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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
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.