NIST's 10-year measurement of gravity's constant deepens a 225-year puzzle

Overview

Updated Sep 20

Stephan Schlamminger spent a decade measuring gravity's most stubborn constant without seeing his own answer. A colleague scrambled the data and sealed the key in an envelope, guarding against bias. When Schlamminger finally opened it in July 2024, his result disagreed with the leading value from France.

The gap is tiny, about 0.0235%, yet notable for a constant that anchors physics. It is the first time a big G experiment has ever been repeated, and the two runs don't agree. The mismatch is probably hidden error, but it could be the first hint that something about gravity is off.

Why it matters

Big G is the least precisely known fundamental constant; if measurements keep disagreeing, every calculation of gravity's strength carries a hidden uncertainty.

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Key Indicators

6.67387 × 10⁻¹¹ m³/kg/s²

Measured value of big G

The NIST team's value, reported in Metrologia in April 2026.

0.0235%

Gap below the BIPM benchmark

Relative difference between the NIST and French measurements, too large for routine error.

3650 days

Duration of the NIST experiment

About 10 years of measurement, followed by two more years of analysis.

225+

Years scientists have tried to pin down G

Experiments have run since Henry Cavendish's 1798 torsion balance.

Voices

Curated perspectives — historical figures and your fellow readers.

People Involved

Stephan Schlamminger

Physicist, National Institute of Standards and Technology (NIST)

Published his group's decade-long measurement result in Metrologia

Patrick Abbott

NIST physicist, keeper of the secret number

Held the only copy of the unscrambling key for a decade

Organizations Involved

National Institute of Standards and Technology

US federal agency

Hosted the replication experiment at its Gaithersburg, Maryland campus

The U.S. federal laboratory responsible for measurement standards, including fundamental physical constants.

International Bureau of Weights and Measures

Intergovernmental organization

Conducted the 2007 benchmark measurement now in dispute

The intergovernmental organization that coordinates global measurement standards, including the SI system.

Timeline

2007 September 2026

6 events Latest: September 20th, 2026 · 2 months ago

Tap a bar to jump to that date

September 2026
ScienceDaily resurfaces the mystery
Latest Statement

ScienceDaily reports the result, renewing attention on the big G discrepancy.
September 20th, 2026
April 2026
Results published in Metrologia
Publication

The NIST team reports G at 6.67387 × 10⁻¹¹ m³/kg/s², 0.0235% below the French value.
April 16th, 2026
July 2024
The envelope is opened
Revelation

At the Conference on Precision Electromagnetic Measurements in Aurora, Colorado, Schlamminger opens the envelope and learns his G value disagrees with BIPM.
July 11th, 2024 • 3:00 PM
2022
Envelope unsealing postponed
Investigation

Schlamminger declines to open the envelope, catching an air-pressure effect that could skew the results.
2022
2014
NIST replication begins
Investigation

Schlamminger starts replicating the BIPM experiment at NIST's Gaithersburg campus, using the same apparatus with modifications.
2014
2007
BIPM sets the benchmark for big G
Measurement

BIPM completes a precision measurement of the gravitational constant with a torsion balance in Sèvres, France.
2007

Scenarios

Systematic error found: the gap closes

Possible Resolves by Q2 2028

Discussed by: Schlamminger and his collaborators themselves

The NIST team already found a new spurious torque from a temperature gradient and residual gas in the vacuum chamber, an effect absent from BIPM's uncertainty budget. If further analysis shows this effect, or another, accounts for most of the 0.0235% gap, the two values reconcile and big G stays a measurement problem rather than a physics mystery. Schlamminger's own estimate, however, suggests the effect is unlikely to explain the whole difference.

Independent labs side with NIST, not BIPM

Possible Resolves by End of 2029

Discussed by: The Debrief's reporting and metrologists following the puzzle

A University of Washington team designed a swing-pendulum G experiment in the early 2000s that Schlamminger called more practical than the torsion balance. If a new independent experiment lands within combined uncertainty of the NIST value, and far from BIPM's, the French result becomes the outlier, pointing to a flaw in the original apparatus that survived replication.

Big G stays the great outlier of physics constants

Likely Resolves by End of 2028

Discussed by: Most metrologists; the likely default

This is the first replication of a big G experiment ever, and the two runs disagree. Without a third measurement to break the tie, the discrepancy simply becomes part of the record. The next CODATA recommended value will carry a wider spread, and the puzzle passes to the next generation of experiments. History suggests most such anomalies end in mundane error, not new physics.

Historical Context

3 moments from history that rhyme with this story — and how they unfolded.

September 2011 - June 2012

OPERA neutrino anomaly (2011-2012)

The OPERA collaboration at CERN announced that neutrinos appeared to travel faster than light, apparently breaking relativity. The claim stunned physicists and triggered months of worldwide scrutiny.

Then

The result was traced to a loose fiber-optic connection in the timing equipment and formally withdrawn.

Now

It became the modern cautionary tale of precision measurement disagreement, nearly always ending in hardware errors rather than new physics.

Why this matters now

The big G gap could resolve the same way — a subtle equipment effect explaining an apparent anomaly without any change to fundamental physics.

1972-2002

Pioneer anomaly (1972-2002)

The Pioneer 10 and 11 spacecraft appeared to decelerate slightly more than gravity predicted, hinting at a flaw in our understanding of gravity. The effect persisted for three decades of tracking.

Then

Analysis showed the craft emitted heat unevenly, producing a small thermal thrust that accounted for the anomaly.

Now

A mundane thermal effect resolved what looked like a deep physics puzzle — a direct parallel to the temperature-gradient torque NIST found in its apparatus.

Why this matters now

Like the Pioneer anomaly, the big G discrepancy may trace to thermal effects nobody had cataloged before, in a measurement of the very same force.

1797-1798

Cavendish experiment (1798)

Henry Cavendish measured the gravitational attraction between lead spheres using a torsion balance, suspending a rod of masses from a fine wire and measuring its twist.

Then

His result gave the first useful value for the density of Earth and the gravitational constant.

Now

The torsion balance method has been refined for 228 years and still underlies how we measure gravity's strength today, including the NIST and BIPM experiments.

Why this matters now

Every modern big G experiment, including the one in dispute, descends directly from Cavendish's apparatus.