NIST gravitational constant work led by physicist Stephan Schlamminger has produced a new value for big G after a decade-long attempt to check a higher result from the International Bureau of Weights and Measures, known as BIPM. The new number, according to NIST, is 6.67387 x 10-11 m3/(kg s2), which is 0.0235 percent lower than the BIPM measurement.
The result does not settle the argument. The accepted value of the fundamental gravitational constant is 6.67430 x 10-11 cubic meters per kilogram per square second, with an uncertainty of ±0.00015 x 10-11 m3/(kg s2), according to NIST’s constants database. For a constant treated as a property of the universe, that is an unusually untidy error bar.
Schlamminger, a physicist at the U.S. National Institute of Standards and Technology, told IEEE Spectrum that his group set out to repeat the BIPM experiment because measurements of big G have long failed to line up cleanly. BIPM, based near Paris, had reported a value that sits higher than many other measurements.
Why is the gravitational constant so hard to measure?
Gravity is weak at laboratory scales, which makes big G a nuisance to measure directly. Schlamminger used the everyday comparison of magnets and coffee cups: a refrigerator magnet produces a force a person can feel, while the gravitational pull between two cups exists but is far too small to notice.
That weakness means the experiment has to account for small effects that would be noise in many other measurements. Schlamminger said measuring big G requires tracking every moving mass: where it is, how large it is, and what it weighs. He contrasted that with other fundamental-constant experiments, including measurements of Planck’s constant, which often have some form of self-calibration built in.
How NIST measured big G
The NIST team used a torsion balance, an old idea with modern bookkeeping problems. The instrument separates Earth’s vertical gravitational pull from horizontal gravitational attraction caused by nearby masses, making it sensitive to the objects arranged around the balance rather than the planet underneath it.
In NIST’s setup, a thin torsion strip held four small cylinders arranged like a plus sign inside a vacuum chamber. Four larger cylinders sat outside. Their gravitational attraction pulled on the inner cylinders. When the researchers shifted the outer masses by a small amount, the suspended plus-shaped assembly rotated. The measured angle gave the gravitational torque, which the team used to calculate big G.
The mechanism sounds clean. The hard part is that every mass and position in the apparatus matters, and the signal is tiny. That is how a measurement of one number can consume 10 years without producing a tidy ending.
What did NIST conclude?
Schlamminger told IEEE Spectrum the experiment did not identify a single flaw that explains the gap between the NIST and BIPM values. He said there was no “smoking gun,” leaving the discrepancy unresolved.
The NIST value also lands slightly below the standard literature value, according to Schlamminger. He said he was disappointed that the result agreed with neither the BIPM number nor the broader accepted value. If the BIPM result is wrong, he said, the literature value that includes it may need to move lower, but he added that an independent group should decide what the new mean value should be.
That is the useful, irritating state of the field: one of physics’ basic constants remains harder to pin down than its textbook status suggests.
This story draws on original reporting from IEEE Spectrum.
