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According to Wikipedia (last accessed September 16, 2020): The kilogram is currently redefined in terms of the Planck constant as approved by the General Conference on Weights and Measures (CGPM) on 16 November 2018.

The kilogram is defined in terms of three fundamental physical constants: The speed of light c, a specific atomic transition frequency ??Cs, and the Planck constant h. The formal definition is:

The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be 6.62607015×10−34 when expressed in the unit J⋅s, which is equal to kg⋅m2⋅s−1, where the metre and the second are defined in terms of c and ΔνCs. This definition makes the kilogram consistent with the older definitions: the mass remains within 30 ppm of the mass of one litre of water.


In Memoriam

Anders J. Thor

(Email June 30, 2011): Dear Colleagues,
As you may know I'm in favour of defining the SI base unit for mass by fixing the value of the mass of carbon 12, not by fixing the value of the Planck constant. You say that many would say that we have lost this battle already. I don't think so.. This is not the end. It is not even the beginning of the end. It is the end of the beginning (Winston Churchhill 1942). For your information I attach my personal views.

Best regards

The International System of Units, Système International d'Unités, (international abbreviation SI), used worldwide both in everyday commerce and in science, is devised around seven independent base quantities: time, length, mass, electric current, temperature, luminous intensity, and amount of substance. In response to ever-increasing demands for accuracy, the definitions of the units for measurement of these quantities evolve over time.

The organizations that maintain the SI system, also known by their French initials, are the BIPM (International Bureau of Weights and Measures, Bureau international des poids et mesures), the CGPM (General Conference on Weights and Measures, Conférence générale des poids et mesures) and the CIPM (International Committee for Weights and Measures, Comité international des poids et mesures).

At its 24th meeting (October 2011), the CGPM adopted a Resolution on the possible future revision of the SI, called the “New SI”. The BIPM has expressed its commitment “to encourage communication, awareness and debate on the possible revision of the SI”, and many scientists and metrologists have raised serious unanswered criticisms about many aspects of the New SI).

The official committees that decide on the New SI Proposal meet behind closed doors, with observers expressly forbidden, and have refused to answer basic questions about their proposal, in particular questions concerning their proposed redefinitions of the kilogram and the mole. Although the official BIPM journal Metrologia has published a few papers describing some of the criticisms of the New SI, many researchers feel the journal is strongly biased toward the New SI Proposal, and papers critical of the New SI are routinely rejected.

Thus a spirited ad hoc online debate on the merits of the New SI has emerged. was established in order to make these concerns about the proposed New SI open to the public, scientists and lay observers alike, to facilitate participation in the debate, and to establish a permanent scientific record of the main issues and concerns.

Frequently Asked (But Never Answered) Questions

These basic questions, posed in numerous scientific publications and online debates, have still not been answered by the advocates of the proposed New SI.

  1. How are "students in all disciplines" supposed to comprehend the Planck kilogram, which requires knowledge of both quantum physics and special relativity?
  2. What is the proposed introductory-level textbook definition of the Planck kilogram?
  3. How is the fixed-Planck kilogram consistent with concrete new evidence that the fine-structure constant, and hence Planck's constant, varies in time and location?
  4. Doesn't the proposed redefinition of the mole violate a basic compatibility condition relating it to the kilogram and the Dalton?
  5. Is kappa (and hence Mu) changing in time or not?
  6. Is kappa (and hence Mu) a new fundamental constant?
  7. Aren't the proposed redefinitions of the SI units inconsistent and/or circular? (For example, the unit amount of substance involves the kilogram, the second is defined in terms of the kelvin and the kelvin in terms of the second, and without the availability of the derived real units joule and watt, the kilogram, kelvin, and candela cannot be realized.)
  8. What is an "idealized atom" in NIST Lecturer's explanation of the New SI?
  9. Doesn't the Planck kilogram require the introduction of a new quantum-mechanical current standard?
  10. How is 10^40, the order of magnitude in the numerical constant implicit in the Planck kilogram, compatible with other SI unit definitions, and how is it physically realistic from a practical standpoint?

Opinion Editorials

"The Avogadro number is an integer multiple of the number '1' ", by P. de Bièvre, Accred Qual Assur 20:41-42, January 2015.
"The mole is an Avogadro number of entities", by B. Leonard, June 2014.
"Constancy and Circularity in the SI", by I. Johansson, June 2014 (replaces "The Avogadro Constant, the Planck Constant, and the New SI")
"The SI's Base Quantities and Units", by W. Emerson, May 2014 (revised July 2014)
"The SI should not ignore the directional nature of vector quantities", by W. Emerson, April 10, 2014
"Two rogue units of the SI: the kelvin and the candela", by W. Emerson, April 7, 2014
"On the need for analytical chemists... to re-think the mole", by P. De Bièvre, posted July 2013, revised August 2013
"Some Illogicalities of the SI", by W. Emerson, October 2012>
"An Exact Value for Avogadro's Number Redux Redux", by R. Fox, January 2012
"The New SI - Draft definitions of the SI Base Units", by A. Thor, February 2012


General Information

"New SI Definitions"
BIPM Page - "On the possible future revision of the SI"
CIPM Resolution on the redefinition of the kilogram
"'Sí', on the New SI: NIST Backs Proposal for a Revamped System of Measurement Units"; document moved from
U.S. Metric Association Page - "Published articles about the metric system, metrication,and related standards"

"New SI" in the News

"Redefining the kilogram" by Bob Grant, The Scientist, July 5, 2012
"The kilogram is dead! Long live the kilogram!" by Simon Hadlington, Chemistry World, October 31, 2011
"Why the World is Losing Weight" by Mary Bowers, The Caravan, September 1-15, 2009.
"This Kilogram Has A Weight-Loss Problem" by Geoff Brumfiel, NPR August 20, 2009.
"Defining a New Kilogram" by S. Spicer, Water Environment Laboratory Solutions, Vol. 15, No. 4, August/September 2008
"A better definition for the Kilogram?" by J. Toon, Georgia Tech News Release, Sept 21, 2007

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