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)