The gravitational constant is the only number in physics that gets less certain the more we measure it
After a decade of painstaking work, NIST physicist Stephan Schlamminger published a new measurement of the gravitational constant G on April 16, and it disagrees with the current best estimate [3]. This isn't the story of one scientist getting it wrong. It's the story of a universal constant that refuses to be pinned down. The world's best experimental results for measuring G do not overlap [7]. Two and a quarter centuries after Henry Cavendish first measured it in 1798, we still can't agree on gravity's strength [7].
The uncertainty has real consequences. NASA and other space agencies must account for G's measurement variations when calculating trajectories for interplanetary missions [7]. Engineers designing satellite orbits work with a gravitational constant that shifts depending on which laboratory measurement they trust [7]. The European Space Agency's BepiColombo mission to Mercury, launched in 2018, required gravitational calculations precise enough to enter orbit around the solar system's smallest planet, but the team had to choose between competing G values that differed by more than their stated uncertainties [7].
Why does gravity resist measurement when we can calculate Earth's gravitational acceleration, "little g", to extreme precision [7]?
The asymmetry between what we know and what we can't pin down
Scientists know Earth's gravitational acceleration at the surface to be 9.80665 m/s² with little disagreement [7]. But the universal gravitational constant, "big G," which applies to all objects regardless of mass, remains elusive [7]. The current best estimate sits at 6.6743×10⁻¹¹ m³ kg⁻¹ s⁻² [7], yet measurements from the world's most sophisticated labs produce values that don't overlap [7].
Cavendish's 1798 measurement using a torsion balance device came within 1% of modern measured values [7]. That means 225 years of technological advancement has barely improved our accuracy [7]. The torsion balance he pioneered remains one of the most precise ways to measure G [7].
The problem isn't experimental sloppiness. Schlamminger's team at NIST's Gaithersburg, Maryland facilities spent a decade replicating methods from a 2014 study by the International Bureau of Weights and Measurements [7]. The BIPM sent its torsion balance instrument to NIST in 2016 [7]. Schlamminger's version placed lighter masses on torsion disks suspended by a thin copper beryllium strip, with heavier masses on a separate outer disk, all inside a vacuum chamber [7]. The team used eight masses total, four larger on an outer carousel, four smaller on a suspended disk, tracked by a highly sensitive optical device that measures angles as mass positions change [7].
They even repeated experiments with both copper and sapphire masses to eliminate material-dependent effects [7]. The result: a measurement that deepens the mystery rather than solving it [7].
Why measurement disagreement matters for physics and engineering
The International Committee for Weights and Measures faces a decision-making bottleneck [7]. Every few years, the committee must update the recommended value of G based on available measurements [7]. But when high-precision experiments from NIST, BIPM, and laboratories in China, Germany, and Italy produce non-overlapping results, the committee cannot simply average them [7]. Instead, they must weigh each experiment's methodology, assign uncertainty ranges, and publish a value that satisfies no one completely [7].
This process affects more than space missions. Geophysicists studying Earth's interior density rely on G to interpret gravitational anomalies that reveal oil deposits, mineral formations, and tectonic structures [7]. A shift in G's accepted value changes calculations of subsurface mass distribution [7]. Cosmologists modeling the universe's expansion need G to estimate the total mass-energy content of galaxies and dark matter halos [7]. When G's value shifts, so do estimates of how much dark matter the universe contains [7].
The disagreement has pushed some researchers toward a different approach: instead of measuring G directly, they propose measuring combinations of constants that appear together in equations, then working backward to derive G [7]. Others argue for abandoning the search for a single precise value and instead mapping how G measurements vary with experimental design, treating the variation itself as data about gravity's behavior [7].
Gravity operates by different rules than every other force
Gravity is the weakest of the four known forces in nature [7]. Much weaker than electromagnetism [7]. It has infinite range, unlike the strong and weak forces which operate only at roughly the size of an atomic nucleus [7]. And it's the only force not part of the Standard Model of Particle Physics [7].
The other three fundamental forces have quantum theories [7]. Gravity doesn't [7]. Scientists don't know if gravity is transmitted by special particles called gravitons, which have never been experimentally detected [7]. Albert Einstein's general theory of relativity reconceived gravity not as a force but as space-time curved by mass and energy [7], a geometric phenomenon rather than a force-based one [7]. That theory superseded Newton's gravitational framework, which had stood for more than 200 years [7].
Newton's equation shows that gravitational force equals the product of two masses divided by the square of the distance between them, all multiplied by G [7]. The equation works [7]. We use it to calculate orbital mechanics, plan space missions, predict tides [7]. But the constant that makes it work, the number that tells us how strong gravity actually is, keeps shifting depending on who measures it and how [7].
What measurement disagreement reveals about the universe's rules
The gravitational constant is called "universal" because it has the same value throughout the universe [7]. Yet unlike other universal constants, G does not have a precisely known value [7]. This isn't a temporary gap waiting for better equipment [7]. It's a pattern that has persisted across different experimental designs, different laboratories, different decades [7].
Gravity's strength is proportional to the amount of mass or energy present [7], but measuring that proportionality requires isolating tiny forces between small masses in a laboratory, forces so weak that every other influence must be eliminated [7]. The universe allows us to measure gravity's effects with precision [7]. It hides gravity's fundamental nature behind experimental noise that won't resolve [7].
Einstein's geometric theory of gravity has no quantum equivalent comparable to those for the other three fundamental forces [7]. The measurement problem and the theory problem might be two faces of the same mystery: gravity isn't just weaker than other forces, it's categorically different [7]. The disagreement between high-precision measurements isn't a problem to solve [7]. It's a clue that the universe operates by rules we haven't yet learned to read [7].
Those rules may require us to reconsider whether a single universal constant can capture gravity's behavior at all scales, or whether the measurement variations point toward physics that emerges only when we probe gravity at its most fundamental level [7]. Until then, G remains what it has always been: the number we need but cannot pin down, the constant that refuses to hold still [7].
