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"Einstein's proof that the speed of light is the most constant dimension in the universe"[edit]

Should "Einstein's proof that the speed of light is the most constant dimension in the universe" be revised?

1) Since the speed of light changes with medium (Jean Foucault 1850) is it a constant?
2) Is the speed of light properly called a "dimension" like height, length, width, & time?
3) "in the universe?" Did Einstein explore every corner of the universe to "prove" this?
4) Proof? Does this statement confuse "the best known explanation based on evidence" with proof? (PeacePeace (talk) 15:27, 21 September 2017 (UTC))

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I think that the colour of caesium is due to relativistic quantum chemistry, but this is not mentioned in the article. Axl ¤ [Talk] 09:32, 6 April 2018 (UTC)

Relativistic effects are fairly small at Z = 55 (only a little earlier Ag is silver as would have been expected without including relativity at all). A better explanation is that the plasmonic wavelength of Cs creeps into the violet end of the visible spectrum (it is in the ultraviolet for most metals) and hence some violet light is absorbed, giving a yellowish colour. A weaker effect may be present for Ca, Sr, Ba, Eu, and Yb, which according to Greenwood and Earnshaw are pale yellow. Double sharp (talk) 09:56, 6 April 2018 (UTC)
Would you add this information to the article, please? Axl ¤ [Talk] 08:39, 7 April 2018 (UTC)
@Axl: I've added it. This incidentally makes me wonder if anyone has tried to calculate what the colour of francium might be, because then relativistic effects should become important. If the transition is to the lowest excited state (which is p1 instead of s1), then relativistic effects widen the 7s/7p gap enough that the energy required to excite the electron goes back up, so that the value of Fr is close to the value for Rb. This bit of OR suggests that francium should be silvery, but unfortunately I can't find an actual source for this (and according to the ref I added the exact electron transition that creates the golden colour of Cs was not known at the time, so it is probably not this simple). Double sharp (talk) 16:59, 7 April 2018 (UTC)
Thank you for adding the statement. Could you perhaps also include the reason why violet (and not ultraviolet) light is absorbed? I shall leave the investigation of francium to you. :-) Axl ¤ [Talk] 20:58, 7 April 2018 (UTC)
Ultraviolet is absorbed too. Ultraviolet and violet are below the plasmonic wavelength of Cs and so for those wavelengths Cs preferentially transmits light rather than reflects it. The transmission is not perfect so you will not see transparency unless you prepare a very thin film of the metal, with a thickness of a few atoms. ^_^ Double sharp (talk) 10:12, 8 April 2018 (UTC)
@Axl: I have expanded the explanation, in the process correcting my awful mistake in typing it up (the lower-frequency colours are preferentially reflected and not absorbed). T_T Double sharp (talk) 15:24, 8 April 2018 (UTC)
Thank you. Axl ¤ [Talk] 17:10, 8 April 2018 (UTC)

Why could it have -1?[edit]

It is the least electronegative... So why??? Alfa-ketosav (talk) 15:59, 7 July 2018 (UTC)

Just because an atom or element itself might be the least electronegative among the elements doesn't mean there could not be other less-electronegative combinations or ways of forcing electrons onto it. The electronegativity is not "zero". See the cited ref to learn about the chemicals that give this result. DMacks (talk) 16:09, 7 July 2018 (UTC)
Says nothing about which compound... I know the EN is always positive, and I see the cited ref about Cs
... and even Fr is more electronegative. Alfa-ketosav (talk) 18:05, 7 July 2018 (UTC)
The ref (doi:10.1002/anie.197905871) seems quite likely to tell you examples of such chemicals. And refs to more articles about them. Somewhere along that chain will definitely be exact procedures for how thay are made. The "why" of your question doesn't make sense..."because in the product that someone made, there are 56 formal electrons assigned to the caesium atom." If you mean "how", then you'll need to follow the cited ref, and refs it cites, and so on... A chemical need not be stable in order to exist. It could be "metastable"...stuck in a high-energy way with no direct way to resolve the problems. Electron configuration can be changed by a variety of factors. Here (doi:10.1038/ncomms5861) is using pressure to raise the energy of lithium's valence electron, giving it the ability to donate to caesium. Possibly up to –2(!). Here (doi:10.1002/rcm.4913) is another approach, using decomposition of carboxylate salts. There is (as I suggested earlier) a more unstable electron donor than a simple "less electronegative" atom. DMacks (talk) 19:58, 7 July 2018 (UTC)
Actually wait, that first ref might actually answer your question as asked: "because under high pressure, Li becomes a better electron donor and Cs does not, so a Li–Cs bond becomes polarized as Liδ+–Csδ-". DMacks (talk) 20:33, 7 July 2018 (UTC)
Alkalides are known, such as [Cs(cryptand-222)]+•Cs-. Burzuchius (talk) 20:28, 23 July 2018 (UTC)