Talk:2019 redefinition of the SI base units

Revocation of 31-Jan-2011

It is a moot point whether one writes 2 x 10^-8 or 20 x 10^-9. The former keeps the mantissa in the range 1.0 to 10.0 while the latter maps onto μg. In such circumstances, I think it appropriate to keep the article in line with the original text. Martinvl (talk) 20:17, 30 January 2011 (UTC)

Rollback of 29 March 2011

I have rolled back a series of changes that were made earlier today. While I agreed with some of the changes, I did not agree with them all. In particular,

• The CCU wrote to the CIPM - see the title given in the reference.
• Hard spaces between numbers and symbols were removed. The hard spaces were put there to ensure that the value and symbol were not separated by line breaks. This is standard practice.
• The way in which negative superscripts was handled - for example changing J·K⁻¹ to J/K. While I agree that these two have the same meaning, I used J·K⁻¹ for the sake of consistency with the rest of the article.
• The symbol "sec" is not the recognised symbol for "seconds" - "s" is.

Martinvl (talk) 21:08, 29 March 2011 (UTC)

Impact on reproducibility

The current text contains a table, introduced with:

"The following table catalogues the improvements"

They don't look like they are all improvements, as in half the cases the change is from an exact value to an uncertain value. I suggest the table catalogues the changes.
—DIV (138.194.11.244 (talk) 07:41, 16 April 2012 (UTC))

Following your observation, I am revising this section to include all constants listd in the reference. Martinvl (talk) 10:12, 16 April 2012 (UTC)

GA Review

This review is transcluded from Talk:Proposed redefinition of SI base units/GA1. The edit link for this section can be used to add comments to the review.

Reviewer: Adabow (talk · contribs) 06:52, 21 June 2013 (UTC)

1. Perhaps link the background section to the History of the metric system using {{main}}
2. Can a citation be added for the paragraph concerning the setting-up of the CGPM, CIPM and BIPM?
3. "In 1889 the CGPM took delivery" - what does "took delivery" mean?
4. "the mandate of the CGPM was extended to provide standards for all units of measure, not just mass and length" - before reading this, it is not clear that the three organisations only dealt with mass and length; please make this explicit earlier on.
5. "In the ensuing years" - vague. Is there a known end date?
6. Can you explain how the conditions set by the General Conference were not fully met?

Adabow (talk) 07:51, 21 June 2013 (UTC)

Proposer

I have taken on board your proposals and have implemented them. I have also ensured that the date passed to the accessdate parameter is consistent across all citations. Martinvl (talk) 10:49, 21 June 2013 (UTC)

Continuing...

1. It is not stated in the background section that/when the speed of light was fixed.
2. Is "an elementary charge" correct, or should it be the elementary charge?
3. Please provide a citation for the current definitions of the base units.
4. There are some great analyses about the consequences of the redefinitions (eg effect of amp redefinition on vacuum permeability, vacuum permittivity and impedance of free space), but it is not all referenced
5. Ditto with the example of potentially defining kg from G - WP:OR
6. Who is Leonard?

GA review (see here for what the criteria are, and here for what they are not)

1. It is reasonably well written.

   a (prose): b (MoS for lead, layout, word choice, fiction, and lists):

2. It is factually accurate and verifiable.

   a (reference section): b (citations to reliable sources): c (OR):

3. It is broad in its coverage.

   a (major aspects): b (focused):

4. It follows the neutral point of view policy.

   Fair representation without bias:

5. It is stable.

   No edit wars, etc.:

6. It is illustrated by images and other media, where possible and appropriate.

   a (images are tagged and non-free content have fair use rationales): b (appropriate use with suitable captions):

7. Overall:

   Pass/Fail:

Placing review on hold now. Adabow (talk) 07:07, 22 June 2013 (UTC)

Proposer

I have taken the liberty of replacing bullet points with numbers to simplify cross-referencing of comments.

The outstanding issues in the first set of comments have been handled as follows:

• Item 4: In this change, the text "under which three bodies were set up to regulate units of measure that were to be used internationally." was replaced by "under which three bodies were set up to take custody of the international prototype kilogram and metre and to regulate comparisons with national prototypes.^[1],[2]" Martinvl (talk) 15:00, 24 June 2013 (UTC)
• Item 5: The updated text reads: "In 1921 the Convention of the Metre was revised and the mandate of the CGPM was extended to provide standards for all units of measure, not just mass and length. In the ensuing years the CGPM took on responsibility for providing standards of time (1956), electric current (1946), temperature (1948), molar mass (1971) and luminosity (1946)." Martinvl (talk) 15:00, 24 June 2013 (UTC)

The issues raised in the second set of comments have been handled as follows:

• Item 1: Done - diffs here. Martinvl (talk) 10:39, 25 June 2013 (UTC)
• Item 2: Done. Martinvl (talk) 16:16, 24 June 2013 (UTC)
• Item 3: The intro paragraph to the section now reads "The current (2008)¹ and proposed (2011)² definitions are given below." Martinvl (talk) 16:16, 24 June 2013 (UTC)
• Item 4: Discussion on the impact of the changes on the ampere has been overhauled and citations provided. Martinvl (talk) 16:16, 24 June 2013 (UTC)
• Item 5: Section Proposed redefinition of SI base units#Impact on reproducibility has been modified - citations were found that paralleled work which could be held to be WP:OR. Martinvl (talk) 10:42, 25 June 2013 (UTC)
• Item 6: Final section has been rewritten - the comments made by the reviewer no longer apply, but the whole section needs to be reviewed.

As each item is addressed, it will be signed.

Martinvl (talk) 14:45, 24 June 2013 (UTC)

Also note the contradiction "In 1960 the metre was redefined in terms of the speed of light, making it derivable from nature" in the lead vs "Similarly, the 17th CGPM (1983) replaced the 1960 definition of the metre[Note 2] with one where the metre is derived from the speed of light" in the background section. Adabow (talk) 06:12, 26 June 2013 (UTC)

Thank you for spotting this - I have corrected it.

When trying to find out more about Leonard, I discovered that a lot more discussion material had become available, so I am rewriting the final section which I am calling "Discussion" rather than "Criticism". I hope to have it in place in a few days. Martinvl (talk) 06:44, 26 June 2013 (UTC)

Sounds great! Take your time - no rush. Adabow (talk) 07:38, 26 June 2013 (UTC)

Hi Adabow,

I have now completed my revisions following your initial review and when you are ready, I would be grateful if you could assess my latest offerings. Martinvl (talk) 20:49, 3 July 2013 (UTC)

There is still no explanation for who Leonard is. Ditto for Chyla. When someone is mentioned, there first name should be included in the first instance. If the first names are not known, then please at least give initials. Affiliation should be given as well. Adabow (talk) 23:27, 4 July 2013 (UTC)

Done. Reading between the lines Chyla appears to be a freelancer so his affiliation carries no weight, however the journal where this paper was published referees its papers, so that has been included in the text (complete with Wikilink).

OK, everything looks good to me now. Passing. Well done. Adabow (talk) 10:45, 5 July 2013 (UTC)

Diagram description

the diagram for current definitions of SI units claims in its description that a meter is defined as the distance traveled by light in 1 second. according to the article on the meter it is equal to 1/299,792,458 of this value, and the claim is sourced. I suggest the source be reused here and the information corrected. — Preceding unsigned comment added by 96.245.192.47 (talk) 20:13, 14 September 2014 (UTC)

The caption doesn't say that the metre is defined as the distance traveled by light in a second, it says "in term of". The point is that it's defined in terms of distance traveled. But you're right, it's clumsy enough to be potentially misleading. I'll try tweaking it (though without putting in the precise value because that isn't the point) - see what you think. NebY (talk) 23:17, 14 September 2014 (UTC)

Fortunately the difference is big enough that it is hard to get wrong in actual use or discussion. It is nice to get it right, though. Gah4 (talk) 23:49, 5 December 2015 (UTC)

gram

maybe it's time to change kg to gram? — Preceding unsigned comment added by 134.7.190.150 (talk) 10:26, 23 June 2015 (UTC)

Spelling

I noticed that the article has the word artefact, and was about to change to artifact. But then again artefact seems to be the British spelling, and so maybe consistent with the rest of the article. But then I find that there are many artifacts, too. Seems to me that we should be consistent, one way or the other. I don't know which way, though. Gah4 (talk) 23:52, 5 December 2015 (UTC)

I would use the word "prototype" anyway. 173.48.62.104 (talk) 04:04, 7 December 2015 (UTC)

unit order

Is there a consistent order that units should be when appended to a quantity? I am mostly used to kg - m - s order, for example usually used in explaining force or energy. I saw some quantities with a different order, though about changing them, then decided to ask here. Gah4 (talk) 23:54, 5 December 2015 (UTC)

Figures are confusing, how to fix?

The diagrams ("directed graphs" to a mathematician) of the current and proposed definitions seem unnecessarily hard to compare. Each has the units in a different order and a different colour code. The m is blue in both, K orange, and the kg red, but s/A swap green/purple and cd/mol swap yellow/turquoise.

This graphic that the first is based on shows the units in the same positions, but I'm not sure

Assuming that having arrows mostly "down" would be most legible, A, mol, K and cd have only in-arrows in both diagrams and so should be placed lower down. s has only out-arrows. kg and m have both.

The other place to look is the changes, which are the arrows we'd like to be noticed most.

• 5 links are unchanged: s→m, s→cd, s→A, m→cd, kg→cd
• 3 links are deleted: m→A, kg→A, kg→mol (A is now based only on the second, mol is standalone)
• 5 links are added: s→kg, s→K, m→kg, m→K, kg→K (kg is no longer primitive, K is no longer standalone)

So perhaps the following positions (expressed in terns of a 12-hour clock)

• 9:26: A
• 11:09: s
• 12:51: m
• 2:34: kg
• 4:17: mol
• 6:00: cd
• 7:43: K

That's like the first diagram, but with the kg-A-cd triangle rotated counterclockwise.

Any other ideas? More importantly, any volunteers to actually draw it? 71.41.210.146 (talk) 15:17, 28 December 2015 (UTC)

Trying to fix the problem, I’ve just replaced the first diagram (current SI) with another one which is color-consistent, position-consistent and size-consistent with the second diagram (new SI), which I have created back in 2011 and which was color-consistent at that time (the color inconsistency dates frome 2015 when the creator of the current-SI diagram updated it and changed its colors for some unknown reason).

As for the optimal unit order in the diagrams, you mention valid points, and one advantage of your proposal would be the absence of upward arrows in both diagrams. Still, I have sticked to my original unit order. Here are some (debatable) reasons :

1. At least, the new-SI diagram has no upward arrow between units.
2. The highest, 12 o’clock position of the unit second emphasizes its major "donor" status in the new-SI diagram, with 5 arrows flowing down from it. (In hindsight, I find it looks like the rays of the sun-god Aten!)
3. This unit order allows a certain vertical symmetry of the arrows in the new-SI diagram.
4. The heptagon figure is visually more "stable" when oriented with two vertices on the base line and one vertex at 12 o’clock position. In your proposal (only one vertex on the base line at 6 o’clock position), the figure would look in an unstable equilibrium. In my new-SI diagram, the stability of the figure is further "weighted" with each of its two lower-edge units receiving three downward arrows.

Those two diagrams are PNG files converted in SVG format, but Wikimedia Commons warns me that "This SVG image contains embedded raster graphics.[1] Such images are liable to produce inferior results when scaled to different sizes". Anyone knows how to convert them in "pure" vector graphics format, without having to redraw them from scratch?

--Wikipetzi (talk) 20:22, 15 January 2016 (UTC)

I redid both images using svg elements from File:SI base unit.svg. --IngenieroLoco (talk) 20:45, 28 June 2016 (UTC)

@IngenieroLoco: I should have said this much earlier, but thank you! 71.41.210.146 (talk) 09:15, 3 January 2017 (UTC)

Date of redefinition

@Quondum: I put the word back to "likely", because "likely" is actually putting the case very mildly. It's basically certain, barring something very unexpected like the appearance of multiple inconsistent measurements of some of the constants. Perhaps the footnotes on that particular statement need improving, but the statement is WP:Verifiably true.

• As the first footnote says, the BIPM director called it a "foregone conclusion" in 2014 already.
• The 26th CGPM has already been scheduled for 13–16 November 2018.[1]. If you want to be specific, the redefinition is scheduled for the morning of Friday the 16th, and the press have already been invited. (Same source, right-hand side of p. 10.)
• The proposal has been around, and accepted in principle, by two previous CGPMs (2011[2] and 2014[3]). At those meetings, they agreed to the redefinitions as soon as the measurements met a defined quality level.[4] That level has now been achieved.
• The CODATA § Task Group on Fundamental Constants have already announced the deadline[5][6] for data to be incorporated into the redefinition: 2017-07-01 for the final fixed SI values.

The entire metrology world is assuming that it will occur. I can find half a dozen more sources (e.g. [7]) which state it in passing or obviously assume it. Can you find any fairly recent source which expresses the slightest doubt?

In fact, I've been thinking seriously about removing the word "likely" entirely, and either saying something like "planned" or "scheduled" or, even bolder, stating flatly that it will occur. Do you have any comments on that idea? 71.41.210.146 (talk) 09:21, 13 January 2017 (UTC)

There is little doubt about it occurring then, but such an assertion remains a judgement. Using "likely" and "will" in the voice of Wikipedia is editorialization. WP should not be drawing conclusions (no matter how "obvious"), but should be citing sources, meaning stating what they say, or better, stating that they say something. To say that it is "expected" or "scheduled" would be fine, because that is what one or two of the sources say (though "expected" should be qualified by whom). My preference would be "scheduled", because that is a statement of fact (about intent), but does not attempt to predict the future. A statement in the future tense in WP is always suspect. —Quondum 16:00, 13 January 2017 (UTC)

@Quondum: I understand your point about future tense being suspect, but not the first point about editorialization. Expressing a value judgement (either overtly or via choice of wording) is editorialization. I don't see how "likely" does that.

The word is just a summary of the sources. (Remember, SYNTH is not just any synthesis.) It's certainly possible to go into great detail about who said what, but when everyone who knows anything about the subject agrees, that's not good sourcing, that's bad writing.

I haven't figured out the right wording for a more specific statement. Formally, only the vote is scheduled, and while the result of that vote is a "foregone conclusion" and everyone is making plans based on the outcome, they're also being polite about it in public. Fitting all of that into something brief is difficult. 71.41.210.146 (talk) 19:34, 13 January 2017 (UTC)

It is a very fine point, but for the lead to say that it "likely" is WP drawing a conclusion about probability that does not appear to be stated by the sources even if that conclusion seems inevitable (let's be guided by how the sources are stating it). I suppose my reaction is because that wording immediately tells me "this could be an editor who really wants this to happen, so maybe they're skewing the interpretation". It just doesn't have the right tone to be convincing. How about something like "... is on track for adoption at ..."? —Quondum 20:44, 13 January 2017 (UTC)

@Quondum: I'm beginning to see the point here. I think that the conclusion is stated in the sources. As I described above, it's implied all over the place, but also stated explicitly in the sources already in the article. (Unless you think that "likely" isn't a reasonable synonym/paraphrase of the sources' words "foregone conclusion" and "expected".)

But even if that's a reasonable word in this context, it raises red flags for you in general and you'd like something which doesn't. That makes sense.

The problem is, as you mentioned earlier, anything that sounds like a prediction raises those flags. We could change the word to "expected" to match a recent source verbatim, but would that address your concern?

I could also add a footnote with details. But if it's going to be discussed in a footnote, I'm tempted to move all the caveats to the footnote, remove all the WP:WEASEL words from the lead, and just say it will happen.

I mean, obviously any statement about the future is subject to very unexpected caveats. There's no need to explicitly state that a major terrorist attack in Paris could preempt the 26th CGPM. But Summer Olympic Games says that "Tokyo, Japan will host the 2020 Summer Olympics" (emphasis added) because there's no reasonable doubt. (Someone added caveats to the lead of 2020 Summer Olympics, but the rest of the article uses the unmodified "will".)

Back to the topic at hand, do we have any remaining disagreement about what the sources say, or are we just trying to find the right wording to say it? 71.41.210.146 (talk) 21:45, 13 January 2017 (UTC)

I think you've characterized it pretty well. Yes, it is probably only wording; I don't think there is any real disagreement with what the sources say, or that adoption is imminent at that date (I'm not well-learned here: it was a pleasant surprise to learn now that there is a definite timeline this time around). It is more that a reader might do what I did: see a red-flag word, check the footnote/references (I am guilty here: notes [1] and [2] don't directly deal with it and I thus barely scanned [3], then jumped to a conclusion about editing; I suggest perhaps keep only [3]; at least remove [1] as too dated and reorder [2] and [3] since [2] only confirms a premise in [3], as I scan it).

I take your point about how one expresses things (one is not expected to list all the caveats). In this case, I think word choice should suggest quoting something rather than an opinion of an editor. "Expected" would have raised less of a red flag for me, since it suggest at least that one is referring to a specific group who expects it and is less suggestive of editorializing. Being able to find the word in the footnote gives one a quick point of purchase. I'm not going to obstruct whatever you choose; I've learned something new and am not disagreeing . —Quondum 22:49, 13 January 2017 (UTC)

The most apropos parts of the sources are actually included in the citations as quotes. [1] was the first overt statement I could find, and the speaker is someone particularly knowledgeable. (Think of the BIPM director as the CEO of a company, the CIPM as the board of directors, and the CGPM as a shareholders' meeting. While the CEO can't speak for the latter, a good CEO knows what they're thinking.) [3] is by the head of the CODATA task group on fundamental constants, who's in charge of producing the numerical values that the CGPM will enshrine. Another good source, but not quite as good as [1]. [2] is the same thing as [3], just in "lay summary" form. Of the three, I'd say it's the most disposable.

It was clear in 2011 that the redefinition was going to go through "soon";[8] the 25th CGPM was held only 3 years after the 24th in anticipation, which turned out to be a bit too optimistic. But everyone who's been following the matter has watched the measurement uncertainties decrease below the specified limits through 2015 and 2016, and could draw the same conclusions as [3] states. (In truth, the hardest thing to track down was the exact date (as opposed to "Q4 2018") of the CGPM. It's not secret, just only communicated to the attendees.)

Anyway, I'll amend that to "expected". 71.41.210.146 (talk) 01:35, 14 January 2017 (UTC)

Specific values in the 9th edition draft

@Petr Karel and Quondum: Sorry for that last undo; I hadn't realized it was a consequence of User:Petr Karel's edit. That's the problematic one, but this will take more discussion than a simple undo. (Feel free to undo my undo pending discussion if you like; I did it in a hurry and then stopped when I realized the situation was more complex.)

The problem is that the draft 9th edition does give those specific values, but those aren't the final values. I think it was done to make the formatting closer to final, but I worry that importing them verbatim will cause confusion among readers.

That those aren't the final values is verifiably true, As http://physics.nist.gov/cuu/Constants/ says, they will be adjusted based on measurement results submitted up to a deadline of 1 July 2017, which is still open. (And the reason the deadline is more than a year before the redefinition is to allow time for criticism of measurements.)

So we need to add some explanatory text... okay, I think I know how to do that. Perhaps consider this just an apology for my too-hasty undo. 71.41.210.146 (talk) 05:51, 14 January 2017 (UTC)

No, I appreciate the undo; it was appropriate. And I was just commenting that the provisional nature of the values in Proposed redefinition of SI base units § Proposal should be made clearer, explaining that they will be finally revised after 2017-07-01, but you beat me to the talk page. —Quondum 05:58, 14 January 2017 (UTC)

@Quondum: Well, I was apologizing for undoing your edit and then stopping to discuss before undoing the previous one that was the really important one. Kind of shooting the messenger. Anyway, edits made. I felt hurried to correct misleading information ASAP, so I'd definitely appreciate a review/copy-edit. 71.41.210.146 (talk) 06:31, 14 January 2017 (UTC)

@Quondum: P.S. Going over your recent edits, the Avogadro constant article actually distinguishes between "the Avogadro constant" and "Avogadro's number". The former is 6.022140857×10²³, the latter is the number of hydrogen atoms is 1 g of gas, about 5.97538324×10²³ based on the atomic mass of protium at 1.00782504(7) u. Although the terms are understood as synonyms today, would it be better to avoid the ambiguity? 71.41.210.146 (talk) 06:50, 14 January 2017 (UTC)

I think it is beautifully clear and up to date now, with little chance of misinterpretation. The only thing is that the proposed values in the table could do with the same footnote, though this might increase the column width even more with the Planck constant and the Boltzmann constant. Perhaps that could be remedied by a forced line break before the footnote?

I kept the distinction between the Avogadro constant and Avogadro's number. All I did was fix minor variants in the terms to match the terms in the article Avogadro constant, specifically only adding/removing "the" and "'s".

A side issue: the table uses the symbol $R_{\infty }$ without ever defining it, or linking to the article Rydberg constant. —Quondum 16:27, 14 January 2017 (UTC)

@Quondum: You're right about the table, sigh.

Regarding the Rydberg constant, the table doesn't define $c$, $\alpha$ or $A_{r}(e)$, either. (Although the last is defined in the text immediately after it.) 71.41.210.146 (talk) 17:41, 14 January 2017 (UTC)

Rationale behind choice of electromagnetic constant

Is there some background on the (proposed) SI's choice of the elementary charge rather than either the electric constant or the magnetic constant to be assigned an exact value in the New SI? They are nominally equivalent ways to specify the same thing in a sense. The latter are arguably more fundamental (even in quantum mechanics), but ultimately the choice in this context would be the one which leads to the most accurate and useful system. Precision of mass constants particularly might be affected. Looking at the table, the dominant source of imprecision for every interesting constant listed is the fine-structure constant, which is in a sense is just the square of e, at least as far as precision is concerned. It seems to me that the history of this choice and comments on it would be appropriate (and definitely interesting) content for this article. —Quondum 22:34, 14 January 2017 (UTC)

I see that there is a recent paper that seems to speak to this: K.A. Bronnikov et al, On the Choice of Fixed Fundamental Constants for New Definitions of the SI Units, Measurement Techniques, November 2016, Volume 59, Issue 8, pp 803–809. —Quondum 04:05, 15 January 2017 (UTC)

It's a bit technical for a lay audience, but we can try. I'll try explaining it here, and see if we can eventually turn it into something suitable for mainspace.

Basically, it's all about shifting the uncertainties around to minimize covariances. If I have a bunch of measurements over here which are accurate relative to each other to parts per trillion, then it's annoying to be measuring them relative to a standard which is only measurable to parts per billion.

That's what was happening in 1960 when the metre was redefined from the bar to a spectroscopic standard. The spectroscopists had all of these wavelengths measured to ridiculous precision relative to each other, but measuring a macroscopic dimension was much harder. The Ångstrom was an agreed-upon standard which was nominally 10⁻¹⁰ m, but that was known much more loosely. When someone finally figured out how to count the number of wavelengths in a metre, the metre bar quickly became the limiting factor.

Another place this has happened is the astronomical unit. It's much easier to measure the relative distances of the planets' orbits than to relate this to a metre bar on Earth. For a long time, the orbits of the planets were known to 3+ decimal places in A.U., but the A.U. was only known to one or two decimal decimal places. (Read up on the transit of Venus for details.)

These sort of situations result in a lot of measurements which are very strongly correlated. A is measured with an uncertainty of 25 ppb, B is measured with an uncertainty of 25 ppb, but all of that uncertainty comes from the 25 ppb uncertainty in the comparison to external standard Z. The ratio of A and B is known with an error of < 1 ppb. This quickly becomes an error-prone pain in the ass.

It would be better to find a standard which could be compared to A and B at their sub-ppb level and move the 25 ppb uncertainty over to Z, since it can't be measured to better than 25 pbb uncertainty anyway.

The interest in Plancks's constant and the elementary charge arise because the field of electrical metrology has surpassed mass metrology, yet the current definitions of the volt, ampere, etc. are all based on the kilogram. Quantum Hall effect resistance standards and and Josephson voltage standards are absolutely fantastic because, as far as anyone has been able to tell, there are no corrections.

When building a real cesium clock, there are all sorts of corrections that must be applied to correct for the non-zero temperature, Doppler effect due to the moving atoms, applied magnetic field, etc. etc. which mean that the atoms inside my clock are not absorbing radiation at quite 9192631770 Hz.

But if you build a Josephson voltage standard, nobody has yet found any source of intrinsic error. (Tested down to 3×10⁻¹⁹!) The only uncertainty is due to sources like thermal EMF and wiring resistance outside the Josephson effect itself. Likewise for QHE resistance standards. This makes them startlingly accurate and practical measurement devices.

And the two effects are defined by the constants K_J = 2e/h and R_K = e²/h. There's been 26 years of research which has been done using conventional electrical units based on assumed exact values of those constants, because the SI-referenced values are not good enough.

Anyway, the reason for defining e and h specifically is to give exact values to those constants, and thus the measurements made with those instruments. 71.41.210.146 (talk) 23:32, 15 January 2017 (UTC)

That is illustrating the underlying process of reasoning for choosing reference standards, not the actual choices made. Someone reading this article (I assume) is interested in which standards are intrinsically more accurately comparable with the quantities of interest. The choice of e versus ε₀ is the one that I am interested in, and I'll use it as an example. The uncertainties in the table that change are:

Constant	Symbol	Proposed definition (e defined)		Alternate definition (ε0 defined)
		Relation to directly measured and fixed constants	Significant factor(s) in uncertainty	Relation to directly measured and fixed constants	Significant factor(s) in uncertainty
Josephson constant	$K_{\text{J}}$	${\frac {2e}{h}}$	exact	${\sqrt {8\varepsilon _{0}c\alpha /h}}$	${\sqrt {\alpha }}$
Von Klitzing constant	$R_{\text{K}}$	${\frac {h}{e^{2}}}$	exact	${\frac {1}{2\varepsilon _{0}c\alpha }}$	$\alpha$
Elementary charge	$e$	defined	exact	${\sqrt {2\varepsilon _{0}hc\alpha }}$	${\sqrt {\alpha }}$
Magnetic constant	$\mu _{0}$	${\frac {2h\alpha }{ce^{2}}}$	$\alpha$	${\frac {1}{c^{2}\varepsilon _{0}}}$	exact
Vacuum permittivity	$\varepsilon _{0}$	${\frac {e^{2}}{2hc\alpha }}$	$\alpha$	defined	exact
Impedance of free space	$Z_{0}$	${\frac {2h\alpha }{e^{2}}}$	$\alpha$	${\frac {1}{\varepsilon _{0}c}}$	exact
Faraday constant	$F$	$eN_{\text{A}}$	exact	${\sqrt {2\varepsilon _{0}hc\alpha }}N_{\text{A}}$	${\sqrt {\alpha }}$

The changes probably amount to just moving uncertainties around (despite the apparent halving of error), and metrologically this is probably inconsequential (in terms of what you say above). But from a practical perspective, an uncertainty of a measured quantity that is not pervasive in physics equations ($R_{\text{K}}$, $K_{\text{J}}$, $e$) is less bothersome or problematic than fuzziness in the definition of a pervasively used constant ($\varepsilon _{0}$, $\mu _{0}$, $Z_{0}$). Having Maxwell's equations exact (in terms of SI units) is probably better than knowing the exact charge of an individual electron, especially since the strength of the field that it generates is uncertain. That the constants chosen to define values for are actually those used at present in measurement is just "something under the hood"; it is the accuracy and usability of the final system that counts.

Anyhow, what I'm getting at is that this article might relevantly summarize references that give how the actual choices were made. —Quondum 04:35, 16 January 2017 (UTC)

"But from a practical perspective, an uncertainty of a measured quantity that is not pervasive in physics equations ($R_{\text{K}}$, $K_{\text{J}}$, $e$) is less bothersome or problematic than fuzziness in the definition of a pervasively used constant ($\varepsilon _{0}$, $\mu _{0}$, $Z_{0}$)."

That's where I disagree with you. How often do people operate circuits in a hard vacuum? There's little point knowing $\varepsilon _{0}$ more accurately than you know the applicable medium's dielectric constant. People normally assume that the relative permitting of air is 1, but it's about 1.00059, and varies with the weather! If that's not stable to better than 1 ppm, uncertainties less than 1 ppm are negligible.

On the other hand, as I said, $R_{\text{K}}$, $K_{\text{J}}$, and $e$ make up the quantum metrology triangle (no article, but there should be one!) which are direct inputs to a lot of high-precision experiments.

71.41.210.146 (talk) 05:24, 16 January 2017 (UTC)

In high-precision experiments, I expect that one would choose the standards one wishes to measure against. For example, if the caesium-133 standard proves to have a higher relative bandwidth or similar impediment to precision than some other source, the experimenter might use the latter. In the case being discussed, these constants are already built into the system, and one can use these instead of the units m and s for the experiment, if they prove superior under the circumstances. This does not seem to me to be a metrological argument, but rather one of allowing sloppy thinking by an experimenter, since correctly analyzed, this instance makes no difference. It comes down to aesthetics (and general usage), really. The accuracy of alpha, for example, stays just as imperfect either way. —Quondum 12:27, 16 January 2017 (UTC)

@Quondum: "For example, if the caesium-133 standard proves to have a higher relative bandwidth or similar impediment to precision than some other source, the experimenter might use the latter."

Yes, but a good global standard is one that is usually the best possible. When a significant area of research builds up around an alternate standard because it can't be compared to the global one, the result is awkward. That's what happened with my historical Ångstrom and a.u. examples, and is the current situation with the Conventional electrical units $R_{\text{K-90}}$ and $K_{\text{J-90}}$, and with the International Temperature Scale of 1990. (And starting to happen with optical frequency standards, but that's not quite as pressing yet.)

It's absolutely true that changing the definition doesn't affect the accuracy of the experiments the tiniest little bit. All it means is that more and more people simply aren't using the unit to describe the result of their experiment because it's too inconvenient. That's the situation that international agreements like the Treaty of the Metre, and International yard and pound are designed to avoid.

As you point out, the two possible definitions are linked by the fine-structure constant, and it's a question of which side of that link to place the definition on, and which side to put the uncertainty on.

The fine-structure constant is known to 0.32 ppb, so that's the uncertainty that the losing side has to deal with.

The question is, which side is more likely to make measurements more precise than 0.32 ppb?

I's sure it's gotten better since then, but The PTB's Josephson voltage standard was achieving 0.15 ppb uncertainty in 2000.

On the other hand, the most accurate possible electric-field-type measurements are done with a gadget called a calculable capacitor. This is a special shape of variable capacitor where the relationship between the position of the plates and the capacitance can be calculated exactly. For realistic dimensions, the change in capacitance is about 0.5 pF. By measuring that change with high accuracy, the electrical standards can be linked to $\mu _{0}$ (and thus the current SI definition of the ampere).

More information:

• Metrology Triangle Using a Watt Balance, a Calculable Capacitor, and a Single-Electron Tunneling Device (2008)
• Progress on the Thompson-Lampard calculable capacitor project at BIPM (2015)

However, the current experiments are struggling to break the 10-ppb uncertainty barrier. This is the limit at which anything defined in terms of the permittivity of free space can be measured.

So, given two domains, one with 0.15 ppb uncertainty, and the other with 10 ppb uncertainty, with a link of 0.32 ppb uncertainty, which side should we place the definition on, and which should be derived?

---

If we were to keep the current fixed $\mu _{0}$, which can only be linked to $R_{\text{K-90}}$ and $K_{\text{J-90}}$ to 0.32 ppb, then we'd need to continue using "conventional" values of the latter for sub-10⁻¹⁰ electrical measurements.

If I had my choice about how to define things, I'd define the metric system in terms of Planck units, which would keep $\mu _{0}$ fixed. But unfortunately, they all depend on the gravitational constant, which is only known to 47 ppm = 47000 ppb.

71.41.210.146 (talk) 23:39, 16 January 2017 (UTC)

On measurements using $\varepsilon _{0}$ defined as exact, it would be effectively determined from measurements of $R_{\text{K}}$ and $\alpha$: there is no reason to require direct a more direct realization of the constant.
I guess the adoption of $R_{\text{K-90}}$ and $K_{\text{J-90}}$ is indicative of the difficulty of tracking uncertainties, what I rather irreverently termed "sloppy thinking", and I guess this carries your argument. But I think the article giving a traceable, notable argument advanced by CIPM would still make sense; this is only the talk page. I don't have access to the reference I cited; I was hoping someone could paraphrase its argument.
I also like a definition in terms of physical constants used in Plank units, but excluding $G$ (it does not enjoy the theoretical status of aspects of the Standard Model aside from the accuracy issues). A nongravitational constant can replace $G$. Is there not some other suitable physical constant that does not rely on gravitation or a system as complex as an atom for its definition? Examples include the cosmological constant $\Lambda$ and $m_{\text{e}}$, though these have similar issues. —Quondum 01:32, 17 January 2017 (UTC)

Status of draft values

Note 3 ("This is a draft value, which will be updated before the redefinition.") was introduced when these values were still due to be updated. The publication of the CODATA 2017 values was presumably the expected update. I imagine a further revision is not impossible, but it seems to me that the values now reflected in the article are expected to be final. Does anyone have more insight so as to accurately revise this note? —Quondum 15:37, 2 December 2017 (UTC)

@Quondum: Indeed, the final numbers are known, and I have made the relevant edits, including removing that note from most places, and greatly softening what remains. On 20 October 2017, the 106th meeting of the CIPM formally approved a Draft Resolution A for consideration by the next CGPM. Although the final text of the resolution is not available from the BIPM web site AFAICT, all of the changes made to previous drafts are described in the meeting minutes and so we know that the 2017 CODATA values have been adopted. (Indeed, publication of the 2017 values was deliberately delayed until the same day the CIPM approved them.) 23.83.37.241 (talk) 10:42, 27 January 2018 (UTC)

Thanks; I've made related tweaks. It is nice that N_A, N_N, Da and u now all have unambiguous draft definitions (given CIPM/106 and the draft ninth SI brochure). SI is becoming a very tight and consistent structure, excluding certain great big gaping flaws with cd and Sv that IMO could easily (and should) have been addressed. —Quondum 19:30, 27 January 2018 (UTC)

Featured Article for 17 November?

For discussion: are other editors interested in turning this article into something suitable for a front-page feature when the CGPM resolution passes? With multiple active editors, an base that has already passed GA, and almost 10 months to prepare, it doesn't seem too strenuous.

The first things seem to me:

• Figure out the eventual article title (I'm thinking 2018 redefinition of SI base units, but not strongly attached)
• Decide if we're going to convert the current article or write a new one in the past tense, and
• Decide if we want to aim for the 16th or 17th
• Request the FA date

The 16th would make writing a good article trickier, as it would hit the front page a few hours before the vote. (What time zone does WP use to roll over featured articles, anyway?) But it would match the news explosion better.

Unless we want to do all the work in Draft space, a mainspace article would require an explanatory hatnote anyway, and maybe that would be good enough for the first few hours of the 16th, but I fear it's too messy and misleading-fine-print-y to satisfy FA review.

But before we get too far into details... anyone else interested? Yea or nay? 23.83.37.241 (talk) 03:11, 28 January 2018 (UTC)

Starting the title with the year suggests that it is a yearly event. Perhaps drop the year altogether and address all historical redefinitions, but with the focus on the most topical: the one that will just have been voted on. I wouldn't mind some involvement. —Quondum 03:49, 28 January 2018 (UTC)

Mole and NIST

Just had my ref to https://www.nist.gov/si-redefinition/redefining-mole reverted. I thought it was a pretty good layman's overview of the overall redefinition, and specifically of the mole. NIST should be a pretty good RS too surely. Thoughts? (I won't myself be re-instating this). - Snori (talk) 22:10, 24 October 2018 (UTC)

I removed the reference. I invite other opinions on the suitability of the linked NIST article as an encyclopaedic reference (in this instance for the expected definition of the mole). Just because NIST is normally considered to be a reliable source does not mean that everything that appears on their website is a suitable reference. Having a general sense of the process that led to the definition, I fail to see it as an overview of anything, historically or otherwise. —Quondum 00:33, 25 October 2018 (UTC)

I like it. Much of the process of (re)defining units isn't easy to follow, but this is fairly easy to follow by less scientific people, but also includes historical information that is often forgotten or ignored. In any case, the idea is to carefully measure a Si sphere, such that one can accurately count the atoms. The lattice constant is known very accurately, so if you can measure the diameter accurately, you figure out how many atoms it has. Gah4 (talk) 09:40, 25 October 2018 (UTC)

one of these things must change

In the Mole section, it says that one of the two must change, suggests that it is the first one, and then says that implies the second is no longer true. If only one must change, and the first one changes, doesn't that mean that the second one doesn't need to change? Gah4 (talk) 00:30, 13 November 2018 (UTC)

The wording is maybe ambiguous. It is intended to say that the first statement is implied (i.e. it will remain valid, not that it will change). The wording in the draft is: "The dalton (Da) and the unified atomic mass unit (u) are alternative names (and symbols) for the same unit, equal to 1/12 of the mass of a free carbon 12 atom, at rest and in its ground state." Your observation is good: I'll look at clarifying it. —Quondum 01:30, 13 November 2018 (UTC)

Change of title

Assuming that the upcoming vote on 16 November is approved, the tag "Proposed" in the current title will start to lose some of its shine. Maybe this page should be retitled to "Planned redefinition..." or "Scheduled redefinition..." or something which makes it clear that this is no longer just a proposal? Episanty (talk) 14:24, 13 November 2018 (UTC)

A change of title as proposed makes sense. Yet, it will have built-in obsolescence: after adoption only six months later, the article will become historical. I would be happy to change the title, or to leave it as is for that interval, and simply update the content. —Quondum 18:35, 13 November 2018 (UTC)

Perhaps from Proposed redefinition of SI base units to 2018 redefinition of SI base units or 2019 redefinition of SI base units? — Preceding unsigned comment added by Richard-of-Earth (talk • contribs) 04:07, 14 November 2018 (UTC)

When they finish this one, will there be new proposed changes? I would expect it to be a never ending task. Gah4 (talk) 05:01, 14 November 2018 (UTC)

When they finish adopting this change? It will stand alone as the only major revision since 1960, and there is a good chance that there will be no major revision again for at least as long. A replacement of the caesium atom to define the second could occur in time, but it could not be for many decades. From an encyclopaedic perspective, it makes sense to capture this event (this particular revision) in this article (which makes Richard-of-Earth's title suggestions look reasonable). —Quondum 15:07, 14 November 2018 (UTC)

I think we should have a moratorium on summarily changing the title of this article without first reaching some consensus on the talk page, as was just done: renaming any article with GA status should be treated this way. The recent change to prepend "2019" to the title may have issues, such as the comment I made under § Featured Article for 17 November?: Starting the title with the year suggests that it is a yearly event. Another point to consider is that it might make perfect sense to consider the scope of this article to be all redefinitions of the SI base units (it already speaks about these, only with the most emphasis on the last). —Quondum 15:24, 18 November 2018 (UTC)

Constants and base units

Under the "Impact on reproducibility" heading, the article currently reads:

"Apart from the candela, all the base units will be defined in terms of universal physical constants, but without a direct one-to-one correspondence between the constants and the base units. Thus six physical constants will be needed to define the six base units."

Those sentences are confusing to me:

• I thought there were seven, not six, base units.
• The diagram labeled "New SI" at the top of the article, File:Unit_relations_in_the_new_SI.svg, appears to show seven constants defining seven units.
• If the second sentence's phrase "to define the six base units" means the six base units other than the candela, then I would suggest wording like "to define six of the seven base units" instead so as to not imply there are only six total base units.
• Also, the first sentence tells us there is no one-to-one correspondence, but the second sentence indicates six constants match six units; while both sentences could be true (a mapping that is not one-to-one could still have the same number of outputs as inputs if some inputs map to more or fewer outputs than other inputs do), the latter sentence as it currently stands is certainly not obviously implied by the former sentence, so "thus", with its meaning of "as a result", is thus the wrong conjunctive adverb to use to connect the two sentences.

—Lowellian (reply) 21:28, 16 November 2018 (UTC)

The goal should be to leave the typical reader with an immediate sense of the meaning, and I agree that this was confusing. I've tried to address your concerns by adapting the text. —Quondum 00:57, 17 November 2018 (UTC)

Criticism in the lead

I removed the mention of criticism of the redefinition from the lead, and it was reverted with the suggestion As a short mention of the criticism has been in the lead for more than 7 years, I guess its removal needs to be discussed on the talkpage first, so I am accordingly opening a discussion here. My primary motivation is that a prominent mention in the lead gives undue weight to the perception of controversy in relation to the redefinition, notwithstanding the long presence in the lead. There will be a large number of new readers visiting the page due to the recent news coverage, and it mention might create the impression that the process was significantly less consensual than it was (it seems to have been exceptional in the degree of consensus). This topic is covered in the article for the more scientifically minded reader, but IMO it would be inappropriate to thrust this in the face of the multitude of readers who will take away little more than what is in the lead. Opinions? —Quondum 17:17, 17 November 2018 (UTC)

As one of the six main sections in the article addresses 'concerns', and it has four subsections (more than all but one of the other sections), then per "The lead serves as an introduction to the article and a summary of its most important contents." from WP:LEAD, it seems reasonable weight to cover it with a one-sentence-paragraph (the smallest of the six paragraphs) in the lead. -- DeFacto (talk). 17:43, 17 November 2018 (UTC)

Diagrams colors and font

I’ve just updated the colors of the two diagrams at the top of the article so they match the colors of the BIPM’s SI Illustration Guidelines. If a SVG specialist knows how to change the font to match the font in the Guidelines (see last page), they’re welcome. The SI Logo graphic files may also be of some help in that purpose. Wikipetzi 22:15, 17 November 2018 (UTC)

@Wikipetzi and IngenieroLoco: Please do not edit the SVG directly without noting that in the metadata. As the file description notes, the original was generated directly from a Mathematica notebook, and direct edits that do not note that change create inconsistencies in the record. Ideally the changes should be to the .nb with an accompanying Pull Request on GitHub (to make for a maintainable image). I'm aware that this does require non-free software so if absolutely needed then direct edits to the SVG cannot be ruled out; just make sure to note that in your metadata.

I've updated the changes in the Mathematica notebook and pushed new versions of the SVG to Wikimedia. This includes a rollback of some incorrect changes to the Old SI scheme - while it makes sense to use the New SI colours for the untouched constants (Δν, c, K_cd), it makes no sense at all to use those colours for the pre-change constants (M_IPK, m(12C), μ_0, T_TPW), so I've switched them back to gray.

As to fonts - this is again best handled from the Mathematica notebook instead of needing an "SVG specialist", though given the state of the style guide this can only be done partially. The bulk of the symbols (those in the Latin alphabet) are in Source Sans Pro Semibold in the BIPM style guide, and those do not represent a problem. However, the Δν is at present completely unclear - the currently-available pdf of the style guide has the Δν as a path (instead of as text) and I can't determine what the original font is. I've chosen a MaTeX-over-xelatex compilation to get the least-ugly symbol possible for the combination that I could find. Alternatives (which are stable and play well with the existing stack over Mathematica) are welcome, particularly if anyone manages to get the BIPM stylists to state what the original font was for that symbol. Episanty (talk) 16:26, 18 November 2018 (UTC)

@Episanty: Thanks for the fonts change, it looks great. While you’re at it, could you fix a tiny minor problem? In both diagrams, the arrow between the "s" unit and its associated constant is the only one to touch the circle of the constant. It needs a narrow gap in between. --Wikipetzi 21:32, 18 November 2018 (UTC)

"Abrogated"

Could someone paraphrased "abrogated" in the Redefinition section to a more common word. It's not helpful even when linking to Wiktionary because there are multiple definitions there so it's unclear which meaning it really means. starship.paint ~ KO 03:13, 18 November 2018 (UTC)

The end of the Redefinition section of the article currently says:

As part of the redefinition:

• The international prototype kilogram was retired and the previous definition of the kilogram was abrogated,
• The current definition of the ampere was abrogated,
• The current definition of the kelvin was abrogated and
• The current definition of the mole was revised.

I thought to replace "abrogated" with "discontinued", but really these definitions are replaced. But then I had a thought; why is this a table anyway. We really should just use prose.

As part of the redefinition the international prototype kilogram was retired and definitions of the kilogram, the ampere, and the kelvin were replaced. The definition of the mole was revised.

Any disagreements or other suggestions? Richard-of-Earth (talk) 07:04, 18 November 2018 (UTC)

I like it. More readable, and still says the same thing. —Quondum 15:10, 18 November 2018 (UTC)

Yes, go for it. Thincat (talk) 15:47, 18 November 2018 (UTC)

 Done Richard-of-Earth (talk) 21:46, 18 November 2018 (UTC)

"Losing" mass x Fluctuation

The beginning of the current version of the article says: "Although designed to not degrade or decay over time these prototypes were in fact losing minuscule amounts of mass over time, even in their sealed chambers." Then, section Impetus for change says "There was no way of determining whether the national prototypes were gaining mass or whether the IPK was losing mass". I propose we change the wording in the beginning of the article. BBC News refers to this difference between the IPK and its replicas as a "fluctuation" and I propose something in that spirit is done. Mateussf (talk) 21:43, 19 November 2018 (UTC)

I agree what is said in the lead is unsatisfactory. The reference given for the "There is no way ..." claim says "The prototype and its copies appear to gain mass over time and lose mass when washed for use in comparisons." But that may not be the best statement to rely on. Any rewording needs to be careful because the IPK has been precisely constant in mass, measured in kilograms, even of not in comparisons. Thincat (talk) 16:36, 20 November 2018 (UTC)

It will be interesting to know the small difference between the old physical standard and the newly adopted one, as sources on that become available. Jonathunder (talk) 16:50, 20 November 2018 (UTC)

Kilogram#Stability of the international prototype kilogram is detailed and nicely written, though rather deficient in its referencing. Note 12 is worth reading. Thincat (talk) 17:19, 20 November 2018 (UTC)

The current text ("Although designed [...] within an acceptable tolerance level") is worse than unsatisfactory; it is essentially incorrect. It would be better to replace it with something to the effect of "The masses of the prototypes have been found to be measurably unstable with respect to each other, becoming less suitable for modern precision requirements", pending sourcing of a more precise statement. —Quondum 17:56, 20 November 2018 (UTC)

Go for it and maybe someone can improve it still further. It is a tricky concept to communicate clearly and precisely. Jonathunder (talk) 19:28, 20 November 2018 (UTC)

Citation date formatting

The reference dates were changed in large numbers from an essentially consistent uniform yyyy-mm-dd format, incorrectly citing WP:DATEFORMAT, which does not even suggest consistency between the three groups Dates in article body text, Publication dates, and Access and archive dates, only within each group, and contrary to WP:STYLEVAR. I propose reverting to yyyy-mm-dd format for the latter two categories – essentially all the citation-related dates that display in § References. With the large (and growing) number of references, there is even the benefit of easier scanning of the list of references and space-saving on the page. For clarity, the dates in the body would remain dmy format. —Quondum 16:12, 20 November 2018 (UTC)

Definition of kg

So does that mean that the kilogram is now defined as 14,755,213,997,352,709,121,298,852,911,707,242,627,072 hν/c²? Rwflammang (talk) 00:19, 21 November 2018 (UTC)

The talk page guidelines discourage discussion of the topic on an article's subject; the science reference desk would be appropriate. But yes, except that after about 18 digits your figure is in error, and the value is not an integer. —Quondum 19:27, 21 November 2018 (UTC)

Shouldn't we add that to the article, as well as the other six base units? Say in a table:

1 Kg = 14,755,213,997,352,709,121,298,852,911,707,242,627,072 hν/c² (or whatever)

1 m = c / 299,792,458

etc.

After all, what's being redefined is one of each particular base unit. --RoyGoldsmith (talk) 21:01, 21 November 2018 (UTC)

Perhaps, if we have sources for the correct values. Jonathunder (talk) 21:03, 21 November 2018 (UTC)

I have doubts that this value would be sourced reliably (even though it is easy to determine), and I fail to see how the actual value is of interest. That value would never actually get used directly. What might be more interesting (but may still lack sources) is simply the point that the kilogram is an exact numeric (rational) multiple of hν/c², without giving the number. A similar approach might apply for all base units. But I am not in favour of presenting this, and it could be regarded as synthesis without a good source with a similar conclusion. —Quondum 23:06, 21 November 2018 (UTC)

Well, there is WP:CALC which should allow for some. They would have to be exact calculations, though, so written as fractions might work, but rounded decimal expansion would not. Well, rounded values with an appropriate ≈ should be fine. Gah4 (talk) 23:25, 21 November 2018 (UTC)

Also, discussions on an article's subject with the intent to improve the article should be fine. I usually try to give reasonable leeway in such cases, unless it is really obviously not useful. Gah4 (talk) 23:25, 21 November 2018 (UTC)

Why isn't the coulomb the base unit and ampere derived unit?

With this redefinition, it seems to me that the coulomb is now more fundamental, in that its definition does not depend on the definition of the second as the definition of the ampere does. So why didn't they change it to make the coulomb the base unit and the ampere a derived unit? — Preceding unsigned comment added by Gene Nygaard (talk • contribs) 15:43, 22 November 2018 (UTC)

If only we could read the minds of the metrologists. It would make sense, but I suspect that the change entails implications (e.g. documentation) that are out of proportion to the benefit: it is not that difficult or problematic to maintain the ampere as the base unit, especially with the style new definitions where each unit is quite naturally be related to multiple constants. The ampere as a base unit has awkwardness/clumsiness though: common qualities, when expressed in terms of base units, often have a factor A^m·sⁿ, where m and n are close, as may be seen by inspecting SI derived unit. But then, the same could be said for the metre: replacing it with a coherent unit of velocity as a base unit (e.g. the benz) would produce a similar simplification. —Quondum 16:11, 22 November 2018 (UTC)

Base unit and derived unit doesn't seem to have much to do with what is derived from what. Since the meter, for some years now, is derived from the second and a defined c, it should be a derived unit. But it isn't. Since we measure current much more often than charge, having ampere as a base doesn't seem so bad. Gah4 (talk) 18:16, 22 November 2018 (UTC)

Why not go the whole way and define only the seven base constants (Δν_Cs, c, h, e, k, N_A and K_cd) and do away with base units entirely? As a matter of fact, isn't that what the SI is, in effect, doing? Making the base units derivable from the defined constants above? The base units, which in the new scheme are entirely derivable, are for convenience: the CIPM and the CGPM are charged with the definition of units of measurement, which scientists need for their work. --RoyGoldsmith (talk) 18:20, 22 November 2018 (UTC)

Yes, maybe they should call them convenience units instead of base units. Gah4 (talk) 18:33, 22 November 2018 (UTC)

Also, why is there no named velocity unit? And no named acceleration unit? They seem to be the rare commonly used units without their own names. Otherwise, we really don't need both ampere and coulomb as named units. Gah4 (talk) 21:27, 22 November 2018 (UTC)

Modification of the Earth's aggregate mass, and its effect on gravity

Intriguing article! Over the span of time and certainly during the existence of standard weights, objects from space ranging from dust particles to meteor/asteroids have fallen on the Earth's surface, modifying its aggregate mass. Wouldn't this have had an effect on the force of gravity, causing a variance, for example, in how far an example weight would compress or stretch a steel spring? Conversely, wouldn't that increase, as it occurs on the Earth's surface, add to lunar tidal braking and decrease by however small a measure the centripetal force near the equator, causing a perceived decrease in the mass of an example weight, depending on where between a pole and the equator an observation was conducted? — Preceding unsigned comment added by Scott Sanchez (talk • contribs) 19:11, 22 November 2018 (UTC)

There are scales that don't have springs. A balance scale does not depend on the value of the gravitational field. As long as both pans are in the same gravitational field, it will give the same reading, no matter where on the planet or which planet it is on. Indefatigable (talk) 17:32, 23 November 2018 (UTC)

20 May 2019

Would it be possible to make this a featured article by 20 May 2019 with the goal of putting it on the main page that day? Jonathunder (talk) 00:27, 7 February 2019 (UTC)

I hope so! I'm hoping to help improve this article, but it looks like I need to do a lot of reading to understand the subject clearly first... Krubo (talk) 11:56, 20 April 2019 (UTC)

Looking at this more, I actually think it would be more useful to make SI the featured article for that day, since it's not a featured article yet. Krubo (talk) 18:57, 22 April 2019 (UTC)

I don't think getting SI to featured article status in that timeframe is achieveable (IMHO, it is not even at GA quality). It would have to be cleaned up and brought up to date with the revision (including completely replacing the tables to match the revision), and all we have for that is the draft of the 9th brochure, which is not ideal as a reference for material that will apply on 20 May, but it is suitable for this article, because we talk about it as a draft. I would focus on this article, which is very topical for the day, being focused on that aspect (i.e. the redefinition) rather than on the SI as a whole. However, the getting the SI to FA status is a good goal, whatever the timeframe. —Quondum 20:44, 22 April 2019 (UTC)

Diagram implies dependencies that do not exist

The diagram at the start of the article showing the dependency of units on other units and on constants is not ideal. A naïve interpretation would suggest that the kelvin and the candela depend on the defined value of the speed of light, but they do not (see [9] – follow the "Click on ..." instruction). I think a diagram that the reflects the dependence of units on constants would be more sensible (and intuitive) than one that reflects the interdependencies of definitions in the wording, which is really just one of several logically equivalent definitions. Is there support for changing the arrows to run only from constants directly to units? —Quondum 23:05, 12 March 2019 (UTC)

The names base unit and derived unit describe dependencies that don't exist, which I noted some time ago. It might be that the diagram follows the same non-dependencies. Gah4 (talk) 23:43, 12 March 2019 (UTC)

Seems to me this is the same principle only. Having a convention on which minimal set in terms of which to reduce any unit is useful, whereas the mutual dependence of the definitions of units has no utility beyond the choice of wording of the definitions. What can be useful is how to express the units in terms of the defining constants. For example, 1 K = βΔν_Csh/k, where β is a rational constant. —Quondum 16:59, 13 March 2019 (UTC)

I looked at generating a diagram. Straight arrows seem to inevitably cross right over colour blobs if we keep the circular arrangement. Unless someone has a bright idea, this suggestion can be shelved, in favour of a mention in text of the dependencies of the units on the constants. —Quondum 16:30, 17 March 2019 (UTC)

derived unit

The article says: around seven base units whose definitions are unconstrained by that of any other base unit. I find the explanation of base unit and derived unit not so obvious, after that statement. (And yes I know that CIPM doesn't care what I write here.). It seems that the meter is defined in term of, or in other words derived from, the definition of the second, yet is still a base unit. In other cases, the derived unit is directly defined, and base units derived from it. I suppose it is convenient not to change which units are base, but it does make the whole system seem not as quite as nice as it should be. Gah4 (talk) 00:05, 21 March 2019 (UTC)

The choice of size of the metre can be freely chosen, despite the second being used as a reference for in its definition for convenience. In a sense, the base units are independent in the same way that a set of vectors that form a basis of a vector space are linearly independent, and this freedom to choose the size of each is what defines what base units are. Any further units (the "derived" units; this is maybe a misleading term) can only have one size and be coherent with the system. I think it says somewhere in the draft brochure that the base/derived distinction no longer has the original significance in the new SI; in fact, s, m, J, C, K, mol might have a neater set of base units to math the new definitions. Do you have a suggestion for expressing the idea of what purpose base units serve that will be clearer? I'd agree that the current wording will not make this immediately clear to everyone. —Quondum 00:58, 21 March 2019 (UTC)

I don't have any suggestions, but just find it a funny (as in strange, not haha) use of the word. I might have chosen primary and secondary, which indicates importance, and not so obviously derivation. Yes, I didn't see the mention of the change in significance in the document. A recent discussion on the diagrams that suggest derivation reminded me of this question. More recently, I was reminded of this in a discussion in Gaussian units, where the connection between E and B, though the electromagnetic tensor, which is less obvious, and less convenient, in SI units. Oh well. Gah4 (talk) 09:56, 21 March 2019 (UTC)

For reference, my comment is based on A concise summary of the International System of Units, SI: "The concept of base units and derived units was used to define the SI until 2018. These categories, although no longer essential in the SI, are maintained in view of their convenience and widespread use.", and Draft of the ninth SI Brochure, 6 February 2019, §1.2 and §2.3. This suggests a strong trend towards defining constants and units expressed in terms of these, and the seven degrees of freedom in SI now takes the form of the number of defining constants being seven. It would make sense for the article to reflect this way of thinking. I may tinker with wording with this in mind. The EM thing is not something I've delved into closely, but would probably look at some point. —Quondum 13:54, 21 March 2019 (UTC)

TFA?

See Wikipedia talk:Featured articles#2019 redefinition of SI base units. --Guy Macon (talk) 15:29, 20 April 2019 (UTC)

The obvious things: The lead is too long. There is some redundancy and some historic things can be shortened a bit. Some references have access dates from ~2012, they have to be checked and updated if something happened at the link target. As the date of the change is approaching the article should be put into a state where it is easy to update. Some future->past updates on May 20 are unavoidable but an article won't get featured if it is clear that several things will have to be rewritten a week later (stable criterion). Some things are too technical. The big table needs references. Overall I think the article is a good starting point but it needs some more work. --mfb (talk) 08:11, 24 April 2019 (UTC)

See 3 sections above. As the only main editor still around, you need User:Quondum's ok to start an FAC. Someone (by the looks of it not him) needs to take it on & really drive it. Johnbod (talk) 02:23, 25 April 2019 (UTC)

I don't see why any 'okay' would be needed from me for anything, though happy to give opinions. And correct, I would not drive any review or editing. This would need someone with more time and experience of this process. I have not noticed much editor interest in getting this article polished to FA status since the flurry of attention in November. —Quondum 16:48, 25 April 2019 (UTC)

It's in the FAC rules: "Nominators who are not significant contributors to the article should consult regular editors of the article before nominating it." You seem to be the only one currently active, and an explicit ok would be required, although you don't have to do anything else if you don't want to. FAC hates drive-by noms, as do many "regular editors". Johnbod (talk) 17:44, 25 April 2019 (UTC)

Ah. FWIW, if there is anyone who is willing to drive the process to get this article to FA status, I would support a FAC nomination. —Quondum 18:24, 25 April 2019 (UTC)

Thanks, that'll do it. Johnbod (talk) 18:44, 25 April 2019 (UTC)

Date specific editing

What time zone does the change take place in? In all time zones at the same time? Gah4 (talk) 16:26, 24 April 2019 (UTC)

The exact moment isn't defined in the decision, so you should assume that it occurs on the first second after midnight UTC and that it occurs at the same moment everywhere on earth. Not that it matters; the new definition gives you the same result as the old at far better accuracy than any of your instruments can give you. --Guy Macon (talk) 18:36, 24 April 2019 (UTC)

Today isn't May 20th, 2019. Gah4 (talk) 19:05, 24 April 2019 (UTC)

Where did I specify the first second after midnight UTC today? Every day has a first second after midnight. If I don't specify a particular day you should pick one that seems appropriate. 20 May 2019 might be a good choice. I'm just saying. --Guy Macon (talk) 03:16, 25 April 2019 (UTC)

Read this section on May 20th, and the above will say: Today is May 20th, 2019! (At least once the cache is rewritten.) You can make edits that will only take effect after a specified date. (I believe the wikipedia servers use UTC, so it will follow UTC in all time zones.) Gah4 (talk) 20:39, 25 April 2019 (UTC)

{{#ifeq:{{#timel:mdy}}|052019|Today is May 20th, 2019!|Today isn't May 20th, 2019.}} Gah4 (talk) 19:05, 24 April 2019 (UTC)

I don't see much value in dynamic text, other than for talk pages. To some extent we can anticipate changes: "On 20 May 2019, these changes take effect", and at some point someone will replace take→took. —Quondum 15:08, 26 April 2019 (UTC)

I suppose that is mostly true here. But in all the other pages that have to change on the same date, maybe it would be nice to put in the automatic selection. Then, after the change but in no hurry, go through and remove the condition, along with the no longer needed text. Small problem in that the effect doesn't take effect until the cache is purged. Gah4 (talk) 02:03, 27 April 2019 (UTC)

"Uncertainty of fundamental physical constants" section tagged as "possibly original research"

I'm not seeing any calculational mistakes it it, but it would definitely be helpful if there were pointers to where this sort of error-propagation work has been done before (e.g., [10]). XOR'easter (talk) 18:21, 18 May 2019 (UTC)

Also, it looks like the uncertainty in the fine-structure constant is the CODATA 2014 value, so I'll add a note to that effect. XOR'easter (talk) 18:37, 18 May 2019 (UTC)

The note makes sense, even though uncertainty in the fine-structure constant should not change much. A citation would be nice, but the hatnote seems excessive. Unfortunately, most of these seem to be hidden behind paywalls. I exhaustively checked the table, and find it very useful for understanding, but I guess it does not quite qualify as WP:CALC. —Quondum 19:39, 18 May 2019 (UTC)

I don't think it is OR in the sense of editors doing too much self-calculation. I can believe that the 2014 part of the table has been carefully edited. My interpretation of the OR tags is that many of the 2019 uncertainty values in the table are purely speculative until the actual values are released. It may be best to blank all the unknown (i.e. inexact) 2019 values until CODATA 2018/2019 drops. --{{u|Mark viking}} {Talk} 21:16, 18 May 2019 (UTC)

Yes, it's not OR in the sense that we're usually worried about, but reporting the relative uncertainties on the "2019 definition" side of the table using the CODATA 2014 values seems a bit confusing. (For example, if the uncertainty in $\alpha$ gets revised down a skosh, then almost all the numbers in the last column would change.) I don't think it would hurt the article if we stashed the table over here until CODATA 2018/2019 drops and we can say something more definitive. It would be nice to have this article as Main Page "in the news" on the 20th, and a temporary (maybe partial) trimming of that section seems the quickest and easiest way to remove the most glaring concern. XOR'easter (talk) 22:24, 18 May 2019 (UTC)

That sounds like a good plan to me. --{{u|Mark viking}} {Talk} 00:37, 19 May 2019 (UTC)

@XOR'easter: This table was a very valuable resource; removing it from this section is a huge loss. If I understand correctly, a suitable CODATA publication is now available which can be used to produce a reliably non-OR version? What is still required for this table to be reinstated? Episanty (talk) 17:07, 18 January 2021 (UTC)

OK, here it is. I think it could be quite useful, but it needs some workshopping first. If there are any references for the formulae, paywalled or not, they'd be helpful. XOR'easter (talk) 01:16, 19 May 2019 (UTC)

The 2018 CODATA values have now been published, including the uncertainty of the fine-structure constant. —Quondum 03:40, 20 May 2019 (UTC)

Great! Thanks for the heads-up. XOR'easter (talk) 16:49, 20 May 2019 (UTC)

I notice that the table says no uncertainty for the kilogram. That doesn't sound right, as uncertainty was one reason for the redefinition. The different secondary standards were changing differently, so there is uncertainty somewhere. Gah4 (talk) 02:00, 19 January 2021 (UTC)

It only says no uncertainty for the time before the redefinition. Before the redefinition the reference kg was defined to be exactly 1 kg so it didn't have an uncertainty. --mfb (talk) 05:15, 19 January 2021 (UTC)

Uncertainty of fundamental physical constants

Fundamental physical constants are often algebraically related through theory. This implies that the most precise estimate of a fundamental physical constant often can be determined by directly measuring other fundamental physical constants, from which the value can be calculated. The uncertainty in these constants is then determined from the uncertainty of the others by the propagation of uncertainty. Constants whose value is fixed, for determining the units in which they are expressed, do not have an uncertainty associated with them, so they do not contribute any extra uncertainty to the value calculated for the constant. One of the reasons that the SI unit system will be changed is that determination of values can often be greatly improved if expressed in terms of the revised unit definitions. This is due to the dependence of their values on physical constants.

The following table catalogues the notable changes in determination of fundamental physical constants. The constants are expressed in direct measurements and fixed constants to minimise and determine the uncertainty. Some constants in that expression do not contribute significantly to the final uncertainty; only the significant factors are noted. The value of the relative uncertainty using the data of CODATA of 2014 is given, and is expressed in the relative uncertainty of the significant factors, noted with u_r(constant). An approximately equals sign (≈) is used if an uncertainty is only approximated by the expression due to insignificant factors or linear approximation.

Constant	Symbol	Previous definition			2019 definition
		Relation to directly measured and fixed constants	Significant factor(s) in uncertainty	Relative uncertainty	Relation to directly measured and fixed constants	Significant factor(s) in uncertainty	Relative uncertainty
Mass of IPK	$m({\mathcal {K}})$	1 kg	None	Exact	$m({\mathcal {K}})$	$m({\mathcal {K}})$	$1.2\times 10^{-8}=u_{\text{r}}(m({\mathcal {K}}))$
Planck constant	$h$	${\frac {8\alpha }{c\mu _{0}K_{\text{J}}^{2}}}$	$K_{\text{J}}^{2}$	$1.2\times 10^{-8}\approx 2u_{\text{r}}(K_{\text{J}})$	$h$	None	Exact
Josephson constant	$K_{\text{J}}$	$K_{\text{J}}$	$K_{\text{J}}$	$6.1\times 10^{-9}=u_{\text{r}}(K_{\text{J}})$	${\frac {2e}{h}}$	None	Exact
Von Klitzing constant	$R_{\text{K}}$	${\frac {c\mu _{0}}{2\alpha }}$	$\alpha$	$2.3\times 10^{-10}=u_{\text{r}}(\alpha )$	${\frac {h}{e^{2}}}$	None	Exact
Elementary charge	$e$	${\frac {4\alpha }{c\mu _{0}K_{\text{J}}}}$	$K_{\text{J}}$	$6.1\times 10^{-9}\approx u_{\text{r}}(K_{\text{J}})$	$e$	None	Exact
Magnetic constant	$\mu _{0}$	4π×10−7 m⋅kg⋅s−2⋅A−2	None	Exact	${\frac {2h\alpha }{ce^{2}}}$	$\alpha$	$2.3\times 10^{-10}=u_{\text{r}}(\alpha )$
Vacuum permittivity	$\varepsilon _{0}$	${\frac {1}{c^{2}\mu _{0}}}$	None	Exact	${\frac {e^{2}}{2hc\alpha }}$	$\alpha$	$2.3\times 10^{-10}=u_{\text{r}}(\alpha )$
Impedance of free space	$Z_{0}$	$c\mu _{0}$	None	Exact	${\frac {2h\alpha }{e^{2}}}$	$\alpha$	$2.3\times 10^{-10}=u_{\text{r}}(\alpha )$
Electron mass	$m_{\text{e}}$	${\frac {16R_{\infty }}{c^{2}\alpha \mu _{0}K_{\text{J}}^{2}}}$	$K_{\text{J}}^{2}$	$1.2\times 10^{-8}\approx 2u_{\text{r}}(K_{\text{J}})$	${\frac {2hR_{\infty }}{c\alpha ^{2}}}$	$\alpha ^{2}$	$4.7\times 10^{-10}\approx 2u_{\text{r}}(\alpha )$
Electron molar mass	$M({\text{e}})$	$A_{\text{r}}({\text{e}})M_{\text{u}}$	$A_{\text{r}}({\text{e}})$	$2.9\times 10^{-11}=u_{\text{r}}(A_{\text{r}}({\text{e}}))$	${\frac {2hR_{\infty }N_{\text{A}}}{c\alpha ^{2}}}$	$\alpha ^{2}$	$4.7\times 10^{-10}\approx 2u_{\text{r}}(\alpha )$
Atomic mass constant, dalton or unified atomic mass unit	${\begin{aligned}m_{\text{u}}&=1\,{\text{Da}}\\&=1\,{\text{u}}\end{aligned}}$	${\frac {16R_{\infty }}{c^{2}\alpha \mu _{0}K_{\text{J}}^{2}A_{\text{r}}({\text{e}})}}$	$K_{\text{J}}^{2}$	$1.2\times 10^{-8}\approx 2u_{\text{r}}(K_{\text{J}})$	${\frac {2hR_{\infty }}{c\alpha ^{2}A_{\text{r}}({\text{e}})}}$	$\alpha ^{2}$	$4.7\times 10^{-10}\approx 2u_{\text{r}}(\alpha )$
Molar mass constant	$M_{\text{u}}$	0.001 kg⋅mol−1	None	Exact	${\frac {2hR_{\infty }N_{\text{A}}}{c\alpha ^{2}A_{\text{r}}({\text{e}})}}$	$\alpha ^{2}$	$4.7\times 10^{-10}\approx 2u_{\text{r}}(\alpha )$
Avogadro constant	$N_{\text{A}}$	${\frac {c^{2}\alpha \mu _{0}K_{\text{J}}^{2}A_{\text{r}}({\text{e}})M_{\text{u}}}{16R_{\infty }}}$	$K_{\text{J}}^{2}$	$1.2\times 10^{-8}\approx 2u_{\text{r}}(K_{\text{J}})$	$N_{\text{A}}$	None	Exact
Atomic mass of carbon-12	$m(^{12}{\text{C}})$	${\frac {192R_{\infty }}{c^{2}\alpha \mu _{0}K_{\text{J}}^{2}A_{\text{r}}({\text{e}})}}$	$K_{\text{J}}^{2}$	$1.2\times 10^{-8}\approx 2u_{\text{r}}(K_{\text{J}})$	${\frac {24hR_{\infty }}{c\alpha ^{2}A_{\text{r}}({\text{e}})}}$	$\alpha ^{2}$	$4.7\times 10^{-10}\approx 2u_{\text{r}}(\alpha )$
Molar mass of carbon-12	$M(^{12}{\text{C}})$	0.012 kg⋅mol−1	None	Exact	${\frac {24hR_{\infty }N_{\text{A}}}{c\alpha ^{2}A_{\text{r}}({\text{e}})}}$	$\alpha ^{2}$	$4.7\times 10^{-10}\approx 2u_{\text{r}}(\alpha )$
Faraday constant	$F$	${\frac {c\alpha ^{2}K_{\text{J}}A_{\text{r}}({\text{e}})M_{\text{u}}}{4R_{\infty }}}$	$K_{\text{J}},\alpha ^{2}$	$6.2\times 10^{-9}\approx u_{\text{r}}(K_{\text{J}})$	$eN_{\text{A}}$	None	Exact
Temperature of triple point of water	$T_{\text{TPW}}$	273.16 K	None	Exact	$T_{\text{TPW}}$	$T_{\text{TPW}}$	$5.7\times 10^{-7}=u_{\text{r}}(T_{\text{TPW}})$
Molar gas constant	$R$	$R$	$R$	$5.7\times 10^{-7}=u_{\text{r}}(R)$	$kN_{\text{A}}$	None	Exact
Boltzmann constant	$k$	${\frac {16RR_{\infty }}{c^{2}\alpha \mu _{0}K_{\text{J}}^{2}A_{\text{r}}({\text{e}})M_{\text{u}}}}$	$R$	$5.7\times 10^{-7}\approx u_{\text{r}}(R)$	$k$	None	Exact
Stefan–Boltzmann constant	$\sigma$	${\frac {256\pi ^{5}R^{4}R_{\infty }^{4}}{15c^{7}\alpha ^{7}\mu _{0}K_{\text{J}}^{2}A_{\text{r}}({\text{e}})^{4}M_{\text{u}}^{4}}}$	$R^{4}$	$2.3\times 10^{-6}\approx 4u_{\text{r}}(R)$	${\frac {2\pi ^{5}k^{4}}{15h^{3}c^{2}}}$	None	Exact

($c$ = speed of light, $\alpha$ = fine-structure constant, $R_{\infty }$ = Rydberg constant, $A_{\text{r}}({\text{e}})=$ electron rest mass in atomic mass units.)

References

When does/did the redefinition take place?

The redefinition is supposed to be effective today, May 20. Does it come into force at some specific hour, or is there a ceremony, or some other event that will bring in the change? -- The Anome (talk) 11:12, 20 May 2019 (UTC)

The way BIPM normally does things is to make such changes at midnight UTC and have them change everywhere in the same instant. The announcement doesn't specify the exact time but it is a safe assumption. There isn't anything specific that anyone has to do at the moment the changes came into force, so the exact time of the change isn't paricularly important. --Guy Macon (talk) 19:20, 20 May 2019 (UTC)

In the captions, the phrase "SI System" is redundant. "PIN number" is an example of a similar redundancy.

So we should remove the redundancy and call it the PI number? The S in SI is Système, and I think the I system doesn't really help. Gah4 (talk) 17:36, 18 June 2019 (UTC)

MOS:BOLDAVOID

@The Anome: SI base units should not be linked in the boldface reiteration of the article title. – Finnusertop (talk ⋅ contribs) 19:51, 21 May 2019 (UTC)

About kilogram and Ampere redefinition.

While I am not User:A.R., I must concur with him/her about for illustration purposes restating the definition of the kg to an equivalent definition that is semantically different. This actually was the proposed redefinition of the kilogram by Taylor and Mohr back in 1999 but the direction of the Metre Convention was to adopt a more terse but equivalent semantic. So I have quoted the Taylor and Mohr article, but have adjusted the actual value since the value of h they were working with back in 1999 was different. The illustrated and semantically different definitions for both the kilogram and Ampere are precisely correct and operationally equivalent. But they are different words and I made that clear in the changes. User:Quondum, I hope that you accept these edits. I believe they are encyclopedic and I know they are correct. I think they help illustrate the new definitions. Sorry that I cannot identify myself more than as IP: 50.47.109.91 (talk) 05:38, 18 June 2019 (UTC)

50.47.109.91, I have no problem with your edits. They are concise, yet contain more information, are illustrative and correct (aside from an inconsequential quibble about the rounding of an exact value). I would only say that the added text could be separated from the definition. A.R.'s edits, while well-intended, had severe issues (and I'm surprised you did not acknowledge the inaccuracy that I pointed out). I'm afraid I did not have the patience to repair the damage after reviewing related edits by A.R. —Quondum 21:39, 18 June 2019 (UTC)

"taking the fixed numerical value of the elementary charge e to be 1.602176634×10−19 when expressed in the unit C, ... this is equivalent to defining the unit charge of one Coulomb to be the equivalent charge of a collection of 6241509074460762608 elementary charges". This assertion of equivalence is arithmetically false, by about 2/9 of an elementary charge (179250736/801088317). If you're going to round it, you may as well round it down to something with a sane number of digits, like 6.24150907e18, which is the number that presumably would have been chosen if the committees had gone this way (if you reciprocate the CODATA 2017 value and uncertainty you get 6.241509074(32)e18). It seems weird and unfortunate to me that the Coulomb is an amount of charge that cannot actually exist (or maybe I don't understand the quantum Hall effect well enough - are fractional charges with 9-digit denominators possible?). — Preceding unsigned comment added by Birdfern (talk • contribs) 03:48, 19 June 2019 (UTC)

Agreed. The precise multiple is not of real significance; I think that for illustrative purposes it is sufficient to say: "this is equivalent to defining one Coulomb to be an exact specified multiple of the elementary charge". —Quondum 13:14, 19 June 2019 (UTC)

Second - frequency of radiation or frequency of transition?

• Previous definition: The second is the duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.
• 2019 definition: The second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the caesium frequency Δν_Cs, the unperturbed ground-state hyperfine transition frequency of the caesium-133 atom, to be 9192631770 when expressed in the unit Hz, which is equal to s⁻¹.

I'm puzzled by this. The previous definition makes reference to radiation. The new definition talks of the "hyperfine transition frequency", which in my mind means the frequency with which the ¹³³
Cs
atom flips back and forth between the two levels, which is saying something quite different from the frequency of radiation from this transition. So if the definition has really changed between these two things, it isn't really "effectively the same" as stated. — Smjg (talk) 22:44, 28 July 2019 (UTC)

This is somewhat unfortunate use of English in the new definition. However, the interpretation is clearly meant to refer to the frequency of the radiation. This should be clear from the SI brochure, which goes on to say: "The effect of this definition is that the second is equal to the duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the unperturbed ground state of the ¹³³Cs atom." —Quondum 23:01, 28 July 2019 (UTC)

Pretty much, this is physics as usual. Quantum mechanics says $E=\hbar \omega$. It is, then, just a unit change between energy and frequency, or specifically energy level difference and transition frequency. Gah4 (talk) 05:42, 29 July 2019 (UTC)

I fail to see any relevance of this comment to the question – or for that matter, to the definition of the second in any sense. —Quondum 11:37, 29 July 2019 (UTC)

Technically, a transition is an energy difference, between two (usually) electron states. That energy difference then translates to a photon frequency. That is why transition energy (difference) is specified as transition frequency. Could CIPM have made this more obvious? I suppose so. You will find many cases where energy and frequency are interchanged in physics descriptions. Also, energy, momentum, and mass, which differ by factors of (ignored) c. I suppose if you find a WP:RS, you can explain it differently. Gah4 (talk) 22:53, 29 July 2019 (UTC)

Ah. Why I did not see any connection is that in this context, the Planck constant has not been invoked in the definition at all, and there has been no need to refer to the energy. However, the energy difference between the two energy states translated to a frequency would be a less ambiguous definition, since there is the matter of recoil to consider when referring to an actual photon, which means that an emitted photon will have a very slightly lower energy than a photon absorbed, assuming the atom starts at rest in both cases :-) —Quondum 00:29, 30 July 2019 (UTC)

I suppose one should read it close enough to see what they say about that. Gah4 (talk) 05:05, 30 July 2019 (UTC)

@Quondum: So the frequency of the radiation is what CIPM meant to say, but it ended up being mistyped to the effect of being the transition frequency? In that case, we have a problem in that the written definition is at odds with that statement from the brochure. When the two contradict each other, which takes precedence? I would imagine the formal written definition. (If there's a law of physics that guarantees that the radiation frequency is the same as the transition frequency then it's a less serious issue. But I've no idea whether this is the case.) — Smjg (talk) 13:19, 29 July 2019 (UTC)

The way I see this is that we develop a shorthand for things we work with a lot. Even "radiation frequency" is technically ambiguous: is it the frequency at which photons are radiated, or the oscillation frequency of the photons? Here "hyperfine transition frequency" is just shorthand for "the frequency of oscillation of the EM field of the photons emitted or absorbed during the transition from one to the other of the two energy levels of the hyperfine structure". In this sense then, there is no internal contradiction, only clarification of the meaning. I don't think there is a well defined "frequency at which transitions occur" in an atom; it particular, when there are no incident photons or other sources of energy, there will not be an upward transition, only a possible downward transition. When bathed in a photon field of a suitable frequency, transitions might be stimulated, and in a quantum-mechanical sense these might even be periodic, but would be at a much lower frequency as individual photons are emitted and absorbed. Without specifying such a bathing field (including its frequency and amplitude), the concept does not apply. In any event, the method of measurement makes this clearer: it is the frequency of the radiation most strongly absorbed by free atoms when it stimulates a transition from the lower to the higher energy state of the hyperfine structure. —Quondum 19:05, 29 July 2019 (UTC)

Quondum nails it above. There is no contradiction or controversy. The concise verbiage is standard compact language to mean pretty much exactly Quondum's interpretation. One reason it is stated in this way is that the frequency (i.e. energy difference) is an inherent property of the atom itself and not just an observable. – Nick Y. (talk) 19:42, 29 July 2019 (UTC)

So the trouble is that "transition frequency" has a "standard compact language" meaning that is at odds with the literal meaning of the phrase. As such, it's liable to confuse anybody who isn't familiar with this area of atomic physics. So on this basis, I suppose we just need something in the article to clarify this. Furthermore, I've just looked at Transition frequency, and the definition given there is "The frequency at which changes in the hyperfine structure of atoms occur". So we have a problem here.... — Smjg (talk) 12:08, 30 July 2019 (UTC)

Pay no attention to that DAB page. That is horribly phrased, and the linked page does not support it. (I changed it.) There is no such thing as "the frequency at which changes in the hyperfine structure of atoms occur". However, a footnote to explain the term in the definition might be helpful. —Quondum 14:48, 30 July 2019 (UTC)

@Quondum: A footnote seems a good idea - would you like to go ahead and write it? You seem to know far more about the subject than I do. Furthermore, currently Hyperfine structure uses the phrase "transition frequency" with no indication whatsoever that it doesn't mean the frequency at which transitions occur, let alone of what it does mean. That page would be a good place for a potentially more detailed/rigorous explanation than would be written in a footnote or a disambiguation page. — Smjg (talk) 11:05, 1 August 2019 (UTC)

I've added clarifying footnotes in both places. —Quondum 23:28, 2 August 2019 (UTC)

Section "Other changes in SI"

The section 2019 redefinition of the SI base units § Other changes in SI introduces the idea of comparison of the SI as a whole before and after the redefinition, specifically to aspects that have no bearing on the redefinition of the base units. I think this risks stretching beyond the scope of this article; I wonder whether this might make more sense in an article such as Non-SI units mentioned in the SI? —Quondum 13:21, 5 November 2019 (UTC)

I, being the author of addition #1, would not mind such a change (and adding "see also"). Main point is that it better be available, sourced.

I note that in the "2019..." article, the dalton (unit) is mentioned in the intro as being problematic wrt the main changes. -DePiep (talk) 13:30, 5 November 2019 (UTC)

This last point is a good observation. I don't believe that a mention of any controversy about the impact on the dalton belongs in the lead of this article, but my earlier removal from the lead was reverted.

I'll look at writing a more expansive comparison between the units accepted for use with the SI in the article Non-SI units mentioned in the SI in the two editions of the SI brochure, after which we can review what is included here. —Quondum 14:10, 5 November 2019 (UTC)

All sounds fine, go ahead. -DePiep (talk) 14:13, 5 November 2019 (UTC)

coulomb and the Daniell_cell

I have wondered for some time how it is that the electrical units, especially the volt and amp, have convenient size. Note, for example, that a common flashlight battery supplies 1.5V at about 1A. Since these are derived from the coulomb, it must be that it was appropriately defined. It is not so hard to get factors of 2, 3, or pi into unit definitions, in addition to an appropriate power of 10. The first reference to this actually being done, that I know of, is Daniell_cell. It seems that at the time, this cell was believe to be about 1V, where now it is closer to 1.1V. Less convenient definitions might have changed this by factors of 10, 100, or 1000. (About 300 for the statvolt.) More details on how this was done would seem useful. Gah4 (talk) 00:49, 6 June 2020 (UTC)

This might be more appropriate to discuss at History of the metric system. I think the QES system was a stepping-stone, in which the mechanical base units were scaled by an effectively arbitrary power of ten, allowing scaling to a convenient size of electrical units to a suitable power of √10 within the Gaussian system. Giorgi's suggested separation of an electromagnetic dimension with the MKSA system then allowed both the "practical" (QES) electrical units to coexist coherently with a separately chosen set of mechanical base units. This is already partly captured at History of the metric system. —Quondum 02:38, 6 June 2020 (UTC)

I wrote here because I thought more would be interested here, but more because I didn't know about that one. Even so, it seems to mostly miss the origin of the volt. I will post there and see if there is more interest. Thanks. Gah4 (talk) 04:35, 6 June 2020 (UTC)

Page change

Make Redefinition of SI base units as a disambiguation page for all redefinitions. 2A02:C7F:31CF:6400:F4DF:CC40:B26C:BA04 (talk) 19:09, 26 August 2021 (UTC)

Link: Redefinition of the SI base units. At least at the moment the 2019 one is by far the most important one. That's generally what readers will look for. The other redefinitions, which were much smaller, don't even have their own articles. --mfb (talk) 10:29, 27 August 2021 (UTC)

IPK

What happened to all the IPKs? They didn't just throw them away, did they? Recycle them? I would suspect that they are still around, though maybe in museums. Gah4 (talk) 19:40, 8 June 2022 (UTC)

About redefinition of mole

It is clear form the Mole part of the Appendix 2 of the SI brochure 9th edition that Molar mass constant is infact calculated from unified atomic mass. Why don't you guys remove the discussion around ambiguity of whether unified atomic mass is defined from Molar mass constant or Molar mass constant is defined from unified atomic mass?? Aakash6022 (talk) 09:07, 10 November 2023 (UTC)