Talk:Speed of light

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Phrasing: "time per unit distance" versus "time for unit distance"

This is a minor point of phrasing, and I don't want to make a big deal out of it. There is a bit of disagreement about whether "time per unit distance" or "time for unit distance" is the better phrase. I say the former sounds more natural in English, just as one would say "miles per hour", not "miles for hour". TimothyRias says "for" is used in literature. I think they mean exactly the same thing, and I don't think it is an official term of art. If I am wrong, I would be curious to read examples of literature in which "time for unit distance" is favored. Otherwise, I think it will distract native English speakers because it just sounds odd. If nobody else cares, I will let go of it. CosineKitty (talk) 21:24, 4 March 2010 (UTC)

This page from the Astronomical Almanac is one such source. I wonder if the phrasing is related to the conventional units – seconds rather than seconds per astronomical unit. Alternatively, it might be a reflection of the fact that it is light times that are measured and then converted into distances. They certainly mean the same thing in any case, whether you use "for" or "per". Physchim62 (talk) 22:25, 4 March 2010 (UTC)

I've never encountered the later use. Per unit distance gets my vote, because I think it is clearer, and IMO, more natural. Headbomb {talk / contribs / physics / books} 22:31, 4 March 2010 (UTC)

The "for" usage may be just as correct, grammatically, but "per" is more typically used, in English.Sebastian Garth (talk) 01:12, 5 March 2010 (UTC)

"Light time per unit distance" gets exactly 0 hits on google scholar, while "Light time for unit distance" gets 18 hits. It very much seems that this constant is called "Light time for unit distance". It may sound a bit odd, but it is the terminology that is used, and which I think Wikipedia should follow. TimothyRias (talk) 08:12, 5 March 2010 (UTC)

This looks like a conflict between specialist literature and everyday English. I finally figured out what bugged me about the phrase "time for". That is idiomatic for "an occasion set aside for the purpose of". An example would be "time for lunch". In that sense, it does not refer to the duration required to eat lunch, but the hour being appropriate for starting to eat lunch. In Spanish there are two different words for these senses of time: vez (occasion), and tiempo (duration). English has to use context to resolve the ambiguity, and we do it in part with surrounding words like for versus per. But like I said, I didn't want to make a big deal out of it, and I can live with it. I won't be surprised though if other editors who happen upon this article to change it to "time per unit distance". My personal preference would still be to avoid awkward wording when precision does not suffer. CosineKitty (talk) 16:28, 5 March 2010 (UTC)

Faster than speed of light?

Einstein's theory no longer valid n space-time continuum out of window? Not really, only in some instances, first few seconds of big bang or creation of black whole or death of superstars or laser ray emitions could be faster than speed of light but they do not exceed 300001 km/ps, speed of light itself is 299973. Then in a second it becomes constant, except of course if it travels thru water, thru other forms! —Preceding unsigned comment added by 64.107.0.74 (talk) 01:16, 12 March 2010 (UTC)

See warnings on your talk pages User talk:64.107.0.74 and User talk:66.99.0.251. DVdm (talk) 07:44, 12 March 2010 (UTC)

SAY THANX INSTEAD!

Physical constant or exact value

The opening sentence declares "The speed of light (usually denoted c) is a physical constant. Its value is exactly 299,792,458 metres per second". But the value can not be both, a physical constant and an exact value. One of these statements must be false. Either it is a physical constant, then it cannot be measured exactly. Or one of both dimensions "metres" and "second" is defined in relation other and this constant, which would render the speed of light to be "exactly 299,792,458 metres per second". So can some please correct the first sentence? --~~ —Preceding unsigned comment added by 62.235.247.69 (talk) 14:23, 12 March 2010 (UTC)

The sentences don't say that the speed of light is a constant and a value. They say that it is a constant with a value.

Regarding your second remark, feel free to have a look at the talk page archives at the top of this page. You can use the search function. Good hunting.

Please sign your talk page messages with four tildes, as indicated at the bottom of the edit window? Thanks. - DVdm (talk) 14:43, 12 March 2010 (UTC)

Did you bother to read the rest of the introduction (specifically the last two sentences of the second paragraph)? In the unlikely event you did but you didn't get the point, did you bother to read the section "Increased accuracy and redefinition of the metre" (specifically the last paragraph)? ― A._di_M. (formerly Army1987) 19:17, 12 March 2010 (UTC)

Last paragraph of "Astronomy"

Do we need this?

A celestial object will exhibit apparent motion due to the motion of the observer and the finiteness of the speed of light. This effect is called stellar aberration. For an observer on Earth, it can be up to 20 arcseconds due to the Earth's motion, and is taken into account for precise astronomical observations. Conversely, if a celestial object itself is moving, it will have moved a certain amount in time that the light needed to reach Earth. The correction needed to obtain the true (current) position is known as Light-time correction.

It's not something most people will care about, and there's already an explanation of aberration (complete with picture) in "Astronomical measurements". ― A._di_M. (formerly Army1987) 19:34, 14 March 2010 (UTC)

It is pretty redundant, so I say scrap it. TimothyRias (talk) 20:27, 14 March 2010 (UTC)

Interferometer image

An idealized interferometric determination of wavelength obtained by looking at interference fringes between two coherent beams recombined after travelling different distances. Top: Constructive interference (in phase); If the difference in path length is a multiple of a wavelength, the recombined beams support one another and reconstitute the original beam. Bottom: Destructive interference (out of phase); If the two paths differ by half a wavelength, the recombined beams are out of phase and cancel each other. The bottom panel in the figure suggests the path length has been increased by half a wavelength by moving the right-hand point of reflection further out.

The article contains this picture to the right. Do others think this picture is clear? For me it is not. The fact that it needs a caption that is larger than the actual picture says quite enough I think. Besides that it is unclear I have some qualms with the actual physics of the picture. First of all the picture uses a light bulb as a light source. Light bulbs produce incoherent light and thus are useless in an interferometer. Second, what is the bottom side of each interferometer trying to show? A standing wave of some sorts? Interferometers normally use a screen of sorts.

Do others agree with this? And if so what should be done to solve it? One option would be to simply leave it out, which would not be such a big loss for the article. A second option would be to find/make a different diagram of an interferometer. TimothyRias (talk) 09:27, 17 March 2010 (UTC)

The bottom part is supposed to show two waves interfering destructively and cancelling each other out. Anyway, I agree that the picture is not vital for this article. ― A._di_M. (formerly Army1987) 17:10, 17 March 2010 (UTC)

See also: Variable speed of light

I was watching the documentary "Einstein's Biggest Blunder" (BBC Horizon, 2000), in which the theory of Albrecht and Magueijo of a variable speed of light is mentioned. Going to the Speed of light Wiki page, there is no reference to any theories about a variable speed of light. So I think there should at least be a link under "See also" to the existing Wiki page on Variable speed of light. Ideally, I would also like some details about what the Albrecht/Magueijo theory actually says (apparently, they use a variying speed of light to argue against the inflation period). Thanks. Tue Sorensen (talk) 03:09, 21 March 2010 (UTC)