# The new SI and fundamental constants

Douglas Scott

The International System of Units (SI) underwent a revolutionary change on May 20, 2019. In October 2017, the International Committee on Weights and Measures met at the International Bureau of Weights and Measures near Paris and recommended a new definition of the SI such that a particular set of constants would have certain values when expressed in the new SI units. In particular, the SI is now defined by the statement:

**The International System of Units, the SI, is the system of units in which**

**the unperturbed ground state hyperfine splitting frequency of the caesium 133 atom nu_Cs is 9 192 631 770 Hz,****the speed of light in vacuum c is 299 792 458 m/s,****the Planck constant h is 6.626 070 15 x 10^-34 J/Hz,****the elementary charge e is 1.602 176 634 x 10^-19 C,****the Boltzmann constant k is 1.380 649 x 10^-23 J/K,****the Avogadro constant N_A is 6.022 140 76 x 10^23 mol^-1,****the luminous efficacy K_cd of monochromatic radiation of frequency 540 x 10^12 hertz is 683 lm/W.**

The numerical values of the constants were determined by a special CODATA adjustment of the values of the constants using data in papers that were accepted for publication by July 1, 2017.

The Convention of the Meter (Convention du Metre), a treaty that species international agreement on how units are defined, was established in 1875 with 17 nations initially signing on, including the US. The SI, established within the treaty in 1960, is more recent and continues to evolve. Currently, the treaty is agreed to by fifty-eight Member States, including all the major industrialized countries. Even though a majority of people in the US still use units such as inches and pounds, the official standards for these units are based on the SI units.

The redefinition has had a signicant impact on the fundamental constants when expressed in SI units. Not only are the defining constants exact, but many others are now also exact, and still others have considerably reduced uncertainties. This reflects a shift from macroscopic measurement standards to quantum based standards.

This talk will describe the new SI, review reasons for the change, and show how units can be based on assigned values of certain physical constants.