NIST Weighs In on the Mystery of the Gravitational Constant

The time had come to open the envelope, but Stephan Schlamminger, a physicist at the National Institute of Standards and Technology (NIST), wasn't sure he wanted to know the secret number that lay inside.

For the past 10 years, Schlamminger had spent most of his working hours trying to measure a single quantity, known as the universal gravitational constant, which determines the strength of gravity everywhere in the universe. The secret number would allow Schlamminger to unscramble his data and get his answer.

Gravity keeps our feet on the ground, holds planets in orbit around the Sun, corrals stars and other matter to create galaxies, and shapes galactic clusters to weave the web of the universe. But its strength, expressed as "big G," is not exactly known.

Despite its importance, big G is notoriously difficult to measure precisely, Schlamminger knew. Scientists have been trying to measure the constant for over 225 years, a century after Isaac Newton published his famous law of gravitation. But its value remains the least well-known of the four fundamental forces in nature, which also include electromagnetism and the strong and weak nuclear forces.

That's in part because gravity is the weakest of the four forces. Even a magnet no bigger than the head of a pin can levitate a paper clip off the floor, exerting an electromagnetic force far greater than the downward pull of Earth's entire gravitational field.

Gravity's inherent weakness is magnified in the laboratory, where researchers can measure big G only by studying the gravitational attraction between masses small enough to be weighed and moved. Those masses are about 500 billion trillion times smaller than Earth, and so the gravitational attraction that scientists need to measure is that much weaker.

Although experiments have become more sensitive and sophisticated, many recent measurements of big G have had slightly different values. Although the differences are slight, about one part in 10,000, they are still too large to be explained by routine experimental errors.

That disparity had created an unsettling mystery. Is there some overlooked experimental error causing the mismatches - the most likely explanation - or is there something fundamentally wrong with our understanding of gravity?

That's what Schlamminger and his colleagues sought to find out by painstakingly replicating a precision experiment conducted by the International Bureau of Weights and Measures (BIPM) in Sèvres, France, in 2007. If Schlamminger could independently reproduce the same results of that study at NIST's campus in Gaithersburg, Maryland, the mystery might be resolved.

Schlamminger worried that he might unconsciously skew his measurement so that it agreed with the value of G that researchers found in the French experiment. To satisfy his own meticulous standards, Schlamminger asked his colleague Patrick Abbott to scramble the data. Abbott did so by subtracting a number - known only to him - from the weights of some of the masses in the NIST experiment that he had carefully measured. By employing that strategy, Schlamminger would not know the actual value of big G that his team had measured. That is, not until he unsealed the envelope and read the secret number inside.

The Big Reveal

Once before, in 2022, Schlamminger had been poised to unmask the hidden number but at the last minute declined, realizing he had overlooked a subtle but important factor related to air pressure that could skew the results. Now, at 3 p.m. on July 11, 2024, Schlamminger was scheduled to report his findings at the annual Conference on Precision Electromagnetic Measurements in Aurora, Colorado.

Too anxious to attend the conference's morning sessions, Schlamminger ruminated about all the possible factors, including variations in temperature and pressure, that might confound his team's measurement. To the best of his ability, he had accounted for each. "I had really dotted all the i's and crossed all the t's of the experiment," he said.

At his afternoon talk, Schlamminger finally read the number Abbott had placed inside the envelope and felt instant relief. To get the results he expected, the secret number needed to be relatively large and negative.

It was.

But over the day, his excitement waned. The number's magnitude was too large for his results to match those of the French experiment.

After two more years of extensive analysis, Schlamminger and his collaborators have reported their result in Metrologia. The team's measured value of G, 6.67387x10^-11 meters³/kilogram/second², is 0.0235% lower than the French result. Given that all the other constants in nature are known to six or more significant digits, that's a notable difference.

The mismatch isn't large enough to change how much you weigh on your bathroom scale or alter the amount of peanut butter you need to make a 16-ounce product. But throughout the history of science, small discrepancies in measurements have sometimes revealed cracks in our understanding of the universe and led to startling new insights about the workings of the cosmos.

An Experiment Rooted in History

The BIPM and NIST measurements relied on a torsion balance, a device that senses minute forces by measuring the twisting angle, or torsion, of a thin suspending fiber. The method harks back to a landmark experiment conducted by English physicist Henry Cavendish in 1798.

Cavendish placed two lead balls on opposite ends of a wooden beam horizontally suspended at its center by a thin wire. Nearby, he positioned two much heavier masses, suspended separately. The gravitational attraction between the smaller and heavier masses caused the wooden beam to rotate, twisting the wire until the torque it exerted counterbalanced the gravitational force. The motion of the wooden beam, measured with a mirror and light pointer, indicated the value of big G.

The more sophisticated BIPM and NIST experiments featured eight cylindrical metal masses. Four of the cylinders sat on a rotating carousel, resembling four candlesticks in an old-fashioned chandelier. The other four smaller masses were placed inside the carousel, on a disk suspended by a ribbon of copper-beryllium about the thickness of a human hair.

When the outer masses attract the inner masses, the torsion balance rotates and the metal strip twists. Precision tracking of the rotation and the gravitational torque, or twisting, provided one measure of G. But both teams went a step further.

In a second set of measurements, the researchers applied a voltage to electrodes placed alongside each of the inner masses. The voltages created an electrostatic torque that twisted the wire in a direction opposite to the gravitationally induced torque. By carefully choosing a voltage that exactly counterbalanced the gravitational torque, the researchers prevented the torsion balance from rotating. The magnitude of the voltage provided another estimate of big G.

Schlamminger's team added one more variant to the study. In an effort to determine whether the composition of the masses somehow influenced their measurement, the researchers conducted their experiment first with copper masses, then repeated the study with sapphire. The team found virtually identical results.

Although the NIST study, a decade-long undertaking, doesn't resolve the problem with big G, it has now been added to the scientific body of evidence. "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 says he's devoted enough time to chasing big G. "I'll leave it to younger generations of scientists to work on the problem," he added.

"We must press on."

Paper: S. Schlamminger, L. Chao, V. Lee, C. Shakarji, A. Possolo, D. Newell, J. Stirling, R. Cochran and C. Speake. Redetermination of the gravitational constant with the BIPM torsion balance at NIST. Metrologia. Published online April 16, 2026. DOI: 10.1088/1681-7575/ae570f