[ Revision of the SI ] The 26th General Conference on Weights and Measures (CGPM) started today

Update: The redefinition of the SI got approved by the 54 member states present at the conference. Effective from May the 20th 2019 (World Metrology Day) all current base units of measurement will be based on fundamental physical constants.


The 26th meeting of the CGPM will take place in Versailles from 13 to 16 November 2018. On Friday Nov the 16th there will be a vote on “Draft resolution A”, in essence the redefinition of the kilogram, the ampere, the kelvin and the mole based on physical constants. “Most notably, this will mark the end of the last remaining physical artefact in the SI system – a cylinder of metal known as the International Prototype of the Kilogram.”

For anyone who is interested, there will be a webcast on Friday:

Here are a couple of interesting links and files:

Personally I am very excited about this and wanted to share it with everyone who might not have heard about it yet. It won’t make any difference in your everyday life but for scientific research it makes life a whole lot easier in some ways. If the vote falls in favour of the resolution it will mark the first time that all units of measurement can and will be defined by fundamental constants.

What do you guys think? :slightly_smiling_face:

SI Definitions in Resolution A

Articles and other content

And also the most highly decorated source of news out there :grin::
On a more serious note: I think it is great that Paul shares this topic with his audience who might not have heard about this otherwise :+1:


It seems like the discussion thread to this topic kind of emerged over there :grin::

Link to the open session livestream:


The quotet text by Max Planck:


At 2:01:45

So think about this, today in the 21st century, the unit of mass is an artifact, a piece of metal that was made in the 19th century, based on an object that was made in the 18th century. This is scandalous. In fact it is such a scandal, that American newspaper comic strips make fun of it.


I love Will Phillips, he is a really cool guy. I had the pleasure of meeting him once and hearing a talk on atomic clocks. … and he has a talent for entertaining presentations :grin:

by the way @wendell : Now that the redefinition of 4 SI units is approved, maybe it counts as newsworthy for next week? The news articles so far are nothing to write home about but you could use the BIPM as a primary source and if you want I can write a short summary.
(not sure if there is enough interest :wink: but it’s probably the last time this will happen)

Otherwise the NIST article is quite good:

1 Like

This has been a really interesting topic, I never gave any of this any thought until now. I’ve always lived under the impression that the definition of 1kg was 1 l (or 1k cubic cents) of water at it’s densest.
Oh what a lie I’ve lived in :rofl:

1 Like

I’m happy you enjoyed it! :slightly_smiling_face:

well, it has been at one point :wink:

If you are interested in the history behind units of measurement, here is a quite entertaining talk at the 26th GCWM: (timestamped)


Voting for Draft Resolution A

At time-stamp 2:14:12 in the main video/stream.

There is also a clip of just this section of the meeting in a separate official video here.

On the revision of the International System of Units (SI)

The General Conference on Weights and Measures (CGPM), at its 26th meeting,

decides that, effective from 20 May 2019, the International System of Units, the SI, is the system of units in which:

  • the unperturbed ground state hyperfine transition frequency of the caesium 133 atom ∆νCs 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 s,
  • 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 of monochromatic radiation of frequency 540 × 1012 Hz,
    Kcd, is 683 lm/W,

where the hertz, joule, coulomb, lumen, and watt, with unit symbols Hz, J, C, lm, and W, respectively, are related to the units second, metre, kilogram, ampere, kelvin, mole, and candela, with unit symbols s, m, kg, A, K, mol, and cd, respectively, according to Hz = s–1, J = m2 kg s–2, C = A s, lm = cd m2 m–2 = cd sr, and W = m2 kg s–3.