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::However, these statements in no way ''suggest to anyone that 299,792,458 is a fundamental constant. '' Well tell me about the way this conclusion flows from the WP statements, please. Will the reader who is not a [[Philadelphia lawyer]] tease out this conclusion from the premises? [[User:Brews ohare|Brews ohare]] ([[User talk:Brews ohare|talk]]) 05:38, 26 August 2009 (UTC) |
::However, these statements in no way ''suggest to anyone that 299,792,458 is a fundamental constant. '' Well tell me about the way this conclusion flows from the WP statements, please. Will the reader who is not a [[Philadelphia lawyer]] tease out this conclusion from the premises? [[User:Brews ohare|Brews ohare]] ([[User talk:Brews ohare|talk]]) 05:38, 26 August 2009 (UTC) |
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:::So you're saying a speed being a fundamental constant of nature implies that its numerical value in SI units is a fundamental constant? Yes, I can see why that would confuse you, since you think of it differently. Oh, well, we'll have to live that that duality. [[User:Dicklyon|Dicklyon]] ([[User talk:Dicklyon|talk]]) 05:44, 26 August 2009 (UTC) |
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== Third paragraph == |
== Third paragraph == |
Revision as of 05:44, 26 August 2009
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Poll on what numerical value or values we show in the lead section
There are three basic options on what value for the speed of light in SI units we should show in the lead section. Note that in all cases further values and explanations can be provided in the main body of the article. Options are:
- A. Show only the approximate speed, as in:
It is normally denoted by c, and is approximately 300,000 kilometres (or 186,000 miles) per second
- B. Show the approximate speed, followed by the exact speed as in:
It is normally denoted by c, and is approximately 300,000 kilometres (or 186,000 miles) per second in the first paragraph, with:
In the International System of Units, the length of the metre is now defined such that the speed of light is exactly 299 792 458 m/s., in the fourth paragraph.
- C. Show the exact speed after the current first sentence (optionally followed by an approximation, I do not think this is a contentious issue) as in:
The length of the metre is defined such that the speed of light is exactly 299 792 458 m/s. optionally followed by something like: For convenience, the approximate value of 300,000 km/s is often used.
Votes
Please state here your preference with only one or two lines of explanation
- Support C It states an impeccably supported fact that is likely to be what many readers actually want to know. This approach has been used through the life of this article until this month. There is no valid reason not to do this.Martin Hogbin (talk) 13:18, 17 August 2009 (UTC)
- Support C. I would open with the sentence like "According to our current theories the speed of light is a fundamental physical constant." The next sentence can mention the definition of the meter, together with the exact value for the speed of light in SI units. DVdm (talk) 13:25, 17 August 2009 (UTC)
- Support C. I agree with DVdm's suggestion. The fact that c is a fundamental constant makes it possible to use it to define the meter in terms of the second in a useful way. So, such a sentence may be helpful to readers. Count Iblis (talk) 13:30, 17 August 2009 (UTC)
- Support A Quite aside from this poll, which attempts to set up majority rule on this topic, the matter should be decided on the basis of what approach actually is logically, historically and semantically correct. In stark contrast to the assertion that C is an "impeccably supported fact", it is a misuse of language to say the SI units provide an "exact" number for the physical speed of light, as that is clearly not the case. The SI units provide an exact conversion factor from time-of-transit to length in metres, as described carefully by Jespersen and Sydenham. Brews ohare (talk) 13:37, 17 August 2009 (UTC)
- Support B – While either A or C would also be acceptable, the history shows that they are polarizing alternatives that lead to a lot of fighting. The important thing is that IF we include the exact value in the lead, we defer the complex and polarizing discussing of the philosophical implications of having a defined fixed value for c until a later section. Dicklyon (talk) 15:07, 17 August 2009 (UTC)
- Support C – c is defined as a specific number in the SI system, the rest is ajusted in consequence. The article should reflect this from the get go, and not wishy-wash it down at first to introduce the real definition later. Mentionning that it is often approximated to 300 000 km/s (or 300 000 000 m/s, whichever pleases people) immediatly following the exact definition will address any familiarity concerns of laymen. Headbomb {ταλκκοντριβς – WP Physics} 15:37, 17 August 2009 (UTC)
- Support B - While I fully sympathize with Brews's reasons for supporting A, there is a problem with wikipedia's rules and regulations that needs to be considered. It would be expected that the up to date SI unit value would appear somewhere near the top of the article. As far as I am concerned, the up to date value in SI units is a tautology, but if that's what they decided at that 1983 conference, then we're stuck with it for the time being. I further support the right of Brews to elaborate on this tautology later on in the article. David Tombe (talk) 19:14, 17 August 2009 (UTC)
- Support C - C is concise and scientifically correct. The metre has been defined to give c the above value, but that does not make c a mere conversion factor. Distance and time are separate physically observable quantities, and c is a measurable physical quantity. Ehrenkater (talk) 20:01, 17 August 2009 (UTC)
- Ehrenkater these entities all are measurable in some systems of units, but not in SI units. Brews ohare (talk) 20:21, 17 August 2009 (UTC)
- This is a poll - Please do NOT comment. Thank you. DVdm (talk)
- Brews ohare It only requires them to be measurable in any ONE system of units to demonstrate that they are separate observable quantities. SI units are a man-made construct which does not alter the physical realities. Your contributions are just a long string of one non sequitur after another. Ehrenkater (talk) 18:05, 18 August 2009 (UTC)
- This is a poll - Please do NOT comment here. Rather use the space below. Thank you. DVdm (talk) 18:10, 18 August 2009 (UTC)
- Ehrenkater these entities all are measurable in some systems of units, but not in SI units. Brews ohare (talk) 20:21, 17 August 2009 (UTC)
- Support C - but include somewhere in the article an explanation that while the speed of light is a fundamental constant, its numerical value is not fundamental as this depends on the choice of units, and we can define unit of length in terms of speed of light and therefore give the speed of light any value we find convenient. Also if the speed of light changed we would notice if we measured it using a stick physically created prior to a change rather than the SI definition - we obviously cannot use the SI definition to measure the speed of light. Charvest (talk) 20:27, 17 August 2009 (UTC)
- Charvest: your statement is accurate, but is it really adequate to bury the important caveat about units while stating a so-called (misnomer) exact value??
- The question of whereabouts in the article to explain all this is a different question than whether the value 299 792 458 should be mentioned in the lead. Charvest (talk) 20:31, 17 August 2009 (UTC)
- This is a poll - Please do NOT comment. Thank you. DVdm (talk) 20:49, 17 August 2009 (UTC)
- The question of whereabouts in the article to explain all this is a different question than whether the value 299 792 458 should be mentioned in the lead. Charvest (talk) 20:31, 17 August 2009 (UTC)
- Charvest: your statement is accurate, but is it really adequate to bury the important caveat about units while stating a so-called (misnomer) exact value??
- Support C - The statement is correct as written, and all the philosphical implications follow from the explicitly stated definition. LouScheffer (talk) 03:24, 18 August 2009 (UTC)
- Support C - I would like to see the exact value in m/s as high up the article as possible. It is surely what many readers to the article are looking for. Philosophical questions of definition and measurement should be deferred (without a teaser in the lead). EdwardLockhart (talk) 11:40, 18 August 2009 (UTC)
Discussion
I agree that the first sentence should state that the speed of light is a fundamental constant, then we state its exact value in SI units. In fact it might be a good idea to say 'dimensional/dimensionful' constant, although this is covered later. Martin Hogbin (talk)
- Majority rule does not decide matters of logic and accuracy. See Jespersen and Sydenham. WP uses sources, not majority rule. Brews ohare (talk) 13:38, 17 August 2009 (UTC)
- Nor Jespersen, nor Sydenham, nor any majority, nor a minority like Bruce will be able to show there is something illogical or inaccurate in a statement like
- "According to our current theories the speed of light is a fundamental physical constant. The meter and the second are defined such that this speed is exactly 299 792 458 m/s".
- DVdm (talk) 13:54, 17 August 2009 (UTC)
- Nor Jespersen, nor Sydenham, nor any majority, nor a minority like Bruce will be able to show there is something illogical or inaccurate in a statement like
To replace opinion with sources:
- Jespersen: "c [is] a conversion factor whose value is fixed and arbitrary"
- Sydenham: "Thus the speed of light as a numerical value is not a fundamental constant...if the speed of light is defined as a fixed number then in principle the time standard will serve as the length standard."
The problem is in the last phrase of DVdm's remark: "defined such that this speed is exactly 299 792 458 m/s" The problem is that the "speed" of light is not defined this way; it is the metre that is defined this way. The number 299 792 458 m/s is not the speed of light; it is a conversion factor selected by international agreement (not measurement) to be close to the speed of light as determined prior to 1983 when c was a measured quantity. That international choice was dictated by convenience, not nature, and could have been chosen instead to be 1 ft/ns (for example) if they felt like it. It was just the pain of conversion that decided continuity with the past was more important than numerical simplification. Brews ohare (talk) 14:12, 17 August 2009 (UTC)
- So what is the numerical value of the speed of light in m/s? Martin Hogbin (talk) 14:19, 17 August 2009 (UTC)
- The speed of light cannot be determined by measurement within the SI system of units as presently set up. Of course, you can compare the speed of light with other speeds (e.g. with the speed of sound in specified circumstances). As Jespersen says: its "value is fixed and arbitrary". You might find the discussion by Bondi "the units of distance" interesting. Brews ohare (talk) 14:23, 17 August 2009 (UTC)
- You are deliberately avoiding the question that I asked. I simply asked what the numerical value of the speed of light was in SI units, I did not ask if it could be measured. What is your answer? Martin Hogbin (talk) 14:36, 17 August 2009 (UTC)
- I'm not ducking you: it is not measurable, has no operational meaning in the sense of some operations that lead to its determination. Assuming a genuine interest in sorting this out, try to explain to yourself how Jespersen and Sydenham and Bondi arrive at their positions. Brews ohare (talk) 14:39, 17 August 2009 (UTC)
- Yes you are ducking me. I just asked what its numerical value is. The value is 'fixed and arbitrary', what is it? Martin Hogbin (talk) 14:44, 17 August 2009 (UTC)
All right. I take it you have no intention of reconciling with the sources I mention. The physical speed of light as the speed of light in the universe (an extant real speed) is not known numerically within the SI system of units; its numerical value is not defined (though it has properties, like isotropy). What is defined is the conversion factor between time-of-flight and distance. I gather that you would like to state that this conversion factor is the speed of light in SI units, but of course, that is a play on words, and suggests that somehow man can dictate to nature what the real speed of light is. Brews ohare (talk) 14:55, 17 August 2009 (UTC)
- You are still refusing to answer my simple question, which is, 'What is the speed of light in SI units?'. I am making no claim that I or anyone else can dictate anything to nature. Please answer the question asked. Martin Hogbin (talk) 15:01, 17 August 2009 (UTC)
The physical speed of light as the speed of light in the universe (an extant real speed) is not known numerically within the SI system of units, and in fact, cannot be determined in principle. For example, if the true speed of light changed tomorrow, the meter would simply expand or contract, and the numerical conversion factor would still be the same. Thus, the conversion factor is not the speed of light. Brews ohare (talk) 15:06, 17 August 2009 (UTC)
- I think you have made your position quite clear, The physical speed of light as the speed of light in the universe (an extant real speed) is not known numerically within the SI system of units. I will say no more. Martin Hogbin (talk) 15:20, 17 August 2009 (UTC)
Dick, There are times to compromise and times not to, and this is one of the times where there should be no compromise, especially just appease one editor who makes comments like the quote above. We need to make a decision. Do you yourself actually believe that there is a problem in stating the numerical value of the speed of light in SI units at the start and, if so, what is the problem? Martin Hogbin (talk) 15:25, 17 August 2009 (UTC)
- Martin: Don't you see a certain intransigence on your part: you will not address the sources, you repeat yourself over and over with no evolution of thought, you take a clear statement backed by sources as somehow a reductio ad absurdum in your mind anyway??? How do you respond to this observation: If the true speed of light changed tomorrow, the meter would simply expand or contract, and the numerical conversion factor 299 792 458 m/s would still be the same. Thus, the conversion factor is not the speed of light. Brews ohare (talk) 15:29, 17 August 2009 (UTC)
- Brews: You are no less intransigent. —Finell (Talk) 18:05, 17 August 2009 (UTC)
- I use sources and logic, Martin uses reiteration with no evolution of thought. Brews ohare (talk) 19:58, 17 August 2009 (UTC)
Finell, Brews can see the tautology. The others can't. David Tombe (talk) 19:21, 17 August 2009 (UTC)
- The 'tautology' as you call it, is obvious, seen by everyone, and intentional. Martin Hogbin (talk) 08:51, 18 August 2009 (UTC)
- If you take the laws of physics and change c into 2 c while leaving all the dimensionless constants the same, you still have the same laws of physics. All you're doing is rescaling your time coordinate. Although someone simulating the laws of physics using a computer could see a difference when looking at his computer screen, that difference amounts to a simple rescaling. He could have left c the same and apply the rescaling to the results of the original computations instead. This means that an observer located inside the universe cannot see the difference between c and 2 c. Count Iblis (talk) 17:00, 17 August 2009 (UTC)
- Yes, for this reason I think we should consider stating at the start that the speed of light is a fundamental dimensionful/dimensional physical constant. To many people this may be unimportant but those who are interested could follow the link to a discussion like yours above. Martin Hogbin (talk) 17:54, 17 August 2009 (UTC)
- If you take the laws of physics and change c into 2 c while leaving all the dimensionless constants the same, you still have the same laws of physics. All you're doing is rescaling your time coordinate. Although someone simulating the laws of physics using a computer could see a difference when looking at his computer screen, that difference amounts to a simple rescaling. He could have left c the same and apply the rescaling to the results of the original computations instead. This means that an observer located inside the universe cannot see the difference between c and 2 c. Count Iblis (talk) 17:00, 17 August 2009 (UTC)
- The sources quoted above state explicitly that the numerical value of the speed of light in SI units is not, repeat not, a fundamental physical constant. The actual, physical speed of light is a physical constant of course. Can you see the difference??? That is why we don't need to know what its numerical value is when we set up a standard: the assumption is that the value of c will be a fixed value for any lab that sets things up, and they actually don't have to measure it to be sure that this is the case. They do have to check they have a "vacuum" or know how to correct to refer to "vacuum". Brews ohare (talk) 20:05, 17 August 2009 (UTC)
- I might add that this fact is not critical to this discussion. If a change in the speed of light should turn out to observable from within this universe there is no experimental evidence that it has changed. Martin Hogbin (talk) 18:32, 17 August 2009 (UTC)
- To reply to Brews' question. It will always be true that if it is found that the means of defining any standard is detectably unstable then there would have to be a rethink or refinement of the standard, depending on the degree of instability. Such a potential problem is not limited to the current standard however, there would have been exactly the same problem if it had been found that the frequency of the krypton light used in the earlier standard was unstable or if it had been found that the length of rods made of the the particular platinum alloy used for the original standard was unstable. Of course the people who design standards generally have a good idea that such things will not happen. This problem is not specific to using light, it is a problem with having standards, see [[1]] Martin Hogbin (talk) 19:07, 17 August 2009 (UTC)
- Martin: The point is not whether action would have to be taken if a change were discovered. The point is that any such action is a matter of international agreement, not physics. The conversion factor is a defined not a measured value. So, for example, if the speed of light changed every year by 10−50%, it possibly would be decided that no action need be taken for the next century. The conversion factor would stay at its present value, and for those applications of extreme accuracy some "good practice" correction would be specified. Who knows what the decision would be, the point is that it is simply a decision, and has no bearing upon what "really" is the speed of light. The conversion factor itself is not the "speed of light". That is why the conversion factor is an exact value: it's only a convention. Brews ohare (talk) 19:49, 17 August 2009 (UTC)
Martin, The ultimate extrapolation of all this is where Headbomb tried to tell us that when you measure the separation distance between the plates of a capacitor, to quote him exactly,
you are not measuring the separation of the plates, but rather are making sure that your ruler is calibrated
He said that on the wiki-physics project page earlier today. When it gets to statements like that, then you need to stand back and seriously consider what is going on. At the 1983 conference, they tried to turn physics upside down by playing around with the fundamental dimensions of length and time and making a compound dimension, L/T, into a simple dimension. When it all came unstuck over the issue of measuring electric permittivity, they tried to wriggle out of it by sacrificing the original meaning of a basic electrical experiment involving a capacitor and a vibrating reed switch. Headbomb's quote above is an example of the Alice in Wonderland world of physics that the 1983 conference has led us into. David Tombe (talk) 19:35, 17 August 2009 (UTC)
- You can dislike the "Alice in Wonderland" physics all you want, this is the definition and what it implies. You can lobby the BIPM to change its definition if you want, but Wikipedia is not the place for this. Headbomb {ταλκκοντριβς – WP Physics} 05:23, 18 August 2009 (UTC)
- To add my tuppence worth: The way to escape the tautology, is to distinguish between an ideal metre as 1/299792458th of a light second and a physical metre stick that has been physically created based on this definition. 299792458 m/s may be the exact conversion figure between the speed of light and an ideal metre, but 299792458 m/s is the approximate measured speed using an actual stick, since all real world measurements are approximate. When it is said that 299792458 m/s is the exact speed of light, that phrase should be taken as shorthand for explaining that this means ideal speed defined with ideal metres in a circular way so that the numerical value is close to the measured value using sticks that were otherwise created prior to 1983. The article could say something to this effect but probably not in the lead. Charvest (talk) 19:49, 17 August 2009 (UTC)
Charvest, I could put it even more simply. The statement from the 1983 conference was "The speed of light is k times the distance that light travels in 1/k seconds, per second, where k can be any number, and we have chosen the number 299792458 in order to disguise the transition from the old system". It's a tautologist's picnic. David Tombe (talk) 20:00, 17 August 2009 (UTC)
- I suggest you phrase it as A meter is one cth of the distance traveled by light in vacuum in 1 second, and see if you have a problem with that one. It's completely and totally equivalent to the previous definition. c is simply a scaling factor, exactly like hbar is. This is why they are often set to 1. The only difference is that we choose the scale for c while we didn't for hbar, although we certainly could (the unit of energy would then be ajusted in consequence to give the proper slope in photovoltaic effect experiments). Headbomb {ταλκκοντριβς – WP Physics} 05:38, 18 August 2009 (UTC)
No Headbomb, I have described the tautology exactly and I don't intend to re-describe it in line with your suggestions. David Tombe (talk) 12:14, 18 August 2009 (UTC)
- Headbomb, I take it that c in your remarks is shorthand for 299 792 458 m/s? Or, is it short for the physically real speed of light that is in principle not measurable in SI units? Brews ohare (talk) 12:22, 18 August 2009 (UTC)
Brews, Headbomb has merely tried to sew confusion on top of my elucidation of the tautology. What he writes above does not appear to make any sense. David Tombe (talk) 12:35, 18 August 2009 (UTC)
Reliable sources
The resolution to this dispute probably lies in what the most reliable sources say on the matter in dispute. How do Encyclopaedia Britannica and Encyclopedia Americanna treat the speed of light in view of the re-definition of the metre? How do tertiary scientific reference works (scientific encyclopedias and dictionaries) treat it? Do they say that the speed of light is now a tautology?
Perhaps I missed it, but I don't see where any of the other editors have directly addressed the sources that Brews cited above. What are the relevant qualifications of the authors and publishers? Are they reliable sources under Wikipedia's stated criteria? (The authors are not the subject of Wikipedia articles, but that is not determinitive.) Are they primary, secondary, or (probably not) tertiary sources? Do their views on this subject represent today's consensus mainstream scientific thought, minority views recognized within the scientifc community, or fringe views? The answers to these questions determine whether and how these sources and what they say should be treated in the article under Wikipedia's content policies and guidelines.
With due respect to the editors addressing this dispute, Wikipedia's content should be determined by what the reliable sources, and the weight of reliable sources, say. Wikipedians' opinions on how Wikipedia should treat the subject, and Wikipedians' conclusions drawn from what the reliable sources say (as opposed what the sources expressly say), fall within Wikipedia's prohibition of original research. Wikipedia follows the sources. Generally, Wikipedia places primary reliance on the conclusions reached by reliable secondary sources. Generally, Wikipedia should be consistent with reliable, up-to-date tertiary sources. —Finell (Talk) 04:31, 18 August 2009 (UTC)
- Jespersen is a Bureau of Standards publication recently updated, so has the authority of NIST, approximately. Sydenham appears also to be very reputable. These sources are preferable to Encyclopedia Britannica, for example, because they are expert opinion, not popularized versions. Popularized versions may use simple formulations, but in this case the distinctions needed are sharper than the demands placed by the Encyclopedias. Brews ohare (talk) 06:02, 18 August 2009 (UTC)
- They are all very good sources, Brews, unfortunately they do not support your claims.
- Finell, welcome to the discussion. The question is whether or not we say that the speed of light is exactly 299 792 458 m/s. It is quite clear from the sources quoted, such as NIST that this is the case. I am not sure whether NIST would be classed as a primary or secondary source but their job is clearly to disseminate reliable information to the public and scientific community. We do, as it happens, also have Britannica, giving this same exact value. Brews himself has found many sources which all state that the speed of light has a fixed numerical value in SI units because of the way that the metre is defined. We are not going to find a source to specifically say that this is not a tautology, they will say that it is true because of the definition of the metre.
- Regarding editors here, we are not just giving our personal opinions but stating the views of every reliable source. There is no up-to-date reliable source anywhere that says that the the speed of light is anything but 299 792 458 m/s. Brews views and claims have been addressed endlessly here before, but no one has been able to convince him. We need to move on.Martin Hogbin (talk) 09:31, 18 August 2009 (UTC)
- Just to make the situation absolutely clear this [[2]] is what a reliable sources says. Please look at the very clear statement in the centre of the page, It follows that the speed of light in vacuum is exactly 299 792 458 metres per second. This is an impeccable a secondary source. The purpose of the BIPM is for experts to study and collate reputable research and understanding on subjects relating to measurements and the definition of units and having done this to to set and disseminate international standards. Martin Hogbin (talk) 12:55, 18 August 2009 (UTC)
- This source defines the meter in the SI system of units. It states that because of this definition the speed of light in vacuum is exactly 299 792 458 m/s, and says that this value is a consequence of the definition of the metre; i.e. is not a measured result. The term "exact speed of light" in this connection clearly refers to a conversion factor (that is, length = 299 792 458 m/s × transit time, a restatement of the definition for an arbitrary length instead of one metre). The number 299 792 458 m/s does not refer to the physical speed of light, which cannot be established by a "definition", of course, but has to be established by observation, and always will have experimental error attached to it. As pointed out in the discussion on this page and by many cited sources (including the BIPM itself), the consequence of this definition of the metre is that the number 299 792 458 m/s will not change, no matter what refinements in measurement technique may occur in the future, simply because such refinements alter the standard metre, not the factor 299 792 458 m/s. That independence of 299 792 458 m/s from measurement is why the number 299 792 458 m/s is exact: because it has no relation whatsoever to measurement, nor to the exact physical speed of light. It is a number that was selected for convenience as a pretty close approximation to the value of the speed of light as measured prior to 1983 when the metre was "1,650,763.73 vacuum wavelengths of light resulting from unperturbed atomic energy level transition 2p10 5d5 of the krypton isotope having an atomic weight of 86" (a speed certainly never considered an exact value). This value of 299 792 458 m/s was adequate to avoid practical issues of accommodation to the new metre. Apart from the need for such accommodation, the value 299 792 458 m/s could have been selected as any number whatsoever. Brews ohare (talk) 15:32, 18 August 2009 (UTC)
Hi Martin: You have yet to address the difficulty that your "exact" speed of light is not measurable within the SI units system, neither in practice nor in principle, as is stated explicitly in many sources. (A partial list provided at the end of this paragraph.) Instead, you provide your own opinion on the matter, and never address these sources. You say "they do not support" my "claims" (although I am quite unsure that you know what these claims are), and I have yet to see any of your reasoning behind your statements. A partial list of sources is: NIST publication, Jespersen, Sydenham and Bortfeldt ("Since 1983 any measurement of the speed of light is neither necessary nor possible"); Brush ("Thus the speed of light ceased to be a measurable constant.") Brews ohare (talk) 12:09, 18 August 2009 (UTC)
- Finell, You are correct in principle about the issue of 'weighting and reliability of sources'. But my experience has shown that groups of editors who push unweighted samples of sourced material are generally not pushing a fringe viewpoint, but rather they pushing their own attachment to a popular fad within the current orthodoxy, to the exclusion of a wider knowledge of the topic.
- Where the issue of weighting becomes particularly acute is in relation to reporting matters to do with the history of science. For example, there has been a recent case in which the views of the 18th century Daniel Bernoulli, arguably one of the greatest scientists ever, has had his views overstamped by the modern orthodoxy on the basis of an opinion expressed by the author of a book written in 1990. These issues will eventually have to be addressed at somewhere like the wiki-physics project page. A clear directive will eventually be needed from the top. As it stands now, I often get suspicious of determined editors who hide behind the party line and claim that their only interest in the matter is to ensure that all edits are backed up by reliable sources. Unless they are countering defensively with this strategy, I generally suspect that the editor in question has an agenda and a point of view which they are not willing to openly expose. It is this kind of silent 'point of view' pushing that causes most of the edit wars. And the edit wars are never won 'de jure' on sources, despite the rallying cry. They are always won 'de facto' on mob rule, by tactical use of the 3RR rule and by abuse of the policy of consensus. David Tombe (talk) 12:31, 18 August 2009 (UTC)
- David: You seem to be pushing every bit at hard as anyone else for your particular point of view. —Finell (Talk) 19:15, 18 August 2009 (UTC)
Finell, Can you please elaborate on that. What is the point of view that you have in mind? On the talk pages here, I have made my opinion on the matter quite clear. But for the purposes of the main article, I have merely been advocating that a clear distinction is drawn between the speed of light as the concept is traditionally understood, and the new post-1983 defined speed of light. This is new territory in physics, and I don't think that the consequences of the 1983 conference have been as yet fully realized, even by many physicists. The non-physics public at large will not understand the 'new speed of light' concept. Brews is correct to want to clarify the difference in the article. This is a case where it is far too presumptious to dominate an introduction with the new Alice in Wonderland physics at the expense of the non-physics readership's expectations. I reluctantly voted B above, because I acknowledge wikipedia's obligations to report the official position. But this is a case where the official position is very new, and that it is so ludicrous that it needs to be complimented with a non SI position on the matter. In principle, I really wanted to vote A like Brews. The lead at vacuum permittivity is one of the most presumptious cases of pushing the new physics that I have ever encountered. It completely deletes an important topic in experimental physics that is of major historic importance, and which was taught at least up until recent times David Tombe (talk) 20:19, 18 August 2009 (UTC)
The initial definition
The initial definition of the speed of light in the first sentence of the article is misleading. For example, should one read "the speed of light in glass" to mean "a fundamental constant in glass"; or as "299... m/s in glass"? The first sentence should define the general concept of the speed of light, and only after that mention any specific examples (like in vacuum). Each point in the article should be clear about whether it refers to the general concept of speed of light, or the speed in a particular medium (such as vacuum). Is there agreement with these statements? Pecos Joe (talk) 21:00, 17 August 2009 (UTC)
- The article's lead defines the term as used in the article. The fact that there is a "speed of light in glass" does not mean that "the speed of light" can't be a fundamental constant. They are different terms, related as discussed in the article. Dicklyon (talk) 23:24, 17 August 2009 (UTC)
Pecos Joe, I made that very same point myself last week. I noted how the initial definition limited the speed of light specifically to the speed of light in the vacuum. I corrected this error, but the correction was immediately reverted by Lou Scheffer. Then a few days later, somebody produced an example lead from Encyclopaedia Britannica. It adopted the correct and generalized approach. This Britannica lead was hailed by some editors until I also hailed it in the specific respect that we are now talking about. Dicklyon then immediately adopted a negative approach to that particular aspect and informed us all that Britiannica was taking a minority point of view in that respect.
Everybody knows that the lead is wrong as it stands at the moment. But nobody wants to correct it because it would be going against Lou Scheffer's reversion of my correction. If I ever correct anything, it is a guaranteed way of ensuring that it will be made wrong again and kept wrong as a crucial matter of principle over and above every other principle. David Tombe (talk) 23:42, 17 August 2009 (UTC)
- Look, I enjoy pedantic nitpicking as much as the next guy (gals are normally more sensible) but there are excellent reasons phrase "the speed of light" refers to the speed of light in a vacuum. If you go up to any physicist (not involved here) and ask how you equate mass and energy, they say "multiply by the speed of light squared". They will also tell you that you can't go faster than the speed of light; you can't send information faster than the speed of light; you use the speed of light to swap space for time in spacetime; and the speed of light is part of the metric in GR. Any normal physicist would agree with these statements, but all are wrong unless the speed of light is used to mean the speed of light in vacuum. Yes, it is *possible* to use the speed of light to mean the speed of light in any medium, then add "in vacuum" or c when the fundamental constant is meant, but that's not the normal usage. Since one particular speed of light is spectacularly important (involved in almost all of physics) and the others are measured values of interest only to specialists, it's very rational for "the speed of light" to refer to the important one. This is just like "the white house", which in theory could refer to any white house, but in normal usage refers to a specific one on Pennsylvania Avenue. LouScheffer (talk) 02:53, 18 August 2009 (UTC)
- When a physicist hears speed of light, the physicist assumes it means the speed of light in a vacuum (unless otherwise stated), because that is the particular speed of light that is fundamental to physics. When most non-scientific, general readers hear speed of light, they make no assumption at all, and that the term means simply how fast light travels. That is probably why general encyclopedias do not limit speed of light to mean its speed in a vacuum, and we should follow suit. Wikipedia's article should address both readerships, should make the distinction in the lead, and explain that it is light's speed a vacuum (without getting into what vacuum means) that is a fundamental physical constant. I may be repeating what Dicklyon proposed awhile ago. Further, I believe that it is more productive to say that the lead needs work than that it is "wrong", since there are opposing opinions on what is wrong with it. —Finell (Talk) 05:05, 18 August 2009 (UTC)
- The vast majority of sources seem to treat "the speed of light" as c. Other uses can be discussed, to, but not such that they dilute this main point. If you want to propose improvements, or repositioning, of the section "Light in transparent media", that would be worth a consideration. But to come in and suggest changing the one thing that all the arguing parties to date have agreed on is just disruptive. Dicklyon (talk) 05:20, 18 August 2009 (UTC)
- I agree, but we should make this clear at the start with something like: 'The term speed of light normally refers to a fundamental physical constant, the speed that light travels in vacuum'. I do not think there is any need to address in the lead the alternative and naturally obvious meaning of the speed that light happens to go in a particular medium, just like we would not have an article on the 'speed of a dog' explaining that this just means the speed that a dog is travelling at. 'The speed of light', unless qualified, has a specific meaning, even in everyday life. Martin Hogbin (talk) 08:48, 18 August 2009 (UTC)
Of course, the main difficulty in the discussion is that the "speed of light" may refer to the real, physical entity, the speed of light as found in the universe. Or, it may refer to the "numerical value of the speed of light in some system of units". What is said about the first, the physical speed of light, does not necessarily apply to the second (its value in some system of units). In fact, the numerical value in the SI system sometimes referred to as "the speed of light" actually is a misnomer, as it does not refer to either the physical speed of light, nor its numerical value, but to a conversion factor that allows the length standard to be replaced by the time standard. The true numerical value of the real speed of light is not accessible from within the SI system of units. For example, if the speed of light changed, the number 299 792 458 m/s would not change, but the metre would. Likewise, if accuracy improved in measurement, the number 299 792 458 m/s would not change, but the metre would. See NIST publication, Jespersen, Sydenham and Bortfeldt ("Since 1983 any measurement of the speed of light is neither necessary nor possible"); Brush ("Thus the speed of light ceased to be a measurable constant.") Brews ohare (talk) 06:24, 18 August 2009 (UTC)
Suggestion: Way back in July, the lead started with "The speed of light normally refers the speed of light in a vacuum, and is an important physical constant in modern physics. Light travels at different speeds through different materials..." (followed by an explanation which said why the speed in vacuum was particularly important). I'd suggest the following minor wording changes "The speed of light normally refers the speed of light in a vacuum, which is an important physical constant in modern physics. Although light travels at different speeds through different materials..." LouScheffer (talk) 13:06, 18 August 2009 (UTC)
- I have no problem with this sort of introduction. My problem is the statement "In the International System of Units, the length of the metre is now defined such that the speed of light is exactly 299 792 458 m/s." Although sources can be found that state such a thing, the notion that the speed of light is exactly known is nonsense, and in fact, in the SI units, the speed of light is not even measurable in principle. Misinformation should be avoided. Brews ohare (talk) 13:14, 18 August 2009 (UTC)
- Possibly a way out of this dilemma would be to state the measured value in 1972, with its error bar, and then describe the transition away from measurement of c in 1983 to the use of a conversion factor for c that removes it from the experimental domain and enables the use of an arbitrary exact value. Brews ohare (talk) 13:22, 18 August 2009 (UTC)
- Putting this info about the switch to a conversion factor into the intro is one way to introduce many important properties of the speed of light, because it is just these properties that make c a candidate for a standard that is readily reproduced. Brews ohare (talk) 15:06, 18 August 2009 (UTC)
- Sorry Brews but your comment, 'Although sources can be found that state such a thing, the notion that the speed of light is exactly known is nonsense' will not wash. It as very reliable secondary source which states this and it is therefore what we must say. Your opinion is of no relevance, especially as most editors here disagree with you. Martin Hogbin (talk) 16:38, 18 August 2009 (UTC)
- We do know that the speed of light is exactly 299 792 458 m/s, and the reason why we know this is because we defined it to be exactly 299 792 458 m/s. We cannot measure this, because it is a defined quantity. This is like saying you can't know that a square has four sides because we defined a square to have four sides.Headbomb {ταλκκοντριβς – WP Physics} 18:39, 18 August 2009 (UTC)
- Sorry Brews but your comment, 'Although sources can be found that state such a thing, the notion that the speed of light is exactly known is nonsense' will not wash. It as very reliable secondary source which states this and it is therefore what we must say. Your opinion is of no relevance, especially as most editors here disagree with you. Martin Hogbin (talk) 16:38, 18 August 2009 (UTC)
I'd like Martin Hogbin & Headbomb to reflect upon this matter further. The speed of light was measurable before 1983 because the metre was defined as an actual physical length (a certain number of wavelengths) determined independently of the speed of light, as was time as well. Hence, c = ℓ/t was found by measuring t and using the known, separately measured ℓ. That is standard practice in determining speed, wouldn't you say?
Post 1983, the metre was not determined separately from the speed of light, but in terms of the distance light traveled in 1/299 792 458 s, as ℓ = 299 792 458 m/s × 1/299 792 458 s = 1 m. Consequently, c = 299 792 458 m/s, no matter what. Do you two think this outline is correct?
Supposing you agree, when the BIPM definition of the metre refers to "the speed of light" as 299 792 458 m/s, I'd say they definitely expect (at a minimum) to use the relation ℓ = c × t in determining length from time-of-transit, and naturally in this context c = 299 792 458 m/s exactly. Do you think you might agree with me on this point?
However, I do not think BIPM or NIST mean that, quite beyond its use in ℓ = c × t, this numerical value for c refers to the exact physical speed of light. (By physical speed of light I refer to the speed of light with the properties explained in the theory of relativity, say.) They mean, rather, to restrict this use of c to be only the exact conversion factor when using the relation ℓ = c × t. Of course, a conversion factor c for use in ℓ = c × t can be defined and never measured (it doesn't have to be measured at all), because it is a convention decided among men to place their comparisons of length upon the same footing. It is therefore exact and does not change with advances in measurement technique. As Jespersen says, it is arbitrary and can take on any value whatsoever. As long as everybody uses the same c in finding lengths ℓ = c × t, everybody is on the same page. I believe that is the position of the sources I have cited.
Would you please discuss these points?? Brews ohare (talk) 03:39, 19 August 2009 (UTC)
Proposed start to the lead.
Bearing in mind what reliable sources say and the opinions of other editors, my suggestion for the start of the lead is:
The term 'speed of light', usually denoted by 'c', generally refers to a fundamental dimensionful physical constant which is the speed that light travels in a vacuum. The length of the metre is defined such that the speed of light is exactly 299 792 458 metres per second but it is often conveniently approximated to 300,000 km/s
Links and references would need to be added. Let me give reasons for my choice of words:
I added 'the term' to 'speed of light' because to say The speed of light generally refers to is not good English... Perhaps 'speed of light' in quotes would be OK.
There is no 'now', 'currently' or 'according to current theory' as this is always expected to be the case in an encyclopedia article.
I mention that it a fundamental physical constant first then give its exact speed in SI units, then the approximation. I have added that it is a dimensionful (or dimensional) constant. This explains a lot for those interested.
I have added a minor tautology 'the length of the metre' because I think it helps understanding of what has been done. I do not think that the additional tautology of 'in SI units' is necessary.
Maybe the English could be polished a but but I think these are the right things to say, in this order.Martin Hogbin (talk) 17:04, 18 August 2009 (UTC)
- It is a step in the right direction. I think the brief comment about the metre would confuse a general reader; and I think the approximate value is better suited to the section on value/notation (are there any sources that go into some detail about the speed of light, and introduce the approximation so prominently?); also, this wording seems to unintentionally invite the question of what the term could also refer to. Pecos Joe (talk) 17:48, 18 August 2009 (UTC)
- I am trying to take account of the comments of other editors. The bit about the metre gives a very brief indication of how the speed can be exact. Perhaps we could do without it?
- I agree that the approximate speed is not essential for the lead.
- If you mean 'generally refers to', I would be quite happy to drop the 'generally'. I think the self evident meaning of the speed that light happens to go in some medium is not particularly relevant to an encyclopedia article.
- Basically, I am trying to cover some of the angles, so that we can end up with a well though out and intro that does not get rewritten every week. Let us see what others think. Martin Hogbin (talk) 18:07, 18 August 2009 (UTC)
- I don't think you can cut out 'generally', unfortunately. For example, in electromagnetism, I think it is fair to say that the speed of light generally means the speed of light in some medium. I don't know a good way to reconcile that yet. I came back to clarify that I support replacing the first paragraph with the one you propose when the protection is gone. Pecos Joe (talk) 18:28, 18 August 2009 (UTC)
A suggestion:
In Physics, the term 'speed of light' generally refers to the speed at which light travels in a vacuum. This is a fundamental dimensionful physical constant, usually denoted by 'c'. In SI units, c is exactly 299 792 458 metres per second.
This adds the field of study, as is conventional. And I hope attaching the statement about exactness to "c" rather than to "the speed of light" might make it a bit clearer that it is an arbitrary choice of value for a constant rather than a happy conincidence. EdwardLockhart (talk) 09:02, 19 August 2009 (UTC)
- Yes, let's go for it. Finally. DVdm (talk) 10:18, 19 August 2009 (UTC)
- That reads very nicely, and I personally have no problem with it. I was trying to cover some of the points raised by other editors, but it is not easy and there is a lot to be said for a simple and straightforward approach like yours. In the future, no doubt, someone will raise the point of how the speed can be an exact number and should we give some indication of this in the lead. The problem is that the only way that I can think of to phrase this neatly starts with, 'The meter is ...', rather than a statement about the speed of light. I am trying to get something that is well written an bomb proof, maybe this is not possible. Martin Hogbin (talk) 11:49, 19 August 2009 (UTC)
- Perhaps making the word "exactly" an internal link to the "speed of light set by definition" section would help. EdwardLockhart (talk) 12:27, 19 August 2009 (UTC)
- That sounds good to me but is it not what you called a 'teaser'. Martin Hogbin (talk) 12:56, 19 August 2009 (UTC)
- That link would not be a teaser in my opinion; just don't write "For more detail see some section." If you wanted to mention the field of study, why restrict it to physics? Chemistry and astronomy also use the term, so maybe it should read "in science'. I don't think it is a convention to mention the field of study, as just about 1/3 of good articles do in what I saw, so either way would be acceptable. Do you have any objection to explaining the other use of the term 'speed of light' as the next sentence of this paragraph? Pecos Joe (talk) 17:05, 19 August 2009 (UTC)
- I'm basically OK with the above, but I'll throw in a suggestion and a comment. The suggestion is to remove the word "dimensionful". It is correct, of course, but most physical constants have dimensions: it is dimensionless constants such as the fine structure constant which really stand out for attention. The comment is that astronomy also uses the speed of light (obviously) but not with the SI definition: I think it would be fair simply to start the lead with "The term "speed of light" usually refers to…" Physchim62 (talk) 17:51, 19 August 2009 (UTC)
- That link would not be a teaser in my opinion; just don't write "For more detail see some section." If you wanted to mention the field of study, why restrict it to physics? Chemistry and astronomy also use the term, so maybe it should read "in science'. I don't think it is a convention to mention the field of study, as just about 1/3 of good articles do in what I saw, so either way would be acceptable. Do you have any objection to explaining the other use of the term 'speed of light' as the next sentence of this paragraph? Pecos Joe (talk) 17:05, 19 August 2009 (UTC)
- I agree. "Dimensionful" sounds pretty awkward, to say the least. DVdm (talk) 17:57, 19 August 2009 (UTC)
- Joe, if you mean by "the other use of the term 'speed of light'" just the speed light happens to go in some particular medium, then I think this should not be in the lead. As I said above this is just the ordinary use of the English language. We would not have an article on 'The speed of a dog'. Martin Hogbin (talk) 18:13, 19 August 2009 (UTC)
- Here is a sentence that describes what I was thinking. It is very poor English, but it adequately describes an approach to answer the question of what else 'speed of light' would refer to. The cleaned-up version would appear as the sentence after the one with the 299... number. I think it would be better to cover 100% of the uses rather than only 90% (or whatever the case may be) of them, especially when the change is relatively small, as a cleaned-up version would be.
- Sometimes, in electromagnetism, when the medium in which light propagates is clear, the term speed of light may refer to the speed of light in that medium, which is slower than the speed in vacuum by a factor called the refractive index of the medium.Pecos Joe (talk) 21:07, 19 August 2009 (UTC)
- I think that we should mention that light travels more slowly in a medium but what we do not, in my opinion, need to say is that in some cases 'the speed of light' can be used to mean simply the speed light is that travelling at. This is just normal English. Of course it is used in this way. Do you see what I mean. It is not that you are wrong, it is that we do not need an encyclopedia article to tell people what 'the speed of light' means when it is not referring to the important case of light in a vacuum. It just means exactly what those words would be expected to mean.Martin Hogbin (talk) 21:52, 19 August 2009 (UTC)
- It seems my suggestion is the minority, so I will not continue suggesting it. Pecos Joe (talk) 17:01, 20 August 2009 (UTC)
So we now have, for the first paragraph:
The term 'speed of light' generally refers to the speed at which light travels in a vacuum. This is a fundamental physical constant, usually denoted by 'c'. In SI units, c is exactly 299 792 458 metres per second.
with a link to the 'Speed set by definition' for 'exactly' and other appropriate links. References are not generally liked in the lead, as I understand it, they should be where the detail is discussed in the main body. Is there anyone who does not like this wording?
Perhaps we are all wasting our time here. Finell has taken it upon himself to rewrite the lead for us, completely ignoring this ongoing discussion. Martin Hogbin (talk) 18:55, 19 August 2009 (UTC)
- I can perfectly live with Finell's edit. And besides, indeed, we are all wasting our time here, but it's fun, isn't it? :-) - DVdm (talk) 19:42, 19 August 2009 (UTC)
- I've changed the lead paragraph to be the one above. Let's see if it sticks! EdwardLockhart (talk) 19:53, 19 August 2009 (UTC)
- ... and Edward's is really ok as well. So, what was al this fuss about? DVdm (talk) 19:55, 19 August 2009 (UTC)
- Maybe I am taking it a bit too seriously, but I am trying to get this article back to being a FA. One of the first criteria for that is that it should be relatively stable. The only way I can see to achieve that is to use wording that is acceptable to as many editors as possible. That is what I was trying to do here. Martin Hogbin (talk) 20:03, 19 August 2009 (UTC)
- Probably you are taking it too seriously, but I appreciate your efforts. Pecos Joe (talk) 21:07, 19 August 2009 (UTC)
- I agree that the goal should be a return to FA. —Finell (Talk) 22:53, 19 August 2009 (UTC)
Well, that is the first paragraph done, how are we going to do the rest. The basic content of the next paragraph is fine but I think it needs rewriting to improve the style and the way that it puts things. Martin Hogbin (talk) 23:05, 19 August 2009 (UTC)
- Replied below in new section. Pecos Joe (talk) 17:01, 20 August 2009 (UTC)
pi is defined to be the ratio of the circumference to the diameter of a perfect circle
Let me see if I understand the dispute with Brews via an analogy. Suppose that for some strange reason we always express lengths of line segments in different units from curved lines. Expressed in these units the ratio between the circumference of a circle and the diameter could be some arbitrary number depending on how we would choose the units for the two type of lengths. In natural units you would choose the same units and then the ratio would be the dimensionless number pi (analogous to c = 1).
But in conventional units the ratio is expressed as 3.001(curved length)/(straight lenght) and a redefinition of the "curved length" unit makes this ratio exactly 3 (curved length)/(straight length).
Then Brews comes along and he argues that however we define things (natural units, or other units), the lengths have a physical meaning independent of our definitions. He argues that the physical ratio could change, but because we define our units so that the ratio is fixed, that would mean that the length standards change their lengths.
The only way what Brews suggests can happen would be if the metric could change from Euclidian to non-Euclidean. If he would not make explicit such a scenario and everyone would by default only consider Euclidean geometry, then Brews objections would be difficult to appreciate.
I think that in the case of speed of light, the laws of physics themselves as we understad them allow one to act according to the 1983 redefinition of the meter. I.e. c is constant and you can assign it whatever value you like, c = 1 being the most natural choice. This choice then fixes the unit for distances in the time direction to be the same as distances in the spatial direction. The "physical speed of light" can differ from c if the photon would turn out to have a small mass.
Anyway, I think it would be helpful if Brews were to specify some unambiguous theory in which making the distionctins he suggests would be relevant. Count Iblis (talk) 19:22, 18 August 2009 (UTC)
- Count Iblis, You said above I think that in the case of speed of light, the laws of physics themselves as we understad them allow one to act according to the 1983 redefinition of the meter. I.e. c is constant .
- But I think that Brews is trying to emphasize the fact that the constancy that you are referring to is not the same as the constancy that comes with the new set of SI units. I suspect that at least some of Brews's opponents have a vested interest in blurring this distinction. Not everybody believes in the physical constancy of the speed of light in the relativistic sense. Hence, to compound this belief with a system of units in which the speed of light has to be a numerical constant, will assist the case of those who do believe in the physical constancy. It would most likely have been a forgone conclusion at the 1983 conference that the distinction between the two types of constancy would eventually become blurred.
- The distinction is important in its own right as part of the general understanding electromagnetism. Brews shouldn't need to point out a theory in which the distinction becomes important. Electromagnetism can be a confusing topic at the best of times for many students, and there is no better way to confuse students even more than to confuse two different concepts into one. David Tombe (talk) 20:34, 18 August 2009 (UTC)
- The answer is that c is constant in both senses. We have no reason to believe that c changes over time, if indeed such a change were to be measurable. It is also true that c is constant in all inertial frames. Thus, as Count Iblis says, we can act according to the 1983 definition, because the speed of light is constant. But I have to repeat, even if this were not the case it would not alter that fact that in metres per second the speed of light is constant by definition. Martin Hogbin (talk) 21:31, 18 August 2009 (UTC)
Martin, The speed of light differs upsteam and downstream in a column of liquid. Fizeau demonstrated this fact in 1851. The speed of light in glass changes with frequency, which is why we have dispersion. The speed of light is different in different media. So how are we supposed to do physics if the speed of light is constant by definition of the units? The situation is a nonsense. This is all new physics. It's not the physics that I was taught, or the physics that I later taught. I live in a world where a trundle wheel measures a distance. But you obviously live quite comfortably in a bizarre world where the distance calibrates the trundle wheel. It is far too presumptious to put this latest new age physics into the lead of an on-line encyclopaedia without putting a clarification alongside it in relation to traditional attitudes to the concept of speed. The public aren't ready for these silly games. You have now confirmed to me that you want to blur the distinction between real physics and new age physics. Basically, you want to delete real physics. And you have already proved to me that you want to delete it from history too. I can see the significance of 1984 coming through very clearly in all of this. The lead to the article vacuum permittivity is pure 1984 material. David Tombe (talk) 00:36, 19 August 2009 (UTC)
- If this is not the physics you were taught, I suggest that you update yourself to take into account the last 26 years of science before coming on Wikipedia to argue you know the truth and everyone else is trying to suppress "real physics". Call the BIPM and complain if you want, in the meantime we'll keep on reporting how the world is rather than how you want it to be. Physicists have had no problem working with that definition of the speed of light, and if they had any you can bet that they would have made a ruckus about it by now. Headbomb {ταλκκοντριβς – WP Physics} 04:38, 19 August 2009 (UTC)
- Obviously, I should have said 'in free space'. I think it is actually the last century of physics that David needs to acquaint himself with. Martin Hogbin (talk) 13:12, 19 August 2009 (UTC)
- If this is not the physics you were taught, I suggest that you update yourself to take into account the last 26 years of science before coming on Wikipedia to argue you know the truth and everyone else is trying to suppress "real physics". Call the BIPM and complain if you want, in the meantime we'll keep on reporting how the world is rather than how you want it to be. Physicists have had no problem working with that definition of the speed of light, and if they had any you can bet that they would have made a ruckus about it by now. Headbomb {ταλκκοντριβς – WP Physics} 04:38, 19 August 2009 (UTC)
Headbomb, The point is that such a cataclysmic change in recent times, which means that we can no longer put a ruler to an object and measure a length, needs to be elaborated on for the benefit of the non-physics readership. It is far too presumptious to slip this nonsense into a mainstream encyclopaedia as if we had always lived in such an Alice in Wonderland world. An average member of the public still thinks of speed in terms of the distance that light travels in a given period of time. There has so far, to the best of my knowledge, been no public awareness campaign instructing the public that since 1984, when we measure a length, we are in fact really just calibrating our measuring device.
You have taken this nonsense on board, hook, line, and sinker. But the public haven't. I would say that 99.9999999% of the public are oblivious to this nonsense. This is all just a fad, and you have become absorbed by the fad to the extent that you want to write these articles as if it had never been any different. Brews on the other hand clearly sees that something badly needs explaining, and you and the rest of your 'consensus' are vilifying him for doing so, because you don't want it to be known that this is actually just new physics that is being slipped in through the back door unnoticed. David Tombe (talk) 12:25, 19 August 2009 (UTC)
NIST and BIPM meaning for c
I'd like Martin Hogbin & Headbomb to reflect further upon the way BIPM and NIST use the term speed of light.
The speed of light was measurable before 1983 because the metre was defined as an actual physical length (a certain number of wavelengths) determined independently of the speed of light, as was time as well. Hence, c = ℓ/t was found by measuring t and using the known, separately measured ℓ. As with all measurements, the speed of light had error bars. That is standard practice in determining speed, wouldn't you say?
Post 1983, the metre was not determined separately from the speed of light, but in terms of the distance light traveled in 1/299 792 458 s, as ℓ = 299 792 458 m/s × 1/299 792 458 s = 1 m. Consequently, c = 299 792 458 m/s, no matter what. Do you two think this outline is correct?
Supposing you agree, when the BIPM definition of the metre refers to "the speed of light" as 299 792 458 m/s, I'd say they definitely expect (at a minimum) to use the relation ℓ = c × t in determining length from time-of-transit, and naturally in this context c = 299 792 458 m/s exactly. Do you think you might agree with me on this point?
Of course, a conversion factor c for use in ℓ = c × t can be defined and never measured (it doesn't have to be measured at all), because it is a convention decided among men to place their comparisons of length upon the same footing. It is therefore exact and does not change with advances in measurement technique. As Jespersen says, it is arbitrary and can take on any value whatsoever. As long as everybody uses the same c in finding lengths ℓ = c × t, everybody is on the same page. A change in c everybody agrees upon simply changes the length of the metre that everybody uses. I believe that is the position of the sources I have cited.
However, I do not think BIPM or NIST mean that, quite beyond its use in ℓ = c × t, this numerical value for c refers to the exact physical speed of light. (By physical speed of light I refer to the speed of light with the properties explained in the theory of relativity, say.) They mean, rather, to restrict this use of c = 299 792 458 m/s to be only the exact conversion factor when using the relation ℓ = c × t.
Such a point of view makes clear why c from the BIPM viewpoint is exactly 299 792 458 m/s with no error bar and no connection to measurement. On the other hand the physical speed of light, like all real, physical properties, is known only by observation and always has error bars and never is exactly known, just as common sense suggests. Moreover, for comparing lengths, one never needs to know what the exact physical speed of light is: one needs only the confidence that, whatever it is, this same speed always is realized in "vacuum".
Would you please discuss these points?? Brews ohare (talk) 03:39, 19 August 2009 (UTC)
- Up to the sentence starting with "However", yes, that is accurate. What follows is more or less unclear. There's no difference between "defining the speed of light" and "defining the conversion factor", these are completely equivalent actions (up to the limit that photons are indeed massless if you want to be picky about it, but that's irrelevant to the discussion at hand). The "physical" speed of light does not have error errors bars on it, it is exactly 299 792 458 m/s, by definition. If you do a measurement, and there are errors bars somewhere, yes, but the error bars are not on the speed of light. Rather they are on the other quantities. For example, if you take a pulse of light traveling in vacuum (speed = c, by definition), and check where it's at three seconds later, then you will probably have errors bars on time because your clock is probably not a single atom of caesium-133, at rest, isolated from the universe, in its ground state, at 0 K. You have an uncertainty on elapsed time, thus will have one on the measured distance. Since t = d/c, then Δt = Δd/c. Since Δt is directly proportional to Δd, then if you are off by 1 part in 10^9 on time, you will be off by 1 part in 10^9 on distance. c however, is unaffected by this, as it is defined to be a certain precise value. Headbomb {ταλκκοντριβς – WP Physics} 04:57, 19 August 2009 (UTC)
Headbomb: It seems to me that it doesn't actually matter what the "exact" speed of light is. Suppose it was 2 × 106 wavelengths of some transition per second. We still can use c=299 792 458 m/s exactly as we do today, with absolutely no effect upon anything we do, even though 2 × 106λ/s was totally different from c=299 792 458 m/s . In particular, we'd still find lengths as ℓ = 299 792 458 m/s × t, with t the transit time in vacuum. Do you agree?? Brews ohare (talk) 05:13, 19 August 2009 (UTC)
- No, we can't use that, we'd have to measure the speed of light in that scenario in order to know it was 299 792 458 m/s, and there would be error bars on the speed of light, and the scaling between meters and seconds would be uncertain. There would also be error bars on the meter, since there is no such thing as as purely monochromatic light source whose spectrum can be described by a delta function located at a certain wavelength. And there would be error bars on the second just as before. So you're worse off doing things this way (and that is why that we are now using a better way). Headbomb {ταλκκοντριβς – WP Physics} 07:21, 19 August 2009 (UTC)
This seems to me to be the exact same situation we have with mass today. The kg is defined as the mass of some metal cylinder kept in a vault in Paris. Now what is the mass of that object? It's EXACTLY 1.000000000000 kg, with no error bars. This is a result of the definition, just as the speed of light is. Of course, just like light, you can (and do) measure the mass (speed) using other realizations of the kg (meter), so at least you have some idea of the uncertainty in your definition. This is a much bigger problem with mass rather than the meter, and they would love to replace the standard kg with a count of some kind of atom, but the technology is not quite there yet. LouScheffer (talk) 11:48, 19 August 2009 (UTC)
The next step in discussion, it seems, is to say, Hey, since we can't tell the difference anyway ... Brews ohare (talk) 05:15, 19 August 2009 (UTC)
- From 1960 to 1983, the wavelength of the orange-red emission line in the electromagnetic spectrum of the krypton-86 atom in a vacuum in SI units had no error-bar on it and could not in principle be determined by measurement - it was by definition 1 / 1 650 763.73 metres. Does this fact bother you as much as the post-1983 situation with the speed of light? Why or why not? EdwardLockhart (talk) 05:16, 19 August 2009 (UTC)
- EdwardLockhart: Off the top of my head I'd say that defining the metre in terms of some reproducible length is logically different from defining it in terms of a standard of time. The precise definition of the metre in terms of some λ does not afford a problem to me: the λ is a well-defined length in principle (Nature's metre, so to speak). When using λ = c t it is the idea that c so-used is supposedly the same thing as exactly the actual speed of light, when in fact the set-up makes measurement of this speed in SI units logically impossible, and c so-used is an arbitrary convention. Brews ohare (talk) 05:27, 19 August 2009 (UTC)
- But it is a reproducible length! Take a pulse of light, wait until 9,192,631,770/299,792,458 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom, at rest, in its ground state, at 0 K have ellasped and the distance light will have travelled is your meter, no matter where you are. This is a well defined length, just as much as the other, only that instance of counting wavelenghts, you count ticks from a clock. Headbomb {ταλκκοντριβς – WP Physics} 07:29, 19 August 2009 (UTC)
- EdwardLockhart: Off the top of my head I'd say that defining the metre in terms of some reproducible length is logically different from defining it in terms of a standard of time. The precise definition of the metre in terms of some λ does not afford a problem to me: the λ is a well-defined length in principle (Nature's metre, so to speak). When using λ = c t it is the idea that c so-used is supposedly the same thing as exactly the actual speed of light, when in fact the set-up makes measurement of this speed in SI units logically impossible, and c so-used is an arbitrary convention. Brews ohare (talk) 05:27, 19 August 2009 (UTC)
- Headbomb, Does that work for the circumference of a trundle wheel too, even though light doesn't travel in circles? David Tombe (talk) 11:58, 19 August 2009 (UTC)
- Brews, along with other editors, I am happy to discuss this topic with you but will you please accept that there is a consensus of editors and reliable sources here to state the exact value.
- Your point that we cannot control or define nature remains true. Although the speed of light in free space is defined exactly in m/s we can never exactly determine the length of (delineate) the metre any more. Under the current definitions there is, and always must be, some uncertainty when the metre is delineated. This will be due to experimental error and due to the fact that, as you have often pointed out, although the speed of light is a defined constant in free space, we can never actually delineate the metre in free space because we do not have any. So, when the meter is delineated, we have to make corrections, as best we can, for the actual experimental conditions. All the experimental and theoretical issues that once applied to measuring the speed of light now apply to the delineation of the metre. Martin Hogbin (talk) 08:07, 19 August 2009 (UTC)
Can I suggest that this discussion is continued on a page in my user space User_talk:Martin_Hogbin/Speed_of_light_set_by_definition so that the rest of us can use this page to discuss improving the article. Martin Hogbin (talk) 13:15, 19 August 2009 (UTC)
- No you can't suggest that. Your suggestion is just a bureaucratic tactic to hide the fact that there is a matter to be discussed. This is the page to discuss ways of improving the article, so we will stick to this page. David Tombe (talk) 13:18, 19 August 2009 (UTC)
- This parrot is no more! He has ceased to be! Martin Hogbin (talk) 14:53, 19 August 2009 (UTC)
- No you can't suggest that. Your suggestion is just a bureaucratic tactic to hide the fact that there is a matter to be discussed. This is the page to discuss ways of improving the article, so we will stick to this page. David Tombe (talk) 13:18, 19 August 2009 (UTC)
This is a dead parrot
The poll and subsequent discussion clearly show a large majority of editors agreeing that we should state the exact value of the speed of light early in the lead. Many reliable sources clearly state that the speed of light has an exact value in m/s and none state that it does not.
I therefore suggest that we accept that there is a consensus, supported by reliable sources, to state the exact value of the speed of light in the lead section of this article. This need not stop discussion of the subject although I would suggest we set up a special talk page for those interested in talking about the subject further.
On this basis, I have proposed a new first paragraph for the lead, with the intention that we come up with well written section that is acceptable to everybody here. I am doing this to try to end the endless rewriting and edit waring over the lead section that we have had recently. I would therefore ask all editors to have a look at the section on the lead above and make their comments and suggestions there. Martin Hogbin (talk) 08:45, 19 August 2009 (UTC)
- Martin, There is one point that I would like to raise. Further up the page, you supplied us with the proceedings of the conference (in your own words). You wrote,
- Taken directly from 17th Conférence Générale des Poids et Mesures - Resolution 1, but in my words, the reasons are:
- (1)The older definition did not allow the meter to be sufficiently precisely realised for all purposes.
- (2)More stable sources of light (stabilized lasers) than the krypton 86 lamp used in the previous definition became available.
- (3)Measurements (delineations of the metre) made with the new light sources agreed well with those made by the old ones
- (4)Realizations of the metre are more reproducible using the newer light sources
- (5)The new defined value is the numerically the same as the older (1975) recommended value
- (6)The new definition agrees well enough with older realizations that there will me no sudden jump in the length of the meter for all practical purposes.
- (7)After consultations and deep discussions on the subject it was agreed that this definition is equivalent to the older one, which was based on the wavelength of a defined light source.
- I would like to draw attention to points (6) and (7). They seem to be saying that the new system will be equivalent to the old system. But I now have Headbomb telling me that I can no longer measure the distance between the plates of a capacitor under the new system. Can you clarify these two points please. David Tombe (talk) 12:14, 19 August 2009 (UTC)
- Although they are in my words the above reasons were taken directly from Resolution 1. There is link to that resolution somewhere on this page that you can follow to get the original words. I suggest that you follow up you discussion with Headbonb directly with him. Martin Hogbin (talk) 13:00, 19 August 2009 (UTC)
As this parrot is now dead I have set up a page in my user space User_talk:Martin_Hogbin/Speed_of_light_set_by_definition where I suggest that this discussion is continued so that the rest of us can use this page to discuss improving the article. Martin Hogbin (talk) 13:07, 19 August 2009 (UTC)
- Well Martin, I'm going to ignore your special page because I was discussing improving the article right here where it is supposed to be discussed. The issue is that there has been too much of a cataclysmic shift in physics as a consequence of the 1983 conference, and it is too recent to allow this new physics to be slipped in without an explanation for the general non-physics readership. Brews wants to make the elaboration and you have been pulling out all stops to vilify him and hinder him because you want the article to be written up as if this new nonsense physics has always existed. David Tombe (talk) 13:14, 19 August 2009 (UTC)
- It is your choice discuss the subject where you like but there is clearly a consensus of editors her who want to state the exact speed in the lead and there is no sign that you have convinced any of them. This parrot is no more! He has ceased to be! Martin Hogbin (talk) 13:59, 19 August 2009 (UTC)
- Well Martin, I'm going to ignore your special page because I was discussing improving the article right here where it is supposed to be discussed. The issue is that there has been too much of a cataclysmic shift in physics as a consequence of the 1983 conference, and it is too recent to allow this new physics to be slipped in without an explanation for the general non-physics readership. Brews wants to make the elaboration and you have been pulling out all stops to vilify him and hinder him because you want the article to be written up as if this new nonsense physics has always existed. David Tombe (talk) 13:14, 19 August 2009 (UTC)
Still dead?
David Tombe is still under an indefinite topic ban from this article and talk page. To those of you who who have extensive experience with Tombe's views and past edits, do the recent edits of the article by 72.84.67.16 (talk · contribs · WHOIS) look like his work? This IP has a total of 4 contribs, all today. The first is a diatribe at AN/I against the admin who topic banned Tombe followed by the 3 article edits here. You may want to initiate an investigation of whether the IP is Tombe violating his topic ban. —Finell (Talk) 20:10, 20 August 2009 (UTC)
- Never mind. I reported it myself at AN/I. The contribs convinced me. —Finell (Talk) 20:44, 20 August 2009 (UTC)
Second lead paragraph
One postulate of the theory of special relativity states that the speed of light is constant in any inertial reference frame. One important consequence of relativity is that neither information nor matter may travel at a speed greater than the speed of light in vacuum.
- It's basically the relativity paragraph, and I had thought the most important things to mention would be the limiting speed of info/matter, and the constancy in inertial frames. I dislike the "believed to be" part (if we can say it is, then say that, but otherwise leave unsettled science out of the lead if possible), and I think mentioning spacetime is confusing to the general reader and unnecessary in the lead. Part of that paragraph could read something like above. Pecos Joe (talk) 16:59, 20 August 2009 (UTC) (modified by Pecos Joe (talk) 17:13, 21 August 2009 (UTC))
- I think we should steer clear of the constant speed in inertial frames, or at least be very careful. It is hard to explain the true situation, as is done in the relevant section, in a sentence or two. If we do say something, I think that it should just be that light travels at the same speed in all inertial frames.
- The other point that would be good to get across is that special relativity, general relativity and spacetime are all essentially the same topic. Martin Hogbin (talk) 21:50, 20 August 2009 (UTC)
- I favor keeping and being careful with the inertial frames part. I'm not quite sure what you mean by equating those topics - could you be more specific? I rearranged the first post, maybe just add to/alter the italicized paragraph up there to propose a particular wording so the section won't become too long to read through the discussion. Pecos Joe (talk) 17:13, 21 August 2009 (UTC)
- The more modern way to treat relativity is to start with Mankowski spacetime, in which c plays the role of the constant relating the space dimensions to the time dimension. All the results that we call SR (maximum speed, composition of velocities, mass-energy equivalence etc) follow from the fact that we live in Minkowski spacetime. In GR this same spacetime is curved by the presence of matter and energy. In this curved spacetime c is still a constant of the spacetime but our intuitive notions of distance and time are no longer applicable. To talk of a speed is meaningless unless you define exactly how you will determine distance and time.
- Because of the above, I do not think that it is a good idea to talk along the lines of,'c is a constant of spacetime...according to SR this happens...according to GR that happens', as though they were three unconnected theories. I would like to say something along these lines, but in better English: c is a constant of the spacetime in which we live. Because of this the following are true: speed of light is constant in all inertial frames; it represents the maximum speed at which matter, energy or information can travel; it is the conversion factor between mass and energy. Martin Hogbin (talk) 22:39, 21 August 2009 (UTC)
299 792 458 m/s a "fundamental constant"
Majority rule has been implemented and the lead statement now says 299 792 458 m/s is a fundamental constant of nature, despite sources (for example Wheeler; Jespersen; Sydenham) that state the contrary.
The fact is that the number 299 792 458 m/s was introduced as "the speed of light" in 1983 by defining the metre to make this so. However, far from being a fundamental constant, this number was decided by committee, and has a fixed and arbitrary value (according to Jesepersen) that (as clearly stated by NIST and CODATA) is beyond measurement, being simply a conversion factor between time and length in the SI units.
It is legitimate to state that (within the context of SI units) the speed of light in SI units is 299792458 m/s because of the definition of the metre; it is absurd to state that it is a fundamental constant, as it is a number determined by committee, as Wheeler says. Brews ohare (talk) 13:04, 21 August 2009 (UTC)
- The lead does not say that "299 792 458 m/s is a fundamental constant of nature". It says that the speed of light is a fundamental physical constant, usually denoted by 'c', and that, in SI units, c is exactly 299792458 m/s. The constant is denoted by 'c', and c has some value in some system.
- Good grief. DVdm (talk) 13:11, 21 August 2009 (UTC)
- Maybe that is what you think it says: I don't agree. Here is the text:
This is a fundamental physical constant, usually denoted by 'c'. In SI units, c is exactly 299792458 metres per second.
- Maybe that is what you think it says: I don't agree. Here is the text:
- To elaborate unnecessarily, the c that is a fundamental physical constant has nothing to do with the number 299 792 458 m/s that is the conversion factor between time and length in SI units, which is set by committee, independent of any recourse to measurement and entirely unrelated (logically) to the actual speed of light, as expressed let's say, in terms of wavelengths of some atomic transition traversed in a second. The closest one can come to connecting the number 299 792 458 m/s to the actual speed of light is that it is a number that was chosen by committee to be pretty close to the measured value pre-1983, when this number still was a measurable entity in the SI system.Brews ohare (talk) 13:16, 21 August 2009 (UTC)
- It says that the speed of light is a fundamental physical constant, usually denoted by 'c', and that, in SI units, c is exactly 299792458 m/s. The constant is denoted by 'c', and c has some value in some system. Period.
- What you think words say, seems to be a personal problem for you. DVdm (talk) 13:27, 21 August 2009 (UTC)
Well, if you say c is the fundamental physical constant, and then say c is exactly 299 792 458 m/s, it's not "my personal problem" if I take that as saying the fundamental physical constant has a value of exactly 299 792 458 m/s in SI units, a blatant untruth, contradictory to sources. Brews ohare (talk) 13:34, 21 August 2009 (UTC)
- Well, It says the speed is "usually denoted by 'c', and that, in SI units, c is exactly 299792458 m/s".
- You see, it is indeed your personal problem. We really can't help you with this. DVdm (talk) 13:41, 21 August 2009 (UTC)
Perhaps you could rephrase it so no confusion could result, even for those of my limited capacities? Brews ohare (talk) 13:52, 21 August 2009 (UTC)
- You now know what the words mean - we explained. You don't have to worry about people with "limited capacities". If such people come along and express their confusion, we'll explain it to them. No worries. DVdm (talk) 13:58, 21 August 2009 (UTC)
It's a bit dubious to leave ambiguous statements in the article that have to be resolved on the talk page. Here's a possible change of phrasing:
“This is a fundamental physical constant, usually denoted by 'c'. Prior to 1983, in SI units the measured numerical value of c was determined as approximately 299 792 458 m/s. Then, in 1983, a switch was made from a measured to a defined value for c, and c was fixed arbitrarily at the value 299 792 458 m/s by forcing the metre to adapt so as to maintain this value regardless of experimental observations.”
References could be added. Brews ohare (talk) 14:03, 21 August 2009 (UTC)
- I said: "No worries". DVdm (talk) 14:09, 21 August 2009 (UTC)
DVdm: Your reassurances are meaningless, of course, and are not a substitute for substantive comment. I'm not a dope, and I do speak English. So the approach should be to fix the language, not to make the reader the problem. Brews ohare (talk) 14:13, 21 August 2009 (UTC)
- I also said that "we really can't help you with this". That should cover the fact that you find the reassurances meaningless. Perhaps another hobby? DVdm (talk) 14:26, 21 August 2009 (UTC)
Apparently you have adopted the responsible position of spokesman for the group "we" who cannot be helpful. I'd suggest that your approach is offensive, contrary to WP policy, and not conducive to improving either the page or the spirit of collaboration. Brews ohare (talk)
- I'm just trying to help keeping the article and the talk page clean, and to explain the meaning of a few simple words. To no avail it seems. Sorry. DVdm (talk) 14:39, 21 August 2009 (UTC)
- If Brews feels that his personal views carry more weight than the consensus of other editors, then there is really nothing "we" can do to help him. We can, however, attempt to minimize the damage. Physchim62 (talk) 14:54, 21 August 2009 (UTC)
- Brews is not conveying his personal views, but reporting the contents of published sources with impeccable credentials. It is your approach to avoid confronting the cited authorities and instead to make backhanded unsupported remarks. Why not attempt to deal with the issues? Is it less fun? Brews ohare (talk) 22:23, 21 August 2009 (UTC)
- If Brews feels that his personal views carry more weight than the consensus of other editors, then there is really nothing "we" can do to help him. We can, however, attempt to minimize the damage. Physchim62 (talk) 14:54, 21 August 2009 (UTC)
If the goal is to restore the status of this page, the lead sentence must avoid ambiguity. To do that, it needs revision. I've tried to suggest the problem. The response I get is that there is no problem. My attempts to clarify the problem are not dealt with, but instead I'm told to get lost. Great way to improve things, eh? Why the animosity? I've provided sources. I've provided logic. I've proposed a possible rephrasing. C'mon, guys. Brews ohare (talk) 15:00, 21 August 2009 (UTC)
- Brews, your proposed wording is good, but it is too detailed for the introduction. The sequence of events that led to the definition can (and should) be elaborated in the article, but doesn't need to be gone over in the first paragraph.
- I would propose a slight addition to the third sentence, reading: "In SI units, c is defined to be exactly 299792458 metres per second." (The italics would not appear in the sentence, of course). DJ Clayworth (talk) 15:08, 21 August 2009 (UTC)
DJ Clayworth: Thanks for your comment. Your suggestion does not solve the problem entirely, however. The difficulty is this: prior to 1983 the speed of light was measurable, due to the metre being a defined number of wavelengths of a particular transition. Consequently, one could determine how long it took for light to traverse this distance, and c was just distance over time. However, post 1983 the value of c became simply a conversion factor, with length = c × time of transit. Evidently, because length is now determined this way, to determine the length of any particular object, the time of transit is measured and then multiplied by c to find the length. The numerical value of c is defined, but obviously is not the fundamental constant termed the speed of light, because it is a number fixed by fiat at the pre-1983 approximate value, and now independent of measurement altogether.
I agree that this state of affairs goes into some detail, and may not be appropriate in the introduction. However, one does not want the introduction to make a statement readily misinterpreted as suggesting the fundamental constant c has an exactly known numerical value, when this is certainly not the case. The problem with this WP article seems to be this: Within the SI system of units, the defined conversion factor is referred to by the wording "the speed of light", and this terminology is unfortunately interpreted outside this SI units context as meaning the actual speed of light. Brews ohare (talk) 16:24, 21 August 2009 (UTC)
Brews, I think your worries may be a bit off. A new reader is more likely to be impressed with the tight linkage between relativity, length-measurement and time via experiment, implied by the ability of a committee (that knew these things) to set the value and be consistent with all of these experiments, than do be thrown off by it, imho. Jok2000 (talk) 17:33, 21 August 2009 (UTC)
- Brews, I think you are getting needlessly philosophical here. We are writing for the general reader, especially in the intro. Let's make the simple statement in the intro, and deal with any possible misunderstandings in the body. DJ Clayworth (talk) 17:46, 21 August 2009 (UTC)
Brews is not alone in doubting the wisdom of the current lead; the problem as I see it is this:
- The speed of light (SoL) has not been defined as 299etc m/s but rather a metre has been defined as 1/299etc of the distance travelled by light in a second.
- It is therefore quite misleading, although completely true, to state in the lead that the SoL is exactly 299etc m/s because this is a circular definition.
- Because even readers with a passing knowledge of science will not realise this circularity, they will believe the SoL is known exactly.
- However, the SoL is not known exactly and never will be; it is measurable with a greater and greater accuracy in any length unit not defined using the SoL.
Now this may seem like semantics or "needlessly philosophical" but imho the lead should be changed to avoid the implication that the SoL is known exactly when it isn't and never will be (except, by definition, in m/s). Abtract (talk) 18:38, 21 August 2009 (UTC)
- Perhaps it would help if you would actually read the lead. Nowhere does it say anything about defining. DVdm (talk) 19:14, 21 August 2009 (UTC)
- Abtract, we have just had a very long discussion on this subject which you can read above, it is a pity that you chose not to take part in it. We seriously considered saying, 'The meter is defined such that the speed of light is exactly 299 792 458 meters per second', but there was a consensus against this. As you have said, what we say in the lead is correct and it is supported by impeccable sources, thus we are entitled to say it. The situation is complicated and, in the end, it was considered not practicable to try to explain it in the lead. You will note that the link on 'exactly' takes you to the section where this matter is discussed in detail. Martin Hogbin (talk) 21:13, 21 August 2009 (UTC)
- It does however say that in the wording I proposed above. I believe that the difference between defining the metre and defining the speed of light is a pretty philosophical and abstract one (again remember we're writing for the general reader and giving an overview, and also remembering that neither space nor time are absolutes). However let me propose another wording:""In SI units, c is exactly 299792458 metres per second by definition". DJ Clayworth (talk) 20:15, 21 August 2009 (UTC)
- This suffers from the same circularity ... it is the metre that has been defined in relation to the SoL, not the other way round. Abtract (talk) 20:31, 21 August 2009 (UTC)
- If you really must say something about definitions, then better would be: "'In SI units, c is exactly 299792458 meters per second, by virtue of the definition of the meter."
- But just "'In SI units, c is exactly 299792458 meters per second" (as it is now) will do perfectly. That is a fact all by itself. Remarks about definitions come later in several remarks and in an entire subsection. No need to overload the lead with this. DVdm (talk) 20:35, 21 August 2009 (UTC)
- There has been a very long discussion on whether we should say that. My original words were, 'The meter is defined such that the speed of light is exactly 299792458 meters per second'.
- There are multiple meanings for the symbol c and for the term speed of light: there's the rub. It can be taken as 299 792 458 m/s in the SI system of units. It can be taken as a fundamental constant of nature. In the above statement by DJ Clayworth, the choice of meaning is not made clear enough.
- It does however say that in the wording I proposed above. I believe that the difference between defining the metre and defining the speed of light is a pretty philosophical and abstract one (again remember we're writing for the general reader and giving an overview, and also remembering that neither space nor time are absolutes). However let me propose another wording:""In SI units, c is exactly 299792458 metres per second by definition". DJ Clayworth (talk) 20:15, 21 August 2009 (UTC)
- Unfortunately, 299 792 458 m/s is not the numerical value of the speed of the fundamental constant of nature, not even in SI units.
- In case that is not clear, here is the point: The number 299 792 458 m/s is defined exactly in SI units as the factor used to determine lengths of objects from the measured transit time of light: length = 299 792 458 m/s × time of transit. One consequence is that measurement of the actual, physical speed of light has to be done in a multi-step process in SI units: (i) one measures a selected wavelength in metres (ii) one measures the time of light to transit this wavelength (iii) one expresses the speed of light in wavelengths/s. Unfortunately, one cannot in principle express the speed of light in m/s because of the tautological nature of the definition of the metre. Brews ohare (talk) 20:43, 21 August 2009 (UTC)
- What does it take to make you understand that we can't help you with this? DVdm (talk) 20:50, 21 August 2009 (UTC)
- This parrot is no more! He has ceased to be! 'E's expired and gone to meet 'is maker! Martin Hogbin (talk) 21:18, 21 August 2009 (UTC)
Brews, I really don't know what your objection is to this, or how it's even possible to believe that the speed of light (c) is anything else but 299 792 458 m/s (or to quote your exact words "299 792 458 m/s is not the numerical value of the [speed of light]). Meters are defined so the speed of light is exactly that number. You cannot "measure a selected wavelength in metres" independently of the speed of light. To measure a wavelength in meters, you need to compare it to the length that a pulse of light travels during 1/299 792 458 seconds. Headbomb {ταλκκοντριβς – WP Physics} 21:03, 21 August 2009 (UTC)
- Headbomb: I understand your frustration with me over this point; it is a consequence of having one end of a stick and not realizing there is another. The point is that within the SI units system 299 792 458 m/s is used to calculate lengths as in length = 299 792 458 m/s × time of transit. I believe you understand this point. I believe you also understand that with length defined this way the speed of light becomes 299 792 458 m/s inescapably. What I think you do not understand is that this number is not related to the fundamental constant of nature referred to by (e.g.) the theory of relativity. The reason: it is not possible given the 1983 definition to express this fundamental constant in m/s: it always comes out as 299 792 458 m/s, and that is a wrong answer. Brews ohare (talk) 21:23, 21 August 2009 (UTC)
- Well then you espouse a WP:FRINGE point of view. The number 299 792 458 m/s is the value of the speed of light / scaling factor represented by c when expressed in SI units. You will find no mainstream source siding with you here.Headbomb {ταλκκοντριβς – WP Physics} 22:47, 21 August 2009 (UTC)
Section on Speed of light by definition
Perhaps it would be helpful to rewrite this section in a context where problems of succinctness are less severe. Then, with a clear statement of the situation in hand, perhaps a more accurate short intro sentence can be constructed?
I'd suggest this section begin with the pre-1983 definition:
“On October 14, 1960 the Eleventh General Conference on Weights and Measures redefined the International Standard of Length as 1,650,763.73 vacuum wavelengths of light resulting from unperturbed atomic energy level transition 2p10 5d5 of the krypton isotope having an atomic weight of 86. The wavelength is
- λ = 1 m / 1,650,763.73 = 0.605,780,211 µm
At different times some national laboratories used light sources other than krypton 86 as length standards. Mercury 198 and cadmium 114 were among these and they were accepted by the General Conference as secondary length standards.”
This beginning point has the merit of being perfectly understandable: define the length standard "metre" in terms of a fundamental length of nature, a particular wavelength.
Given this metre and a time measurement, the speed of light in m/s is readily understood as a length / time of transit, and is roughly 299 792 458 m/s.
That introduction then can be followed by an explanation of the 1983 switch to a defined speed of light and a length standard based directly upon time-of-transit. That explanation would clarify why the term "speed of light" in SI units is then to be understood only in the context of length determination as in length = 299 792 458 m/s × time of transit. The number 299 792 458 m/s is explained to be a convention adopted by CODATA, and caution given that it should not be confused with a numerical value for the physical speed of light, which can no longer be expressed in m/s, but can be expressed, for example, in wavelengths of some atomic transition / second.
Of course, references to all critical statements are to be provided, such as Wheeler; Jespersen; Sydenham. What do you all think? Brews ohare (talk) 21:14, 21 August 2009 (UTC)
- I think you already know what we all think. Can you please stop disrupting and harassing this talk page? Thank you. DVdm (talk) 21:18, 21 August 2009 (UTC)
- The reliable sources you quote are not critical of the current state of affairs, that is just a misconception on your part. Martin Hogbin (talk) 21:20, 21 August 2009 (UTC)
- If you want to discuss this subject further, I have set up a page for it at User_talk:Martin_Hogbin/Speed_of_light_set_by_definition Martin Hogbin (talk) 21:22, 21 August 2009 (UTC)
- Martin: You state I am under a misconception, but do not say what that misconception is. I have provided sources, and you do not address them. I have outlined an approach and you dismiss it without comment. Humbug. Brews ohare (talk) 22:15, 21 August 2009 (UTC)
- This clearly goes under wp:nor#Synthesis of published material that advances a position. DVdm (talk) 21:24, 21 August 2009 (UTC)
- Perhaps you could elaborate by stating exactly what has been said that goes beyond the cited sources? You have never addressed the sources, the issues; go beyond bald accusation and put some meat on the bones, eh?. Brews ohare (talk) 22:13, 21 August 2009 (UTC)
Well Martin et al.: I have explained this matter carefully to you all. I have made above a simple and clear outline of a proposed change. You all simply will not engage on a detailed basis, but continue to harangue in abstractions. That is not how to advance. The article is in flat contradiction to sources despite your unsupported claims to the contrary. It is logically unsound. It is factually incorrect. It makes the absurd claim that the speed of light is known exactly, in distinction (contradiction) to all other observations of nature. All this will become clear in due course, I hope. Brews ohare (talk) 21:31, 21 August 2009 (UTC)
- The speed of light is known exactly because it is exactly defined. You're claiming that we can't know a square has four sides because we defined squares as having four sides. Headbomb {ταλκκοντριβς – WP Physics} 01:04, 22 August 2009 (UTC)
- The analogy with a square is a false one because we have defined "four" and "side" quite independently of "square" so when we make a statement about the square using four and side it is useful and meaningful. However, when we define the metre as 1/299.. of the distance travelled by light in a second and then state that the SoL is 299.. m/s we have a circular statement that is not a definition at all because, although we do have an independent definition of a second, we do not for the metre. To state it another way, "The SoL is 299.. m/s" says absolutely nothing scientific about the speed of light because we do not know what a metre is withought first knowing the SoL. Of course the statement is meaningful to a layman (and indeed to us all) because we all "know" what a metre is from its previous definition.Abtract (talk) 09:33, 22 August 2009 (UTC)
- The numerical value of the speed of light in SI units is known exactly. The mechanical length of the meter has, as far as can be measured or detected, not changed.
- Your statement:' "The SoL is 299.. m/s" says absolutely nothing scientific about the speed of light because we do not know what a metre is without first knowing the SoL' is not correct, we do not need to know the SoL, all we need to delineate the meter is some light, which we have. Martin Hogbin (talk) 09:47, 22 August 2009 (UTC)
- Thanks. Perhaps you could enlighten (sorry couldn't resist) me as to how we delineate a metre using some light. Abtract (talk) 09:55, 22 August 2009 (UTC)
- You, obviously, also need a means if realizing the second. In principle, all you then do is let the light travel for the appropriate fraction of a second and see how far it goes. That is your metre. Of course, in practice, this is done with interferometry and light of a measured frequency. Martin Hogbin (talk) 10:50, 22 August 2009 (UTC)
- Thanks for confirming the circularity and apologies for previouly stating the problem badly. What I should have said is:
- "The SoL is 299.. m/s" says absolutely nothing scientific about the speed of light because we do not know what a metre is without first defining it in relation to the SoL.
- Abtract (talk) 13:17, 22 August 2009 (UTC)
- Thanks for confirming the circularity and apologies for previouly stating the problem badly. What I should have said is:
[outdent]I do not understand what you mean by 'nothing scientific'. The current article gives the speed in SI units. Obviously, if you do not know what a second and a meter are this will mean little to you. This applied just as much when the meter was defined by a metal rod. If you had never seen the rod or a meter rule, you would have no idea what the number meant. Martin Hogbin (talk) 15:12, 22 August 2009 (UTC)
- If we had defined "four" as the number of sides a square has, and then state that a square has four sides, that would be a circular statement and not very helpful. Imho defining the metre as the distance travelled by light in 1/299.. metres and then stating that the SoL is 299.. m/s is equally circular and should be avoided without some reference to the way a metre is defined (which we have now done). That's all I'm saying. Abtract (talk) 15:29, 22 August 2009 (UTC)
- As you say, the current lead briefly explains why the speed of light is an exact value so I suggest we leave drop the subject. Most people are getting rather fed up with. Martin Hogbin (talk) 15:51, 22 August 2009 (UTC)
- Agreed, and thanks for your patience. Abtract (talk) 17:44, 22 August 2009 (UTC)
Proposed lead first para
In an attempt to overcome the circularity problem, I offer the following wording (second part taken more or less verbatim from the source):
- The term speed of light generally refers to the speed at which light (or any other electromagnetic wave) travels in vacuum. This is a fundamental physical constant, usually denoted by the letter "c".
- The metre has been defined (SI units) so that the speed of light in vacuum has the exact value:
- c = 299 792 458 m/s ... which can be used as a standard. [1] Abtract (talk) 22:29, 21 August 2009 (UTC)
- This is close to what we once had before, but I am not sure why it is better that the wording we have now. There was a feeling that we wanted the fundamental constant bit first. Also, I do not understand what you mean by the 'circularity problem'. Also, I would prefer to say 'The metre is defined' rather than 'The metre has been defined'. Martin Hogbin (talk) 22:46, 21 August 2009 (UTC)
- I have explained what I mean by the 'circularity problem' in the section above (Section on Speed of light by definition) Abtract (talk) 09:36, 22 August 2009 (UTC)
- This is close to what we once had before, but I am not sure why it is better that the wording we have now. There was a feeling that we wanted the fundamental constant bit first. Also, I do not understand what you mean by the 'circularity problem'. Also, I would prefer to say 'The metre is defined' rather than 'The metre has been defined'. Martin Hogbin (talk) 22:46, 21 August 2009 (UTC)
- Is or has been, makes no difference to me; I think this phrasing is superior to what's there now, though I abhor staring with "the term". My suggestion would be:
- In physics, the speed of light is a fundamental physical constant, the speed at which light (or any other electromagnetic wave) travels in an ideal vacuum. It is usually denoted by the letter "c". In the SI system of unit, the metre has been defined such that the speed of light has the exact value c = 299 792 458 m/s.[2]
- Abtract, I can see why you do not like saying 'the term', it is rather ugly, but I think we have to say something like that. 'The speed of light' is, even in everyday usage, is a phrase with a specific meaning, namely the speed of light in free space. The natural English phrase 'the speed of light' can be used to mean many things, just like saying, 'the speed of a train'.
- Until we have defined our terms, I do not think that we can say, for example 'the speed of light is 299 792 458 m/s' because this is not always the case, the speed of light in glass is less, for example. My alternative suggestion was to say something like, "the speed of light is generally taken to refer to ...". — Preceding unsigned comment added by Martin Hogbin (talk • contribs)
- Martin, that was me. So why did you put back "the term"? The lead defines the meaning in the context of the article, without that. I recommend we put back the more standard-form lead, and you tell us what it is in it that you "strongly object" to. You can certainly say, as there is a strong consensus to say, that 'the speed of light is 299 792 458 m/s' once you've said that 'the speed of light is a fundamental physical constant, the speed at which light (or any other electromagnetic wave) travels in an ideal vacuum'. Was I wrong in thinking that we had a consensus to go this way? Does anyone else object? Dicklyon (talk) 22:08, 22 August 2009 (UTC)
The problem is that the words "the speed of light" in the phrase The term "speed of light" generally refers to the speed at which light (or any other electromagnetic wave) travels in vacuum does not have the same meaning as it has in the phrase: The metre has been defined (SI units) so that the speed of light in vacuum has the exact value: c = 299 792 458 m/s .
The first occurrence refers to the physical, observable, real speed of light. The second occurrence refers to the factor used in the SI system of units to relate length to time-of-transit. This last number agrees with an approximate value for the physical speed of light as measured prior to 1983, now enshrined by definition to lie outside observation, forever immutable. Brews ohare (talk) 22:59, 21 August 2009 (UTC)
- Brews, please stop dragging dead parrots into every discussion on this topic. Martin Hogbin (talk) 09:32, 22 August 2009 (UTC)
The numerical value 299 792 458 m/s is not measured in SI units
In SI units the number 299 792 458 m/s is used to relate time-of-transit to length in the formula length = 299 792 458 m/s × time.Jespersen and in Sydenham. Given this relation, the number 299 792 458 m/s is established by definition, and is not subject to confirmation by experimental observation. (See same references.) In short, the number 299 792 458 m/s in the SI system of units is not measurable.
In contrast, if a length is expressed in wavelengths of some particular atomic transition (for example, as per the definition of the metre prior to 1983), the time it takes light to traverse this length can be measured (as is done for the metre), and the speed of light then is measurable in terms of wavelengths/s. Needless to say, if the wavelength is expressed in metres, this number for the speed of light will be close to 299 792 458 m/s, but unlike 299 792 458 m/s, this measured speed of light will have an error bar associated with the errors of observation.
This measured value of the real speed of light is not used in the modern SI definition of length because (at the moment, anyway) the error bar introduced by use of the measured value of c is larger than that incurred using 299 792 458 m/s with the time-of-flight methodology. Brews ohare (talk) 23:17, 21 August 2009 (UTC)
- Yes, so you've told us, and we get it. How does this relate to the article? I don't see anything about "measured" that needs to be fixed. Dicklyon (talk) 23:34, 21 August 2009 (UTC)
The number 299 792 458 m/s is not the measured speed of light in SI units, and it is not the "exact" speed of light in SI units; it's just the number in the formula length = 299 792 458 m/s × time of flight. The measured speed of light has an error bar, and is not known exactly, but is approximately 299 792 458 m/s. The statement in the lead In SI units, c is exactly 299,792,458 metres per second. is not correct, or perhaps I should say it is not correct if c is interpreted as the actual speed of light, and not just as the proportionality factor in the equation for length. Brews ohare (talk) 00:06, 22 August 2009 (UTC)
- Brews, the scaling constant and the speed of light are the exact same things. I suggest you drop the stick before someone files an ANI request and get topic-banned like David Tombe just got. Headbomb {ταλκκοντριβς – WP Physics} 08:07, 22 August 2009 (UTC)
- Or you can continue the discussion somewhere else. Martin Hogbin (talk) 08:56, 22 August 2009 (UTC)
What was the measured speed of light in 1982 in m/s, before the meter was redefined to be based upon the speed of light? —Finell (Talk) 12:47, 22 August 2009 (UTC)
- 299 792 458 m/s. Martin Hogbin (talk) 14:43, 22 August 2009 (UTC)
- Thanks, that's what I thought. Martin, I am sure that you can explain the fallacy of the DT-BO position in terms that they are able to understand, including demonstration (not bare assertion) of how they either misinterpret their sources or that they are not WP:RSs. Whether they will agree with it is a separate matter. While I would welcome their understanding and agreement, that is not my primary concern; my primary concern is the process. The explanation belongs on this Talk page and is the the appropriate way to close this discussion topic. As tendentious as they have been, they have been trying in good faith to improve the article and their thesis is relevant to the article's topic. Therefore, the error in the thesis should be explained, here. FYI, Jimbo looked at this talk page and agrees that neither side has addressed the other's points or supported their own position by showing what the RSs actually say on the subject (he doesn't claim to understand the topic, but he got the gist of the dispute). The explanation will serve in the future to show how the issue was resolved, if it arises again. Thank you. —Finell (Talk) 22:10, 22 August 2009 (UTC)
- Reply to Finell.
I will make an attempt here to state my case in response to Finell with supporting references as a summary of how I see the matter. I invite the other editors on this page to support their views in a similar manner. The following is not intended to be argumentative, but a straightforward response to Finell.
- Background
There are several different points to make. The first is that there does exist in the universe something called the speed of light that, according to relativity, relates to the structure of spacetime and the bounds on the transport of information and of matter. Second, the numerical value of this speed can be measured. One way to do this is to take a set length, for example the wavelength of an atomic transition, and determine the time it takes for light to transit this distance. The speed of light is then c = λ/t. That approach was the methodology used in the SI units prior to 1983. See this NIST timeline for details: definition:
“On October 14, 1960 the Eleventh General Conference on Weights and Measures redefined the International Standard of Length as 1,650,763.73 vacuum wavelengths of light resulting from unperturbed atomic energy level transition 2p10 5d5 of the krypton isotope having an atomic weight of 86. The wavelength is
- λ = 1 m / 1,650,763.73 = 0.605,780,211 µm
At different times some national laboratories used light sources other than krypton 86 as length standards. Mercury 198 and cadmium 114 were among these and they were accepted by the General Conference as secondary length standards.”
It seems to me doubtful that any of the parties disagree with this background. They probably also agree that this pre-1983 approach resulted in a value for c of approximately 299 792 458 m/s, and that this value was obtained only to within observational error (that is, this number is not an exact value).
- SI units post 1983
In view of the status of the speed of light as a physical constant, independent of frequency, direction etc., it is possible to set up a laboratory to realize the speed of light (subject to some caveats about corrections needed to adjust for "nonideal vacuum"). That being so, lengths can be compared by comparing the times of transit of light along the lengths using a formula length = c × time of transit. Actually, one does not have to know the numerical value of c in order to make such length comparisons: one has to be sure only that the speed of light in vacuum has been realized. Thus length comparisons are simply ratios of transit times, and the actual value of c simply divides out in such ratios. For this reason, as pointed out by Jespersen of the Bureau of Standards (underscores are mine):
One fallout of this new definition was that the speed of light was no longer a measured quantity. The reason is that, by definition, a meter is the distance light travels in a designated length of time, so however we label that distance - the speed of light is automatically determined. ... turning c into a conversion factor whose value is fixed and arbitrary.
As stated by Sydenham in a rather definitive article that is often cited (underscores mine):
The numerical value of the speed of light c = 299 792 458 m/s is the result of numerical standards chosen for the standards of time and of length. Thus the speed of light as a numerical value, is not a fundamental constant.
- Summary
We see then, that while the "speed of light" was initially a measurable quantity pre-1983, post-1983 is has become a "arbitrary" value that is not "a fundamental constant". This odd situation is understood by noticing that the speed of light still is a physical quantity, but the terminology "speed of light" has two different meanings: pre-1983 it was a measured value, and post-1983 it is a "conventional value" of 299 792 458 m/s.
This change in meaning is a peculiarity of the SI units. The speed of light is used in two senses: the actual physical experimental speed of light and the SI units conventional value for the speed of light. Because of the post-1983 definition of the meter, the actual physical speed of light can no longer be expressed in m/s; all that is possible is the conventional, defined value 299 792 458 m/s. To state the actual physical speed of light in SI units, that is the physically observable behavior of light, one must resort to expressing the speed of light in terms of some physical length, for example, wavelengths of some transition/second.
The debate on this WP page stems from confusion between the conventional defined value for the speed of light of 299 792 458 m/s, (which is an exact number chosen by definition to be a value chosen for convenience to be close to the measured value prior to 1983), and the actual physical speed of light, which cannot be expressed any longer in m/s because of the convention that the metre is defined so this number always is 299 792 458 m/s.
Within the new definition, the actual physical speed of light must be expressed not in m/s (because that results in tautology) but in terms of some physical length, for example in terms of wavelengths of some atomic transition/s, as was done prior to 1983. That is, c can be expressed as some number of λ's per second. The real, physical speed of light is not calculable from some basic theory, and so cannot be stated as a definite numerical value, but must result from measurement with an observational error bar attached to it.
I hope that this exposition lays out the situation clearly. It is not intended to be argumentative, but simply to point out the confusing usage of the term "speed of light" to refer both to a fact of physical nature (that is beyond human capacity to measure without an error bar) and also to an exact conventional "conversion factor" of 299 792 458 m/s Brews ohare (talk) 16:04, 23 August 2009 (UTC)
- ... and if everyone politely ignores this clearly laid out exposition of your confusion, will you then finally stop repeating it on this talk page? DVdm (talk) 16:47, 23 August 2009 (UTC)
- DVdm: You have made what is simply a nasty uncooperative response to a sincere effort on my part. Your failure to be constructive and your snarky attitude does nothing to advance matters. My view is that your ideas about these matters are misinformed, and if you disagree, you should support your views, not badmouth earnest effort. Brews ohare (talk) 19:23, 23 August 2009 (UTC)
- Perhaps I should have ignored it myself to begin with. My apologies. DVdm (talk) 21:23, 23 August 2009 (UTC)
- True apology is accompanied by changed behavior. False apology is just a form of sarcasm. Brews ohare (talk) 02:01, 24 August 2009 (UTC)
- This was not an apology to you but to the others, for not following my own hint to politely ignore your repetitive talk page disruptions. DVdm (talk) 10:00, 24 August 2009 (UTC)
Rest
I think that the speed of light, and specially its talk page, desperately needs some rest ;-) - DVdm (talk) 10:55, 22 August 2009 (UTC)
- I completely agree that we should stop discussing the speed of light being fixed by definition. This is an impeccably sourced fact that must remain in the article. The fact that some people seem to be having difficulty getting their heads round this fact does not change it. I have offered a home for further discussion of the subject by those interested in it. Martin Hogbin (talk) 11:09, 22 August 2009 (UTC)
Martin's lead
In this diff, Martin took us back to saying "the term", removed a ref, adding a dangling "this", and made some other subtle wording changes. Does anyone understand or support this change, or can I put back the previous one? Dicklyon (talk) 22:12, 22 August 2009 (UTC)
- Where is the dangling this and what was the previous version? Abtract (talk) 23:28, 22 August 2009 (UTC)
- Click the diff link to see both versions, including "This is..." with no subject noun. Dicklyon (talk) 23:31, 22 August 2009 (UTC)
- I have no preference either way. Abtract (talk) 23:38, 22 August 2009 (UTC)
- We could change 'This is' to 'It is', perhaps and maybe make it one sentence.
- Regarding 'the term', I believe that it is important to make clear that we are talking not about the normal English meaning of 'the speed of light' but a specific although widely used meaning of 'the speed of light in a perfect vacuum and an inertial frame (and maybe other conditions) that is the fundamental constant of the spacetime in which we live'.
- I do not think that it is good English to write 'The speed of light generally refers to the speed at which light travels in free space'. Maybe we could have, 'The speed of light is a term that generally refers to the speed at which light travels in free space'. Martin Hogbin (talk) 12:20, 23 August 2009 (UTC)
- In view of comments made elsewhere, I should also point out that this is not my lead it is the result of collaborative editing in the section above 'Proposed start to the lead'. Martin Hogbin (talk) 12:35, 23 August 2009 (UTC)
- I don't think we want "generally" or "refers" or "term" in there at all; just say what it is, as in most other articles. And if any of those collaborators have an opinion, they should let us know. Dicklyon (talk) 15:07, 23 August 2009 (UTC)
- I can live with the new version, but I prefer the original one, so afaiac, go ahead. - DVdm (talk) 12:51, 23 August 2009 (UTC)
- I put back the standard-form opening. If anyone besides Martin think that "the term" is a better approach, they should say so here. I left Ruslik0's addition of "partly to allow more precise measurements of distance" in the lead paragraph, even though I think it's a bad idea to go off on this tangent in the opening paragraph. Dicklyon (talk) 16:05, 23 August 2009 (UTC)
- I have no problem with the first sentence but I have restored the second one which says all we need to say. We do not need the date when the standard was changed in the lead, only the current standard. As Dick has said, we do not want just one of many reasons for the change. Also, as has been pointed out by Abtract, the speed of light is not strictly a defined quantity, it is fixed by virtue of the definition of the metre. Unsigned comment probably due to Martin Hogbin
- OK, sounds like we're converged. I re-wikified, and changed "exact" to "defined" to avoid giving the false impression that the speed of light has been exactly measured or something like that. Hopefully, any further expansion or clarification of this bit of complication can be kept out of the lead. Dicklyon (talk) 19:55, 23 August 2009 (UTC)
- Sorry, that last bit was left there by me by mistake. I have now deleted it. Have a look at the article. 86.142.238.242 (talk) 20:23, 23 August 2009 (UTC)
The statement "In SI units, the metre is defined such that the speed of light has the exact value of 299,792,458 metres per second." is technically accurate. However, this statement is not a simple one, involving as it does the notion that one actually is permitted to set an exact value for the speed of light, while normally one would think of it as a measured quantity. If one tries to explain this matter as a decision to redefine the metre so as to make it so, that again appears highly non-intuitive, and possibly circular. I do not find it satisfactory to leave the reader dangling, hoping they might find and read by some accident the subsection "Speed of light by definition". Brews ohare (talk) 20:34, 23 August 2009 (UTC)
- Actually, the statement itself is simple, and that's what's good about it, given its position in the lead. Any physical, metaphysical, semantic, and philosophical complexity associated with it can well be deferred to the section about how and why it is a defined quantity. You have spent much of the last year pushing to include such complexity in the lead, and the consensus is clearly against you on that. Dicklyon (talk) 20:38, 23 August 2009 (UTC)
Call it simple to suggest that one can define the speed of light when intuition suggests the speed of light should be measured? Brews ohare (talk) 20:46, 23 August 2009 (UTC)
I guess I spoke too soon about convergence. Abtract has thrown it out and started over again. Abtract, can you please explain what and why? Dicklyon (talk) 23:05, 23 August 2009 (UTC)
- Sorry about that but I started to make a couple of minor clarifications, then it came to me that the paragraphs were not in the ideal order and one thing led to another ... but it does read much better now imho. Abtract (talk) 00:27, 24 August 2009 (UTC)
- The opening paragraph of three choppy little sentences reads better to you? And you're OK ignoring the long-fought consensus to put the exact value in the opening paragraph? Dicklyon (talk) 03:17, 24 August 2009 (UTC)
Definition and measurement
The WP article states:
In the International System of Units the metre is defined such that the speed of light in vacuum has an exact value where c = 299,792,458 metres per second.
That is an accurate statement, although it omits explanation of how it is possible to have an exact value for a physically observable entity the speed of light despite the fact that no observable entity has an exact value when that value cannot be calculated from scratch, but must be measured. That conundrum is only partly fixed by statement that this feat is accomplished by definition of the metre as a length that satisfies c = 299,792,458 metres per second. The natural response is "Huh? Isn't c measurable, and subject to measurement error? Isn't this 'definition of the metre' argument pure sophistry?".
Of course, to actually measure c in using the SI system, as opposed to defining c one must introduce a length other than the metre. One way to do that is to follow the older definition of length and use the wavelength of some specified atomic transition to do that, call it λatomic. Then one measures the time of transit of light through distance λatomic and expresses the speed c as c = λatomic / time-of-transit. That gives the actual speed of light in units of λatomic/s.
Is this number in principle 299,792,458 m/s? The answer is: "No, but it is pretty close". If we could measure λatomic in metres exactly and time-of-transit exactly, then we could convert this measured c to m/s, and it would not be exactly 299,792,458 m/s, but nearly so, because 299,792,458 m/s is the measured value (within experimental error) found before the 1983 definition of the metre. See NIST time line.
An interesting point is that if the present definition of the metre were altered so that the defined value for c were 300, 000, 000 m/s instead of 299,792,458 m/s, say, and with this new definition for the meter the wavelength λatomic were remeasured in terms of the new metre, the measured speed of light would still be exactly the same number of λatomic /s as before with the old metre, because λatomic would have the same transit time as before, so c in units of λatomic/(time-of-transit) would still be the same. However, in terms of the new metre, now c = 300, 000, 000 m/s. That is what Wheeler means by answering the question "A fundamental constant of nature?" by "Hardly! Rather, the work of two centuries of committees."; and what Jespersen; and Sydenham mean by statements like: the numerical value of c in SI units is a "conversion factor whose value is fixed and arbitrary." Brews ohare (talk) 00:53, 24 August 2009 (UTC)
- Yes - 299,792,458 is not a fundemental constant of nature. However, 299,792,458 m/s is. EdwardLockhart (talk) 07:33, 24 August 2009 (UTC)
- Your own words about the disputed statement in the lead are, 'That is an accurate statement'. Bearing in mind the length of discussion that there has been on this subject, do you really think that it is desirable or even possible to have an explanation of why the statement is correct in the lead? We have a statement that you agree is accurate. Let us leave it at that and work on an explanation elsewhere. Martin Hogbin (talk) 08:12, 24 August 2009 (UTC)
Hmm I'm beginning to see what the problem is here. Lockhart's statement above that "Yes - 299,792,458 is not a fundemental constant of nature. However, 299,792,458 m/s is." is surely incorrect. The SoL is a fundamental constant of nature (FCoN) ... yes; "c" is a FCoN ... yes; but this FCoN surely cannot be stated in SI unit because the standard unit of length has itself been defined such that the SoL has the exact value 299,792,458 m/s in SI units. This is the circularity problem I referred to previously and I think is what ohare keeps banging on about.Abtract (talk) 09:22, 24 August 2009 (UTC)
- There is no circularity problem any more than there is if we define the metre using a metal rod. We mark up a rod with two lines which we define to be a meter apart. So, how far apart are the lines? One metre! How long is a metre? The distance between the lines! This 'circularity' is a consequence of our defining arbitrary standards. Martin Hogbin (talk) 10:33, 24 August 2009 (UTC)
- Indeed but, if we then go on to ask how tall am I in metres or how fast does light travel in m/s, there would be no circularity because the metre had been defined independently of me or light. Abtract (talk) 12:06, 24 August 2009 (UTC)
- Asking how tall you are is the same in both cases. If we define a meter with a metal rod then we can measure the speed of light based on our standard metre, we cannot measure the metre, it is set by definition. If we define the meter based on the speed of light we can measure out (delineate) the meter but we cannot measure the speed of light, it is fixed by definition. We choose to define some things, others are results of our definitions. Martin Hogbin (talk) 12:46, 24 August 2009 (UTC)
Martin: In the new SI units, how tall you are is how long it takes for light to travel your height in seconds. Converting this to metres by multiplication with 299,792,458 m/s (or any other exact conversion factor) adds absolutely nothing to the experimental info in use. It is still a time of transit, not a measured length. You have not dealt with the three sources Wheeler, Jespersen and Sydenham. If, however, you measure your height in wavelengths, you have added some experimental info and the real, physical, entity speed of light is included in that new info. See the last paragraph of the above comments. Brews ohare (talk) 13:52, 24 August 2009 (UTC)
Do we have a consensus on the first paragraph?
Abtract and I have just made some minor changes to the first sentence.
Abtract removed the brackets, which I agree do not look good in the first sentence.
My changes were:
Change 'electromagnetic waves' to 'electromagnetic radiation'. I think that this is a more general term in that it covers the quantum model better.
Change 'ideal vacuum' to 'vacuum' with a link to 'free space'. This is another case where we cannot hope to explain a much discussed topic in one or two words, best to have a link to the page that discusses the topic fully. I do not think the term 'ideal vacuum' is that standard, it is not used in the vacuum article.
If there are objections to my changes can we please discuss then here and, if there is a consensus, just revert them rather than rewriting the whole paragraph from scratch again.
- Yes, we have my consensus :-)
- I changed "in an vacuum" to "in vacuum" though. Feel free to change into "in a vacuum" if you prefer that. DVdm (talk) 09:09, 24 August 2009 (UTC)
- I have made a few clarifying changes. In addition I think the statement of the actual number should be moved down until after the history para ... there would then be a natural progression. Abtract (talk) 09:20, 24 August 2009 (UTC)
- As Dick has pointed out there is a strong consensus not to move the numerical value from the first paragraph. Martin Hogbin (talk) 09:36, 24 August 2009 (UTC)
- Although my own preference was to start with the approximate value in the first paragraph, since all that many readers will want to know. The exact value, and the fact that it is exact, is an extra complication that I'd just as soon defer. But I think I lost that argument already. If someone wants to hold another referendum, we can revisit that; otherwise, let's don't. The best we can hope for at this point is to satisfy everyone except Brews and David, I think. Dicklyon (talk) 02:03, 25 August 2009 (UTC)
Unfortunately, there is not unanimity; clarity of exposition has not been reached. The natural response to the lead paragraph as now constituted is: "Huh? Isn't c measurable, and subject to measurement error? Isn't this 'definition of the metre' argument pure sophistry?" Brews ohare (talk) 13:58, 24 August 2009 (UTC)
- I agree that readers may find the fact that the speed of light has an exact value surprising. That's why we explicitly say that it is a consequence of the way the metre is defined, and why we supply links to metre and SI units for interested readers to follow. EdwardLockhart (talk) 14:07, 24 August 2009 (UTC)
- Brews, that may be your reaction but it is not everyone's. You have agreed that what is written is correct. Considering the pages of discussion we have had here, you must surely accept that it is simply not possible to provide any kind of explanation or justification for the change in the standard in the lead section. Martin Hogbin (talk) 14:08, 24 August 2009 (UTC)
Martin: The main source of this impossibility is a lack of understanding of the real role of 299,792,458 m/s as explained by the three sources Wheeler, Jespersen and Sydenham. If the editors would concede that this number is an arbitrary value set by committee and is not a physical constant of nature, the wording could be rearranged to explain that fact, and the sources provided. However, a number of editors are not yet aware of this fact, and continue to think 299,792,458 m/s is somehow an exact value for the real physical speed of light. This situation could be improved if all editors addressed the three sources instead of inventing new protests of personal construction. Brews ohare (talk) 14:17, 24 August 2009 (UTC)
- You see a contradiction where there is none. Yes, 299,792,458 is an arbitrary number set by committee which could equally have been something else. But nontheless, 299,792,458 m/s is an exact value for a fundamental constant of nature. EdwardLockhart (talk) 14:23, 24 August 2009 (UTC)
- It is a source of amazement to me that no-one (other than ohare?) seems to understand the circularity of that last statement. A degree is 1⁄360 of a full rotation; a full rotation is 360 degrees ... duh. Abtract (talk) 14:36, 24 August 2009 (UTC)
- EdwardLockhart: As I understand what you are saying, it is within the power of a committee to set a fundamental constant of nature. In other words, if we are dissatisfied with the limitations on space travel set by the speed of light, we can simply declare it to be faster? If that is not what you mean, how do we determine what is the fundamental limitation upon space travel? I've suggested that it can be measured independent of man's machinations as c = λatomic/time-of-transit. This suggestion is not my invention: it is exactly what was done prior to 1983, and is the origin of the number 299,792,458 m/s (although post-1983 the error bars have been stripped). Again, I plead with you to address the three sources and to avoid shooting from the hip based upon your immediate understanding of the situation. Brews ohare (talk) 14:40, 24 August 2009 (UTC)
How about taking the parallel with temperature? The Celsius scale was originally defined with the freezing point of water at zero. Using this scale, it was not possible to measure the freezing point of water - it had been arbitrarily set by committee. It was, however, possible to measure the triple point of water, which was approximately 0.01 degrees. However, in 1954, the situation was reversed - the triple point became the defined value (at 0.01 degrees), and the freezing point was now able to be measured (it's about zero).
None of this is committees dictating to nature - we are merely making convenient choices about how to define our arbitrary units. Since we use physical phenomena as our yardsticks, some phsyical phenomena will end up with defined values rather than measured values. Which phenomena these are will depend on the choice of definitions for our units. EdwardLockhart (talk) 14:59, 24 August 2009 (UTC)
- EdwardLockhart: We could digress to discuss temperature scales and just what is the analogy between that and the speed of light. However, that strikes me as the long way around. It would be simpler and more direct to address the specific points raised, address the three sources, and state why you do or don't agree. Brews ohare (talk) 15:15, 24 August 2009 (UTC)
- I agreee with the sources, e.g. Sydenham's "the speed of light as a numerical value is not a fundamental constant", and Jespersen's "the speed of light was no longer a measured quantity; it became a defined quantity".
- You said to Martin:
- "If the editors would concede that this number is an arbitrary value set by committee and is not a physical constant" - I think we are all in agreement on this point.
- "a number of editors [...] continue to think 299,792,458 m/s is somehow an exact value for the real physical speed of light" - yes, this is the consensus of the editors. It is also what the sources say. EdwardLockhart (talk) 15:34, 24 August 2009 (UTC)
EdwardLockhart Do you also agree with Wheeler answering the question: "A fundamental constant of nature?" by "Hardly! Rather, the work of two centuries of committees."; and Jespersen "c is a conversion factor whose value is fixed and arbitrary"?" Doesn't your acceptance of Sydenham ""the speed of light as a numerical value is not a fundamental constant" stand in contrast to your earlier remark that " But nonetheless, 299,792,458 m/s is an exact value for a fundamental constant of nature." ? Brews ohare (talk) 15:39, 24 August 2009 (UTC)
- I agree with Wheeler that there is nothing special about the number 299792458 except that a committee happens to have picked it to be, by definition, the speed of light in SI units.
- There is no contradiction between the two statements of mine you quote, or between them and the sources you mention. EdwardLockhart (talk) 15:46, 24 August 2009 (UTC)
Well, EdwardLockhart, I hesitate to parse the two sentences ""the speed of light as a numerical value is not a fundamental constant" (Sydenham) and "But nonetheless, 299,792,458 m/s is an exact value for a fundamental constant of nature." (EdwardLockhart), but in my mind they are absolutely contradictory. Perhaps you can bring out the nuance that separates their meanings? Brews ohare (talk) 15:52, 24 August 2009 (UTC)
- The first statement refers to the number 299792458. There's nothing remotely special about this number - it is a purely arbitrary choice. And although there were good reasons to pick this particular number, they too arose from previous arbitrary decisions.
- The second statement refers to the speed of light, which is a fundamental constant of nature. And in SI units, it has an exact value, by virtue of the way the definitions have been chosen. This value is 299,792,458 m/s.
- But all this has been explained before, so I am not hopeful that this will help any. EdwardLockhart (talk) 16:02, 24 August 2009 (UTC)
- EdwardLockhart: Here is the difficulty: the number 299,792,458 m/s was selected by committee. It is fixed. This number was measurable pre-1983 and was 299,792,458 m/s ± Error. It was not known exactly. Fixing this number by fiat in 1983 at 299,792,458 m/s does not make it an exact numerical value for "c"; it makes the conversion factor from transit time to metre an exact value. These are not the same things. That is what the sources say: see the words arbitrary value, not a fundamental constant? Brews ohare (talk) 21:04, 24 August 2009 (UTC)
There is a pretty strong consensus to leave the lead section as it is with respect to this subject. Perhaps we should discuss how the relevant section should read to make the exact situation as clear as possible. My first question is this. We start with:
In 1983 the 17th Conférence Générale des Poids et Mesures defined the metre to be the length of the path travelled by light in vacuum during a time interval of 1⁄299,792,458 of a second. The reasons for using this definition are stated in Resolution 1.[104]
The first sentence is a simple statement of fact, whether or not we like it,but should we also state the reasons given in resolution1 in the text: verbatim, in our own words, not at all? Martin Hogbin (talk) 16:28, 24 August 2009 (UTC)
- There is no unanimity upon the first sentence beyond the belief that it will spark incredulity among some readers. That reaction could be avoided by simply pointing out that the approximate value for the speed of light of 299,792,458 m/s that resulted from measurement at an earlier time has been adopted today as a conversion factor for exact translation of times-of-transit to lengths in metres, thereby changing the definition of the metre. Brews ohare (talk) 21:26, 24 August 2009 (UTC)
- You must know that your "demand" for unanimity goes against WP:CONSENSUS, and so will be ignored: no single editor has an absolute right of veto over changes. You do not OWN this article, and you would do well not to obstruct its improvement. Physchim62 (talk) 04:17, 25 August 2009 (UTC)
- Physchim62: There is no demand for unanimity made here; just the rather mild observation that unanimity has not been achieved. I am not obstructing the improvement of this article, but inviting actual discussion of sources in place of belligerence and attempted intimidation. Brews ohare (talk) 05:05, 25 August 2009 (UTC)
- Instead of attitude, how about addressing the issue: There is no unanimity upon the first sentence beyond the belief that it will spark incredulity among some readers. Brews ohare (talk) 11:23, 25 August 2009 (UTC)
Last paragraph of lead
Here is my suggestion for the last paragraph of the lead, which I suggest is made the second:
The speed of light is an important constant connecting space and time in the unified structure of spacetime. It defines the conversion between mass and energy[3] and is an upper bound on the speed at which matter and information can travel.[4][5] It is the speed of travel of all electromagnetic radiation in free space,[6] and is believed to be the speed of gravitational waves.[7] In an inertial frame, light in vacuum always travels at c. However, when light passes through a transparent medium, such as glass or air, its speed is less. The ratio between the speed of light in vacuum and the speed of light in a medium is called the refractive index of the medium.
I also suggest that, 'regardless of any differences in the frequency, polarization, intensity, or propagation direction of light' is too detailed for the lead. Martin Hogbin (talk) 15:55, 24 August 2009 (UTC)
Maybe it should be moved to the 'Constant speed in inertial frames' section. We currently have more detail in the lead than the body. Martin Hogbin (talk) 15:58, 24 August 2009 (UTC)
- Now edited to reflect my comments above. If nobody objects I will insert this section as the second paragraph. Martin Hogbin (talk) 21:29, 24 August 2009 (UTC)
IMO, the intro reads better with this paragraph at the end, rather than as second paragraph. The presently preceding paragraphs are more qualitative and general and easier to read. This paragraph is more detailed and requires more of the reader. Brews ohare (talk) 23:01, 24 August 2009 (UTC)
- That is one 'no' so far. The reason I would like to change this paragraph is that it currently states many facts as if they were disconnected, 'The speed of light is a constant of spacetime', 'according to special relativity that', 'EM radiation travels at c', etc. These effects are all manifestations of the fact that the speed of light is a constant of our spacetime. Martin Hogbin (talk) 10:37, 25 August 2009 (UTC)
- I did not vote "no" on the paragraph restructuring, only on a change of placement. Brews ohare (talk) 10:52, 25 August 2009 (UTC)
- Sorry, I misunderstood you. Martin Hogbin (talk) 16:33, 25 August 2009 (UTC)
Terrell-Penrose rotation
Should we have a brief mention of this subject in 'Practical effect of the finite speed of light'? Martin Hogbin (talk) 16:00, 24 August 2009 (UTC)
- This subject is found on the WP page Terrell rotation and the effect refers to the apparent geometry of a moving object. I think a mention is useful. Brews ohare (talk) 23:04, 24 August 2009 (UTC)
- I don't think so - it's a relativistic effect, not a finite-light-speed one. EdwardLockhart (talk) 09:11, 25 August 2009 (UTC)
- Quite the reverse. It was years before it was noticed that if we were to look at a passing, rapidly moving, object we would not see it Lorentz contracted, as predicted by relativity, but rotated (simply speaking) due to the combined effect of the Lorentz contraction (the expected relativistic effect) and the time delay in light reaching our eyes from different parts of the object (a simple and easily calculated effect due only to the finite speed of light). It is similar, in principle, to aberration of starlight. Martin Hogbin (talk) 10:35, 25 August 2009 (UTC)
- So it is. Yes, I think it would make sense to include it. EdwardLockhart (talk) 21:40, 25 August 2009 (UTC)
- I have added a paragraph on the subject. Martin Hogbin (talk) 22:28, 25 August 2009 (UTC)
- There are some nice simulations of this effect. Should we add a link to one? Martin Hogbin (talk) 22:30, 25 August 2009 (UTC)
Approximate value of c in first paragraph
Quote from above discussion:
Although my own preference was to start with the approximate value in the first paragraph, since all that many readers will want to know. The exact value, and the fact that it is exact, is an extra complication that I'd just as soon defer. But I think I lost that argument already. If someone wants to hold another referendum, we can revisit that; otherwise, let's don't. The best we can hope for at this point is to satisfy everyone except Brews and David, I think. Dicklyon (talk) 02:03, 25 August 2009 (UTC)
I believe this view, to start with an approximate value of c, to be the best compromise I have seen. It avoids sparking incredulity among readers induced by startling claims about defining the metre so that the speed of light cannot be measured in SI units, and allows this topic to be deferred to the section on "Speed of light by definition" where there is room to go into matters.
I see absolutely no necessity for introducing the number 299,792,458 m/s in the lead, and in fact many discussions of speed of light simply use an approximate value, because the main point is that it is fast. See Google search.
I believe the desire to introduce 299,792,458 m/s at bottom stems from a misplaced notion that this number has fundamental significance. The underlying reason for this belief is that it is referred to as an exact value, which confers a mystical awe among some editors. This despite the fact that 299,792,458 m/s is an arbitrary value, exact only because it is defined by committee as a conversion factor between length and time-of-transit. Half a dozen sources have been cited and quoted that say this, and explicitly indicate the lack of any mystique associated with this number. It could just as well be 300, 000, 000 m/s. Brews ohare (talk) 11:32, 25 August 2009 (UTC)
- Congratulations! You just confirmed Dicklyon's phrase "The best we can hope for at this point is to satisfy everyone except Brews and David, I think."
- Now please stop harrassing this talk page? Thank you. - DVdm (talk) 12:09, 25 August 2009 (UTC)
Your input is not relevant to the issues outlined above. You seemingly are unable to distinguish between harassment and discussion. Of course, to participate in discussion one must be able to articulate a point of view, not just rely upon hostility. Brews ohare (talk) 12:27, 25 August 2009 (UTC)
- There is nothing more to discuss here, Brews. We have two definitive sources BIPM and NIST stating: 'The meter is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second', and Note that the effect of this definition is to fix the speed of light in vacuum at exactly 299 792 458 m·s-1.
- This is clearly what must go in the article. If you want to discuss why this decision was made, the science behind it, why you do not agree with it, or your bizarre interpretation of the three sources which repeatedly quote then please do so on my talk page. Martin Hogbin (talk) 16:45, 25 August 2009 (UTC)
Martin; Of course BIPM and NIST say that, but they are talking about the SI system of units within which c refers only to the conversion factor between transit time and length. This is a classic case of a term being used in two senses: the speed of light as a fundamental constant of nature and the speed of light as a conversion factor in the SI units. Please consult Jespersen and Sydenham and Wheeler as I have suggested to you innumerable times without success. They point out that 299 792 458 m/s is an arbitrary value, a conversion factor set by committee decision that is not a fundamental constant of nature. Of course the actual speed of light is a fundamental constant of nature and has a specific value, not an arbitrary one. As Tombe, Brews-ohare and Abstract have said over and over again, you cannot measure this fundamental constant of nature within the SI system of units in units of m/s, because it is a defined value. You could, however, measure it in units of wavelengths per second. Brews ohare (talk) 19:26, 25 August 2009 (UTC)
- Brews the definitive sources are quite clear. I will only continue discussion on my talk page. Martin Hogbin (talk) 21:46, 25 August 2009 (UTC)
First, your definitive sources (NIST and BIPM) are only one source, as both sources simply quote the same 1983 committee determination. Second, this 1983 determination is specified by this source as placing 299 792 458 m/s out of the reach experimental determination as "never to be measured again". Third, the three sources Wheeler, and Sydenham, and Jespersen classify 299 792 458 m/s as arbitrary, a conversion factor and not a fundamental constant of nature. Thus, reference to 299 792 458 m/s is clearly not a reference to the fundamental constant of nature, and yet still is called in the SI units context the speed of light.
Is there any reason to impugn these three sources? I'd say they are at least as definitive as the committee source, and more general as they discuss the speed of light in a broader context than the SI units conversion factor. Brews ohare (talk) 00:34, 26 August 2009 (UTC)
- While I agree with Brews that it would be better to start with the approximate number, I don't agree with his reasoning or logic for why. The present lead says "In physics, the speed of light is a fundamental physical constant, the speed at which light and all electromagnetic radiation travels in vacuum. It is usually denoted by the letter "c". In the International System of Units, the metre is defined so that c has the exact value of 299,792,458 metres per second." which certainly does not suggest to anyone that 299,792,458 is a fundamental constant. And I agree that this number has to be in the article, and almost certainly in the lead; I just don't think that the somewhat distracting fact of the defined value should appear in the first paragraph. Many readers would be better served the way many books do this (not like Brew's sources, but some gentler sources), starting with the approximation that a reader can remember and use. Dicklyon (talk) 03:53, 26 August 2009 (UTC)
- Let me understand this carefully: the speed of light is a fundamental physical constant ... usually denoted by the letter "c". Moreover, in the SI units, "c" has the exact value of 299,792,458 metres per second.
- However, these statements in no way suggest to anyone that 299,792,458 is a fundamental constant. Well tell me about the way this conclusion flows from the WP statements, please. Will the reader who is not a Philadelphia lawyer tease out this conclusion from the premises? Brews ohare (talk) 05:38, 26 August 2009 (UTC)
- So you're saying a speed being a fundamental constant of nature implies that its numerical value in SI units is a fundamental constant? Yes, I can see why that would confuse you, since you think of it differently. Oh, well, we'll have to live that that duality. Dicklyon (talk) 05:44, 26 August 2009 (UTC)
Third paragraph
What does this paragraph add to the lead? It seems to me that it is of passing interest only and should be relegated to the body of the article. Abtract (talk) 20:34, 25 August 2009 (UTC)
- The lead is meant to be a summary of the article. In this case it does seem that the lead has some odd points in it that are not covered in the article. So, I agree in general. The questions are, where should the information now in the lead be put and should we have a very brief summary of it in the lead? Martin Hogbin (talk) 21:52, 25 August 2009 (UTC)
Google summary
A google search for speed of light returns this page (good), but the extract is: "Here, laser light in air is traveling at about 99.97% the speed of light in a vacuum (the refractive index of air being about 1.0003). ..." (bad). I assume because this is the first mention in the article text. I tried moving the picture & caption to after the lead paragraph, but that made the layout worse. Does anyone know how to fix this? EdwardLockhart (talk) 04:24, 26 August 2009 (UTC)
- I don't think that "fixing" the google snippets is among the wikipedia editing guidelines; leave it alone. Dicklyon (talk) 04:29, 26 August 2009 (UTC)
- ^ "Fundamentals of Physics" Halliday, Resnick and Walker. edition 8E 2008
- ^ "Fundamentals of Physics" Halliday, Resnick and Walker. edition 8E 2008
- ^ Uzan, J-P; Leclercq, B (2008). The Natural Laws of the Universe: Understanding Fundamental Constants. Springer. pp. 43–44. ISBN 0387734546.
- ^ Greene, G (2003). The Elegant Universe. WW Norton & Co. pp. 55–56. ISBN 0393058581.
- ^ Davies, PCW (1979). The Forces of Nature. Cambridge University Press. pp. 127–128. ISBN 052122523X.
- ^ Duke, PJ (2000). "Electromagnetic waves in free space – no electric charges or currents". Synchrotron Radiation: Production and Properties. Oxford University Press. p. 53. ISBN 0198517580.
- ^
Schwinger, JS (2002) [1986]. "Gravitational waves". Einstein's Legacy: The Unity of Space and Time (Reprint ed.). Courier Dover. p. 223. ISBN 0486419746.
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