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# Peter Mohr (NIST) "The New SI and Fundamental Constants"

## March 15, 2018 @ 4:00 pm - 5:00 pm

The International System of Units (SI) is expected to undergo 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 new SI would be 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 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× 10−34 J/Hz,
- the elementary charge e is 1.602 176 634×10−19 C,
- the Boltzmann constant k is 1.380 649×10−23 J/K,
- the Avogadro constant NA is 6.022 140 76×1023 mol−1,
- the luminous efficacy Kcd of monochromatic radiation of frequency 540×1012 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 M`etre), a treaty that specifies international

agreement on how units are defined, was established in 1875 with 17 nations initially signing

on, including the U.S. 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 U.S. still

use units such as inches and pounds, the official standards for these units are based on the

SI units, so the U.S. national measurement standards will also be redefined, although the

change will be imperceptible in every-day use.

The redefinition will have a significant impact on the fundamental constants when ex-

pressed in SI units. Not only will the defining constants be exact, but many others will also

be exact, and still others will 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.