NIST considers the mystery of the gravitational constant

It was time to open the envelope, but National Institute of Standards and Technology (NIST) physicist Stephen Schlamminger wasn’t sure if he wanted to know the secret number inside.

For the past decade, Schlamminger has spent most of his working hours measuring a single quantity known as the universal gravitational constant, which determines the strength of gravity everywhere in the universe. Using a secret number, Schlamminger is able to decipher the data and get answers.

Gravity keeps our feet on the ground, holds planets in orbit around the sun, corrals stars and other matter into galaxies, and forms galaxy clusters to weave the web of the universe. However, its strength is described as “great.” G,” is not exactly known.

Despite its importance, a large G is notoriously difficult to accurately measure, Schlamminger knew. A century after Isaac Newton published his famous law of gravity, scientists have been trying to measure the constant for more than 225 years. However, its value is the least known of the four fundamental forces of nature. These forces also include electromagnetism and the strong and weak nuclear forces.

Part of the reason is that gravity is the weakest of the four forces. A magnet no larger than the head of a pin can lift a paperclip off the floor and exert an electromagnetic force far greater than the downward pull of Earth’s entire gravitational field.

Gravity’s inherent weakness is magnified even further in the laboratory, where researchers can measure large gravitational forces. G This is only possible by studying the gravitational force between masses small enough to weigh and move. Their masses are about 500 billion times smaller than Earth’s, so the gravity that scientists need to measure is that much weaker.

Although experiments have become more sensitive and sophisticated, many recent measurements G had slightly different values. The difference is small, about 1 in 10,000, but still too large to be explained by routine experimental error.

This difference created a disturbing mystery. Is there some overlooked experimental error causing the discrepancy — the most likely explanation — or is there something fundamentally wrong with our understanding of gravity?

This is what Schlamminger and colleagues set out to find out by painstakingly replicating a precision experiment conducted in 2007 by the International Bureau of Weights and Measures (BIPM) in Sèvres, France. If Schlamminger can independently replicate the results of that study at NIST’s campus in Gaithersburg, Maryland, the mystery may be solved.

Schlamminger worried that his measurements would be unintentionally distorted to match the next value. G What researchers discovered in a French experiment. Schlamminger asked his colleague Patrick Abbott to scramble the data to meet his meticulous standards. Abbott did this by subtracting a number known only to him from the weights of several masses he had carefully measured in NIST experiments. Adopting that strategy would prevent Schlamminger from knowing the real value of big things. G That’s what his team measured. That is, until he opened the envelope and read the secret number inside.

serious exposure

Once, in 2022, Schlamminger was looking to uncover hidden numbers, but backed out at the last minute after realizing he had overlooked a subtle but important factor regarding air pressure that could skew the results. Now, at 3 p.m. on July 11, 2024, Schlamminger was scheduled to report his research results at the Annual Conference on Precision Electromagnetic Measurements to be held in Aurora, Colorado.

Schlamminger was too anxious to attend the conference’s morning session, thinking about all the factors that could disrupt the team’s measurements, such as fluctuations in temperature and barometric pressure. He explained each thing to the best of his ability. “I dotted all the i’s and crossed all the t’s in the experiment,” he said.

During his afternoon speech, Mr. Schlamminger finally read the number that Mr. Abbott had placed in the envelope and was immediately relieved. The secret number needed to be relatively large and negative to get the results he expected.

it was.

But as the days passed, his excitement wore off. The size of the numbers was too large for his results to match those of the French experiment.

After two more years of extensive analysis, Schlamminger and his collaborators reported their results in the following paper. Metrology. The team measurements are G, 6.67387×10^-11 meter³/kilogram/second²0.0235% lower than the French result. This is a notable difference, considering that all other constants in nature are known to six or more significant figures.

This discrepancy is not large enough to change the weight on the scale or change the amount of peanut butter needed to make 16 ounces of product. But throughout the history of science, small errors in measurements have sometimes revealed cracks in our understanding of the universe, leading to surprising new insights into how it works.

Experiments rooted in history

The BIPM and NIST measurements relied on torsion balances, devices that sense tiny forces by measuring the twist angle, or twist, of thin suspended fibers. This method is reminiscent of a groundbreaking experiment performed by British physicist Henry Cavendish in 1798.

Cavendish placed two lead balls at each end of a wooden beam suspended horizontally in the center by a thin wire. Nearby, he placed two much heavier masses suspended separately. The gravitational attraction between the smaller and heavier masses rotated the wooden beam, twisting the wire until its torque balanced the force of gravity. The movement of the wooden beam measured with mirrors and optical pointers showed large values. G.

The more sophisticated BIPM and NIST experiments used eight cylindrical metal blocks. Four of the cylinders were placed on a rotating carousel and resembled the four candlesticks of an old-fashioned chandelier. The other four smaller chunks were placed inside the carousel, on a disk suspended by a hair-thick ribbon of copper beryllium.

When the outer mass attracts the inner mass, the torsion balance rotates and the metal strip twists. Accurately tracking rotation and gravitational torque, or torsion, provides one of the following metrics: G. But both teams went one step further.

In the second set of measurements, the researchers applied voltage to electrodes placed along each internal mass. The voltage created an electrostatic torque that twisted the wire in the opposite direction to the gravitational torque. The researchers prevented the torsion balance from rotating by carefully choosing a voltage that precisely balanced the gravitational torque. The magnitude of the voltage provided an even larger estimate. G.

Schlamminger’s team added one more variant to their research. To determine whether the composition of the ingot affected the measurements in any way, the researchers first conducted the experiment with a copper ingot and then repeated the study with sapphire. The research team found virtually identical results.

NIST’s research is a 10-year effort, but major problems remain. G, It is now being added to the scientific evidence. “Every measurement matters because the truth matters,” Schlamminger said. “For me, making accurate measurements is a way to bring order to the universe, whether or not the numbers match expectations,” he added.

After many years of work, Schlamminger says he has plenty of time to pursue big goals. G. “I will leave it to the younger generation of scientists to tackle this problem,” he added.

“We have to move forward.”

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