Talk:Speed of light

Visually off-putting math symbol

From the Notation section:

"In 1856, Wilhelm Eduard Weber and Rudolf Kohlrausch had used c for a different constant that was later shown to equal √2 times the speed of light in vacuum."

Please replace with \sqrt{2}. LOWERCASEUSERNAME (talk) 23:27, 22 June 2026 (UTC)

See MOS:RADICAL. NebY (talk) 23:46, 22 June 2026 (UTC)

If we go by the book, "hours" could become "days"

The lede says When communicating with distant space probes, it can take hours for signals to travel. It might be time to change this soon, as NASA says Voyager 1 will be one light day from Earth in November this year . ☆ Bri (talk) 14:16, 7 July 2026 (UTC)

One day is still easily expressed as a two-digit number of hours. We can wait a handful of years till we get to three digits . - DVdm (talk) 13:39, 9 July 2026 (UTC)

Luminal speed - short and correct name

The name luminal speed for the speed of light in vacuum is allowed by iternational IEC standards. It is shorter, exact and it also fits to broadly used superluminal speed and subluminal speed. Will you add it into the first line just after (or maybe even before) speed of light in vacuum?

See official page controlled by IEC, Electropedia:

113-01-34 speed of light in vacuum

luminal speed

speed of light

DEPRECATED: light speed

...

Note 3 to entry: The term “luminal speed" is used in relativity theory. Related concepts are "subluminal speed" and "superluminal speed". JOb (talk) 20:44, 10 July 2026 (UTC)

Thanks! This didn't really workout in the first sentence and since the source explicitly associated the term with relativity I put it in that part of the intro. Johnjbarton (talk) 01:49, 11 July 2026 (UTC)

Thanks too! And bmho the term superluminal for speed is so oftenly (mis)used in scifi that corresponding luminal speed need not be limited to relativity theory only and could be mentioned just in the first paragraph together with other names. JOb (talk) 08:58, 11 July 2026 (UTC)

PS: Even in Electropedia, luminal speed stays before speed of light. JOb (talk) 09:00, 11 July 2026 (UTC)

I used the source you provided. I can't use sources you don't provide. Johnjbarton (talk) 16:09, 11 July 2026 (UTC)

Above mentioned 113-01-34 can be found in Electropedia. the official page of International Electrotechnical Committee IEC. Put down https://electropedia.org

or directly

https://electropedia.org/iev/iev.nsf/display?openform&ievref=113-01-34

Is it what you needed? JOb (talk) 18:28, 11 July 2026 (UTC)

That is the source I already used. Per Note 3: The term “luminal speed" is used in relativity theory. I think the article is fine as is. Johnjbarton (talk) 19:30, 11 July 2026 (UTC)

Perfect. JOb (talk) 23:32, 11 July 2026 (UTC)

At the address http://jobdr.gilhad.cz you will find till 16.7.2025 my simple PPTX and mp4 to demonstrate how to create a superluminal spot by spheric wave of light and a rod. Please use it for Wiki freely as you like. JOb (talk) 22:50, 12 July 2026 (UTC)

Just two comments: 1) It might be better to refer to a vacuum as a “medium” rather than a “material”. 2) "Luminal speed" used in relativity as a synonym for "speed of light in vacuum" could be placed closer to this term.

Would be acceptable to start with e.g.

The speed of light is the speed of electromagnetic waves. The speed depends upon the medium the light travels in. The speed of light in vacuum (called also luminal speed) is a universal physical constant... JOb (talk) 21:06, 18 July 2026 (UTC)

I think broadly "medium" is the opposite of "vacuum", so don't think it is a good way to start the article. Johnjbarton (talk) 22:06, 18 July 2026 (UTC)

Of course it is up to you and I suppose, English is your mother-tongue-language. In our language, we feel more the Latin origins: "medium" = generally something between, like in "intermedial", whereas from "material" we feel materia = mass. Thus, for us, vacuum will be easier considered as a special medium rather than a special material (e.g. between electrodes in a capacitor). JOb (talk) 13:44, 19 July 2026 (UTC)

The word "medium" is used in the context of Aether theories which make it unsuitable here. It isn't necessary or desirable to require vacuum to be a special case. It is an extreme lack of any material, not a special material. Being "something between" is also unsuitable, since vacuum is specifically nothing between. Johnjbarton (talk) 02:37, 20 July 2026 (UTC)

Vacuum is not specifically nothing between. It is space-time inbetween and the electromagnetic field (thus the light as well) is its property. JOb (talk) 07:52, 20 July 2026 (UTC)

Proposal to move to "Speed of light in vacuum"

I just noticed that the lede and the article are almost entirely about "speed of light in vacuum". Of course in any case where we discuss "speed of light in vacuum", we switch to "speed of light" once that context is established. That is what we should do in this article: start with the fully specific term as the title, explain the issue and use "speed of light" as normal. One of our sources (https://electropedia.org/iev/iev.nsf/display?openform&ievref=113-01-34) puts it this way:

• The term "speed of light" shall be used only when the context indicates without ambiguity that the full term “speed of light in vacuum” is meant, because "speed of light" is also observed in matter where its value is always less than that in vacuum.

Coming into the article the context is not established: we should begin with "speed of light in vacuum" as the title. Johnjbarton (talk) 19:45, 15 July 2026 (UTC)

• Oppose Per WP:COMMONNAME: For titles, Wikipedia "...generally prefers the name that is most commonly used (as determined by its prevalence in a significant majority of independent, reliable, English-language sources)..." The term "speed of light" is almost universally used for the value in a vacuum, unless specifically stated otherwise. To the extent there is any ambiguity, we clear that up in the lede. We shouldn't change the title to solve a non-problem.--agr (talk) 20:07, 15 July 2026 (UTC)

  In my experience, the same "significant majority of independent, reliable, English-language sources" begin with "speed of light in vacuum" then use "speed of light". This is an abbreviation and it is only more common for that reason. For example, I don't think you will find a top quality source the claims "c" is the "speed of light" without discussion of "speed of light in vacuum". That is why our article starts with speed of light in vacuum as the first highlighted topic.

  I suppose we could start out with the explanation for the abbrevation. Johnjbarton (talk) 20:24, 15 July 2026 (UTC)

Approximate values (to three significant digits)

After looking over the talk page archives, I can see that this has been debated repeatedly, but I can't for the life of me wrap my head around the logic that has led to the current situation in the infobox. Why are these values so dramatically reduced in precision compared to the exact figure of 299,792,458 m/s?

The primary argument in favor of the status quo seems to be that most visitors to the page won't need any more precision than that. In that case, why provide the 299,792,458 figure in the first place? Just say it's approximately 300 million, and anybody who wants more precision can click on the footnote to get it straight from the source. Wikipedia is a work of reference, and it should provide reference values. These numbers are not so complicated that they require dumbing down. And the article as a whole is clearly not written to be understood by people who would find such numbers overwhelming.

Let's say there are two types of people who come to this article looking for an imperial value for the speed of light: people who want to see 186,282 (or, to match the precision of the original value, 186,282.397), and people who will be satisfied with the precision level of 186,000. As it currently stands, the latter group will be okay, and the former will be annoyed that they have to do the calculation themselves. If more precision was added to the values in the infobox, the former group would be satisfied, and the latter would have the far simpler task of mentally replacing the "extraneous" digits with zeroes. Even if the former group is very small, which I doubt, providing more precision would be a net benefit to our readers.

Also, why are the approximate values cited to a high school algebra textbook? Surely, regardless of the chosen level of precision, converting the original value into other units falls under WP:BASICMATH and requires no citation. I can't see the cited page on Google Books, does it have something to say about the significant digits themselves? – Reschultzed (talk) 03:05, 22 July 2026 (UTC)

About Talk:Speed_of_light/Archive_17#Precision_of_values_in_infobox 11 years ago there was a consensus to list converted values for the speed of light to 3 significant figures. You could propose a different approach but you need to get input.

I actually think there is a case for a different solution. List miles per second at six figures because the six figure speed of light in miles/second appears in a lot of old textbooks, then delete the other conversions altogether. There is no case for including them other than editor whimsy I guess.

In any case, I disagree with your claim about no citation needed. Unless we have a direct source, the conversion factor should be documented, ie via https://www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors/nist-guide-si-appendix-b8, and the significant figures need to correspond to the arithmetic used. This is not WP:BASICMATH as you can see from https://www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors#B72. Johnjbarton (talk) 03:24, 23 July 2026 (UTC)

Universal constancy of C shall not be presented as fact, rather than theory!

> The speed of light in vacuum is a universal physical constant

This is a wish or theory at best, not a fact and so should not be presented as a fact! There is no way for mankind to explicitly verify that in each and every corner of the Universe, at any given time in the past, C was the same as in the present. Even if current scientific theory says so, the theory may or may not be correct in light of future experimental discoveries.

Thus BIPM / SI painted themselves into a corner by defining the meter as a derivative of C. If oscillation, slowing or quickening of C is ever discovered, they'd need to go back to the X-shaped rod, which is still kept in a safe. ~2026-48980-54 (talk) 00:06, 10 September 2026 (UTC)

Right, if that is ever discovered, the world will happily go back to anything. Wikipedia reflects the current consensus, not any other possible future. - DVdm (talk) 08:59, 10 September 2026 (UTC)

I think you are confused. The value of the speed of light is not relevant to any physics theory. The value is simply a conversion factor from natural units to International System of Units. It has no more meaning than the number "3" in the conversion from Tablespoons to Teaspoons. Its role in physics is to be part of chain of comparisons needed to compare data from different types of experiments. Sadly our current article misrepresents the sources on this issue. I will try to make some improvements. Johnjbarton (talk) 23:07, 10 September 2026 (UTC)

Inappropriate sources

The article has a sentence: In the early 1860s, Maxwell showed that, according to the theory of electromagnetism he was working on, electromagnetic waves propagate in empty space[152]... where source 152 is: See, for example:

• Giordano, Nicholas J. (2009). College physics: reasoning and relationships. Cengage Learning. p. 787. ISBN 978-0-534-42471-8. Extract of page 787
• Bergmann, Peter Gabriel (1992). The riddle of gravitation. Courier Dover Publications. p. 17. ISBN 978-0-486-27378-5. Extract of page 17
• Bais, Sander (2005). The equations: icons of knowledge. Harvard University Press. p. 40. ISBN 978-0-674-01967-6. Extract of page 40

However none of these sources are discussing Maxwell. It seems that these sources purport to verify "electromagnetic waves propagate in empty space" which is inappropriate since it would not have been possible for Maxwell to create "empty space". The links in these citations do not reveal any content. I think these sources should be removed. Johnjbarton (talk) 23:25, 10 September 2026 (UTC)

Proposal to cut part of the content on special relativity.

The section "Role in physics" starts with 4 paragraphs on special relativity. Not all of that content is directly about speed of light. Special relativity has many counterintuitive and experimentally verified implications. is true and Terrell rotation is one of them but this kind of detail gets in the way of plainly describing the speed of light as a boundary for a "domain of validity" as described by

• Uzan, J. P. (2025). Fundamental constants: from measurement to the universe, a window on gravitation and cosmology: J.-P. Uzan. Living Reviews in Relativity, 28(1), 6.

The special theory of relativity explores the consequences of this invariance of c with the assumption that the laws of physics are the same in all inertial frames of reference. is an anthropomorphic repetition of the first paragraph and is not directly supported by the sources given (which are just sources on invariance of c in special relativity, no exploring discussed). This invariance of the speed of light was postulated by Einstein in 1905... is true but it is History, not directly related to "Role in physics".

If we say less about special relativity details we have more punch on "Role in physics". Johnjbarton (talk) 03:42, 12 September 2026 (UTC)

Ok I went ahead. I think the result is more focused on the speed of light now. Johnjbarton (talk) 04:10, 13 September 2026 (UTC)

miles per second

The speed of light is a well defined constant, exactly c = 299792458 m/s, and it is a trivial matter to convert it to miles per second: $$c={\frac {299{,}792{,}458~\mathrm {m} }{\mathrm {s} }}\times {\frac {100~\mathrm {cm} }{\mathrm {m} }}\times {\frac {1~\mathrm {in} }{2.54~\mathrm {cm} }}\times {\frac {1~\mathrm {ft} }{12~\mathrm {in} }}\times {\frac {1~\mathrm {mi} }{5{,}280~\mathrm {ft} }}=186{,}282~{\frac {39{,}937}{100{,}584}}~\mathrm {miles/second}$$ Since this final value is exact and easily representable as such, I see little motivation to have the article only approximate it. @Johnjbarton et al. what do you think? —Quantling (talk | contribs) 13:44, 2 October 2026 (UTC)

Presumably this relates to these edits, in which you changed

For example, in imperial units, the speed of light is approximately 186282 miles per second.

to

For example, in imperial units, the speed of light is 186282 ⁠39937/100584⁠ miles per second.

It's not immediately obvious why we should present such a precise formulation for a conversion that you describe as trivial. As WP:NOTEVERYTHING says, Information should not be included solely because it is true or useful, and its utility is in any case doubtful; someone wishing to carry out exact calculations in miles will find it trivial to use the exact values already provided together with routine conversions, whereas the form you provide is not easy to use in calculation and not otherwise meaningful for general readers. Most of all, this does not make the article easier to read or understand, but quite the opposite. NebY (talk) 14:10, 2 October 2026 (UTC)

I agree that general readers don't need exact values, whether metric or imperial units, and that readers who want the exact values will likely want metric. But I don't see that providing the exact value in imperial units impedes any of these readers. On the other hand, by using a trivially little amount of space, the exact value provides information that some readers may be seeking and others will find interesting even though it is not of direct use to them. So, yes, the upside isn't dramatic, but the downside IMHO is even smaller. I'm seeing providing this exact value as a net win. —Quantling (talk | contribs) 14:56, 2 October 2026 (UTC)

I agree with NebY, this unusual fractional form is distracting, not representative of sources on the topic, and has no practical value. Johnjbarton (talk) 16:00, 2 October 2026 (UTC)