Talk:Speed of light/Archive 1

message: The article currently gives "The speed of light is of relevance to communications. For example, given that the equatorial circumference of the Earth is 40,075 km and c, the theoretical shortest amount of time for a piece of information to travel half the globe is 0.067 seconds."  Though not particularly well-versed in physics, I believe the circumference of the Earth shouldn't play into this calculation (see this section's heading title).  I don't disagree with that the quotation apparently gives the theoretical shortest amount of time for light to travel across the Earth circumferentially.  However, since AFAIK a piece of information could be encoded in any number of frequencies of electromagnetic radiation (and that radiation could penetrate the Earth), the actual shape of interest is a straight line between opposite poles of the planet, not the circumference between them. 07:04, 22 Dec 2004 (UTC)
thread_title: Theoretical limitations on the speed of communications on Earth

message: As a layman, I don't understand how it's possible for something to travel faster than c but not carry information faster than c. Could someone explain this? -- User:Evercat
:A very rough explanation is that the "something' that travel faster than light doesn't carry energy. -- looxix 00:43 Apr 19, 2003 (UTC)
:Also, it is only in a vacuum than nothing (no information) can travel faster than light; in a medium thing can travel faster than light (see Cherenkov effect) -- looxix 00:49 Apr 19, 2003 (UTC)
Irrelevant. The electrons involved in the Cherenkov effect are still going slower than c. (It is true that light travels even slower than those electrons in that medium).
:The fact that Group velocity is greater than c doesn't mean that there is really a particle moving (travelling) at that speed but rather that something is changing at this speed, in this case the waveform. (See also Phase velocity) -- ReiVaX 16:44, 18 Jul 2004 (UTC)
thread_title: Faster than ''c''

message: "It is a solution to the wave equation"
:How is the speed of light be a solution to a vector equation? Κσυπ Cyp 08:08, 13 Nov 2003 (UTC)
replies: text: As my electromagnetics professor explained it (and you'll have to bear with me - it's been almost 2 years), it's not that the speed is the solution, per se. It just doesn't have a solution for any other speed besides C. --Raul654 08:28, 13 Nov 2003 (UTC)
replies: text: One more thing. Here is the exact derivation you are looking for: http://people.ccmr.cornell.edu/~muchomas/P214/Notes/OtherWaves/node18.html --Raul654 08:36, 13 Nov 2003 (UTC)
thread_title: Wave equation?

message: Wile, why are you removing the scientific notation approximation? In 99% of cases when I'm performing a calculation involving the speed of light, the number I'm looking for is 3 × 108. I'm pretty sure this approximation, in this format, is useful to other people as well. Fredrik (talk) 18:49, 8 Jun 2004 (UTC)
replies: text: Well, it is cluttering, as it doesn't serve any obvious purpose. Anyone who is actually making use of c in computations is surely capable of approximating it as 3 times 10^8 or 0.2998 times 10^9 or whatever they please. The vast majority of the remainder of the readers will be much more at home with "thousands of somethings" instead of scientific notation. -- I feel pretty strongly that the introductory sentences of an article must get right straight to the point. Naturally it is quite possible that the introduction still isn't getting there, so let's work in that direction. Regards, Wile E. Heresiarch 20:45, 8 Jun 2004 (UTC)
replies: text: How would you know that anyone actually making use of c is capable of approximating it? Every high school student? It isn't immediately obvious to everybody that 100,000 kilometers equals 108 meters. And sure, they might be able to figure it out, but our job is to make this information available as conveniently as possible. Perhaps the note should be placed elsewhere, but there's no reason not to provide it. Fredrik (talk) 21:02, 8 Jun 2004 (UTC)
replies: text:  We don't seem to be discussing anything very fundamental here; the article is about the speed of light, not scientific notation. That 299,792,458 is close to 3 x 10^8 seems to be a footnote that is useful in some contexts. Maybe there is a place for it somewhere in the article (or maybe not) but it seems far from central. Wile E. Heresiarch 21:24, 8 Jun 2004 (UTC)
replies: text:  This is not an article about scientific notation, but it is neither an article about obsolete imperial units (the mile). The question is indeed whether the information is central (useful to someone), and I hold that it is. It would be nice to get input from a few other people with regard to this matter. Fredrik (talk) 07:36, 9 Jun 2004 (UTC)
replies: text:  I've put the 3 × 10^8 m/s just before 30 cm/ns (under the heading "Overview"). It is appropriate since both are convenient approximations. Wile E. Heresiarch 16:21, 9 Jun 2004 (UTC)
replies: text:  Thank you. Fredrik (talk) 16:30, 9 Jun 2004 (UTC)

text:  I'd like to support Fredrik on this point. It's far simpler to get a quick idea of the scale of c without having to count the number of digits in 300,000,000. Cederal 16:52, 9 Jun 2004 (UTC)
thread_title: Scientific notation

message: I think that perhaps it should be clearly stated that by adding two velocities with the Einstein velocity addition formula we cannot get a speed greater than c. (Because the interval (-c,c) with that operation is an Abelian group.) I know that its said that c is a "speed limit" but what its not said is that the formula is consistent with that. -- ReiVaX 18:09, 18 Jul 2004 (UTC)
thread_title: Adding velocities

message: According to the theory of special relativity, all observers will measure the speed of light as being the same, regardless of the reference frame of the observer or the velocity of the object emitting the light.  A simple three-step analysis is sufficient to show that this is the case:
# the speed of light in vacuum can be derived from Maxwell's equations; 
# special relativity requires the laws of physics, such as Maxwell's equations, to be identical in all unaccelerated frames, and so 
# observers in all such frames must observe the same speed of light.  
The constant speed of light derives from the Lorentz transforms, which distort distances and times for observers travelling at large velocities in the same way.''
Isn't 3 precisely what we are trying to show is the case?  "all observers will measure the speed of light as being the same" because "observers in all such frames must observe the same speed of light"?  Also, as the speed of light in a vaacum is a defined value, the fact that it can be derived from Maxwell's equations needs to be explained. anthony (see warning) 19:00, 12 Aug 2004 (UTC)
replies: text: Er - to précis - "ToSR says SoL is constant.  How?  Because (i) SoL comes directly from Maxwell, (ii) Maxwell is a law of physics, (iii) laws of physics must be the same in all inertial frames; hence, (iv) SoL same in all inertial frames.  Plus Lorentz gives you a way to work out how it all works."  I don't think this is begging the question, is it?  -- ALoan (Talk) 19:14, 12 Aug 2004 (UTC)

text: Feel free to copyedit. :) -- ALoan (Talk) 19:48, 12 Aug 2004 (UTC)

text: I've tweaked slightly  - is that better? -- ALoan (Talk) 02:05, 15 Aug 2004 (UTC)
replies: text: I guess.  I'm still confused as to what we are showing to be the case.  Are we showing that "all observers will measure the speed of light as being the same" or are we showing that this is a result of the theory of relativity?  Because it's not really a result of the theory or relativity, it's a postulate of the theory or relativity (I think, I'll double-check that one). anthony (see warning) 02:21, 15 Aug 2004 (UTC)

text: Yes, "We will raise this conjecture (the purport of which will hereafter be called the "Principle of Relativity") to the status of a postulate, and also introduce another postulate, which is only apparently irreconcilable with the former, namely, that light is always propagated in empty space with a definite velocity c which is independent of the state of motion of the emitting body." Well, sort of.  Einstein goes beyond saying that "all observers will measure the speed of light as being the same", and actually posits that the speed of light always is the same.  anthony (see warning) 02:27, 15 Aug 2004 (UTC)
thread_title: begging the question

message: Hello all. This is a great article, thanks to everyone who has worked on it. I wonder if we really need the picture at the top of the article. It shows two nerdy guys playing with a laser, which is fun, but beside the point. Can we find another picture that directly addresses the topic? The caption does state something interesting about the speed of light but, strangely enough, doesn't describe something illustrated in the picture. Maybe someone can state what it is about the speed of light that it illustrates. Regards & happy editing, Wile E. Heresiarch 13:52, 11 Oct 2004 (UTC)
replies: text: Well, if you have a better picture, be my guest!  Although light itself is quite visual, the speed of light is rather hard to see.  Cherenkov effect?. -- ALoan (Talk) 19:09, 11 Oct 2004 (UTC)
replies: text: Thanks for the suggestion. I've replaced the laser guys with the picture from Cherenkov effect. Although it is a very lovely shade of blue, the picture is a little fuzzy and hard to read. We could improve it with a sharper image; I guess I'll keep my eyes open. Wile E. Heresiarch 03:44, 12 Oct 2004 (UTC)
replies: text: I've put the guys back lower down, in the "slower than light" section where it makes more sense. -- ALoan (Talk) 11:36, 12 Oct 2004 (UTC)
replies: text:  I guess I'm a bit mystified by the longevity of the laser guys picture in this article. The caption is correct, the speed of light is less in air than vacuum, but one can't see that from the picture. The picture is also somewhat misleading since it might be construed as an illustration of the light-slowing experiments referenced in the text. A picture of a glass lens or a diamond would show an observable effect of the different speeds of light, come to think of it. I'll keep my eyes open. Wile E. Heresiarch 13:38, 12 Oct 2004 (UTC)
thread_title:  Replace picture at top of article? 

message: I have noticed that the article uses both spellings. Which is the appropriate spelling for this article?
:Thanks - metre is far more common in the article, so I've changed the other couple of occurences.  -- ALoan (Talk) 19:09, 11 Oct 2004 (UTC)
thread_title:  Meter or metre? 

message: I've reverted to put back the image of Cherenkov radiation as the lead image: a featured article ought to have a lead image, and this is the best example I can think of the image the actually shows "the speed of light" in a visible form.  But I'd happily defer if someone can think of a better example. -- ALoan (Talk) 11:30, 16 Dec 2004 (UTC)
thread_title: Lead image 

message: Substituted at 22:04, 3 May 2016 (UTC)
thread_title: Assessment comment