# Talk:Speed of light

Speed of light is a featured article; it (or a previous version of it) has been identified as one of the best articles produced by the Wikipedia community. Even so, if you can update or improve it, please do so.
This article appeared on Wikipedia's Main Page as Today's featured article on October 29, 2004.
WikiProject Physics / Relativity  (Rated FA-class, Top-importance)
This article is within the scope of WikiProject Physics, a collaborative effort to improve the coverage of Physics on Wikipedia. If you would like to participate, please visit the project page, where you can join the discussion and see a list of open tasks.
FA  This article has been rated as FA-Class on the project's quality scale.
Top  This article has been rated as Top-importance on the project's importance scale.
Wikipedia Version 1.0 Editorial Team / v0.5 / Vital (Rated FA-class)
FA  This article has been rated as FA-Class on the quality scale.

## Why Round?

What is the point in rounding the speed of light to 186,000 miles/sec? No one ever needs to know the number rounded, and if I'm wrong, they will be capable of knowing where the comma is and just saying "thousand" after that. An article about the speed should present it accurately. Because of this, one of my students got the incorrect number when searching speed of light through Google. It was correct in meters/sec but showed the incorrect miles/sec. Rounding it serves no constructive purpose. I see people have been fighting over that on the page, so I'm not going to participate in that war, but I am going to say this is harmful. Doczilla @SUPERHEROLOGIST 08:22, 2 March 2017 (UTC)

The infobox has a number of approximate expressions with 3 significant digits, which is clearly marked. The opening sentence of the lead says: "Its exact value is 299792458 metres per second (approximately 3.00×108 m/s, approximately 186,282 mi/s)". We can't list an exact value in mi/s in the infobox, because there isn't one, alas—see note 3. Besides... who uses miles anyway? . - DVdm (talk) 08:35, 2 March 2017 (UTC)
Liberia and Myanmar use miles, and one other backwater country whose name escapes me at the moment... Stigmatella aurantiaca (talk) 18:44, 3 March 2017 (UTC)
Mile#Comparison table aka OUCH! - DVdm (talk) 19:10, 3 March 2017 (UTC)
Per our article on "Mile," miles have in exact value and is defined in meters, so the speed of light does have an exact value in miles. You could have also just read note 3 for this article, as well. It is exactly 186282.3970(51220870118507913783504334685437047641772 repeating) miles/second, the non-repeating portion only has one sig-fig more than in m/s; I don't see why it couldn't be added. Gormadoc (talk) 18:56, 22 September 2017 (UTC)
Never mind adding, it seems that there have been bitter battles over this in the past and it doesn't seem worth another. But either way, I feel that your response was needlessly belittling. Gormadoc (talk) 19:02, 22 September 2017 (UTC)
How can it belittling with such a huge number of flavours of mile? Sorry, could't resist - DVdm (talk) 19:09, 22 September 2017 (UTC)

Hello fellow Wikipedians,

I have just modified 10 external links on Speed of light. Please take a moment to review my edit. If you have any questions, or need the bot to ignore the links, or the page altogether, please visit this simple FaQ for additional information. I made the following changes:

When you have finished reviewing my changes, you may follow the instructions on the template below to fix any issues with the URLs.

You may set the `|checked=`, on this template, to true or failed to let other editors know you reviewed the change. If you find any errors, please use the tools below to fix them or call an editor by setting `|needhelp=` to your help request.

• If you have discovered URLs which were erroneously considered dead by the bot, you can report them with this tool.
• If you found an error with any archives or the URLs themselves, you can fix them with this tool.

If you are unable to use these tools, you may set `|needhelp=<your help request>` on this template to request help from an experienced user. Please include details about your problem, to help other editors.

Cheers.—InternetArchiveBot 19:09, 20 May 2017 (UTC)

Hello fellow Wikipedians,

I have just modified 2 external links on Speed of light. Please take a moment to review my edit. If you have any questions, or need the bot to ignore the links, or the page altogether, please visit this simple FaQ for additional information. I made the following changes:

When you have finished reviewing my changes, you may follow the instructions on the template below to fix any issues with the URLs.

You may set the `|checked=`, on this template, to true or failed to let other editors know you reviewed the change. If you find any errors, please use the tools below to fix them or call an editor by setting `|needhelp=` to your help request.

• If you have discovered URLs which were erroneously considered dead by the bot, you can report them with this tool.
• If you found an error with any archives or the URLs themselves, you can fix them with this tool.

If you are unable to use these tools, you may set `|needhelp=<your help request>` on this template to request help from an experienced user. Please include details about your problem, to help other editors.

Cheers.—InternetArchiveBot 19:18, 23 September 2017 (UTC)

Hello fellow Wikipedians,

I have just modified one external link on Speed of light. Please take a moment to review my edit. If you have any questions, or need the bot to ignore the links, or the page altogether, please visit this simple FaQ for additional information. I made the following changes:

When you have finished reviewing my changes, you may follow the instructions on the template below to fix any issues with the URLs.

You may set the `|checked=`, on this template, to true or failed to let other editors know you reviewed the change. If you find any errors, please use the tools below to fix them or call an editor by setting `|needhelp=` to your help request.

• If you have discovered URLs which were erroneously considered dead by the bot, you can report them with this tool.
• If you found an error with any archives or the URLs themselves, you can fix them with this tool.

If you are unable to use these tools, you may set `|needhelp=<your help request>` on this template to request help from an experienced user. Please include details about your problem, to help other editors.

Cheers.—InternetArchiveBot 19:29, 6 October 2017 (UTC)

## This is the speed of light in miles per hour, if anyone just wants to know if they would like to put it in the article.

671075955.8428776 m/h 115.164.179.29 (talk) 01:55, 27 October 2017 (UTC)

## Constancy of the speed of light

Dear all

I have recently added the following subsection:

The constancy of the speed of light is due to current research. Theories like loop quantum gravity indeed postulate that the velocity of a photon is not constant, but depends on its frequency or equivalently energy[1]. Other authors like Albrecht and Magueijo [2] assume the speed of light varying in the early universe and show that in this case one might solve the horizon, flatness and cosmological constant problem. Köhn assumes a spacetime with three spatial dimensions and two local time coordinates. Applying the Euler-Lagrange formalism he derives the constancy of the speed of light in the observable universe and a time dependence of the speed of light in the early universe [3].

I agree that the first sentence should rather be "The constancy of the speed of light is a matter of current research."

Despite that, this edit (and its previous version) were deleted by John Blackburne and DVdm with the remark that a) the constancy of the speed of light is widely accepted, b) the wording would be weired and c) the references are broken.

Ad a) The crucial point is that the speed of light is widely accepted to be constant in the observable universe. What about the early universe? What about small scales (as in loop quantum gravity)? Is there any explanation for why the speed of light is constant?

Ad b) I have reformulated the text!

Ad c) For me, the references work fine. Please elaborate why you think the references do not work.

The text above does NOT question the constancy of the speed of light in the current observable universe, but questions its constancy in the early universe (with a reference) plus gives a possible explanation for its constancy in the present state of the universe (also with a reference). Thus, I would ask to repost the edit above (with necessary changes).

Darkch2 (talk) 18:24, 5 November 2017 (UTC)

a) the answer to 'why the speed of light is constant' is in the article. If you do not understand it then you do not understand this topic and should not be editing this article
b) the text was better than the illegal copyright text, now removed from the page history, but it does not matter it does not belong
c) see the references, which appear below automatically, which contain two errors in red.--JohnBlackburnewordsdeeds 23:19, 5 November 2017 (UTC)
Dear all, dear John Blackburne
Let me comment:
a) I do not think the text says why the speed of light is constant. Please point me to where exactly the reason for its constancy is given. I am citing from the current version of the article about the speed of light "It is generally assumed that fundamental constants such as c have the same value throughout spacetime, meaning that they do not depend on location and do not vary with time. However, it has been suggested in various theories that the speed of light may have changed over time.[4][5] No conclusive evidence for such changes has been found, but they remain the subject of ongoing research.[6][7]
" saying "assumed", "suggested in various theories that the speed of light may have changed over time" and "subject of ongoing research" as I state in my editing. (Of course, I see that the speed of light is being defined, but this is purely a human definition and does not say anything about the why.) As a compromise, I thus suggest to add "Theories like loop quantum gravity indeed postulate that the velocity of a photon is not constant, but depends on its frequency or equivalently energy[8]. Other authors like Albrecht and Magueijo [9] assume the speed of light varying in the early universe and show that in this case one might solve the horizon, flatness and cosmological constant problem. Köhn assumes a spacetime with three spatial dimensions and two local time coordinates. Applying the Euler-Lagrange formalism he derives the constancy of the speed of light in the observable universe and a time dependence of the speed of light in the early universe [10]." at the end of the cited paragraph above.
ad c) I do not see any red marks in the reference, but I think, I know where the problem is, i.e. missing links. I have added them. However, I would advise you to update your browser or try a different one.
Thx
Darkch2 (talk) 08:15, 6 November 2017 (UTC)
And again, I can no longer look at your change as it’s been removed from the history for copyright infringement. You can’t add to an article by copying the abstract of a paper, even lightly paraphrased. That’s not how Wikipedia works. Wikipedia is written by editors who understand the topic, who write about it in their own words, using references to support their assertions. You clearly do not understand this topic well, or you have an understanding that is well at odds with the mainstream understanding, which this article describes. Your fringe views do not belong in this article.--JohnBlackburnewordsdeeds 04:18, 7 November 2017 (UTC)
Dear John Blackburne
If you have to comment, please do so by commenting on my (new) remarks. Your previous answer was too general and did not relate to the present topic.
Thx — Preceding unsigned comment added by Darkch2 (talkcontribs) 15:35, 15 November 2017 (UTC)

References

1. ^ Amelino-Camelia, G. "Potential Sensitivity of Gamma-Ray Burster Observations to Wave Dispersion in Vacuo". Nature vol. 393 pp. 763--765 (1998).
2. ^ Albrecht, A.; Magueijo, J. "A Time Varying Speed of Light as a Solution to Cosmological Puzzles". Phys. Rev. D vol. 59 043516 (1999).
3. ^ Köhn, Christoph. "The Planck Length and the Constancy of the Speed of Light in Five Dimensional Spacetime Parametrized with Two Time Coordinates". J. High Energy Phys., Grav and Cosm. vol. 3 no. 4 635-650 (2017).
4. ^ Ellis, GFR; Uzan, J-P (2005). "'c' is the speed of light, isn't it?". American Journal of Physics. 73 (3): 240–7. arXiv:. Bibcode:2005AmJPh..73..240E. doi:10.1119/1.1819929. The possibility that the fundamental constants may vary during the evolution of the universe offers an exceptional window onto higher dimensional theories and is probably linked with the nature of the dark energy that makes the universe accelerate today.
5. ^ An overview can be found in the dissertation of Mota, DF (2006). "Variations of the fine structure constant in space and time". arXiv: [astro-ph].
6. ^ Uzan, J-P (2003). "The fundamental constants and their variation: observational status and theoretical motivations". Reviews of Modern Physics. 75 (2): 403. arXiv:. Bibcode:2003RvMP...75..403U. doi:10.1103/RevModPhys.75.403.
7. ^ Amelino-Camelia, G (2008). "Quantum Gravity Phenomenology". Living Reviews in Relativity. 16. arXiv: [gr-qc]. Bibcode:2013LRR....16....5A. doi:10.12942/lrr-2013-5.
8. ^ Amelino-Camelia, G. "Tests of quantum gravity from observations of gamma-ray bursts" (PDF). Nature vol. 393 pp. 763--765 (1998).
9. ^ Albrecht, A.; Magueijo, J. "A Time Varying Speed of Light as a Solution to Cosmological Puzzles". Phys. Rev. D vol. 59 043516 (1999).
10. ^ Köhn, Christoph. "The Planck Length and the Constancy of the Speed of Light in Five Dimensional Spacetime Parametrized with Two Time Coordinates". J. High Energy Phys., Grav and Cosm. vol. 3 no. 4 635-650 (2017).

## Semi-protected edit request on 8 November 2017

Eleaticus (talk) 23:36, 8 November 2017 (UTC)

Not done: Empty request, nothing to do. - 00:23, 9 November 2017 (UTC)

## "Exact" is not exclusive to integers

Within the opening abstract, it is noted that "It is exact because the unit of length, the metre, is defined from this constant and the international standard for time." "Exact" at best implies rationality but does not preclude decimal values, yet this sentence was intended (given the reference of metres) to confirm that [i]no[/i] decimal value exists. Given this, it would be more precise to say, "It is an integer because the unit of length, the metre, is defined from this constant and the international standard for time." — Preceding unsigned comment added by 24.138.37.174 (talk) 03:12, 28 November 2017 (UTC)

"Exact" is the right word here, as then main point is that there are no digits beyond the ones shown. It is somewhat coincidental that this means that when expressed in m/s the numerical value is an integer. If more significant digits where known at the time of the definition, the definition probably would also include some digits after the decimal point.TR 08:13, 28 November 2017 (UTC)

Today exact measurements of speed of light is: US National Burau of Standards 299792.4574 ± 0.0011 km/sec ; The British National Physical Laboratory 299792.4590 ± 0.0008 km/sec and 299,792.458 km/s is the adopted value for speed of light at the General Conference on Weights and Measures, 1983 Oct 21.