Talk:Speed of light

Speed of causality?

"The speed of light is not about light" or so I've seen youtube video titles ( https://www.youtube.com/watch?v=msVuCEs8Ydo ). IIRC I've seen it called "speed of causality". Can/shall we include something like this? Darsie42 (talk) 10:04, 18 February 2022 (UTC)

The article already says that anything else massless will also travel a the speed of light, and that c is the upper limit for the speed at which ... any signal carrying information can travel through space. I don't think we need to say more than the article already does, but that's just my take. XOR'easter (talk) 19:19, 18 February 2022 (UTC)

Maybe specifying that light only moves at c when in a vacuum would be better. There are lots of cases where photons move slower than c due to a medium or gravity or whatever, from my understanding. 50.124.244.111 (talk) 22:02, 30 June 2022 (UTC)

This is already explained at the end of the intro. XOR'easter (talk) 22:17, 30 June 2022 (UTC)

oh yeah, it does, my bad lol 50.36.160.182 (talk) 16:25, 1 July 2022 (UTC)

No worries! :-) XOR'easter (talk) 16:31, 1 July 2022 (UTC)

Which units need to be in the infobox?

Bearing in mind that an overlong infobox means more for mobile users to scroll past, which units and numerical values should we report? The case for the exact SI value is pretty obvious, but "astronomical units per day" feels more like trivia. Does it provide additional insight to say that c is roughly 0.307 parsecs per year as well as saying that light takes about 3.26 years to travel one parsec? XOR'easter (talk) 22:33, 16 June 2022 (UTC)

Trivia multiply on WP like a disease. Some sort of guideline or criterion to restrict this should be in the MoS, IMO. Here, as you point out, reciprocals are trivially redundant and hence space-wasters. In general, we should avoid illustrative stuff in infoboxes: this is not their function, so the entire "Approximate light signal travel times" list should be removed or moved into the article. Some of the signal travel time entries have essentially no merit anyway, such as "from geostationary orbit to Earth", "the length of Earth's equator" (the person who wants this would relate better to "7.5 times around the Earth in one second"), "one light-year" (we do not need to be reminded in an infobox about the obvious), "one parsec" (the speed in parsecs per year gives this), "from the nearest galaxy to Earth" (worse than bad; whether it is a galaxy is disputed and it is unrelatable, since almost all readers will have to research what is meant by this). 172.82.46.195 (talk) 15:30, 7 August 2022 (UTC)

Descartes' eclipse argument is nullified by Bradley's discovery of aberration -- should this be noted?

Descartes' oft-cited argument, that any propagation delay of light would result in a misalignment of Sun/Earth/Moon during a lunar eclipse, has an oft-overlooked flaw that is evident in retrospect since Bradley's time -- that the supposed misalignment would be cancelled by aberration of light, so that the alignment would appear straight regardless of light speed. Thus, the absence of discernible misalignment does not impose a lower limit on light speed, but instead has no bearing on the matter.

A note about this has been added to the page on aberration of light. Should a brief note be given here also? (There may be some value in pointing this out, as an online search on this point turns up more confusion than clarity -- some students and even professors seem to think that such misalignment is still present, and inconspicuous only because of the short timescale, not absent altogether.)

If the above considerations are deemed convincing, please consider augmenting your historical section, excerpted below, with a brief note along the lines of the new sentence in parentheses there.

[If unconvinced, or if the cited reference is difficult to access or follow, perhaps the following approximate gedanken experiment will help: Imagine yourself (like Gamow's Mr. Tompkins) in a world where lightspeed is much slower, say only about 10 times as fast as you can run. Suppose you are on a flat field facing due North, while on your left the Sun is just setting due West, and on your right, your shadow is visible on some white fence (Tom Sawyer memorial) that runs north/south. Standing still, you see your shadow perfectly abreast of you, but if you start running North, your shadow begins to fall behind a little, as Descartes would have expected. After you have been running at a steady 0.1c for a bit, an overhead observer (in the Goodyear Blimp high above you and your shadow and roughly equidistant from both) would see your shadow lagging you by about 0.1 radian. Since your view of your shadow involves a round trip, you might expect to see a lag of 0.2 radian instead. But since you are moving North at 0.1c, the aberration of light will make the sun and your shadow each appear to be 0.1 radian north of their "geometric" locations, thereby cancelling out the lag effect, so that your shadow still appears diametrically opposite from the sun after all.]

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Present text excerpt (with proposed new note in parentheses):

Early history … In the early 17th century, Johannes Kepler believed that the speed of light was infinite since empty space presents no obstacle to it. René Descartes argued that if the speed of light were to be finite, the Sun, Earth, and Moon would be noticeably out of alignment during a lunar eclipse. (Although this argument fails when aberration of light is taken into account, the latter was not recognized until the following century.) Since such misalignment had not been observed, Descartes concluded the speed of light was infinite….

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DAL47 2601:601:1501:8320:ED21:7264:3A2:E1E4 (talk) 00:22, 27 June 2022 (UTC)

Good addition. I did not know that. Roger (talk) 17:56, 27 June 2022 (UTC)

References

Semi-protected edit request on 8 July 2022

विश्वमा भासिरोचनम 103.119.245.102 (talk) 12:26, 8 July 2022 (UTC)

 Not done: it's not clear what changes you want to be made. Please mention the specific changes in a "change X to Y" format and provide a reliable source if appropriate. Also, any requested edits to the text must be in English. ComplexRational (talk) 13:00, 8 July 2022 (UTC)

Grammar fix (request for semi-protected edit)

Please insert a word: "one can thus a standard for the metre" should be "one can thus define a standard for the metre" (or any similar correction). 172.82.46.195 (talk) 19:01, 1 August 2022 (UTC)

 Done I went with "establish a standard for the metre", which is equivalent to "define the metre", also noting that "define" is used in the previous sentence. Thanks for catching this! ^Complex/_Rational 19:51, 1 August 2022 (UTC)

It would be better English to say "thus, one can..." than "one can thus...". Also, "thus" itself is usually a very pedantic and amateurish word. Try and make a sentence without "thus". cheers Billyshiverstick (talk) 05:17, 16 August 2022 (UTC)

Esplaining in Plain English

It would be great if a physicist with some teaching chops could put some real world examples here, for instance "the Lorenz Factor means that a person travelling headlong at 85% of the speed of light would be 100 feet tall", or that if a space ship travelled away from Earth at 50% of the speed of light for a year its clocks would be six months ahead of Earth clocks, but if it returned to Earth at the same speed for a year, the clocks would re-synchronise. Thanks Billyshiverstick (talk) 05:15, 16 August 2022 (UTC)

The idea is introduced in a section (Speed of light § Fundamental role in physics), but to get further detail like this is beyond the scope of this article. The link at the top of the section (to Special relativity) is sufficient: that is where detail of this nature belongs. 172.82.46.195 (talk) 11:38, 16 August 2022 (UTC)

Definition

"Its exact value is defined as 299792458 metres per second"

That's the wrong way around really. C is an absolute constant - you can't define it in terms of metres and seconds. Indeed the metre is defined in terms of c, so that would be a circular definition. I think it should just say something like "It's value is 299792458 m/s" Bagunceiro (talk) 14:15, 16 August 2022 (UTC)

Maybe I'm missing something, but isn't that what the article already explains? The note at the end of the line goes into more detail; the point is that the value is exact by definition. XOR'easter (talk) 14:24, 16 August 2022 (UTC)

Maybe this is an opportunity to consider the wording, so that more people read the intended meaning into the statement: what is being defined might be made clearer. What is being defined is a relationship between units (the second and metre) through the choice of a numeric value for the physical quantity when c is expressed in terms of those units. The footnote does a good job of explaining it, but the sentence could be revised. I'll give thought to a rewording. 172.82.46.195 (talk) 15:00, 16 August 2022 (UTC)

I made a little adjustment to it. XOR'easter (talk) 15:10, 16 August 2022 (UTC)

I'm not going to fight this corner - it's probably a bit too pedantic. But I would nevertheless suggest that this still implies that the speed of light is what it is by definition. This is not so - c is a fundamental constant, you can define units in terms of it, but not the other way around.Bagunceiro (talk) 15:31, 16 August 2022 (UTC)

It is that value by definition of the meter. Headbomb {t · c · p · b} 15:33, 16 August 2022 (UTC)

So maybe the article can avoid all confusion by saying "by the definition of the metre" instead of just "by definition".  Dr Greg  talk  16:44, 16 August 2022 (UTC)

I think "by definition" is good enough, what with the elaboration in the endnote and later. XOR'easter (talk) 18:18, 16 August 2022 (UTC)

By pedantic interpretation, yes, but this is the lead and that is not how readers might first interpret it. Since we do not say by definition of what, and to keep it simple, I think the footnote is sufficient, and we should just omit any mention if "by definition" in that sentence. That is, I would suggest "Its exact value is 299792458 metres per second ...", along the lines suggested by Bagunceiro. What are we actually adding by including "by definition"? Only this who know already will understand from the sentence what it is saying. 172.82.46.195 (talk) 19:13, 16 August 2022 (UTC)

BC or BCE

I see an older version from 2007 used BCE. Is there a reason it was changed to BC? Great article, by the way. John (talk) 16:15, 16 August 2022 (UTC)

I guess it's no big deal. I changed it back per MOS:ERA. John (talk) 16:49, 16 August 2022 (UTC)