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Speed of light in a medium

The current opening paragraph in this section seems rather confused to me and surely does not represent Feynman's words on the subject as the reference suggests. We currently have:

When light enters materials, its energy is absorbed. In the case of transparent materials, this energy is quickly re-radiated. However, this absorption and re-radiation introduces a delay. As light propagates through dielectric material it undergoes continuous absorption and re-radiation. Therefore the speed of light in a medium is said to be less than c, which should be read as the speed of energy propagation at the macroscopic level. At an atomic level, electromagnetic waves always travel at c in the empty space between atoms. Two factors influence this slowing: stronger absorption leading to shorter path length between each re-radiation cycle, and longer delays. The slowing is therefore the result of these two factors

This seems to mix up two models into one confused explanation. In classical electromagnetic theory, the original wave is extinguished by interaction with the bound electrons in the atoms of the substance and replaced with another wave moving at a lower velocity. This is a bulk phenomenon, representing the interaction of an EM wave with the huge number of electrons in the material.

Where is the confusion? The EM-wave encounters a field of bound-charges. You then have a simple case of a field of driven oscillators. The new wave, travelling at c, is in turn extinguished like the original. As with any driven oscillation (with a mass) each generation undergoes a phase-delay - thus the phase-velocity of the propagation is reduced.

You suggest that the 2nd generation wave propogates through the field of bound electrons without further interaction - which is not possible.

I think your interpretation of the text above is somewhat charitable. I think that whoever wrote it was trying to explain the quantum behaviour of light in a medium in layman's terms, as TimothyRias says below. The text has been changed now anyway. Martin Hogbin (talk) 23:10, 15 June 2010 (UTC)

In QED photons are repeatedly absorbed and emitted by atoms resulting in a delay in the signal propagation through the material. As photons are quantum entities, I would stop short of saying that they travel at c between the atoms although this may be the mental model that many people have. Martin Hogbin (talk) 14:08, 13 May 2010 (UTC)

Stopping short implies that photons might be travelling at some other velocity...

I think the text is trying to explain the quantum behaviour of light in a medium in laymen's terms. Talking about virtual processes as if they were real always makes me cringe a little, but it is exactly the sort of thing Feynman does all the time. Someone with quick access to the Feynman lectures should check that this paragraph accurately reflects his explanation, though. TimothyRias (talk) 08:34, 17 May 2010 (UTC)

No, IIRC, Feynman gives a purely classical explanation in that section (treating each electron as a harmonic oscillator, essentially; he only mentions at the end that for quantum reasons each one actually has several frequencies). He assumes an incoming wave on the left of a plane layer of transparent material, and describes the outgoing wave at the right a superposition of the incoming one and a wave generated by oscillating electrons, and the result has (at sufficiently low densities) almost the same amplitude but different phase than the original wave. If it's exp(ikx - i\omega t + i\phi_1) on the left, it's exp(ikx - i \omega t + i\phi_2) on the right, then he shows that \phi_2 - \phi_1 is proportional to the thickness, so that inside the material it can be written as (i(k+k')x - i\omega t + i\phi_1) and calls \omega/(k+k') + c/n. (When I get back home, I'll check whether it's actually as I remember it.) A. di M. (talk) 09:35, 17 May 2010 (UTC)

Yes, I checked this out and Feynman is discussing the classical model in which the original wave is extinguished and a new one created.

As I suggest above, I am not against giving a layman's QED answer but we must be careful not to describe photons as classical particles. Martin Hogbin (talk) 22:19, 18 May 2010 (UTC)

I definitely agree on the last bit. Any ideas for a clear exposition? TimothyRias (talk) 07:46, 19 May 2010 (UTC)

I wish I had more insight. I don't know if I like any qualitative description of how the speed of light varies in a material before the article tells me which speed of light it is talking about. The mechanism for the phase speed is different then for the group speed and the front speed. My first instinct is that description fits the description of the front velocity and not the phase velocity since phase velocity can be and is for certain circumstances greater than c. The explanation for phase velocity almost certainly needs to involve waves since it is a wave phenomenon.

This article has progressed a lot since I last saw it, but this section is still its Achilles's heal. (The only other significant item is the amount of physics speak in the relativity section.) I will see if I can find something better somewhere. TStein (talk) 06:37, 22 May 2010 (UTC)

As this article is not about light, it is somewhat out of the scope of the article to provide an explanation of why light travels at a different speed in a medium. I've been bold and rework the section to omit this, and focus on relaying the facts about light propagation in a medium. I've also added a good source discussing in detail the bussiness of group and phase velocities being superluminal or negative, and showing that the front velocity is equal to c. TimothyRias (talk) 15:48, 25 May 2010 (UTC)

Looks good. I've copy-edited it a little. A. di M. (talk) 18:21, 27 May 2010 (UTC)

Am I interpreting this wrong?

'From the observation that the periods of Jupiter's innermost moon Io appeared to be shorter when the earth was approaching Jupiter than when receding from it, he concluded that light travels at a finite speed, and was able to estimate that would take light 22 minutes to cross the diameter of Earth's orbit.'

Light would take about 1/23 of a second to cross the diameter of the earth. What's this 22 minutes all about? —Preceding unsigned comment added by 91.108.4.98 (talk) 14:02, 29 May 2010 (UTC)

The time needed to cross the diameter of the Earth's orbit. Not the time needed to cross the diameter of the Earth.TimothyRias (talk) 20:04, 29 May 2010 (UTC)

Misusing of refs

Jagged 85 (talk · contribs) is one of the main contributors to Misplaced Pages (over 67,000 edits; he's ranked 198 in the number of edits), and practically all of his edits have to do with Islamic science, technology and philosophy. This editor has persistently misused sources here over several years. This editor's contributions are always well provided with citations, but examination of these sources often reveals either a blatant misrepresentation of those sources or a selective interpretation, going beyond any reasonable interpretation of the authors' intent. Please see: Misplaced Pages:Requests for comment/Jagged 85. The damage is so extensive that it is undermining Misplaced Pages's credibility as a source. I searched the page history, and found 39 edits by Jagged 85 (for example, see this series of edits). Tobby72 (talk) 20:45, 14 June 2010 (UTC)

From the edits above, the information that needs to be checked is the following text that is currently in the History section:

Early Islamic philosophers initially agreed with the Aristotelian view that light had no speed of travel. In 1021, Islamic physicist Alhazen (Ibn al-Haytham) published the Book of Optics, in which he used experiments related to the camera obscura to support the now accepted intromission theory of vision, in which light moves from an object into the eye.

This led Alhazen to propose that light must therefore have a finite speed, and that the speed of light is variable, decreasing in denser bodies.

Also in the 11th century, Abū Rayhān al-Bīrūnī agreed that light has a finite speed, and observed that the speed of light is much faster than the speed of sound.

I have seen the discussion of this editor's work, and there is wide consensus that enormous misrepresentations of sources have occurred, so this text needs to be confirmed. Johnuniq (talk) 23:55, 14 June 2010 (UTC)

Well, the MacTutor and other sources quoted support the claims made almost verbatim. Since those sources are generally reliable the facts seem OK. The only issue remaining is the somewhat undue emphasis of the Islamic nature of these philosophers. TimothyRias (talk) 08:24, 15 June 2010 (UTC)

Agreed. Are there any reliable sources suggesting that the religion of the philosophers was relevant to their scientific theories. If not the reference religion should be removed. Martin Hogbin (talk) 23:14, 15 June 2010 (UTC)

The point is the culture, not the religion. People of this culture are often referred to as Islamic (or Muslim) or Arabic (that is, by religion or by language) to distinguish them from other cultures that existed at about the same time in the same region. This should not be taken as an indication that the religion had much to do with the science. The early islamic philosophy article refers to Alhazen as an "Arab polymath"; some sources call him "Persian"; he was born in what's now Iraq and lived mostly in Egypt. Dicklyon (talk) 02:46, 16 June 2010 (UTC)

Are we ready for FA?

When I returned to this article last week, after about a month I hadn't significantly edited it, I found it even better than I remembered. I am quite positive that it is ready for FA status, but can anyone find any issue with it before I nominate it again? ― A._di_M. (formerly Army1987) 18:41, 10 July 2010 (UTC)

I have an issue with the 'Fundamental role in physics section. In this section there important facts about the subject of the article have been relegated to footnotes. Why are they not in the main text? Martin Hogbin (talk) 23:40, 10 July 2010 (UTC)

The Doppler effect and the Terrell rotation are quite irrelevant to the point being made, so I would keep them where they are. The Scharnhorst effect is so small that it might well be never observed during the lifetime of anyone around here, so it was agreed that it was undue weight to put it in the main text.

Anyway, I'm going to move the reference to the relativity of simultaneity and the tachyonic antitelephone back into the main text (being sent to a footnote only comprising one link distracts more than seeing it in the text, IMO). As for the note on one-way vs two-way speed, I think it should be in the main text too, but I seem to remember there once was an opposition to that and was moved to the footnote as a compromise. ― A._di_M. (formerly Army1987) 13:56, 11 July 2010 (UTC)

Measurement

The introduction says about the speed of light: “Its value is exactly 299,792,458 metres per second”. Sydenham is cited in this connection, as is Jespersen “turning c into a conversion factor whose value is fixed and arbitrary” (p. 280). I'd suggest that some reconciliation of this viewpoint be attempted in the section Measurement which explains many methods for determining the speed of light and cites values like 299,710±22 km/s with error bars, as is appropriate only for an uncertain quantity, not an exact value.

I do not feel comfortable in pursuing a discussion of these matters, which caused me a great deal of difficulty with ArbCom in the past, and brought the most extreme invective and vituperation upon me that I have experienced in my 71 odd years of life.

However, it still seems to me that a naive reader is likely to wonder what is going on here. I hope that some enterprising soul can brave this wilderness to bring some clarity to the Measurement section. That involves at most the addition of a few sentences of reconciliation. Brews ohare (talk) 20:49, 15 July 2010 (UTC)

As for the introduction, the reader is going to understand what is going on by the time they get to the end of the third paragraph. Maybe a paragraph could be added after the first paragraph of "Measurement" stating that today measuring c in metres serves no purpose (other than verifying that your measuring instruments are properly calibrated and properly working) and that measuring it in some other unit is equivalent to measuring the length of other unit in metres ... but I'm not sure of how to word it. What would you propose? —Preceding unsigned comment added by A. di M. (talk • contribs) 08:06, 16 July 2010 (UTC)

A. di M.: I wouldn't hazard an attempt at wording given the history of conduct among editors of this article.

The underlying difficulty with the Speed of light article is short shrift given to how a system of units based upon replacing ‘distance’ with ‘time-of-flight’ compares with a different system where distance and time are kept separate. That comparison could be done in a general manner for any speed standard, explaining the need for reassurance that the "standard" speed has been realized in any given measurement, and the role of definitions in making that speed "exact". That presentation could then be narrowed to describe why light-speed is a good choice. Brews ohare (talk) 11:36, 16 July 2010 (UTC)

But there aren't two or more systems: modern physics is based on special relativity, from which we get that the speed of light is fixed, so distance can be defined in terms of time. Historically there were many different ways to understand it, all covered in the article, but the current system has been settled science for about a hundred years. The definition changed more recently, but standards bodies are much more conservative than scientists, and have to take account of how easily a definition is to use.--JohnBlackburne_deeds 13:53, 16 July 2010 (UTC)

The idea of a fixed, finite speed of light dates back to Rømer (who also used time of flight as his measure of distance, as astronomers still do to this day). This subject has been discussed ad nauseam on this page, there is no need for any additional clarification, and certainly no need for philosophical ponderings of what might have been had physcics been different. Physchim62 (talk) 14:52, 16 July 2010 (UTC)

FWIW, the idea of a fixed speed of light only dates back to special relativity, actually. Before then, it was believed to depend on the frame of reference, which left Michelson and Morley scratching their heads. A. di M. (formerly Army1987) (talk) 02:07, 17 July 2010 (UTC)

"I do not feel comfortable in pursuing a discussion of these matters, which caused me a great deal of difficulty with ArbCom in the past, and brought the most extreme invective and vituperation upon me that I have experienced in my 71 odd years of life." — well don't do it then! Physchim62 (talk) 14:52, 16 July 2010 (UTC)

John, and Physchim62: Your remarks are not at all responsive to the matters raised. To recapitulate, the section on Measurement describes many attempts to measure the speed of light and quotes the results with error bars appropriate to an inexactly known quantity, which appears to be in conflict with the statement of the Introduction that the speed of light is an exact value. That is confusing, and could be clarified with some remarks reconciling these two views in the Measurement section. This is a matter of making the logic clear, and is not a question about historical development or modern practice. I'm sorry to see your intemperate reaction, Physchim62. Brews ohare (talk) 15:10, 16 July 2010 (UTC) -

(edit conflict) And I'm sorry that you have chosen to ignore Coren's good advice. The logic is clear: after a couple of centuries of ever-decreasing the error bars on measurements of the speed of light against standards of the metre, the uncertainty in the definition of the metre became the limiting factor. Rather than stop improving measurement techniques, the definition of the metre was changed so that the speed of light became the defining factor. This is no different from, say defining the ampere in terms of the magnetic constant and not in terms of a number of electrons which passes a given point per second, or saying that the mass of a carbon-12 atom is exactly 12 atomic mass units. All of these are definitions which have changed over the years, without any logical inconsistency. Physchim62 (talk) 15:33, 16 July 2010 (UTC)

Perhaps the last sentence of the lead "...the numerical value of c in metres per second is now fixed exactly by the definition of the metre." could be moved up... Count Iblis (talk) 15:31, 16 July 2010 (UTC)

And again, Physchim62, your are beside the point. There is no question of logical inconsistency in the historical record or in modern practice. The question is simply one of clarity of presentation in a WP article called Speed of light of two different approaches: the “time-of-flight” approach that uses a definition with an exact value for c, and the approach that uses separate ‘time’ and ‘distance’ units and therefore a measurement of c as ‘distance/time’. Brews ohare (talk) 15:50, 16 July 2010 (UTC)

But there aren't two different approaches, there's only one, the one given in the article. In what we think of as modern physics it's one of the best established facts, that the speed of light in a vacuum is a constant, which fixes the definition of length in terms of time. Any other approach is historic (and covered already) or wrong (and so has no place here).--JohnBlackburne_deeds 16:46, 16 July 2010 (UTC)

John, you are mistaken: please re-read the article itself carefully. The standards methodology changed in 1983 to achieve greater reproducibility and accuracy. Read this section and take a look at the references, which provide a more detailed discussion, in particular, Sydenham is cited in this connection, as is Jespersen. That is why error bars appear on the numbers in the Measurement section. I'd say that if a person can come away from the Speed of light article without this awareness, the article is lacking. Brews ohare (talk) 17:19, 16 July 2010 (UTC)

Brews, you have just plunged headlong into the same discussion that, 'brought the most extreme invective and vituperation upon me that I have experienced in my 71 odd years of life'. I am sure that you have been right about many things in your life but in this particular case please just consider the possibility that you might be wrong before going any further. Martin Hogbin (talk) 17:32, 16 July 2010 (UTC)

Thank you Martin, for that advice. Frankly, I see nothing at all controversial in what I have said here. My view of the reaction to these rather simple remarks is that in the minds of several (among whom Physchim62 clearly is one), old, ingrained arguments that have not been brought up here at all, dominate their thoughts and prevent them from reading what is before them. Like Pavlov's dogs, certain sounds cause salivation, even when no food is present. So, the Speed of light article is free to continue in its present confusing form, that even an erudite editor like John Blackburne will misinterpret. I will follow your advice, which I take as urging my withdrawal. Brews ohare (talk) 17:48, 16 July 2010 (UTC)

Well, distances could be and were measured through transit times before 1983, and they still can be and are measured by other means today. The only thing that changed was which of the conversion factors needed to express one's measurement in metres is exact. Section 6.3 of the current version already clearly explains the purpose of the redefinition; what is confusing with it? A. di M. (formerly Army1987) (talk) 02:07, 17 July 2010 (UTC)

BTW, the "the approach that uses separate ‘time’ and ‘distance’ units and therefore a measurement of c as ‘distance/time’" was not quite what the most precise measurements in the 1970s were about: the experiments described in "Laser interferometry" were essentially measuring the ratio of two frequencies. A. di M. (formerly Army1987) (talk) 10:29, 17 July 2010 (UTC)

Gentlemen, the sentence should state, by international agreement, for purposes of measurement in the SI system of units, the value of the speed of light is taken to be exactly... This is a rather simple solution to the difficulty and Brews was correct to raise the point.72.64.55.156 (talk) 21:26, 16 July 2010 (UTC)

It already does say that, except in more detail and more clearly, in the third paragraph. But most of the time the history of the definition is inimportant - when you use c in a formula like E=mc for example you should know it's a constant, and to do calculations you need to know it's value, but that's all. A curious or attentive reader might wonder why it's exceptionally an exact value, and can find the theory behind and the practical reasons for this later in the article.--JohnBlackburne_deeds 21:46, 16 July 2010 (UTC)

The suggested sentence gives the impression that there is a 'real' definition of distance that has somehow been ignored by the definition of the metre. Such notions have long since been abandoned by physicists. Even so, the metre is still defined as a unit of proper length. Martin Hogbin (talk) 08:34, 17 July 2010 (UTC

You gentlemen seem to be throughly confused. There is no constant of physics that is known exactly, except by a definition as to the agreement of the value to be used. All of this talk is beside the point. You have said that a constant is known exactly through measurement and that is impossible. Please stop putting impossible physics into Misplaced Pages articles, we have enough of that already.

Who said it is "known exactly through measurement", exactly? A. di M. (formerly Army1987) (talk) 14:04, 17 July 2010 (UTC)

A. di M.: Your comment above about "measuring the ratio of two frequencies" is the kind of detail that requires more attention in this article. Statements like Blackburne's “aren't two different approaches, there's only one, the one given in the article” are indicative of the confusion generated around this topic. Replies like Physchim62's “there is no need for any additional clarification, and certainly no need for philosophical ponderings of what might have been had physics been different.” are total irrelevancies.

I hope you can clear up and explain better how the measurement situation led to the decision to change the basis of the units from a ‘time and space units’ approach to a ‘time-of-flight’ approach requiring only the second, and connect that to today's definition that doesn't employ error bars like 299,710±22 km/s, but an exact value 299,792,458 m/s. It's a big challenge, given that so many want to believe that they understand perfectly what happened in 1983 but cannot tolerate any attempt to explain it more carefully. Brews ohare (talk) 16:48, 17 July 2010 (UTC)

It may be that a separate article should be written to discuss at length the comparison of “time and space” units approaches and “time-of-flight” approaches using only the second. That would allow for a more detailed discussion of measurement techniques, historical developments, and the difference in logical basis allowing the use of a defined speed of light. It also would allow for the treatment of issues such as how we can check that c is a universal constant even though we use a defined value for it. And above all, it would allow referral to this article from Speed of light and thereby remove all this furor to a different venue. Brews ohare (talk) 16:58, 17 July 2010 (UTC)

If we move this to a different venue it will be to return to ArbCom; all your points have been discussed at great length before, and the discussions can be found in the archives of this page. That you choose to to return here with exactly the same arguments once your topic ban from physics articles was lifted is a sign that you do not wish to improve this page, simply to find an outlet for your minority views. Physchim62 (talk) 17:25, 17 July 2010 (UTC)

Physichim62: Threats of ArbCom action as a response to a mild suggestion to create a separate article for more in-depth treatment is exaggerated and hostile. Brews ohare (talk) 17:49, 17 July 2010 (UTC)

Again there is no "difference in logical basis", there is only the current correct and universally accepted approach. Everything else is wrong and/or historical, and is already covered at length in the article. There's certainly no need for a separate article, especially not to explore alternative (i.e. incorrect) interpretations of the science.--JohnBlackburne_deeds 17:07, 17 July 2010 (UTC)

John: An approach using both length (metre) and time (second) units is not "incorrect". It is the system in use prior to 1983. It requires a speed to be established by measuring a distance traveled and the time of travel, and as both of these measurements are indeed measurements, the speed has an experimental error bar. An approach using a standard speed also is perfectly viable. It could be the speed of sound in a specified medium, for example. Then a distance is determined as a "time-of-flight", that is, how long it takes the sound wave to travel the length. Such a system requires only a time unit, the second, and all lengths are determined in terms of times-of-flight in seconds. In choosing between these two systems, it is not their "correctness" that is at stake. What is involved is practical matters, most notably, how much trouble it is to insure that the standard speed has actually been realized. In the case of a standard speed in terms of the propagation of sound, the issue is how readily and accurately one can ascertain that the standard medium has been realized. It is when considering this matter that the speed of light becomes so highly recommended, because experiment has shown the speed of light to be readily realized in a great variety of circumstances, available to all observers without undue concern and preparation. Brews ohare (talk) 17:23, 17 July 2010 (UTC)

Suggestion

Might I suggest that this discussion is continued on a separate talk page where those that wish to discuss the subject can do so and this page can continue to be used for improving the article in other respects. I have started a page at Talk:Speed_of_light/Definition_of_the_metre for those interested and have copied Brews paragraph above there. Martin Hogbin (talk) 18:52, 17 July 2010 (UTC)

That's a good idea. I think whether Brews is right or wrong about how to present the definition of the meter etc. can be gauged better by improving the article on other points to make the article ready for FA review. Then we'll get comments from univinvolved editors who may or may not notice the same issues as Brews is raising. Then, if the FA review is successful and the article appears on the main page, we can expect a lot of comments from a much larger audience. So, if people are really surprised that c has an exact value and want the article to explain this in a clearer way, then we'll get that feedback. Count Iblis (talk) 19:03, 17 July 2010 (UTC)

I like this idea myself. I make this suggestion: in a day or so I'll set up a user page along the lines of Speed of light pre- and post-1983 and present what seem to me to be the salient topics. On its discussion page all are invited to comment. How does that sound? Brews ohare (talk) 19:23, 17 July 2010 (UTC)

Fine. Martin Hogbin (talk) 20:12, 17 July 2010 (UTC)

Exact value for c in miles per second.

In SI units the mile is defined exactly in terms of the meter, which means c has an exact rational value in units of miles per second:

${\displaystyle \textstyle c={\frac {18737028625}{100584}}}$ mi/s.

It is impossible to convert meters/sec to miles/second using a decimal conversion factor. —Preceding unsigned comment added by NOrbeck (talk • contribs) 11:13, 17 July 2010 (UTC)

This is already mentioned in footnote 1. A. di M. (formerly Army1987) (talk) 14:06, 17 July 2010 (UTC)

Proposal for a new article concerning time-of-flight and length-and-time standards

A rough, preliminary draft of a proposed article can be found here. Comments are invited on its discussion page.

This article is intended for the naive reader to help them understand the change in SI units that occurred in 1983. The article is far from final form, and some help in its construction would be appreciated. Brews ohare (talk) 16:50, 18 July 2010 (UTC)

Aside from the way it is written - after the first paragraph is seems largely devoid of encyclopaedic writing, and reads more like an essay than an article making arguments largely unsupported by the references - I don't see the need for such an article. Anything that needs to be said of the definition of the metre should be in the article Metre, or on the speed of light in this one. The history of it is interesting, as it tells us a lot about how our understanding of the universe has developed, but that's already covered.--JohnBlackburne_deeds 16:59, 18 July 2010 (UTC)

The article is not about the metre per se, but about two definitions of the metre, a ‘time-of-flight’ definition and a ‘length’ definition. The idea is to contrast the approaches and point out why one provides better reproducibility and accuracy than the other. Improvements are expected and recommendations and comments are invited. Brews ohare (talk) 05:22, 19 July 2010 (UTC)

Re the 1st sentence of the draft, "A change in the meaning of the term speed of light as used in the SI system of units occurred in 1983." – Perhaps you meant the physical constant c, which changed to have a specific fixed value because of the change in the definition of the meter? My feeling is that the speed that light travels in a vacuum is what it is, regardless of what humans do, whereas the value and units of the physical constant c are derived from definitions made by humans. (BTW, I'm not sure if this view of mine is in the consensus of opinion here.) I recognize that you might have had this distinction in mind when you wrote "term" but this subject can easily get confusing if it isn't made as clear as possible.

Also, assuming this article isn't a POV fork, it seems that the material in it could be covered in existing articles, if it isn't already. Regards, --Bob K31416 (talk) 08:13, 19 July 2010 (UTC)

This is nothing but a WP:FORK. I also note that you are again try to push the same idiosyncrasies as you did before you got banned. You'd think that after the Arbcom kurfuffle you would have learned your lesson and stop beating deadhorses, but I guess this is impossible for you (as we all feared). You yourself said this was the worse experience of your life, and yet you're back asking for more. Brews, please drop the stick and find yourself a less controversial article for you to edit. Because I'm really not looking forwards to ARBCOM/Speed of Light 2, which will inevitably happen if you keep at it. Headbomb {talk / contribs / physics / books} 10:25, 19 July 2010 (UTC)

Does light always travel at c, or does it never travel at c?

These are not rhetorical questions. I'm curious, if c in is the speed of light in a vacuum, and pure vacuums do not exist in nature, why is it claimed that that physical light travels at c? Does light always travel at c as claimed in section 4, or does it never travel at c as claimed in section 5? Isn't it common knowledge that c is a concept distinct from "the speed of light", and that it's just called that for historical reasons? Is anyone willing to argue that c is in fact "the speed at which light travels"? The question is, does light travel at c (y/n)? NOrbeck (talk) 07:32, 19 July 2010 (UTC)

I think the article deals with the issues that you have raised. In current models of physics c is a fundamental constant representing, among other things, the speed of light in a vacuum. You might say, as you suggest, that there is no such thing as an absolute vacuum, indeed there can be no such thing except in a completely empty universe. However, most of physics is based on concepts that do not actually exist so the speed of light is no different in that respect.

Bear in mind also that the article is entitled 'The speed of light' and should therefore have some basis in the natural meaning of these words. The philosophy of this subject has been discussed at some length and we could continue to discuss it ad infinitum but I think the article in its current state reflects the current state of physics regarding the subject reasonably well. That is not to say it could not possibly be improved. Do you have any suggestions? Martin Hogbin (talk) 08:12, 19 July 2010 (UTC)

Re "The question is, does light travel at c (y/n)?" – y --Bob K31416 (talk) 08:24, 19 July 2010 (UTC)

Yes. We 'know' that light travels at c in a vacuum as well as we 'know' most other things in physics. Martin Hogbin (talk) 08:34, 19 July 2010 (UTC)

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