After a grueling 10-year effort, physicists at NIST have completed a highly precise measurement of the universal gravitational constant, known as big G, only to find their result doesn't quite align with previous findings, deepening the mystery surrounding this fundamental force.
Physicist Stephan Schlamminger and his team dedicated a decade to meticulously measuring big G, a number that dictates the strength of gravity across the universe. Despite gravity being a familiar force, scientists still struggle to pin down its exact strength with the precision seen in other fundamental forces like electromagnetism. This difficulty stems from gravity's extreme weakness; a small magnet can easily overpower Earth's gravitational pull on a paperclip.
The challenge in measuring big G lies in its faintness. Scientists have to detect the gravitational pull between small, precisely positioned objects, which are about 500 billion trillion times smaller than Earth. Even with today's sensitive instruments, measurements of big G have consistently yielded slightly different results, with discrepancies larger than expected experimental uncertainty. This has led to two possibilities: either a subtle experimental factor has been missed, or there's something fundamental about gravity that scientists don't yet grasp.
To tackle this, Schlamminger's team aimed to replicate a 2007 experiment from the International Bureau of Weights and Measures (BIPM) in France. To avoid unconscious bias, Schlamminger had a colleague "blind" the experiment by adding a secret number to some of the data, with the correction factor sealed in an envelope. After years of analysis, including accounting for subtle effects like air pressure, Schlamminger finally opened the envelope during a presentation in July 2024. While the correction was substantial and negative as anticipated, the restored NIST measurement ultimately did not match the French result.
The NIST team's reported value for G is 6.67387 × 10-11 m³/kg/s², which is 0.0235% lower than the BIPM's figure. While this difference is minuscule in everyday terms, it's significant for fundamental physics, where other constants are known to much higher precision. The persistent disagreement keeps the possibility of new physics alive, though experimental error remains the more likely culprit.
The experiments utilized a torsion balance, an instrument dating back to Henry Cavendish's 1798 experiment, which detects tiny forces by measuring the twist of a suspended fiber. Both the BIPM and NIST experiments involved precisely measuring the gravitational attraction between carefully positioned masses, and even employed electrostatic forces to balance gravitational torque for more accurate readings. Schlamminger's team also tested different materials, copper and sapphire, for the masses, finding no significant difference, which ruled out material properties as the cause of the discrepancy.
