Talk:Speed of light/Archive 13

Are we serious about style and quality here?
In the lead we have, '... a vacuum where there are no atoms, molecules or other types of matter to slow it down'., and later on we have, 'In non-inertial frames (gravitationally curved space or accelerated frames), the local speed of light is constant and equal to c, but the speed of light along a trajectory of finite length can differ from c, depending on how distances and times are defined'.

Am I the only one that thinks this language and terminology is not suitable for a high quality encyclopedia? Martin Hogbin (talk) 22:55, 27 November 2009 (UTC)
 * See WP:NOT PAPERS, points 5 and 7. Being able to be understood by readers is more important than using the most precise terminology possible, especially in articles such as this one which are likely to be read by laymen. (Of course, we shouldn't take this to the extreme of making false statements, but I don't think there's any such issue in the article.) OTOH, I agree that the definition of "vacuum" in the lead is unnecessary: that light is slowed down by matter is the whole point of the fourth paragraph; I can't see the point of mentioning atoms and molecules here; and, to be really pedantic, neutrinos are matter according to most definitions of "matter", but AFAICT they can't slow light down. -- _ _ _ A. di M. 12:07, 28 November 2009 (UTC)
 * In addition to the other reasons I gave in my edit summary, "... very concisely clarifies vacuum, matter, and effect of matter on the speed of light for the general public, which is appropriate for the lead" I should add another important reason, which is that it gives the reader an understanding of why a vacuum is used for specifying the physical constant c, the speed of light. Here's the phrase, which is the last part of the following sentence from the lead.


 * Also, I have addressed the precise point mentioned by A. di M. re neutrinos by changing "to" to "that can". --Bob K31416 (talk) 16:29, 28 November 2009 (UTC)


 * I have no objection to a clear and understandable explanation of the affect of matter on the speed of light but a single sentence in the lead is not the way to do it. Most people know what a vacuum is and I would suggest link for those who want more detail.  The current wording also takes it for granted that matter would slow light down.  My comment was also referring to style as well as technical accuracy.  I make no claim to have the perfect writing style but the quoted sections do not make the grade in my view.


 * I really would like to know what others think about this. If most others think that the quoted sections are OK then I will leave the matter, and probably this page.  Martin Hogbin (talk) 12:34, 28 November 2009 (UTC)

I am reading the following sentence in this article: "In models of the expanding universe, the farther things are from Earth, the faster they move away from us". So, we must be living in a geocentric universe, right? —Preceding unsigned comment added by 78.139.5.39 (talk) 07:23, 28 November 2009 (UTC)
 * No, we mustn't. Draw a few dots on a balloon and inflate it. Consider a particular dot A. The farther another dot is from A, the faster it moves away from A. But this is true no matter which dot we call "A". BTW, article talk pages are for discussing improvements to articles; for questions like that one, there's the WP:Reference desk. (OTOH, I'm going to replace "things" with "galaxies": bound systems don't enlarge due to the expansion of space.) -- _ _ _ A. di M. 12:07, 28 November 2009 (UTC)


 * I think the comment was meant as criticism of the text, rather than a request for information. I made a corresponding edit in the article to address the criticism. --Bob K31416 (talk) 22:36, 28 November 2009 (UTC)

For simplicity, consider the universe as an expanding 3-dimensional hypersphere with radius r(t) in a 4-dimensional space. The distance s between two points would be measured along a great circle of the hypersphere and would be given by the radius r(t) times the angle θ in radians that is subtended by the arc between the two points.
 * $$ s = r(t) \ \theta $$

If the radius of the universe was expanding at a rate linear in t,
 * $$ r(t) = r_0 + v_0 \ t $$    with constants  r0 and  v0,

then the two points would be moving away from each other at a constant velocity
 * $$ v = \frac{ds}{dt} = v_0 \ \theta $$.

Note that the velocity depends on the angular separation θ between the two points, so that at any particular time, points farther away from each other are  moving away from each other faster than points that are closer to each other. However, note that θ is constant for two given points and that the points are moving away from each other only because the radius r(t) of the hyperspherical universe is increasing.
 * --Bob K31416 (talk) 19:10, 29 November 2009 (UTC)

Physics getting worse
'The speed at which light propagates in vacuum is independent of both the source of the light and the frame of reference (1) of the observer. This was first postulated by Albert Einstein in 1905, motivated by Maxwell's theory of electromagnetism and the results of the Michelson–Morley experiment (2), and has since been confirmed by various experiments (3).The theory of special relativity explores the consequences of the existence of such an invariant speed c and the assumption that the laws of physics are the same in all inertial frames of reference. One particular immediate result is that all massless (4) particles and (5) waves, such as light, must always travel with the speed c, which justifies calling c the "speed of light" '.

Where have all the physicists gone?

1 Only true for inertial frames.

2 It is not clear that Einstein knew about the MMX.

3 The constancy of the one-way speed of light is a convention that cannot be confirmed by experiment. This was stated previously.

4 This is not an 'immediate result' of SR

5 Is that all waves that must always travel at c or just 'massless waves', whatever they are. Neither statement makes any sense.

There are several other dubious points. Martin Hogbin (talk) 23:29, 1 December 2009 (UTC)
 * Agreed. I'm waiting for Bob K31416 get bored and leave, then we can correct all of this. --Michael C. Price talk 07:28, 2 December 2009 (UTC)
 * Michael, In addition to the remarks at the end of TimothyRias's message below (thanks Tim) please note that all my changes to the section under discussion were reverted previously by Sebastiangarth here and he explained his reasons for doing that here. You might have been confused by the link that Sebastian gave in his message which wasn't to my version, but to TimothyRias's version. I tried to clarify that a couple of messages later here. Anyhow, I am taking a break from the article. --Bob K31416 (talk) 10:05, 2 December 2009 (UTC)
 * I apologise for implying that you were the author of the passage. Martin's substantive criticism of the physics content remains (see my response to Tim below). --Michael C. Price talk 10:32, 2 December 2009 (UTC)
 * Reply
 * 1) Actually true in any local frame, since those are always (considered to be) inertial. It might be better to just write velocity instead of frame of reference though. That is less jargony anyway, and less ambiguous.
 * 2) The second paragraph of Einsteins 1905 paper where he states: "Examples of this sort, together with the unsuccessful attempts to discover any motion of the earth relatively to the ``light medium,''" strongly suggest that he was. After all MMX was the most prominent of the attempts to measure the speed of the earth relative to the ether. Moreover, SRT texts generally assume that he was.
 * 3)As is stated in the footnote to that statement.
 * 4)That anything with a massless energy-momentum must travel with the speed of light IS an immediate result of SR. (as you can again find in any SRT text book).
 * 5)A massless wave, is a wave in which the wave (4-)vector is massless. This makes perfect sense physically.
 * Also, Michael, Bob had little to with that paragraph since I wrote it. It would be a lot more productive if you guys comment on such changes when they are proposed instead of just bitching from the sideline. TimothyRias (talk) 08:49, 2 December 2009 (UTC)
 * Okay, well here is some more specific feedback.
 * 1) Frames are not always local or inertial. Light slows down in a gravitational field; the gravitational field disappears in a local frame, but not in all frames; ergo light is only constant in inertial frames, not all frames.
 * 2) This reference by Einstein in his 1905 paper is probably not about MMX, but to other attempts to detect the ether. Einstein is on record, early on, as saying that he hadn't heard of the MMX null result until after 1905, although later in his life, when we can presume his memory would not be so clear about distant events, he contradicted himself on this point.  (Cf A P French's standard textbook (or see Michael Polanyi on this point) - French concludes that Einstein had not heard about the MMX -- and although you can find many texts that assume the reverse, they are wrong, IMO.)
 * 4 & 5) was a point about poor wording, not about the background physics. --Michael C. Price talk 09:34, 2 December 2009 (UTC)
 * So, basically I see some points about poor wording and one about history (I must confess I wasn't aware that Einstein was not aware of the MMX result. I'm a theoretician not an historian, Jim!). I see no substantial criticism of the underlying physics. Nobody ever claimed the wording was perfect (in fact when I introduced this version I explicitly assumed (and said) that it wasn't.) Nobody is stopping you from making edits to improve the wording. So, why aren't you? TimothyRias (talk) 11:11, 2 December 2009 (UTC)
 * Because editing this article has become a frustrating business. Perhaps I'll have a go again. --Michael C. Price talk 11:28, 2 December 2009 (UTC)
 * About point 1), I've tweaked the wording to circumvent the problem. (Which just is the usual trouble of staying concise and correct at the same time. TimothyRias (talk) 11:11, 2 December 2009 (UTC)


 * I guess I will leave you guys to it. Martin Hogbin (talk) 14:27, 2 December 2009 (UTC)

I was also thinking of mentioning the Lorentz factor in the second paragraph. (Or at least, even if we don't name it and give its expression, somehow making the point that time dilation etc. are totally negligible at "human" speeds and diverge as v approaches c.) (I'm gonna write a draft of that later this evening.) -- _ _ _ A. di M. 17:25, 2 December 2009 (UTC)


 * The best context for metioning the Lorentz factor would IMHO be a paragraph explaining that c is the supremum speed that massive objects can reach. This might work well in the current FTL section. (which might need a rename then). This way where staying close to the core topic of the speed of light without digressing too much. TimothyRias (talk) 21:52, 2 December 2009 (UTC)
 * Agree about the title of the FTL section. Right now, the TOC mentions the exception but not the rule. In my earlier draft, there was a section "Maximum speed" including the current first paragraph of the FTL section plus two now-deleted paragraphs about light cones, and then a subsection "Faster-than-light observations and experiments" containing the stuff in the second and later paragraphs of the current FTL section. (OTOH, re-adding the sentence "These effects are very small when the speeds involved are much slower than c, in which case special relativity is closely approximated by Galilean relativity." from the 29 November version back to the now-second paragraph of the "Fundamental role" wouldn't hurt—going to do that right now. -- _ _ _ A. di M. 22:43, 2 December 2009 (UTC)
 * BTW, this is roughly what I had in mind, but I think your proposal might be better. -- _ _ _ A. di M. 22:43, 2 December 2009 (UTC)

Einstein and the MM experiment
So there seems to be a problem here. It is not completely clear whether Einstein new about the MM experiment. It seems unlikely that he was completely unaware of this result, as it was quite general knowledge at the time. It may be that he was aware of the result (movement with respect to ether had not been detected) without knowing the details of the experiment. (Much like most physics students (and a significant number of non-physcists) nowadays will be aware that observations indicate the universe is expanding but many will not know the details of either WMAP or of the high redshift supernova results.) What is clear though is that whether he knew about it or not it did not play a large part in his own motivation. This makes the current statement in the article an overstatement at the very least.

On the other hand, the result probably was very important to his contemporaries in accepting the postulates of SR. Moreover, most textbooks list the MM experiment as a major motivation for SR. As such, it is good to mention the connection of the postulate to MMX. Anybody have good idea how to word this without digressing to much into historical nuances. TimothyRias (talk) 15:42, 2 December 2009 (UTC)
 * I think the current wording is OK. I cannot believe that Einstein didn't know the result of the MM experiment, which was carried out eighteen years before the description of SR. If he didn't mention it, maybe it was because he didn't think it was important to prove his theory: we shouldn't judge this on our current standards of scientific proof and publication. Physchim62 (talk) 16:22, 2 December 2009 (UTC)
 * Yes, I think the current wording is OK, thanks to the presence of "the results of" in the phrase "...and the results of the Michelson–Morley experiment...". This leaves the question right in the middle - where it seems to belong. DVdm (talk) 16:31, 2 December 2009 (UTC)
 * More correct but also more wordy would be "... and by the failure of experiments to detect differences in the speed of light due to the motion of the Earth (such as the Michelson–Morley experiment), and ...". One more advantage would be that even readers who have never heard of the MMX could get the point of that part of sentence; but I'm not sure whether that justifies the greater wordiness. (And if there still are people who could take "such as" to imply "including", just replace it with "like", although it might be a tad too informal.) _ _ _ A. di M. 17:17, 2 December 2009 (UTC)
 * To Physchim62: there was no Internet in 1905, so that he might have been unaware of an experiment performed on the other side of the Atlantic 18 years earlier is strange but not completely implausible. Our Annus Mirabilis papers article claims, "At the time the papers were written, Einstein did not have easy access to a complete set of scientific reference materials, although he did regularly read and contribute reviews to Annalen der Physik." (After all, until a few days ago I was unaware that photon–photon scattering had been experimentally observed more than half a century ago.) -- _ _ _ A. di M. 20:18, 2 December 2009 (UTC)
 * And also in another language. --Michael C. Price talk 20:48, 2 December 2009 (UTC)

If we are going to talk about the motivation of SR and the speed of light (which we should) then we should say what Polanyi so clearly reports; namely that it was motivated by reflection upon Maxwell's equations not the MMX. What we should not be doing is perpetuating simplistic unhistorical myths, either by omission or deliberate obfuscation. What we can say is that MMX played a big role in the acceptance of SR by others - but not in Einstein's case. --Michael C. Price talk 17:55, 2 December 2009 (UTC)
 * The experiments weren't the main motivating factor, but he did state, "Examples [about electrodynamics] of this sort, together with the unsuccessful attempts to discover any motion of the earth relatively to the ``light medium, suggest that the phenomena of electrodynamics as well as of mechanics possess no properties corresponding to the idea of absolute rest." I don't think that the and in "motivated by Maxwell's theory of electromagnetism and the results of ..." suggests anything significantly different than the together with'' in Einstein's paper. (OTOH, he didn't name Michelson and Morley.) -- _ _ _ A. di M. 20:18, 2 December 2009 (UTC)
 * I'm having trouble parsing the formatting of that statement, but I hope I've addressed your points. The article now refers to the non-detection of the ether as well as Maxwell's equations as motivators for SR, but excludes MMX, except as playing a role in its wider acceptance.--Michael C. Price talk 00:43, 3 December 2009 (UTC)
 * I've tried something slightly different by just leaving out MMX altogether. This avoids making statements about whether Einstein knew about it or not, since there is no clear evidence either way. Einstein isn't exactly the most reliable in remembering past events and both statements he made on the subject were made years after the fact. The current version is perfectly inline with what Einstein writes in his 1905 paper. Since this article is not about Einstein's motivation for SR lets just leave it that.
 * A mention of MMX might be in order for the history section though. TimothyRias (talk) 09:01, 3 December 2009 (UTC)
 * Yes, somewhere in the history section. --Michael C. Price talk 10:16, 3 December 2009 (UTC)

Faux faster-than-light communication
While recently working on the sweeping laser beam paragraph, I thought of a fun example of faux faster-than-light communication that is similar to the faster than light movement of the laser beam spot. Consider two people carrying on a conversation in a room on Earth. Additionally, each one has a microphone in which they speak and their words are recorded on separate recording devices. After the conversation, a timer on each recording device is set to go off at the same time and play back each recording. This may be as long as 100 years later. Now suppose each recording device is taken in the same type of space ship at the same speed in the same amount of time to two places that are separated by a light-year. The timers were originally set at a long enough time so that they would go off sometime after the space ships arrived at their destinations. When they go off, there is a faux communication between the two people through the recording devices that is a faux faster-than-light transmission of information. --Bob K31416 (talk) 17:49, 5 December 2009 (UTC)

Here's some more fun. Suppose we have two lasers that each can make a spot with the same characteristics on a distant object. We start with both lasers off. Before turning on the lasers, we point one laser to a place on the distant object and point the other laser to another place on the distant object. Now we turn on the first laser. We then turn on the other laser. Then we turn the first laser off. After the time it takes for the light to reach the distant object, a spot will appear at one place, then a spot will appear at the other place, and then the spot at the first place will disappear. One could say that the spot has moved from one place to another place before it left the first place. Of course it doesn't have to be lasers and distant objects. It could be flashlights making spots on a wall.--Bob K31416 (talk) 01:08, 6 December 2009 (UTC)
 * I heard of a similar example: if a group of people agree that person A will raise his hands at noon, person B, who is three feet east of A, will raise his hands at two nanoseconds past noon, person C, who is six feet east of A, will raise his hands at four nanoseconds past noon, and so on, they can make a superluminal Mexican wave. -- _ _ _ A. di M. 11:09, 6 December 2009 (UTC)

Doppler effect
The "practical effect of a finite speed of light section" (we really must find a better section title) currently has a subsection "Doppler effect". First, do we really need this section? Does it need to be here? (We could also mention the Doppler effect as a result of SR.) Second, if we do want it then it needs to explain what this effect is and how it results from the speed of light being finite, currently it doesn't do this at all. TimothyRias (talk) 16:13, 7 December 2009 (UTC)
 * We need to decide what the "practical effects" section is about, first. As for me, I'd replace its two-sentence introduction with something like this. (It shouldn't be very hard to source those claims.)
 * The GPS needs to be mentioned, as the fact that GPS works and the fact that gold is yellow and mercury is liquid are about the only consequences of SR which most people will directly experience in their lives (and IMO the latter is way out of the scope of this article, although it might be added to Introduction to special relativity). I've no opinion about Doppler effect. It isn't indispensable, but it's not harmful, either. The aberration section can be safely merged into the last paragraph of "Early astronomical techniques", IMO. Calling the Terrell rotation a "practical effect" is quite weird, as I don't think it was ever seen experimentally. (Personally, I'd move the paragraph about the Terrell rotation into a footnote after the first mention of length contraction.) _ _ _ A. di M. 16:33, 7 December 2009 (UTC)


 * Re "...has a subsection "Doppler effect". First, do we really need this section?" - It looks like a digression that doesn't even mention how it's connected with the speed of light. Other subsections here could bear scrutiny and rewrite too, so that they are more about the speed of light than a digression to another subject. --Bob K31416 (talk) 19:38, 7 December 2009 (UTC)

Note-footnote
In the current Speed of light article, there is the style of having a Note followed by a footnote that refers to the citation for the material in the Note. Here is an example from the article, where I think there is this connection between [ Note 1] and [ 3], which may not be apparent to a reader.

The speed at which light propagates in vacuum is independent of both the source of the light and the inertial frame of reference of the observer. The constancy of the speed of light was postulated by Albert Einstein in 1905, motivated by Maxwell's theory of electromagnetism and the lack of evidence for the luminiferous ether; it has since been confirmed by various experiments.

Notes and the ? How would that help? -- _ _ _ A. di M. 22:45, 15 December 2009 (UTC)
 * Then the picture could be moved higher. Here is an example in my sandbox. (Also, the Cherenkov paragraph needs to be rewritten if this is done.) --Bob K31416 (talk) 02:24, 16 December 2009 (UTC)


 * Something like that is necessary anyway, since the MoS does not like one paragraph sections. An alternative to merge/rewrite is just to leave it out. It is non-essential fluff mostly, so if we have trouble finding a suitable way to include it, not including it is an alternative. TimothyRias (talk) 07:13, 16 December 2009 (UTC)

I may have fixed it, see what you think. Abtract (talk) 07:16, 16 December 2009 (UTC)
 * That option goes against the MoS, as well. Anyway, we should not be trying to fix a symptom of a larger problem, being that we have a section that is too short. TimothyRias (talk) 09:57, 16 December 2009 (UTC)
 * I've edited the paragraph to usefully append to the "in a medium" section. It needs some more work, but feels less digressive then the subsection was. I have also introduced a to prevent the picture bleeding into the next section. This should fix most layout issues at the cost of some empty white space. TimothyRias (talk) 10:43, 16 December 2009 (UTC)

Formulat Format
This article includes the formula: γ = 1/√1 − v2/c2 with a very crude line over the argument for the square root (not shown here). Wouldn't it look nicer to put this argument in parentheses and just have it raised to the 1/2 power? Jmdeur (talk) 19:33, 17 December 2009 (UTC)
 * I made a change using your suggestion and a little more:  . --Bob K31416 (talk) 03:47, 18 December 2009 (UTC)

This line does not make sense
The FTL subsection contains the following line:
 * In some interpretations of quantum mechanics, certain quantum effects may seem to be transmitted not just faster than c, but instantaneously as in the EPR paradox.

As explained just a few paragraphs above this line something moving faster than light will move instantaneously in some frame. So the not just ..., but clause makes very little sense. Me changing this to more accurately reflecte the implication was revert by Bob, so I'd like to see how he proposes to improve this.TimothyRias (talk) 09:21, 18 December 2009 (UTC)


 * I presume that what you were referring to by "just a the few paragraphs above" was regarding the relativity of simultaneity? Anyhow, the above sentence makes sense since in the context where it appears, it implies that the transmission is instantaneous when viewed in any reference frame. If you would like that to be mentioned explicitly, that could be done by adding the phrase "when viewed in any reference frame" like this:
 * In some interpretations of quantum mechanics, certain quantum effects may seem to be transmitted not just faster than c but instantaneously when viewed in any reference frame, as in the EPR paradox.


 * BTW, the edit you made was not correct for at least the reason that it pertained to only some reference frame, whereas it implied that it pertained to any reference frame. Regards, --Bob K31416 (talk) 16:20, 18 December 2009 (UTC)

Laboratory demonstration section
From the FAC: "Why does the "Laboratory demonstration" section matter? Is there a benefit to that capability?"

I have no strong opinion about that. It is not vital, but it is not harmful either. I would not object to remove it, but I'd want to hear other opinions first. What do you think? ― A._di_M.2nd Dramaout (formerly Army1987) 16:36, 14 January 2010 (UTC)


 * I don't think the heading makes sense. The content belongs in a small subsection of the History section, in chronological sequence, in my opinion.—Finell 01:21, 15 January 2010 (UTC)


 * Maybe the heading does make sense, now that I understand the content. Do the sources give the margin of uncertainty in this technique?—Finell 01:29, 15 January 2010 (UTC)
 * The abstract of the second of the sources says "to an accuracy of a few percent", and that of the third says "with errors of less than 1% [...] on the order of 0.25%". ― A._di_M.2nd Dramaout (formerly Army1987) 21:09, 15 January 2010 (UTC)

Early astronomical techniques
The Early astronomical techniques section make it sound like Rømer was trying to compute the speed of light and used observations of Io to do it. My understanding was, and I think this is borne out by the link in hat note and the references there, that Rømer was studying Io as an aid to terrestrial navigation and his computation of the speed of light was the result of resolving discrepancies in that data. I don't mean to be picky but the phrasing seems a bit misleading as to how Rømer came to be making this computation. In particular it fails to mention Cassini's role in the computation.--RDBury (talk) 22:45, 18 January 2010 (UTC)
 * I massaged the text slightly to clarify the primary role of the group. (Hope that's okay with everybody.) I'm not sure that a mention of Cassini's opposition to Rømer's conclusions is needed for this summary, but it is described in some detail in the Rømer's determination of the speed of light article. Cassini's membership in the group of astronomers is also listed in a footnote. Thanks.&mdash;RJH (talk) 16:47, 19 January 2010 (UTC)
 * Look good, thanks.--RDBury (talk) 13:07, 20 January 2010 (UTC)

Backwards in time, or imaginary time?
The following appears in this article, and I have seen similar assertions elsewhere:
 * ... if something were travelling faster than c relative to an inertial frame of reference, it would be travelling backwards in time relative to another frame.

However, if one plugs a value for v > c into the time dilation equation, you get not a negative real value but an imaginary value, because of the square root. Are there any crackerjack physicists here who can clarify if going faster than light leads to going backward in time, or is it an even crazier concept of imaginary time? Thanks! CosineKitty (talk) 02:34, 19 January 2010 (UTC)
 * Look at the picture in "Fundamental role in physics". If something goes from A to B, it goes forward in time faster than light according to the red frame, instantaneously according to the green frame, and backwards in time according to the blue frame. ― A._di_M.2nd Dramaout (formerly Army1987) 08:41, 19 January 2010 (UTC)


 * The time dilation equation says something about the time measured between two events occuring at the same place in one of the frames. Putting v > c in the equation would result in another reference frame measuring an imaginary time between these two events, i.o.w. in the impossibility of measuring the time between these events. This is not related to travelling backwards in time in any way. The Lorentz transformation equations, and by extension all equations carrying a gamma-factor, already assume that v < c, so they are useless when one wonders about "what would happen if/when v ≥ c". What we need is just two spacelike related events (E1 and E2), for which two observers (O1 and O2) can be found to disagree about which event happened first. So let's have that O1 measures E1 to occur before E2 and that O2 measures E2 to occur before E1. But assume now that O1 is able to actually send a faster than light signal from event E1 to cause event E2, then observer O2 feels unhappy because he knows that E1 caused E2, but he also knows that E1 occured after E2, so the cause came after the effect, so finally, the signal travelled backwards in time. He didn't travel - the signal did. DVdm (talk) 09:08, 19 January 2010 (UTC)


 * Thanks, NoDrama and DVdm. What you are saying about the Lorenz equation being based on an assumption of v < c is a point well taken, and the other reasoning is persuasive also.  I am left thinking that newcomers to the subject (like me) who have had some college physics will be left in the same state of confusion I was.  This article already explains that the v = c for a particle with positive rest mass leads to infinite mass and would require infinite energy to attain the speed.  It seems like the article might be improved by mentioning the imaginary gamma values for v > c.  Our discussion here suggests to me that (if we restrict ourselves to special relativity) superluminal, non-accelerating travel of a mass-carrying object in a flat spacetime is fraught with problems.  The flat spacetime constraint would exclude ideas like the Alcubierre drive.  I am still left trying to do thought experiments, even if as a proof by contradition, where I imagine some spaceship literally coasting faster than light.  It seems patently nonsensical even beyond travelling backward in time.  Is it possible to generalize the Lorenz equation beyond its original limits, even if the result is to shoot a hole in superluminal travel?  CosineKitty (talk) 15:02, 19 January 2010 (UTC)
 * Tachyons, if they existed, would have a negative squared mass (i.e. an imaginary mass), so they must travel faster than c or their energy would be imaginary. Did you mean something like this? ― A._di_M.2nd Dramaout (formerly Army1987) 15:16, 19 January 2010 (UTC)


 * Yes, the tachyon article contains exactly what I was talking about. My hypothetical superluminal object would have to be a tachyon, and that article covers the wackyness quite well! :) CosineKitty (talk) 17:00, 19 January 2010 (UTC)


 * I personally don't like the kind of reasoning of the type "... and v=c with positive rest mass would need infinite energy to attain the speed..." This is the kind of reasoning which puts the horse behind the cart, as if nature would be obliged to obey the equations of the theory. For me it goes like this: (1) Nature does what it does. (2) For some unknown reason we can't have massive objects move at (or beyond) the speed of light. (3) We have found some equations describing what nature can do under certain circumstances and with certain approximations. I.o.w. the equations obey nature, and not the other way around. So, I'm certainly not happy with these "infinite energy" hypotheticals in the article. Of course I'm even less in favour of talking about imaginary gamma values and such. If you are interested, there's a rather nice FTL FAQ here in part 4. Make sure you thoroughly understand parts 1, 2 and 3 before you go to part 4. DVdm (talk) 15:35, 19 January 2010 (UTC)
 * I fully agree with (1). (2) should instead be "We have never seen a massive object moving at or faster than c", and (3) should be "we believe that it is impossible that we'll ever see a massive object moving at or faster than c, because, as far as we know, that would be incompatible with some postulates (causality, principle of relativity, and all that) which are entirely reasonable and of which we have never seen any other violation. OTOH, special relativity is so widely established, even empirically, that putting "it is believed that" everywhere wouldn't be that useful; it is believed that the Universe already existed last month, but there's can be no definitive proof of that, and since this belief is so widely accepted, explicitly marking it as such would be silly.
 * I'll take a read to the FAQ now, thanks for the link. ― A._di_M.2nd Dramaout (formerly Army1987) 17:19, 19 January 2010 (UTC)
 * Enjoy the site. By the way, I stand by my (1), (2) and (3), but I also fully agree with your comments. Paradox :-) . Cheers - DVdm (talk) 18:20, 19 January 2010 (UTC)

Wrapping and orphan control
I replaced some of the spaces with no breaking spaces to prevent equations from being broken in the middle and to prevent new lines from beginning with a symbol. Wikipedia talk:WikiProject Mathematics has a recent discussion on this with examples. This is a purely typographic edit and the effect may not be visible on all browsers. I wouldn't bother with this issue for most articles but since this is being reviewed for FA I thought it would be nice to sure everything looks good.--RDBury (talk) 16:00, 20 January 2010 (UTC)


 * I don't agree. Widows and orphans have to do with a page break, where someone has to turn a page, not line wrapping, where someone merely moves their eyes (as they do at the end of every line). I do not believe that it is typographically awkward for a single character (such as c) to begin a line. The only reason to use non-breaking spaces is to prevent a line break between words that should remain a unit, such as 325 m or 27 in or Dr. Jones.—Finell 20:01, 21 January 2010 (UTC)

Astronomical usage 2
I don't think that the following one-sentence paragraph belongs in this section: "The light time per unit distance is effectively the same quantity that was measured by Rømer and Cassini in the late 17th century, where they gave a value of "ten to eleven minutes",[8] slightly longer than the currently accepted value of 8 minutes 19 seconds." It might belong (as a sentence within a "real" paragraph) in the history section, where that measurement is already discussed. Also, while the relative accuracy of the measurement is impressive for its day, "slightly" is an exaggeration today.—Finell 20:12, 21 January 2010 (UTC)
 * Agree. Removing one sentence paragraph. TimothyRias (talk) 08:50, 22 January 2010 (UTC)

Subheadings for "Practical effect of the finite speed of light" section
The FAC instructions ask that article headings not be changed during the review, so reviewers can use the existing headings in their comments. I added a "temporary" sub-heading (without wikimarkup), "Astronomy", in the "Practical effect of the finite speed of light" section. I propose that, after the FAC review is complete, this should be converted into a level 3 heading and eliminating the "Stellar aberration" heading. The stellar aberration material, one short paragraph, fits nicely within an "Astronomy" sub-section.—Finell 00:27, 14 January 2010 (UTC)


 * Seems reasonable. With that change it might also be a good idea to change "transit time" to something like "navigation". IMO these titles make the scope of the section much more clear. TimothyRias 08:57, 14 January 2010 (UTC)
 * I agree with both proposals. ― A._di_M.</i><sup style="font-family: fantasy">2nd Dramaout (formerly Army1987) 15:41, 14 January 2010 (UTC)
 * I agree with Timothy, too. But I think the heading should be something like "Measuring distance". Although GPS is navigation, radar is used for shooting at things (among other purposes), and I don't think that we're using radar astronomy or measuring the distance to planets for navigation—yet. Or maybe we should ditch all the subheadings in that section; it's a pretty short sections, and paragraph structure may suffice. In preparation for whatever happens, I'm deleting the See also's for that subsection; the terms are wikilinked and that's enough.—Finell 01:53, 15 January 2010 (UTC)


 * Since the FAC has failed I've implemented these suggestions. I've also made some changes to the leading paragraph to clearify what this section is about. Wording can probably use some more work though. TimothyRias (talk) 10:57, 26 January 2010 (UTC)

Astronomical usage
In the following text:
 * In astronomy and satellite communication, it is useful to use standards based on the mass of either the Sun or the Earth, defining a standard length as the distance from the centre of the body at which a planet or satellite would have a given orbital speed. Carl Friedrich Gauss first used this method in 1801 to calculate the orbit of the dwarf planet Ceres. In 1895, Simon Newcomb refined this method in his Tables of the Sun.

Isn't the use of a standard length an assumption rather than a method? I.e. the method he used was to fit a Keplerian ellipse to the three observed points (and the time between the observations) using least squares. Computing the orbit required the assumption of some standard unit length. I think this text needs a citation.&mdash;RJH (talk) 17:18, 19 January 2010 (UTC)
 * I'm not sure I follow your logic. You don't need to assume anything for choosing a unit of measurement; you are free to choose to use astronomical units or inches or smoots or whatever to describe Ceres's orbit and you'd better obtain equivalent descriptions each time, as Ceres does not know what units you're using. Agree about the citation, I'll look for one. ― <i style="background: white; color: blue; font-weight:600; font-family: monospace">A._di_M.</i><sup style="font-family: fantasy">2nd Dramaout (formerly Army1987) 19:19, 19 January 2010 (UTC)
 * FWIW, I have added some citations for these facts. DVdm (talk) 20:47, 19 January 2010 (UTC)
 * Surely assuming an arbitrary unit for a baseline orbital distance doesn't give you a full solution for the orbital parameters? Giving a single distance and velocity doesn't provide a unique solution to the orbit; you've still got the orientation to work out, for example. Yet this is how it reads. I see this as providing an approach to deriving an orbit from the data, but not as the method for providing a unique solution.&mdash;RJH (talk) 15:52, 20 January 2010 (UTC)
 * I still don't follow you. What does the choice of units of measurement have to do with the fact that you can't normally estimate n parameters from m < n independent observations? Of course if you can more easily measure the distances in AUs than in Planck units you'd better use AUs, but that's another issue. BTW, I agree that "method" isn't the best word for that, and have just replaced it with "choice". ― <i style="background: white; color: blue; font-weight:600; font-family: monospace">A._di_M.</i><sup style="font-family: fantasy">2nd Dramaout (formerly Army1987) 22:48, 20 January 2010 (UTC)

I think the contention might be the implication that calculating the orbit of Ceres was simply a matter of assuming a circular orbit, which is clearly false. The phrase "used this choice to ..." kind of implies this. I suggest some kind of rewording like "used this choice when he calculated ..." or "made this assumption as part of his calculation of ...", or something like that. I.e. just reword it so it is clear that the distance assumption was part of the calculation, but not the sole step in doing so. CosineKitty (talk) 23:35, 20 January 2010 (UTC)
 * Yes, that would work. Thank you.&mdash;RJH (talk) 17:43, 22 January 2010 (UTC)

I cannot check the cited source, but the claim that Gauss was the first to use an astronomic distance as a unit for length is just flat out wrong. For example, Aristarchus of Samos expressed the Earth-Sun distance in units of the Earth-Moon distance in the third century B.C. So, there might have been something that Gauss did for the first time when determining the orbital elements of Ceres (There were quite a few actually), but using an astronomical distance as a unit of length was not among them. He might have been the first to define this in terms of a given orbital speed (shouldn't this be angular speed?) of an infitesimal mass in a circular orbit around an object of a certain mass, though. Any way this paragraph seems to have major issues. TimothyRias (talk) 10:31, 21 January 2010 (UTC)
 * Gauss was certainly not the first to use an astronomical distance as a unit of length: such usage is implicit in Kepler's calculations, for example, and explicit in Rømer's measurement of the speed of light. However, Gauss's method of orbital calculation does appear to be novel, and the constant he introduced is certainly called the Gaussian gravitational constant, whose value is taken as that chosen by Newcomb, even though we know that it is not "exactly right" if we want the AU to be equal to the average distance between the Earth and the Sun. Physchim62 (talk) 13:25, 21 January 2010 (UTC)
 * Huh? I've always taken that paragraph to indeed mean that Gauss was the first to use an unit of measurement defined as the distance at which a body would orbit circularly around the Sun at a given speed (as the modern astronomical unit is). How else had you three interpreted it? ― <i style="background: white; color: blue; font-weight:600; font-family: monospace">A._di_M.</i><sup style="font-family: fantasy">2nd Dramaout (formerly Army1987) 13:19, 21 January 2010 (UTC)
 * Gauss certainly didn't assume circular orbits! What he did realise was that you don't need to know the distance from the Earth to the Sun to calculate where Ceres will appear in the sky. Gausss just set the Earth–Sun distance as "one" on an arbitrary scale of units, and that was sufficient. That arbitrary scale of units is still used in astronomy more than two hundred years later! Physchim62 (talk) 13:39, 21 January 2010 (UTC)
 * Thanks. So I'm not nuts, as I wasn't the only one to understand it that way. ― <i style="background: white; color: blue; font-weight:600; font-family: monospace">A._di_M.</i><sup style="font-family: fantasy">2nd Dramaout (formerly Army1987) 15:50, 21 January 2010 (UTC)


 * But Gauss certainly was not the first to use that scale. TimothyRias (talk) 16:16, 21 January 2010 (UTC)
 * OK, I'm removing "first". ― <i style="background: white; color: blue; font-weight:600; font-family: monospace">A._di_M.</i><sup style="font-family: fantasy">2nd Dramaout (formerly Army1987) 16:46, 21 January 2010 (UTC)

Just a thought, but is this paragraph really necessary? It seems to delve way too much into the definition of the AU. Deleting the whole paragraph does not seem to detract from the point of the section at all. TimothyRias (talk) 18:43, 21 January 2010 (UTC)
 * Well, it explains what the point is of measuring c in an unit other than the metre rather than measuring lengths in metres so that one doesn't have to measure c. ― <i style="background: white; color: blue; font-weight:600; font-family: monospace">A._di_M.</i><sup style="font-family: fantasy">2nd Dramaout (formerly Army1987) 19:50, 21 January 2010 (UTC)

I agree with TimothyRias. If the relevance of the paragraph is simply that the AU can be used to measure the speed of light, and this paragraph relates to the principle underlying the AU, then I agree that it does not belong in this article (it would belong in the AU article, if it isn't already there). If the paragraph is deleted, then ", based on the solar mass" should be removed from the first sentence of the paragraph that now follows it. The first sentence would then read, "The astronomical unit is one example of such a length standard" (i.e., a standard that is not based on c), which is directly relevant to the speed of light.—Finell 19:49, 21 January 2010 (UTC)


 * In reading this section again, I agree with Finell and TimothyRias. It seems like we are getting way out in the weeds with something that isn't really important to the speed of light as a topic.  I say remove the paragraph, take out ", based on the solar mass".  Also there needs to be some rewording where it says "it is the inverse of c in astronomical units per second".  I think that may better be expressed as "it is the inverse of c, expressed in seconds per astronomical unit".  Note I added the comma, and flipped around the ratio to be units appropriate for 1/c.   CosineKitty (talk) 20:23, 21 January 2010 (UTC)
 * Dunno, I think that now that that paragraph has been removed, some readers would just think, "So why don't you just redefine the astronomical unit in terms of the metre, then, so that c is exact in that units too?" I think at least a piece of sentence explaining that it is convenient to define the AU in a particular way (even without giving details about that particular way) would be very useful. ― <i style="background: white; color: blue; font-weight:600; font-family: monospace">A._di_M.</i><sup style="font-family: fantasy">2nd Dramaout (formerly Army1987) 11:10, 22 January 2010 (UTC)
 * The usefulness of using the AU in astronomy arises from the fact that it IS the mean Earth-Sun distance. The effect of that paragraph was more to obscure this than to elucidate it. I guess nowadays you could redefine the AU in terms of the meter, I think the measurement technology is at a level that this could be done accurately. So, the question "why don't they?" is in principle valid. The fact of the matter is though that they haven't done that. TimothyRias (talk) 11:30, 22 January 2010 (UTC)
 * I thought it was useful because the Gaussian gravitational constant has an exact value in that system, and so you can use the trick described by Physchim62 with no uncertainty in the conversion factors. Is that wrong? I'm not an expert in these things... ― <i style="background: white; color: blue; font-weight:600; font-family: monospace">A._di_M.</i><sup style="font-family: fantasy">2nd Dramaout (formerly Army1987) 12:21, 22 January 2010 (UTC)
 * The usefulness of the astronomical unit is that you don't know the mass of the Sun, you don't know the Newtonian gravitational constant G (at least, not very accurately), but you do know their product to extremely high precision. Hence, it makes sense to use the product as the basis of your system of units. These days, you could redefine the AU as a fixed number of metres – indeed, there have been calls from very senior astronomers (eg Nicole Capitaine of the Paris Observatory, member of the international committee which decides such things) for exactly such a change to be made – but it wouldn't make any practical difference. That's probably why nobody can be bothered to change the 115-year old definition!
 * As for the paragraph in question, it was inserted around the time of the big wars over this article kast year, to counter the claims that "you can no longer measure the speed of light since the 1983 redefinition of the metre". You can still measure the speed of light, and the speed of light is still measured, but in different unit systems. Physchim62 (talk) 14:20, 22 January 2010 (UTC)
 * It seems to be that the section still conveys that message with the digressive bit taken out. On the other hand, if people feel that the section was better with the paragraph, than I have no real problem with putting it back either. TimothyRias (talk) 15:46, 22 January 2010 (UTC)

Slight change to note system
Would anyone object to me changing the note system from numeric to alpha? So notes appear as [Note a], or just [a], etc instead of [Note 1]. The reason is some readers may find using the same set of symbols to denote two sets of notes confusing. Cheers, Ben (talk) 02:33, 23 January 2010 (UTC)
 * I wouldn't, but how do you implement that? ― <i style="background: white; color: blue; font-weight:600; font-family: monospace">A._di_M.</i><sup style="font-family: fantasy">2nd Dramaout (formerly Army1987) 02:50, 23 January 2010 (UTC)
 * Done in these four edits. I just ended up using Cref2. This being a science article, my inner nerd compelled me to use Greek letters but this can be changed to Latin if you prefer. Cheers, Ben (talk) 13:52, 23 January 2010 (UTC)
 * I don't mind it being Greek or Latin letters, but unfortunately it isn't quite working on my browser. (I use Google Chrome; don't know if that's a factor.)  When I go to the bottom of the article, the notes show up as Greek letters, but in the text, the references to them appear as Latin.  For example, the inline text shows up as [c], but when I click on it, it takes me to the note labeled &gamma;.  They really need to be consistent symbols of some kind, even if happy faces, bunnies, and rainbows!  :)  CosineKitty (talk) 14:03, 23 January 2010 (UTC)
 * I've no problem using letters for notes rather than numbers – as is an accepted, if minority, usage in scientific writing – but I think they should be Latin rather than Greek! We don't want to seem as if we're being deliberately geeky! Physchim62 (talk) 14:14, 23 January 2010 (UTC)
 * Yeah sorry I stuffed it up, but I think I've fixed it. Can you try again? Cheers, Ben (talk) 14:19, 23 January 2010 (UTC)
 * I just checked again using Internet Explorer, Google Chrome, and Firefox, and all three are working correctly now: symbols are consistent, and links going both ways (text-to-note and note-to-text) work. I think the Greek letters may be a bit distracting to some readers, but I can live with it now that they work.  CosineKitty (talk) 14:26, 23 January 2010 (UTC)
 * Cool. I'll change them to Latin characters after a quick break from this computer then .. I'm working on a computer that has trouble dealing the cards in game of Microsoft Hearts at the moment, so editing Wikipedia is making me want to Hadōken it. Cheers, Ben (talk) 14:33, 23 January 2010 (UTC)
 * De-geekified I think. Cheers, Ben (talk) 14:53, 23 January 2010 (UTC)
 * Thanks! ― <i style="background: white; color: blue; font-weight:600; font-family: monospace">A._di_M.</i><sup style="font-family: fantasy">2nd Dramaout (formerly Army1987) 17:26, 23 January 2010 (UTC)

I just want to voice my strong opposition to using for notes in this article. This system makes maintaining the article in the future much harder for a questionable benefit for the readers. The problem here is that for any future note added to the article we have to check that the labelling of the notes remains correct. This pure foolishness if there is a system that automatically numbers notes. I also like the explicit "note" text since it tells the reader exactly how these notes differ from the other refs. TimothyRias (talk) 00:12, 24 January 2010 (UTC)


 * I agree with TimothyRias. That was a pretty drastic change to make with discussion open for only 12 hours and comments by so few of the editors who have been active on this article.


 * I agree that having more than one footnote with the same number is confusing to readers. As long as the note system is being changed to solve this problem, I would prefer the opposite solution to the one just implemented: Use the  markup for all notes, both explanatory and citations. That system is the easiest for readers to navigate and for editors to use and maintain. It has the additional advantage of being the system used in the real world. Structurally, that would require making "Citations" a Level 2 heading and renaming it "Notes" or "Footnotes". I considered recommending this a long time ago, but refrained because I thought it was too big a structural change to make in a long article that was otherwise in such good shape.—Finell 03:50, 24 January 2010 (UTC)


 * is the "preformatted text" element... what did you mean? If you mean merging them into one list, I don't think it's a good idea: almost all readers want to read explanatory footnotes, whereas very few will want to read all the bibliographic ones. As for the "real world", they are differentiated in some media: the bibliographic ones as [1], [2], ... and the explanatory ones as *, †, ‡, ..., or 1, 2, 3, ... without brackets (but I don't think this is a very good idea), or similar; in addition, often the former are endnotes and the latter footnotes. I have a slight preference for the new system, but I agree the older one is much easier to maintain; both would do it, for me. ― <i style="background: white; color: blue; font-weight:600; font-family: monospace">A._di_M.</i><sup style="font-family: fantasy">2nd Dramaout (formerly Army1987) 11:24, 24 January 2010 (UTC)


 * Sorry, I quickly and carelessly inserted the wrong codes. I did mean one numerical list using  markup. Ideally, I can understand wanting to help the reader distinguish between explanatory notes and citations. I do not see how the current system does that. What tells the reader that a superscript bracketed lower case letter is an explanatory note, while a superscript bracketed number is a citation? What should the reader understand from the string of footnote calls at the end of the first paragraph of the "Numerical value, notation and units" section? Do we expect the reader to click on each kind to see what happens? Does the reader read the whole article and then discover the explanatory notes at the bottom, which might have helped while reading the text? Considering accessibility, what does someone using a screen reader understand from the superscript letters? The footnote system also uses superscript lower case letters to indicate multiple calls to the same footnote, which adds to the confusion; capital letters would avoid that ambiguity, and they have the advantage of being the same height as the numerals. The previous system at least gave the reader a clue by using[Note #] in the footnote calls from the text; I think that[Explanatory note #] would really do the job. And manually assigning and maintaining footnote letters is soooo 1961 IBM Selectric typewriter.


 * Also, less experienced editors will have a problem with any non-standard system (although I agree that serving readers is a higher priority than making life easy for editors). It took me some time to figure out how the old explanatory note markup worked, and I've been around for awhile.


 * Yes, the real world does use different calls to distinguish footnotes from endnotes. In print, however, the reader sees the footnotes and their symbols immediately upon looking at a page. A Wikipedia article can't have this distinction because all notes are equally footnotes and endnotes (an article is one logical page), and the reader has to scroll down to the bottom of the article to see what is going on (the Notes section begins 12 screenfulls from the top on my computer, with the current settings).


 * Most Wikipedia articles, even long ones, get along fine without distinguishing 2 kinds of footnotes. If the consensus is that the distinction is important in this article, then I favor the old system, but use "Explanatory note" in the call and as the name of the section.


 * Slight change?—Finell 23:46, 24 January 2010 (UTC)
 * Well, on seeing [1] and [Note 1], or [1] and [a], or whatever, the difference is not predictable a priori, but after seeing one or two of those notes, one will likely figure out what the difference is. OTOH, if they are merged into one list, the reader can't know in advance whether [50] is bibliographic or explanatory even if they have read all of the 49 preceding ones.
 * "[Explanatory note 1]" is longish for the inline references; what about "[nb 1]" or something for them, while using "Explanatory notes" as the title of the section with the note texts? ― <i style="background: white; color: blue; font-weight:600; font-family: monospace">A._di_M.</i><sup style="font-family: fantasy">2nd Dramaout (formerly Army1987) 12:35, 25 January 2010 (UTC)


 * It is longish. I would favor the old "[Note #]" over "[nb]" on the grounds that a larger percentage of Wikipedians will understand it and is it will stand out more. The word "Note" conveys more information than any single symbol or character. I'm really not trying to be difficult about this (it comes natrually).—Finell 03:19, 26 January 2010 (UTC)
 * Thanks to TimothyRias for restoring the old system so quickly. I think it would have taken me half a dozen edits to do that without screwing everything up. ― <i style="background: white; color: blue; font-weight:600; font-family: monospace">A._di_M.</i><sup style="font-family: fantasy">2nd Dramaout (formerly Army1987) 11:26, 26 January 2010 (UTC)

FAC
The article's candidacy died with a whimper. That's disappointing. Apparently, it can be re-nominated again in a week.—Finell 03:33, 26 January 2010 (UTC)
 * OTOH, if it hasn't gained "garnering support" now, I think it's unlikely it will in a week. Any ideas? ― <i style="background: white; color: blue; font-weight:600; font-family: monospace">A._di_M.</i><sup style="font-family: fantasy">2nd Dramaout (formerly Army1987) 11:19, 26 January 2010 (UTC)
 * I guess there a couple of issues, first being that the FAC did not get explicit support from the major contributors except A di M. Speaking for myself, I've not posted a support !vote, because I'm somewhat on the fence whether this article is really FA at the moment. Then there are some image issues with the diagrams made by Brews, which are hard to fix because he is banned at the moment. Some issues that still need work (IMHO)


 * "Practical effects of finiteness" section was still a bit a vague. (Did some work on that today.)
 * Images/captions for some of the diagrams.
 * The last two minisections "Modern astronomical measurements" and "laboratory demonstration" do not really provide content on the same level as the other top-level sections.
 * The history section is missing information on some of the historical events pertaining to c as a fundamental constant. Some mention should be made of the series of developments that lead to c becoming recognized as a fundamental constant effecting stuff other than just light.


 * The latter two issues can be remedied by splitting the current history section in a measurement section and a real history section. (much like it was a year ago). The history section can then be expanded by the other stuff that is necessary, while the measurement section can abosrb the orphan sections as sub sections.TimothyRias (talk) 12:53, 26 January 2010 (UTC)
 * I didn't explicitly support beyond nominating, either... :-) ― <i style="background: white; color: blue; font-weight:600; font-family: monospace">A._di_M.</i><sup style="font-family: fantasy">2nd Dramaout (formerly Army1987) 14:49, 26 January 2010 (UTC)

Arbitration motion regarding Arbitration/Requests/Case/Speed of light
Per a motion at Arbitration/Requests/Amendment: 1) Exception to topic ban

is permitted to participate in featured article candidacy discussions for "Speed of light" for the sole purpose of discussing the images used in the article. This shall constitute an exception to the topic ban imposed on him (remedy #4.2). 2) Second exception to topic ban

is permitted to edit images used in the "Speed of light" article to address issues regarding the images that arise in connection with the article's featured article candidacies. This shall constitute an exception to the topic ban imposed on him (remedy #4.2).

On behalf of the Arbitration Committee, ~ Amory ( u •  t  •  c ) 02:21, 27 January 2010 (UTC)
 * Discuss this

Figures
I have been cleared to discuss changes to figures used in this article. If there are some changes to be made in images I have made, what are the figures needing changes, what are these changes and what are the reasons behind them? Brews ohare (talk) 04:26, 27 January 2010 (UTC)


 * I would like to ask editors here if they would consider removing the block on Brews ohare. He has contributed more than his fair share to this article, and it seems small minded to block him indefinitely.Likebox (talk) 05:37, 27 January 2010 (UTC)


 * The editors here have no control over this. It is an arbitration decision. Further, Brews is not blocked; he does have a topic ban on the topic of physics.—Finell 06:15, 27 January 2010 (UTC)


 * I meant try to un-topic-ban him, or at least put in a word at the arbitration committee's door. Now is a good opportunity. Figures are badly needed for many articles, and it's a thankless job, and he has a knack for this.Likebox (talk) 06:21, 27 January 2010 (UTC)

Brews: Thanks very much for the offer. Unfortunately, the FAC failed for failure to gain sufficient support within a 2-week period (see above). If the FAC is re-submitted, some of the editors may take you up on your offer.—Finell 06:15, 27 January 2010 (UTC)

Anomalous index and Kramers-Kronig Dispersion relations
In this article it says that the index of refraction is always greater than 1. This is false. There are materials where the index of refraction is less than 1 (meaning a superluminal speed of light). There is no paradox, because the index of refraction is defined by phase velocity, and the disturbances in the field always travel at less than the speed of light.

This was historically important, because Kramers and Kronig were motivated to figure out exactly what were the restrictions on the refractive index to forbid faster-than-light communication. The answer was a subtle relationship between the real and imaginary part of the refractive index (the imaginary part of the index is the absorption coefficient) requiring that as an analytic function, the singularities have to be on a certain half-plane of the full complex plane.

This result spawned a field of study, called dispersion relation theory, which evolved into S-matrix theory in the 1960s, the granddaddy of modern string theory. The Kramers Kronig dispersion relation by itself is very interesting.Likebox (talk) 05:37, 27 January 2010 (UTC)


 * This is discussed briefly in index of refraction, but it should also be mentioned here.Likebox (talk) 06:08, 27 January 2010 (UTC)
 * Feel free to make a suggestion for a change. TimothyRias (talk) 10:48, 27 January 2010 (UTC)


 * I was about to change it, but then I noticed that it says transparent materials. In order to have an index less than 1, you need absorption that is quickly varying with the wavelength. So I think the lead is OK. A section might be added to "superluminal effects". While doing this, I found interesting articles on the Kramers Kronig relation in curved space.Likebox (talk) 14:32, 27 January 2010 (UTC)
 * Section "In a medium" already mentions this, but I think it is a rather advanced topic for the lead. I might try to add a sentence or two about Kramers–Kronig relations in that section. ― <i style="background: white; color: blue; font-weight:600; font-family: monospace">A._di_M.</i><sup style="font-family: fantasy">2nd Dramaout (formerly Army1987) 16:18, 27 January 2010 (UTC)


 * We could have a dumbed down explanation of how a frequency dependent index of refraction could give rise to causality violations and then mention that if the K&K relations are satisfied, you don't get such causality violations. The dumbed down textbook example are the goggles that let through part of the spectrum, allowing you to see the future. Count Iblis (talk) 16:45, 27 January 2010 (UTC)

Comment on an old dispute
There was a bruhaha regarding some material here. The real dispute, as I understand, was whether you could simultaneously say that the speed of light "c" is a physical constant and a defined exact value at the same time.

There are exactly 2.54 centimeters in an inch. So the ratio inch/cm is exact. It is the same in all times and all places. But you probably would not call this ratio a physical constant. On the other hand, the charge on an electron is measured in a laboratory, and in its dimensionless form, it is called the fine-structure constant. This quantity is definitely not a conversion factor--- its a bona-fide physical constant.

The standard version of this dispute in the literature is whether you can call a dimensional constant a constant of nature. If you have two different dimensional constants of nature with the same dimensions, their ratio is a dimensionless constant of nature. But what about just one?

A parable: on the Neutron star N3221C, there lived a species of intelligent life of centimeter width and length, but of nanometer-scale height. These nearly two-dimensional people defined a unit of length, and called it a meter. They also defined a unit of height, and called it a nano-inch. The unit of length was defined by a neutronic-matter rod kept in a special flatland vault, while the unit of height was defined by the energy with which objects impacted the surface after a neutron-star glitch hit, or when, with heroic effort, they were able to stack objects on top of their wafer-thin bodies.

Eventually, one day, an experiment convinced them that the world is three dimensionally rotationally invariant. They then discovered a new constant of nature, which they called the "verticality constant", which had a value which they measured to be "2.54 10^{-11} meter/nano-inch" to one part in a thousand. Later on, the measurement of the number of inches per meter became too precise for the standard definition of the inch (one twelth the height of the emperor's left tentacle, times 10^9 to represent the number of subjects in the empire), so they decided to redefine the inch so that the verticality constant was fixed to be exactly .0254 m/inch. Is the verticality constant a constant of nature?

A question: is mu-0, the permeability of free space, a constant of nature? It is defined as exactly 4pi x 10^-7 in SI-units, so that all electrical units are fixed by the operational definition that two wires 1 meter apart carrying 1 Ampere of current exert a force per unit length of 2 10^-7 Newtons per meter. On the permeability page, it calls mu-0 a constant, but not a constant of nature.

A second question: Boltzmann's constant is also fundamentally a conversion factor, but it is a measured quantity, so it is called a physical constant (but not a constant of nature). Avogadro's number (which is a conversion factor if ever there was one) is also measured, and it is called a "constant" (not even a physical constant).

So one way to make it even is to say c is a "dimensional physical constant". Then there are three of these, c hbar and G, or 5 if you want to count Boltzmann's constant and Avogadro's number. The number of dimensional constants of nature is always dependent on the system of measurement, the neutron star people would include the inch.

Some people will chortle when they hear the term "dimensional physical constant", while the some people will say "well, yes, it is dimensional, i'nn't it". Does this resolve everything?Likebox (talk) 09:09, 27 January 2010 (UTC)
 * I think that more than 90% of disagreements in these areas are about the meaning of words, rather than about physics. For example, ISO says, "Some physical quantities are considered to be constant under all circumstances. Such quantities are called universal constants or fundamental physical constants." Under this definition, g0 is a fundamental physical constant, because it is the product of the constant pure number $9.807/(299,792,458 × 9,192,631,770)$, the speed of light in vacuum, and the frequency of a certain 133Cs transition, all of which are "fundamental physical constants" in that sense. NIST follows this definition and includes g0 in their list of fundamental physical constants. OTOH, John Baez and John Barrow (IIRC) only consider dimensionless constants to be fundamental, so they exclude c, ħ and the like.
 * Measured in sane units, verticality is 1, as are the speed of light and the Boltzmann constant. Now, is 1 a constant of nature?
 * Two more things about the Boltzmann constant: 1) my BSc thesis's source file included the line, as I seldom used either factor of that expression alone... in essence, I treat kBT as a single symbol. 2) I think the kelvin will be redefined in terms of kB within this decade. ― <i style="background: white; color: blue; font-weight:600; font-family: monospace">A._di_M.</i><sup style="font-family: fantasy">2nd Dramaout (formerly Army1987) 10:13, 27 January 2010 (UTC)
 * BTW, do you think something is wrong with the article, as far as that matter is concerned? (I don't.) If so, what? ― <i style="background: white; color: blue; font-weight:600; font-family: monospace">A._di_M.</i><sup style="font-family: fantasy">2nd Dramaout (formerly Army1987) 10:15, 27 January 2010 (UTC)


 * This is NOT about the meaning of words, at least not entirely.


 * "The frequency of a certain Cs transition" is not a dimensional constant, because it can be defined as the ratio of this frequency to the Planck frequency. This is dimensionless. It is certainly conceivable that this number could be different tommorrow because something happened. If something happened to the Cs atom, it would emit a different frequency.


 * On the other hand, it is not conceivable that the number of inches per meter could change tomorrow, no matter what. The number of inches in a meter is a constant which is a unit-conversion factor. In the modern SI definition, the speed of light is the same, and it is not conceivable that it could change. If the actual speed of light did change, but nothing else did, you would say that the actual lenght of a meter in terms of all atomic wavelengths seemed to have changed.


 * One point of this is to clarify what Brews ohare/David Tombe were arguing about, and to get people to go to Admin-board and apologize for hounding them out, and try to get them back. The other point is to clarify what a "dimensional physical constant" is, and to explain that the number of these depends on the system of units.Likebox (talk) 14:50, 27 January 2010 (UTC)
 * What applies to the Cs wavelength applies to anything else. By that logic, the speed of light is not a dimensional constant, because it can be defined as the ratio of this speed to the speed of one Planck length per Planck time. But that would be a particularly dull dimensionless constant... ― <i style="background: white; color: blue; font-weight:600; font-family: monospace">A._di_M.</i><sup style="font-family: fantasy">2nd Dramaout (formerly Army1987) 16:05, 27 January 2010 (UTC)


 * The ratio of one Planck length to one Planck time is the speed of light. Unlike other quantites, there is no other speed which you can use to compare it too, it is the only absolute speed. In that regard, it is no different from inches/meter, which for the Neutron star dwellers, is the only natural verticality constant.Likebox (talk) 21:53, 27 January 2010 (UTC)


 * By the way, when I used the phrase "the actual speed of light did change" in the above construction, I was using the second definition of the speed of light, the one where it is not a unit-conversion factor, but the result of an experiment.Likebox (talk) 14:53, 27 January 2010 (UTC)
 * The result of which experiment? If you mean a measurement of c in AD 1960 metres per second, and you saw that such a result today differs from what you got yesterday, the only thing you could be sure of would be that the ratio between the krypton frequency and the caesium frequency changed; but you could not be sure of which changed, and short of defining another standard for lengths and times, the question is unanswerable. (Also I don't understand the point of your parable, as you seem to be suggesting exactly the opposite of its consequence. Would it be possible for the inhabitants of N3221C to find that the verticality constant changed? Assuming that the world is rotationally invariant, no. Would it be possible for the inhabitants of Earth to find that the spacetime constant changed? Assuming that the world is Lorentz-invariant, no. What's the difference?) ― <i style="background: white; color: blue; font-weight:600; font-family: monospace">A._di_M.</i><sup style="font-family: fantasy">2nd Dramaout (formerly Army1987) 16:05, 27 January 2010 (UTC)


 * The verticality constant measurement on the neutron star would be a heroic thing, involving tilting a meterstick far enough so that the change in horizontal length due to the tilt could be observed. The change in length would be -C h^2/l, where C is the verticality constant, h is the height in inches, and l is the length in meters, to lowest nonvanishing order. The effect would be tiny for the slopes that the neutron star folk could acheive, so they probably would have centuries of 2-d physics which would suddenly be swept away in the 3-d revolution.


 * The verticality constant defined by this experiment could change in principle, but only under the implausible scenario that the laws of physics are not really rotationally invariant, and a big "mush" happens that changes the radial coordinate on the neutron star in a way centered on the center of neutron star. But the inhabitants of the neutron star would not consider rotational invariance between height and length obvious, only rotations between width and length. So for them, it would be concievable that the verticality constant depends on time, in a way that isn't conceivable for us.


 * I don't think you could say that the neutron star people are wrong to think this, only that they don't consider it self-evident that the laws of physics are fully rotationally invariant. Likewise, we consider rotational invariance self-evident, but we don't always consider Lorentz invariance self-evident. So some people will say that the speed of light is a measured property (meaning Lorentz invariance is not an a-priori truth to be taken for granted), while others will say that the speed of light is best defined.


 * I don't personally consider rotational invariance not as an a-priori self-evident truth, but as a scientific truth, and that it is true. But if someone denied that rotational invariance is obvious, and insisted that height was not the same as width or length (which is never going to happen, but lets suppose), I wouldn't know how to convince them. In principle, they are right.


 * If these people then demanded that we use different units for height and for length, and claimed that the verticality constant be called a measured constant, the ratio of the horizontal length of a rod to its vertical height, it would be a self-consistent point of view. So I think the speed of light, like the verticality constant, is a quantity to be measured, and only the discovery of rotational/Lorentz invariance turns it into a quantity best thought of as a defined conversion factor.Likebox (talk) 23:19, 27 January 2010 (UTC)


 * The only change to the article I am suggesting is to say "dimensional physical constant" in the lead, and to use the same phrase for hbar, for G, for mu-0, for Boltzmann's constant, and perhaps for Avogadro's number (although the last one is only dimensional if you consider a mole a dimension).Likebox (talk) 15:12, 27 January 2010 (UTC)
 * The first section after the lead uses that phrase and explains its significance; it could be added to the lead, too, but I don't think it's vital. ― <i style="background: white; color: blue; font-weight:600; font-family: monospace">A._di_M.</i><sup style="font-family: fantasy">2nd Dramaout (formerly Army1987) 16:05, 27 January 2010 (UTC)

Note to likebox: This is all I will say on the matter, since neither of them is allowed to respond here. (And do enjoy the peace that it has brought to this page.)
 * 1) David and Brews were not banned for what they were arguing, but for there misconduct both on this page and in the ArbCom case.
 * 2) The point your making here is not what they were arguing at all. (In fact, it is almost the diametrical opposite of what David was arguing).

More on topic, there is not much concensus in the literature about calling constants fundamental, dimensional, etc. The current option in the lead of just calling c a physical constant, i.e. a constant used in physics, is the most neutral thing to do. Adding addjectives like fundamental or dimensional to the lead is not going to help a lay reader. Moreover, it is likely to spark controversy with readers holding a different opinion on the subject.


 * Unfortunately, I went through the archives for hours yesterday, and looked at everything they were saying, and the arguments about "misbehavior", and everything else. Aside from not being very political, and from arguing forever and making their text unclear through over-sourcing, they did not misbehave. They were arguing for a subtle point, which could have been recognized by people.


 * What they were saying is that there are two notions of the speed of light. One notion is a conversion factor, and this you don't measure. The second notion is the laboratory number. The idea is that these are two philosophically different quantities--- one converts units, the other is defined as the ratio of the speed of light. One cannot change, even in principle, and the other can change in principle. This distinction is sort of nifty, it has been made before, and it could have been made here.


 * It's not a point I care about at all--- it's too obvious to waste time on. But I am trying to keep this alive to shame the others here. Shame shame.Likebox (talk) 21:53, 27 January 2010 (UTC)
 * It's history, there's no point revisiting it now. The fact is it ended up in arbitration where it was resolved. Anyone who seriously thinks the outcome was wrong can take it to arbitration again, as that's the only way to challenge such a decision. Otherwise there's nothing discussing it here will achieve, except distract us from the task of improving WP, the reason all of use are here.-- JohnBlackburne words<sub style="margin-left:-2.0ex;">deeds 22:03, 27 January 2010 (UTC)


 * I was hoping those who took it to arbitration can take it back to arbitration and say "enough is enough, we want to welcome these people back".Likebox (talk) 00:39, 28 January 2010 (UTC)


 * Anyone can take it to arbitration. It's not the identity that's important it's the argument: the why, the evidence, lack of people opposing or evidence against. That's what was done to get Brews's ban partially lifted for the FAC for this article, and it had a successful outcome. But that was a narrow request for a particular purpose. I suspect it will be harder to end the broader topic ban early.-- JohnBlackburne words<sub style="margin-left:-2.0ex;">deeds 01:01, 28 January 2010 (UTC)


 * It shouldn't be too hard--- just get the folks here to say "let bygones be bygones". The arbitration comittee is a service provided for editors, to implement their consensus.Likebox (talk) 01:41, 28 January 2010 (UTC)

Archiving a one-day-old talk-page discussion, without the consent of all editors, because you find it either annoying or embarassing, violates policy. I would request that you unarchive this material.Likebox (talk) 14:07, 29 January 2010 (UTC)
 * Archiving a thread because it is off-topic, and its instigator has made it clear that he has no intention of making it on-topic, however is not. Likebox, you are very narrowly brushing the lines with WP:DISRUPT here. Your insinuations also are not inline with WP:AGF. I'll leave this be for now, anyway.TimothyRias (talk) 14:56, 29 January 2010 (UTC)

Where things really went wrong with Brews (and certainly with David) was the lack of discussing things from well defined models (or classes of models). You need to specify that in order to make clear what your assumptions are, like Likebox discusses above (e.g. rotational invariance, Lorentz invariance etc. etc.). Two other examples of what happens if you don't do this:

http://arxiv.org/abs/hep-ph/0306245

http://arxiv.org/abs/hep-ph/0505250

Count Iblis (talk) 14:28, 29 January 2010 (UTC)
 * But special relativity is so well established that writing "assuming that special relativity holds" wherever we write "the speed of light in vacuum is constant" is akin to writing "assuming the universe wasn't created five minutes ago" wherever we write "John is 54 years old". (Now I'm not saying we should never mention we're assuming that SR holds, but I think what is now in the article is sufficient. See WP:MNA.) ― <i style="background: white; color: blue; font-weight:600; font-family: monospace">A._di_M.</i><sup style="font-family: fantasy">2nd Dramaout (formerly Army1987) 17:28, 29 January 2010 (UTC)

Discussion on the speed of light

 * My goal: given that recently Brews was unblocked for the narrow purpose of changing figures on this page, I was hoping some people could go here and give support to unblocking Brews and David permanently, so as to put this dispute behind us once and for all. Blocking people for their point of view and impolitic behavior, rather than for violations of rules, is not a good precedent. The comment is just as on-topic as the "Arbitration motion regarding Wikipedia:Arbitration/Requests/Case/Speed of light" section just above it, which was placed here for the same reason.


 * I know that everyone was acting in good faith. The editors here did not choose the draconian sanctions, they just wanted to put the dispute on this page behind them. This forum is the only place I know which will get the attention of the proper people.


 * It's the wrong place and editors here are not the proper people. This is the place to discuss improving Speed of light. If you want to contest an arbitration decision take it to the relevant arbitration board. If you have a good argument you might get support, as happened for Brews on the ban related to this article. But this is not the place.-- JohnBlackburne words<sub style="margin-left:-2.0ex;">deeds 20:46, 29 January 2010 (UTC)


 * I do not want to disrupt--- just to explain these editors' position and see if it can be (briefly) accomodated on the page. Special relativity is an assumption, a truth established by theory and experiments together. If you do not accept it as self-evident (just as if you were not to accept rotational invariance as self evident) you would have grave doubts about defining the meter in terms of the speed of light, since the speed of light is a derived quantity.


 * While I agree with A di M. that relativity is too well established to be casually thrown into doubt, it is easy enough to say "the four-dimensioal point of view of relativity makes the speed of light into a conversion factor between the units second and the unit meter, and is the same type of constant as the number of inches in a meter. Those who do not accept relativity as a-priori true can take issue with thinking of the speed of light as a conversion factor." I don't care about this, but when editors who do care are silent, who will speak for their position?Likebox (talk) 20:27, 29 January 2010 (UTC)


 * Even without relativity there is nothing wrong with using the speed of light as the basis for the definition of the meter. It is just as good a dimensifull quantity to base a unit of length on as say the mean distance to the sun. TimothyRias (talk) 21:15, 29 January 2010 (UTC)


 * Agreed. Except you need that the speed is constant at all times of year, at all times, at all places etc. Which is implied by relativity, but strictly speaking, slightly weaker.Likebox (talk) 21:19, 29 January 2010 (UTC)


 * They are laws of physics, so don't change, or at least that's the idea, and that includes relativity. So there's no need to add the extra 'all times, all places'. That they are laws of physics implies that.-- JohnBlackburne words<sub style="margin-left:-2.0ex;">deeds 21:33, 29 January 2010 (UTC)


 * Sure to make it a good definition you need that the speed of light is constant, which is an empirical fact. (Of course, in the context of GR you can make it true by definition by absorbing any change in c in the metric, which is nice) Itis not more of worry, than assuming that all caesium atoms have the same ground state hyperfine levels for the definition of the second. TimothyRias (talk) 21:39, 29 January 2010 (UTC)


 * This is subtler for speeds--- the speed of sound, for example, changes with your motion. The relativity assumption is that the speed of light doesn't depend on the motion of the earth, that "c" is constant independent of your motion. The assumption that c is constant for all times and for all motions is equivalent to relativity if you assume rotational invariance. It is slightly weaker, because rotational invariance could fail in principle.Likebox (talk) 21:42, 29 January 2010 (UTC)
 * The laws of relativity are as solid as you'll get in physics. Simple, consistent, easily tested. It's not to modern physics an assumption, it's a fact - the geometry of our universe is based on it. There are no cases where it's weaker, or less likely. It's always true.-- JohnBlackburne words<sub style="margin-left:-2.0ex;">deeds 21:46, 29 January 2010 (UTC)


 * I am not a relativity doubter. When I said "weaker", I didn't mean that relativity is weaker, all I meant is that the assumption that the speed of light is independent of the motion of the observer is equivalent to relativity under standard implicit assumptions, but is weaker than full blown relativity.Likebox (talk) 21:52, 29 January 2010 (UTC)

I don't see what you're trying to say. The speed of light is a fact, not an assumption. It's well understood, well tested, and all of modern physics is based on it. It's not weaker in certain circumstances, nor is relativity.-- JohnBlackburne words<sub style="margin-left:-2.0ex;">deeds 22:01, 29 January 2010 (UTC)
 * Replying to Likebox (talk) 21:19, 29 January 2010 (UTC): The same applies with any other standard: to define your unit of length in terms of a metal bar, you have to assume that the length of that bar won't change, or at least that variations in it will be much smaller than you'll ever need to measure anything in terms of that unit. (Also, a conversion factor can be a measured quantity, if two units have unarguably the same dimensions but are defined independently; for example, the electron volt and the joule.) ― <i style="background: white; color: blue; font-weight:600; font-family: monospace">A._di_M.</i> (formerly Army1987) 13:22, 30 January 2010 (UTC)


 * What I was saying is that the two statements:


 * light moves at the same speed at all places and all times, in all reference frames.
 * Special relativity is true


 * Are equivalent. Statement 1 imlies statement 2, and statement 2 is based on statement 1. They are the same. So saying "the speed of light doesn't depend on the observer" and saying "I believe special relativity" are the same.


 * However, if you do not assume rotational invariance, or translational invaiance, then statement 1 can be weaker than statement 2. Meaning, statement 2 always implies statement 1, but without some additional assumptions which are usually too obvious to mention, statement 1 does not imply statement 2.Likebox (talk) 22:20, 29 January 2010 (UTC)


 * Well, if you want to be picky 1 is just something we've measured a lot (in a rotating, moving frame in a gravity well inside another, etc.) and it's always been the same. 2 is just a hypothesis that explains it. Neither is a priori true, both could be challenged by a repeatable experiment that e.g. gave different results for 1. But physics works via laws, not logic. Physicists devise laws, apply them, test them again and again, and if they stand up to all tests and explain many phenomena they become generally accepted as true.


 * I.e. statement 1 is an experimental fact. But it's one we've tested a lot, and have got very good at testing it. In a sense every time you use a GPS you're verifying it, as if the speed of light varied by even a little your GPS would give wrong answers. Statement 2 is about a theory, which explains 1 rather well, and a lot of other testable things, so is accepted as true. The theory explains the constancy of c, the constancy of c is evidence for the theory, as are many other things. No-one doubts either, so to say one is weaker is meaningless, at least in the context of modern physics.-- JohnBlackburne words<sub style="margin-left:-2.0ex;">deeds 23:10, 29 January 2010 (UTC)


 * If by no one, you mean you and me, you are right. I don't doubt that special relativity is true. But there are people who propose modified relativity theories in the literature. I agree that this page should treat relativity as true, and the speed of light as constant, but it might be OK to explain that this is not the most intuitive point of view for those who are not well acquainted with relativity.Likebox (talk) 23:14, 29 January 2010 (UTC)


 * If anyone doubts the constancy of c when they start reading the article hopefully they have fewer doubts after reading it. Adding in information on fringe theories, even if to refute them, would just confuse people.-- JohnBlackburne words<sub style="margin-left:-2.0ex;">deeds 23:34, 29 January 2010 (UTC)


 * Doubting special relativity is not always fringe. For example, Sidney Coleman and Sheldon Glashow proposed that relativity could be an accidental symmetry, and that it would be violated at high energy. Lee Smolin has proposed a doubly special relativity theory that would be different at high energies. Some people have claimed that gamma-ray bursts contain evidence that SR fails. None of these proposals are accepted as correct, but they are not fringe either. It is important to cover them, even if they are not correct as physics, so that they do not have to be indepedently rediscovered by every generation. See Le Sage ether for a great historical example of a wrong non-fringe theory. I don't think any of them belong on this page, though.Likebox (talk) 07:00, 30 January 2010 (UTC)
 * The article already has a paragraph about that, and it also mention the 2009 experimental result putting the upper bound to any violation of Lorentz invariance at lP/1.2. ― <i style="background: white; color: blue; font-weight:600; font-family: monospace">A._di_M.</i> (formerly Army1987) 13:22, 30 January 2010 (UTC)
 * Replying to JohnBlackburne words<sub style="margin-left:-2.0ex;">deeds 23:10, 29 January 2010 (UTC): Well, actually that's backwards. Einstein started assuming that the physical laws and the speed of light are the same for all inertial observers, and from that he deduced special relativity essentially from first principles. It's more like "special relativity is the theory predicting that a bunch of interesting things would happen if c were constant, and these things turn out to agree with the experiment", than vice versa. ― <i style="background: white; color: blue; font-weight:600; font-family: monospace">A._di_M.</i> (formerly Army1987) 13:22, 30 January 2010 (UTC)

Trying to clarify a distinction
(deindent) I do not agree that this thread has no bearing on the article--- this thread is trying to clarify the distinction between two concepts:
 * 1) The speed of light--- the conversion factor between the units "meter" and the unit "second", both units of either time or space, assuming relativity is a-priori true.
 * 2) The speed of light--- the quantity which measures the maximum speed with which influences travel, and where the "second" and the "meter" are units of time and space, where you do not assume that relativity is a-priori true.

The distinction between these is illustrated by the verticality constant. Assuming that height is no different from length and width, the distinction between inches and meters is only a unit conversion factor. If they are fundamentally different, the distinction is not a unit coversion factor, but a constant of nature which can in principle change. If you call the unit of height the "inch" and the unit of length the "meter", then inches and meters are separate concepts unless you accept rotational invariance.

This discussion went on for a long time, and until the principal authors (Brews Ohare and David Tombe) agree that the current article is fine, I don't see any reason that the discussion should not remain on the talk page. As for a suggestion to this article: you can incorporate the previous two paragraphs into the text.Likebox (talk) 09:30, 28 January 2010 (UTC)
 * No one's removed anything, just archived it so we don't get sidetracked. I've added a new section head to also make it clear we're here to improve the article. OK, so if you think the article should be changed then change it. If someone disagrees they will revert it and we can discuss it here, but at least we'll be talking about the article. And you never know, if your change makes sense it might be kept, or used as the basis for other improvements.-- JohnBlackburne words<sub style="margin-left:-2.0ex;">deeds 09:41, 28 January 2010 (UTC)
 * OK, find a source that explicitly makes this distinct. Then we will talk again. But even then I'm not convinced that this is something the article needs to make note of. The distinction is a statement about the equivalence of time and space, and actually has little bearing on the speed of light. This quantity can be defined in both cases and has the same properties in both cases. TimothyRias (talk) 10:23, 28 January 2010 (UTC)


 * As I said--- it's not something I care deeply about. But I am not happy with the talk page not mentioning this, because it is an important point for a few editors. The distinction is real, but entirely philosophical: "the speed of light" can be a conversion factor like inch/meter, which means it is meaningless to measure it, or to claim that it has changed. Or the same quantity can be thought of as a measurable quantity referring to quantities of space traversed in quantities of time, where the distinction between the two is assumed, and the operational definition of time/space uses atomic lenths and times as fundamental, and does not assume relativity as an a-priori truth. For me, relativity is a-priori true, so I don't care. The note here is for the benefit of future editors.Likebox (talk) 11:04, 28 January 2010 (UTC)


 * First, I disagree that is (a priori) meaningless to measure a conversion factor. If inch and meter had independent physical definitions (which in reality they don't), measuring their ratio tells you something useful. In particular, looking for a change over time is then a crucial test for the physical assumptions going into the definitions.
 * Second, once there is talk page concensus that something as non-issue for an article, the corresponding thread should be archived. TimothyRias (talk) 11:30, 28 January 2010 (UTC)
 * (Or, for an explicit example, the conversion factor between the electron volt and the joule is a conversion factor, but it is a measured quantity. ― <i style="background: white; color: blue; font-weight:600; font-family: monospace">A._di_M.</i><sup style="font-family: fantasy">2nd Dramaout (formerly Army1987) 17:02, 28 January 2010 (UTC))


 * It seems to me that there are two quantities that are not obviously constant in the definition of the speed of light: the relationship between space and time that is used to define the meter ("the length of a path travelled by light in vacuum during a time interval of 1/299792458 of a second"), and the wavelength of caesium 133 used to define the second. This article already links to the article Variable speed of light, which discusses these issues in great detail.  Because both are empirically constant as far as anyone can tell, and there is no known experiment that rivals them in precision, the existing brief mention and the link to an external article seem appropriate to me.  CosineKitty (talk) 17:38, 28 January 2010 (UTC)
 * The relationship between space and time, to people acquainted with special relativity, is as obvious as the relationship between length and height; it might not be to other people much like the relationship between length and height might not be obvious to lay "people" on the star in Likebox's parable. As for the wavelength of caesium-133... fair point. If there are reliable sources expanding on that, that should be explained in the Second article, if it's not already. ― <i style="background: white; color: blue; font-weight:600; font-family: monospace">A._di_M.</i><sup style="font-family: fantasy">2nd Dramaout (formerly Army1987) 21:22, 28 January 2010 (UTC)

I just looked at the article for Second, and it appears there is a distinction being made there between quantum vacuum and free space, and the implication that the definition of both the meter and the second is based on the latter, said there to be an unattainable abstraction, which I find surprising. This seems a bit like the controversy that caused such a ruckus here, if I am understanding right. When I read the SI specification document, I don't see any mention of free space in either definition, but perhaps I am missing something. CosineKitty (talk) 21:47, 28 January 2010 (UTC)
 * The difference is it's defined in free space but that's impossible to achieve on Earth so the next best thing is used. This is less of an issue than it seems as the thing being measured was chosen as its a very stable transition, so even in less than perfect conditions it gives the right result (though I can't remember where I read this).-- JohnBlackburne words<sub style="margin-left:-2.0ex;">deeds 22:12, 28 January 2010 (UTC)


 * Interesting. It would make sense that those smart people would pick something that's easy to reproduce &mdash; at least, easy for crackerjack physicists!  :)  But I'm still skeptical about the claim being made in the article Second.  They just don't say that, at least not explicitly.  The SI specification (the one I linked above) defines a meter based on "light in a vacuum", and for the definition of a second, it just refers to a caesium 133 atom at 0K.  One presumes that 0K is unattainable, but it is approachable as a limit, in which case there are no other atoms colliding with the caesium atom, and thus it is in a de facto vacuum.  In either case, I don't see how vaccum means anything other than a literal laboratory container with (almost) all of the air pumped out of it.  The gravitation in the laboratory distorts spacetime a little bit (as seen from someone far from Earth), but we assume that distance and time durations are both relative to an observer in the same frame.  You would want to measure your distance and time standards horizontally, if done on Earth, to minimize the effects of climbing out of (or into) the gravity well.  But once you made your calibrated meter-stick and second-clock, I would assume you could take them out into intergalactic space and use them, and they would work just as well as ones that were made in an intergalactic laboratory. CosineKitty (talk) 22:55, 28 January 2010 (UTC)


 * Don't worry about relativistic effects. As long as the Caesium and the observer are in the same frame of reference they can be ignored. E.g. you set up your Caesium, measure it, set your watch by it and you have an accurate watch. You take it to some other frame of reference it will still be an accurate watch and you'll still be able to tell the time and measure distances well. It may run slow if observed by someone in another frame of reference, due to time dilation, but the watch is still accurate.-- JohnBlackburne words<sub style="margin-left:-2.0ex;">deeds 23:13, 28 January 2010 (UTC)


 * Yes, I think we are saying the same thing about relativistic effects. I'm not worried about them.  I may have gotten out in the weeds there; my real point was that the cited source talks about a vacuum, and there is no hint there that they mean anything other than an honest-to-goodness real-world container with as much air pumped out of it as possible, not some unattainable abstraction as free space.  Am I right or wrong (or just confused)?  CosineKitty (talk) 23:22, 28 January 2010 (UTC)
 * Well, they mean at least slightly more by vacuum. In the context of these definitions it is also meant to imply the absence of any background fields. TimothyRias (talk) 09:18, 29 January 2010 (UTC)
 * Yes, but the Second article seems to imply (but comes short of explicitly saying) that the definition is supposed to apply to (classical) free space rather than (quantum) vacuum so that you have to correct it for Lamb shift and similar. That sounds strange to me... I thought it just meant "no particles, including photons". Anyway this belongs to Talk:Second not here. ― <i style="background: white; color: blue; font-weight:600; font-family: monospace">A._di_M.</i><sup style="font-family: fantasy">2nd Dramaout (formerly Army1987) 11:30, 29 January 2010 (UTC)
 * I've copied this discussion to Talk:Second. Let's continue it there, except for matters affecting this article (if any). ― <i style="background: white; color: blue; font-weight:600; font-family: monospace">A._di_M.</i><sup style="font-family: fantasy">2nd Dramaout (formerly Army1987) 11:37, 29 January 2010 (UTC)

Meta discussion
Oh not this again. When will we stop discussing a meta-discussion and start talking about the actual content of the article? I wanted to get back on this page after it failed the FAC, so we could address real problems (structure, presentation, order of ideas...), but there's no way I'll stay around if we're going back to the same debate that lead to that ARBCOM case. Headbomb {{{sup|ταλκ}}<sub style="margin-left:-4.0ex;">κοντριβς – WP Physics} 22:43, 29 January 2010 (UTC)


 * I have nothing further to say on the matter, and I think everyone else is done too.Likebox (talk) 22:52, 29 January 2010 (UTC)
 * Since everybody seems done here, mind if I archive these discussions to make room for the discussions about some actual changes to the article? TimothyRias (talk) 16:33, 31 January 2010 (UTC)
 * Sounds good and is fine by me. -- JohnBlackburne words<sub style="margin-left:-2.0ex;">deeds 17:31, 31 January 2010 (UTC)
 * Me too. ― <i style="background: white; color: blue; font-weight:600; font-family: monospace">A._di_M.</i> (formerly Army1987) 21:55, 31 January 2010 (UTC)


 * I strongly oppose any form of archiving, at least at the moment, for the following reasons:


 * This discussion is pretty fresh. The fact that I'm done, and I don't see many changes that can be made doesn't mean that somebody else won't get inspired to make changes based on this.
 * This thread is here to briefly represent a position which is not consensus at the moment, but might be consensus at some point in the future. It is important to leave a brief mention of possible ideas for change on the talk page, so that articles can grow. If you want to refactor out the discussion of "the second" that was copied to another page, that's OK.
 * I believe that the ideas here are relevant to improving the page on a longer time scale. Just keep on with the usual business around it for now. This is not causing any trouble.Likebox (talk) 00:36, 1 February 2010 (UTC)
 * Likebox, by that logic no talk page comment should ever be archived. Also if an idea is not relevant to improving the article now (when it is near becoming FA), it is not ever gonna be relevant to improving article. If new editors want to draw inspiration from past discussions on the talk page they can wade through the archive, that is what they are for in the first place. And yes, this thread staying here is causing trouble; it is forcing me to scroll through a lot more talk page to view new comments. TimothyRias (talk) 08:11, 1 February 2010 (UTC)
 * Compromize: Likebox could write a small abstract that remains visible after archiving. We are used to having to browse all the abstracts on some parts of arXiv.org every day anyway, so one small abstract by Likebox here on Wikipedia is not going be a problem. Count Iblis (talk) 13:55, 1 February 2010 (UTC)


 * Count Iblis' compromise is completely fine in terms of content. But this premature archiving is setting a bad example. You must always bend over backwards to make sure that you are not hiding points of view, and to scroll a little more is not much of a sacrifice for this purpose.


 * As far as being near FA, who cares? Even if it were FA, the article should be able to grow, and discussions should not be hidden away prematurely. If a year passes, and this page becomes crowded, then you can archive the discussion. At that point, I will introduce a short paragraph summarizing this disputed point of view, and point readers to archived discussions.Likebox (talk) 17:06, 1 February 2010 (UTC)


 * I think archive after 100 days and 200K, in chronological order is the normal procedure.Likebox (talk) 17:11, 1 February 2010 (UTC)
 * It is nice of you to think so, but the guidelines leave this completely up to the talk page consensus. The current status quo on this talk page is to automatically archive threads 14 days after they have become inactive. If I recall correctly automatic archiving was original initiated by Brews (at a 7 day interval). You can try to get consensus to change this status quo, but I will probably oppose. TimothyRias (talk) 17:40, 1 February 2010 (UTC)


 * Honestly, I don't care what the details of archiving policy here is, so long as it is basically POV neutral. In this case, that means archive in chronological order. I hope you can see the danger of archiving certain points of view before others. This is not something that editors can force on others. It requires consensus from all editors who put comments up on talk.Likebox (talk) 17:46, 1 February 2010 (UTC)
 * So what are we saying, leave the automated archival at 14 days and no manual archiving? I would support that. --Michael C. Price talk 12:09, 2 February 2010 (UTC)
 * OK, so let's keep this section here until it's archived automatically, unless someone else chimes in in the next 14 days. ― <i style="background: white; color: blue; font-weight:600; font-family: monospace">A._di_M.</i> (formerly Army1987) (timestamp intentionally omitted) —Preceding undated comment added 14:57, 2 February 2010 (UTC).

Inconsistencies
"Its value is exactly 299,792,458 meters per second,[1][2] often approximated as 300,000 kilometers per second"

Is it METERS per second, or KILOmeters per second? —Preceding unsigned comment added by 99.142.23.132 (talk) 23:02, 4 February 2010 (UTC)


 * The figure in m/s is exact, and makes more sense in metres as then there's no decimal point. The approximation is in lkm/s as that's how it's popularly given, simply as we usually use km (or miles, when it's 186,000 miles/sec approx.) for large distances. -- JohnBlackburne words<sub style="margin-left:-2.0ex;">deeds 23:06, 4 February 2010 (UTC)


 * Maybe I should add: and 299,792,458 is almost 300,000 × 1000 as there are 1000 m in a km.-- JohnBlackburne words<sub style="margin-left:-2.0ex;">deeds 23:08, 4 February 2010 (UTC)


 * I sincerely hope we don't have to explain something as simple as 1 km = 1000 m. Otherwise, this article is going to expand in size by orders of magnitude.  If we really think people are going to be confused by that, most of the content in this article will be hopelessly difficult.  CosineKitty (talk) 16:06, 5 February 2010 (UTC)

Reworking measurement and history stuff
I agree that also stuff such as Maxwell's theory of electromagnetism predicting 1/c^2 = eps_0 mu_0 and the birth of special relativity should be explained. OTOH, I'm not sure that completely pulling measurements out of that section would be practical. I've seen the last revision before David Tombe came along, and it has a "Ancient, medieval and early modern history" section and a "Measurement of the speed of light" section. Maybe we could do something like this: But there could be slight problems, because it's not obvious how to split the current "Early astronomical techniques", the current "Redefinition of the metre" assumes the reades has just read "Heterodyne laser measurements", etc. Who wants to give it a try? ― <i style="background: white; color: blue; font-weight:600; font-family: monospace">A._di_M.</i><sup style="font-family: fantasy">2nd Dramaout (formerly Army1987) 17:03, 26 January 2010 (UTC)
 * History
 * Ancient, medieval and early modern speculation
 * First measurement attempts
 * First evidence of finiteness (first half of current "Early astronomical techniques")
 * Maxwell's theory of electromagnetism
 * [] (Fizeau experiment, Michelson–Morley experiment)
 * Birth of special relativity
 * Redefinition of the metre
 * Measurements
 * Early astronomical techniques (latter half of current section)
 * Fizeau–Foucault apparatus
 * Cavity resonance
 * Heterodyne laser measurements
 * Modern astronomical measurements
 * Laboratory demonstration
 * Sounds like a reasonable outline, and I agree that there are some problems there. I will try some experiments in my sandbox this week to see if I can make it work. TimothyRias (talk) 20:07, 26 January 2010 (UTC)
 * I've made a suggested outline of the new sections on my sandbox page. As you can see realizing it would require refactoring a lot of content, some of which needs to rewritten to adjust to the new context.
 * The "measurements" section should focus on explaining the physics behind the various techniques, while the history should stay clear of that subject and just name the kind of technique shortly while focussing on the result and its implications. Make sense? Any suggestions? TimothyRias (talk) 16:24, 31 January 2010 (UTC)
 * The outline for "History" is OK. Some rearranging for "Measurement techniques" could be useful. (Except for aberration, the astronomical measurements *are* based on time of transit... Maybe the first subsec should be "Earth-bound time-of-flight techniques". Also, within "Astronomical measurements", Rømer's technique should come before stellar aberration -- but I guess that was a slip-up -- and the explanation about astronomic units should come before the techniques which use them, as in the current version of the article modulo possible minor trimming.
 * I recall someone once mentioning the absence of demonstrative methods not using light; assuming they meant "visible light" rather than "any EM wave", I once read an article on a popular science magazine's website explaining how to measure the wavelength of a microwave oven using chocolate chips, and multiply it by its frequency to obtain c; if I find it, maybe we could consider adding it to the article. (Motion Mountain mentions the same experiment, but I'd not consider it a RS.) ― <i style="background: white; color: blue; font-weight:600; font-family: monospace">A._di_M.</i> (formerly Army1987) 16:36, 1 February 2010 (UTC)
 * I think it would be best to mention in the "astronomical measurements" sub that these are also time of flight measurements. That removes the need for awkwardly long section titles. My thinking on putting the stuff about the au after the actual techniques is that it comes less as a surprise that we are talking about the au. You would get a paragraph that goes something like this:
 * "The results of the above techniques are most natural expressed in astronomical units; the mean distance of the Earth to the Sun au and the time it takes for the Earth to revolve about it axis, the day.(footnote with details about actual definition of those units). These units are defined independently from the SI and the speed of light expressed in these units is thus not effected by the fixed value in the SI. The best determination of the speed of light using modern astronomical methods is ..."
 * Don't mind the wording too much, but I hope you get the idea.
 * About a technique measuring c not involving light. I think I once mentioned the idea. My intent was to have something completely unrelated to electromagnetic waves. Since c shows up in so many areas of physics, it must be possible to determine c without using EM waves. For example, particle accelarators should be able the give a fairly accurate estimate on the upper limit of speeds for massive particle. But I'm not aware of any notable cases where such a measurement was actually published. Wouldn't be surprised if it does exist though.TimothyRias (talk) 09:43, 2 February 2010 (UTC)
 * Section 6 of this mentions something like that, but nothing which can be done in an undergrad class. ― <i style="background: white; color: blue; font-weight:600; font-family: monospace">A._di_M.</i> (formerly Army1987) 10:18, 2 February 2010 (UTC)
 * After looking around a bit, it seems there is enough literature on measuring the limiting velocity for a small subsection. It will be easy to add such a section after the proposed restructuring is done. So lets try to focus on that first. Any editors around that disagree with the proposed restructuring? If not, I suggest we get to it. It will involve quite a bit of work all of which will be hard to do as incremental edits. It might be best to split up the tasks between different editors. One doing the history and another doing the measurements section. TimothyRias (talk) 10:53, 2 February 2010 (UTC)

History draft
I've created a draft for the history section in my sandbox. Please criticize mercilessly, especially the statements marked "citation needed" since those are basically approximations to the truth done from memory. TimothyRias (talk) 15:57, 5 February 2010 (UTC)
 * It seems good overall; I'll take a more careful read at it and suggest any improvement I can think of. Meanwhile, I'm going to write a draft for the Measurement section. ― <i style="background: white; color: blue; font-weight:600; font-family: monospace">A._di_M.</i> (formerly Army1987) 16:37, 7 February 2010 (UTC)
 * Thanks for the comments, how is your draft coming along? Any other comments? TimothyRias (talk) 10:27, 11 February 2010 (UTC)
 * It's at User:A. di M./Sandbox, but it's in a very early state (essentially made up by pieces copied and pasted from the article). I think that a non-negligible amount of redundancy with material in your "History" section will be hard to avoid, though. ― <i style="background: white; color: blue; font-weight:600; font-family: monospace">A._di_M.</i> (formerly Army1987) 11:17, 11 February 2010 (UTC)

Measurement draft
Here it is; feel free to comment and edit it. There are redundancies with TimothyRias's draft of the History section, but they can be "ironed out" after the two are put together. ― <i style="background: white; color: blue; font-weight:600; font-family: monospace">A._di_M.</i> (formerly Army1987) 00:18, 15 February 2010 (UTC)


 * Looks good as a basis. There indeed some redundancies with the other draft, but those will be much easier to iron out after adding both to the main article. More eyes and hands, etc. Also some of the phrasing still reads like a history, but we can iron those out later as well. If there are no major objections in the next day or so, I'd lets add the whole thing to the main article and work from there. TimothyRias (talk) 09:59, 15 February 2010 (UTC)
 * Yep, that looks OK. I wonder if we could find another section heading instead of "time of flight", but that's just a minor quibble. Physchim62 (talk) 10:25, 15 February 2010 (UTC)
 * Good work. Definite improvement. Perhaps the title should be changed from "Measurement techniques" to "Measurement of the speed of light", to distinguish it from the speed of light being used to measure other things. --Bob K31416 (talk) 18:17, 15 February 2010 (UTC)
 * There once was a section called something like that, but it was suggested to change it per WP:HEAD: "Section names should not explicitly refer to the subject of the article, or to higher-level headings, unless doing so is shorter or clearer." What about just "Measurement"? ― <i style="background: white; color: blue; font-weight:600; font-family: monospace">A._di_M.</i> (formerly Army1987) 18:54, 15 February 2010 (UTC)
 * Just "Measurement" is fine by me. --Bob K31416 (talk) 20:34, 15 February 2010 (UTC)

I've implemented both drafts into the article. Let the shaving, rubbing and messaging begin. TimothyRias (talk) 21:27, 15 February 2010 (UTC)


 * It's looking pretty good! I made a few tweaks.  A couple of notes:
 * It looks like the copy-and-paste introduced some carriage return characters. Some of these landed inside the double-square-brackets for a wiki-link, thus breaking them.  I fixed the ones I found.  I may go back and remove all the extraneous carriage returns, so that word wrapping works as expected for future editors (if nobody objects to me doing this).
 * I got the feeling that the "measurement" section and the "history" section are (to a small extent) chronological forks. It might be nice to merge them into a single chronological stream.
 * I would like to propose a small rewording of this phrase: "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 ...". My understanding is that the discrepancy was noticed over the span of time between opposition and nearing conjunction of Jupiter (when it is lost in the Sun's glare).  The time of certain events happening with Jupiter's moons (e.g. passing behind the planet) were off by several minutes.  It wasn't that the Earth was approaching or receding from Jupiter, but that we were closer or further away, thus increasing or decreasing the amount of time the light took to get from there to here.
 * CosineKitty (talk) 22:32, 15 February 2010 (UTC)
 * As for the last point, the fact is, if we are approaching Jupiter, then we will be closer to it on the next eclipse of Io than we were on the previous one, so the two eclipses appear to be closer in time; vice versa if we are receding from it. ― <i style="background: white; color: blue; font-weight:600; font-family: monospace">A._di_M.</i> (formerly Army1987) 23:08, 15 February 2010 (UTC)


 * Yes, I agree with that. My point was more about the ability to measure this historically.  I believe the original discrepancy was noticed based on being near or far from Jupiter, not approaching or receding from a comparable distance.  The way it is currently worded, it makes it sound like the discrepancy was first noticed due to a Doppler effect.  If you double the Earth's orbital speed, you get 60 km/s, or about 0.0002c.  This would work out to a discrepancy on the order of half a minute, if my math is right (using classical physics, since Lorenz factor is almost exactly 1, and with Io's orbital period of about 2 days).  Much easier to notice would be the extra 2 AU of light travel time when the Earth was near or far from Jupiter, on the order of 18 minutes.  Does this make sense?  CosineKitty (talk) 23:26, 15 February 2010 (UTC)


 * Follow-up: I did some research about this in my college physics textbook.  Here is a quote I found: "Roemer observed a systematic variation in Io's period during a year's time.  He found that the periods were larger than average when the earth receded from Jupiter and smaller than average when the earth was approaching Jupiter.... Roemer attributed this variation in period to the fact that the distance between the earth and Jupiter was changing between the observations.  In three months (one quarter the period of the earth), the light from Jupiter has to travel an additional distance to the radius of the earth's orbit."  So, based on this source, and after re-reading the article text, it seems fine; Doppler shift and average period observed over long times are really just different ways of saying the same thing anyway.  I was just curious about the nitty-gritty details of the discovery, and it sounds like it was an ongoing investigation over a long period of time.  CosineKitty (talk) 02:43, 16 February 2010 (UTC)
 * Can you give me a cite to that college physics textbook, and I'll drop a line to the author! You're correct that the effect that Rømer can be analysed as a Doppler phenomenon, and this is currently a "trendy" way of doing so since . However, it is historiographical nonsense, just as is the claim that "Roemer observed a systematic variation in Io's period during a year's time": Rømer's timekeeping simply wasn't up to such measurements. If you take Rømer's observations and calculate the period of Io based on pairs of successive measurements, you won't see any trend at all.
 * In modern terms, Rømer started with a null hypothesis that light was propagated instantaneously (ie, at infinite speed) compared with a test hypothesis that light had a finite speed and hence a delay in it's propagation which depends on the distance (the mora luminis which is referred to in Rømer's writing). He calculated the times of eclipses of Io based on the null hypothesis (that was his day-job, in fact), and showed that the observed times could be better explained using his test hypothesis. In particular, he realised that he needed an interval as long as possible between observations (which cannot be more than 3½ months for practical reasons) to demonstrate the effect and hence to estimate a numerical value for the speed of light. Physchim62 (talk) 15:31, 16 February 2010 (UTC)
 * Hi Physchim62, my college textbook was published in 1986 by Raymond Serway. It is called Physics for Scientists and Engineers.  It looks like there is an updated edition for 2004, so maybe there is a better explanation of this now?  Anyway, my 1986 edition says that Roemer only established a lower bound for the speed of light, not an upper bound or error bars.  I agree that measuring time that accurately would have been difficult in the late 1600s.  The key seems to be the word "average" in "average period"; I doubt he could have measured a shift in a single orbit of Io.  It had to be a discrepancy over many months that added up to multiple minutes, which could have been measured via telescopic observation of stars, i.e. sidereal time.  CosineKitty (talk) 16:48, 16 February 2010 (UTC)
 * I can't be too harsh on Dr. Serway then, as he was writing before the 1988 paper which reviewed over 100 different college textbooks (in English, German, Polish and Russian) and found almost 100 different explanations of Rømer's work! The key is the "average", yes, but it's also which average. If you take measurements over a whole synodic period (the 13-month period between successive oppositions of Jupiter), you will get a good value for the orbital period of Io: the value that Rømer was using was correct to within 10 ppm. On the other hand, if you take an average over a single series of measurements taken in the four months after an opposition, you will get a slightly different answer. If you use your mean orbital period of Io to predict the time of your last measurement, you will be out by about ten minutes. Timings, by the way, were made in solar time and then corrected to mean solar time using an Equation of Time: that's not quite the same as sidereal time, but it is proportional (assuming your EoT is good enough). Physchim62 (talk) 17:32, 16 February 2010 (UTC)
 * That is very interesting! Here is another tidbit (look out, here comes some OR!).  If you are an amateur astronomer like myself, you know that even with WWV sounding out an atomic clock, it's not that easy to time the occultations of the Galilean satellites, simply because it's hard to decide when it really started/finished.  The image is shimmering because of atmospheric interference, and the similar light level of the tiny moon to the very bright planet makes it hard to decide the exact moment of the event.  The error can be on the order of 15 or 20 seconds, which would drown out the expected error in a single period of Io's orbit.  That's what brought my attention to this, along with doubts about limited timekeeping in that era.  CosineKitty (talk) 19:41, 16 February 2010 (UTC)
 * Funny you should mention the figure of 15–20 seconds: that's exactly what Cassini thought was the uncertainty in their measurements of eclipse timings! He was slightly optimistic, but not by much: a recent analysis of the data gives an uncertainty of about 30 seconds, which will combine the observational problems you mention with the instrumental uncertainty in the clocks. The timekeeping at the Paris Observatory was pretty good, even between different observers: Rømer was only 44 seconds of time out in his determination of the longitude of Uraniborg based on eclipses of Io, despite the many difficulties he faced. Jean Richer, another of the Paris observers, determined that the length of a seconds pendulum in Cayenne was 0.285% shorter than the length of a seconds pendulum in Paris (the modern value is 0.297% shorter)! Anyway, we digress, perhaps we should take this to usertalk pages? Physchim62 (talk) 21:26, 16 February 2010 (UTC)

In a vacuum
In the interest of maintaining complete accuracy in the intro (while retaining accessibility to the layman), would it perhaps be better to change:
 * ''The speed of light is a physical constant. Its value is exactly 299,792,458 metres per second, often approximated as 300,000 kilometres per second or 186,000 miles per second."

..to say..
 * "The speed of light is a physical constant in any given medium. Its value is exactly 299,792,458 metres per second, often approximated as 300,000 kilometres per second or 186,000 miles per second in a vacuum (such as in outer space). Light speeds up and slows down as it passes through the interface between different mediums (never exceeding it's speed in a vacuum)."

- Eric (talk) 17:39, 4 February 2010 (UTC)


 * In my opinion, that amount of detail in the first sentence or two of the lead is more confusing than helpful to the general reader. It makes some later statements in the lead section redundant (you can't tinker with one part of the lead in isolation from the rest of it). I also have quibbles about its accuracy: the only physical constant is the speed of light in vacuum (i.e., in the reference state). Outer space is not a perfect vacuum (damn close though, if you are talking about the almost-empty regions). I prefer the current content and structure of the lead section.—Finell 18:09, 4 February 2010 (UTC)


 * I don't like it either--- the "speed of light" has nothing to do with light. It is a conversion constant between units of time and units of space. As such, even if light were found to travel at less than the speed of light (this is not a contradiction in terms) the speed of light would not be affected.Likebox (talk) 18:35, 4 February 2010 (UTC)


 * The "speed of light" is, as the lede says, the speed of light (i.e. electromagnetic radiation). That's the generally accepted understanding of it. That light propagates at c is not a coincidence or accident, it's a well established (understood and tested) fact. So changes based on the possibility it might change have no basis in physics.


 * As for the more subtle point, that c to physics is a ratio of two dimensions in spacetime first, and because of this it's the speed of electromagnetic waves, that's also in the lede but after the general point is made. If I were writing this article I would probably move the special relativity stuff down - or make the lede shorter with same effect. It's important but the speed of light and things like measurement, refraction, can be understood without SR.-- JohnBlackburne words<sub style="margin-left:-2.0ex;">deeds 19:35, 4 February 2010 (UTC)
 * You disagree with point 3 of this text? Count Iblis (talk) 00:10, 5 February 2010 (UTC)
 * No as I'm not sure what it's about. This page is for talking about this article, or possibly useful sources of which that is definitely not an example.-- JohnBlackburne words<sub style="margin-left:-2.0ex;">deeds 00:18, 5 February 2010 (UTC)
 * I don't think the text that Count Iblis cites is at all relevant to the discussion. Physchim62 (talk) 00:42, 5 February 2010 (UTC)
 * It is very relevant for this and other wiki-physics articles. Count Iblis (talk) 14:46, 5 February 2010 (UTC)
 * Huh? If I understand correctly, it's intended to be a list of bad features of textbooks; are you suggesting that this article should do the same? ― <i style="background: white; color: blue; font-weight:600; font-family: monospace">A._di_M.</i> (formerly Army1987) 10:24, 5 February 2010 (UTC)
 * This article should follow Warren Siegel's advice of abandoning the historical approach. JohnBlackburne claimed that you don't need to mention special relativity when discussing properties of light. That's the traditional but flawed way of teaching/explaining physics.Count Iblis (talk) 14:46, 5 February 2010 (UTC)
 * Ah sorry, I misunderstood you. I agree with that, but I think it should continue to mention EM radiation in the lead as it does now. I take it to be more of a "rationale" to call c the speed of light, than anything else. ― <i style="background: white; color: blue; font-weight:600; font-family: monospace">A._di_M.</i> (formerly Army1987) 16:09, 5 February 2010 (UTC)

(deindent) Unlike the glue strong interactions, or gravity waves, light is not theoretically established to be exactly massless (at least not until now--- I think I can personally make an argument, but its not in the literature). It could have a really tiny mass, and a galactic-scale compton wavelength, without contradicting theory or experiment. So it is wrong to say that "c" is the speed of electromagnetic waves, because it isn't. It's the "c" in E=mc^2 and relativity, and it happens to also be the speed of electromagnetic radiation.Likebox (talk) 00:29, 5 February 2010 (UTC)
 * If it's not in the literature it's WP:OR, so cannot be used here. According to the generally accepted laws of physics electromagnetic radiation propagates at c. You will need more than your personal views to change that.-- JohnBlackburne words<sub style="margin-left:-2.0ex;">deeds 00:38, 5 February 2010 (UTC)


 * Please read the comment more carefully: What is OR is your claim that electromagnetic radiation propagates at c. Light is not known to propagate at exactly c. There can be no experimental argument, and there is no theoretical argument. I can make one, but it does not belong in the encyclopedia.


 * What is NOT OR, is that light is known to travel at approximately the speed of light. There is no reason that light shouldn't have a small nonzero mass, like neutrinos. This is what the literature says.Likebox (talk) 00:41, 5 February 2010 (UTC)


 * To be 100% clear, because I am not sure this point has been understood: Your statment "According to the generally accepted laws of physics, electromagnetic radiation propagates at c" is not quite correct. According to the generally accepted laws of physics, light either propagates at exactly c, or is a massive particle, which propagates at less than c. At the moment, the experimental evidence is overwhelming that light is massless, but it is difficult to establish that m is exactly zero by experiment.


 * On the other hand, "According to the generally accepted laws of physics, gravitational radiation propagates at c" is correct. There is no reasonable way to make gravity travel at less than c.


 * Also, "According to the generally accepted laws of physics, gluon radiation propagates at c for distances much less than one proton radius" is also correct. There is no reasonable way to make gluons massive which does not conflict with experiment.Likebox (talk) 00:48, 5 February 2010 (UTC)


 * This article is not about gluon radiation or gravitational waves, so disregarding all that, it comes down to you. You say "light either propagates at exactly c, or is a massive particle", but then say "evidence is overwhelming that light is massless". It's either one or the other. It's either undecided, as the first quote suggests, or there's overwhelming evidence for the current interpretation. I would say the latter.-- JohnBlackburne words<sub style="margin-left:-2.0ex;">deeds 12:27, 5 February 2010 (UTC)

(deindent) The "speed of light" c has nothing to do with light. It is the speed of gluons and gravitons. Light can be massive without altering physics as we know it, but gravity can't and neither can gluons. The evidence that light has a very small mass is overwhelming. The evidence that the mass is exactly zero is nonexistent.

I can't understand why trivial points such as these cause arguments.Likebox (talk) 13:24, 5 February 2010 (UTC)
 * I'm confused. Isn't the (theoretical) reason for gluons and gravitons being massless exactly the same as the one for the photon being massless: any mass term will break gauge invariance? This discussion is getting a bit off topic, but I do agree with your point that this article is about the physical constant called the "speed of light" that a priori has nothing to do with light. TimothyRias (talk) 13:48, 5 February 2010 (UTC)
 * A massive photon won't break gauge invariance, see e.g. here. Count Iblis (talk) 14:41, 5 February 2010 (UTC)
 * That article talks about a photon mass obtained through the Higgs mechanism, i.e. through spontaneous breaking of the U(1) gauge symmetry. Through the same mechanism you can give mass to gluons by breaking the SU(3) symmetry. (This in fact happens in colorsuperconductivity). So, I still don't get the distinction being made by Likebox between photons and gluons being massless. TimothyRias (talk) 14:57, 5 February 2010 (UTC)


 * For Thor's sake people, focus. No one in the world would be taken seriously if they went around saying photons have mass. As far as anyone is concerned, they don't. The upper bound is something like 10&minus;50 g. This is mentionned in the article thrice, once in the lead, and twice in the main text"According to the theory of special relativity, c connects space and time in the unified structure of spacetime, and its square is the constant of proportionality between mass and energy (E = mc2).[3] In any inertial frame of reference, independently of the relative velocity of the emitter and the observer, c is the speed of all massless particles and associated fields, including all electromagnetic radiation in free space,[4] and it is believed to be the speed of gravity and of gravitational waves.[5][6] It is an upper bound on the speed at which energy, matter, and information can travel,[7][8] as surpassing it 'would lead to the destruction of the essential relation between cause and effect.'[9] Its finite value is a limiting factor in the speed of operation of electronic devices.[10]"
 * and in the Fundamental physics section:"The theory of special relativity explores the consequences an invariant speed c and the assumption that the laws of physics are the same in all inertial frames of reference.[22][23] One consequence is that c is the speed at which all massless particles and waves, including light, must travel."
 * and again in the progatation of light section: In modern quantum physics, the electromagnetic field is described by the theory of quantum electrodynamics (QED). In this theory, light is described by the fundamental excitations (or quanta) of the electromagnetic field, called photons. In QED, photons are massless particles and thus, according to special relativity, they must travel at the speed of light.

Extensions of QED in which the photon has a mass have been considered. In such a theory, its speed would depend on its frequency, and the invariant speed c of special relativity would then be the upper limit of the speed of light in vacuum.[29] To date no such effects have been observed,[46][47][48] putting stringent limits on the photon mass. The limit obtained depends on the used model: if the massive photon is described by Proca theory,[49] the experimental upper bound for its mass is about 10−57 grams.[50] If photon mass is generated by a Higgs mechanism, the experimental upper limit is less sharp, m ≤ 10−14 eV/c2 [49] (roughly 2 × 10−47 g).
 * Could we please stop forgetting what the article already says? Headbomb {{{sup|ταλκ}}<sub style="margin-left:-4.0ex;">κοντριβς – WP Physics} 14:58, 5 February 2010 (UTC)


 * Seriously, the article is called "Speed of light". It evidently is about whatever speed light actually does travel.  If actual photons do turn out to have mass, that will be fascinating, but this article will still be about how fast they go (even if it is a range of speeds).  Until then, the overwhelming evidence is that photons have exactly zero mass, and there is no issue.  CosineKitty (talk) 16:06, 5 February 2010 (UTC)
 * You are very much wrong. This article is about the physical constant c also called the "speed of light". A priori that constant has no connection to light whatsoever. TimothyRias (talk) 16:24, 5 February 2010 (UTC)
 * You start from a very strange a priori! Physchim62 (talk) 16:32, 5 February 2010 (UTC)


 * Would it be more accurate if I said that the speed of light "is the speed of electromagnetic radiation (such as radio waves, visible light, or gamma rays) in vacuum, where there are no atoms, molecules or other types of matter that can slow it down"? CosineKitty (talk) 16:36, 5 February 2010 (UTC)
 * Accurate would be: the speed of light is the speed of massless particles and fields.TimothyRias (talk) 16:51, 5 February 2010 (UTC)
 * Nothing to do with light? All those measurements of the speed of light have nothing to do with the speed of light (or, if you prefer, c)? Let's not inflict speculative possibilities and quibbles on the high school and college students of the world in this article, when they read it to learn about the speed of light.—Finell 17:45, 5 February 2010 (UTC)
 * I said nothing to do with light 'a priori'. Since light is massless (for all intents and purposes anyway) it has again everything to do with c and the article currently accurately reflects that accurately. It is simply not true that the "speed of light" is whatever speed photons travel at even if they do have mass, as CosineKitty claimed. (or the starter of this thread for that matter) Anybody who thinks that has been confused by the unfortunate historical name. TimothyRias (talk) 18:15, 5 February 2010 (UTC)

Note that my quote was copied verbatim from the lead paragraph of this article. If it is "wrong", then this whole article is in deep trouble. CosineKitty (talk) 18:11, 5 February 2010 (UTC)
 * There is nothing wrong with the quote. It is however wrong to say that that quote defines c, which is what I presumed you were trying to see. TimothyRias (talk) 18:19, 5 February 2010 (UTC)
 * I think I understand the distinction you are making; yes, c is the speed of a massless particle, as in the ultimate universal speed limit according to relativity. I think we would agree that if there is ever significant evidence that actual photons in a vacuum travel at some lower speed (i.e., they have positive mass), that would require a serious overhaul of this article.  As we currently understand it, photons are massless, and this is a distinction without an empirical difference.  I guess I was operating on the assumption that in context (a discussion page about improving this article), there was some desire to change the article.  If this is just a philosophical discussion about what the term speed of light means, but no actual problem with the article's current wording, I have no quibble.  CosineKitty (talk) 18:33, 5 February 2010 (UTC)
 * Overhauling this article would be the least of it. I hate to say it, but this is starting to sound like the discussion that tied up this page for about a year and led to an arbitration.—Finell 00:59, 6 February 2010 (UTC)


 * Just to be clear: this is NOT that discussion. That discussion is over and done. This is a different discussion about what "c" means really. The other discussion was about whether it is better defined or measured. This one is about whether it is a property of light or a property of space. This one, unlike the other, has a clear answer in the literature, and this is the answer that TimothyRias is giving.


 * I want to quickly explain the difference between a photon mass and a nonabelian mass for TimothyRias--- because it is a major headache to find this clearly explained in the literature. You are right, all mass terms should be thought of as breaking gauge invariance, and generically you can see this in a gauge theory because the moment you add a mass term, the current is no longer conserved, and the theory stops being renormalizable. But if you actually add a mass term to QED, or any abelian gauge theory, and you make the Feynman rules, you will find that the extra terms that normally give you current non-conservation don't do that. They drop out instead. One way of saying this is that you can make a photon massive by replacing the denominator of the Feynman propagator for the photon with "k^2-m^2+ie" instead of "k^2+ie". You don't need to touch the numerator, and the numerator term proportional to k_u k_v in the massive propagator is what gives rise to all the trouble. It vanishes in QED because each k_u is always contracted with a transverse source.


 * But this mass term should be breaking gauge invariance, and should be screwing up everything. So what gives? This was historically extremely confusing, because people knew that massive QED was OK long before they learned that massive gauge theory is generally not OK. This is part of why 'tHooft's work was such a bombshell--- nonabelian gauge theory requires zero mass.


 * The real reason is explained in the "affine Higgs mechanism" section in the Higgs mechanism page. The U(1) gauge theory, unlike the nonabelian gauge theory, can be noncompact. This means that you can add a Higgs field with any charge, not just quantized multiples. If you take a limit as the Higgs field charge goes to zero, keeping eH constant, where H is the Higgs field VEV, you get a consistent limit where there is a massive QED, but the Higgs boson is infinitely massive. In this case, since the Higgs is not required to appear, you can think of it as massive U(1) without breaking gauge invariance, although of course gauge invariance is explicitly broken by the mass term.Likebox (talk) 02:01, 6 February 2010 (UTC)
 * Of course it is not the same issue. It is the nature of the discussion, and the nature of the issue, that is similar: discussing an obscure point of speculative physics theory in this article, which is not aimed at physicists. It is a question of emphasis. Discussion of it in this article would confound more readers than it would inform, by a wide margin. The only readers who would appreciate the point are those who are already aware of it, and who don't use Wikipedia to learn more about physics.—Finell 09:38, 6 February 2010 (UTC)

C'mon. Right now, by large the mainstream view is that photons are massless; so c is the speed of light. Speculations about what would happen if the photon were massive belong where they belong, and are already there. In the event that evidence were found that the photon is massive (as was with the neutrinos in the last decades), then maybe a new name will be coined for c, and when such a name becomes mainstream we would move this article there. (Anyway, only hard-core theoretical physicists would really care, as to obtain a difference in speed in the fifteenth significant digit (299,792,458.999 999) you'd need a gamma factor of about 12 million, which for a $eV$ photon (the PDG's 2009 limit) would mean a wavelength of over $100 km$, unless I goofed with the calculations. By comparison, experimental physicists often say that neutrinos go at c, even if they know that's not exactly true.) Until then, this discussion is nearly pointless. ― <i style="background: white; color: blue; font-weight:600; font-family: monospace">A._di_M.</i> (formerly Army1987) 11:11, 6 February 2010 (UTC)

Approximate
Hi. I thought the table would look nicer without the approximation signs (≈). They don't seem necessary, especially since two entries are noted as "exact". --Bob K31416 (talk) 20:59, 13 February 2010 (UTC)


 * Hi. I think the table looks just fantastic with the approximation signs (≈). DVdm (talk) 22:07, 13 February 2010 (UTC)


 * I agree with BobK; there seems no need to state approximate when exact is stated for some ... and it does look a tad inelegant imho. Abtract (talk) 00:16, 14 February 2010 (UTC)


 * How's this? ― <i style="background: white; color: blue; font-weight:600; font-family: monospace">A._di_M.</i> (formerly Army1987) 02:49, 14 February 2010 (UTC)


 * Utterly unimportant, good grief :-) - DVdm (talk) 10:23, 14 February 2010 (UTC)


 * Thanks for the compromise. --Bob K31416 (talk) 11:16, 14 February 2010 (UTC)

AU and measurement of c
Re part of opening paragraph of new section "Measurement": "therefore, to measure the speed of light, one must use a standard of length that is not based on the SI metre and that is defined independently of c. The astronomical unit is one such standard" - Are there reliable sources for these remarks, and the implication that the AU may be used in measuring c with a greater precision, or were they originated by an editor? --Bob K31416 (talk) 02:52, 16 February 2010 (UTC)


 * It is obvious from the definition of the AU, which is well sourced, that it is not based on the SI metre and is defined independently of c. Further, no one is talking about measuring c in AUs being more accurate than some other measure. The point is that it is possible, whereas measuring the speed of light directly in true SI metres is not (although one could, in principle, calibrate a measuring rod in SI metres, and use the rod to make repeated measurements of the speed of light).—Finell 06:11, 16 February 2010 (UTC)


 * I do think that this remark is somewhat out of place in the first paragraph about measurement. I think it might be better to incorporate it in the astronomy subsection. I'll see if I can message the text a bit. TimothyRias (talk) 09:19, 16 February 2010 (UTC)


 * Here's a possible replacement for the first paragraph,
 * To measure the speed of light, various methods can be used which involve observation of astronomical phenomena or experimental setups on Earth. The setups could use mechanical devices (e.g. toothed wheels), optics (e.g. beam splitters, lenses and mirrors), electro-optics (e.g. lasers), or electronics in conjunction with a cavity resonator.
 * --Bob K31416 (talk) 12:32, 16 February 2010 (UTC)


 * It looks like the opening paragraph has been replaced with the above. Good. --Bob K31416 (talk) 03:58, 19 February 2010 (UTC)

The part mentioned at the beginning of this discussion, of the former opening paragraph, has been modified and moved to the opening paragraph of the Astronomical measurements section, and now looks like this,
 * "Since the most used reference length scale in modern experiments (the SI metre) is determined by the speed of light, the value of c is fixed when measured in metres per second. Measurements of c in astronomical units provides an independent alternative to measure c."

I'm not sure what the second sentence means and it looks like OR. Is there a reliable source for the second sentence? --Bob K31416 (talk) 03:59, 19 February 2010 (UTC)
 * What the sentence is trying to say, is that astronomical measurements of c expressed in AU/day know nothing about the definition of the meter and thus provide measurement that does not return a fixed value and can be used as an independent check that c is indeed constant. In this respect the sentence is supported by the last paragraph of the section. There is definite room for improvement of phrasing though. TimothyRias (talk) 09:38, 19 February 2010 (UTC)
 * No, it only tells that any variation in c cancels out any variation in GMSun. (IOW, philosophically I continue to deem such measures as "weighing the Sun" rather than measuring c, but that's just my view.) ― <i style="background: white; color: blue; font-weight:600; font-family: monospace">A._di_M.</i> (formerly Army1987) 12:24, 19 February 2010 (UTC)
 * This is somewhat offtopic, but variations in GMSun would also alter the geometry of the various orbits themselves, allowing you to pick those out of other measurements. You would have to work really hard to make variations in c and GMSun cancel out precisely. If the measurement of c using astronomical measurements is constant, this is a very strong indication that indeed both c and GMSun remain constant. TimothyRias (talk) 12:45, 19 February 2010 (UTC)
 * Re "In this respect the sentence is supported by the last paragraph of the section." - So far, it appears there isn't a reliable source for the second sentence. --Bob K31416 (talk) 17:43, 19 February 2010 (UTC)
 * There's plenty of reliable sources at our astronomical unit article: the statement is not contentious. However, I don't think those two sentences add anything to the section: indeed they are clumsily switching between between a historical discussion and the modern definition of the metre. We can just bin them as far as I'm concerned. Physchim62 (talk) 09:08, 20 February 2010 (UTC)
 * Re "There's plenty of reliable sources at our astronomical unit article..." - Please give the source for the second sentence and the excerpt from the source. --Bob K31416 (talk) 13:04, 20 February 2010 (UTC)
 * Not if you can't be bothered to read the article, no, I shaln't. I'm not your secretary, nor your brain. Physchim62 (talk) 13:06, 20 February 2010 (UTC)
 * I looked there yesterday and I didn't find a source. If you prefer not to give a source and excerpt, that is of course your choice. But again, so far it appears there isn't a reliable source for the second sentence. --Bob K31416 (talk) 13:15, 20 February 2010 (UTC)
 * Erm, which part of the article – which contains two (mathematically equivalent) definitions for the astronomical unit – makes you believe that it is defined in terms of the metre? If you refuse to engage your brain when reading Wikipedia, there is little point in continuing this discussion. On the off chance that I'm wrong in my assessment, you can also take a look at this paper, which is also referenced in the astronomical unit article. Otherwise, please stop wasting everybody's time. Physchim62 (talk) 13:39, 20 February 2010 (UTC)
 * I've said about all I can here regarding the OR issue and the lack of sourcing. I'll leave it to the editors here to decide what to do with the second sentence. I will also try not to let one editor's somewhat rude and evasive tone reflect negatively on my perception of the general editing environment here. I usually tolerate unpleasant behavior but I think I'll just go away. Also, I think there are some problems here regarding the level of understanding by the editors, and it may be too much to deal with.  We are all volunteers and I think I understand the limitations of Wikipedia and maybe it's better not to generate more hard feelings by  staying here.  It will be a useful article in any case.  --Bob K31416 (talk) 14:44, 20 February 2010 (UTC)

Position of the history section
In the splitting of the history and measurement section I, somewhat arbitrarily, made the history section the last section in the article. Is this the best position for this section? Does anybody have strong feelings about this? I don't really care all that much. But some tweakings of the text do depend on what section comes first, so it would be nice to know of others had particular ideas about this now, instead of later. TimothyRias (talk) 09:31, 16 February 2010 (UTC)
 * In other articles about fundamental constants, we usually put the history section before the measurement sectionas it is more likely to be of interest to the non-specialist reader. Physchim62 (talk) 09:53, 16 February 2010 (UTC)

Physical constant c and measurement of the speed of light
I'm interested in checking what the thinking is here regarding measurement of the speed of light.

The physical constant c, the speed of light in vacuum, is exactly 299,792,458 metres per second. If one were to measure the speed of light in vacuum in a college student lab for instance, one might measure some other value in metres per second, because of less accurate instrumentation and measurement technique.

Is this thinking consistent with the consensus here? Any comments are welcome. --Bob K31416 (talk) 16:05, 17 February 2010 (UTC)
 * That would seem to be about correct. But how does this relate to improvement of the article? This not a general discussion forum after all. TimothyRias (talk) 16:12, 17 February 2010 (UTC)


 * Thanks. It relates to this excerpt from the article, "Since the most used reference length scale in modern experiments (the SI metre) is determined by the speed of light, the value of c is fixed when measured in metres per second." It seems that this statement needs clarification since a college student could make a measurement of the speed of light in his class's lab session and might simply give for the result "299,792,458 metres per second", based on this statement in the article. --Bob K31416 (talk) 16:22, 17 February 2010 (UTC)


 * And he would be technically correct. He will also he fail his class, since he was asked to show experimentally how long it takes light to cross the distance crossed by light in 1/299,792,458 seconds, and failed to show how he obtained his answer experimentally. Also quoting wikipedia is the road for a speedy flunk anyway. TimothyRias (talk) 16:32, 17 February 2010 (UTC)
 * Well, if it were my student in my lab class, he or she would even get an F (as I'm in Europe, and we don't mark that way)! An A-class student would give me the result in nanoseconds per laboratory standard length, and then convert the laboratory standard length into SI metres, both with measurement uncertainties. Physchim62 (talk) 16:35, 17 February 2010 (UTC)
 * That hypothetical was just meant to raise the question of whether or not the sentence in the article needs clarification, not to get any teacher's juices going, LOL. --Bob K31416 (talk) 16:42, 17 February 2010 (UTC)
 * I think the section is fine as it is. There are plenty of exact things that somebody might measure wrong.  For example, the diagonal of a square that is 1 meter on a side is exactly √2 meters.  Somebody might measure that wrong too, but I wouldn't feel a need to add a disclaimer about it.  CosineKitty (talk) 17:11, 17 February 2010 (UTC)
 * Perhaps the clarification would involve some mention that the metre is not realised by just any measurement of the speed of light, such as the college student's measurement, but a measurement sufficiently precise, such as the measurement in 1972 and potentially any equally or more precise measurements after 1972. Otherwise the length of the metre would depend on any speed of light measurement that is used and the college student would measure 299,792,458 metres per second every time that student conducted the experiment, because the college student would use that student's measurement to determine the length of the metre to use in reporting the result of that student's measurement of the speed of light. So maybe that sentence needs clarification? I think I've said all I can on this matter. If you folks see my point, OK. If you don't, OK. --Bob K31416 (talk) 21:02, 17 February 2010 (UTC)
 * Bob makes a very good point in metrological terms: that there is a difference between a unit and the realization of a unit. The metre is defined exactly, well, by definition, but we can't usually measure length to any better than about 2 parts in 1011 (which is already subatomic scale at the level of the metre). But what does this have to do with the speed of light? Physchim62 (talk) 21:10, 17 February 2010 (UTC)
 * Any measurement of c will realize the metre, but an imprecise one will realize it imprecisely. Using as an example the oscilloscope thing, you can set the mirror at a distance such that the round-trip time will be 6.67... nanoseconds, but this will be likely less precise than using a ruler, so you'd better measure the former in terms of the latter than vice versa. (Or, any measurement whatsoever is a measurement of a ratio of two quantities, so strictly speaking measuring A in terms of B is the same thing as the other way round; but typically one treats the more precise quantity as the measurement standard ans the less precise one as the thing being measured.) Hope this helps... ― <i style="background: white; color: blue; font-weight:600; font-family: monospace">A._di_M.</i> (formerly Army1987) 21:31, 17 February 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!

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". ― <i style="background: white; color: blue; font-weight:600; font-family: monospace">A._di_M.</i> (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


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. ― <i style="background: white; color: blue; font-weight:600; font-family: monospace">A._di_M.</i> (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)
 * There's already such a paragraph in the article. ― <i style="background: white; color: blue; font-weight:600; font-family: monospace">A._di_M.</i> (formerly Army1987) 11:11, 21 March 2010 (UTC)

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)


 * (I see that this is old, but...) This is exactly the sort of thing that I'm trying to get at, below. Yes, it's "specialist language", but there's a reason for it. The phrase "time for unit distance" has specific meaning, within this context, and so for really shouldn't be substituted with per, since the special meaning is then lost. Believe me, I understand that "time per unit distance" can be considered to be more grammatically correct to most, but I'm just trying to explain (without writing a whole textbook here) that there's a legitimate reason to use for in the sentence. I mix them up myself quite often, but "time for unit distance" really is more correct, here. — <span style="font-family: Courier New, monospace ;font-style:italic">V = IR (Talk&thinsp;&bull;&thinsp;Contribs) 23:02, 29 March 2010 (UTC)

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)? ― <i style="background: white; color: blue; font-weight:600; font-family: monospace">A._di_M.</i> (formerly Army1987) 19:17, 12 March 2010 (UTC)
 * The reason people continue to have trouble with, and question this, is that the editors here have somehow either forgotten, left out, or removed a key word (I've been avoiding this article like the plague since the issue went to arbitration, so forgive me for not saying something earlier. There's no way that I'm personally touching this article, either). The sentence stating the velocity of light needs to use the word "defined", which has a specific meaning in pretty much every scientific textbook on the planet now. So, instead of Its value is exactly 299,792,458 metres per second., it really should be Its value is defined to be exactly 299,792,458 metres per second.. There's a key difference between the two sentences, to anyone who's received a physics or chemistry education in the last... probably 20 years or so. I realize that it's explained in a satisfactory manner later on, but there really is a distinct difference in meaning between the two sentences. — <span style="font-family: Courier New, monospace ;font-style:italic">V = IR (Talk&thinsp;&bull;&thinsp;Contribs) 21:44, 29 March 2010 (UTC)
 * While I'm here, I might as well mention a style problem as well. The two choppy sentences to open the article really suck. I'd like to suggest: The speed of light (usually denoted c) is a physical constant, with a value which is defined to be exactly 299,792,458 metres per second. Its value is often approximated as 300,000 kilometres per second, or 186,000 miles per second (see the table on the right for values in other units). (Yea, the new 2nd sentence is sort of short as well, but the aside that it contains makes it stand alone much better. I've always found that people have an easier time digesting a shorter sentence after a longer one, anyway.) — <span style="font-family: Courier New, monospace ;font-style:italic">V = IR (Talk&thinsp;&bull;&thinsp;Contribs) 21:50, 29 March 2010 (UTC)
 * I don't see a problem with the current wording. An analogous assertion would be A litre of pure water weighs exactly 1kg. That is (or at least was) true because of the definition of a kg, but it's still the weight of water: you would not say A litre of water is defined to weigh exactly 1kg. The metre is defined, not the speed of light. It just happens to be a definition that makes light travel at 299,792,458 m/s, as is clarified a short distance down in the next paragraph.-- JohnBlackburne words<sub style="margin-left:-2.0ex;">deeds 21:59, 29 March 2010 (UTC)
 * I completely understand that, from the perspective of literature, both sentences have the same meaning. Within technical documentation in the sciences however (such as textbooks), the use of the word/term "defined to be" has specific meaning, and educators stress its importance. That's why the IP who opened this thread is mentioning it at all. (incidentally, your comment about the liter and the meter actually illustrate exactly why this is important. It doesn't really make a difference within the context of this discussion, but you've got the relationship exactly backwards. There's nothing wrong with that, until you get into more advanced topics in physics, and some advanced chemistry, where [and this is an extreme simplification] things like the measurement of a liter or meter can sort of change [but the constant c doesn't]. None of this really needs to be in the article, but I wanted to give a small bit of background for those of you who obviously live more in the Humanities world.) — <span style="font-family: Courier New, monospace ;font-style:italic">V = IR (Talk&thinsp;&bull;&thinsp;Contribs) 22:41, 29 March 2010 (UTC)
 * The problem with putting "defined to be" at that point is that many readers (we've tried it!) get confused as to what is being defined. You and I know that we're defining the value of the speed of light in a certain system of units, but many people will read it as defining the speed of light itself. The definition of the "speed of light" (in the sense we're using it in this article, the physical constant) is the speed of electromagnetic radiation in vacuum and, I think that sentence should come before the numbers. The exact fixed value comes from the definition of the metre, the unit of measurement. Physchim62 (talk) 23:03, 29 March 2010 (UTC)
 * Yes, OK, I see the problem. humm... well, if you think about it "correctly" (or incorrectly, depending on your preferred POV), then we essentially have defined the speed of light itself. But, ugh... This is why I tend to avoid technical articles here, because the vocabulary for the general audience is really just inadequate. In terms of this article, and Wikipedia in general, I guess that this sort of thing sort of becomes a "chicken or the egg" type question... the article will never need to be technically correct enough that the language will make a difference, but to people who actually know it's obvious that the article is sort of "wrong". *shrug* I guess that it doesn't really matter, but I thought that I'd try to help out some people with understanding the issue. — <span style="font-family: Courier New, monospace ;font-style:italic">V = IR (Talk&thinsp;&bull;&thinsp;Contribs) 23:25, 29 March 2010 (UTC)
 * "Its value is exactly 299,792,458 m/s" is not wrong (well, apart from those bloody commas, but I lost that battle ages ago!): take a look at |search_for=speed+of+light NIST... The sentence does beg the question as to how the figure can be exact, but that is addressed in the very next paragraph and in more detail at the end of the article. Physchim62 (talk) 23:53, 29 March 2010 (UTC)
 * Hey, I did put the word in quotes... — <span style="font-family: Courier New, monospace ;font-style:italic">V = IR (Talk&thinsp;&bull;&thinsp;Contribs) 11:55, 30 March 2010 (UTC)

← The problem with the statement "A equals by definition B" is that, while it actually just means "A is exactly equal to B because of some definition", it strongly implicates "A is exactly equal to B because of the definition of A". (See pages 36 and following of CGEL for what I mean by "meaning" vs "implicature".) So, while $$c \stackrel{\scriptstyle \mathrm{def}}{=} 299\,792\,458~\mathrm{m}/\mathrm{s}$$ is true, it is very misleading unless the reader already knows about the definition of the metre, and hence we'd better not use the word "definition" before the sentence where we state which way the metre is defined. IOW, I think the current lead is just fine. ― _ _ _ A._di_M. (formerly Army1987) 12:16, 30 March 2010 (UTC)

Definition of c
I've reverted the change to the lead, that put the statement that c is the speed at which light travels in the first sentence.

The argument to move the sentence was that this is the definition of c. The problem is that it is not the definition. The physical constant c plays many roles in physics. Some of these are related to light others are not. One particular result is that light travels with the speed c. We have very much faith in this result, so much so that we call c the "speed of light", and for many intents and purposes this can be used an operational definition.

It is already confusing enough for readers, to grasp that c is called the speed of light, but does in principle not have anything to do with light. The misconception that c is defined as the speed at which light propagates, for example, causes a lot of confusion with readers, when they next read what fundamental role it plays in in relativity. "How can the behaviour of light effect to systems not involving light (or EM in general)?" The misconception makes it hard to except that c is a limiting velocity for all matter, for example.

If we really want to move the sentence up, I advocate a form like
 * "It plays in important role in many areas of physics such as electrodynamics, in particular it is the speed at which electromagnetic waves such as light propagate through the vacuum."

(TimothyRias (talk) 09:45, 30 March 2010 (UTC))

What about something like "... the ultimate speed, at which only massless 'particles' such as photons (e.g. light and radio waves) can travel." Abtract (talk) 10:10, 30 March 2010 (UTC)


 * That would be better. But suffers from the same problem that is a physical result rather that the definition of what c is. More correct would be something like: "c is that velocity of a null path in relativity". Obviously such a statement is much to technically dense for this article. TimothyRias (talk) 10:18, 30 March 2010 (UTC)


 * I'm not sure it's as clear which is the primary definition as you're implying. The metre is defined as the distance travelled by light in vacuum in \frac{1}{299792458} s, defining the speed of light as 299792458 m/s. That's a practical definition relating to actual photons in a real-world vacuum.  From this perspective, relativity shows that the universal speed limit is equal to the speed that photons actually travel at, rather than vice versa.  Djr32 (talk) 11:07, 30 March 2010 (UTC)
 * I think that is exactly the reason why no attempt at a definition first in the lead. What the definition of c is, depends on your POV. (And for the record, the thing that is shown by relativity is: "If there exists an invariant speed c, then this is the maximum speed any massive object, and the speed at which massless particles (such as photons) propagate.) TimothyRias (talk) 11:29, 30 March 2010 (UTC)
 * If we are talking about a defined value of 299,752,458 m/s for c, then we are talking about c in its role as the speed of electromagnetic radiation in vacuum. That is the only value which is exactly defined. Of course, c is many other things as well, as the lead already mentions and as the article discusses at some length. If we want to say "The speed of light is a physical constant, the velocity of a null path in relativity", we would have to follow it with "its value is exactly 1" ;) Physchim62 (talk) 13:22, 30 March 2010 (UTC)

OK let's have some options. Other suggestions welcome. TimothyRias (talk) 13:58, 30 March 2010 (UTC)
 * 1) The speed of light (usually denoted c) is a physical constant, which plays an important role in many areas of physics such as electrodynamics and relativity. In particular, it is the speed at which electromagnetic waves (such as light) propagate through vacuum. Its value is ...
 * I like this option because it immediately identifies the two most important contexts in which c appears, without suggesting that it is defined as the speed at which light travels.
 * 1) The speed of light (usually denoted c) is a physical constant, which is equal the speed at which electromagnetic waves (such as light) propagate through vacuum. Its value is ...
 * The problem I have with this it that it suggests that c is defined by the propagation speed of EM waves. The usual object to this is that if tomorrow it is discovered that the photon has a mass, this will not effect c, but will mean that light does not propagate at a speed of c.
 * 1) The speed of light (usually denoted c) is a physical constant, which denotes the invariant speed of special relativity.
 * While correct, this is (IMO) to technical to start this article off with.
 * The speed of light (usually denoted c) is a physical constant; among other things, it is the speed at which electromagnetic waves (such as light) propagate through vacuum. Its value is ... (Or maybe with something less informal than "among other things".)
 * The speed of light (usually denoted c) is a physical constant; it gets its name from the fact that it is the speed at which electromagnetic waves (such as light) propagate through vacuum, but it also plays an important role in many areas of physics such as relativity. (Too cumbersome?)
 * The speed of light (usually denoted c) is a physical constant playing an important role in many areas of physics such as electrodynamics and relativity; it gets its name from the fact that it is the speed at which electromagnetic waves (such as light) propagate through vacuum. Its value is ... (Doesn't sound quite right to me, but I'm not sure why.) ― _ _ _ A._di_M. (formerly Army1987) 15:32, 30 March 2010 (UTC)
 * 2a. The speed of light (usually denoted c) is a physical constant that is equal the speed at which electromagnetic radiation (such as light) travels through vacuum, and which also plays an important role in other areas of electrodynamics and in relativity. Its value is ...
 * I think we need to give lay readers a least a clue as to what electrodynamics is. If tomorrow it is discovered that photons have mass, we will have to rewrite the lead ;) but, until then, I think WP:CRYSTAL applies. The question of massive photons is already treated later in the article, as is VSL. Physchim62 (talk) 15:39, 30 March 2010 (UTC)
 * Whether or not photons have mass or not, is not the point. The point of the hypothetical case is that it makes clear that the speed of photons is not the definition of c. Take that as a definition and any discussion of light not travelling at speed c turns in an oxymoron.TimothyRias (talk) 15:48, 30 March 2010 (UTC)
 * I can't quite get your point here. It's important to say that the Speed of Light (c0) is more than just the speed of light, and it's important to say that light does not always travel at the Speed of Light. Both these points are already addressed in the lead section. But to say that the Speed of Light is not the speed of light seems to be going further than is warrented... Physchim62 (talk) 16:22, 30 March 2010 (UTC)

I still like "The speed of light (usually denoted c) is a physical constant; it is the ultimate speed, at which only massless 'particles' such as photons (e.g. light and radio waves) can travel." This is brief, not incorrect and informative imho. Abtract (talk) 16:28, 30 March 2010 (UTC)
 * Yep, I can go with that. I would replace "ultimate" with "greatest" for pedantic semantic reasons, and I wouldn't use the scare-quotes around 'particle', but otherwise it is correct, readable and informative to the lay reader. Physchim62 (talk) 16:57, 30 March 2010 (UTC)
 * Something like that is fairly acceptable to me as well. There is some wording to be tweaked though. I guess the two pieces of information the second part is trying to convey is 1)It is the speed of massless particles 2)It the upper limit for the speed of massive particles. The second part is not quite coming through at the moment. May be something like "it is the greatest speed at which objects may travel, which is on attained by massless particle such as..." TimothyRias (talk) 18:05, 30 March 2010 (UTC)
 * Yes, that would work.
 * The speed of light (usually denoted c) is a physical constant. It is the fastest speed at which energy or information can travel, and is only attained by massless particles and waves such as electromagnetic radiation (e.g. radio waves, visible light, or gamma rays) in vacuum, where there are no atoms, molecules or other types of matter that can slow it down. Its value is exactly 299,792,458 metres per second, often approximated as 300,000 kilometres per second or 186,000 miles per second (see the table on the right for values in other units).
 * That would require tweaking the third paragraph to remove redundancy, though. ― _ _ _ A._di_M. (formerly Army1987) 14:57, 31 March 2010 (UTC)
 * I've added A di M's last suggestion to the article, since it appears that everybody can live with that. TimothyRias (talk) 12:04, 1 April 2010 (UTC)

Hi everyone, I am back again. The lead has got even worse now. Bearing in mind Physchim62's comment, 'If we are talking about a defined value of 299,752,458 m/s for c, then we are talking about c in its role as the speed of electromagnetic radiation in vacuum. That is the only value which is exactly defined'. I would propose this as a starting point:

The speed of light (usually denoted c) is a physical constant. It is the speed that light travels in vacuum, the speed of massless particles, the velocity of a null path in relativity, and the fastest speed at which energy or information can travel. Its value is exactly 299,792,458 metres per second, often approximated as 300,000 kilometres per second or 186,000 miles per second. Martin Hogbin (talk) 22:24, 12 April 2010 (UTC)
 * Looks good (but it needs more links – most readers will wonder "WTF is a null path?" – and I'd say "at which" rather than "that"). ― _ _ _ A._di_M. (formerly Army1987) 14:29, 13 April 2010 (UTC)


 * May I suggest this as being a bit cleaner. I have removed all the brackets which I think look awkward in the first paragraph. The bit about the table is not necessary, readers can see this for themselves.  I have removed the reference to null paths (which I suggest it explained in a bit more detail in the body of the article) and replaced it with, 'a fundamental speed in the theory of relativity'.


 * The speed of light, usually denoted by c, is a physical constant. It is the speed at which electromagnetic radiation, including light, travels in vacuum.  It is a fundamental speed in the theory of relativity, the fastest speed at which energy or information can travel, and the speed of massless particles. Its value is exactly 299,792,458 metres per second,[1][2]  often approximated as 300,000 kilometres per second or 186,000 miles per second. Martin Hogbin (talk) 16:13, 13 April 2010 (UTC)


 * I've removed the reference to null path, since this basically is jargon for the path of a massless particle, anyway. In that sense it was not adding anything for anybody. Either you know what a null path is, in which case you will no that its speed is c, or you don't know what a null path is, in which case the mention is unhelpful.TimothyRias (talk) 12:49, 19 April 2010 (UTC)

Lead order
I note that the first four physical constants that I looked at, gravitational constant, Planck's constant, electric constant and the elementary charge, each start with the definition before the value is mentioned. I suggest that this article be brought in line with those. Abtract (talk) 10:35, 30 March 2010 (UTC)


 * Well, in general I don't think the state of various C-class articles is what we should base this article on. On the other hand, I'm not generally opposed to having some form of definition before the value. I do see some difficult in getting a good NPOV definition. This exactly the reason, I started the discussion above. To see, I we can come the some kind of consensus. TimothyRias (talk) 11:35, 30 March 2010 (UTC)


 * What the definition should be is nothing to do with where it should sit and, so far as I am aware, all articles should start with the definition. Abtract (talk) 13:15, 30 March 2010 (UTC)
 * It does seem rather silly to give a value for something when we haven't even said anything about what it is! Physchim62 (talk) 13:23, 30 March 2010 (UTC)

Context for infobox values?
I am assuming that the values given in the infobox are for light in a vacuum. If this is the case, I feel it needs to be explicitly stated somewhere within the infobox itself - the reader should not have to read the article to understand this very important facet of the speed of light. -- as 67.58.229.153 (talk) 16:26, 31 March 2010 (UTC)
 * At the level of approximation they are given to, they are also correct for light in air. ― _ _ _ A._di_M. (formerly Army1987) 21:42, 31 March 2010 (UTC)
 * If I have the correct impression from reading the lead, at least the meters-per-second value is an exact figure, due to the nature of the official definition of a meter. Needless to say, though, a short note should still be possible to add, something like "Non-exact values apply to light in a vacuum or in air". And before I forget, assuming all other properties of the medium are identical, does density have any effect on the speed of light? -- as 67.58.229.153 (talk) 02:35, 1 April 2010 (UTC)


 * Note that this article is not about the propagation of light, but about the physical constant called the "speed of light". Which is equal to the speed at which light propagates through vacuum. I've tweaked the infobox to better reflect this. TimothyRias (talk) 12:08, 1 April 2010 (UTC)
 * Thanks, Timothy, that helps some! =) -- as 67.58.229.153 (talk) 17:01, 1 April 2010 (UTC)

Unreliable source - Muslimheritage.com material
I attempted to remove questionable material about early estimations of the speed of light based on the following reasoning:

Content from Muslimheritage.com / FSTC is an unreliable source, as discussed on Reliable_sources/Noticeboard/Archive_18. None of its publications are peer-reviewed, and its authors often exhibit a strong bias and incomplete or flawed citation practices. The site has been used as a source in numerous science and history of science articles to make extraordinary claims about Islamic invention and discovery. I am working to remove these extraordinary claims where they stem directly and solely from a Muslimheritage.com reference. Many of these claims were added by a user who has a history of using flawed sources for extraordinary claims, as discussed on Requests_for_comment/Jagged_85. That page details numerous examples where claims from these sources contradict more reliable sources, on a scale which casts the entirety of the material originating from the site into doubt. If you would like to discuss this or any related removal with me, please leave a note on my talk page,

Dialectric (talk) 01:36, 26 April 2010 (UTC)


 * If you can't be bothered to check the claim, then don't remove it. Just tag claim and possibly remove the unreliable reference. As for the claim itself, it is clear that Taqi al-Din published a book on optics in which he discussed refraction as the result of differences in the speed of light in different media. What is not immediately clear to me is how much of this was actually novel at the time. TimothyRias (talk) 07:44, 26 April 2010 (UTC)
 * My thoughts also - although since the claim is dated it doesn't have to be novel. --Michael C. Price talk 08:06, 26 April 2010 (UTC)

However, if this source has been demonstrated to be unreliable, Dialectric is perfectly entitled to remove claims based solely upon it. Indeed it appears he may be doing wp a service in so doing. And using phrases like "If you can't be bothered ... " may be considered by some to be a tad aggressive to a, presumably, good faith editor. Abtract (talk) 22:53, 1 May 2010 (UTC)

Statement needing clarification
In addition to the above statements needing a ref the following statement is tagged as needing clearification:


 * The speed of light additionally affects wireless communications design.

This statement immediately follows the statement about ping. To me this is a very vague. Undoubtedly, light having a finite speed will affect wireless communications, but without explaining how it is affected this statement is essentially meaningless. Does anybody know what the intention of this statement was supposed to be?TimothyRias (talk) 12:37, 21 April 2010 (UTC)
 * I've removed the statement since it was not adding anything to the article. TimothyRias (talk) 14:52, 7 May 2010 (UTC)

== If gravity exists in vacuum, and it also distorts time, then if one says that a metre is how far light travels in vacuum in 1/299792458 seconds, must one also include "devoid of gravity" in the defin ==

If gravity exists in vacuum, and it also distorts time, then if one says that a metre is how far light travels in vacuum in 1/299792458 seconds, must one also include "devoid of gravity" in the definition?165.212.189.187 (talk) 14:03, 10 May 2010 (UTC)kgb
 * In 2002, the CIPM decided "that in the context of general relativity, the metre is considered a unit of proper length. Its definition, therefore, applies only within a spatial extent sufficiently small that the effects of the non-uniformity of the gravitational field can be ignored (note that, at the surface of the Earth, this effect in the vertical direction is about 1 part in 1016 per metre)." ― _ _ _ A._di_M. (formerly Army1987) 14:16, 10 May 2010 (UTC)

Statements needing references
The article currently contain the following statements that are in need of a reference:


 * According to special relativity, the energy of an object with rest mass m and speed v is given by γmc2, where γ is the Lorentz factor defined above. When v is zero, γ is equal to one, giving rise to the famous E = mc2 formula for mass-energy equivalence. Since the γ factor approaches infinity as v approaches c, it would take an infinite amount of energy to accelerate an object with mass to the speed of light. The speed of light is the upper limit for the speeds of objects with non-zero rest mass.


 * Furthermore, straight lines rarely occur in global communications situations, and delays are created when the signal passes through an electronic switch or signal regenerator. A typical time as of 2004 for a U.S. to Australia or Japan computer-to-computer ping is 0.18 s.


 * The refined method of Foucault replaces the cogwheel by a rotating mirror. Because the mirror keeps rotating while the light travels to the distant mirror and back, the light is reflected from the rotating mirror at a different angle on its way out than it is on its way back. From this difference in angle, the known speed of rotation and the distance to the distant mirror the speed of light may be calculated.

None of these statements are very controversial and should be ease to source. (Although we all know how hard it can be to find a source to support the well known.) Finding sources for this couple of statements should be one of the last steps needed for this article to go to FAC again. Also a kind request to all participating editors to comb over the article to look for statement that need a citation, but are not marked as such. TimothyRias (talk) 13:02, 19 April 2010 (UTC)
 * None of the experiments found any hint of the motion, finding that the speed of light was the same in every direction. (about experiments trying to detect etherwind)
 * In 1905 Albert Einstein proposed that the speed of light was independent of the motion of the source or observer. Using this and his principle of relativity as a basis he derived the special theory of relativity, in which the speed of light c featured as a fundamental parameter, also appearing in contexts unrelated to light.
 * None of these statements are really controversial, but it might be helpful to rephrase them to find sources (or, for the ping, simple mathematical explanations). The first and the last in particular could probably be better phrased, but I won't touch them unless I can put a source in there as well ;) Physchim62 (talk) 13:23, 19 April 2010 (UTC)
 * The last could even be sourced to Einstein's own article, I think. The first and the fourth could be sourced to anywhere, and when I get home I'll look them up on Feynman's Lectures and/or on Jackson's Classical electromagnetism. ― _ _ _ A._di_M. (formerly Army1987) 14:08, 19 April 2010 (UTC)
 * I've done the last one, and changed the sentence to read "the theory of relativity" and "his special theory of relativity": other editors please amend as necessary or desirable! Physchim62 (talk) 14:53, 19 April 2010 (UTC)
 * Michelson–Morley Experiment done as well.My source cites Shankland et al., Rev. Mod. Phys., 27, 167 (1955). Physchim62 (talk) 15:14, 19 April 2010 (UTC)
 * While the formula for relativistic energy can indeed be found pretty much everywhere, finding a source explicitly stating that "[t]he speed of light is the upper limit for the speeds of objects with non-zero rest mass" is proving trickier than I thought. (Meanwhile, I'm replacing "non-zero" with "positive", just in case tachyons existed...) ―  _ _ _ A._di_M. (formerly Army1987) 14:48, 20 April 2010 (UTC)
 * Tachyons have imaginary mass (i.e. negative mass squared). Negative mass objects also would be restricted to sublightspeeds. Still, I guess "positive" is not wrong. It does imply that its real after all. And by all means lets not make the statement more complicated than necessary. TimothyRias (talk) 15:37, 20 April 2010 (UTC)
 * I've added a source for the description of the Foucault method. Moreover, it occurred to me the sentence preceding the ping statement also needs to be referenced. That might be easier to find, that reference is bound to give an example which may or may not coincide with the ping statement we have now. TimothyRias (talk) 12:32, 21 April 2010 (UTC)
 * I've added sources to the remaining claims. (and removed the unsourced ping example) TimothyRias (talk) 14:54, 7 May 2010 (UTC)

Yesterday, I tagged the following statement in the FTL section: Although this is common knowledge for anybody who has followed a SR course, it also defies common logic for anybody who hasn't. As such it should have a source. It shouldn't be to hard to find a SR book that specifically discusses this case. TimothyRias (talk) 08:23, 12 May 2010 (UTC)
 * The rate of change in the distance between two objects in a frame of reference with respect to which both are moving (their closing speed) may have a value in excess of c. However, this does not represent the speed of any single object as measured in a single inertial frame.