The Most Elusive Number in Physics Just Got Even More Mysterious

A decade-long effort to measure one of physics’ most fundamental constants culminates in a moment of uncertainty and revelation.

The moment had arrived to open the envelope, but Stephan Schlamminger, a physicist at the National Institute of Standards and Technology (NIST), hesitated. Inside was a hidden number that would reveal the final result of his decade-long experiment.

For 10 years, Schlamminger had focused on measuring the universal gravitational constant, the quantity that defines the strength of gravity across the universe. The concealed number would allow him to decode his data and determine his result.

Gravity governs everything from keeping our feet on the ground to holding planets in orbit and shaping galaxies and cosmic structures. Yet its strength, known as “big G,” remains uncertain.

Despite its importance, big G is extremely difficult to measure accurately. Scientists have worked to determine its value for more than 225 years, beginning about a century after Isaac Newton introduced his law of gravitation. Even today, it is the least precisely known of the four fundamental forces, which also include electromagnetism and the strong and weak nuclear forces.

One challenge is that gravity is far weaker than the other forces. A magnet the size of a pinhead can lift a paper clip, producing a force stronger than Earth’s entire gravitational pull on that object.

This weakness becomes even more problematic in laboratory experiments. Researchers must measure the attraction between relatively small objects that can be weighed and moved. These masses are about 500 billion trillion times smaller than Earth, so the forces involved are extremely faint.

Although modern experiments are highly sensitive, recent measurements of big G do not fully agree. The differences are small, about one part in 10,000, but still too large to be explained by normal experimental uncertainty.

This inconsistency has raised a troubling question. Are unseen experimental errors responsible, or could there be a deeper issue with how gravity is understood?

To investigate, Schlamminger and his team set out to replicate a precise experiment conducted in 2007 by the International Bureau of Weights and Measures (BIPM) in Sèvres, France. If they could reproduce the same result at NIST in Gaithersburg, Maryland, it might help resolve the discrepancy.

To avoid bias, Schlamminger took an unusual step. He asked his colleague Patrick Abbott to alter the data by subtracting a secret number from some of the measured masses. Only Abbott knew this value. This approach ensured that Schlamminger would not know the true result until the very end, when he opened the envelope.

The Big Reveal

Schlamminger had nearly revealed the number once before in 2022, but stopped when he realized he had missed an important correction related to air pressure.

On July 11, 2024, at 3 p.m., he was scheduled to present his findings at the Conference on Precision Electromagnetic Measurements in Aurora, Colorado. Too anxious to focus on the morning sessions, he reviewed every possible factor that could affect the results, including temperature and pressure. “I had really dotted all the i’s and crossed all the t’s of the experiment,” he said.

During his presentation, he finally opened the envelope. He felt immediate relief. For his expectations to hold, the hidden number needed to be fairly large and negative.

It was.

However, as the day went on, his confidence faded. The number was larger than expected, meaning his results did not match those from the earlier French experiment.

After two more years of detailed analysis, Schlamminger and his collaborators published their findings in Metrologia. Their measured value of G, 6.67387×10^-11 meters³/kilogram/second², is 0.0235% lower than the BIPM result. Compared to other constants in physics, which are often known to six or more significant digits, this difference is notable.

The gap is too small to affect everyday measurements, such as body weight or food packaging. However, small discrepancies in science have sometimes led to major discoveries about how the universe works.

An Experiment Rooted in History

Both the BIPM and NIST experiments relied on a torsion balance, an instrument that detects tiny forces by measuring the twist of a thin fiber. This technique dates back to 1798, when English physicist Henry Cavendish first used it to estimate the value of G.

Cavendish suspended a wooden beam with small lead spheres at each end using a thin wire. He placed larger masses nearby. The gravitational attraction caused the beam to rotate, twisting the wire until the restoring force balanced the pull of gravity. By tracking this motion with a mirror and light, he could calculate the constant.

Modern versions of the experiment are far more advanced. In the BIPM and NIST setups, eight cylindrical metal masses were used. Four larger masses were arranged on a rotating structure, while four smaller masses were suspended inside on a disk attached to a thin copper-beryllium ribbon about the thickness of a human hair.

As the outer masses attracted the inner ones, the system rotated and twisted the ribbon. Measuring this motion provided one way to calculate G.

The researchers also used a second method. They applied voltages to electrodes near the inner masses, creating an electrostatic force that opposed the gravitational pull. By adjusting the voltage until the system stopped rotating, they could determine G from the electrical balance.

Schlamminger’s team added another test. They repeated the experiment using both copper and sapphire masses to see if material composition affected the results. The measurements were nearly identical.

Although this decade-long study did not resolve the difference in values of big G, it adds important data to the ongoing effort. “Every measurement is important, because the truth matters,” Schlamminger said. “For me, making an accurate measurement is a way of bringing order to the universe, whether or not the number agrees with the expected value,” he added.

After years of work, Schlamminger has decided to move on. “I’ll leave it to younger generations of scientists to work on the problem,” he said. “We must press on.”

Big G, Little g

Big G isn’t the only g in Newton’s law of gravitation. There’s also a little g, and there’s a big difference between the two.

Little g describes the acceleration that an object experiences due to the gravitational pull of a large mass, such as Earth, and it varies from location to location. For instance, the value of little g is approximately 9.8 m/s² at Earth’s surface but only 1.62 m/s² on the Moon because the Moon has a lower mass and therefore exerts a weaker gravitational pull than Earth.

In contrast, big G is universal: Its value is the same everywhere in the universe, to the best of scientists’ knowledge. It can tell you the gravitational force between any two objects, whether it’s a person and a planet, or a pair of weights in a laboratory. Calculating the gravitational force between two masses, m₁ and m₂, requires taking the product of the two masses and dividing by the square of the distance r between them, then multiplying that value by the gravitational constant, big G. Written as an equation, Newton’s law states that the force equals Gm₁m₂/r².

Reference: “Redetermination of the gravitational constant with the BIPM torsion balance at NIST” by Stephan Schlamminger, Leon Chao, Vincent Lee, Craig Shakarji, Antonio Possolo, David Newell, Julian Stirling, Robert Cochrane and Clive Speake, 16 April 2026, Metrologia.
DOI: 10.1088/1681-7575/ae570f

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