Talk:Speed of light/Archive 16

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Speculative interpretation of a religious text, not a scientific result at all.

Wouldn't [1] have helped readers realize the mention of Speed of Sun-light being clearly present accurately in the 1300's? Again I wanted to know why one cannot cite quotations from the religious texts?Ganesh J. Acharya (talk) 05:39, 21 February 2011 (UTC)

I had cited the following

Sayan. a (c. 1315-1387) while commenting on Rigveda stated "tatha ca smaryate yojananam. sahasre dve dve sate dve ca yojane ekena nimis.ardhena kramaman. a namo stu ta iti" which when translated is [O Sun,] bow to you, you who traverse 2,202 yojanas in half a nimesa", this speed turns out to be 186,413.22 miles per second which being near to the speed of light, is speculated to be speed of light it self. Src: http://www.ece.lsu.edu/kak/sayana.pdf, (accessed 15 Feb 2011), the referenced information is accessed from Indian Journal of History of Science, vol. 33, 1998, pp. 31-3 Ganesh J. Acharya (talk) 05:41, 21 February 2011 (UTC)

That sources doesn't appear to me to be claiming that they knew or estimated the speed of light, just that there was a numerological coincidence and some astronomy traditions that were lost and worth investigating. Dicklyon (talk) 06:40, 21 February 2011 (UTC)

Yes, out of who knows how many big numbers in historic texts it's one that can be made close to the speed of light. But it's not an estimate or calculation of the speed of light, and making that connection is pure speculation by the writers of that paper, relying on unconvincing assumptions about the precise sizes of the units involved.--JohnBlackburne^words_deeds 10:44, 21 February 2011 (UTC)

ok, again the reference is talking about "speed of light" and may be they are speculating. But then as per what guidelines of wikipedia it should not be included. Ganesh J. Acharya (talk) 06:11, 22 February 2011 (UTC)

The reference is actually arguing that Sayana is in all likelihood not referring to the speed of light , but to the speed of the Sun as it transverses the sky. This makes the interpretation of this text off-topic in this article. More so, since, whatever what was meant, this phrase seems to have had no influence on the historical development of the subject. Inserting a phrase like "some scholars have suggested that some phrase in the Sarayana may be interpreted as giving a value of the speed of light" would give undue weight to a singular interpretation of a religious text.TR 08:57, 22 February 2011 (UTC)

I agree, this text is not appropriate here. Martin Hogbin (talk) 09:24, 22 February 2011 (UTC)

• Is sun-light part of sun? even if it is written sun in travelling, is it wrong?
• Currently we could have a preconceived mindset that sun and its light are different. Why do we see them different?
• If I blow a balloon, does the balloon travel while it expands, or it does not travel?Ganesh J. Acharya (talk) 12:59, 22 February 2011 (UTC)

I don't see how the speed at which the sun transverses the sky, and the spee butd of the light travelling from the sun to us could be seen as the same thing, especially since the two are perpendicular. Nor do I see how it is relevant to discuss that on this page, since it has nothing to do with improving the "speed of light" article.TR 13:08, 22 February 2011 (UTC)

even in case of balloon if I reduce my size to that of an atom I will see balloon's sub atomic particles and space. But we still call these particles to be part of the expanding balloon and say the balloon is travelling?Ganesh J. Acharya (talk) 04:12, 25 February 2011 (UTC)

Only a fool would call such a discovery by hindus as a co-incidence!!How can such a stuff be something irrelevant to this topic??sun means light,you guys are so ignorant about Hinduism and everything and think that you know everything.I strongly recommend to include this in the topic. — Preceding unsigned comment added by Shaardu (talk • contribs) 13:14, 11 June 2011 (UTC)

What wiki guidelines say? Its very apparent that the speed of light (i.e. part of sun) was accurately declared by Sayan. Ganesh J. Acharya (talk) 03:03, 7 July 2011 (UTC)

Again, it becomes a valid inclusion in the article sun in any case. Ganesh J. Acharya (talk) 04:06, 7 July 2011 (UTC)

Dear Mr. Ganesh J. Acharya, Here is my thought, if you blow air into the balloon, it expands of course. If you blow balloon with air, you are providing potential energy. The air inside obviously has high pressure than the air outside. So the potential energy is stored as difference in air pressure (you can imagine so). And when you release the air, the pressure difference makes the air to blow out, which is nothing but the conservation of energy (potential energy to kinetic energy of air molecules). My argument is that, the balloon has expanded and retained to its original position, so that the displacement (I'm considering vectorial quantity) in regard with time is zero. Coming to Sun, the Sun is not travelling at 186,413.22 miles per second as you have mentioned. This would be against relativity. May be the Rig-Vedic people didn't know relativity so didn't mention its sunlight. Sunlight is just an EM radiation over the visible range. You can say that they are just emitted by the sun's chemical reactions but not a part of it. Shriram (talk) 07:48, 7 July 2011 (UTC)

Various guidelines apply here, including WP:UNDUE, Wikipedia:Exceptional claims, WP:SECONDARY. To include this claim we would need an exceptional secondary reliable source. That: a) confirms this claim. b) puts this claim in a proper historical context. TR 08:58, 7 July 2011 (UTC)

In those days or for that matter even today Sun is seen as a living soul. If you start illuminating by yourself like Sun and your parts (rays emitted) move at 186,413.22 miles per second which a Risi sees and admires with the above words would that be wrong?Ganesh J. Acharya (talk) 03:06, 10 July 2011 (UTC)

Irrelevant.TR 10:22, 10 July 2011 (UTC)

Ganesh, you are perfectly entitled to your religious beliefs but they have no place in a scientific article. If you want to add this interpretation of a religious text to Wikipedia, I suggest that you look for a more appropriate article. Martin Hogbin (talk) 10:53, 10 July 2011 (UTC)

Rounded speeds

What is the point in having speeds rounded to 1 significant figure? Surely it would be better to have them to at least 3 or 4? I tried doing this but my edits were reverted on the grounds that it was intentional. Xtremerandomness (talk) 23:17, 22 February 2011 (UTC)

The article gives both extremely accurate figures and, in the "Approximate values" section, rounded ones that will be convenient and sufficient for many readers and casual purposes. Hertz1888 (talk) 23:31, 22 February 2011 (UTC)

People are perfectly capable of rounding figures to one or two significant figures in their heads. It's far easier for the majority to do that than to make the few looking for a more accurate figure have to delve through the article at large. Xtremerandomness (talk) 23:33, 22 February 2011 (UTC)

In my experience, that's not so true. Most people who have grown up since the demise of slide rules are relatively clueless about significant figures, and get distracted by digit values as if they mean something more than they do. The conventional rounded values are what we should be using in contexts where simple info is appropriate; the more-exact or exact numbers have their place, too. Dicklyon (talk) 04:23, 23 February 2011 (UTC)

I would disagree with you on that one - almost all people understand that these values are best rounded to 1 or 2 significant figures in most instances, even if they wouldn't know what a "significant figure" was if it was mentioned in conversation. Why not include 'metres per second' in the 'approximate values' section for the ease of others too? It is inconsistent not to do so. This is on top of the fact that it is not fair to patronise the community, presuming that they are unable to work out for themselves what is useful, whilst telling those that do need the more accurate information that they have to search through the article. I couldn't find them all in one place, so for some it would be quite a pain for them to have to do so. Xtremerandomness (talk) 14:06, 23 February 2011 (UTC)

299,792,458 is 300 million when rounded to three significant figures. --137.43.105.17 (talk) —Preceding undated comment added 14:17, 23 February 2011 (UTC).

Common misunderstanding should be cleared up in the article

I believe that it's a common and misguiding understanding that one light year is one year's worth of travel. Won't a paragraph or footnote emphasizing the perspective - ie to the traveler, it's teleportation as no time passes, to the viewer, it's one year? I know if I was just explained this at a young age I would have understood it and not had to wonder about it for the best part of 10 years. —Preceding unsigned comment added by 196.210.241.222 (talk) 22:28, 18 March 2011 (UTC)

It's just a distance, not a year's worth of travel, but how far light goes in a year. Like any other distance, if you could travel it at the speed of light, no time would pass for you. Dicklyon (talk) 06:14, 19 March 2011 (UTC)

I think WP should explain about this kind of thing but I am not sure whether this is the best place to do it. There is a fundamental problem with explaining things like relativity in the non-technical manner preferred by WP and this is that ordinary language is not able to adequately describe the subject. Intuitive understandings of terms like 'time' and 'distance' are based on our everyday, non-relativistic experience and are bound to be misleading. That is why we have to talk in technical terms like 'proper length' and 'coordinate time' if we want to discuss the subject in any detail. Martin Hogbin (talk) 10:03, 19 March 2011 (UTC)

It's definitely not the most obvious place to reference this; "Light year" does however references a unit of time "Year". Sure, clarify that it's a distance; I certainly find it notable that "year" has altered significance depending on the perpspective, being a fundamental property of light, seldomly explained simply...

change to c?

"Speed of light" is a bit misleading, as it makes it seem like light dictates everything else in the universe. c isn't really the "speed of light", it's just the maximum speed at which anything can travel in our universe. Light doesn't always travel at c, and not everything that can travel at c is light. I think this should at least be made clear in the article. Serendi^podous 09:03, 28 March 2011 (UTC)

From your question I infer that you think that the article does not make this clear?TR 11:11, 28 March 2011 (UTC)

No I don't think it does. Certainly not the first few paragraphs, which place the emphasis onto the particles, rather than the structure of the universe, which is what c is really about. Serendi^podous 11:47, 28 March 2011 (UTC)

But c is not really about the structure of the universe either. From a relativity perspective, c is just a gauge freedom of the metric with it's only physical significance being the fact that it has a finite value (rather than being zero or infinite).TR 13:37, 28 March 2011 (UTC)

[quibble] “not everything that can travel at c is light”, well, last time I checked the only massless particles were photons and gluons, and it's not like gluons by themselves can “travel” at all. :-) Gravitation too, of course. Shouldn't comment when I'm not fully awake yet.[/quibble] Anyway, I think I once proposed something along the lines of “this constant was named "speed of light" because it is the speed at which electromagnetic radiation travels in vacuum, but it has since been shown to have a much broader significance” (well, not with such a lousy wording), but I can't remember exactly what became of that. --A. di M. (talk) 11:55, 28 March 2011 (UTC) (amended at 12:00, 28 March 2011 (UTC))

The lead makes this pretty clear to me.

The speed of light, usually denoted by c, is a physical constant important in many areas of physics. Light and all other forms of electromagnetic radiation travel at this speed in empty space (vacuum), regardless of the motion of the source or the inertial frame of reference of the observer. Its value is exactly 299,792,458 metres per second (approximately 186,282 miles per second). In the theory of relativity, c interrelates space and time, and appears in the famous equation of mass–energy equivalence E = mc². It is the speed of all massless particles and associated fields in vacuum, and it is predicted by the current theory to be the speed of gravity (that is, gravitational waves) and an upper bound on the speed at which energy, matter, and information can travel.

No? Headbomb {talk / contribs / physics / books} 12:48, 28 March 2011 (UTC)

Maybe rearranging the sentences could “solve” the OP's problem, like this:

The speed of light, usually denoted by c, is a physical constant important in many areas of physics. Its value is exactly 299,792,458 metres per second (approximately 186,282 miles per second). It is the speed of all massless particles and associated fields in vacuum, and it is predicted by the current theory to be the speed of gravity (that is, gravitational waves) and an upper bound on the speed at which energy, matter, and information can travel. Light and all other forms of electromagnetic radiation travel at this speed in empty space (vacuum), regardless of the motion of the source or the inertial frame of reference of the observer. In the theory of relativity, c interrelates space and time, and appears in the famous equation of mass–energy equivalence E = mc².

(Or some other permutation thereof.) --A. di M. (talk) 13:17, 28 March 2011 (UTC)

That is certainly much better, though I would reemphasise the first line:

The speed of light, usually denoted by c, is a physical constant important in many areas of physics. Its value is exactly 299,792,458 metres per second (approximately 186,282 miles per second). It is the maximum speed at which all energy, matter, and information in the universe can travel. It is the speed not only of light but of all massless particles and associated fields in vacuum, and it is predicted by the current theory to be the speed of gravity (that is, gravitational waves). Such particles travel at c regardless of the motion of the source or the inertial frame of reference of the observer. In the theory of relativity, c interrelates space and time, and appears in the famous equation of mass–energy equivalence E = mc².

Serendi^podous 13:28, 28 March 2011 (UTC)

That would work for me (although some others may disagree, god knows how much we have discussed different perturbations of that paragraph.) Minor quibble, the start of the sentence "It is the speed not only of light but of" suggests that the reader should already know that light travels at that speed, which is not particular good form for an encyclopedia. TR 13:46, 28 March 2011 (UTC)

Well the article is called "Speed of light"; the reader pretty much goes into the text with that idea in mind. Serendi^podous 13:59, 28 March 2011 (UTC)

I agree with TR; that bothers me as well: it's not like light has already been mentioned (except in the phrase “speed of light” itself). How about “It is the speed of all massless particles and associated fields – including electromagnetic radiation such as light – in vacuum, and it is ...”? --A. di M. (talk) 14:44, 28 March 2011 (UTC)

OK, that works. Serendi^podous 15:09, 28 March 2011 (UTC)

Right hand side info bar needs fix

There's a problem considering the info bar at the top right corner. The numbers should not be represented using commas, as they are wrong when mathematically expressed. Spaces should be used instead. Even though they are correct in at least USA, they are incorrect in many, many other countries. This causes misunderstanding and qualifies as a reason to change them. Many people tend to read the English page of a Wikipedia article, too, for better and more information even though their English and their understanding of English text wouldn't be so good (Small, trivial things, like using commas, like this article does.). It's not nice when you, for example, lose points for your presentation because you copy pasted one small part of the text and it caused misunderstanding. That's a bad example, I know, but it's still one reason, and who hasn't copy pasted from Wikipedia? Was it one small line or huge section... FinFihlman (talk) 14:10, 14 April 2011 (UTC)FinFihlman

Commas are overwhelmingly the most common character used for this in English, and this is the English Wikipedia. There are non-native speakers around, but this doesn't mean we shouldn't be using e.g. actual to mean ‘real’ because the cognate words in many other languages mean ‘current, present’, does it? See WP:MOSNUM#Delimiting (grouping of digits). ― A. di M.​_plé​^dréachtaí 23:40, 14 April 2011 (UTC)

File:Sun to Earth.JPG caption revert

I have reverted the change by 67.161.54.63 which added the following to the main image caption: "Moonlight takes about 1.17 seconds (average distance) to reflect from the Moon to the Earth." While that may be true, the addition, which brings it from one line to three, adds little to the understanding of the image which wasn't already present. Without a distance from the Earth to the Moon, it is less informative than what was already there and appears to violate the succinctness portion of WP:CAPTION. Furthermore, there is already an image displayed further down showing how long it takes moonlight to reach Earth. It was almost immediately undone by the user who added it. Are there any objections to reverting it back to its original form? ScottSteiner ✍ 09:10, 16 April 2011 (UTC)

I agree with all of that and have restored it. It would be misplaced anywhere, especially so in the lead/infobox image.--JohnBlackburne^words_deeds 09:44, 16 April 2011 (UTC)

Scott, would adding (to the image) the Earth - Moon distance be productive? -161

If the image included that distance, maybe. But the new image and the caption still wouldn't add anything to the understanding of the article. The current image/caption combo that is there is concise and informative. Just adding one more calculation/distance/time duration wouldn't really improve that. ScottSteiner ✍ 10:22, 16 April 2011 (UTC)

Removal of "in vacuo" in lede

Are we really proposing that the speed of light is constant regardless of the medium it is travelling through. Those first couple of sentences conflate the speed of light and c but it seems clear that it is the speed of light rather than the constant that is the subject there. Crispmuncher (talk) 00:25, 11 May 2011 (UTC).

The physical constant c, called the speed of light, is by definition exactly 299,792,458 m/s. The speed at which light propagates in a medium is less than c. The article is correct, with the above information given in more detail in the first two paragraphs.--JohnBlackburne^words_deeds 00:33, 11 May 2011 (UTC)

I understand the distinction, but it seems clear to me that the subject of the first sentence (and therefore by extension the second, which refers to "it") is the speed of light, not c. Therefore the "in vacuo" clarification is necessary. We shouldn't be talking about it as a fundamental constant without clarification either, for that matter. Crispmuncher (talk) 01:01, 11 May 2011 (UTC).

The first sentence reads "The speed of light, usually denoted by c, is a physical constant important in many areas of physics.". I'm not sure how you can come to the conclusion that it is not about the physical constant c, called the speed of light.TR 05:49, 11 May 2011 (UTC)

Because if for the sake of clarity we remove the subclause (which should not affect wider sentence structure) the sentence starts "The speed of light is..." How then, can it not be about the speed of light? Crispmuncher (talk) 15:05, 11 May 2011 (UTC)

Even without the subclause, it is clear that it is about the physical constant called "the speed of light". (Cause that is pretty much literally what the first sentences says.) So, yes it is about "the speed of light". Yet, it is not about the speed at which light propagates, as is clarified somewhat further in the opening paragraph.TR 15:20, 11 May 2011 (UTC)

Personally, I'd much rather have a clarification early on than until the fourth sentence. c is the speed of light, in vacuum, and that is a very important fact (whoever wrote in vacuo needs to be trouted). Also, currently, the second sentence reads like the speed of light is c regardless of the medium, which is simply false. I'd clarify it as soon as possible, preferably in the first sentence. Something like

The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is exactly 299,792,458 metres per second (approximately 186,282 miles per second). It is the maximum speed at which all energy, matter, and information in the universe can travel. It is the speed of all massless particles and associated fields—such as electromagnetic radiation such as light—in vacuum, and it is predicted by the current theory to be the speed of gravity (that is, gravitational waves).

Headbomb {talk / contribs / physics / books} 16:09, 11 May 2011 (UTC)

I agree that this is an improvement. Count Iblis (talk) 16:30, 11 May 2011 (UTC)

Me too. I was WP:BOLD and changed it, since it doesn't hurt, and we can save the fine distinction that c is not only "the speed of light in vacuum" anymore, though is equal to it, for later. Also, I thought it was necessary to use SOME of the excerpted ref in the lede to remove the immediate question which otherwise arrises of why the speed of light is EXACTLY some value in m/sec. Answer: because we fiddle with the metre to require that it be so, having picked and fixed our arbitrary standard of time (which is easier to do, since there are so many good frequency standards out there for various processes). SBHarris 16:37, 11 May 2011 (UTC)

why light slows in a medium

I think this article misses the important point of why the speed of light is lower in a medium, or, how light travels in a medium. —Preceding unsigned comment added by 84.120.147.185 (talk) 07:54, 23 May 2011 (UTC)

The propagation of light is not the subject of this article. The subject of this article is the constant c, the only relevant point is that light does not always travel at that speed, even though it is called the "speed of light". Why that is so, is a topic for another article, such as light or maybe refractive index.TR 09:09, 23 May 2011 (UTC)

Er... There already is a section Speed of light#In a medium. ― A. di M.​_plé​^dréachtaí 10:27, 23 May 2011 (UTC)

Superluminosity of c?

I was thinking about today's OPERA findings (disclaimer: IANAPP, I Am Not A Particle Physicist) and its possible implications, if the experiment is confirmed valid. Now if indeed a neutrino traveled at velocity greater than lightspeed, my understanding is that should result in a paradox (or a contradiction) within special relativity. Now special relativity is so fundamental, that something has to be broken. What if it is just that c (the universal top speed) is different from that of speed of light in a vacuum by an amount detected in this experiment (0.002%), and there is a hypothetical "universal web of aether" slowing down all interactions, except neutrinos (who have very few interactions anyway). Causality, which I find soothing, in special relativity would not be broken, but it would hold that c > v_light. The true value for c given by this experiment would be in the neighborhood of 299,799,922 m/s. Could we have missed such tiny inaccuracy, or have there been experiments (eg. E=mc² experiments) that should have detected this anomaly with c = 299,792,458 m/s? If not, could such experiment be devised? --hydrox (talk) 18:18, 23 September 2011 (UTC)

The first thing I thought when I heard that was that there must have been some kind of glitch with the measurement, and a part of me still hopes/believes that, but apparently people have known about this and trying to figure out what was wrong for years. On Monday I'm attending a talk about that by someone who has worked in that experiment, so I'll see what they think might be going on. If such a result was confirmed (e.g. a different experiment got similar results), it would imply... I dunno what, but it would have to be some really crazy stuff.
― A. di M.​_plé​^dréachtaí 19:18, 23 September 2011 (UTC)

There really is no point in speculating about this. It should be evident to anyone reading this article that a confirmed case of matter or energy travelling faster than c would have profound implications. At the moment we do not have that, we have a news story that is extremely relevant to this article and should therefore, in my opinion, be mentioned in this article in the relevant section. Beyond that we must wait and see. There have been similar claims before that have turned out to be errors. Martin Hogbin (talk) 08:36, 24 September 2011 (UTC)

Indeed. Even the authors themselves say, “Despite the large significance of the measurement reported here and the stability of the analysis, the potentially great impact of the result motivates the continuation of our studies in order to investigate possible still unknown systematic effects that could explain the observed anomaly. We deliberately do not attempt any theoretical or phenomenological interpretation of the results.” Hey, I might quote that in the article.
― A. di M.​_plé​^dréachtaí 11:32, 24 September 2011 (UTC)

IANAPP either, but the report claims Sigma 6. I think this is extraordinarily significant, so that it is scientifically accepted to regard it as a discovery, not a signal deviation. The report is signed by 174 scientists, so they either risk their reputation en masse, or have valid reasons for their claim. Still it might be some as yet undiscovered systematic error, so let's wait and see... Rursus dixit. (^mbork³!) 09:17, 24 September 2011 (UTC)

Systematic error isn't that unlikely, the way they needed to correctly calculate the time of flight using different clocks at different locations is extremely difficult to get right. Also, this is the first experiment in which this is done to such an accuracy, so there is no well established routine that they could adopt knowing that it would give the desired result. Count Iblis (talk) 18:33, 24 September 2011 (UTC)

Replying to my above hypothesis that c > v_light. I calculated the cumulative error due to an incorrect value of c in GPS. The oldest GPS satellite currently in use is Navstar 2A-01, launched about 21 years ago. If GPS receivers were calculating the clock drift aboard this satellite with an incorrect value of c (0.99998c), it would result in an error of about 14,746 ns in 21 years, corresponding to a positioning error of over 4.4km. More recent satellites, like those launched in 2009, would still be off by at least 0.4km. So GPS seems to prove that Einstein's c has to be equal to the speed of light in a vacuum to a relative accuracy far below 0.002%. --hydrox (talk) 00:03, 25 September 2011 (UTC)

Just a thought

The 'neutrino' news has prompted me to wonder if we should have something like this in the article.

Because of its fundamental significance to physics, the speed of light continues to be a subject of speculation and investigation. From time to time, reputable laboratories have published results which have initially appeared to show energy, matter, or information travelling faster than light. Because of the significance that these results would have if confirmed they are often widely reported in the media. To date no such observations have been confirmed, and in cases where investigations have been complete the results have been found to be due to errors or misunderstandings. For these reasons, media claims of superluminal effects should be treated with caution until confirmed by the scientific community Martin Hogbin (talk) 09:20, 24 September 2011 (UTC)

I don't think this could fit in the article. After all, it boils down to a warning about media claims, so the essence would be off-topic in the article. I think such a warning could at best belong on top of the talk page, but I'm not sure. O.t.o.h. we already have a tag something like {{recentism}}:

DVdm (talk) 09:40, 24 September 2011 (UTC)

I realise that the "upper limit on speeds" section has to be greatly simplified for people like me to understand it - but in view of the Opera neutrino speed experiment, I have to ask the following question: given that the measured speed of the neutrinos is only a little faster than the speed of light (around 1000 Km/s IIRC), is it not possible that light travels a little less quickly than the physical maximum speed limit? New Thought (talk) 11:54, 25 September 2011 (UTC)

Yes that is possible, along with many other things but it is not our job to speculate here. Please read the section that I have now put in bold above. If and when there is a confirmed result, scientists will start to propose theories to explain it. Martin Hogbin (talk) 12:16, 25 September 2011 (UTC)

FWIW (& perhaps somewhat related) - Harvard Physicist Lisa Randall Seems To Have Presented A Worthy Perspective On All This - Enjoy! :) Drbogdan (talk) 16:10, 25 September 2011 (UTC)

I also have the feeling that the new speed is just a "correction" of the 1983 standard, that due to the age and the possibly gravitational mistakes, was just less accurate that the today neutrino measures... just my opinion. — Preceding unsigned comment added by 95.236.123.84 (talk) 18:02, 25 September 2011 (UTC)

• also, read here : http://arxiv.org/ftp/arxiv/papers/1109/1109.4897.pdf

they are really talkin about 20 cm ???? are they kidding ???? it looks an enourmous distance...for me. bah. — Preceding unsigned comment added by 95.236.123.84 (talk) 20:20, 25 September 2011 (UTC)

Incorrect statement

I have deleted the words in bold from this quote from the article because they are clearly wrong. The speed of sound waves is independent of the motion of the source but not that of the observer relative to the medium. "The speed at which light waves (or any wave for that matter) propagates in a vacuum (or otherwise) is independent both of the motion of the wave source and of the inertial frame of reference of the observer. This is a property of waves, be they electromagnetic (light waves or radio waves) or even mechanical (sound waves)." Martin Hogbin (talk) 17:16, 12 April 2011 (UTC)

I'M SURE THERE IS AN ERROR ON THE LEFT WITH YOUR GROUP AND PHASE VELOCITY ANIMATED GRAPHIC. THE SPEED OF THE ENVELOPE IS THE GROUP VELOCITY NOT THE PHASE. — Preceding unsigned comment added by 188.28.239.158 (talk) 10:09, 26 September 2011 (UTC)

What error? The green dot is anchored to the maximum of the envelope and labelled as the group velocity, and the blue dot is anchored to a point where φ = π/2 and labelled as the phase velocity.
― A. di M.​_plé​^dréachtaí 19:30, 26 September 2011 (UTC)

OPERA experiment image

Result of the OPERA experiment

The result of OPERA experiment: The left plot shows δt as a function of the energy for νμ CC internal events, where δt (low-energy: 13.9 GeV) = (53.1 ± 18.8 (stat.) ± 7.4 (sys.)) ns and δt (high-energy: 42.9 GeV) = (67.1 ± 18.2 (stat.) ± 7.4 (sys.)) ns. The right plot shows the global result of the analysis including both internal and external events (for the latter the energy cannot be measured), where δt (average-energy: 17 GeV) = (60.7 ± 6.9 (stat.) ± 7.4 (sys.)) ns.

I just removed the image (at right as removed), for the second time so am mentioning it here. As an image it very unclear with the text too small and unexplained or unlabelled features. But on inspection it's not really a graph or plot of anything: it just presents the results, which are given in the article already. Because it is so confusing the image does need a lengthy explanation to make sense of it, but much too much for the caption; and there's already a paragraph on the experiment with the results in the article, another is not needed. Images are meant to illustrate the topic, not need lengthy explanations of their own, especially when they are little to do with the topic.--JohnBlackburne^words_deeds 16:41, 29 September 2011 (UTC)

Not to mention that there is no proof that it actually is public domain, and therefore probably a copyright violation.TR 18:05, 29 September 2011 (UTC)

Hi Timothy, I've updated the referred license of the image included in the paper being hosted on ArXiv to WP:CC-BY-SA.

Visuall (talk) 19:48, 29 September 2011 (UTC)

At the very least, this article should mention the experiment. Currently the word OPERA is not mentioned anywhere in this article. --_{Piotr Konieczny aka Prokonsul Piotrus| talk to me} 20:54, 29 September 2011 (UTC)

Yes it is, at the end of the Faster-than-light observations and experiments section.--JohnBlackburne^words_deeds 20:57, 29 September 2011 (UTC)

Hi John,

The rm comment 1: "rm very unclear image: needs a lengthy explanation in text

1. Just as you stated I added a long description to the image although the paragraph explains it clearly.

"but this is not the article for that"

2. But the whole large section is.

The rm comment 2: "images is still unclear and confusing: caption is far too long"

3. What should I do ? Restore the image description to my previous version, and then moved the image description to the paragraph ?

4. If just the image text is a little smaller, why not adjust its size ?

5. I don't see a reason for removing only (but not updated).

"and doesn't relate the image to the speed of light"

6. The fact is, the image is highly relevant to the large section "Faster-than-light observations and experiments".

The talk page:

"As an image it very unclear with the text too small"

7. As stated in issue 3, why not adjust the image size ?

"and unexplained or unlabelled features."

8. What did you mean by "unexplained or unlabelled features" ?

"But on inspection it's not really a graph or plot of anything: it just presents the results

9. But it includes some facts the paragraph doesn't mention.

"which are given in the article already."

10. The fact is, both the energy value and the whole left plot of the image are unmentioned in the article.

"Because it is so confusing the image does need a lengthy explanation to make sense of it, but much too much for the caption"

11. A "lengthy explanation" was written in the paragraph, excepted for "the energy value and the left plot of the image" mentioned above, maybe previous simple description is enough ? Otherwise adds some concise text to the paragraph ?

"there's already a paragraph on the experiment with the results in the article, another is not needed."

12. First, a picture is always more friendly than text, it's very useful to any readers. Again, "But it includes some facts the paragraph doesn't mention".

"Images are meant to illustrate the topic, not need lengthy explanations of their own,"

13. The image + previous concise description explains the whole paragraph via a more friendly way, and also refer to issue 6.

"especially when they are little to do with the topic."

14. Refer to the fact stated in issue 6.

Thanks,

Visuall (talk) 21:33, 29 September 2011 (UTC)

Briefly: we already have details of the experiment, with the results, and links to where it is covered in much more detail. As all the diagram does is display the results it doesn't add anything: even if fixed and much clearer I don't see the point of it, as the results are already in the text. But as I've already made my views clear perhaps someone else could provide a third opinion on the content issue?--JohnBlackburne^words_deeds 21:46, 29 September 2011 (UTC)

Speed of light broken?

Might be worthy of note. http://www.bbc.co.uk/news/science-environment-15017484 --92.30.2.203 (talk) 19:16, 22 September 2011 (UTC)

Definitely worth mentioning, but with a caution that this needs more research. Pine^talk 21:35, 22 September 2011 (UTC)

Wait until it's properly sourced; a reliable scientific source. There's far less to it than some of the more excited headlines out there.--JohnBlackburne^words_deeds 21:40, 22 September 2011 (UTC)

John, you know not what you speak of. Return to your chalkboard. LaRouxEMP (talk) 21:42, 22 September 2011 (UTC)

Just read the articles, not just the headlines. From the BBC article

"we are not claiming things, we want just to be helped by the community in understanding our crazy result - because it is crazy".

I.e. the scientists know it is wrong to jump to conclusions. It's not a published result, never mind peer reviewed. It's the press trying to create headlines.--JohnBlackburne^words_deeds 22:22, 22 September 2011 (UTC)

There’s also this report, which gives an exact figure of 300,006 kps. But it then gets into extremely murky waters when it describes this figure as "6 kps faster than the speed of light". They’ve taken the exact known speed 299,793.458, rounded it up to 300,000, then subtracted this from 300,006, to arrive at a 6 kps difference. As mathematicians, they’d make good journalists. It’s impossible to tell from this article whether the newly measured speed is 299,793 + 6 = 299,799, or whether the 300,006 speed is correct, in which case the difference is not 6 kps but 300,006 – 299,793 = 213 kps. So much for quality science reporting. -- Jack of Oz ^{[your turn]} 23:04, 22 September 2011 (UTC)

A pretty good summary from Ars reports the 'news', gives various reasons to be sceptical, and says the results won't be made public until tomorrow when it will be easier to see what the fuss is about.--JohnBlackburne^words_deeds 23:18, 22 September 2011 (UTC)

There is also this experiment Minos (http://arxiv.org/pdf/0706.0437v3) which seems to suggest neutrinos can go faster than light, by roughly the same amount as in the CERN Opera experiment. Maybe empty space is not really transparent to light, but slows it down slightly so it cannot go the true speed limit. And I guess neutrinos aren't slowed in the same way? Doubledork (talk) 00:08, 23 September 2011 (UTC)

That's actually be theorised for quite some time; indeed, it is the basis of the theory underlying the Scharnhorst effect. Crispmuncher (talk) 00:18, 23 September 2011 (UTC)

Of course we do - Scharnhorst effect. hydnjo (talk) 03:32, 23 September 2011 (UTC)

I have replaced the BBC-ref with the arxiv source. DVdm (talk) 05:30, 23 September 2011 (UTC)

By the way, fwiw, see "Why CERN’s claims for faster-than-light neutrinos is wrong." DVdm (talk) 11:30, 23 September 2011 (UTC)

I suspect he may soon be very embarrassed he didn't double check that in his rush to get it out. I'm an electronics engineer, not a theoretical physicist, and it took me no more than five minutes to dismiss it. His whole argument is premised on the difficulty knowing exactly when the neutrinos are actually emitted, and he contends that the error is much greater than that used in the Opera paper. However, while he devotes his entire paper to the 500kHz coarse structure he completely neglects to account for the 200MHz fine structure. Opera are using a much more refined model of the neutrino emission than he is: that is why they can claim greater precision than he suggests.

That doesn't prove the Operas findings of course, but it does disprove Costella's assertions. Crispmuncher (talk) 16:27, 23 September 2011 (UTC).

He himself retracted it. Now it's titled “Why OPERA’s claim for faster-than-light neutrinos is not wrong”. :-)
― A. di M.​_plé​^dréachtaí 21:10, 26 September 2011 (UTC)

(Though I'm surprised that now he's that confident that the experiment is right – more than almost all physicists I've talked to.)
― A. di M.​_plé​^dréachtaí 00:40, 28 September 2011 (UTC)

I think the current article has the balance about right. We should mention the news that a reputable laboratory has results that suggest FTL travel but is should only be in the FTL section, not the lead. We should not add any more detail or discussion until the result is confirmed, if that happens. Martin Hogbin (talk) 17:18, 23 September 2011 (UTC)

Would anyone object to the removal of the following statement from the article:

One possibility is that the scientists overlooked that neutrinos travel through the Earth and not around the Earth’s circumference; taking such a "shortcut" shortens the travel time between CERN and LNGS by just about the right amount.

It's unreferenced and is pretty much WP:OR (the original citation was to a blog, and while it's an interesting observation, it doesn't reach the level of a reliable source or a notable explanation for the results). 107.10.43.91 (talk) 04:01, 25 September 2011 (UTC)

I agree. The Italians would have to be pretty stupid not to have considered the fact that neutrinos travel in straight lines thru the Earth. I also some speculation about extra dimensions, especially the part that it is the "current suggestion". Roger (talk) 05:56, 25 September 2011 (UTC)

Agreed. In fact, the experiment was actually done by firing the neutrinos THROUGH the Earth. There's no overlooking here. — Preceding unsigned comment added by 220.255.1.69 (talk) 08:30, 25 September 2011 (UTC)

I’m more a social scientist than a physical scientist and I was concerned that the way neutrinos travel through the Earth could be overlooked from what I understand about human thinking. Most fundamental particles that scientists study can’t pass through the Earth and that is the type of thing that can be overlooked, I overlooked it till I read the blog. I was worried that scientists from other organisations seeking confirmation might overlook that as well and scientists might waste research time studying something that isn’t happening. I don’t know if neutrinos are travelling faster than light or not, I know scientists should ask CERN if they took into account that neutrinos travel through the Earth. Incidentally the Schlafly blog Conservapedia isn’t highly regarded over relativity, see Conservapedian relativity. Proxima Centauri (talk) 09:26, 25 September 2011 (UTC)

They may have overlooked something, but they definitely didn't overlook that. Also, I guess the relative difference in length between a straight line and a great circle from CERN to LNGS is much more than 2.5×10⁻⁵.
― A. di M.​_plé​^dréachtaí 10:11, 25 September 2011 (UTC)

I get an arc to chord ratio of 1.0005466, given a distance between emitter and detector of 730 km, as the archivx article reports, and earth radius of 6378.1 km, i.e. the great circle distance would be 730.399 km.--agr (talk) 23:59, 26 September 2011 (UTC)

Here are some comments on the subject from scientists quoted in Scientific American. All are skeptical. http://www.scientificamerican.com/article.cfm?id=ftl-neutrinos Pine^talk 19:11, 30 September 2011 (UTC)

"In 1905, Albert Einstein postulated that the speed of light in vacuum was independent of the source or inertial frame of reference,"

Wasn't Maxwell first to postulate that? — Preceding unsigned comment added by 193.62.111.31 (talk) 14:32, 26 September 2011 (UTC)

No, just like in case of Einstein and Dirac, the equation Maxwell discovered was smarter than himself. Count Iblis (talk) 14:39, 26 September 2011 (UTC)

You have to be careful with terms like "postulate" since they are frequently misinterpreted. A postulate is not a conclusion nor even a hypothesis. A postulate is a precondition. This appeared to be the case even before Einstein started work on SR and for the purposes of developing the theory he accepted it without question. This is the starting point for SR, not a conclusion of it. Crispmuncher (talk) 15:13, 26 September 2011 (UTC).

A sense of "postulate" is "propose the existence of" and that seems to be the sense used here. And it was Maxwell that first postulated the constant speed of light (in 1898 I believe), which is necessarily independent of inertial frame of reference. — Preceding unsigned comment added by 193.62.111.31 (talk) 15:50, 26 September 2011 (UTC)

Maxwell died in 1879. Yes, he said that the speed of light and other electromagnetic waves was independent of the source, and that had been common knowledge for a long time when Einstein said it in 1905. The current text is misleading and should be fixed. Roger (talk) 17:14, 26 September 2011 (UTC)

Put Maxwell was hardly the first to propose this. The idea that light should travel at a speed independent of the source is inherent to any wave approach to light and thereby goes back at least as far as Huygens and probably further. For this reason the sentence does not claim that he was the first to do so.TR 16:13, 27 September 2011 (UTC)

How is it misleading? Here is the full sentence

In 1905, Albert Einstein postulated that the speed of light in vacuum was independent of the source or inertial frame of reference, and explored the consequences of that postulate by deriving the theory of special relativity and showing that the parameter c had relevance outside of the context of light and electromagnetism.

It does not say he was the first to think the speed of light was independent of its source or the frame: after all that's what the Michelson–Morley experiment strongly suggested. He was though the first to work out a theory that explained it.--JohnBlackburne^words_deeds 17:35, 26 September 2011 (UTC)

No, Einstein was not the first to work out such a theory. See Lorentz ether theory. The sentence is misleading because Einstein was not the first with either the postulate or the theory. Roger (talk) 20:08, 26 September 2011 (UTC)

Should we replace with

"In 1865, James Clerk Maxwell postulated that the speed of light in vacuum was independent of the source or inertial frame of reference. Lorentz explored the consequences of that postulate by deriving the theory of relativity and showing that the parameter c had relevance outside of the context of light and electromagnetism. Einstein later adopted this into the "special" theory of relativity."

I am at least sure about the Maxwell part. This article is about the speed of light, not relativity, and Maxwell gets the credit for that. (I was the IP OP I made a user name) 이방인 얼라이언스 (talk) 15:18, 27 September 2011 (UTC)

I have just restored the sentence on Einstein and relativity that was removed: Although others did work before Einstein he is largely credited with the discovery of SR, which established our modern understanding of the speed of light and its significance in Physics. I kept the information about Maxwell with a slight reworking of the information that was added about him. I'm aware though this makes that paragraph a bit longer in an already long lead, which could do with editing for length if anyone can find a way.--JohnBlackburne^words_deeds 22:40, 27 September 2011 (UTC)

Yes, Einstein is usually credited with SR, but this article is about the speed of light. The speed of light was known to be constant and important before Einstein, and the article should reflect that. I disagree with your change. Roger (talk) 00:02, 28 September 2011 (UTC)

It now has Maxwell's insight, but he did not establish the theoretical basis for the speed of light being constant, or its significance as not only the speed of electromagnetic waves but it's importance as a more general physical constant due to special relativity.--JohnBlackburne^words_deeds 00:18, 28 September 2011 (UTC)

Einstein's postulate was that the speed of light was independent of the source. Maxwell certainly did establish the theoretical basis for that. Your description of the postulate is inaccurate, and the rest of the sentence describes things done by others before Einstein. Roger (talk) 04:28, 28 September 2011 (UTC)

Your going to need to provide reliable secondary sources for your claims.TR 07:58, 28 September 2011 (UTC)

We don't need a reliable source for the fact that 1865 is before 1905. 이방인 얼라이언스 (talk) 12:21, 28 September 2011 (UTC)

We need a RS that someone discovered (special) relativity before Einstein.--JohnBlackburne^words_deeds 12:57, 28 September 2011 (UTC)

Here is Lorentz in 1904 showing (for the second time) "that the parameter c had relevance outside of the context of light and electromagnetism."[2] Since this article is about c (or the speed of light?), not "special" relativity (whatever that is), I think we should credit Lorentz here. 이방인 얼라이언스 (talk) 13:02, 28 September 2011 (UTC)

Furthermore, the constant c, which had been widely accepted since Maxwell and was used by Lorentz, is necessarily independent of inertial frame of reference. In fact that is the definition of "constant" - "independent of other things"; so the article phrase "Albert Einstein postulated that the speed of light in vacuum was independent of the source or inertial frame of reference" gives credit where it is not due. 이방인 얼라이언스 (talk) 13:33, 28 September 2011 (UTC)

That's not a secondary source, but leaving that aside that's Lorentz writing about the effect on solid bodies of "their motion through the aether", so repeating the same mistake of others before him. It was Einstein's insight that there is no privileged frame, so no ether, and all frames are relative to each other, from which with the constancy of the speed of light he was able to derive the laws others had arrived at and much more.--JohnBlackburne^words_deeds 13:43, 28 September 2011 (UTC)

Well, ignoring the aether is a good move and doubtless can be mentioned in the "special relativity" article. However, the sentence "In 1905, Albert Einstein postulated that the speed of light in vacuum was independent of the source or inertial frame of reference, and explored the consequences of that postulate by deriving the theory of special relativity and showing that the parameter c had relevance outside of the context of light and electromagnetism." simply gives credit where it is not due.

• Maxwell was the first to "[postulate] that the speed of light in vacuum was independent of the source or inertial frame of reference"
• Lorentz was the first to "[explore] the consequences of that postulate by [...] showing that the parameter c had relevance outside of the context of light and electromagnetism." 이방인 얼라이언스 (talk) 14:12, 28 September 2011 (UTC)

From Lorentz' 1904 paper:

"Poincaré has objected to the existing theory of electric and optical phenomena in moving bodies that, in order to explain Michelsons's negative result, the introduction of a new hypothesis has been required, and that the same necessity may occur each time new facts will be brought to light. Surely, this course of inventing special hypothesis for each new experimental result is somewhat artificial. It would be more satisfactory, if it were possible to show, by means of certain fundamental assumptions, and without neglecting terms of one order of magnitude or another, that many electromagnetic actions are entirely independent of the motion of the system."

It is clear that Lorentz was aware that attempts to detect the aether were negative, and was working from there. "[E]lectromagnetic actions are entirely independent of the motion of the system", sounds kind of like "independent of the source or inertial frame of reference", or even exactly equivalent. Lorentz' use of the word "aether" in the prose of this paper is functionally equivalent to "space", the concept doesn't factor into the equations, effectively discarding the concept. 이방인 얼라이언스 (talk) 14:26, 28 September 2011 (UTC)

JohnBlackburne says, "It was Einstein's insight that there is no privileged frame, so no ether, and all frames are relative to each other,..." There are many other WP articles about frames, aether, relativity, etc. This is an article about the speed of light, not Einstein's insights. It should accurately reflect developments in the speed of light, and the speed was thought to be constant long before Einstein. Roger (talk) 15:05, 28 September 2011 (UTC)

But Einstein based one of the most successful physics theories to date on the formal postulate of the invariance of the speed of light, whereas all others were struggling to precisely avoid just that. I think this is highly notable and it should be mentioned — specially — in an article about the speed of light. This is essential. DVdm (talk) 16:05, 28 September 2011 (UTC)

The constant speed of light was a given throughout Lorentz' papers. Take a look at[3] for example. I am at a loss as to why you would claim that this was not treated as a postulate, let alone that "all others were struggling to precisely avoid just that." 이방인 얼라이언스 (talk) 16:19, 28 September 2011 (UTC)

Stachel in [4]: "Einstein also postulated that the speed of light with respect to an inertial frame is independent of the motion of the light source". I have added this ref to the text. DVdm (talk) 16:35, 28 September 2011 (UTC)

Yes, no question. But Lorentz did that before him. 이방인 얼라이언스 (talk) 16:38, 28 September 2011 (UTC)

Colin Howson [5]: "Lorentz was justified in asserting that: . . . the chief difference [is] that Einstein simply postulates what we have deduced ..." 이방인 얼라이언스 (talk) 16:49, 28 September 2011 (UTC)

My point exactly. QED :-) - DVdm (talk) 17:10, 28 September 2011 (UTC)

Credit is usually given to the person making the initial deduction rather than some randomly selected scholar using it as a postulate later. 이방인 얼라이언스 (talk) 20:44, 28 September 2011 (UTC)

Yes, that is a funny thing about relativity. There are historians and philosophers who have written entire essays on how, in Einstein's case only, it is better to credit the one who postulated what others proved. Roger (talk) 22:40, 28 September 2011 (UTC)

Although this is really getting off-topic here, I think the both of you are precisely missing the entire point here. DVdm (talk) 22:52, 28 September 2011 (UTC)

No, this is spot on topic, which is the incorrect sentence in the lead. We have a source saying that Lorentz deduced that the speed of light is to all purposes the same in all inertial reference frames. Of course anybody after that would then take that as a postulate, as any number of scholars have done. Lorentz gets the credit. "Deduce" trumps "postulate". 이방인 얼라이언스 (talk) 10:08, 29 September 2011 (UTC)

Since you insist, the point you are missing is this: "As to the ether... though the conception of it has certain advantages, it must be admitted that if Einstein had maintained it he certainly would not have given his theory, so we are grateful to him for not having gone along old-fashioned roads." (Lorentz, 1922). Lorentz seems to know who gets the credit. Resting case, DVdm (talk) 10:43, 29 September 2011 (UTC)

"Einstein gets the credit"...for what exactly? Your quote says nothing about the speed of light being independent of inertial reference frame, which is what we are discussing. The existence or non existence of the ether is irrelevant to that. The quote just praises Einstein for discarding the ether. This is a good step, but deducing that that the speed of light is independent of inertial reference frame was the basis for discarding the concept of the ether, and Lorentz did that. 이방인 얼라이언스 (talk) 11:57, 29 September 2011 (UTC)

That quote is from 1922, and by then Einstein had reversed himself. In 1920, he said, "More careful reflection teaches us, however, that the special theory of relativity does not compel us to deny ether." [6] The history of the aether is detailed elsewhere, such as Luminiferous aether and History of special relativity, and slightly off-topic here. This article is about the speed of light. Roger (talk) 15:49, 29 September 2011 (UTC)

I corrected the statement of Einstein's postulate. Yes, Einstein postulated it, but the text should not imply that a constant speed of light was a new idea in 1905. Roger (talk) 18:27, 28 September 2011 (UTC)

Einstein was the first to postulate the speed of light was the same in all inertial frames. Neither emission theory or aether theory predict this.Martin Hogbin (talk) 18:50, 28 September 2011 (UTC)

Actually, the Lorentz aether theory did predict that the speed of light was the same in all inertial frames. That is how it explained Michelson-Morley. But you have gotten Einstein's postulate wrong. Just look at the Stachel quote above to see what Einstein postulated. Roger (talk) 18:58, 28 September 2011 (UTC)

LET does not predict that the speed of light is the same in all inertial frames, it predicts that the speed of light is constant in the aether frame but that changes to rulers and clocks, due to their motion through the aether, are such that the speed of light is measured to be the same in all inertial frames. Lorentz himself keeps the distinction between [real] time and what he calls 'local time' in his theory. There is an important philosophical difference, although LET is, as we all know, experimentally indistinguishable from SR. Martin Hogbin (talk) 20:11, 28 September 2011 (UTC)

So correct your misstatement of Einstein's postulate, and add that Lorentz had a theory for why the speed of light is measured to be the same in all inertial frames. This is an article about the speed of light, and not obscure philosophical differences. Roger (talk) 21:20, 28 September 2011 (UTC)

It is not my statement but it says that Einstein postulated that the speed of light was the same in all inertial frames. He did, so the statement is correct. He was also the first to do this. Lorentz postulated that certain effects would occur that would make the speed of light appear to be the same in all inertial frames - not quite the same thing. Poincare got close too, but not quite there. Martin Hogbin (talk) 21:45, 28 September 2011 (UTC)

No, Einstein said, "and also introduce another postulate, which is only apparently irreconcilable with the former, namely, that light is always propagated in empty space with a definite velocity c which is independent of the state of motion of the emitting body." [7] Or see the Stachel quote above. Just say what Einstein said, and what the other said, if you wish. Roger (talk) 22:40, 28 September 2011 (UTC)

Yes, but he is referring to 'all frames of reference for which the equations of mechanics hold good', that is to say inertial frames. Martin Hogbin (talk) 22:53, 28 September 2011 (UTC)

That quote is from the other postulate. The conclusion follows from both postulates put together. Of course it also follows from what Lorentz and Poincare said years earlier. This is all detailed in other WP articles. This article should just stick to simple facts, and the current text is in error. Roger (talk) 23:05, 28 September 2011 (UTC)

If you would actually bother to read the article instead of whining about a sentence in the lede, you would see that this is actually already mentioned in the article.TR 06:17, 29 September 2011 (UTC)

Please refrain from terms like "whining", which are uncivil. The issue here is what is thought to be a misleading sentence in the lead. Whether the correct information can be found in the body of the article is irrelevant. Martin, you claim correctly that Lorentz deduced that the speed of light is to all practical purposes the same in all inertial reference frames, while not actually being so. I find this a bit odd. Can you reference that? 이방인 얼라이언스 (talk) 10:15, 29 September 2011 (UTC)

Then you don't quite understand the nature of the lede. The lede simply serves as a brief summary of the article. Do the size constraints the lede cannot cover all subtleties of a subject, leaving these to the body of the text. The discussion here should thus here be if the body text covers all aspects of this history adequately. I think there may very well be some room for improvement of the paragraph we have on this in the history section. (Obviously, any additions to that paragraph should be adequately sourced from reliable secondary sources.)

Based on any improvements there, the lede may be adjusted as well.TR 13:01, 29 September 2011 (UTC)

The body has "In 1905 Einstein proposed that the speed of light in vacuum, measured by a non-accelerating observer, is independent of the motion of the source or observer" which misleadingly gives the impression that Einstein was the first to think of that, and the lead amplifies this distortion even further. 이방인 얼라이언스 (talk) 13:36, 29 September 2011 (UTC)

TR, you have been asked politely to refrain from uncivil remarks. If you have any comment about the accuracy of the sentences at issue, then go ahead and post it. I think that the article unnecessarily wanders into a priority issue, and then says something that is demonstrably false. Roger (talk) 15:15, 29 September 2011 (UTC)

There was a discussion of some of these issues over at notice board, where it has been shut down due to a sockpuppet allegation. I do not know what that is all about. I suggest that the discussion be continued here. Here is my last comment. I think that the Stachel quote is correct, and is better than the previous text that misstated the postulate. However I also think that it is very misleading to imply that the constant speed of light was something new in 1905. My textbooks all say that the crucial experiment was in 1887, following theory by Maxwell. I am not pushing a minority view. While those books sometimes criticize the views of Lorentz and Poincare, they all agree that Lorentz and Poincare were explaining the 1887 experiment and constant speed of light before Einstein. Roger (talk) 20:43, 3 October 2011 (UTC)

Then provide (reliable, secondary) sources for your claims.

(Also note that Lorentz did not deduce that all observers would measure the same value for the speed of light regardless of their velocity until 1909. In his 1904 paper hem merrily deduced that interference experiments as done by MM would give the same result regardless of the motion of the apparatus. He did this by (effectively) showing that zeros of the EM field would be transformed into zeroes of the transformed EM field (and thus that interference fringes are the same).)TR 09:39, 4 October 2011 (UTC)

I suggest that there is nothing to be added to the Stachel quote, or to the hundreds of quotes we can find with this books search to demonstrate the validity of the particular statement in the article, namely that the invariance of the speed of light was postulated by Einstein. This opened the way to one of our most succesfull physical theories, whereas others struggled with their old fashioned ether concept, thus missing the entire point, and failing to get the final credit for the theory. DVdm (talk) 21:07, 3 October 2011 (UTC)

I agree with DVdm. This has all been settled long ago. Others came close but did not quite get there. Einstein is universally given the credit for postulating the constancy of light speed. Martin Hogbin (talk) 21:41, 3 October 2011 (UTC)

However, something about the History of Lorentz transformations, and that Poincaré used a clock-synchronization convention making the "apparent" one-way speed of light equal in all inertial frames, should be mentioned in the article. --D.H (talk) 15:02, 4 October 2011 (UTC)

I think that would be more appropriate for the One-way speed of light article. At present this article does not discuss the one-way speed of light at all. I think a bit more about that subject should be included in the text here. At the moment, all we have is a note (Note 4) and a link. Martin Hogbin (talk) 17:47, 4 October 2011 (UTC)

Yes, for now I only alluded in a general manner to the connection of the constancy of light speed and synchronization, not the one-way speed. This topic should probably be discussed in a separate section of the article. --D.H (talk) 18:21, 4 October 2011 (UTC)

Now this discussion is being censored because of some comments by someone who is accused of using an alias to evade a WP ban that he got a couple of years ago for offending people on some other topic. I tried to find out what he said or did that was so offensive, but I failed. Other editors here seem to want to consider "final credit for the theory" and other peripheral matters. I just wanted accurate info about the speed of light. Roger (talk) 21:54, 6 October 2011 (UTC)

Well, one of the usernames of the banned user was User:Juden Raus (talk · contribs · deleted contribs · logs · filter log · block user · block log). That gives a pretty good hint of the wind direction. DVdm (talk) 22:00, 6 October 2011 (UTC)

Amount of detail in the History section.

Last week user:D.H added the following to the history section:

Because of this experiment Hendrik Lorentz proposed that the motion of the apparatus through the aether may cause the apparatus to contract along its length in the direction of motion, by a factor such as to ensure that there is no interference fringes detected by the interferometer. Lorentz further assumed, that the time variable for moving systems must also be changed accordingly ("local time"), which led to the formulation of the Lorentz transformation. Based on Lorentz's aether theory, Henri Poincaré (1900) showed, that this local time (to first order in v/c) is indicated by clocks moving in the aether, which are synchronized under the assumption of constant light speed. In 1904, he speculated (provided that the assumptions of Lorentz's theory are all confirmed), that the speed of light could be a limiting velocity in dynamics. In 1905, Poincaré brought Lorentz's aether theory into full observational agreement with the principle of relativity.

This is certainly an improvement over that which was there before it. I do however think that it is maybe a bit too detailed compared to the other paragraphs of the section. I think that it can probably be condensed to about half the size by sticking to the developments that are truly relevant to this article. For example, the last sentence is certainly relevant to the history of relativity, but not so much to the historical developments regarding the speed of light.

Since me being WP:BOLD about it would probably rub some people the wrong way, I'm asking for talk page input first.TR 09:08, 7 October 2011 (UTC)

The constancy of the speed of light in all inertial frames and the validity of the relativity principle are closely connected. So I think the last sentence should stay in the article. However, I agree that the sections concerning the aether, length contraction and local time are indeed a little bit too detailed.--D.H (talk) 09:31, 7 October 2011 (UTC)

That is all essential info to how the speed of light got to be so important. What did you want to remove? Roger (talk) 09:59, 7 October 2011 (UTC)

As I suggested above, perhaps we could move that section to the One-way speed of light article and have an abbreviated version here. I also suggest that we move note 3 (which is about the one-way speed of light) into the main body of the text as this relates directly to the subject of the article. Martin Hogbin (talk) 10:29, 8 October 2011 (UTC)

I do not agree with these attempts to remove Michelson-Morley from the article. This was done to the lead section, over my objections, and now TR and Martin want to do it to the body of the article. Historically, that experiment, along with the analyses of it by FitzGerald, Lorentz, and Poincare, is what convinced everyone that the speed of light is constant. For example, a recent neutrino article said:

IN 1887 physicists were feeling pretty smug about their subject. They thought they understood reality well, and that the future would just be one of ever more precise measurements. They could not have been more wrong. The next three decades turned physics on its head, with the discovery of electrons, atomic nuclei, radioactivity, quantum theory and the theory of relativity. But the grit in the pearl for all this was a strange observation made that year by two researchers called Albert Michelson and Edward Morley that the speed of light was constant, no matter how fast the observer was travelling. Economist magazine article

I propose that the article prominently say something similar to this paragraph. Roger (talk) 18:53, 8 October 2011 (UTC)

Although this is widely believed, it is not correct. The MM-experiment (alone) doesn't prove that the speed of light is "constant", because it is also compatible with the old emission theory of light, in which light speed depends on the source speed. The strength of special relativity rather lies in the fact, that it is the only theory which can explain the combination of all experiments, for example the Sagnac effect, the Fizeau experiment, the MM-experiment, the Ives-Stilwell experiment etc.. --D.H (talk) 20:41, 8 October 2011 (UTC)

Just for the record, I have not mentioned the MMX and I have not expressed any opinion about where it should be in the article. Martin Hogbin (talk) 21:57, 8 October 2011 (UTC)

The article on emission theory says that experiments were still being done in 1964 to rule it out. And the Italians are still trying to figure out whether neutrinos can go faster than light. Nevertheless, the M-M and other experiments did convince Lorentz and others in the 1890s that that speed of light was constant, and that is how he discovered the Lorentz transformations. The Economist article is correct. Roger (talk) 07:14, 9 October 2011 (UTC)

Whatever one thinks about the experiment: Its importance in ruling out an observable preferred frame (or anisotropic light speed) is correctly described in the article. Also the development of Lorentz transformation and clock synchronization is shortly mentioned. Coming back to the original topic: I think the section is neither too long nor too short. --D.H (talk) 08:38, 9 October 2011 (UTC)

I am not going to argue with you DH, especially as there is a more important issue (see below) where your comments would be welcome. Martin Hogbin (talk) 09:02, 9 October 2011 (UTC)

Schafly, please point towards I ever suggest removing anything about MM from the article?TR 09:56, 9 October 2011 (UTC)

At the beginning of this section, you proposed cutting a section about the M-M experiment, and the consequences that Lorentz and Poincare deduced from it. If I overstated your position, then I accept your correction. Roger (talk) 01:02, 10 October 2011 (UTC)

I did no such thing. Please read again.TR 05:52, 10 October 2011 (UTC)

Experimental evidence of isotropy of light speed.

I have removed the short section claiming experimental evidence that the speed of light is isotropic because it is, at best, unclear exactly what is being claimed. We know that the one-way speed of light cannot be measured independently of clock synchronisation. We also know that theories that are experimentally equivalent to SR assume that light travels different speeds in different directions. The cited experiments can therefore only show isotropy of the two-way speed of light, which is not true isotropy. Either we should make the true situation clear or we should leave out the section that I have deleted. What is the general view here? Martin Hogbin (talk) 09:00, 9 October 2011 (UTC)

Martin, I think you are doing heavy original research here. For instance, read PD's comment here. There is no reason to remove this, so I have restored it. The source is solid. (Perhaps it would be a good idea to have a chat about this with PD this overthere...) Cheers - DVdm (talk) 09:31, 9 October 2011 (UTC)

DVdm, I would have thought that you know that I understand the physics by now, that is not in dispute. No experiment can show that the speed of light from A to B is the same as that from B to A without a clock synchronisation convention. I assume you agree with that. The problem is readers' understanding of the word 'isotropic'. It means 'the same in all directions'. The experiments do no show this. Martin Hogbin (talk) 10:03, 9 October 2011 (UTC)

It is not up to us to decide that. If the sources say that it does, then for Wikipedia it does. Simple, no? :-) Martin, it really might help trying to clear this up with PD overthere. I'd be interested in seeing how this goes. Really... Cheers - DVdm (talk) 10:38, 9 October 2011 (UTC)

Martin: The resonator experiments (like that of Müller et al.) are synchronization-independent measurements of the isotropy of the two-way speed of light (see for example this explanation by Claus Lämmerzahl, a well-known expert). Why do you think that two-way isotropy is not "true isotropy"? --D.H (talk) 09:50, 9 October 2011 (UTC)

Because 'isotropic' means the same in all directions and the experiment does not show that. Light travelling at completely different speeds in opposite directions would be consistent with the cited experimental results.

At the very least we must add 'two-way speed' to the section otherwise it is misleading. Martin Hogbin (talk) 09:55, 9 October 2011 (UTC)

It seems that we are using the word "isotropy" differently. To me, two-way-isotropy is sufficient to speak about "isotropy" per se, so the experiments clearly show isotropy in all directions. But I won't quarrel about words. At least, it is certainly undisputed that MMX and Resonator experiments are using two-way paths. --D.H (talk) 10:33, 9 October 2011 (UTC)

Yes, we are not arguing about the physics, that is well known and understood, but without adding 'two-way' the section is going to mislead most readers. The word 'isotropic' means 'the same in all directions'. Most reader will assume that to mean that the speed of light from A to B is the same as the speed from B to A (or for that matter the speed from A to C), which we both know is not shown by the experiments. When used without qualification, the word 'speed' is naturally taken to apply to a single direction not a round trip average. We must make clear what we are saying. For some reason this important fact about the subject of the article has been relegated to a footnote thus making parts of the article, such as this one, meaningless without additional explanation.Martin Hogbin (talk) 10:44, 9 October 2011 (UTC)

I'd say, make sure that this "additional explanation" is very properly sourced, or it will have to go per wp:NOR. I think that it is safer (and obligatory!) to closely stick with the sources. - DVdm (talk) 10:49, 9 October 2011 (UTC)

There is no difficulty in finding impeccable sources that tell us the the one-way speed of light cannot be experimentally determined without a clock sychronisation scheme. Martin Hogbin (talk) 10:57, 9 October 2011 (UTC)

Perhaps, but combining such sources with the other sources that do not mention clock sychronisation (like the one which you felt should be removed), would be a truly classic example of wp:original research. DVdm (talk) 11:05, 9 October 2011 (UTC)

This is not original research it is basic physics, which I assume you understand. Are you claiming that there is some doubt that the cited experiments do not show the one-way speed of light to be isotropic are are you saying that we cannot mention this well known and understood fact without finding a source which uses those exact words?

If you read the summary of the first source you will see that it refers to the Robertson-Mansouri-Sexl test theory in which the one-way light speed in not, in general, equal to the two-way speed, thus the source actually makes no claim about one-way isotropy.

My main point is this. If there is this degree of confusion and misunderstanding here, what will our readers make of it.Martin Hogbin (talk) 11:20, 9 October 2011 (UTC)

I don't think it is up to us to interpret —let alone combine (aka wp:SYNTH)— the sources. Our readers are supposed to read the article and verify with the sources without us telling them how to interpret the sources — Wikipedia is not a textbook. DVdm (talk) 16:05, 9 October 2011 (UTC)

It is up to us to write an encyclopedia. Are you suggesting that we should knowingly write something that is incorrect?

In any case, the first quoted source does not say that the speed of light is isotropic, the title is, 'Test of the isotropy of the speed of light ...' and the summary says, 'Within the Robertson-Mansouri-Sexl test theory, our measurement restricts the isotropy violation parameter...'. Nowhere does it say that the experiment shows light speed to be isotropic, that is merely someone's interpretation of the paper and, unfortunately, it is an incorrect one. Martin Hogbin (talk) 17:00, 9 October 2011 (UTC)

The second reference refers to the JPL experiment by Krishner et al which, at the time, claimed to show that the one-way speed of light was isotropic. This was later shown by Zhang to be an incorrect conclusion; the experiment determined only the isotropy of the two-way speed. Martin Hogbin (talk) 17:09, 9 October 2011 (UTC)

I am not suggesting that we should knowingly write something that (we know—or think) is incorrect. I'm suggesting that we should not synthesise or interpret. In this case Wikipedia should say that within the Robertson-Mansouri-Sexl test theory, their measurement restricts the isotropy violation parameter.... We are here to report what reliable sources say, nothing more and nothing less, per elementary wp:NOR and wp:SYNTH. DVdm (talk) 17:14, 9 October 2011 (UTC)

Please DVdm you seem to be arguing just for the sake of it. Let us take it bit by bit. Firstly, regardless of Wikipedia, do you believe that there are any experiments that show the isotropy of the one-way speed of light? Martin Hogbin (talk) 17:22, 9 October 2011 (UTC)

Martin, you seem to want to get rid of historically important developments, and instead you want to push esoteric one-way speed of light theories. Those theories have never been taken seriously. If someone really wants to read about them, they can follow the link to one-way speed of light. Roger (talk) 17:45, 9 October 2011 (UTC)

Sorry Roger, I have no idea what you are talking about. Martin Hogbin (talk) 18:56, 9 October 2011 (UTC)

Martin, this is not "for the sake of it". It is for the sake of Wikipedia. What I (or you) believe about there being any experiments that show the isotropy of the one-way speed of light, doesn't matter one bit. Really. We are not here to discuss the subject but the article. Something that cannot be found in a single source, does not belong in a Wikipedia article. There is a very similar thread now at Talk:Richard Dawkins#Adding something like 'activist' to lede. Do have a look over there. Combining sources to put forward a position (as in wp:SYNTH) is a big NONO, and luckily it is very easy to spot. Cheers - DVdm (talk) 20:32, 9 October 2011 (UTC)

DVdm, I know perfectly well how WP works but I asked you a simple question that you have refused to answer. The fact that isotropy of the one-way speed of light cannot be demonstrated experimentally is not something I have just put together from the two sources cited here, it is a well known fact that I am surprised you are not aware of. However, I do not need to quote any sources because I want to remove an inaccurate statement from WP that is not actually supported by any of the cited sources. I propose to delete it again. If you want to put it back you must find a reliable source that has not since been discredited that clearly says exactly what is claimed. Martin Hogbin (talk) 21:11, 9 October 2011 (UTC)

Martin, I don't think that this properly sourced content should be deleted. If the article reports what the source says, it can stay. Deleting it again would be "removal of sourced content against consensus". If other sources say something conflicting, you can report what they say. But combining them is original research. Do have a look at the Dawkins thread — it is very similar. DVdm (talk) 21:22, 9 October 2011 (UTC)

Note. You just proposed to delete it, and then you did delete it. I have restored it. DVdm (talk) 21:29, 9 October 2011 (UTC)

You have just agreed a few lines above that the first source does not justify the wording used. You suggested within the Robertson-Mansouri-Sexl test theory, their measurement restricts the isotropy violation parameter.... By all means put this in if you like if you think it will be useful. Regarding the JPL source, not only that has been discredited by Jhang but Jhang's analysis has been agreed by the original analyser Will. We do not cite old and out of date sources we cite information that agrees with the current scientific consensus. No doubt there is a source somewhere (once reliable) to say the Earth is flat. Martin Hogbin (talk) 21:36, 9 October 2011 (UTC)

Martin, it is about most recent and modern resonator experiments such as arXiv:0508097 or arXiv:1002.1284, which are simply stating that they measure the isotropy of the speed of light, based on the Robertson-Mansouri-Sexl-model. So the most recent and important publications are using the word "isotropy" for their findings, thus it's your choice of wording which is not reflecting current terminology. As I said above: isotropy of two-way speed is all which is necessary to speak about isotropy. The synchronization problem (together with the artificial construction of anisotropic one-way speeds) doesn't affect the outcome of those two-way resonator experiments. In general, I think you are overestimating the importance of this one-way issue... --D.H (talk) 23:02, 9 October 2011 (UTC)

As I said, there is no argument about the physics, the two-way speed can be shown to be isotropic by experiment the one-way speed cannot. The question is, how do we express this to our readers? To talk of a two-way or round-trip speed being isotropic is meaningless, far better to say the average speed for a round trip is independent of the path taken. 'Isotropic' means 'the same in every direction' a two-way trip does not have to have a direction, the path could be circular, for example. Also, the natural understanding of the word speed would be a one-way sped from A to B. To put it in WP terms there are no currently accepted sources that actually say the speed of light is isotropic so we must not say that here.

Regarding the difference between two-way and one-way speeds, this is a very important distinction that is made very clearly in Zhang's book. Facts relating to the two-way speed can be verified by experiment, the constancy and isotropy of the one-way speed are matters of convention only. Einstein knew this and made it clear right from the start. There was a brief period when some physicists though they had measured one-way speed but, as you know, these conclusions were found to be in error. It is important not to overstate what we know about the subject and to give the impression that what is just a convention is supported by experiment degrades the integrity of the subject. Martin Hogbin (talk) 08:13, 10 October 2011 (UTC)

Martin, you overly emphasize the importance of the one-way speed of light, which is little more than a philosophical footnote to the subject of the speed of light, since there is no physical way to distinguish between "one way" or "two way" speed of light. Consequently, from a physical point of view it is not reasonable to even make the distinction.

Also note that the direction of a two way speed is actually very well defined. (It simply is an element of the projective plane).TR 10:24, 10 October 2011 (UTC)

I do not understand what you mean by, 'there is no physical distinction'. The one-way speed is from A to B and requires two clocks, synchronised by some means, the one-way is from A to A by some arbitrary closed path, that seems fairly physical to me. This is not something I have just made up, it is a important point in Zhang's book (Zhang, YZ (1997). Special Relativity and Its Experimental Foundations. Advanced Series on Theoretical Physical Science. 4. World Scientific. pp. 172–3. ISBN 9810227493. http://www.worldscibooks.com/physics/3180.html.) which is widely regarded as the definitive work on the subject. Martin Hogbin (talk) 13:45, 10 October 2011 (UTC)

There is no physical experiment you can do to establish that the two-way speed of light is different from/equal to the one-way speed of light. In fact you can always define your clock synchronization scheme, to make them equal by definition. The consequence is that any statement about the one way speed of light belongs to the realm of philosophy rather than physics. This is the reasons that most physicists simply do not make the distinction. TR 14:19, 10 October 2011 (UTC)

I fully agree with your statement, 'any statement about the one way speed of light belongs to the realm of philosophy rather than physics', and that is exactly my point. To most readers, a statement in this article that just refers to 'the speed of light' would be taken to refer to the one-way speed, simply because that is how the word 'speed' is generally understood. When you talk of the speed of sound or the speed of a train you naturally mean the speed from A to B. In this article we make the statement that experiments have shown that the 'speed of light' is isotropic. Nearly everyone, probably including many physicists, will take this to mean that light travels at the same speed in all directions, but as you quite rightly say, the speed of light from A to B is purely conventional and cannot be measured or verified by experiment.

To say that physicists do not make the distinction between the one-way speed (which we agree is purely conventional) and the two-way speed (which can be measured experimentally) is just plain wrong. The constancy of the (one-way) speed of light remains a postulate rather than an experimentally verified fact. Text books on the subject, such as Zhang and Wills, clearly distinguish between the two speeds and even one of the cited sources (the JPL experiment) erroneously claims to have experimentally verified that the one-way speed of light is isotropic. It is absolutely essential to distinguish between these two different concepts to properly understand the subject. Martin Hogbin (talk) 14:45, 10 October 2011 (UTC)

(edit conflict)Martin, perhaps you could add a little comment —maybe in a subsection— on this matter, quoting from Zhang's book, sticking dryly and closely to what they say, without removing or modifying existing content, and, specially, without going into textbook- or lecture-mode (aka wp:SYNTH). I personally don't think it matters much though, but if others can live with it, I don't mind. A plan? DVdm (talk) 14:48, 10 October 2011 (UTC)

I am glad that you have remembered that I am not one ofthe crackpots from the old Usenet days ;-) I think the best plan would be to bring note 4 into the body of the article so we mention the one-way thing in the right place. In the 'isotropic' section we should drop the JPL reference, which is wrong and reword to make things clearer. Martin Hogbin (talk) 15:31, 10 October 2011 (UTC)

I don't think we should drop the JPL reference and/or reword things, for the reasons stated before. (And of course I remember. How could I forget? :-)) Let's see what others say... DVdm (talk) 15:57, 10 October 2011 (UTC)

Regarding the JPL reference, the conclusion is universally agreed by the physics community to be wrong. It is not that there are two schools of thought about the subject, as I said, even Clifford Will, the well-regarded expert on the subject who did the original analysis, has accepted that he made a mistake in this case (although I cannot find a reference for that particular fact). Martin Hogbin (talk) 18:25, 10 October 2011 (UTC)

"The constancy of the (one-way) speed of light remains a postulate rather than an experimentally verified fact." It is not a postulate. In the conventional definition of the one way speed of light, it is equal to the two way speed of light, per definition". Consequently, the one-way speed of light is constant because the two way speed of light is. Of course, you can define "one way speed of light" in such a way that this isn't true, but that is kinda of a pointless philosophical tangent.

The word 'postulate' comes from a translation of Einstein's original German. Whether we call it a postulate/stipulation/convention/definition is irrelevant, we agree that the one-way speed of light is set by convention and is not subject to experimental verification.

"To say that physicists do not make the distinction between the one-way speed (which we agree is purely conventional) and the two-way speed (which can be measured experimentally) is just plain wrong." You might want to notice my use of the word most.

The problem here is that "one way speed of light" is not a well defined quantity. It depends on a measurement convention, which can be chosen such that it is always equal to the "two way speed of light". Consequently, there is no sense in trying to make the distinction, which is why most physicists simply speak of the speed of light with no further qualification.TR 14:29, 11 October 2011 (UTC)

Exactly! The one-way speed is defined only by convention but the two-way speed can be measured experimentally thus they are two distinct concepts.

Physicist generally tend to use language appropriate to the context. In discussions about one-way and two-way speeds, such as those in Zhang and other works on the subject, physicists are very careful to make clear which concept they are referring to. In many other contexts it would naturally be assumed that the Einstein convention is being used so just 'speed' of light would be used. This does not mean that physicists consider the one-way and two-way light speeds to be the same thing, it is just that usually it is understood what is meant by 'speed of light'.

This is an article about the very subject 'speed of light' thus it is essential that we make clear to our readers exactly what is meant by the term. It is far from obvious here that 'speed' refers only to the two-way speed, nearly everyone, on seeing the words 'speed of light' will assume that we are talking about the speed from A to B - the one-way speed, thus when they read that the speed of light is isotropic they will assume that it cam be measured to take an equal time to travel and equal distance in all directions. Martin Hogbin (talk) 17:34, 11 October 2011 (UTC)

This is an obscure philosophical topic. The textbooks do not even mention it. There is already a link to an article explaining it, One-way speed of light. What more do you want? Roger (talk) 17:44, 11 October 2011 (UTC)

This subject is hardly obscure for an article entitled 'Speed of light'. As you will see from above, I know, and Timothy knows, and maybe you know, that the speed (as understood by most of the population) of light is fixed by convention and is not subject to measurement or experimental verification but nowhere in the article (apart from a footnote) does the article tell readers this rather surprising fact. Martin Hogbin (talk) 15:26, 12 October 2011 (UTC)

You are still misrepresenting this. The one way speed of light as commonly defined, is subject to measurement en experimental verification. (Because it is the same physical quantity as the two way speed of light.) The slightly surprising fact is that the two are not seperate quantities. This however is not surprising to the overwhelming majority, because this is what they would assume.TR 15:45, 12 October 2011 (UTC)

As you say, the one-way speed is commonly defined to be equal to the two-way speed. To nearly everyone it is a surprising fact that that a speed cannot be measured but can be defined. This is not true of the speed of sound or the speed of a bullet for example. We must tell our readers this important fact.

Also, let me repeat, excellent secondary sources (such as Zhang) do talk about the two different speeds, this is not something I have concocted. Martin Hogbin (talk) 15:28, 14 October 2011 (UTC)

A vacuum vs. vacuum

OK, clearly a lot of us feel differently about this. I know it's been talked about before, but I'd like to hash things out here again if that's possible.

The reason I object to the term without "a" is that I believe it's overly technical language right in the lede. WP:BEGINNING says "The article should begin with a declarative sentence telling the nonspecialist reader what (or who) is the subject" (emphasis mine). WP:MOSINTRO says about the same thing: "In general, specialized terminology and symbols should be avoided in an introduction" (emphasis mine).

The issue here is that "a" vacuum is not technically incorrect, so I believe it should stay. But perhaps we can reword it so it avoids the term altogether? Magog the Ogre (talk) 05:02, 2 November 2011 (UTC)

I agree. If the poor novice reader is foolish enough to click on vacuum, he will get a confusing first paragraph there, along with explanations of long historical debates about whether there is any such thing as a vacuum. Roger (talk) 05:35, 2 November 2011 (UTC)

"In vacuum" has been working fine for a long time. Why the thrashing today? Dicklyon (talk) 05:48, 2 November 2011 (UTC)

I doubt that WP:BEGINNING advocates or encourages pandering to the nonspecialist by using substandard or dubious grammar. "In vacuum" is the scientific norm; if that's "overly technical language", there could be a lot more dumbing down ahead. Shouldn't WP represent the convention, rather than flouting it?

The term can hardly be avoided, being an inseparable part of the definition of "c". Hertz1888 (talk) 06:09, 2 November 2011 (UTC)

Okay, Magog, explain to me why "a" is not technically correct. Why no article, definite or indefinite? Why is "vacuum" any different from, say, "fog" or "storm". You can go for a walk in "the storm", or we might say so-and-so died during "a storm", but not just "storm". -- cheers, Michael C. Price ^talk 06:35, 2 November 2011 (UTC)

Re: Michael: reread what I said. I said it's not technically incorect.

Re: Hertz: is the term incorrect as used? If it is, then we do have issues; we don't want to be factually incorrect. Magog the Ogre (talk) 06:44, 2 November 2011 (UTC)

Thanks, Magog! -- cheers, Michael C. Price ^talk 07:07, 2 November 2011 (UTC)

Storm is a count noun, water usually isn't, vacuum can be either count or non-count. ― A. di M.​  10:20, 2 November 2011 (UTC)

This has nothing to do with technical usage, but everything correct English. "In vacuum" and "in a vacuum" are both correct English phrases, but with a different meaning. Compare: the "speed of light" and the "speed of a light", both are correct English, but the second one is simply not what is meant.TR 06:50, 2 November 2011 (UTC)

Speed of "the light" would be correct, though. You were simply using the wrong article in that example. -- cheers, Michael C. Price ^talk 07:07, 2 November 2011 (UTC)

As far as I can tell, "in vacuum" is no different grammatically than "in space". The use of an article is awkward and unnecessary, if not downright incorrect. Hertz1888 (talk) 07:12, 2 November 2011 (UTC)

Space here is referring to outer space, but if you were using it to refer to a specific space (e.g. that under the floorboards) then you would say "the space under the floorboards". The vacuum here does not just refer to the vacuum of outer space, but vacuums everywhere, including any under the floorboards (or in labs). -- cheers, Michael C. Price ^talk 07:18, 2 November 2011 (UTC)

Correct, we're concerned with space, or vacuum, as a generic, a universal. Preceding it with a, denoting one, would indicate a specific space or vacuum. Hertz1888 (talk) 07:30, 2 November 2011 (UTC)

In a vacuum is apparently more common overall, but if there's a semantic distinction (cf in empty space vs in an empty space) that's not a definite argument. On the other hand, English grammar is weird (it's usually in the air but in water), so semantics alone won't give a definite answer, either. FWIW, speed of light in vacuum is the only possibility occurring with sufficient frequency^∗ in the Google Book corpus.[8] (As for “overly technical language”, anyone who can parse in water can parse in vacuum too.) ― A. di M.​  10:14, 2 November 2011 (UTC)

---

∗ When a phrase is found in fewer than 40 books in a given year in a given subcorpus, the frequency of that phrase in that year in that subcorpus is reported as zero.

There is a semantic difference. The "speed of light in a vacuum" would mean the same as the "speed of light in an empty space", rather than the "speed of light in empty space" which is what is meant here. (Also, the second google ngram link you give quite clearly indicates that in this context "in vacuum" is the common usage.)TR 10:52, 2 November 2011 (UTC)

So, what about putting "in (a) vacuum" there? DVdm (talk) 09:24, 2 November 2011 (UTC)

Ew. I'd rather flip a coin than have that. ― A. di M.​  10:14, 2 November 2011 (UTC)

Excellent idea, let's toss one ;-) - DVdm (talk) 10:19, 2 November 2011 (UTC)

OK, I've just flipped an Austrian 1-euro coin, and the result of the toss (either heads or tails) followed by a space and a few random characters has an SHA-512 hash of
1a89206a9c7f0c4b045a5c2642b1ce4a14e666db1ea9691363ff3438fd6dec0e7e1e65cac3b0bdbc001069471e0a0d1ee118b498dd1745435ef2cff7c35b50d2.
Now, you decide whether heads is a vacuum and tail is vacuum or vice versa, and I'll reveal the result of the toss and the random characters. (Or let's just have {{#ifexpr: {{CURRENTTIMESTAMP}} mod 2|a vacuum|vacuum}} so that it's chosen every time the page is rendered.) :-) ― A. di M.​  18:52, 2 November 2011 (UTC)

Finally. Good call. But please make another toss, this time a triangular one. We must choose between "In vacuum", "in a vacuum" and "in the vacuum". DVdm (talk) 19:06, 2 November 2011 (UTC)

Sadly, I'm short of triangular coins right now. ― A. di M.​  19:27, 2 November 2011 (UTC)

To add to the fray: I think that the choice of medium is what is significant: "I fell into snow" vs. "I fell into the snow". The former emphasises the medium. "Vacuum" and "a vacuum" is not as clear, but that the choice of medium is being communicated argues for "vacuum", not "a vacuum". Quondum talk
 contr 10:44, 2 November 2011 (UTC)

Yes. Also note that the Inuit will tell you that "they fell into a snow". DVdm (talk) 10:52, 2 November 2011 (UTC)

In Italian, the rules about when to use articles are sometimes even more messed up. (You'd say nel [in the] Lazio but in Abruzzo, and apparently no-one knows why.) ― A. di M.​  17:04, 2 November 2011 (UTC)

┌─────────────────────────────────┘
I think the difference here is entirely cultural. Scientific culture says it one way, while everyone else says it another. This isn't unique to science, BTW; my church friends have a bunch of idiosyncratic sayings (my favorites are to "love on" someone instead of just loving them, to talk about God as "seated on high" instead of just "sitting up high". But I digress).

Nevertheless, I will try to break this down a bit:

• in the vacuum: implies there is only one vacuum (or only one that matters).
• in a vacuum: implies there are numerous vacuums. Alternate way of saying "in any given vacuum".
• in vacuum: implies that vacuum is uncountable, like milk. One cannot refer to one milk.

The problem is that language is not quite so exact with articles. That's why people that don't speak English natively will mess them up so often (e.g., in Spanish, one says "I live in the Spain"). Outer space is uncountable, but space is countable (such as the space under my bed, where the monsters hide).

The issue here is that in common speech, a person will say "in a vacuum", and it is understood (all linguistic parsing aside) to mean "in any given vacuum" (countable), or in the amorphous concept called vacuum (uncountable). The phrasing is not ambiguous enough for anyone to think it could mean "it behaves this way in one vacuum, but differently in another vacuum."

To be honest, this reminds me a debate I came across on Wikipedia that had the article for starfish actually located at sea star, because it isn't technically a fish, and the scientific community couldn't stand a name that sounds wrong (see Talk:Starfish). The problem is, that everyday usage still calls it starfish (fixing it is a hypercorrection), and the manual of style proscribes the most common English usage (I don't have the link immediately handy). Magog the Ogre (talk) 11:26, 2 November 2011 (UTC)

I think you meant prescribes, not proscribes? Quondum talk
 contr 11:37, 2 November 2011 (UTC)

Well, if your argument is common English usage, then AdiM's link above closes the case. The only usage that is significant to be measurable in the corpus collected by google is "speed of light in vacuum". Anything else simply is not common. Since it also is not likely to be misunderstood, there simply is no case for any other phrasing.TR 12:33, 2 November 2011 (UTC)

I think that "the speed of light in vacuum" is correct colloquially as well as technically: consider "He's fallen in the water" as opposed to "sound travels faster in water than in air" (the distinction is specific versus generic as already mentioned above). We would use the article in a phrase such as "one way of creating a vacuum is to pump air from a container and open a cold trap". In "Do you have a vacuum flask for our picnic?", "vacuum flask" is a compound noun so that usage does not count. An article would though be used in phrases like "You cannot breathe in a vacuum" or "His departure left a vacuum" where the vacuum is associated with the subject of the phrase even though, in the first case not necessarily physically bounded and in the second an abstract concept. --Mirokado (talk) 11:41, 2 November 2011 (UTC)

It should be "a vacuum" and then there should follow a precise description of exactly what conditions the vacuum should satisfy for the speed of light to be c. Boundary conditions and background fields will alter the speed of light, see e.g. here. Count Iblis (talk) 16:50, 2 November 2011 (UTC)

Such a discussion is far too much for the lede, and fails to address the grammatical primary issue (with its physical implications) that we've been dealing with here. Perhaps it has a place elsewhere in the article. Hertz1888 (talk) 17:06, 2 November 2011 (UTC)

(edit conflict)In the lead section? Seriously? ― A. di M.​  17:08, 2 November 2011 (UTC)

That is one reason why we should not write "in a vacuum", which suggests the the type of quantum ambiguity you refer to. The traditional phrase "in vacuum", can only be interpreted as the classical vacuum (i.e. the groundstate of the classical field theory). (Note that even in the cases you refer to the constant called "the speed of light in vacuum" is still the same. In the referenced exotic vacua, light simply does not propagate at that speed. TR 17:15, 2 November 2011 (UTC)

I have these following dictionary definitions, all of them the first definition for the word "Vacuum":

Merriam-Webster: "a space absolutely devoid of matter"

Random House Unabridged: "a space absolutely devoid of matter; an enclosed space entirely devoid of matter"

American Heritage: "a space entirely devoid of matter"

All of them define "vacuum" as "a space." Under those definitions, vacuum isn't the substance, like "water," it's the space in which there is no substance. — Sam 63.138.152.135 (talk) 17:45, 3 November 2011 (UTC)

The speed of light in a vacuum may be changing

CERN has messured neutrinos going faster than the speed of light.

Albert Einstein says that nothing can go faster than the speed of light in a perfect vacuum. The neutrino speed is not changed by the refractive index. The photon is slowed by the refractive index of light. Even the vacuum of space is not perfect. The refractive index in the messurment of light must not have been 1 but something higher. Dsmith7707 (talk) 18:37, 18 November 2011 (UTC)

Expansion VS SOL Limit

Is there any possible way, that someone could put together a nice addition for the front page on how the Universe is expanding faster than the speed of light? I think this information should be shown in greater detail because it shows a very big flaw in the argument that the Speed of Light is absolute.

Thank you. —Preceding unsigned comment added by 68.96.242.243 (talk) 01:49, 13 February 2011 (UTC)

The universe could expand a thousand times faster than the speed of light with no contradiction, since no information, mass, or energy would be moving faster than the speed of light. "The universe", in this cosmological context, means the measured extent of the universe. In cosmological expansion, everything expands equally (the coordinate system is expanding), so there is no physical movement involved. See Metric expansion of space for details. David Spector (talk) 02:16, 28 November 2011 (UTC)

Recent changes

I have just undone changes made by User:Drift chambers to the lede. The main reasons were that it broke the formatting of the lede, undoing the version currently there arrived at by consensus on this talk page, as well as adding a nonsensical sentence. As they were so disruptive and added nothing of value I had to revert.--JohnBlackburne^words_deeds 12:50, 20 September 2011 (UTC)

from http://www.britannica.com/EBchecked/topic/559095/speed-of-light. (re-worded) The speed of light refers to the rate of change of distance of light waves in motion through varying substances. Drift chambers (talk) 14:13, 20 September 2011 (UTC)

I'm sorry but the sentence you added, which I quote below in full, was simply nonsensical. The EB is a tertiary source, not a reliable secondary one, so should not be used anyway but you have clearly misunderstood it and the topic. --JohnBlackburne^words_deeds 15:08, 20 September 2011 (UTC)

The speed of light is the value calculated for the time taken for light to travel from a source emitting energy within the visible spectra to a hypothetical observer, an example is the light from the star known as the sun to an observers position within or upon earth.

John, I'm not familiar with your editing history. Writing "you have clearly misunderstood it and the topic" may or may not be true, but it is an incredibly insensitive way to make a correction. It attacks the person instead of the information. You might consider other ways of expressing yourself that do not so easily risk offending a possible neophyte who is trying in his or her own and possibly uninformed way to improve WP. You might, for example, analyze the actual proposed content, explaining the mistakes. Or you might compare with a correct revision of the proposed content. This was probably a lapse on your part, as I'm sure you agree that we should be welcoming, or at least polite, to all contributors. There is even a policy to that effect (WP:CIVIL), as you no doubt know. Please forgive this bit of chiding; I have similarly lapsed myself and have had to learn a similar lesson. David Spector (talk) 02:46, 28 November 2011 (UTC)

Substitute "an electrical insulator" → "a dielectric material"?

The last paragraph in section In a medium reads:

It is possible for a particle to travel through a medium faster than the phase velocity of light in that medium (but still slower than c). When a charged particle does that in an electrical insulator, the electromagnetic equivalent of a shock wave, known as Cherenkov radiation, is emitted.

It seems appropriate to replace "an electrical insulator" with "a dielectric material". I do not have access to the reference. Any opinions? Quondum^talk_contr 07:31, 17 November 2011 (UTC)

I have had now made this change. Quondum^talk_contr 04:26, 6 December 2011 (UTC)

Fonts in infobox

I changed some of the fonts in the infobox from bold to normal using labelstyle = font-weight:normal because the Distance items didn't seem to be labels (although one could look at them as labels) and it looked better to me in normal font. However, there were items in the sections Exact values and Approximate values which might be more appropriately called labels. The label items in those two sections can be changed back to bold font manually using ''' ''' but I'm not sure whether the infobox would look better with them in bold. But rethinking all of this, my basic motivation was simply that I thought that the infobox looked better with the changes to normal font. Interested in thoughts about any of this. --Bob K31416 (talk) 13:30, 25 November 2011 (UTC)

The change looks fine to me. TR 14:51, 25 November 2011 (UTC)

Trimming list of signal times

While we are on the subject: Maybe the list of signal times could use some trimming. At the very least there is no reason to have entries for both proxima and alpha centauri. Other possible points: Do we need both the length of the equator and the distance to geostationary orbit? The distance to the canis major dwarf galaxy? I think that the main point of these times is to give an indication of the order of magnitude of c. This purpose is somewhat defeated by including relatively obscure distances.TR 14:51, 25 November 2011 (UTC)

Delete Proxima. Keep Alpha as more notable and essentially the same distance. I'll have to think some more about what to do with the others you mentioned. --Bob K31416 (talk) 16:01, 25 November 2011 (UTC)

I would argue the precise opposite: Proxima is the more notable as the nearest star. There is nothing notable about Alpha at all apart from the fact it is nearby. If that is significant it makes sense to go for the nearer one. As for the other units, I think we can ditch the figures for kilometre and statue mile since they essentially replicate the earlier figures for both Imperial and metric systems. As for the others most appear to me to relate to the object under comparison than the speed of light and so probably deserve to go, however some are so often used it may be as well keeping them. The ones I'd ditch are:

• Length of the Earth's equator (ambiguous)
• Moon to Earth
• Voyager 1
• The galaxies, with the exception of the Milky Way's diameter.

Crispmuncher (talk) 16:48, 25 November 2011 (UTC).

My mistake for using the word notable, although Alpha Centauri is notable for being close, the third brightest star in the night sky, and "known for as long as recorded history".[9] When I wrote my previous message I meant that Alpha Centauri is the more familiar. For example, "Proxima Centauri" googles 454,000 hits and "Alpha Centauri" gets 3,270,000. Although I would prefer using Alpha Centauri, I could accept either one. --Bob K31416 (talk) 17:14, 25 November 2011 (UTC)

The Alpha Centauri system contains Alpha Centauri B, a star much like our own, around which Earth-like planets may exist (Alpha Centauri A will make for interesting nights, but should not disturb any planetary system). Alpha Centauri C (proxima) may be a silly bit closer, but nobody will be traveling to it as it's a red dwarf and uninteresting. Alpha Centauri B is notable for being probably a good first target for interstellar flight (though it's a safe bet that nobody will send a probe without good telescopic survey's showing an oxygen-bearing planet which also has methane, not too large, and residing in the liquid-water "life-zone." SBHarris 19:12, 25 November 2011 (UTC)

Here is a possible change for the list of signal times.

Speed of light

Sunlight takes about 8 minutes, 19 seconds to reach the Earth (based on the average distance).
Exact values
Metres per second	299792458
Planck units	1
Approximate values
kilometres per second	300,000
kilometres per hour	1,080 million
miles per second	186,000
miles per hour	671 million
astronomical units per day	173
Approximate light signal travel times
Distance	Time
one metre	3.3 ns
across an average-sized continent (6000 km)	20 ms
from Moon to Earth	1.3 s
from Sun to Earth (1 AU)	8.3 min
from nearest star* to Earth	4.24 years
from nearest galaxy† to Earth	25,000 years
from furthest observed galaxy to Earth	13 billion years
_______       * other than the Sun       † other than the Milky Way

--Bob K31416 (talk) 17:34, 28 November 2011 (UTC)

I like it, but I'd keep “one foot” (1 ft/ns is a somewhat common approximation/mnemonic, plus if we have metric “one metre” we can have the imperial analogue as well), replace “across an average-sized continent” with “halfway along Earth's circumference” (i.e. the longest possible distance between two points on Earth's surface; a better wording might be found), and maybe keep the “from geostationary orbit to Earth” (it's a limiting factor in satellite-based communications; might be replaced with “from Earth to geostationary orbit and back”). ― A. di M.​  19:01, 28 November 2011 (UTC)

1) re foot - How about instead of metre, having:     one foot     (0.305 metres)         1ns ?

2) re halfway along Earth's circumference - That's about 20,000 km which is close to distances to orbit (Geostationary 36000 km) and thus repetitious, if we want to use an orbital distance. However, I think it would be useful to have a purely terrestrial distance, e.g. continent which is a little less repetitious at 6000 km. Also, the purpose of using continent was to relate to long distance communication within continents. I tried to get something that was nation neutral, like you did.

3) re geostationary - Instead of Geostationary, how about a more general classification, such as: "between a mid-Earth orbit and Earth (20,000 km)" ? However, whichever orbital altitude is used, it would be somewhat close in distance to continent or half the length of the equator. So we may want to choose between the orbit and the terrestrial item (e.g. continent). The terrestrial item might be better because it would be the only terrestrial distance we have in the list; the others are all extratrestrial distances, except for foot or metre.

--Bob K31416 (talk) 03:23, 29 November 2011 (UTC)

I'm not entirely convinced by the "nearest galaxy" thing. This is somewhat misleading since the Canis Major Dwarf Galaxy is a satellite galaxy to the milky way (it is in fact nearer to the earth than the center of the milky way.) If you want something in this length scale "distance to the center of the milky way" may be a better choice. (I also agree with AdiM on the foot value.)TR 22:18, 28 November 2011 (UTC)

An alternative to "nearest galaxy" is "across the Milky Way galaxy", which is what is currently in the infobox, except for the addition of "galaxy". --Bob K31416 (talk) 03:23, 29 November 2011 (UTC)

Yeah, the galactic centre might be a good idea too. ― A. di M.​  10:34, 2 December 2011 (UTC)

The advantage of mentioning the distance to center of the Milky Way instead of the diameter, is that the diameter of the Milky Way not that well defined (there is no clear outer boundary). — Preceding unsigned comment added by TimothyRias (talk • contribs) 12:21, 2 December 2011 (UTC)

Well, since the discussion has fallen off, I'll test the waters and make the change corresponding to the Centauris and see what happens. --Bob K31416 (talk) 00:33, 2 December 2011 (UTC)

While you're at it, you may wish to consider the last entry: "from furthest observed galaxy to Earth: 13 billion years". This is the light travel time from a point in the past to reach Earth now, but it does not reflect the current distance between the Earth and that galaxy, which is considerably larger than 13 billion light years. Nor does it reflect the time a signal emitted now by that galaxy will take (it'll never reach us). So this entry actually needs qualification. Perhaps a rewording to the effect of: "from furthest observed galaxy to Earth, arriving now"? Quondum^talk _contr 12:35, 2 December 2011 (UTC)

Since it is a list of light travel times, this is not that ambiguous.TR 12:59, 2 December 2011 (UTC)

Re "from furthest observed galaxy to Earth, arriving now" – might confuse. Instead, maybe a footnote.

from furthest observed galaxy* to Earth                     13 billion years

...

...

_____________

* position of galaxy when it emitted the light that reaches Earth today

In general, footnotes don't affect the flow of reading for those who aren't concerned with more detail, and one isn't constrained as much by the need for being concise, which can result in lack of clarity in some cases. --Bob K31416 (talk) 15:36, 2 December 2011 (UTC)

I added a footnote similar to the above to the infobox of the article, because it addresses the point mentioned by Quondum and it is informative, keeping in mind the above comments re footnotes in general. --Bob K31416 (talk) 14:04, 3 December 2011 (UTC)

I noticed that a couple of edits changed the footnote from "position of galaxy 13 billion years ago when it emitted the light that reaches Earth today" to "For light reaching Earth today". I agree that the latter is sufficient for those of us discussing the subject on this talk page, but I don't think the point that the light travelled from the position of the furthest observed galaxy as it was 13 billion years ago would be evident to most readers. Also, note that in the left column of distances, the form of many of the entries is essentially the distance "from position A to position B". --Bob K31416 (talk) 18:16, 3 December 2011 (UTC)

You're right. As suggested by my edit summary, I am not quite happy that it is clear. The problem arises from the fact that this last measurement is quite different from the rest: it is not the time light takes to travel a set distance; it is simply a measurement of the time that the light has travelled since it was emitted. This makes it in some sense tautological and quite misleading. Please feel free to make it more detailed again; maybe you'll be able to get some sense into the wording that a distance is not being indicated? The column is headed "Distance", but how we define distance when it changed from much less than to much more than 13 bn light years while the light was travelling is problematic, to say the least, especially for the reader not versed in cosmology. I almost feel the entry does not belong in the table at all because of this. Quondum^talk_contr 20:23, 3 December 2011 (UTC)

And you're right. I now see the problem with the original footnote and the furthest observed galaxy entry re distance. --Bob K31416 (talk) 21:08, 3 December 2011 (UTC)

Here's another try at a footnote. Interested in comments.

This distance is complicated by the expansion of the universe during the light's travel time.

--Bob K31416 (talk) 21:45, 3 December 2011 (UTC)

This is a major improvement. It captures what is needed in simple language. Quondum^talk_contr 21:51, 3 December 2011 (UTC)

Seems like I should have gotten an edit conflict but didn't. Were you referring to the present one I just put up or the previous one that I put up just before your recent message? Anyhow, thanks. --Bob K31416 (talk) 22:01, 3 December 2011 (UTC)

Since the history shows I posted before your revision, I was clearly referring to the earlier version. But I like your latest version better. Quondum^talk_contr 22:11, 3 December 2011 (UTC)

I just made the change to the footnote in the article. If anyone objects, feel free to revert and explain here. Thanks. --Bob K31416 (talk) 22:24, 3 December 2011 (UTC)

On a light-hearted note, under "Exact values", perhaps "phoots per second: 10⁹"? On second thoughts, perhaps not: you were trimming the list. Quondum^talk_contr 12:55, 2 December 2011 (UTC)

I was indeed surprised that that unit had no name as far as I knew... ― A. di M.​  17:31, 2 December 2011 (UTC)

On article page, combined parsec and nearest star entries with the result

"from nearest star to Sun (1.3 pc)                    4.24 years"

--Bob K31416 (talk) 16:09, 2 December 2011 (UTC)

Furthest observed galaxy

I think we should consider deleting this entry from the infobox unless a reliable source is found that gives the light travel time. At present, there is a link for "furthest observed galaxy" to the Wikipedia article UDFj-39546284. That article does not comment on the light travel time and its source for the distance of the galaxy doesn't either.[10] From our discussions, we see there are complications in determining the light travel time. Without a reliable source, an entry for the light travel time would be questionable original research. (Credit is due Quondum for originally considering that this entry should be deleted.) --Bob K31416 (talk) 14:10, 5 December 2011 (UTC)

Hm, I have just added a source for the 13 billion light years distance, but I hadn't seen this section. A quick skimming of the above suggests that we can't ever use light travel times for anything, since we never know whether the object in question will still be there? Duh, I think I'll keep away from that type of discussion :-| - DVdm (talk) 14:45, 5 December 2011 (UTC)

For the furthest observed galaxy, the main problem is that the universe was expanding during the travel time for the light that reaches Earth today. For the other entries it seems reasonable to neglect this effect, to the approximation that is given. --Bob K31416 (talk) 15:00, 5 December 2011 (UTC)

That kind of decision sounds like wp:OR to me. — See source and quotation here below. I think this whole thing smells a bit like, as we would say and badly translate, "looking for nails at low tide" :-) - DVdm (talk) 15:06, 5 December 2011 (UTC)

Re OR, my general approach is that if information isn't questionable to me or anyone else, I leave it in by remembering the policy WP:IAR, which states

"If a rule prevents you from improving or maintaining Wikipedia, ignore it."

This approach is somewhat consistent with the following excerpt from WP:V

"This policy requires that all quotations and anything challenged or likely to be challenged be attributed in the form of an inline citation that directly supports the material."

although this excerpt assumes that a reliable source probably exists. --Bob K31416 (talk) 16:34, 5 December 2011 (UTC)

Update: See light travel times at [11]: "Of course, the light-travel implies the distance in light-years that the light has traveled to get to you, but noy the distance that the object would now be, which would be much greater. Table 5.1 shows the light-travel time for various vales of z." - DVdm (talk) 14:57, 5 December 2011 (UTC)

(To be nitpicky, the source [12] actual does directly translate the observed redshift in travel time for the light. That is rather standard cosmological statement to boot anyway.)

More to the point, I actually agree that it may be a good idea to remove it. Including it, does not give any interesting information about the speed of light. Both the distance and the travel time are very big numbers, that effectively register as "huge". As such, the entry gives information about the size of the observable universe rather than the speed of light. A similar thing may be said about the other entries regarding other galaxies and the size of the Milky Way.TR 15:03, 5 December 2011 (UTC)

Not to neglect your other worthwhile points, but for now re "the source [13] actual does directly translate the observed redshift in travel time for the light" - I didn't see what you were referring to in that article. Could you give the relevant excerpt here? --Bob K31416 (talk) 15:37, 5 December 2011 (UTC)

It is located at a redshift of 10.3 – redshift is a measure of how much the expansion of space has stretched the light from an object to longer (redder) wavelengths – equivalent in this case to 13.2 billion years.

Note that the last part is a time. TR 16:23, 5 December 2011 (UTC)

Thanks. That sentence got my attention too, but to me it's unclear that it is saying that the travel time is 13.2 billion years. --Bob K31416 (talk) 17:17, 5 December 2011 (UTC)

The quote does seems like bad English. Yet 13.2 bn y is the travel time, i.e. time since emission, and not directly related to distance. As I understand it, the redhift also directly gives the ratio of comoving distances: approx 30 bn l-y now, approx 3 bn l-y then. The difficulty of interpretation reminds me of the Robotics article that confusedly spoke of a contaction of 400%... Quondum^talk_contr 17:59, 5 December 2011 (UTC)

The quote is formulated badly, but it is general knowledge (in cosmology) that redshift directly measures travel time. (The longer a foton travels the more it is redshifted.)TR 22:42, 5 December 2011 (UTC)

I guess we should decide what the table is intended to say. The first entries illustrate the speed of light in terms of known distances and times. The next (e.g. to the moon, sun and nearest star) in a sense give large distances in terms of light travel times, but since these are everyday ideas this becomes an illustration of how light speed can make large distances interpretable. After that, as TR says, time is "huge", and distance is "ultrahuge", so nothing is illustrated aside from the hugeness of "out there". Unless we can find what an entry illustrates in relation to the speed of light, I think should be treated as a waste of space. Quondum^talk_contr

One of the points of the SoL article is that c is the maximum speed that information travels. The table can be viewed as giving examples of how this limit affects the time it takes to receive information from various objects, including the furthest observed galaxy. Whereas this latter example doesn't affect every day life, it does affect science and philosophy, which may be of interest to students, teachers and practitioners who are part of the audience that the article is trying to reach. However, with this comment I'm just addressing this particular point and there are other points to consider regarding whether or not to keep the furthest observed galaxy entry in the table. --Bob K31416 (talk) 16:11, 5 December 2011 (UTC)

I don't think that is an appropriate point in the infobox at the start of the article.TR 16:32, 5 December 2011 (UTC)

I don't see anything wrong with "13 billion years". Remember that we don't call it "huge". We quantify it to 13 'somethings', in this case "billions of years". So what? What is wrong with that? Listing the light-travel time to/from that galaxy as "huge", would indeed be a waste of space — and downright unencyclopedically silly. Omitting it in its quantified state, would raise the question whether we should mention 2.5 million years. Isn't that huge? Or is it merely enormous? Or very very great? No, it isn't. Is it quantified. The way the entry is listed now, together with the note about the expansion complication, I think it is very appropriate and (unlike this discussion perhaps) not a waste of space at all. - DVdm (talk) 16:23, 5 December 2011 (UTC)

Note, that I suggested that the other huge entries should be removed as well. I think the current footnote looks kind of silly. (The grammar alone is funky. The distance is "complicated"? How do you complicate a distance?TR 16:32, 5 December 2011 (UTC)

@TR: Okay, perhaps it should read "The definition of this distance...".

@DVdm: Numbers might be encyclopedic, but relevance should be justifiable. What do they illustrate in this context?

@Bob: Okay, you've identified a new point (or two) being illustrated: how the speed of light results is relevant to waiting for signals (from people via satellite, from a planetary probe) and from what age we can glean info from astronomical objects. Perhaps the table should be split into sections so that the reader is clear on what is being illustrated.

Back to the point of inclusion/exclusion of the item being debated, what is it illustrating aside from the age of the universe? Quondum^talk_contr 17:11, 5 December 2011 (UTC)

I wouldn't split the table. In any case, I'm uncomfortable with that entry in the table because I don't understand how to define and determine travel time and the corresponding distance when the expansion of the universe during the travelling is a non-negligible consideration, and so far there doesn't seem to be a reliable source that explicitly states what the travel time is for the light from the furthest observed galaxy. --Bob K31416 (talk) 18:27, 5 December 2011 (UTC)

@TR: You put a finger on the source of my unease with that wording (which prompted me to do this) better than I could have myself. ― A. di M.​  17:45, 5 December 2011 (UTC)

Traveling at the speed of light

I think it's relevant and in order to put a sentence such as the following in one of the first two paragraphs: Ironically, from light's perspective, no time passes as it travels, only for the surrounding matter, time seems to speed up. — Preceding unsigned comment added by 196.215.115.224 (talk) 09:08, 28 November 2011 (UTC)

Please sign your talk page messages with four tildes (~~~~)? Thanks.

This might be ironic if it were even true, but this is an encyclopedia, and we are not supposed to editorialise. Such statement also would need to be properly backed by a reliable source. - DVdm (talk) 09:31, 28 November 2011 (UTC)

(edit conflict) Well, the idea that the proper time along a light ray vanishes might be worth mentioning, but I can't think of a decent way to word that. ― A. di M.​  09:33, 28 November 2011 (UTC)

Quite, without a reference frame it's hard to talk about time, even about its non-existence. Tricky :-) - DVdm (talk) 09:48, 28 November 2011 (UTC)

There is nothing tricky about saying that the proper time along a light ray is zero. (Of course, a sentence that includes a phrase "from light's perspective" is a lot more tricky because it presumes there exist a reference frame for the light.) I say little reason to mention this in the lede of this article.TR 11:28, 28 November 2011 (UTC)

Agreed on all accounts. - DVdm (talk) 11:40, 28 November 2011 (UTC)

Who talked about the lede? I was thinking more of the section “Fundamental role in physics” or its subsection “Upper limit on speeds”. I can't make up my mind whether that belongs, though. (And the tricky part would be to word that in a way that kind-of sort-of makes sense to someone who doesn't know what proper time means but is not technically incorrect or misleading. If I come up with something decent, I'll post it here.) ― A. di M.​  16:25, 28 November 2011 (UTC)

I think something along those lines should be mentioned in the “Fundamental role in physics” section. Martin Hogbin (talk) 19:09, 28 November 2011 (UTC)

Could you explain why you think this appropriate in an article about the speed of light?TR 12:13, 29 November 2011 (UTC)

Well, the slope of null geodesics is, like, the simplest definition of c (not mentioning light) I can think of. :-) — Preceding unsigned comment added by A. di M. (talk • contribs) 14:58, 29 November 2011 (UTC)

That statement is physically equivalent to "c is the velocity of a massless particle" which already appears in the article. (And the latter is more accessible to a general audience.) TR 15:26, 29 November 2011 (UTC)

A. di M., In any case, from the discussion so far it looks like the suggestion in the first message isn't suitable. As you mentioned, "If I come up with something decent, I'll post it here", so we can wait for that to discuss this further. In general, everyone should be careful not to digress into the topic of special relativity, since there is overlap between that topic and this article. Re "slope of null geodesics" - editors like me who need to review the subject, can look at the lead of Geodesic (general relativity) for "null geodesic", and the beginning of the section Mathematical expression for the relevant definition of slope (tangent). --Bob K31416 (talk) 16:00, 29 November 2011 (UTC)

Speed of light

The article begins by defining the speed of light in vacuum, which links to "absence of matter", a description that ignores the virtual particles generated in any vacuum, continues with "Its value is 299,792,458 metres per second", which applies only in the reference medium of the SI units which is classical vacuum, and in the next paragraph says: "According to special relativity, c is the maximum speed at which all energy, matter, and information in the universe can travel" To my count, these are three different things confused as one.

The first might be modeled as QCD vacuum as that allows vacuum fluctuations and comes closest to a model of a physical vacuum that actually might be approached in principle by experiment, the second is an idealized value in a medium that doesn't allow vacuum fluctuations and so can never be realized in any physical apparatus (even though it is an accurate model in some cases), and the last refers to a physical limit on certain phenomena that take place in the real universe, which might refer to QCD vacuum but maybe not, as the underlying concept of a limit doesn't actually require a detailed description of just how this propagation occurs, and gravitational effects are not included in QCD.

In short, this introduction is really bad, and judging from the history of this article, will be subject to unending modification without benefit for the rest of time. Brews ohare (talk) 18:05, 29 November 2011 (UTC)

This could be mentioned somewhere in this article or in another article. This is then only a theoretical issue; in principle, the speed of light in a vacuum depends on he boundary conditions, the presence of (elctromagnetic) fields, but these effects are so small, that you don't need to specify this in practice when defining the metre in terms of the speed of light. I think the Scharnhorst effect is mentioned in this article, and we also have an article on the Euler-Heisenberg Lagrangian, and that latter article needs to be expanded by including the formuae for the effective index of refraction for light propagating through a vacuum containing magnetic fields. Count Iblis (talk) 18:48, 29 November 2011 (UTC)

Is there any reliable source that discusses the effect of any of these considerations on the speed of light, and how much they would affect its measurement? This does bring to mind recent work on neutrino measurements (OPERA neutrino anomaly) that got a result faster than the speed of light that was obtained previously from the best photon measurements. Maybe a reliable source useful for these considerations might come out of work explaining that neutrino result. In any case, as editors we have the luxury of not having to come up with our own ideas, but rather get ideas from reliable sources. --Bob K31416 (talk) 20:49, 29 November 2011 (UTC)

<nitpick>It was not “the speed of light that was obtained previously from the best photon measurements” (how do you send photons across 730 kilometres of rock?), it was the speed of light as one of the two fundamental constants of general relativity, which the GPS assumes is valid in order to work.</nitpick> ― A. di M.​  22:15, 29 November 2011 (UTC)

By "best photon measurements" I was trying to refer to the ones at the end of this section. --Bob K31416 (talk) 22:36, 29 November 2011 (UTC)

AFAIK, the GPS just assumes GR to be exact and uses the SI definition for the metre and the second, so it doesn't need a measurement of c at all. ― A. di M.​  15:12, 30 November 2011 (UTC)

I think we are talking about two different things, probably because my original comment about OPERA wasn't appropriate, due to my lack of knowledge at the time of the numbers involved. So you can disregard the part of my original message about OPERA. --Bob K31416 (talk) 18:01, 1 December 2011 (UTC)

Animation — light from furthest observed galaxy

There's a nice animation on the webpage The Distance Scale of the Universe which might be useful in the section Speed_of_light#Astronomy which discusses light from faraway galaxies. From the copyright information at that website, it appears that the animation can be copied to Wikimedia Commons for use in Wikipedia. So there it is, if anyone would like to move it into Wikimedia Commons and then into the article. --Bob K31416 (talk) 15:28, 6 December 2011 (UTC)

It doesn't belong to this article IMO. If we really think that some reader would take distance to mean anything else than ‘light travel distance’ in that table (which I don't; they would have knowledgeable enough to be aware that other distance measures exist and stupid enough to fail to realize they are not what is meant here), then we'd better not have such an entry at all. ― A. di M.​  16:02, 6 December 2011 (UTC)

I'm not sure you understood my last message, which wasn't about the entry that was recently removed from the table, but rather about an animation for the section Speed_of_light#Astronomy. --Bob K31416 (talk) 16:11, 6 December 2011 (UTC)

I feel that this is entirely off-topic for the article: this would amount to illustrating the findings of cosmology. I don't think it is appropriate to have more than the brief mention that is already in the section. It'll raise far more questions for the average reader than it'll answer. On a side-note, the illustration gives a sense that things are distorted by the idea of the light "trying to catching up", but little more. Quondum^talk_contr 16:19, 6 December 2011 (UTC)

Wow, do we see things differently! I could respond but I sense there is so much resistance here that it's not worth it for me to continue discussing it, and if I did the only thing I think I would accomplish is to annoy people. I'll wait to see if anyone finds the animation worthwhile for the article. [Note added 17:58, 6 December 2011 (UTC): Be sure to read the section Speed_of_light#Astronomy before deciding whether the animation would be useful there.]--Bob K31416 (talk) 16:52, 6 December 2011 (UTC)

Sorry, I do not mean to antagonize anyone or to dampen your enthusiasm. I suppose I feel articles are best concise and focused, not pedagogical or full of information covered in other articles. Judging by the style of many articles, mine may be a minority view. I'll also wait for other views. Quondum^talk_contr 17:29, 6 December 2011 (UTC)

Maybe that animation, with a very short caption, could be added to the penultimate paragraph of Section “Faster-than-light observations and experiments”, but even that would be a stretch. (The article is already 108 KB, about 5% more than when it was promoted to FA, and even then people agreed that it was already comprehensive enough.) ― A. di M.​  14:46, 7 December 2011 (UTC)

A. di M., I see why you considered that penultimate paragraph somewhat appropriate for the animation, and I understand your concern about the size of the article. However, the Astronomy section presently discusses the light travel time of 13 billion years from faraway galaxies and the animation depicts that 13 billion year trip, so that would seem to be a good place for it, except that the article is already too large. With this in mind, instead of putting the animation itself in the Astronomy section, a link to it could be put in a footnote that is added to the existing Notes section. Here's the subject sentence as it presently appears in the Astronomy section of the article, along with the proposed footnote.

"For example, it has taken 13 billion (13×10⁹) years for light to travel to Earth from the faraway galaxies viewed in the Hubble Ultra Deep Field images."

Notes

1. An animated depiction of the light's 13 billion year trip can be found in the lead section of The Distance Scale of the Universe.

--Bob K31416 (talk) 20:08, 7 December 2011 (UTC)

I don't see what that would add. (Not to mention that the quoted website is not a WP:RS.)TR 22:15, 7 December 2011 (UTC)

Me neither. As for RSness, hell, the light doesn't even redshift in that picture! (Plus, I would have used a grid with closer spacing.) ― A. di M.​  10:59, 8 December 2011 (UTC)

Conversion for kilometre and miles per hour is NOT incorrect on chart in infobox

If the speed of light is 299,792,458 metres per second this works out to 299,792.5 Km/s or 17,987,547.5 km/h. I believe the chart stated this as being 1,080,000 km/h. Similarily, when you convert the km/h to mph you should arrive at 11,176,943.8 mph.

Can someone, verify this calculation and then edit the chart as I am unable. — Preceding unsigned comment added by 207.195.52.1 (talk) 18:28, 15 December 2011 (UTC)

You seem to have converted from seconds to hours with a factor of 60. There are 3600 seconds in an hour. Quondum^talk_contr 18:34, 15 December 2011 (UTC)

Yes it appears that I did thank you kindly sir — Preceding unsigned comment added by 207.195.52.1 (talk) 18:08, 21 December 2011 (UTC)

The listing gives the speed as approximately 1,080 million km/h, i.e., 1,080,000,000. Hertz1888 (talk) 18:48, 15 December 2011 (UTC)

Questions

a very very interesting article.

I have 2 questions and would much appreciate any replies or observations. (nb. i am a historian, not a physicist, so in lay terms please)

A. why is C such an exact value.

B. assuming a source of photons, what distance is estimated between the emission of a photon and its achievement of velocity C and what stimulii create this acceleration.Miletus (talk) 19:49, 14 January 2012 (UTC)

A. It's a constant and because of the way it relates distance and time it's defined to be an exact value. This fixes the metre in terms of the definition of the second

B. They attain that speed instantaneously and always go at that speed. They just do: it's an intrinsic property of light (and other electromagnetic waves) that it goes at this speed.

Sorry, that's maybe not in lay terms but it's difficult to explain properly without using pretty advanced theory. E.g. the answer to A depends on an understanding of special relativity.--JohnBlackburne^words_deeds 20:34, 14 January 2012 (UTC)

Actually, the answer to can be put in very simple terms. The meter is defined is the distance that light travels in a certain fraction of a second. Consequently, the speed of light (the distance travelled divided by the time taken) is exactly this fraction. This is just a consequence of the way the units are defined. Compare this to the easier to understand situation of the speed of light measured in lightyears per year. Since a lightyear (by definition) is the distance traveled by light in a year, the speed of light is exactly 1 lightyear per year. (What requires more knowledge to understand, is why this is a good definition of the meter.)

B. is indeed almost impossible to explain in laymans terms. The thing here is that photons always travel at the speed of light, they are created at their source travelling at that speed. There is no acceleration involved.TR 22:47, 14 January 2012 (UTC)

Miletus, you have two good answers above. I hope they make sense to you. Let me just add that photons are quantum entities. If you start thinking of them as little balls travelling at a specific speed and following a trajectory you have misunderstood what they are. Martin Hogbin (talk) 00:48, 15 January 2012 (UTC)

Isn't this something for the wp:reference desk/science? - DVdm (talk) 10:00, 15 January 2012 (UTC)

Strictly, yes, or at least for user's talk pages. I would be happy to continue to discuss this on my talk page. One thing I agree with Brews on is that statements from readers that they do not understand something in an article can be very useful in improving the article, so there is a logic to at least starting the discussion here. Martin Hogbin (talk) 10:09, 15 January 2012 (UTC)

One further thought on B. The reason why things take time and energy to accelerate is because they have mass and inertia, and it takes time to overcome that inertia. The heavier something is the longer it takes to accelerate it. Conversely the lighter something is the less time it takes. Light has no mass, no inertia, so needs no time to get up to speed or change direction.--JohnBlackburne^words_deeds 11:05, 15 January 2012 (UTC)

B is a tough question. Questions like that are humbling because it's hard to give a satisfying answer. You don't learn them in physics class. By definition, a photon travels at the speed c. If it doesn't, as when accelerating to the speed c, then it's not a photon. All I can add is that the microscopic world of quantum mechanics that describes photons is not accessible through our common sense experience. It's quite a different world that our common sense can't explain and is constructed with theories expressed by mathematics. The only reason we believe it, is that it ultimately predicts results that we can check by doing experiments. --Bob K31416 (talk) 03:52, 20 January 2012 (UTC)

We should be careful of even of statements such as "By definition, a photon travels at the speed c." I think that c is best defined as a well-established physical constant, and would remain unmodified even if we discovered that photons had a rest mass. For instance, the experimentally known upper limit on the rest mass of a photon is rather small, but it is nonzero. And we could perhaps in principle find a new ultrashort-range force that the photon couples to, which might result in a sub-lightspeed propagation near some objects. Photons also "have" a rest mass inside superconductors; with an energy below that they can only penetrate it as a virual particle. And before you write off a superconductor as "not vacuum", vacuum is treated in some standard approaches as a superconductive quark condensate. — Quondum^☏✎ 04:45, 20 January 2012 (UTC)

Parts of your comment seemed to be in the context that anything's possible. Other parts:

Re "Photons also 'have' a rest mass inside superconductors..."— I guess the quotes around "have" mean that it's not true. Was your comment based on the following?

"A particle of mass M has associated with it a Compton wavelength given by $\hbar /Mc$, where $\hbar$ is Planck’s constant and c is the speed of light. The Compton wavelength tells how fast the field of a particle falls off with distance. For example, the exponentially falling penetration of the magnetic field into a superconductor means that the photon has acquired a mass $M_{\gamma }=\hbar /\lambda c$. (The expert reader will object that this is not a true photon mass, because the condensate is nonrelativistic, and the electric field behaves differently from the magnetic field. But it’s close enough for our purposes.)"[14] (p. 28/6)

Re your last sentence, "And before you write off a superconductor as "not vacuum", vacuum is treated in some standard approaches as a superconductive quark condensate."— Would you happen to have a link to the approaches you are referring to? There's the Affine Higgs mechanism which has a non-zero photon mass, but that came out well before quarks.

--Bob K31416 (talk) 17:29, 20 January 2012 (UTC)

You are correct about the effective source of "photon mass" in a superconductor (I read a different article, though). It is merely an analogy, and is not to be taken literally. The presumed quark condensate would give mass to the photon's sister particles W and Z, but not to the photon itself; sorry about any confusion. I was only suggesting that it is plausible to have some unknown mechanism giving the photon a small rest mass: my point is merely that we cannot define the rest mass of the photon. — Quondum^☏✎ 19:32, 20 January 2012 (UTC)

wave length = rotation

please note there is no wave length as such but the rotation of the electron charge, think of the riffeling effect of a round bullet. the bullet travels forward in a straight line but the charge rotates giving the effect of an up and down motion ie wave length. the faster the rotation the shorter the apparent wave length this solves the wave particle problem. I am interested in this stuff and do not know how to use this forum you can email me at tom@thehomeofmusicandsound.com — Preceding unsigned comment added by 60.229.175.8 (talk) 12:35, 31 January 2012 (UTC)

Note that this is not a forum. Article talk pages on wikipedia are exclusively for discussing improvements to the article. For general discussions on physics topics like this please visit a physics forum such as [physicsforums.com].

Internal Propogation delay of the observer system

Lets imagine a micro processor based intelligent digital system that can detect and measure electro magnetic radiation.

Assume that the system has not been calibrated to any human standard of measurement units.

So the system's unit of time, is dictated by its own clock speed (t).

Assume that the system starts with no memory or knowledge of earlier data to compare its current readings, but it has memory to store readings from the time it is switched on.

Let us assume that the system's scale of measurement is 0 to 65536 counts.

Let us assume an environment in which the electromagnetic radiation is a perfect sine wave of a constant frequency and constant amplitude.

Assume that the sensor that detects the AC radiation produces a average DC signal which when read by the digital system is equal to 30,000 counts.

Let us assume that the intelligent system takes a sample reading once in 10,000 clock ticks and saves it in its memory.

As long as there is no change in the frequency and amplitude of the radiation, the intelligent system will keep recording the same 30,000 counts after every sampling.

What is the speed "c" of the radiation in this environment?

It is irrelevant because, the concept of "speed" is applicable only for the propagation of a "variation" of either frequency or amplitude. Since both are constant, the concept of "speed" does not arise.

Now let us assume, that there is a 10%change in amplitude of the waveform.

If this change occurs and dies down (restores to previous level) within the sampling time of 10,000 clock ticks, the system will never know. However, if the change persists, over multiple sampling periods, then the system will record it as such.

Now let us assume that the sampling interval is reduced to 1 clock tick from 10,000 clock ticks.

Now if an event occurs in the environment, which caused the sensor's signal (and hence the count) to increase from 30,000 to 30,300 the earliest the system will know about it is after "One clock tick".

Assume there is no other means to know, when the event occured, and at what speed the impact of the event propagated in the environment and reached the sensor of the digital system.

So if the system wants to measure the speed of propagation of a "Change" event, it has to induce an event by itself and measure the return signal.

Assume that the system has a means to disturb the raditation in its environment.

However the speed of propagation it will eventually measure will be limited by

a) the "t" of its own clock tick b) the response speed of its "digital to analog to radiation transducer" c) the response speed of its "radiation to analog to digital transducer".

There is no way to know, whether the propagation speed measured by the system is its own internal speed of propagation only, or the propagation speed of the radiation per se in the environment, or a combination of both.

Let us now hypothesise that a "radiation changing impact event propagates instantaneously and simultaneously across the entire environment". Then the speed of propagation measured by the system will be nothing but its own internal propagation speed.

If we replace the intelligent digital system, with the human system, then the speed of light c measured by us, includes

(a) the speed of propagation internal to the human system + (b) the propagation speed of the radiation in the environment. Is there a way to prove or disprove that b = 0 or b <> 0

(----) Ramkumar R S

Note that this is not a forum. Article talk pages on wikipedia are exclusively for discussing improvements to the article. For general discussions on physics topics like this please visit a physics forum such as [physicsforums.com].TR 06:59, 3 February 2012 (UTC)

Wow, mind blowing. I think this is a good place to link to / advertise a forum, due to the fact that many people will naturally tend to discuss related issues here. 41.164.8.107 (talk) 23:08, 23 March 2012 (UTC)

speed of light and dark

speed of light is 299,792,458
speed of dark is ? — Preceding unsigned comment added by 122.174.159.124 (talk) 10:55, 17 April 2012 (UTC)

There are many things that can move faster than light, in fact at unlimited speed, including shadows. Sometimes this is informally referred to as the 'speed of dark'. See Faster-than-light Martin Hogbin (talk) 13:00, 17 April 2012 (UTC)

It makes no more sense to call the speed of a shadow the “speed of dark” than to call the speed of the spot of red light made by a laser point the “speed of red” IMO. ― A. di M.​  21:44, 17 April 2012 (UTC)

As I say above it is an informal, not entirely serious, use of language. Martin Hogbin (talk) 22:15, 17 April 2012 (UTC)

Off values in table

What do you guys think about this? IMO that's just a waste of space, but I'm willing to listen to arguments for its usefulness. (Anyway, if we do that the smaller percentages should be in parts per million/billion, and we should give them in standard deviations too, like this.)

History of measurements of c (in km/s)

1675	Rømer and Huygens, moons of Jupiter	220000	−27% off
1729	James Bradley, aberration of light	301000	+0.4% off
1849	Hippolyte Fizeau, toothed wheel	315000	+5% off
1862	Léon Foucault, rotating mirror	298000±500	−0.6% (−3.6σ) off
1907	Rosa and Dorsey, EM constants	299710±30	−0.028% (−2.7σ) off
1926	Albert Michelson, rotating mirror	299796±4	+12 ppm (+0.9σ) off
1950	Essen and Gordon-Smith, cavity resonator	299792.5±3.0	+0.14 ppm (+0.014σ) off
1958	K.D. Froome, radio interferometry	299792.50±0.10	+0.14 ppm (+0.4σ) off
1972	Evenson et al., laser interferometry	299792.4562±0.0011	−6 ppb (−1.6σ) off
1983	17th CGPM, definition of the metre	299792.458 (exact)	definition

(I've just noticed some of them were wrong, too.) ― A. di M.​  14:16, 15 March 2012 (UTC)

I agree, this is a waste of space.TR 10:16, 25 March 2012 (UTC)

I feel the added column should be removed. The normal way of indicating uncertainties as done in the previous column already gives an immediate sense of the error over time. Adding these redundant percentages detracts from the quality of the article. — Quondum^☏✎ 15:10, 25 March 2012 (UTC)

Note that the "offs" are not uncertainties. They are errors w.r.t. the official value. I think these values have real merit, and I'd keep them. - DVdm (talk) 15:16, 25 March 2012 (UTC)

It seems interesting and informative to me to know how far earlier measurements were off the current value. Martin Hogbin (talk) 15:18, 25 March 2012 (UTC)

(To make things clear, I had already reverted the edits when I started this discussion.) OK, so we're three people against it and two in favour of it, so maybe some sort of compromise might be in order:

History of measurements of c (in km/s)

1675	Rømer and Huygens, moons of Jupiter	220000, (−27% off)
1729	James Bradley, aberration of light	301000 (+0.4% off)
1849	Hippolyte Fizeau, toothed wheel	315000 (+5% off)
1862	Léon Foucault, rotating mirror	298000±500, −0.6% (−3.6σ) off
1907	Rosa and Dorsey, EM constants	299710±30, −0.028% (−2.7σ) off
1926	Albert Michelson, rotating mirror	299796±4, +12 ppm (+0.9σ) off
1950	Essen and Gordon-Smith, cavity resonator	299792.5±3.0, +0.14 ppm (+0.014σ) off
1958	K.D. Froome, radio interferometry	299792.50±0.10, +0.14 ppm (+0.4σ) off
1972	Evenson et al., laser interferometry	299792.4562±0.0011, −6 ppb (−1.6σ) off
1983	17th CGPM, definition of the metre	299792.458 (exact), definition

This way, the “off” values occupy space which (with my screen resolution) would be unused anyway. ― A. di M.​  18:48, 25 March 2012 (UTC)

Good with me. Perhaps parenthesising the offs? It does give a problem with the sigmas though. I did the first 3 as a test. Feel free to... um... whatever :-) - DVdm (talk) 19:27, 25 March 2012 (UTC)

The normal way of expressing things may be fine among professionals, but I feel we should try for optimal clarity for a wide spectrum of readers. The second table is hard to read because it is so cluttered. The additional, separate column for the uncertainty is needed. I'd also suggest: instead of "-27% off", or "+12 ppm off", we can say "27% low" and "12 ppm high". It would be more immediately comprehensible, slightly more compact, and avoid the repetition of "off". Hertz1888 (talk) 00:14, 26 March 2012 (UTC)

Just another suggestion: this level of detail would be readily accessible to a general audience as well as compact (if not as accurate) if displayed graphically as the 1σ range on a graph against time instead of putting all this detail into a table. Maybe I just like graphs... — Quondum^☏✎ 12:19, 26 March 2012 (UTC)

Yeah, a graph would be much better than this. Maybe I'll make one myself if I have enough time. ― A. di M.​  15:52, 26 March 2012 (UTC)

Here it is. It is not very clear because Rømer's measurement was so off that it screws the vertical scale, and even if we only included measurements since 1700, Fizeau's measurement would screw the vertical scale, and even if we only included measurements since 1850, Focault's measurement would dominate the vertical scale, etc. ― A. di M.​  17:24, 31 March 2012 (UTC)

Quite tricky. Would it not be better to have arrows indicating the wayward measurements, with a reduced scale so that error bars would be visible on all the other measurments? Or as a further thought, some non-linear scale centred on the true speed of light, with some nearly logarithmic scale away from it? — Quondum^☏✎ 18:41, 31 March 2012 (UTC)

One way to graph values that cluster around a 'correct' value is to use a logarithmic scale that magnifies the differences between values inside the cluster and diminishes the differences between outlier values. Granted, it is not a common coordinate system, but it is quite useful in providing detail of the measurements near the correct value while sacrificing detail of the measurements made far away from the correct value. The usefulness lies in presenting all the information within a nicely bounded rectangle, using the space in the graph efficiently. David Spector (user/talk) 13:40, 18 April 2012 (UTC)

I fear such a graph would be a bit too complicated for a general audience.TR 14:39, 18 April 2012 (UTC)

The slowness of matter

While the article is about the speed of light, it fails to clearly explain an intrinsic property of light due to relativity: speed is distance over time. But when moving at the speed of light, this breaks down. To a photon, no time passes: it merely teleports. IMHO this is a fundamental concept to grasp to understand the speed of light. I believe this can be explained with something simple as saying that according to general relativity, light teleports. 41.164.8.107 (talk) 23:08, 23 March 2012 (UTC)

Stating that in a non-misleading way requires the reader to understand the difference between proper time and coordinate time, and explaining that would be outside the scope of this article IMO. (But sooner or later I should eventually get around turning the article “Introduction to special relativity” into something decent.) ― A. di M.​  19:06, 25 March 2012 (UTC)

Yes... preferably into something within Special relativity itself. That would be very nice. - DVdm (talk) 19:29, 25 March 2012 (UTC)

Like Special relativity isn't already big enough. Plus, the “Introduction to”/main article scheme is working nicely for General relativity and Introduction to general relativity (both FAs). But let's leave it at that. ― A. di M.​  15:48, 26 March 2012 (UTC)

Come now, just hinting that something more might be at play here, might actually entice people who would not do so otherwise, to go and read up more about special relativity. Assuming that everything happens from our frame of reference is analogous to the medieval population believing that the earth is the center of the universe. To me, this explanation makes 100 times more sense than the stupid train example. Perhaps two sentences: "Speed is distance over time, as measured from a specific vantage point. Special relativity dictates that speed actually affects time - when taking this into consideration, and measuring the speed of light from the vantage point of the light itself, it means that no time passes from the vantage point of the light itself and that light simply teleports to the furthest point possible, slowed only slightly in the vicinity of matter." (When you think about it, it almost works like a type of compression.) I still don't know if these assumptions are 100% correct because it's not stated clearly anywhere and nobody seems to be able to confirm this :( — Preceding unsigned comment added by 196.215.100.239 (talk) 07:08, 27 March 2012 (UTC)

Please sign your talk page messages with four tildes (~~~~). Thanks.

That's just the problem. I.m.o. a significant part of layman's confusion about special relativy is rooted in this misconception that light can/should/must have a vantage point. Stating something in the article from the "vantage point of light" would just be wrong and could only add to the confusion. We don't have any model to describe what happens from its —alleged— vantage point. That might be the reason why nobody seems to be able to confirm it :) - DVdm (talk) 07:43, 27 March 2012 (UTC)

By the way, I noticed this edit at Teleportation. I have removed it per wp:UNSOURCED. - DVdm (talk) 07:51, 27 March 2012 (UTC)

I find the word 'teleport' objectionable, since outside of science fiction and specialized areas of physics, the word has no precise meaning. For example: imagine yourself teleporting. Oops, you can't, because no one has ever done this. If you can't teleport, why is imagining a photon teleporting a good analogy? It isn't. A better analogy is that there is only one photon. From its vantage point, it exists in a finite set of locations and energy states forever. Sure, that makes no sense, because the photon's vantage point is outside of our experience. Most people want to apply our experience to all of physics. But the differences between special relativity and Newtonian mechanics are outside of our unaided experience. Most people find that hard to accept. Advanced physics, like ethical history (example: understanding the value of school integration to the USA of the 1950s), should expand the consciousness. David Spector (user/talk) 13:33, 18 April 2012 (UTC)

Wrong Footnote? (108)

Article text: "Descartes speculated that if the speed of light were found to be finite, his whole system of philosophy might be demolished.[108]"

Footnote 108: ^ a b c MacKay, RH; Oldford, RW (2000). "Scientific Method, Statistical Method and the Speed of Light". Statistical Science 15 (3): 254–78. doi:10.1214/ss/1009212817.

However, when you click on the footnote link, the resulting page says nothing about Descartes. — Preceding unsigned comment added by 209.197.169.54 (talk) 01:29, 7 May 2012 (UTC)

I get something relevant if I click on the Historical background section. Materialscientist (talk) 01:42, 7 May 2012 (UTC)

Thank you Materialscientist.

But then should the footnote reference not be changed to http://sas.uwaterloo.ca/~rwoldfor/papers/sci-method/paperrev/node2.html ?

There are three places in the article that indicate footnote 108:

1) The Descarte one which should actually point to http://sas.uwaterloo.ca/~rwoldfor/papers/sci-method/paperrev/node2.html

2) Aristotle "light is due to the presence of something, but it is not a movement". This can be found at the new footnote reference

3) "This led Alhazen to propose that light must have a finite speed" I cannot find Alhazen at either http://sas.uwaterloo.ca/~rwoldfor/papers/sci-method/paperrev/ or at http://sas.uwaterloo.ca/~rwoldfor/papers/sci-method/paperrev/node2.html

Hm .. the problem lies in the coding of that LaTeX copy - if we link http://sas.uwaterloo.ca/~rwoldfor/papers/sci-method/paperrev/node2.html then we lose the table of contents. Thus I left a comment in ref. 108 instead, as a quick solution. Materialscientist (talk) 03:34, 7 May 2012 (UTC)

Not sure if I'm seeing a different layout than you. When I first go to the link, I have "Next: Historical Background" at the top of the page. Clicking on that link takes me to the wrong page for Descarte. But if instead I scroll down to the bottom, I see a list of links with two for "Historical Background". It is the second "Historical Background" link that is the correct one. But perhaps it's just my version of IE. At any rate, assuming you get to the node2 link, I don't think this takes care of the Alhazen reference... I don't see any reference to Alhazen at that link (see the three items listed above: Note 108 covers (1) and (2), but not (3) as far as I can see. — Preceding unsigned comment added by 209.197.169.54 (talk) 04:14, 7 May 2012 (UTC)

Upper limit

As I understand it, the Lorentz Factor is a coefficient derived from the fact that there's an upper limit. Therefore, saying that there's an upper limit because of the Lorentz Factor is like saying there's an upper limit because there's a upper limit... not very insightful/scientific.

Pakito15191 (talk) 06:44, 20 March 2013 (UTC)

The closest I can find in the article to saying that there is an upper limit because of the Lorentz Factor is, 'Since the γ factor approaches infinity as v approaches c, it would take an infinite amount of energy to accelerate an object with mass to the speed of light'. Is this what you are objecting to? Martin Hogbin (talk) 09:39, 20 March 2013 (UTC)

You are right. The factor is usually derived from the demands that (1) laws should be the same in inertial frames and (2) that light speed is invariant. Saying that there is an upper limit because of the factor, is just silly. I made a little correction to the text. Good find. - DVdm (talk) 13:18, 20 March 2013 (UTC)

Although I have no real objection to your change I do not think it was actually necessary. The original wording does not say that there's an upper limit because of the Lorentz Factor it merely points out how the LF shows that infinite energy is required to accelerate an object with rest mass to the SoL. It does not assert that this is 'because of' the LF, which is a meaningless statement in physics anyway. Martin Hogbin (talk) 14:04, 20 March 2013 (UTC)

DVdm's change is minor, but it makes it a wee bit more difficult for a nitpicker to intentionally misread the text, so I'm in favor of the rewording. Stigmatella aurantiaca (talk) 14:36, 20 March 2013 (UTC)

Error in main table

Under exact values, the numbers for metres per second and for kilometres per second are listed as the same. This should be corrected. — Preceding unsigned comment added by 69.51.218.38 (talk) 22:36, 20 March 2013 (UTC)

There is a difference. Look closer. - DVdm (talk) 16:28, 21 March 2013 (UTC)

Yeah. Just noticed it. My mistake. I think I need a new monitor... or a new set of eyes! Sorry :) — Preceding unsigned comment added by 69.51.218.38 (talk) 01:56, 6 April 2013 (UTC)

Only a suggestion: I would insert into the infobox something like "one mile 5.38 μs" to bridge the ~10^8 gap between 3.3 ns and 119 ms, and also "major axis of Pluto's orbit 10.9 hours" for that ~10^5 gap. 80.98.239.192 (talk) 17:41, 27 April 2013 (UTC)

Must admit, made the same mistake, thinking there was an Error in Main Table. Just a suggestion as do not know if possible : but could we bold the dot . exceptionally for this article ? I appreciate if you make the calculation it should trigger the answer, but just for convenience.... ? — Ludopedia^(Talk) 14:58, 19 May 2013 (UTC)

Seems a good idea. I've edited the figure to use a boldface decimal point. It looks a bit funny, but it's probably better to have a somewhat funny looking number than to have the number easily misread. Stigmatella aurantiaca (talk) 18:30, 19 May 2013 (UTC)

User:Syed Wamiq Ahmed Hashmi thinks that use of scientific notation is a better solution to the problem, and overwrote my edit. I thought that we had better bring this matter up in talk to reach a consensus. If we adopt scientific notation for the speed of light, perhaps we should convert all the numbers in this table to scientific notation for consistency? What do people here think? Stigmatella aurantiaca (talk) 19:00, 19 May 2013 (UTC)

The idea of consistency came to my mind, but I did not do it myself for the fear that others won’t deem it appropriate... —Syɛd Шαмiq Aнмɛd Hαsнмi (тαlк) 19:03, 19 May 2013 (UTC)

The table provides information for a general audience, so scientific notation is not desirable. I would suggest using spaces, not commas, as thousands separators. We can also add the words "note decimal point" to the km text.--agr (talk) 19:09, 19 May 2013 (UTC)

Agree. Scientific notation would defeat a primary purpose of the infobox. I support both suggestions of agr and would like to see them implemented. Maximum clarity should be the goal here. Hertz1888 (talk) 20:17, 19 May 2013 (UTC)

Yes, spaces are good suggestion. Words "note decimal point" only in an HTML-comment. No scientific notation in infobox. - DVdm (talk) 21:18, 19 May 2013 (UTC)

I would Agree with the use of spaces, but wouldn't that imply using a comma for the decimal point? I guess that is why agr suggested also using the words "note decimal point", since spaces with decimal point do not really conform with any language "locale", certainly not with British English, with which this article is supposed to conform. In the end, I suppose the need for maximum clarity transcends the need to be consistent. Both Syed and I were going for clarity over consistency. Stigmatella aurantiaca (talk) 21:38, 19 May 2013 (UTC)

Yep - DVdm (talk) 21:49, 19 May 2013 (UTC)

Indeed. Along with the earlier bolding, that looks to be a simple, elegant solution. Hertz1888 (talk) 22:10, 19 May 2013 (UTC)

Here's another thought: move kilometers per second to the "Approximate values" section as 300,000 km/s. That eliminates the confusion and would be more useful to a general reader. Anyone who needs the exact value in km/s is likely capable of moving the decimal point themselves.--agr (talk) 11:04, 20 May 2013 (UTC)

This is also a good idea. In fact, it may be a better idea than anything else that we've discussed so far. Stigmatella aurantiaca (talk) 11:49, 20 May 2013 (UTC)

 Done. If anyone doesn't like that, feel free to revert. — A. di M.  14:21, 20 May 2013 (UTC)

Is the distance appeared in 299 792 458 m/ s bona fide in both space and time?

The veritable motion of pulse in its heuristic world line accrued to a discernible slant edge of light cone per se as the space-time interval is always zero between two events connected by a light speed path, ergo, is the distance (299 792 458 m) used in standard Celeritas specious and in unison rectilinearly in space-time continuum with aforementioned adduced spatial distance? 74.200.19.65 (talk) 00:42, 22 April 2013 (UTC)Eclectic Eccentric Kamikaze

You should stay away from big words that are too hard for you. SBHarris 00:57, 22 April 2013 (UTC)

Veritably! Studious pretenders to perfection should eschew such obfuscation, and make a flaired approach to the kernel of truth that substantiates the underlying conundrum! In reality (within some epistomology, as it were), the exact Celeritas value is none other than that which gives the aforementioned apothem of the light cone its definition! Dicklyon (talk) 23:31, 19 May 2013 (UTC)

Pleas do not misconscrue the posts original intensions. P0M (talk) 02:48, 12 July 2013 (UTC)

New theory seeking to explain speed of light in vacuum

This paper The quantum vacuum as the origin of the speed of light suggests an explanation for the observed limit on the speed of light in a vacuum. The limit is described as being caused by interaction of light with virtual particles, which according to quantum theory exist in the vacuum in large numbers. Interaction with the virtual particles (virtual meaning "very short-lived" here) takes time. In between these interactions, light moves at infinite speed. As the number of particles along the path of light will vary stochastically, the paper predicts that the speed of light in a vacuum will also show slight variations. The article should be extended to include this theory. Objections? --Kurt.thomas (talk) 21:04, 11 July 2013 (UTC)

Yes. The paper is a primary source that has not stood the test of time, and which has little if any evaluation from secondary sources, experimental backup, etc. Wikipedia is not a forum for the rapid publication of cutting-edge theories that may or may not pan out. See Wikipedia:What Wikipedia is not — Stigmatella aurantiaca (talk) 22:21, 11 July 2013 (UTC)

WP:PHYSICS review: A-level article

I'm beginning a sort of WP:Expert review process for articles independent of the featured article system which I've realized has problems. As such, I've rated this article a level 'A' which means it is of the quality that would be expected from a professional reference work on the subject. I say this as a person with graduate degrees in astrophysics, but I encourage others who have similar qualifications to make comments if they believe my judgement to be incorrect.

jps (talk) 02:20, 12 September 2013 (UTC)

Tach One means the speed of light

Tach One means the speed of light http://tach-one.com Rvansteenberg (talk) 16:58, 15 September 2013 (UTC)

What should we use as time for light to travel from the Sun to the Earth?

I just modified the infobox to read "Sunlight takes about 8 minutes 17 seconds to travel the distance from the surface of the Sun to the Earth." This is based on the nearest point of the Sun to the Earth. In reality, light coming from different parts of the Sun take different amounts of time to reach the Earth. The limb of the Sun may be considered as being situated 1 AU from the Earth, and the the time it takes for light to travel 1 AU is 8 minutes 19 seconds.

Perhaps an integrated average time over the entire surface of the Sun would be best?

Or perhaps we should go back to 8 minutes 19 seconds, noting that this is the time for light to travel 1 AU?

Looking over the history of this article, there's been a lot of disagreement over just what figure need to be placed in the infobox. Stigmatella aurantiaca (talk) 18:23, 15 September 2013 (UTC)

Hertz1888 just added the word "average" to the modified caption that I introduced, to emphasize that we are talking about the mean distance of the Earth from the Sun. But "average" can also mean an average of the distance that light travels integrated over the entire half of the photosphere visible from the Earth. This is an undesirable ambiguity. Stigmatella aurantiaca (talk) 18:39, 15 September 2013 (UTC)

Thank you for your calculations and thoughts. An integrated average time may be more rigorous, but not for this article, where such strict precision should not matter. The inexactness of the average is covered by the qualifier "about" (which I have italicized in the caption for emphasis. I also restored "average" for the distance). The travel time varies considerably more than a few seconds over the course of a year. I would be inclined to stay with 8:19 as a solidly representative value. Any ambiguity can be explained in a footnote if there is a need. Debating about a two-second (0.04%) difference seems pointless when the approximateness is noted by "about". Hertz1888 (talk) 19:13, 15 September 2013 (UTC)

Quite true. This is one of those nugatory matters that raises a lot more debate than justified. If you think 8 min 19 sec is the best figure, then by all means use that figure, modifying the caption appropriately to indicate that this is the distance for light to travel 1 AU. Stigmatella aurantiaca (talk) 19:38, 15 September 2013 (UTC)

'Speed of light -> Length of the Metre -> Speed of Light' is twisted logic.

"The speed of light in vacuum, commonly denoted c, is a universal physical constant important in many areas of physics. Its value is exactly 299,792,458 metres per second, a figure that is exact because the length of the metre is defined from this constant and the international standard for time." --------------- I find a problem with starting such an important section with a statement that is blatently wrong. The above figure is a rounded version of the most accurate measurement currently known. It is rounded because of known measuring uncertainty in the last digit. That is sensible. The length of the metre has been standardised to that figure. That is sensible. Now, if experimenters find a problem with their measurement techniques and produce a measurement that is significantly different from this accepted figure, the speed of light will then be recognised as changed. The standard length of a metre will not have changed. To say that "The speed of light ..... is exactly 299,792,458 metres per second, a figure that is exact because ..... (of some human agreement)." is wrong. The speed of light "...is a universal physical constant..." and not the outcome of human decisions. Having defined the metre based on the known speed of light, you cannot then fix for all time the speed of light based on the length of the metre unless you are also prepared to change 'the standard metre'.. 83.104.248.202 (talk) 11:22, 28 October 2013 (UTC)

On the contrary, by defining the metre based on the known speed of light, you are indeed committing yourself to being prepared to change the standard metre (or rather your view of it) should our best measurements of the speed of light change. And that's what the scientific community has done. So the statement seems entirely correct. W. P. Uzer (talk) 11:42, 28 October 2013 (UTC)

You surprise me. Yous say that the 'Standard metre' could possibly change every year. That is not the impression I have got from TV programs on these standards. Nor it it what I read in the above when it says "a figure that is exact because the length of the metre is defined from this constant" because from what you say it is not guaranteed to be a 'constant' in the sense that the metal bars were. I have to accept your word for it. I read another article from a university that actually said that the speed of light can no longer be change but better experimental values. I was thinking of this when I read yours, and thinking about that left me with the same wrong impression. 83.104.248.202 (talk) —Preceding undated comment added 13:44, 28 October 2013 (UTC)

The length of the meter is defined in terms of the speed of light and the second so it could, even in principle, change as ambiguities in the definitions are clarified. In addition, the realisation of the standard metre could indeed change every year as newer methods realisation are found.

This is not that different from how things were when there was a standard metre rod when you get down to the fine detail. For example, the engraved lines on the Paris standard will have finite width so a decision would have to be made as to where within the engraved line the end points were. Just stating 'the centre', for example, would not be sufficient without stating exactly how the centre was to be defined. When that has been done, there is still the question of experimental uncertainties in the actual realisation.

As it is now, the uncertainties in realising the metre using the current light-based standard are much less than they would be if the old metal bar standard were used. Martin Hogbin (talk) 13:58, 28 October 2013 (UTC)

I slept on it and still think that you have it wrong. Martin is right. The measurement of the metre is now much more exact. Better than the marks on some bar stored carefully somewhere. But doing this does not make the speed of light more exact. Saying "Its value is exactly 299,792,458 metres per second, a figure that is exact because the length of the metre is defined from this constant " is wrong. The speed of light is no more exact because it has been used to define the metre. The metre is more exact because it uses the speed of light, but the converse is not try. If we look up the latest measurements, they should certainly specify some measure of the error. What is scary, I'm finding several references implying that the speed of light is now known 'exactly', as you are. Take your reference (10). The NIST now records the uncertainty in the measurement of the speed of light as "Exact". It is NOT. The accuracy of measuring the speed of light has NOT been changed just because someone redefined the metre using the latest measument rounded to make the uncertainty irrelevant. I must repeat. The metre is now more accurately known, but NOT the speed of light. You are clearly saying the the speed of light is exact because etc, when you say " because the length of the metre is defined from this constant " 83.104.248.202 (talk) 14:31, 29 October 2013 (UTC)

You are wrong. For about 100 years now, since Einstein's Special Relativity, we have understood space and time to be not separate things but one thing, called Spacetime, where they are interchangeable by e.g. Lorentz transformations. But for historical and practical reasons the units for space and time are quite different. The ratio of those units must be fixed, and so we choose to define it as 299,792,458. It's no different from how inches, once a separate measurement from metres, are now defined in terms of them as 1" = 2.54cm (which then defines them in terms of seconds).--JohnBlackburne^words_deeds 16:08, 29 October 2013 (UTC)

83.104.248.202, I was not saying that the speed of light was not exact. The speed of light is exact by definition. A metre is defined to be however long it need to be to make the speed of light exactly 299,792,458 m/s. That is what the word metre means. This still provides a better (but not exact) realisation of the metre than the metal bar, as I explained above. Martin Hogbin (talk) 16:45, 29 October 2013 (UTC)

I want to close this cleanly. To do this, I will point out the problem in a different way. Look at the table of measured speeds. What instrument was used to make the last one? None. it was a decision of the CGPM committee to produce a standard metre in terms of a standard number of wavelengths. The exact number of waves in the measured speeds or lengths is not known precisely, as the other measurements show. Now, in order to let Cosmologists and others communicate consistently, certain features need to be standardised. Then a Chinese scientist can compare some complex calculations with a Nigerian friend and know that if they have any differences, it is not because of their standard metre, because both have standard metres based on a standard number of wavelengths. However,a Finish physicist and an Argentinean physicist may well be measuring something more precisely and they may want to know the differences due to a different measurements. So they do not use the Standard metre and Standard speed of light. So what CGPM has done is NOT a new measurement of the speed of light as the table almost implies, but have created a standard metre for use by those scientists who do not need an exact speed of light but do need a consistant speed of light. No one, not even the CGPM, can changed the speed of light. Nor have they measured it 'exactly' as your article says. Even Martin agrees with me on this; we can expect future measurements of the speed of light to change and that is because we know it is not exact. No Committee can ever make it is exact. What they have done is set up an exact Standard metre and in the future they may choose to change that. (Incidentally, the NIST table for the speed of light now also says that this speed of light is 'exact' without saying it is the standard that is exact). What is exact is the Standard length of the metre in terms of an artificial but standard number of wavelengths. That is a big step forward. 83.104.248.202 (talk) 18:31, 29 October 2013 (UTC)

@83.104.248.202 The speed of light is defined as exactly 299,792,458 meters per second. If you want to know how long a meter is, find out how far light travels in 1/299,792,458 seconds. These are not measurements. These are definitions. There are no problems here. Likewise, ever since that definition got implemented, you can no longer measure the speed of light in m/s. If you try to measure the speed of light in meters per second, you are either verifying how good your meter stick is, or how good your clock is. If you measure a speed of light 1% above the definition, your meter stick is 1% shorter than the definition of the meter. If you are measuring a speed of light 4% under the definition, your meter stick is 4% longer than the definition of the meter. (Or your clock is off.)Headbomb {talk / contribs / physics / books} 18:40, 29 October 2013 (UTC)

I think this discussion has now moved away from improving the 'Speed of light' article to 'explaining to 83.104.248.202 why the speed of light is exact when measured in metres per second'. No doubt someone will complain soon. I suggest that anyone who wishes to continue does so on my talk page. Martin Hogbin (talk) 18:59, 29 October 2013 (UTC)

I think the above discussion suggests we are not telling the story as well as we could. Here is a ref that I think does a better job http://www.nist.gov/pml/div683/upload/museum-length.pdf. The speed of light is a defined constant only in the metric system (and systems derived from it such as the English system) and only as it has been constituted since 1983. Between 1960 and 1983 the meter was defined in terms of the wave length of a particular spectral line of Krypton-86. In that era, measuring the speed of light consisted of measuring the frequency of that Kr-86 line, not an easy thing to do since it is in the 495,000 GHz range, compared to the 9.19 GHz Cesium frequency used to define the second (which has not changed since 1960). Once those frequency measurements became accurate and repeatable enough, it was possible to define both the second and the meter with just the Cesium frequency, and that was done in 1983. However, the process of measuring the frequency of Kr-86 line, as well as other spectral and laser lines, to ever higher accuracy did not stop after 1983, but continues to this day. But these same experiments which once measured the speed of light, now measure the wavelengths of these lines to higher accuracy. Those improved measurements, in turn, allow such light sources to be used as a practical method to measure distances in terms of the meter. This is how accurate distance measurement are made in practice. The BIPM calls them a mise en pratique [15] and provides a long list of them. The BIPM has plans to define other SI base units by fixing the value of fundamental constants, a process it calls "explicit constants" http://www.bipm.org/en/si/new_si/explicit_constant.html. Here too there will be mise en pratique where experiments that were once used to measure the fundamental constants are not used to calibrate practical measurement tools.

There are other units of measure that are not defined in terms of the meter, for example the astronomical parsec. For these, the speed of light is still a value that can be refined by more accurate measurement. It is at least conceivable that some future discovery in physics would make advisable an independent definition of the meter and return the speed of light to an experimental status.--agr (talk) 20:52, 29 October 2013 (UTC)

I don't follow the part about the parsec. What would be an operational definition of that unit of length? Knowing that two objects are a parsec apart involves being able to measure an angle precisely. If I were to measure an angle with a protractor and use that measurement to calculate the length of the opposite side of a triangle I'd have to have an awfully big protractor to even get the accuracy demanded by a surveyor. I think what physicists would like to find and hang onto is a natural unit of length that is precise and which does not change. The meter stick as a physical thing has the disadvantage that it can change over time just as stone stairs wear down after centuries of passage by people in leather shoes. Being able to measure how long it takes for light to get from one place to another, and thereby to calculate the distance between the two demands an accurate clock, and people continue to improve on those devices.

I can't see any way of finding an object of some kind that could serve as an unvarying standard length. Anything large enough to see will expand and contract depending on temperature, wear off at the ends, etc. Anything small enough to be a potential natural unit, such as the radius of the lowest orbit of a hydrogen atom, will have quantum uncertainty as one of its central characteristics, so it will differ in length between any two measurements.

We could imagine a carpenter's jig that would fabricate secondary standards of length such as a laser-carved meter stick based on seeing where on the stick light had reached after a certain time as measured by our best atomic clock. Whether we could see the difference or not, if we got a better clock that would probably change the exact length measured out on new laser-carved meter sticks. In practice, the errors in the fabricating machinery would be greater than the differences between atomic clock mark 3 and atomic clock mark 4. P0M (talk) 01:44, 30 October 2013 (UTC)

See JohnBlackburne's comments above. In the modern view, 299,792,458 meters/second is a proportionality constant that relates distance to time, just as 4 cups/quart is a proportionality constant that relates two different measures of volume in the English system. It makes no sense to try to experimentally evaluate the number of cups in a quart. The number is 4, exactly. It makes no sense to try to experimentally evaluate the number of meters in a second. The number is 299,792,458, exactly. It does, however, make sense to try to create accurate means of determining a "one-cup volume", and to determine the errors in extant experimental realizations of "one cup". Stigmatella aurantiaca (talk) 21:16, 29 October 2013 (UTC)

STOP!!

This is not a forum. Even if we get this IP to accept that the speed of light is exact in m/s, this would in no way help imrove the article.—Шαмıq✍ 02:31, 30 October 2013 (UTC)

Anyone who wishes to continue this discussion is welcome to do so on my talk page. There is some relevance to improving this article in that this is not the first time that we have had a reader who had difficulty understanding this subject. If we do manage to explain it to one reader it may help us make the article clearer for the benefit of others.

On the other hand, I agree that this is not the right place to continue this discussion. Martin Hogbin (talk) 09:04, 30 October 2013 (UTC)

Motivation for the current definition of the speed of light and other improvements to the introduction

1) The initial lede paragraph may be improved by providing some context as to why the current definition of the speed of light is framed as it is. To judge from the large amount of commentary above, the current wording in the lede that "Its value is exactly 299,792,458 metres per second, a figure that is exact because the length of the metre is defined from this constant and the international standard for time" is unsatisfactory. I believe that something based on John Blackburne's comment could be a useful addition to the lede:

For about 100 years now, since Einstein's Special Relativity, we have understood space and time to be not separate things but one thing, called spacetime, where they are interchangeable. For historical and practical reasons, however, the units for space and time are quite different. By international convention, the proportionality constant between the units of space and time is currently defined as 299,792,458 metres per second. This proportionality constant represents an exact number. By analogy, inches, once a separate measurement from metres, are currently defined as 1" = 2.54cm.

2) The current introduction is far too long. The current second, third and fourth paragraphs should be sectioned off. Stigmatella aurantiaca (talk) 12:01, 30 October 2013 (UTC)

Stigmatella, although I agree with the facts that you have stated regarding spacetime I do not think that this was the motivation for the change in definition of the meter and the speed of light. The reasons given at the time were mainly practical in that the change allowed the most stable light source at any time to be used.

I am also not sure that your proposal will address the difficulty that some readers have had in understanding the concept of fixing the speed of light in a given system of units. Referring to spacetime is likely to make some readers think that it is all hocus pocus with no basis in everyday measurement. Exactly where the difficulty in understanding arises is not clear. Several editors have tried to explain the current status to 83.104.248.202 but so far without success. That is why I would like to continue the discussion with 83.104.248.202 on my talk page so that we can find out how the misunderstanding arises. Martin Hogbin (talk) 12:58, 30 October 2013 (UTC)

Both theoretical and practical considerations played vital roles in SI 1983. Unfortunately, a link that I had to a paper dealing with these considerations no longer works, but here is a quote from a paper on "FC SI" which is still under development:

The units of time, length and mass in the FC SI are established simultaneously, in a single definition, which puts the second, the metre and the kilogram on equal footing. This is quite different from clear seniority of base units in the present SI and in the New SI, where the unit of time is (implicitly) defined first, then the second is explicitly used to define the metre, and finally (in the New SI) the new definition of the kilogram makes use of the previously defined units of time and length. Formally, this feature of the FC SI results from the fact that h, c and G are associated with at least two of the three fundamental physical notions; in other words, there are no fundamental constants that could have been associated exclusively with time, or distance, or mass. In physical terms, this is the manifestation of the deep connection between these notions in the physical realm: space and time are two different aspects of a single physical entity, the spacetime, which has first been noticed in the special relativistic regime; the interconnections between all three notions (space, time and mass) manifest clearly in the general relativistic realm. We need to recognize that our macroscopic world, where we have got used to treat the space, time and mass as separate entities, independent of each other, is just the weak field approximation of the actual, general relativistic situation, where these three notions are (in general) inseparable. The physical dimensions of fundamental physical constants c, h and G reflect that relationship and this implies that the corresponding fundamental base units (s, m, kg) must be defined simultaneously (otherwise, the first definition would have referred to a previously undefined unit). This feature of the FC SI should be interpreted as a manifestation of the profound physical relation between space, time, and mass, rather than an inconvenience of no physical significance.

As I recall, SR considerations played an important role in the discussions leading up to SI 1983. The New SI definition of the metre includes special caveats stemming from GR considerations. FC SI treats mass, length and time as closely interrelated entities. Stigmatella aurantiaca (talk) 16:18, 30 October 2013 (UTC)

BTW, I see no point in continuing the discussion with 83.104.248.202. Stigmatella aurantiaca (talk) 17:30, 30 October 2013 (UTC)

I should clarify that whereas it is only a matter of time before "New SI" is accepted by the scientific community and approved by the CGPM, "FC SI" is a radical proposal for a system of units that is entirely free from any reference to material entities. It is not a system of units that is likely to be realized within any of our lifetimes. Stigmatella aurantiaca (talk) 20:46, 30 October 2013 (UTC)

My goal is to improve the article, not convince 83.104.248.202. Here are key points I think this article needs make, the first in the lede, the rest in an appropriate section:

1. The speed of light is a constant in the metric system as the metric system is currently constructed.
2. Prior to 1983, there were separate definitions of the second and the meter and the speed of light was a measured value.
3. In 1983, after the measurement of speed of light had become accurate enough, the BIPM chose to fix the speed of light, making it what the BLPM calls an “explicit constant”. Doing this allows the same physical source (currently a Cesium resonance) to be used to define both the second and the meter.
4. The constancy of the speed of light in vacuum, part of special relativity, allows this approach to defining the meter and the second. (I would add “but does not require it” if I had a cite.)
5. Experiments that in the past were used to measure the speed of light, are now used to calibrate lasers and other light sources so they can be used practical tools for length measurement. --agr (talk) 16:06, 30 October 2013 (UTC)

I feel there is a real problem with continuing to associate the name "speed of light in vacuum" to what has become a fundamental constant now. The constant c is essentially a model coefficient in the Lorentz metric which is used in tangent spaces to space-time. Perhaps, it would have been better to call this c Maxwell's constant or Lorentz' or Einstein's constant. The speed of electromagnetic waves under certain circumstances may be observed to come close to this c but it will actually never be identical to it. Bas Michielsen (talk) 00:15, 19 November 2013 (UTC)

The term "Einstein's constant" but not this one has been proposed but has never gained widespread acceptance. I think the issue that you talk about, the fact that the symbol c and the term 'speed of light' are used for conceptually different things, is discussed in the article, although maybe this could be improved.

My feeling, from the way things are going, is that the fundamental constant with a defined fixed value is intended to be Einstein's constant, rather than 'the speed at which light travels in a vacuum'. At the moment, from a metrological point of view there is no difference (although the principle has started to be addresses by BIPM) but when there is a measurable difference I would guess that realisations of the metre using light might have some 'corrections' in them. There would also need to be a new definition of exactly what is meant in metrology by 'distance'. Martin Hogbin (talk) 13:12, 19 November 2013 (UTC)

Semi-protected edit request on 27 December 2013

the numerical expression of the speed of light is incorrect. It is a typographical error, however the result is inconsistent. ie... 299,792,458 meters per second (as shown on the primary page) reads as Two Hundred and Ninety Nine MILLION Seven Hundred and Ninety Two THOUSAND Four Houndred and Fifty Eight meters pers second, should read as Two Hundred and Ninety Nine THOUSAND Seven HUNDRED and Ninety Two (point) Four Five Eight meters per second The error has occured by using a comma instead a full stop between the sixth number (2) and the remaining numbers (458). The APPROXIMATE value is shown as 300,000 which is correct. The correct numerical value for the above should read as 299,792.458. Under the guidelines as stated above, all reference(s) to 299,792,458 meters per second must be replaced with 299,792.458 meters per second. 60.231.77.92 (talk) 00:34, 28 December 2013 (UTC)

Not done: I believe the numbers in the article are correct. You seem to be confusing two different units commonly used to state the speed of light, which can be written as 299,792,458 metres per second or 299,792.458 kilometres per second. Adrian J. Hunter^{(talk•contribs)} 02:47, 28 December 2013 (UTC)

You have the actual speed of light WRONG! Way wrong!

I'm bad. If I could delete page I would. sorry for missing the Metric vs the KM.

class="autosigned">— Preceding unsigned comment added by Gunnymorris23 (talk • contribs) 20:01, 8 February 2014 (UTC)

No harm done. Hertz1888 (talk) 20:10, 8 February 2014 (UTC)

bad link

The link under the heading "Distance measurement", which should point to a main article also called "Distance measurement" (as the tool tip also states) actually takes you to the article "Rangefinder". I thought I might fix the link, so I searched for "Distance Measurement" and was brought again to the Rangefinder article. Maybe someone else knows how to correct this, especially in the search results. — Preceding unsigned comment added by 80.120.134.230 (talk) 13:05, 20 January 2014 (UTC)

The 'Distance measurement' article has been redirected to Rangefinder. I do not see this as a real problem but, if it is considered to be one, it would be easiest to change the main article to Rangefinding. Martin Hogbin (talk) 09:35, 9 February 2014 (UTC)

Clarification of invariability of massless particles' speed in vacuum

Was not until recently I gained the understanding that was never made clear that particles without mass _always_ travel at exactly c through vacuum. I've noticed this is not made clear in almost any description of the nature of such particles. Recommend this addition for insight & clarity, from 1st paragraph:

"light) travel in vacuum."

to:

"light) always travel in vacuum." — Preceding unsigned comment added by 97.114.74.199 (talk) 19:55, 27 April 2014 (UTC)

introduction

The intro says: "the expansion of light wavefronts from individual flashes of lightning is clearly visible". Could this be explained further? It's not clear (to this layman at least) what this means. -71.34.93.102 (talk) 19:41, 29 April 2014 (UTC)

I've reworded this. See whether it makes more sense now. —Quondum 23:37, 29 April 2014 (UTC)