For more than two centuries, scientists have tried to determine one of the most important numbers in physics: the universal gravitational constant, known as “big G.” It defines the strength of gravity throughout the universe, influencing everything from falling objects on Earth to the motion of galaxies. Yet despite its importance, researchers still cannot agree on its exact value.
That uncertainty weighed heavily on Stephan Schlamminger, a physicist at the National Institute of Standards and Technology (NIST), as he prepared to open a sealed envelope containing a crucial secret number. For nearly 10 years, Schlamminger had devoted much of his career to measuring big G with extraordinary precision. The hidden number inside the envelope would finally allow him to decode his team’s results.
Why Measuring Gravity Is So Difficult
Gravity may shape the cosmos, but it is surprisingly weak compared to the other fundamental forces of nature. Electromagnetism, for example, is far stronger. Even a tiny magnet can lift a paper clip against the pull of Earth’s entire gravitational field.
That weakness becomes an enormous challenge in the lab. Scientists must measure the gravitational attraction between relatively small objects, and those forces are incredibly faint. The masses used in experiments are roughly 500 billion trillion times smaller than Earth, making the gravitational pull between them extremely difficult to detect accurately.
Researchers have spent more than 225 years trying to improve measurements of big G since Isaac Newton first described gravity mathematically. Despite increasingly advanced equipment, modern experiments still produce slightly different answers. The differences are tiny, about one part in 10,000, but they are larger than expected experimental uncertainties.
That has raised an uncomfortable question. Are scientists overlooking subtle flaws in their experiments, or is there something incomplete about our understanding of gravity itself?
Recreating a Landmark Gravity Experiment
To investigate the discrepancy, Schlamminger and his colleagues decided to replicate a highly regarded experiment performed in 2007 by the International Bureau of Weights and Measures (BIPM) in Sèvres, France. The goal was simple in principle: see whether an independent team at NIST in Gaithersburg, Maryland, could obtain the same result.
Schlamminger also wanted to avoid any possibility of bias. He worried that knowing the expected value might unconsciously influence his analysis. To prevent that, he asked colleague Patrick Abbott to scramble part of the data.
Abbott secretly subtracted a hidden value from measurements involving some of the experimental masses. Only Abbott knew the number. Until the envelope was opened, Schlamminger had no way of knowing the true value his experiment had produced.
The Moment of Truth
The envelope had almost been opened once before. In 2022, Schlamminger was ready to reveal the result but stopped at the last moment after realizing that a subtle air pressure effect could influence the measurement. He postponed the unveiling and continued refining the analysis.
Finally, on July 11, 2024, at the annual Conference on Precision Electromagnetic Measurements in Aurora, Colorado, the moment arrived.
Schlamminger skipped the conference’s morning sessions, preoccupied with worries about temperature fluctuations, pressure changes, and other tiny effects that might distort the results. “I had really dotted all the i’s and crossed all the t’s of the experiment,” he said.
During his afternoon presentation, he opened the envelope and read Abbott’s hidden number. At first, he felt relieved. The secret value needed to be large and negative for the experiment to align with expectations.
It was.
But as the day went on, that relief faded. The number was too large for the NIST results to match the earlier French experiment.
A New Discrepancy in Big G
After two additional years of detailed analysis, Schlamminger and his collaborators published their findings in Metrologia. Their measured value for G was 6.67387×10-11 meters3/kilogram/second2, which is 0.0235% lower than the French measurement.
That may sound insignificant, but physicists take such differences seriously. Most other fundamental constants are known to six or more significant digits with far greater agreement.
The discrepancy is not large enough to affect everyday life. It will not change your weight on a bathroom scale or alter how manufacturers measure ingredients like peanut butter for a 16-ounce jar. However, throughout scientific history, tiny inconsistencies have sometimes pointed to major discoveries and revealed hidden gaps in existing theories.
How Scientists Measure Gravity
Both the BIPM and NIST experiments relied on a device called a torsion balance, which detects extremely small forces by measuring how much a thin fiber twists.
The technique traces back to English physicist Henry Cavendish, who conducted a pioneering gravity experiment in 1798. Cavendish suspended two lead spheres from a wire and positioned larger masses nearby. The gravitational attraction between them caused the suspended beam to rotate slightly, twisting the wire. By measuring that motion, Cavendish estimated the strength of gravity.
The modern versions used by BIPM and NIST were far more advanced. The setups included eight cylindrical metal masses. Four larger cylinders sat on a rotating carousel, while four smaller masses were suspended inside on a copper-beryllium ribbon about as thick as a human hair.
As the outer masses attracted the inner ones, the torsion balance rotated and twisted the ribbon. Measuring that tiny motion provided one estimate of big G.
The teams also used a second technique involving electricity. Researchers applied voltage to electrodes near the inner masses, creating an electrostatic force that counteracted gravity. By carefully adjusting the voltage until the balance stopped rotating, they obtained another independent measurement of G.
Testing Copper and Sapphire Masses
Schlamminger’s team added an extra step to the experiment. To determine whether the material itself could influence the measurement, they repeated the study using both copper and sapphire masses.
The results were nearly identical, suggesting that the composition of the masses was not responsible for the discrepancy.
Although the experiment did not solve the mystery surrounding big G, it added another important data point to the growing 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 spending a decade pursuing the problem, Schlamminger says he is ready to move on.
“I’ll leave it to younger generations of scientists to work on the problem,” he added.
“We must press on.”
Big G vs. Little g
Newton’s law of gravity contains both a “big G” and a “little g,” but they describe different things.
Little g refers to the acceleration caused by gravity near a large object such as Earth. On Earth’s surface, little g is about 9.8 m/s2. On the Moon, where gravity is weaker because the Moon has less mass, little g is only about 1.62 m/s2.
Big G, on the other hand, is considered universal. Scientists believe it has the same value everywhere in the universe. It determines the gravitational force between any two objects, whether that involves planets, people, or laboratory weights.
Newton’s equation calculates gravitational force by multiplying two masses together, dividing by the square of the distance between them, and multiplying by big G. Written mathematically, the law is expressed as Gm1m2/r2.
