We still don't have a more precise value for 'Big G'
Source: Ars Technica
“Such experiments bring order to the universe, whether or not the number agrees with the expected value.”
Credit: R. Eskalis/NIST
The gravitational constant, affectionately known as “Big G,” is one of the most fundamental constants of our universe. Its value describes the strength of the gravitational force acting on two masses separated by a given distance—or, if you prefer a relativistic view, the amount a given mass curves space‑time.
Physicists have a solid ball‑park figure for the value of Big G, but they have been trying to measure it ever more precisely for more than two centuries. Each effort yields a slightly different value; the spread is roughly one part in 10 000.
Other fundamental constants are known far more precisely, so Big G is the black sheep of the family—a source of frustration for precision metrologists. Gravity is by far the weakest of the four fundamental forces, and the ever‑present background field of the Earth (the “little g”) adds significant noise, especially in a laboratory setting.
The latest NIST effort
In the most recent attempt to resolve the discrepancy, scientists at the National Institute of Standards and Technology (NIST) spent a decade replicating one of the most divergent recent experimental results. Their findings were announced in a paper published in Metrologia — see the article.
The new measurement does not settle the long‑standing disagreement, but it provides another data point for physicists striving to nail down a more precise value for Big G.
A brief history
- Isaac Newton introduced the concept of a gravitational constant when he published his law of universal gravitation in the late 17th century. The notation “Big G” did not appear until the 1890s. Newton imagined measuring gravity by swinging a pendulum near a large hill, but he never attempted the experiment, judging the effect too small to detect.
- By 1774, the Royal Society formed a committee to determine Earth’s density as an indirect measurement of Big G, using a variation of Newton’s pendulum idea.
It was Henry Cavendish in 1798 who achieved the first direct laboratory measurement of the gravitational attraction between two bodies, using a torsion balance. His apparatus consisted of:
- A small dumbbell with two‑inch lead spheres on a six‑foot wooden rod, suspended by a central wire so it could rotate.
- A larger dumbbell with two 12‑inch, 350‑pound lead spheres that, when brought near the smaller ones, caused the suspended rod to twist.
Cavendish painstakingly recorded the resulting oscillations, allowing him to infer Earth’s density. His torsion‑balance design has since become a workhorse for physicists seeking ever‑more precise determinations of Big G.
Updating the Cavendish Experiment
Developing ever‑more precise experiments has long been the dominant strategy for resolving the discrepancies. The authors of this latest paper realized that simply adding more measurements to the dataset would not be sufficient, since earlier inconsistent results would still dominate. So they came up with the idea of taking a closer look at one of the largest outliers—specifically a 2007 experiment by physicists at France’s International Bureau of Weights and Measures (BIPM) that employed a much‑more sophisticated version of Cavendish’s torsion‑balance apparatus.
The NIST team replicated the original BIPM experiment, building a torsion balance with eight metal cylinders: four on a rotating carousel and four smaller masses inside the carousel, sitting on a suspended disk held by a thin ribbon of copper‑beryllium. The torsion balance and ribbon would twist when the outer masses attracted the inner ones, and physicists measured G by tracking the cylinder’s rotation and the resulting gravitational torque.
They also performed a second set of measurements by applying a voltage to electrodes beside the inner masses. This twisted the wire in the opposite direction to the gravitational torque, and the voltage magnitude provided another estimate of G.
The NIST scientists added an extra twist: they ran two versions of the experiment, one with copper masses and one with sapphire masses, achieving nearly identical values for both. This ruled out the possibility that the specific materials used were affecting the measurements. After all that, they reported a value of
[ G = 6.67387 \times 10^{-11}\ \text{m}^3\ \text{kg}^{-1}\ \text{s}^{-2}, ]
which is 0.0235 % lower than the original BIPM result.
Why Keep Measuring G?
Some might question why physicists continue to try to measure the gravitational constant with ever greater precision. One benefit is that it drives the development of better instruments for measuring tiny forces, torques, and other subtle effects—advances that benefit science in general.
“Every measurement is important, because the truth matters,” said co‑author Stephan Schlamminger, a physicist at NIST. “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.”
Metrology, 2026. DOI: 10.1088/1681-7575/ae570f
About the Author
Jennifer Ouellette is a senior writer at Ars Technica with a particular focus on where science meets culture, covering everything from physics and related interdisciplinary topics to her favorite films and TV series. She lives in Baltimore with her spouse, physicist Sean M. Carroll, and their two cats, Ariel and Caliban.