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Title: Same apparatus design, different G: the NIST versus BIPM discrepancy
Plain: 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: The NIST measurement (2026, Metrologia), using a torsion-strip balance of the BIPM design in both servo and Cavendish modes, gives $G = 6.67387e-8\,\mathrm{cm}^3\,\mathrm{g}^{-1}\,\mathrm{s}^{-2}$, 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.
Settled by: 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.
Kind: well-posed
Answer type: identification
References: (empty)
Literature status: open
Status note: NIST's blinded result was unblinded in July 2024 and published in April 2026.
Posed since: 2024