Same apparatus design, different G: the NIST versus BIPM discrepancy
This text mentions work from 2024 on. Those parts were partly written without a live literature search and have not been checked by an editor yet.
In plain words
NIST rebuilt the instrument used by the international measurement bureau (BIPM) and got a value about 2 parts in 10000 lower. Since the design is the same, the difference should be traceable to a specific cause.
Precise statement
The NIST measurement (2026, Metrologia), using a torsion-strip balance of the BIPM design in both servo and Cavendish modes, gives $G = 6.67387 \times 10^{-8}\,\mathrm{cm}^3\,\mathrm{g}^{-1}\,\mathrm{s}^{-2}$, about 235 ppm below the BIPM 2013 value. Identify the effect (mass metrology, torsion-strip anelasticity, source-mass density inhomogeneity, alignment, environment) that accounts for the difference.
What would settle it
A cross-check with exchanged source masses or fibers, or a third realization of the same apparatus, that isolates the component responsible for the 235 ppm offset.
Status in the literature
Unverified note: it names recent work and was partly written without a live literature search; it awaits a check by an editor.
NIST's blinded result was unblinded in July 2024 and published in April 2026.
Parent problems
- Special case of Why laboratory values of G scatter beyond their stated uncertainties · open · from revision 1 The parent was revised since revision 1: it now states another question.
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