Talk:Speed of light

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Precision of values in infobox[edit]

There’s a hidden comment in the infobox reading “This section lists various values for c, to three significant digits. Please do not change to more exact values!”, but then values are given to six or seven significant digits. Shall I round off the extra digits, or shall I remove the comment? I have no strong preference either way. — A. di M.  09:26, 5 November 2014 (UTC)

Thanks for catching this. It appears that in early October an editor ignored the comment and put in more precise values. The pre-October values should be restored (and the notice left in place). Other changes were made simultaneously in the infobox that may be desirable to retain. I would suggest examining them carefully with that in mind, rather than doing a full revert. If you would like help I would be happy to assist. Hertz1888 (talk) 10:42, 5 November 2014 (UTC)
I've rounded them off to three sigfigs and left the rest of the table alone. — A. di M.  11:16, 7 November 2014 (UTC)
I restored the extended values becuase:
  • They give more significant digits, so that it can be used in calculations in addition to giving a mental picture of the scale of c.
  • Use less space, so that some lines aren't wrapped in the infobox.
  • Avoid mixed representation of numbers with both digits and letters
  • An hybrid representation just use more space for no benefit. Expressing numbers with digits is precise, concise and gives all the required information it in a format that can be easily understood.
  • Representing numbers with letters or using an hybrid representation makes it harder to people just learning English to understand. Digits on the other hand are used almost worldwide with only the decimal separator possibly changing between languages and cultures. Also, it makes the representation non uniform and needs more arbitrary choices: For instance, 1.07×1011 may be written “one hundred seven million”, “107 million”, etc...
Regards. Mario Castelán Castro (talk) 14:40, 7 November 2014 (UTC).
You make some good points. However, the approximate values section has been there a long time, is provided for the convenience of those who don't need high accuracy or may lack familiarity with scientific notation, and was discussed previously at Talk:Speed of light/Archive 16#Rounded speeds, and possibly prior to that. Your change (which escaped notice initially on 2 October) represents a shift in policy. I think we need to allow ample time for others to comment and a consensus to develop. I oppose the change because I feel the readership, overall, is best served by the existing structure and content. Hertz1888 (talk) 15:27, 7 November 2014 (UTC)
I agree with Hertz1888. - DVdm (talk) 15:36, 7 November 2014 (UTC)
So do I. Stigmatella aurantiaca (talk) 16:36, 7 November 2014 (UTC)
DVdm, Stigmatella aurantiaca: Please note that according to (policy) Wikipedia is not a democracy, (policy) Wikipedia:Consensus, (policy) Wikipedia:MAJORITY and (essay) Wikipedia:Polling is not a substitute for discussion, comments stating an opinion without arguments are meaningless in what regards to reaching consensus for the resolution of disputes.
Hertz1888: Your edit message contains a mistake in the part it says “No consensus for change in policy.”. Nobody is trying to change a policy regarding this article, at least not me or anyone else as far as I can tell. We had reached consensus for the version of the infobox previous to your edit by virtue of the fact that it was undisputed between 2 October and 7 November of this year. Note that we currently have 553 users watching the page. We can count on some of them them being active. As a summary in chronological order:
  • Previous to the events listed there, consensus had been reached on the digit/words hybrid and very rough figures for the infobox.
  • 2 October 2014: I make an edit overhauling the infobox. Immediately after the edit is saved it's not know yet whether we will have consensus on this.
  • More than 1 month elapses, and among the 553 watchers and among the humans and machines that cumulatively viewed (or at the very least, downloaded the page) this page from Wikipedia more than 100 000 times (some days in the interval are not counted there) nobody disputed my change, thus we have reached a new consensus. In spite of this, Hertz1888 later claims that this edit “escaped notice”.
  • 5 November 2014: A. di M. asked whether to change infobox to the previous deprecated consensus.
  • 5 November 2014: Hertz1888 effectively answers yes.
  • 7 November 2014: Today A. di M. changes the infobox. His proposed version is the same as the one used previous to this summary. Immediately after the edit is saved it's not know yet whether we will have consensus on this.
  • Same day: I dispute the edit above and revert it while giving several arguments in this talk page, meaning that we do not have consensus on the above edition.
  • Same day: Hertz1888 restores A. di M. version for which there wasn't consensus and citing a talk page corresponding to the time of the infobox version previous to 2 October, whose consensus has already been superseded by consensus on the 2 October version.
I have exposed my arguments for keeping the version for which we had consensus just before this dispute. Currently, those arguments remain unchallenged. Please expose your arguments on this point. Given that you're concerned that scientific notation is not understandable by a wide audience, I will put link to scientific notation in a note. This gives those readers a pointer to something useful and new for them to learn about, just as we currently talk about relativity, quantum mechanics, and electrodynamics. Note that we don't explain either of these subjects in this article but we link to their respective pages.
Regards. Mario Castelán Castro (talk) 17:49, 7 November 2014 (UTC).
Because no one caught and commented on the major change when first made does not make it acceptable. That is not how consensus is achieved. We are, perhaps belatedly, discussing the change currently; meanwhile please refrain from repeatedly reverting and then accusing others of edit warring. What you have referred to as "my" proposed edit is essentially how the infobox stood for longer than four years, except for recent weeks. You have been reverted; please be guided by WP:BRD. Hertz1888 (talk) 18:28, 7 November 2014 (UTC)
Hi Mario, regarding those pointers to various policies, please note that in such matters the only relevant policy is wp:CONSENSUS. There seems to be no consensus on this talk page for the changes that you propose. Therefore, per the second bullet of wp:NOCONSENSUS, this commonly results in retaining the version of the article as it was prior to the proposal or bold edit. Cheers - DVdm (talk) 19:37, 7 November 2014 (UTC)
It is in the nature of Wikipedia that we don't always get what we want. I recently made a change to this article that was reverted, and in the ensuing discussion on this Talk page, it was clear that although a significant number of editors agreed with my views, a significant number disagreed as well. As you pointed out, Wikipedia is not a democracy, and there was no means of, or indeed, any point to attempting an exact poll of how many editors agreed with me, and how many disagreed. I just had to accept the fact that there was no consensus supporting my edit, and went on to other things. Not always getting what we want is just one of those things that takes a bit of getting used to. Stigmatella aurantiaca (talk) 09:09, 8 November 2014 (UTC)
I also support the appropximate values. We have provided the exact value in SI units, someone wanting precise calculations in other units can convert the exact value to the precision they need as necessary. The rounded figures are much easier to think about or for quick calculations. I imagine the first thing most readers would do (if anything) with six-figure numbers would be to work out an approximation anyway. --Mirokado (talk) 18:37, 7 November 2014 (UTC)
Mirokado's last point is especially well-taken. Approximate values are best for the infobox. EEng (talk) 20:33, 7 November 2014 (UTC)

Dimensionless value[edit]

Can the speed of light be expressed in a dimensionless way or unit? (by comparison to the speed of sound)-- (talk) 15:17, 19 January 2015 (UTC)

exactly 299792458 metres[edit]

Is this really exact? What about differences in micrometres, nanometres and so on? -- (talk) 20:28, 3 February 2015 (UTC)

The last two sentences of the lead read: "In 1983, the metre was redefined in the International System of Units (SI) as the distance travelled by light in vacuum in 1/299792458 of a second. As a result, the numerical value of c in metres per second is now fixed exactly by the definition of the metre." Exact means mathematically exact, zero error at any level. I don't see that this can be made clearer in the article. —Quondum 20:46, 3 February 2015 (UTC)
They measured the speed of light first, so they must've based the distance light travels in one second on the metre defined before. Then they used this to redefine the metre itself and the new metre just happened to work with all previous derived calculations without causing decimals? What about the measurement uncertainty mentioned? -- (talk) 21:40, 3 February 2015 (UTC)
Despite the history, the metre was defined afresh. While the second and hence the metre have measurement uncertainty associated with them, the ratio of the two now has no uncertainty in terms of the speed of light, and vice versa. This page is not the place to discuss this; if you want help with this ask at Wikipedia:Reference desk/Science. —Quondum 22:02, 3 February 2015 (UTC) Yes, it is exact, by definition. Once you define an unit to be 1/X of Y, then Y is exactly X of that unit. About uncertainty: When an unit is redefined in terms of a different physical constant, the ratio of the new physical constant to the old physical constant (on which it was based) is measured to the best available accuracy and that's taken as the base of the new definition, so as to minimize the change in magnitude of the unit. This makes the value of the old physical constant subject to uncertainty (it was exact with the former definition), and the new physical constant exact (it was subject to measurement uncertainty). The impact of the redefinition on the uncertainty and magnitude of other physical constants is non-trivial but it is negligible for all every-day purposes, most engineering and most scientific purposes. See New SI#Physical constants used in the new definitions and other physical constants. Also bear in mind that only dimensionless numbers can be measured (E.g: the ratio between one's own mass and that of the IPK), even if they're multiplied by a dimensional constant for presentation (E.g: the kilogram in this case, which is by definition the mass of the IPK, of course). Regards. Mario Castelán Castro (talk) 01:08, 4 February 2015 (UTC).

Exact values[edit]

I was looking for the exact value in km/h and mph of the speed of light and couldn't find it in its article, so I edited it. Hertz1888 reverted the edit.

Though it's fair that the lede is not the appropriate place for it, as it's one of the fundamental constants, and both km/h and mph are common units, does anyone think the exact values should appear somewhere in the article?

By the way, the infobox contains the values to only 3 significant figures.

Thanks, cmɢʟeeτaʟκ 19:38, 10 February 2015 (UTC)

Exact km/h gives 1,079,252,848.8, with that decimal. Afaiac, we could add that in the infobox, but there are other opinions—search talk archives.
For "exact" mps, see Speed of light#cite note-14 and Speed of light#Numerical value, notation, and units. For mph multiply by 3600, and recalculate. Problematic, if not ugly, if not silly, right? Face-smile.svg DVdm (talk) 22:31, 10 February 2015 (UTC)
Maybe I am missing the point, but I fail to see any practical value to providing the exact value converted into units other than the official m/s. Anyone who would need those to the ultimate decimal place presumably would have the ability to do the necessary multiplication. Hertz1888 (talk) 05:28, 17 February 2015 (UTC)


To DVdm: Just curious as to why there are 26 occurrences of poor grammar in this article, and you would incorrectly "correct" the one instance I changed rather than to correctly correct the other 25 instances? Just to be clear, the Latin "in vacuo" translates in English as "in a vacuum" (not "in vacuum"). – Paine  14:48, 21 April 2015 (UTC)

Not necessarily poor grammar. I believe it is simply conventional scientific (and other) usage, just as one would say "in space", "in air", or "in water". If I recall correctly, this topic has been discussed here before, with the consensus evidently to leave "in vacuum" in place. You might want to check the page/page archives. As used in the phrase, vacuum is a condition, not a location. Hertz1888 (talk) 15:10, 21 April 2015 (UTC)
To Paine Ellsworth: This has been discussed before. See (for instance) Talk:Speed of light/Archive 16#A vacuum vs. vacuum. F.w.i.w, see NIST. And compare Google books "speed of light in vacuum" (81.100) vs "speed of light in a vacuum" (97.600)
By the way, I don't think that you can know for sure that the Latin "in vacuo" translates in English as "in a vacuum" (not "in vacuum"), as there are no articles in Latin. "In vacuo" can just as well translate as "in the vacuum". - DVdm (talk) 15:32, 21 April 2015 (UTC)

──────────────────────────────────────────────────────────────────────────────────────────────────── Thank you both for your responses! Not to put too fine a point on it, I've read hundreds of science papers and heard dozens of lectures and have never seen nor heard it used this way, so I seriously doubt that its usage is "conventional" among scientists. Einstein often used "in vacuo" in his writings, but I've never seen any translation that used "in vacuum". It is always either in vacuo or in a vacuum. The usage should probably be reliably sourced, at least here on the talk page, but I've grown too old to fight those battles. The bottom line for me is that it is either poor grammar or it is too-technical language for general readers. To average readers, I would bet that when they come across "in vacuum", they often say to themselves "that doesn't look right". (rhetorical ?:) Is that what we want them to be thinking when they should be thinking about the article's subject? Thank you! and Best of everything to you and yours! – Paine  15:46, 21 April 2015 (UTC)

Did you have a look at the results of "speed of light in vacuum" with Google Scholar (31.100) and Google Books (81.100)? Plenty of reliable sources there, right? Face-smile.svg. - DVdm (talk) 15:59, 21 April 2015 (UTC)
Given the rate of occurrence of the phrase "in vacuum" in the literature (Google books gets 1,150,000 hits), you might consider that this perception might just be confirmation bias? Face-smile.svgQuondum 16:04, 21 April 2015 (UTC)

──────────────────────────────────────────────────────────────────────────────────────────────────── That last one was a real good bait. Reliance upon Google over standard grammar teachings and practice is folly to say the least. Thank you! and Best of everything to you and yours! – Paine  16:54, 21 April 2015 (UTC)

.... but but... we don't rely on Google... We rely on the 31.100 + 81.100 listed sources, which are directly relevant to this article. If all these sources would call it "da zpeed of lit in vac", we would be talking here about "da zpeed of lit in vac". And grammar could go to hell, right? Face-smile.svg. - DVdm (talk) 17:37, 21 April 2015 (UTC)
"speed of light in vacuum" sounds more familiar to me, and the expression means the concept "vacuum", not a specific vacuum in some lab. But as the reference search shows, both are used. --mfb (talk) 17:57, 21 April 2015 (UTC)