Talk:International System of Units/Archives/09/2022

Electromechanical units?

Citation: “a CGS-based system for electromechanical units (EMU)…” — shouldn’t it be “electromagnetic” units? — Wassermaus (talk) 14:51, 11 January 2022 (UTC)

That's just one of many problems with that section, which contains quite a few statements that are at best misleading. For example, it is misleading (at best) to say that the ESU system of units is 'a system for electrostatic units': in fact, the ESU and the EMU systems each provide units for all electric and magnetic quantities. --Reuqr (talk) 05:29, 21 January 2022 (UTC)

They do. But units have to agree with Maxwell's equations. ESU units are based on the force between charges, while EMU units are based on the force (per unit length) between currents. You get one or the other, not both. Gah4 (talk) 19:27, 1 September 2022 (UTC)

RfC on entries in tables

Should entries in tables ("the speed of light"; not a constant) that conflict with title of the table ("si defining constants") include extra context ("in a vacuum"), as is done in the paragraph of the article body, for clarity? Marrew (talk) 18:21, 13 August 2022 (UTC)

• Support For 'speed of light in a vacuum' only. It won't add much to the table and some people are quite likely to quibble over it. NadVolum (talk) 19:38, 13 August 2022 (UTC)
• Support for entries for which it applies, but better done as footnotes instead of repetitive or wordy extra content inside the table.  — SMcCandlish ☏ ¢ 😼  23:57, 13 August 2022 (UTC)
• Oppose The article makes the matter quite clear. Xxanthippe (talk) 00:29, 14 August 2022 (UTC).
• Oppose The article is clear and c is the speed of light. This is standard vocabulary. The unqualified speed of light is the speed of light in vacuum, unless otherwise specified. In prose we have more room, so we can choose to expand if we want, but it is not required. Headbomb {t · c · p · b} 01:09, 14 August 2022 (UTC)

  The addition of "in a vacuum" would still leave this entry shorter than two other items in the same column of the table. Should they be somehow shortened as well? Marrew (talk) 15:03, 14 August 2022 (UTC)

  Also, insisting that "this is standard vocabulary" does not help those who are unfamiliar with the topic. Marrew (talk) 15:48, 14 August 2022 (UTC)

• Oppose. You are confusing the name of the constant with its description. Please don't clutter the table. Tercer (talk) 06:28, 14 August 2022 (UTC)

I think 'Name' in the heading for the column should be replaced by 'Defining constant' as in the standard in ref 1 pages 127 and 128. They never give names to the defining constants though they do give symbols and in the table and most other places refer to 'speed of light in vacuum'. The units are explicitly given names. What kind of 'name' is 'luminous efficacy of 540 THz radiation' or 'hyperfine transition frequency of Cs'? NadVolum (talk) 07:42, 14 August 2022 (UTC)

Indeed, "constant" or "defining constant" would work better there. Tercer (talk) 08:53, 14 August 2022 (UTC)

The "speed of light" without the extra context of "in a vacuum" also makes the heading and entry of the third column problematic. The speed of light without the extra context of "in a vacuum" does not have an "exact value." Marrew (talk) 15:01, 14 August 2022 (UTC)

"speed of light" is the name of the fundamental constant. By definition, it has an exact value, 299 792 458 m/s. You are confusing this with the speed at which a photon travels, which will depend on the medium it is travelling through and the curvature of spacetime. Do we need to also include the qualification "in flat spacetime"? Tercer (talk) 15:36, 14 August 2022 (UTC)

You are trying to confuse the issue with something unrelated here. The issue is that the speed of light without extra context is not a constant, and listing it in a table of constants is misleading at best. Marrew (talk) 15:43, 14 August 2022 (UTC)

• Support (Summoned by bot) for entries for which it applies including 'speed of light in a vacuum' or as a footnote as suggested. Whilst physicists may take it as a given that the speed refers to 'in a vacuum' - those of us whose physics is very rusty would benefit from the reminder/clarifier and it adds three short words only. Pincrete (talk) 07:35, 14 August 2022 (UTC)
• Oppose In tables, it's needless and counterproductive clutter. The explanation is in the text, where it belongs. Footnotes that just recapitulate what the text already says are also clutter. XOR'easter (talk) 12:39, 14 August 2022 (UTC)
• Support, but in the table it should be a footnote. For someone working in optics the distinction is crucial. --Shmuel (Seymour J.) Metz Username:Chatul (talk) 13:05, 14 August 2022 (UTC)

  Wouldn't someone actually working in optics ... already know? What audience is benefited by the addition? Surely not the people already going around with tables of refractive indices in their heads. XOR'easter (talk) 13:18, 14 August 2022 (UTC)

  What about people unfamiliar with the topic? Aren't they the target audience for any article on wikipedia? Marrew (talk) 15:05, 14 August 2022 (UTC)

  No, they're not the only target audience, and overcomplicating a simple thing does not serve them well, either. XOR'easter (talk) 19:48, 14 August 2022 (UTC)

• Oppose When people say "speed of light", they almost always mean the maximal speed of causality which is the speed of light in a vacuum. The speed of light in a material medium only appears to be less than c because some of the radiation is absorbed and re-emitted after a short delay or reflected at a shallow angle (thus taking a longer path) off imperfections in the medium. JRSpriggs (talk) 19:50, 14 August 2022 (UTC)

  Causality? What if there are tachyons? --Shmuel (Seymour J.) Metz Username:Chatul (talk) 07:03, 15 August 2022 (UTC)

To Chatul: What if 2+2=5? JRSpriggs (talk) 02:47, 16 August 2022 (UTC)

Are you claiming to have proven that tachyons don't exist? If not, your question is irrelevant. Absence of proof is not proof of absence. --Shmuel (Seymour J.) Metz Username:Chatul (talk) 11:47, 16 August 2022 (UTC)

The absence of a rational argument is not evidence for alleged existence of a non-existent. JRSpriggs (talk) 15:35, 16 August 2022 (UTC)

• Oppose. The RFC statement above is overbroad. We should be discussing the specific issue raised in the section above, not this RFC.--Srleffler (talk) 23:16, 14 August 2022 (UTC)
• Oppose There is an interest in keeping tables concise for ease of reading and display. When "speed of light" is used without qualification, it universally refers to the speed of light in a vacuum. Qualification would only be necessary if speed of light in a media were being used. Making the point explicit is fine in the body of the article where there is unlimited space, but is absolutely unnecessary or desirable in the table PianoDan (talk) 17:51, 15 August 2022 (UTC)

• Oppose. Speed of light in a vacuum is a universal constant, and as such was included in the International System of Units. That it is in a vacuum is tacitly implied in its definition. It is not necessary to complicate the table by making redundant clarifications. It is possible to make a clarification in the text. But the clarity and simplicity of the tables is important for their quick and correct verification. AteneaZ3 (talk) 11:49, 19 August 2022 (UTC)

If you have a look at the standards document [1] 'in a vacuum' is not assumed even in their tables in the standards part. It is only left out sometimes in the introduction and the listing of their resolutions. NadVolum (talk) 12:50, 19 August 2022 (UTC)

This example has the luxury of being a PDF document (or printed as a booklet) so it has plenty of space to construct definitions in any format the author(s) choose. This is not the same as the limited space available for a table in this article. Also, the table in this article is for quick reference and is good for summation-at-a-glance. ---Steve Quinn (talk) 19:38, 21 August 2022 (UTC)

• Oppose as phrased - I oppose extra context inside the table as clutter, but suggest changing the name to “speed of light in vacuum” (not in “a” vacuum) to match what is given as the phrasing in BIPM (here) and NIST (here). Cheers Markbassett (talk) 22:29, 19 August 2022 (UTC)

• Oppose. c is understood to be the speed of light in a vacuum and is a constant. No need to unnecessarily clutter the table. Also, JRSpriggs has aptly explained what the nominator seems to be describing in their above posts. ---Steve Quinn (talk) 19:59, 21 August 2022 (UTC)

• Oppose. excessive for a compact table in which it's helpful to our readers to be succinct. It's correctly linked there, so any reader sufficiently informed to find it ambiguous can find instant clarification, and we carefully supply "vacuum" seven times in the text and another larger table. The risk that readers who are aware that the speed of light is different in different in different media will imagine that the metre might be defined according to its speed in anything but a vacuum and that honest confusion would survive further reading of the article is negligible. NebY (talk) 10:45, 24 August 2022 (UTC)