Wikipedia:Reference desk/Archives/Science/2022 June 5

= June 5 =

Why is it called lime?
None of the articles referring to limestone or Agricultural lime explain the term. I found a definition but don't know what to do with it. It seems like it should be mentioned on all those pages but that would be too much.— Vchimpanzee  •  talk  •  contributions  •  18:38, 5 June 2022 (UTC)


 * It gets a mention in lime (material). First paragraph. Heh, and I see the short answer is "because it's slime", like when you're using it as cement. Card Zero  (talk) 18:50, 5 June 2022 (UTC)
 * I guess that's enough for Wikipedia.— Vchimpanzee  •  talk  •  contributions  •  21:15, 6 June 2022 (UTC)
 * I feel faintly irritated that the limestone article is useless to the visitor who wants to know "why is it called limestone". Does it have a citrus aroma? Is it sometimes bright green? Instead of easily finding out about the name, they will have to read about CaCO3, 540 million years, dolomite and magnesium and karst and toothpaste, before finally getting to the little link to lime in the middle of the third paragraph, which is only likely to be noticed by people who already know the answer. I also dislike that it says "a chemical feedstock for the production of lime", which is probably technically correct, but seems to me to be needlessly opaque jargon. I want it to say something like "limestone is used for making lime", near the top. OK I made the change. I have a cynical feeling that somebody will be upset by change and will revert it. Card Zero  (talk) 02:26, 7 June 2022 (UTC)


 * Further info, and it may in fact be cognate with "slime". --←Baseball Bugs What's up, Doc? carrots→ 18:55, 5 June 2022 (UTC)


 * The entry in OED for lime has a note: "(In Old English any adhesive substance, e.g. glue, paste.)" See for example birdlime. But there is no specific mention in the etymology of a relation to slime as in the link above. A second note states: "another grade of the root occurs in loam n., lair n." The slime entry, however, states: "The stem is probably related to that of Latin līmus." Which google translate claims is the latin word for clay. An example usage from 1530 is "That slyme was a fatnesse that issued out of the earth, like vnto tarre; and thou mayst call it cement, if thou wilte." --mikeu talk 19:51, 5 June 2022 (UTC)

Improving the accuracy of certain Planck units
While reading through the above article it occurred to me that one should ostensibly be able to calculate the speed of light (exactly 299792458 m s^-1) by simply dividing out Planck length (1.61625518e-35 m) by Planck time (5.3912476e-44 s). In fact the absolute difference between the computed value of 299792422.815 m s^-1 and the current fixed value turns out to be -35.1849172711 m s^-1 so a respectable relative error of just -1.17364250942e-05%. Which isn't terribly bad obviously. But perhaps it would be possible to use something like the Von Klitzing constant or the fine structure constant to work our way backward (so to speak) to obtain even more precise values for these two Planck units? Earl of Arundel (talk) 20:10, 5 June 2022 (UTC)
 * First note that the "exact" character of the speed of light in SI units is an artifact of the definition of SI units. It isn't true, in practice, that we know the speed of light exactly.  Because of the SI definitions, that uncertainty needs to be phrased in terms of our uncertainty as to how large a meter and/or a second is against our real-world artifacts.  But this is a fact about SI; it's not a fact about the speed of light.
 * The Planck units are defined in terms of the speed of light c, Planck's constant h, and the gravitational constant G. The one we have the poorest estimate of is G, which per our article we know to about four significant figures.  That is the limiting factor as to how well we know the Planck units relative to real-world artifacts, and you can't do better than that until you have a better estimate of G. --Trovatore (talk) 21:36, 5 June 2022 (UTC)
 * I do understand the inexact nature of measurement with respect to the speed of light itself, insofar as precise lengths and durations go. As you say, a much better approximation of G would help. But what are the chances of improving upon that? Earl of Arundel (talk) 00:33, 7 June 2022 (UTC)
 * Dimensional analysis shows that knowledge of the values of either the Von Klitzing constant or the fine structure constant, or for that matter of both, is not of any use here. --Lambiam 19:13, 6 June 2022 (UTC)
 * Perhaps it could be approached like a maximization problem then? We have a fixed value for c, so for example just start from the current hypothesized value of Planck length then gradually adjust until its ratio with Plank time approaches the digits of c? Or vice versa. Either way, an error rate even better than ~0.00002% should be possible. No? Earl of Arundel (talk) 00:33, 7 June 2022 (UTC)
 * Just to illustrate, fix the Plank length at 1.61625518e-35 m then adjust the Planck time to 5.391246967e-44 s. That yeilds an error of less than 0.000000005%. Conversely, by adjusting the former to 1.61625537e-35 m and holding the latter instead fixed at 5.3912476 s, we achieve an error rate slightly below 0.00000002%. Maybe there is some intelligent way to explore the entire space of solutions? I don't know, but just fixing one of the two does at least bring their ratio a little bit closer to the speed of light. Earl of Arundel (talk) 00:48, 7 June 2022 (UTC)
 * No one has measured the number of Planck times in a second. We only know its magnitude in seconds by measuring G, taking the square root of the magnitude obtained, expressed in units N m$2$⋅kg$–2$, and multiplying the result by 6.59912452×10−39. Since the precision to which G is known is rather limited, so it is for the magnitude of t&thinsp;P. It could be as little as 5.3911×10−44&thinsp;s or as much as 5.3913×10−44&thinsp;s. Since the ratio between t&thinsp;P and ℓP is fixed by definition, it is pointless to attempt to tweak their ratio by tweaking one of the two, just as you cannot tweak the ratio between h and ħ by tweaking the value of h. --Lambiam 05:52, 7 June 2022 (UTC)