Scientists Spent 10 Years Measuring Gravity’s Elusive Constant. The Answer Still Doesn’t Match.

For a decade, physicist Stefan Schlamminger chased one of the most stubborn numbers in science: the universal gravitational constant, known as G. After years of experiments, revisions and painstaking analysis, the final answer was sealed inside an envelope.

Schlamminger, a physicist at the National Institute of Standards and Technology (NIST), was not entirely sure he wanted to open it.

The hidden numbers contained the key to restoring the experimental data and revealing what his team had actually measured. But the result could also deepen one of physics’ persistent mysteries: why do carefully conducted experiments continue to produce slightly different values for G?

Why gravity’s constant is so difficult to measure

Gravity is one of the most familiar forces in everyday life. It keeps people on Earth, holds planets in orbit around the Sun, helps form stars and galaxies, and plays a central role in shaping the cosmic web of galaxy clusters across the universe.

Yet scientists do not know the fundamental strength of gravity as precisely as they know the constants associated with the other three fundamental forces: electromagnetism and the strong and weak nuclear forces.

The strength of gravity is represented by the universal gravitational constant, G. Scientists have tried to measure it for more than 225 years, beginning about a century after Isaac Newton introduced his law of universal gravitation.

Despite generations of increasingly sophisticated experiments, G remains unusually difficult to determine. The problem is partly simple: gravity is exceptionally weak.

A small magnet illustrates the challenge. A magnet about the size of a pinhead can lift a paper clip against the gravitational pull of the entire Earth. In that basic contest, electromagnetism easily overcomes gravity.

Laboratory measurements are even more demanding. Scientists cannot move planets around to conduct controlled experiments, so they must measure the gravitational attraction between much smaller objects that can be weighed and positioned with extraordinary precision.

The test masses used in these experiments are about 500 billion times smaller than Earth. The gravitational forces researchers are trying to detect are therefore incredibly small.

Why different experiments keep producing different values

Modern equipment is highly sensitive, but measurements of G still produce slightly different answers.

The disagreement is small—about 1 part in 10,000—but it is larger than researchers would expect from normal experimental uncertainty.

This persistent discrepancy has raised uncomfortable questions for physicists. The most likely explanation is that subtle experimental effects have been overlooked. A more intriguing possibility is that scientists do not yet fully understand gravity itself.

Schlamminger and his colleagues set out to investigate the problem by carefully replicating a precision experiment conducted in 2007 by the International Bureau of Weights and Measures (BIPM) in Sèvres, France.

The plan was straightforward: if an independent team at the NIST campus in Gaithersburg, Maryland, could reproduce the French measurements using essentially the same approach, the comparison might help explain why different experiments disagree about G.

But Schlamminger was concerned about another possible source of error: his own expectations.

When scientists know the result they expect, they can unintentionally influence how measurements are interpreted or analyzed. To reduce that risk, Schlamminger asked his colleague Patrick Abbott to blind the experiment by secretly scrambling the data.

Abbott subtracted a secret number from the carefully measured weights of several masses used in the experiment. Because only Abbott knew the correction, Schlamminger could analyze the data without knowing the true value his team was producing.

The corrections needed to recover the actual result were sealed inside the envelope.

A decade of work reduced to one envelope

Schlamminger was expected to open the envelope in 2022. At the last moment, however, he realized the team had not fully considered subtle effects caused by air pressure.

Because even minor disturbances can affect such a delicate experiment, he postponed the reveal and returned to the analysis.

Two years later, the moment finally arrived.

At 3 p.m. on July 11, 2024, Schlamminger was scheduled to present the results at the Annual Conference on Precision Electromagnetic Measurements in Aurora, Colorado.

He was so anxious that he missed the morning session. Instead, he mentally reviewed the many factors that could have affected the measurements, including small changes in temperature and pressure.

By the afternoon, he believed the team had accounted for everything reasonably possible.

“I dotted all the i’s and crossed all the t’s in the experiment,” he said.

During his presentation, Schlamminger finally revealed the hidden numbers. He immediately felt relieved.

For the experiment to produce the result he expected, Abbott’s secret correction needed to be relatively large and negative.

It was.

At first, that seemed like good news. But over the following days, Schlamminger realized there was a problem: the correction was too large. Once the blinded data was restored, the NIST measurement did not agree with the French result.

A tiny difference with major consequences for physics

After two more years of detailed analysis, Schlamminger and his collaborators reported their measurement in a subsequent paper.

Their value for G was:

6.67387 × 10⁻¹¹ m³ kg⁻¹ s⁻²

This result was 0.0235% lower than the value obtained in the French experiment.

In everyday life, a difference this small has no practical meaning. It would not significantly change a scale reading or affect how much peanut butter is needed to fill a 16-ounce container.

In fundamental physics, however, the discrepancy is significant.

Other fundamental constants are known to six or more significant figures. The accuracy of G remains comparatively low, even after more than two centuries of effort.

History also gives physicists a reason to take small discrepancies seriously. In the past, differences between measurements and theoretical expectations have sometimes revealed that scientists were missing an important part of how nature works.

That does not mean the disagreement over G points to new physics. Experimental error remains the more likely explanation. Still, the mystery continues because precise experiments have not yet converged on one value.

How a 1798 experiment still helps measure gravity

The technology used by both the BIPM and NIST teams is based on an experimental method more than two centuries old.

Their experiments relied on a torsion balance, a device that detects extremely small forces by measuring how much a suspended fiber twists.

The basic idea dates back to a famous experiment performed by British physicist Henry Cavendish in 1798.

Cavendish placed two lead balls at the ends of a wooden beam suspended horizontally from its center by a thin wire. He then positioned two heavier masses nearby.

The heavier masses gravitationally attracted the smaller lead balls, causing the suspended beam to rotate. As it rotated, the wire twisted until its resistance balanced the gravitational force.

By measuring the beam’s tiny movements with mirrors and light pointers, Cavendish determined the gravitational interaction between the masses and obtained information corresponding to the value of G.

More than 200 years later, the same basic principle remains useful, although the equipment is far more sophisticated.

Measuring forces that are almost invisible

The BIPM and NIST experiments used eight cylindrical metal blocks.

Four large cylinders were placed on a rotating carousel in a configuration similar to the four arms of an old-fashioned chandelier. Four smaller cylinders were positioned on a separate carousel mounted on a disk suspended from a copper-beryllium ribbon about as thick as a human hair.

The gravitational force between the outer and inner masses caused the suspended torsion balance to rotate, twisting the thin metal ribbon.

By precisely measuring that movement and the associated gravitational torque, the researchers calculated a value for G. Torque is a twisting force, similar to the force used to turn a wrench.

The team did not rely on only one measurement method.

In another series of measurements, the scientists placed electrodes next to the internal mass and applied a voltage to them. The resulting electrostatic force produced a torque in the opposite direction of the gravitational torque.

The researchers adjusted the voltage until the electrostatic and gravitational effects balanced exactly, preventing the torsion balance from rotating.

The voltage required to achieve that balance could be measured with great precision, providing another way to calculate G.

Copper and sapphire produced the same result

Schlamminger’s team also conducted additional tests to determine whether the materials used for the experimental masses could influence the result.

The researchers first performed measurements using copper. They then repeated the experiment with sapphire.

The results were essentially the same for both materials.

That finding eliminated one possible explanation for the disagreement, but it did not solve the larger mystery.

After a decade of research, the NIST experiment has become another important data point in scientists’ continuing effort to determine the true value of G.

“Every measurement matters because the truth matters,” Schlamminger said. “For me, making accurate measurements is a way to bring order to the universe, whether the numbers match expectations or not,” he added.

After working on the problem for years, Schlamminger said he was ready to leave it to others.

“We will leave it to the younger generation of scientists to tackle this problem,” he added. “We have to move forward.”

Big G is not the same as little g

G is not the only letter associated with gravity. Physicists also use lowercase g, but the two quantities represent completely different things.

Lowercase g refers to the acceleration an object experiences because of the gravitational force of a nearby massive body, such as Earth.

Unlike universal G, the value of g changes depending on location.

Near Earth’s surface, g is about 9.8 meters per second squared. On the Moon, it is about 1.62 meters per second squared because the Moon’s much smaller mass produces weaker gravitational acceleration.

By contrast, G is considered universal. As far as scientists know, it has the same value everywhere in the universe and determines the gravitational force between two objects—whether they are laboratory masses, a person and Earth, or distant celestial bodies.

Newton’s law of universal gravitation uses G to connect mass, distance and gravitational force. For two masses, m₁ and m₂, the force is calculated by multiplying the masses, multiplying by G, and dividing by the square of the distance between them, r:

F = Gm₁m₂ / r²

More than two centuries after scientists began trying to measure it precisely, this seemingly simple constant remains one of the most difficult numbers in physics to pin down.