LaTeX symbol for c_0

[VintageRoses]

There has been a lengthy discussion on SE that may be relevant for the siunitx package here on GitHub.

The issue has to do with how to properly represent the speed of light (typically represented by an italic 'c') in a vacuum (which according to the accepted answer provided on SE) could be represented by c_{\scriptscriptstyle 0}.

The SE discussion includes a graphic from 'The International System of Units (SI)'. The graphic shows how the SI portrays the 'speed of light in a vacuum' symbol, but the OP is not convinced that the LaTeX solutions presented on SE, although very close to the symbol that appears in the graphic, are the most appropriate solution.

Given the unique significance of the 'speed of light in a vacuum' and how the meter itself is defined relative to the 'speed of light in a vacuum' as:

The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.

Would it be appropriate for the siunitx package to include a proper LaTeX symbol to represent the 'speed of light in a vacuum'?

The image presented for what appears in the SI as a ‘reference rendering’ is:

Thanks for reading.

[josephwright]

The symbols I include in siunitx are those needed for units. In the latest edition of the SI, the speed of light does not feature as a unit, and therefore this is out-of-scope.

At the same time, I think one should be wary of reading too much in to the typography of the SI Brochure. It's focussed on the metrology, and it's not typeset to a particularly high standard. For example, typographic tradition is that a value and unit are separated by a thin space: this reflects the fact this is one mathematical entity. However, the Brochure simply uses a full-width space. I suspect this reflects the tools used to create the Brochure. I would look at the IUPAC Gold book: https://goldbook.iupac.org/terms/view/S05854. There they use the same sized subscript as I'd expect: c_{0}.

[josephwright]

You might also look at the Green Book: same situation - https://iupac.org/what-we-do/books/greenbook/.

[VintageRoses]

Thank you for the updated reference links. Very informative.

Online, the IUPAC Gold Book displays both symbols for the 'speed of light in a vacuum', e.g.: c _0, and c.

While the BIPM, The International System of Units, SI Brochure, 9th ed. (2019), p. 127ff makes no mention that I can see of the speed of light through a vacuum as c _0, the Online PDF release of the Green Book (Quantities, Units, and Symbols in Physical Chemistry, IUPAC Green Book, 3rd edition, IUPAC 2007) has almost two dozen mentions of the c _0 nomenclature in their symbols list index (p. 184), e.g.:

Given the more recent SI publication date, does this indicate that there a shift away from the c _0 nomenclature, or is the difference simply a result of the tenor between the physics and chemistry of c? I'm also a bit confused as to the Gold Book's use of both c _0, and c.

Since 'the speed of light does not feature as a unit, and therefore this is out-of-scope', is there a LaTeX repository that exists for constant symbols such as the speed of light in a vacuum, either as c _0 or c?

Thank you very much for the links and your comments, (still confused!)