Talk:Q83327

Unit of mass?

@Infovarius: In the SI, the electronvolt is a unit of energy (SI Brochure (9th edition) (Q68977219), page 145). Energy and Mass in the SI have incompatible dimensions, therefore this item can't be a unit of energy (Q2916980) and unit of mass (Q3647172) at the same time. Toni 001 (talk) 09:22, 8 March 2020 (UTC)

But it is used as such. How to reflect this? --Infovarius (talk) 20:45, 9 March 2020 (UTC)

Energy and mass have the same dimensions in units of c=1 (in fact, even without resetting c, one can think, due to the energy-mass equivalence principle that they measure the same thing, differing only in a factor). I agree, it's not SI, but SI is only a standard and it's not uncommon for mass values to be given in natural units, as in Review of Particle Physics (Q56810046), for example. I personally favour keeping the possibility of using electronvolt as a unit of mass, since that allows for faithfully transcribing the values given in those type of sources (going to the extreme of using megaelectronvolt (Q72081071) instead of converting to electronvolt (Q83327)), thus avoiding introducing further conversion errors or altering the display of the data unnecessarily. All that being said, this question does raise the question of which standard is in use. I unconsciously used this item as if it was obvious from the context, but it's not something I have the right to assume. I presume this is a case when a qualifier would come handy. - Sarilho1 (talk) 20:58, 10 March 2020 (UTC)

Or do you think electronvolt for energy and electronvolt for mass should be two separate items? I usually don't like to dwell much on those almost philosophical question in Wikidata, but I guess that's a possibility too. - Sarilho1 (talk) 21:03, 10 March 2020 (UTC)

I don't have a suggestion yet. The overall question seems to be: How do we represent different unit systems side by side? Toni 001 (talk) 08:04, 11 March 2020 (UTC)

About 5 years ago I started this discussion https://www.wikidata.org/wiki/Wikidata_talk:WikiProject_Physics#Rethinking_the_way_of_describing_units. It looks like not much has changed since then. For stating quantities I would always stick to SI dimensions, in this case writing eV/c^2. Imagine a length in seconds in a system of units that has speed of light = 2pi. It seems a stupid example, but for different electromagnetic unit systems you have things like that. Does anyone really think the average editor would be capable of guessing the name of the system of units that was used in the source add it as a qualifier and then some algorithm would be able to pick that up and e.g. sort items by length?--Debenben (talk) 21:22, 24 June 2021 (UTC)

Physical constant?

@Toni 001:As far as I know the distinction between a unit and a constant quantity is only a somewhat loose convention. In case of electronvolt, especially NIST should have lots of publications with statements of the form $a=123\,\mathrm {eV}$ where eV is written upright as a unit and a italic like a physical constant / quantity. I don't know, maybe it was inconvenient to make a distinction in that list. However if eV is a physical constant then every other unit is a physical constant, too and from that viewpoint the label as a unit (as a special form of a physical constant) is sufficient.--Debenben (talk) 20:55, 24 June 2021 (UTC)

I agree that the distinction between unit and constant seems to be a convention (as each constant - and products of powers of them - could be used as unit and units are constant). From a practical point of view, items which are units of measurement have properties conversion to SI/standard unit (P2370 or P2442) while physical constants have a numerical value (P1181). I'm wondering whether we could update our model somehow to not depend that much on convention. Toni 001 (talk) 08:07, 25 June 2021 (UTC)