attraction between manageable test masses.
A small magnet can lift a paperclip. In labs, the masses used are about 500 billion trillion times smaller than Earth, making the gravitational forces incredibly faint.
A Blinded Decade-Long Experiment
Schlamminger's team aimed to replicate a 2007 experiment conducted by the International Bureau of Weights and Measures (BIPM) in France. To prevent unconscious bias, colleague Patrick Abbott blinded the data by subtracting a secret number from measured weights. The correction needed to recover the true answer remained sealed until the final analysis was complete.
Schlamminger nearly opened the envelope in 2022 but delayed for two more years to account for a subtle air pressure effect. He finally revealed the result on July 11, 2024, at the Conference on Precision Electromagnetic Measurements in Colorado.
A Persistent Discrepancy Emerges
The unblinded NIST measurement did not match the French result. After further analysis, the team reported a value for G of 6.67387 × 10^-11 meters^3/kilogram/second^2. This is 0.0235% lower than the BIPM's 2007 value.
While tiny, this discrepancy is significant for a fundamental constant. Other constants are known to six or more significant digits, but big G remains stubbornly imprecise. Schlamminger stated, "I had really dotted all the i's and crossed all the t's of the experiment." The mismatch keeps alive the question of whether hidden experimental errors or an unknown aspect of gravity is at play.
An Experiment with Historic Roots
The technique used by both teams employs a torsion balance, an instrument dating back to Henry Cavendish's 1798 experiment. Cavendish measured the twist of a suspended wire caused by gravitational attraction between lead balls. Modern versions are far more sophisticated but operate on the same principle.
The NIST and BIPM experiments used eight cylindrical metal masses. Four larger cylinders rotated on a carousel, while four smaller masses hung inside on a disk suspended from a thin copper-beryllium ribbon. The gravitational pull between these sets of masses caused a measurable twist.
History shows that small measurement discrepancies can sometimes reveal new physics, though experimental error is considered more likely in this case. The continued lack of consensus after generations of effort means the mystery of gravity's fundamental strength endures.
