The New SI and the CODATA recommended values of the fundamental constants 2017 compared with 2014, with a Comment to Possolo et al., Metrologia 55 (2018) 29

This note comments on the special CODATA 2017 adjustment of the fundamental constants of July 2017 involved in the revision of the SI (based on: P.J. Mohr et al., Data and Analysis for the CODATA 2017 Special Fundamental Constants Adjustment for the Revision of the SI, Metrologia 2018, September 2017 preprint), with a comparison to the CODATA 2014 adjustment. In Appendix, a comment is also added on the paper Possolo et al., Metrologia 55 (2018) 29 on the Planck constant. The previous versions of this manuscript (also available on ArXiv) were aiming at pointing out some standing features of the present and future CODATA method in the light of the CODATA table of 2014 recommended values for the fundamental constants. A comprehensive discussion on this and related issues is becoming very important in view of the foreseen revision of the International System (SI) of measurement units in 2018. The present features may still raise doubts on a possible mixing of physical reasons of general validity in science with some needs specific of legal aspects of metrology concerning the SI. This illustration was adjourned in version 3 (v3) according to the CCU 2016 draft of the 9th SI Brochure and to the outcomes of its 22th meeting, with a note about the intended need of a 8th digit in the stipulated value of k.

💡 Research Summary

The paper provides a detailed comparative analysis of the CODATA 2017 special adjustment of the fundamental physical constants with the earlier 2014 adjustment, focusing on the implications for the 2018 redefinition of the International System of Units (SI). The author begins by outlining the context of the SI revision, which fixed the numerical values of seven constants—Planck’s constant (h), the elementary charge (e), the Boltzmann constant (k), the Avogadro constant (N_A), the cesium hyperfine transition frequency (Δν_Cs), the speed of light in vacuum (c), and the magnetic constant (μ0)—as the basis for redefining the kilogram, ampere, kelvin, mole, second, metre, and ampere, respectively.

The core of the analysis contrasts the 2014 CODATA recommended values, which were derived from a weighted statistical combination of experimental data and retained finite uncertainties, with the 2017 values, where the seven defining constants are assigned exact values with zero uncertainty. This shift transforms the role of CODATA from a purely data‑driven evaluator to a body that also implements legal metrological decisions. The paper presents tables and graphs that illustrate the numerical differences, the changes in relative uncertainties for derived constants such as the electron mass (m_e), the Bohr magneton (μ_B), and the Rydberg constant (R∞), and the methodology used to propagate the zero‑uncertainty definitions into the uncertainties of dependent quantities.

A significant portion of the discussion is devoted to the decision to specify the Boltzmann constant to an eighth decimal place. The author argues that this level of precision exceeds the current experimental capability and appears motivated more by the desire for a clean legal definition than by scientific necessity. The paper highlights the tension between the need for a stable, unambiguous legal framework for the SI and the principle that scientific constants should reflect the best available measurements, including their uncertainties.

In the appendix, the author critiques the recent article by Possolo et al. (Metrologia 55, 2018, 29) on a new measurement of the Planck constant. The critique points out that the value reported by Possolo et al. does not align with the exact value fixed by the 2017 CODATA adjustment, and that the authors of the Possolo paper appear to conflate experimental uncertainty with the legally defined value. The commentary stresses that, even after the SI redefinition, independent experimental determinations of h remain essential for validating the consistency of the system and for future refinements.

The conclusion emphasizes that while the 2017 CODATA special adjustment successfully provided the numerical foundation for the SI revision, it also exposed a need for clearer separation between scientific measurement practice and legal metrological decisions. The author calls for future CODATA adjustments to adopt transparent criteria for data selection, uncertainty budgeting, and the relationship between defined constants and experimental verification. Such guidelines would help maintain confidence in the SI while respecting both scientific rigor and the legal stability required for worldwide measurement standards.