Talk:International System of Units/Archives/03/2020

how can the SI be based on the ISQ when the SI existed 50 years before the ISQ?

Discuss. Dondervogel 2 (talk) 09:54, 6 February 2020 (UTC)

The present SI is predicated logically on the ISQ, and the historical sequence in which this dependence manifested does not necessarily relate. It all comes down to interpretation of the meaning of "based on". I do not interpret it to mean "arose from". The 9th SI Brochure mentions of "The system of quantities underlying the SI ..." and "noting also that the ISO/IEC 80000 series of Standards specify base and derived quantities which necessarily correspond to the SI base and derived units defined here" is a reference to this dependence, even though historically the underlying quantities might not have been formalized in the ISQ (or indeed at all). Would you prefer another choice of wording, e.g. "depends upon" or "the ISQ underlies"? —Quondum 17:45, 24 February 2020 (UTC)

I do think it should be reworded. If the BIPM uses wording like "underlies" and "corresponds to" then I think WP should reflect that choice of words. Dondervogel 2 (talk) 19:06, 24 February 2020 (UTC)

I would be happy with a rewording to use the term "underlies" or similar, which I would consider to be clearer. A direct (and grammatically correct) replacement would substitute "based on" → "underlain by". However, I would make it "and is underlain by" "which is underlain by" or preferably a fresh sentence "It is underlain by". —Quondum 21:33, 24 February 2020 (UTC)

There is a scientific relationship, but not a legal relationship. No doubt the scientists and engineers who were behind the creation of SI had in mind scientific and mathematical principles which were eventually standardized as ISQ, but at a later date.

On the other hand, the CGPM is a treaty organization, and various governments sent representatives to periodic conferences. The resulting agreements are incorporated into national and sub-national laws. The local implementation is weights and measures inspectors who visit local retail outlets, impose fines for violations, seize inaccurate measuring devices, and seize packaged goods with inaccurate mass or volume claims on the label. If the shop keeper resists, the inspectors will call the police and the police will use violence to enforce the law.

I don't think we should be too quick to say that ISQ underlies SI, because you can violate the ISQ to your heart's content, and no one will put you in handcuffs. The same is not true for SI. Jc3s5h (talk) 01:51, 25 February 2020 (UTC)

Maybe, maybe not. SI defines radians as the ratio of two length measurements. If I publish the quantity arc length divided by diameter in radians, I will not be violating the SI definition of units, only its underlying system of quantities, but I could be prosecuted for criminal misrepresentation. On the other hand, there is something in the gist of what you say: there is an underlying system of quantities for the SI, and ISO/IEC 80000 attempts to describe and standardize part of this system. ISO/IEC 80000 evidently does not define the ISQ per se, despite what is said in International System of Quantities. ISO/IEC 80000-1:2009 does say: "ISQ is a shorthand notation for the “system of quantities on which the SI is based”", which is pretty much what the lead here currently says. —Quondum 05:26, 25 February 2020 (UTC)

   The SI was named in 1960, the ISQ was named in 2009, which doesn't mean that length, mass, time, current, temperature, amount of matter and luminous intensity were unknown before that. On the contrary, AFAICT both the SI and the ISQ were “in the making” when the MKSA system was born in 1946, based on four quantities and their respective units, with two more “dimensions of measure” added no later than the SI's birth in 1960, and the mole added as the seventh unit even 11 years after that; and if they hadn't yet “solidified” before 1946, it was because there was a lot of hesitation about exactly which electromagnetic quantity to regard as fundamental and what exactly to use as its unit. IMHO the “ISQ naming” event in 2009 simply consisted in formalising the system of separate measurable and macroscopically independent magnitudes which had underlied the philosophy of the system of units all the way since the acceptance of the MKSA system; IMHO the fact that three additional quantities and their respective units were added since then is secondary to that philosophy: if someday a new physical quantity is discovered which is (macroscopically) independent of the current seven, and if some way is found to compare measurements of it with a small enough experimental error, I don't doubt that this new quantity will be added to the ISQ and a unit chosen from it and added to the SI; neither of these additions would change anything to the philosophy of how physicists (and lay people) measure quantities.
   Now how should we formulate all this to make it clear without using the kind of long-worded jargon which would sound like gobbledygook to a lay person? What do you think of the following?

The International System of Units […] is the most widely used system of measurement, based on what is known since 2009 as the International System of Quantities. It comprises a coherent system (etc.)

…

The system was published in 1960 as a result of an initiative that began in 1946. It is based on the metre–kilogram–second–ampere system of units (MKSA) rather than any variant of the CGS; the latest fundamental unit officially added to the system was the mole in 1971.

(About the mole officialy becoming part of SI in 1971 and no earlier, see Giovanni Giorgi#The Giorgi system and the reference mentioned there in fine.) — Tonymec (talk) 14:59, 29 February 2020 (UTC)

I've now had a go at fixing this, by specifying that the SI is based on a set of fundamental quantities, (something that has always been true), and that that set of quantities is now known as the International System of Quantities. This also gives the opportunity to link the fundamental quantities in the first section of the article, separately from the units that measure them. -- The Anome (talk) 15:13, 29 February 2020 (UTC)

I'm sorry, I disagree with the detail of this fix. Since the 2019 redefinition, base units are no longer central to the definition of the SI. Secondly, the ISQ is not a system of seven base quantities. —Quondum 18:28, 29 February 2020 (UTC)