Talk:Speed of light/Archive 7

Light and the expansion of the universe
Due to the expansion of the universe, the distance travelled by a photon isn't the same as time elapsed. I don't know how to do the calculation, but you can't say that light 13 billion light years away was emitted 13 billion years ago. 204.214.148.1 (talk) 06:41, 15 February 2009 (UTC)


 * I think it means light emitted 13 billion years ago was emitted from an object whose position at that time was 13 billion light years from where we are now, assuming we're in an inertial reference frame (which might not quite be true). And no time has elapsed for the photon, as it travels at the speed of light.  But no matter what inertial frame you look at the photon's emission and absorption in, those events will be at zero interval in spacetime, meaning the distance and time will be in correspondence at the speed of light. Dicklyon (talk) 06:45, 15 February 2009 (UTC)


 * A light year is defined as the distance light would travel in a year assuming that the speed of light in a vacuum is 300,000kms. The light year is a unit chosen so as not to end up with massive numbers when measuring distances to stars and galaxies and other objects in the Universe. A light year is roughly equivalent to 5.87849981 x 1012 miles. The figures obtained for these distances are based on the objects red-shift. As a distant object is moving away from us the light emitted is stretched and when viewed appears to be shifted over to the red end of the visable spectrum. Look it up on here. Therefore, the light that we measure left the object 13 billion years agoOopsISwearALots (talk) 00:36, 16 February 2009 (UTC)


 * I guess he's referring to the difference between the comoving distance and the light travel distance: see Distance measures (cosmology). --A. di M. (talk) 22:19, 16 February 2009 (UTC)


 * True, I see where you're coming form. Though The Light Year is an S.I. Unit used to measure astronomical distances. It is given as the distance light travels in one year. When a distamce is given as, take his example, 13 billion ligh years. The light has in realistic terms taken 13 billion years to reach the Earth. Therefore no matter what we do, we will always see the Universe as it was, not as it is today. The deeper you look into the Universe, the further back in time you go. OopsISwearALots (talk) 02:14, 18 February 2009 (UTC)

Lead section
I have removed several bits from the lead because this is meant to be a summary of the article as a whole. We should not therefore have excessive detail or put material there that is not covered in the body of the article. Martin Hogbin (talk) 18:07, 17 February 2009 (UTC)

Faster than light
The article on redshift contains these unsupported remarks:
 * As a consequence, popular literature often uses the expression "Doppler redshift" instead of "cosmological redshift" to describe the motion of galaxies dominated by the expansion of spacetime, despite the fact that a "cosmological recessional speed" when calculated will not equal the velocity in the relativistic Doppler equation.[27] In particular, Doppler redshift is bound by special relativity; thus v > c is impossible while, in contrast, v > c is possible for cosmological redshift because the space which separates the objects (e.g., a quasar from the Earth) can expand faster than the speed of light.


 * This is because the expansion of the spacetime metric is describable by general relativity and dynamically changing measurements as opposed to a rigid Minkowski metric. Space, not being composed of any material can grow faster than the speed of light since, not being an object, it is not bound by the speed of light upper bound.

This all seems a bit weird to me, as the space between Earth and the quasar is not empty entirely of all matter and energy. If the distances between all dust particles increase faster than the speed of light, don't the dust particles move faster too? In any event, should this be brought up in the Faster than light section? Brews ohare (talk) 13:43, 18 February 2009 (UTC)


 * There being no reaction to this observation, I propose to add a new section about speed of light and space-time metric. Brews ohare (talk) 14:48, 19 February 2009 (UTC)

Speed of light and cosmology
This section needs tidying up. I do not really know where to start, we need someone who understands this stuff. Also it is the third time that the bit about astrophysical events at high redshifts now appears in the article. Martin Hogbin (talk) 23:01, 18 February 2009 (UTC)


 * Hi Martin: You could be more helpful by describing what you want to change here. I believe your main concern is that you don't understand this section, but nothing will change that except some effort on your part to read the cited work.


 * The appearance of redshift in this section is unavoidable, as all cosmology is tightly associated with redshift. Brews ohare (talk) 14:40, 19 February 2009 (UTC)


 * It is not redshift in general that I am referring to, it the fact that the same paper by Schaefer and its conclusions are mentioned three times in the article. Do we really need these same results presented presented three times?


 * My concern is not that I do not understand the section but that the writer does not understand the subject matter of the cited papers. Martin Hogbin (talk) 21:06, 19 February 2009 (UTC)

Any evidence suggesting the import of these papers is misconstrued? No claim is made that this work is anything more than exploratory, and no controversy is raised, nevermind any "slant" being placed upon matters. All that is done is to point out the potential interest in these issues because of the possibility of very fundamental implications. Brews ohare (talk) 22:34, 19 February 2009 (UTC)

The paper by Schaefer is mentioned several times because this evidence supports several different points: (i)limitations upon dispersion in c; (ii) limitations upon possible QED variations in c; (iii) limitations upon the variation in c with the age of the universe Brews ohare (talk) 22:21, 19 February 2009 (UTC)

"Other Models" section
The movement of material from the discussion of refractive index (to which it applies) to a new section Other models is inadvisable, as it is now a subsection of "Light as EM radiation" to which it has no relevance. I moved it back and added a new title and two sentences to make the subject more evident. Brews ohare (talk) 14:45, 19 February 2009 (UTC)

Stopped light
I know that many physics people will probably disagree with this and say it is impossible, but light has come to a complete stand still. Is it emntioned in the article that origianly the slowest light has been that we know of is 60 kph (38mph)? If not this was done by putting it through sodium at -272 degrees centigrade. But the same team of scientist (at Harvard uni) who did the previous experiment also stopped light by putting it 'bec' of rubidium. You could say this is impossible as it has stopped so has no energy so is no longer light, and I don't understand it either that is all I know about it, but you could say it has just completely resticted it's movement. This could change its energy type but would this mean it isn't light? If you are going to reply to this please do so at my talk page if it is directed at me. Thanks!  ' The Ninja  lemming '  19:49, 19 February 2009 (UTC)

hi i completely agree to your statement that light can be stopped. but my question is why the light should travel or what is the necessity that light should travel? why cant be it at a perticular place? —Preceding unsigned comment added by 59.178.217.232 (talk) 16:43, 12 May 2009 (UTC)

tushar-9868311150 —Preceding unsigned comment added by 59.178.217.232 (talk) 16:44, 12 May 2009 (UTC)


 * I'm familiar with that experiment. I don't think "they stopped light" is a great description of it, I would say "they stored and released light" or something like that, but whatever. Anyway, some stuff on this used to be in this article, but isn't anymore, probably because it's less important than the other topics in the article. You'd probably be interested in the article slow light. :-) --Steve (talk) 21:48, 12 May 2009 (UTC)

External Link/Reference in Lead
I was bold and removed the reference in the lead for the speed of light and made it a citation instead of an external link. WP:LINKS says that "external links should not normally be used in the body of an article." I also checked WP:LEAD but it doesn't make any reference to external links in the lead. Ol Yeller '''Talktome 05:31, 26 February 2009 (UTC)

Use of Italics for Emphasis
I removed the italic face of "exactly" as it doesn't fall into WP:ITALICS. I imagine the author was trying to show quotation marks to imply that the exact speed is speculated but it is not. User Martin Hogbin reverted my change stating that "Italics are used for emphasis, but sparingly." I reverted his change as emphashis implies a PoV which has no place in an encyclopdia and MH used no references to policy in his claim.

If you would like to discuss this issue, please do so here. Ol Yeller '''Talktome 16:53, 26 February 2009 (UTC)
 * I'm neutral as to whether italics should be used here or not, but I think that Martin could justify their use. The emphasis here does not imply a PoV: it shows that the numerical value of the speed of light is exact by definition (just as the atomic mass of carbon-12 is exact by definition, for example), and that there is no measurement uncertainty. There are only a handful of physical constants whose values are defined in this way, so I could support the use of italics to emphasize the fact. On the other hand, I don't really think it's hat important, as the use of the adjective "exactly" should convey the same meaning! Physchim62 (talk) 17:09, 26 February 2009 (UTC)
 * I see what you're saying. I guess I just feel that the word speaks for itself and adding anything else is like saying, "the sun is really really really big" and I think that implies PoV since its only something that a person would say and not something you'd read in an encyclopedia.  Thanks for your input. Ol Yeller  '''Talktome 17:16, 26 February 2009 (UTC)
 * I can see your point as well, but I think the very best solution would be to revise the prose to explain why it is important, instead of simply reverting the italics. If we can't get the very best solution of excellent prose, I don't think we should be worrying about italics one way or the other. If either party wants a list of articles which need working on more seriously than this page, I can provide them with it :P Physchim62 (talk) 18:50, 26 February 2009 (UTC)
 * I like it. There's no POV involved, this is an uncontroversial statement. Remember, people don't read and absorb every single word in an article one after another. Italics helps a reader read, the same way that intonation helps a listener listen. "Exactly" is a single word that conveys a lot, and is easy to miss and important, so I think it should be italicized. --Steve (talk) 18:54, 26 February 2009 (UTC)
 * This is apparently more of a controversial issue than I thought and like Physchim62 pointed out, we probably have better things to work on. I did some more searching and found this in the manual of style.  It says, like MH said in his revert that, "Italics are used sparingly to emphasize words in sentences."  I think that the word should speak for itself and disagree with this policy but I'm not a policy maker.  I'm reverting the reverted reverts (brain ouchy).  Ol Yeller  '''Talktome 19:04, 26 February 2009 (UTC)


 * Looks like it is settled then. The reason the word needs to be emphasized is that an exact value for the speed of something is somewhat unexpected in physics. Normally there are expected to be some measurement uncertainties, and these should be quoted with the value.  Because the speed of light (in free space) is set by definition there is no uncertainty and the figure is exact. Martin Hogbin (talk) 19:22, 26 February 2009 (UTC)

Waves do not have mass
Hi Martin: I do not think that your recent revision (Martin Hogbin (talk | contribs) at 17:44, 3 March 2009) is well justified. (i) An EM wave (for example) is one way of seeing a photon, and photons might have mass (it's under experimental scrutiny) (ii) Gravitational waves are conjectural, but might be carried by gravitons with some mass (iii) The speed of light is a defined value only in "free space", which means real measurements have to be corrected for the effects of gravity; it isn't clear to me that gravitational waves can propagate in free space even in principle (that is, "free space" is a region containing only EM waves, and gravitons are excluded.) (iv) It is a matter for experiment (as the editor you reverted pointed out) to decide whether gravitational waves in vacuum are supported by gravitons with mass and what speed they propagate at.

A more satisfactory statement would be:


 * By definition, in free space it is the speed of all forms of electromagnetic radiation including visible light. According to special relativity, it also is the speed of anything having zero rest mass.[3] To within experimental accuracy it is the speed of all electromagnetic radiation in vacuum, and is anticipated to be the speed of gravitational waves as well. Brews ohare (talk) 20:03, 3 March 2009 (UTC)


 * Firstly you have agreed previously there is no definition that says c is the speed of all EM radiation. The definition clearly refers to light.
 * Waves do not have mass, they are not localized, particles may have rest mass. Martin Hogbin (talk) 22:36, 3 March 2009 (UTC)
 * SR does not say that anything with zero rest mass travels at c either.Martin Hogbin (talk) 22:38, 3 March 2009 (UTC)
 * A statement with so many errors is not acceptable, especially in the lead section. Martin Hogbin (talk) 22:39, 3 March 2009 (UTC)

Martin: In free space c is defied to be c0 regardless of frequency. That means c is c0 for all EM waves in "free space" according to this definition.


 * There is no such definition. (All references that follow refer to  free space.) The length of the metre is defined in terms if the speed of light, not EM radiation in general.   We have had this discussion some time ago and you accepted that he speed of all EM radiation is not fixed by definition.  It is, in my opinion, correct to say that the speed of all radiation is c.  That is because according to the well established theory of classical electromagnetism all EM radiation travels at the same speed and there is, as yet, no evidence to support any theory which says otherwise. Martin Hogbin (talk) 09:26, 4 March 2009 (UTC)


 * Martin: yes all definitions refer to free space. However, the defined "vacuum of free space" is not a realizable medium, and any realizable medium (for example, a region of space containing only EM radiation might be realizable someday ) is only verifiable to within experimental error to be "like" free space. The properties of a realizable medium are measured, not defined.


 * The frequent confusion in this article between "vacuum" (a vague designation) and "free space" (a hypothetical medium with defined properties, and therefore not subject to experimental verification) is a bad thing. Given Maxwell's equations, free space has the same properties at all frequencies, is linear, isotropic and polarization independent. These properties are not expected theoretically for any realizable vacuum, including quantum vacuum. The distinction between a model medium, which is a hypothetical entity like free space, and its comparison with an observable medium, which can be realized and measured, is the basic distinction here.


 * The basis of the comparison is questions like: Does outer space have the same speed of light as free space? Does outer space exhibit zero dispersion, like free space? Is outer space isotropic, like free space? Substitute "quantum vacuum" or "QCD vacuum" or "quantum gravity vacuum" for "outer space" in these questions, if you like. These questions can be asked theoretically, but differences can be confirmed only experimentally and confirmed only to within measurement accuracy. Definitions may enter the theoretical models, but not the experimental results.


 * I feel it likely that you agree with all this, and the disagreement lies in my opinion that the present language is at variance with these remarks, while you do not see any such discrepancies. In part this may be due to the compelling image of vacuum as "the absence of matter" and our intuitive acceptance of such a void as an obvious concept. Unfortunately, it isn't. Brews ohare (talk) 14:29, 4 March 2009 (UTC)

I am not tied to saying anything about zero mass particles: the reference supposedly covers that matter. Have you looked at it?


 * My point was that this does not come from SR. We agree that it is true that zero rest mass particles are believed to travel at c, so let us try to get a good reliable source to use as a reference. Martin Hogbin (talk) 09:26, 4 March 2009 (UTC)


 * Look at the cited reference. Brews ohare (talk) 14:29, 4 March 2009 (UTC)

How do you distinguish between photons (particles)and waves? They are two facets of the same phenomena. If a particle is a wave packet and has mass, how can that happen if all its constituent waves are massless?? In any event this argument does not affect the article, as it never comes up. Brews ohare (talk) 05:08, 4 March 2009 (UTC)


 * When we use the wave model of light it is pointless to use the word 'mass'. Mass of what? We can talk of momentum transfer but waves are not particles and therefore the term mass is inappropriate.  The text been changed since I made my comment this so I agree that there is no longer any problem.


 * A while ago, we had a form of words that was acceptable to both of us and the others here. I suggest that we revert to that.

Martin Hogbin (talk) 09:26, 4 March 2009 (UTC)

We need to get the introduction under control
Here is today's introduction:


 * The speed of light in a vacuum is an important physical constant usually denoted by the symbol c.  Experimentally, to the limits of measurement accuracy, all electromagnetic radiation travels in a vacuum at exactly the same speed.  Theoretically, Maxwell's equations of classical physics also predict that all light travels exactly at the same speed.  In practice, this speed is so consistent and repeatable that it makes sense to speak of the speed of light, and the meter is now defined as the distance light travels in 1/299,792,458 of a second (so the speed of light is exactly 299,792,458 metres per second, by definition).   According to special relativity, c also is the speed of anything having zero rest mass, and is anticipated to be the speed of gravitational waves as well.  Some theories hold that when  quantum mechanics of vacuum is taken into account, the speed of light may not be precisely constant.  Such variations are of great theoretical interest, but are at most very tiny corrections, and have not yet been observed.


 * Einstein's theory of relativity together with the principle of causality requires that no matter or information can travel with a speed larger than c. Speeds faster than that of light in a vacuum are encountered in physics but, in all such cases, no matter or information is transmitted faster than c.

Lets compare this to a version picked out of random (the last one in February).


 * The speed of light in a vacuum is an important physical constant usually denoted by the symbol c. The speed of light in free space is defined as exactly 299,792,458 metres per second (due to the definition of the metre).


 * The speed of light is of fundamental importance in physics. It is the speed of not just visible light, but of all electromagnetic radiation, and it is believed to be the speed of anything having zero rest mass, and of gravitational waves. Einstein's theory of relativity together with the principle of causality requires that no matter or information can travel with a speed larger than the speed of light.   Speeds faster than that of light are encountered in physics but, in all such cases, no matter or information is transmitted faster than c.

Whatever the benefits of the purity of being absolutely correct to the last letter that the current version may have it cannot possibly make up for its lack of clarity and its excess of obtuse jargon (at least to the layman). Is there any way we can simplify the introduction by pushing more subtle arguments to their appropriate section in the article?

TStein (talk) 06:20, 5 March 2009 (UTC)


 * I was just about to say the same thing. The lead section should be a summary of the body of the article.  References are not necessary as they should be present in the full discussion below.  People should not use the article, particularly the lead section, to make points; they should be discussed here and a consensus agreed before they are incorporated into the article.  This was once a FA and I would like to try to get that status back.  A long and rambling first paragraph is not the way  to do it. Martin Hogbin (talk) 09:51, 5 March 2009 (UTC)


 * I'm the editor who made the most recent changes, and I agree 1000%. I've been watching the discussion of quantum vs classical vacuum, and tried to move this from the first sentence to at least the end of the first paragraph.  But the above version is much b etter for the reader, and the point that there should be no footnote in the lead is well taken.  I strongly support this.   LouScheffer (talk) 11:41, 5 March 2009 (UTC)

The present introduction is fine with the exception of the last paragraph, which introduces sub rosa an incorrect treatment of the definition of the speed of light, which either should be deferred to later paragraph where it can be presented at length, or rephrased to distinguish between the physically measured speed of light in realizable media like outer space and the speed of light defined in free space. As I have said above:
 * All definitions refer to free space. However, the defined "vacuum of free space" is not a realizable medium, and any realizable medium (for example, a region of space containing only EM radiation might be realizable someday ) is only verifiable to within experimental error to be "like" free space. The properties of a realizable medium are measured, not defined. The frequent confusion in this article between "vacuum" (a vague designation) and "free space" (a hypothetical medium with defined properties, and therefore not subject to experimental verification) is a bad thing. Brews ohare (talk) 13:46, 5 March 2009 (UTC)

In case the notion of vacuum is not as clear as it might be to you all, consider these questions and sources: These questions can be asked theoretically, but differences can be confirmed only experimentally and confirmed only to within measurement accuracy. Brews ohare (talk) 13:55, 5 March 2009 (UTC)
 * Does outer space have the same speed of light as free space? Does outer space exhibit zero dispersion, like free space? Is outer space isotropic, like free space? Does information travel in outer space in the same manner as in free space? Do Lorentz transformations apply in outer space the same way they do in free space?
 * Substitute "quantum vacuum" or "QCD vacuum" or "quantum gravity vacuum" for "outer space" in these questions, if you like.

See also Unruh_effect, QCD vacuum, vacuum state, electric constant, free space, Outer space, Interplanetary medium, Interstellar medium, and speed-of-light itself at this location and here. Brews ohare (talk) 14:09, 5 March 2009 (UTC)

Internal cross links in Intro
I've set up some internal cross links in the intro that take the reader to the part of the article where more detail is found. It seems like a good approach in principle for making the intro more useful: sort of an embedded TOC. What do you think? Brews ohare (talk) 19:50, 5 March 2009 (UTC)

Introduction
The section now titled Introduction is very much a repetition of material in the first few paragraphs. This repetition is a result of the idea that no references should be cited in the first few paragraphs, so instead all the material is repeated with the citations in this section instead. That is not sensible. It should be eliminated, or possibly pared down and whatever is different put back into the first few paragraphs. The citations should be put in the subsections where they belong. Brews ohare (talk) 20:15, 5 March 2009 (UTC) I have implemented this suggestion. Brews ohare (talk) 21:14, 5 March 2009 (UTC)

Lead now truly awful
The lead section has now degenerated into a rambling exposition of an idiosyncratic point of view of the subject. Martin Hogbin (talk) 09:33, 6 March 2009 (UTC)


 * Hi, Martin! The lead section is supposed to give an overview of the topic suitable for an intelligent but non-technical reader.  So what do you think the reader should take away from this section?  For a thought experiment, compare the speed of light to the speed of sound.  What would you point out that a layman could easily grasp that is special about the speed of light?  I'd propose
 * The speed of light is fast, but finite.
 * It does not depend on wavelength or amplitude in a vacuum, but in other materials it does
 * It determines the structure of space-time (based on Einstein's relativities)
 * It provides an absolute speed limit (even a casual reader has probably encounter faster-than-light ships in sci-fi)
 * It shows up in the equivalence of mass and energy (E=mc^2 is probably the only equation the average user has ever seen with the speed of light in it.)
 * It's important to lots of technologies (as opposed to the speed of sound, for example, which is mostly of interest to aerodynamicists)
 * Historically, it's so fast it's hard to measure, with the first measurements in the 1600s.
 * It's really, really, constant. I don't know what the tightest bounds are (lunar ranging in different colors agreeing to cm level, or gamma ray bursts, implications from the constancy of alpha as determined by atomic clocks) but they are very tight.  I'd like to see something here for the average person (same to a part in a million billion, or similar)
 * It was not explained until the late 1800s ( by Maxwell, though we could leave that out.)
 * I have no particular attachment to the prose, but strongly prefer that the intro makes these points. What are your views?  Thanks,  LouScheffer (talk) 13:29, 6 March 2009 (UTC)

The lead section looks good to me. It doesn't cover everything in the article, but hits the high points. I see nothing "idiosyncratic" (personal, quirky) or "rambling" (disconnected, wordy) about it. Maybe Martin could elaborate? Brews ohare (talk) 14:21, 6 March 2009 (UTC) The Wiki guidelines are located at Lead section. An organized critique could be based upon comparing the present lead with those recommendations. Brews ohare (talk) 14:42, 6 March 2009 (UTC)

In this connection, the guidelines do recommend some citations in the lead section: The verifiability policy advises that material that is challenged or likely to be challenged, and quotations, should be cited. Because the lead will usually repeat information also in the body, editors should balance the desire to avoid redundant citations in the lead with the desire to aid readers in locating sources for challengeable material. It may become necessary to do this to defend against the "idiosyncratic" claim, which might be shorthand for POV or might be shorthand for "bogus". Brews ohare (talk) 14:46, 6 March 2009 (UTC)


 * Before I reply to the points raised above, I will wait to see what others say. Martin Hogbin (talk) 18:00, 6 March 2009 (UTC)

The lead is genuinely horrible. I particularly dislike the statement that "theory shows" something about reality, but the whole thing is a rambling and highly idiosyncratic presentation which completely fails to get the key ideas across. Lou's outline looks like a good starting point. EdwardLockhart (talk) 10:40, 9 March 2009 (UTC)


 * The present lead does what Lou suggests. Allegations like "rambling" and "idiosyncratic", with no specifics, aren't helpful.
 * So "theory" cannot say anything about reality: exit Newton, Maxwell and Einstein, eh?
 * The intro paragraph states the grand claims of Maxwell's equations when applied to ideal vacuum, and then compares them with experimental data on realizable vacuum. That is standard operating procedure. Brews ohare (talk) 12:10, 9 March 2009 (UTC)

Sixteen digit accuracy
This claim is stated as though it refers to independence of the speed of light with respect to all variables: frequency, field, passage of time etc. However, that is unlikely. For example, in the 30Hz range independence from frequency is established only to 0.8%. At 10um it is a few parts in 109. At 633-nm it is several parts in 1011 1981. I believe your statement applies to the limits on the cosmological variation in c over the life of the universe (the Webb observation of d ln α /dt ≈ 2 × 10−16/year). Maybe a more limited statement could be made? Brews ohare (talk) 20:44, 6 March 2009 (UTC)


 * You are absolutely correct. The specific claim I was refering to is the current limit of 1x10^-16 of alpha/year, by comparing atomic clocks. (Of course it is still possible that C does vary, but so does the meter and the second, and in a way that cancels).  But other limits are less well known (position independence, or logn wavelengths, for example).  Others are pretty good (gamma rays vs optical from GRBs match within a few seconds out of 10 billion years, also 10^16 type of number).  A way to write this that gives the correct expression that constancy is quite good, but that does not overstate the case, would be appreciated. LouScheffer (talk) 21:43, 6 March 2009 (UTC)


 * I have taken a shot at this. The most recent data refers to frequency standards, not speed of light. I assume the same meters are obtained at all frequencies so the error in transition frequency translates to a limit on the uncertainty in c at a given frequency. Brews ohare (talk) 22:43, 6 March 2009 (UTC)


 * The limits on dispersion are much tighter, at least in some cases. A recent gamma ray burst, for example, was about 10 billion light years away.  The suspected dispersion (though there are other explanations) resulted in a 16 second mismatch between the high and low energy gamma rays.  Out of 3x10^17 seconds, that's less than 1 part in 10^16.  And as far as I know the gamma to optical is always less than one minute, so that's 1 part in 5x10^15.   LouScheffer (talk) 00:37, 7 March 2009 (UTC)

Hi Lou: You have more confidence in the astronomy than I do. From my jaundiced viewpoint, the astronomers are still arguing over whether dark matter exists, is c a function of the age of the universe, are the red shifts Doppler or gravitational or expansion of space, etc. etc. I think there are just too many things going on in interpreting these results for credibility. So I'd stick with the lab results. Brews ohare (talk) 01:15, 7 March 2009 (UTC)


 * Check out this one Astrophysical Tests of Lorentz and CPT Violation with Photons by V. Alan Kosteleck´y and Matthew Mewes.  Pulses differing in wavelength by a factor of 5 arrive within 0.005 seconds of each other after traveling for 4 billion years.   "Over energies from 3 to 17 MeV, arrival-time differences are no more than Δt < 4.8 ms for this source at z ≃ 0.3 (Boggs et al. 2004).".  That's 5 parts in 10^20.

Double the distance?
The distances to the moon, planets, and spacecraft are determined by measuring the round-trip travel time and dividing by the speed of light.

Wouldn't this give double the distance, the distance the light covers going there and back? Am I missing somehing here? 19:30, 11 March 2009 (UTC) —Preceding unsigned comment added by 86.74.122.183 (talk)


 * Just a bit of careless writing. Martin Hogbin (talk) 22:04, 11 March 2009 (UTC)


 * No, this is not careless, it was deliberate to avoid a long explanation. To a first order approximation, the questioner is right.   However, a first order approximation cannot be used here, precisely due to the finite speed of light and the long travel times involved.  The observable is the total distance traveled by the signal, which is computed exactly as specified.  This is *not* twice the distance to the spacecraft - imagine (for example) a signal sent to a spacecraft 3 light hours away from a station on the equator, when the spacecraft is directly overhead (and ignoring the motion of the Earth around the Sun, for now).  When the signal gets back, 6 hours later, the earth has made a quarter turn, and the spacecraft is on the horizon as seen by the transmitting station.  In this case the return path is one Earth radius longer than the transmit path.  If you just divide the round trip time by 2, your position will be off by 1/2 the radius of the Earth, or about 3000 km, likely a deadly error if you are orbiting something.  This is why the original says "determined by" and not "equal to" - the "determined by" step includes a *lot* of complexity in this case (Location of the Earth itself, station locations, Earth rotation, relativity, equipment delays, delays through the atmosphere, ionosphere, corona and so on).


 * All this being said, it certainly looks wrong on a casual reading, so I changed it to add a more complete explanation.  LouScheffer (talk) 13:30, 12 March 2009 (UTC)

Relevant Image?
I fail to see the relevance of the second image to this article. Someone care to elaborate?

dawmail333 (talk) 11:11, 16 March 2009 (UTC)


 * The text mentions E=mc^2. The figure illustrates the application of this equation, providing a concrete example of the application of this formula, and a citation where more can be found. Brews ohare (talk) 16:09, 16 March 2009 (UTC)

In this case the measurement is one-way
Here we have another confusion due to too short a discussion: it is a tenet of special relativity that no one-way measurement is possible, apparently contradicting this statement in the article. Maybe something has to said about how one insures "an accurate local time on both ends". Brews ohare (talk) 18:39, 16 March 2009 (UTC)


 * The full explanation is fairly complicated. To first order, first everyone must first agree on a reference frame. See National Geodetic Survey Precise GPS Orbits.  Then the orbital clocks are all synchronized using radio signals sent from  the ground.  (to an observer in any other reference frame, of course, the clock would not appear to be synchronized).  Next, the receiver gets signals from at least 4 satellites, each signal containing where the signal was sent from, and the time it was sent.  Without an accurate local time t, (the usual case, since hardly anyone carries an atomic clock around), and 1-3 satellites, the receiver would be out of luck, and unable to compute the distance.  But if you can observe at least 4 satellites then the receiver can solve for x,y,z, and t.  Thus it derives a local time (and location) that makes all measurements consistent.   LouScheffer (talk) 19:18, 16 March 2009 (UTC)

Hi Lou: I took a whack at fixing this up by linking to trilateration; take a look, Brews ohare (talk) 20:30, 16 March 2009 (UTC)

Article issues
I'm thinking I'm going to be bold soon and start translating some of the article from the Spanish one: es:Velocidad de la luz. I'm not very fond of how the current one reads: the introduction, for example, does not properly introduce the subject. If anyone has any objects to any changes I may make, please state so here. Magog the Ogre (talk) 21:27, 1 April 2009 (UTC)

What if....
What if information cannot pass the speed of light because it get canceled. Just as the same that information cannot have a speed of total 0. Like interference of two waves? —Preceding unsigned comment added by 190.140.38.151 (talk) 01:18, 24 April 2009 (UTC)

"Exact" speed of light???
Defining one meter as 1/299792458 of the distance that light travels in a "vacuum" in one second is perhaps an exact definition of the meter. (Though *not* an exact measurement thereof.)

But this certainly does not mean that 299,792,458 m/sec is the "exact speed of light" -- as the article calls it several times -- because the meaning of one meter has been measured only depending on the accuracy with which the speed of light has been measured.

So: What I am wondering is

a) why is there no mention of how the measurement of the speed of light was improved between 299,792,500 +- 100 m/s to (presumably, before the meter was defined as I cite above) the speed 299,792,458 (and what were the error bounds?)

b) How was this improvement achieved?

And surely, defining the meter in terms of the speed of light has not ended physicists' attempts to measure the speed of light ever more accurately!

c) What about the most recent attempts to measure the speed of light -- their results, and the methods used?Daqu (talk) 18:05, 18 April 2009 (UTC)


 * See, and more specifically
 * "In 1983 the international standard for the meter was redefined in terms of the definition of the second and a defined value for the speed of light. The defined value was chosen to be as consistent as possible with the earlier metrological definitions of the meter and the second. Since then it is not possible to measure the speed of light using the current metrological standards, but one can still measure any anisotropy in its speed, or use an earlier definition of the meter if necessary."
 * DVdm (talk) 21:53, 18 April 2009 (UTC)


 * Sorry if I was unclear, but it's perfectly obvious that -- as you say -- if the meter is defined in terms of the speed of light, then one cannot improve the accuracy of the figure for the speed of light by using that definition of meter along with the same definition of one second.


 * But that has nothing to do with the (presumed) continued attempts to improve the accuracy in the measurement of the speed of light!!!  The more accurate measurements just need to be cited in terms that used a fixed length unit (as well as a fixed time unit).


 * My comment opening this section was not requesting references. It was intended to suggest that someone knowledgeable on the subject (which I am not) answer these rather natural questions (a, b, c above) in the article.


 * In case this is *still* unclear, let me use an analogy. If one defines pi as the ratio of the circumference to the diameter of any perfect circle, that does not end the search for a increasingly exact value of pi.Daqu (talk) 00:01, 26 April 2009 (UTC)


 * There are no continued attempts to improve the accuracy in the measurement of the speed of light because the speed of light is fixed by definition; there are attempts to improve the precision with which the metre is delineated. Martin Hogbin (talk) 21:43, 4 May 2009 (UTC)


 * It's crystal clear to me that with the definition of the speed of light as a fixed number (299,792,458) of meters per second -- and the definition of the second fixed (in terms of a fixed number of vibrations of a cesium atom) -- the two endeavors of improving the precision of the meter, and improving the precision of the speed of light are logically equivalent.Daqu (talk) 08:36, 6 May 2009 (UTC)


 * They are much the same thing, but to go back to your original point, as the speed of light is now fixed by definition and the length of the metre is not, information on experimental improvements in measurement technology after 1983 might be better put in the article on the metre rather than this one. Martin Hogbin (talk) 10:50, 6 May 2009 (UTC)

&larr;This comes down to an equation with three unknowns. One of those unknowns has to be fixed by definition in order to define the system of units; the second (time) can be measured with great accuracy, so its uncertainty doesn't really enter into the experiment. Hence, the measurement uncertainty is concentrated in the third of the variables. By fixing the speed of light by definition, it becomes explicit that no measure of length in SI units can be more accurate than the best determinations of the speed of light: all the experiments that, prior to 1983, were determining the speed of light are now determining the length which is equal to one metre. Physchim62 (talk) 13:21, 6 May 2009 (UTC)
 * Before 1983, the length of the metre was fixed by definition, and the uncertainty was expressed in the value of the speed of light: the question being asked was "how long does it take light to travel a given distance?"
 * Since 1983, the speed of light is fixed by definition, and the uncertainty is expressed in the length of the metre: the question being asked is "how far does light travel in a given time?"
 * no measure of length in any unit -- since, for example, Imperial units are defined in terms of SI. Yes, your point is correct.  To put it differently, an experiment that measures the speed of light is in effect an experiment that realizes the meter.  And the resulting uncertainty in that realization of the meter will be the uncertainty of the experiment or the uncertainty in the realization of the second -- whichever is larger.  (Today, the former will certainly be the larger.)  Furthermore, the reason the CGPM adopted the new definition is that this inaccuracy (realization of the meter via speed of light measurement) is now better than the inaccuracy of the previous definition.  That, after all, is the basis on which unit definitions are chosen and changed. Paul Koning (talk) 16:51, 6 May 2009 (UTC)
 * You can actually measure length in astronomical units to a comparable level of accuracy with measures in SI units (within the same order of magnitude of relative uncertainty, that is a few parts in 1011, roughly a hair's breadth between New York and Los Angeles), even though the two systems of measurement have very different bases. The stability of atomic clocks, for any single Earth-based experiment, is about 10,000 times greater: hence my comment that the uncertainty in time measurement is negligible. On the other hand, at this level of precision, you have to take account of General Relativity: this teaches us that the measurement of distance and time are not actually independent of one another as one might assume from everyday measurements, and is a huge headache for people devising astronomical units! Physchim62 (talk) 00:10, 7 May 2009 (UTC)
 * Ok, but AUs are defined in terms of the meter -- exactly as the inch is. An AU is either a measured distance, or an agreed on value; either way, the value is in meters. Paul Koning (talk) 00:51, 8 May 2009 (UTC)
 * But AUs are not defined in terms of the metre! See Astronomical unit for the actual definition. You can express the value of the AU in metres, of course, and it's instructive in the context of this discussion to see how you do it. Firstly, you measure the time taken for light to be reflected from various astronomical objects (such as the Moon and the inner planets). Then you calculate the positions of these objects using a scale based on astronomical units – you can't do the calculation in SI, because one of the constants (the solar mass) isn't known to sufficient precision in SI, so it is given a conventional value (of unity) in the astronomical system of units. With a measure of time and a calculation of distance, you have a speed: the speed of light in astronomical units! It is usually quoted as a reciprocal speed, that is light time per unit distance, and has a relative uncertainty of 4 parts in 1011. If you so wish, you can then calculate the length of the astronomical unit in metres by using the fixed value of the speed of light in the SI. Physchim62 (talk) 06:48, 8 May 2009 (UTC)

I am a bit shocked to see that some folks don't understand the concept of velocity, or of precision. A velocity depends on more than purely a number!!!!! It depends on a number and a unit of length and a unit of time. Basically I am reiterating what I said earlier and (as I understand him) what Physchim62 wrote above.

If the speed of light is said to be "fixed" at "exactly" 299,792,458 meters per second, this velocity is specified only to the extent that the length of a meter and the duration of a second are.

Since the length of a meter is no longer specified (except in the sense that the velocity of light is 299,792,458 meters per second), the speed of light is not specified. Any more than the fact that π = C/D (the circumference of a circle divided by its diameter) is "specified". For we know π only to a certain number of digits. That number of digits can always be increased by further calculation. So, having an exact definition of π is not the same thing as being unable to increase the precision to which we know it.

Likewise, the speed of light can still be determined to increasing precision by more and more careful experiment. (Since by definition, it is equal to 299,792,458 meters per second, this increasing of precision would take the form of increasing the precision of the determination of the length of a meter.)Daqu (talk) 16:49, 13 May 2009 (UTC)


 * I agree with you that there are legitimate questions on this topic, for example: (1) "the speed of light in astronomical units per second is known to how many decimal places?" and (2) "a meter (with its standard, speed-of-light-based, definition) can be reproduceably created/defined to what precision"? That is, if two labs created two ideal meter-sticks after independently using light and caesium atoms, how close could those meter-sticks be in length? On the other hand, I think (1) isn't very important, and (2) is more a topic for the meter article (as Martin suggested). How reproduceable is a measurement of the speed of light? Moreso than almost any other measurement, which means that the small imprecision has little or no practical consequence, since it's dominated by the larger imprecision of whatever the speed of light is being compared to. --Steve (talk) 22:01, 13 May 2009 (UTC)
 * The relative uncertainties are 4 for (1) and 2 for (2). The former is discussed in astronomical unit; the latter should be discussed in metre. I only brought up the example of the AU to demonstrate that the speed of light is only defined with respect to a system of units, even if it is fixed (ie, unchanging) with respect to all observers. You are quite welcome to define the speed of light as 1 unit of speed, and several systems of natural units do exactly that. Physchim62 (talk) 12:01, 16 May 2009 (UTC)

The missing point in all this discussion is that the defined value for the speed of light applies only in free space which is not a realizable medium. (In fact, free space is a medium where c has its defined value, else you don't have an example of free space. Free space also has other defined properties like isotropy, linearity, and frequency independence.) There is zero interest in refining this defined value of c. The experimental interest is in measuring the speed of light in real, obtainable media, such as outer space or ultra-high vacuum as that information tells us something about physical nature of the Universe. Our definitions about the speed of light in free space are really only a matter of convenience in comparing measurements made by different parties on different media in varying circumstances. Brews ohare (talk) 05:33, 16 May 2009 (UTC)

It may be added that the "speed of light" as the maximum speed of transfer of information might be separate from the properties of free space. That is an experimental issue. Brews ohare (talk) 05:45, 16 May 2009 (UTC)

Article Restructuring
I think this article loses focus of its main topic in several sections, and would benefit from a restructuring. Also, the article seems a little too long, which may lead to more confusion as it is being read. I would like to propose that we refactor the article into different sections, then condense those sections, to improve the article. I think sections with titles like: History, The Speed of Light in Scientific Fields, Scientific Uses, Measurements, and Superluminal Experiments, would be appropriate. Although these would result in drastic changes to the article, I think they would result in a much clearer presentation. If anyone has any comments or other suggestions, please leave them here. I'll check back in a few days to see if the community agrees the proposed changes would benefit the article. Pecos Joe (talk) 20:37, 20 April 2009 (UTC)


 * There is a lot that needs to be cut and some that needs to be added to this article IMO. Personally I don't like either The Speed of Light in Scientific Fields or Scientific Uses.  Neither of them say anything useful.  (To be fair the current heading are not any better.)  I prefer the Faster then light to Superluminal for a heading since those reading the ToC might not know what it means.  Also, again IMO, speed of light and causality needs to be a prominent section (just before the Faster than light section.  Condensing the History into one section sounds good, though.


 * Stuff that needs to be cut, IMHO. The michelson morely (check to see if you can merge it with the main article first it looks good it just doesn't belong here). Addition of velocities near the speed of light needs to be drastically shortened or eliminated.  It seems silly to me to include an equation just because it has c in it.  It would be more appropriate for an article such as List of equations that depend on c.  I don't know why the fussion picture is there for the same reason.


 * Stuff you should consider adding is a section about the different 'speeds of light' in a material. I heard that there are something like 10 different 'speeds of light' that a material can have.  Group and phase velocities are the most well known.  There is also the wavefront velocity which determines the speed that information travels in the material.  These are of course different then the speed of light c which is the limit that any information can travel.  (Those are 4 that I am aware of.) TStein (talk) 22:10, 20 April 2009 (UTC)


 * Agree. This article gets far too involved in abtruse and irrelevant areas, to the point that it barely even defines what the speed of light is. The historical details are relegated to the end of the article, when this page is the most obvious place to put them: instead, the reader is treated to a whole load of tad relativity theory which could be dealt with elsewhere. Faster-than-light phenonema have their own page (justifiably), and so should be treated in summary style (or "See also") here. Physchim62 (talk) 23:40, 20 April 2009 (UTC)

I agree that the article loses focus. I think one of the reasons for the loss of focus is a statement in the initial summary; "When light is traveling in a medium, its speed is less than c and becomes a function of the refractive index of the material.". This statement is really a trap door for the article--because it leads it into explanations to try and justify or explain the apparent contradiction of the earlier statement in the summary; "The speed of light in free space is a physical constant defined as 299,792,458 metres per second.". This gets the article into trying to explain variations, of which there are many, in speed relative to frequency across the electromagnetic spectrum, and material refractivity. It would be better if the summary stated; "Light is simply an electromagnetic wave and the speed of an electromagnetic wave is always constant at 299,792,458 meters per second. Depending on the waves frequency and the medium it is traveling through it can be absorbed and re-emitted resulting in a delay in its propagation. So while the actual speed of an electromagnetic wave never varies, because all mediums contain some free space, its apparent or observed speed propagating through a medium does." Then the article could be linked to separate articles on material refractivity, the electromagnetic spectrum, and other articles that are relevant to the discussion. I think this new summary is what the article is actually saying, just in a very long format. So if the summary was changed to give a more accurate initial understanding and then there were links added to more detailed discussions—this would help to clarify the article to make the restructuring better, and still allow for the detailed discussion to continue under their own heading. Hope this helps. Thanks, for allowing my input.User:68.114.14.80 (talk) 09:05, 21 April 2009 (EST)


 * You accidentally overwrote the previous discussion of this section. I restored it and indented your paragraph. TStein (talk) 14:04, 21 April 2009 (UTC)

In response to User:68.114.14.80, I think most of us are in agreement that the article needs to be pared down by cutting many sections and paring down other sections with links to the relevant articles. I also agree that the article devolves too quickly into discussing minutia. I am not so comfortable, though, with "Light is simply an electromagnetic wave and the speed of an electromagnetic wave is always constant at 299,792,458 meters per second." While it is true that photons only travel at c it is false to say that electromagnetic waves travel at c.

The real problem is that the term 'speed of light' is used for at least 4 (that I know of) very distinct quantites that are seemingly identical. (1. Maximum speed limit of universe and speed of a photon, 2. phase speed of an electromagnetic wave 3. group speed of a wave pulse. 4. wavefront speed of a wave pulse.) Somehow we need to structure the article to make that distinction apparent as early as possible while keeping the structure simple and organized from simplest material to more complicated material. Further we need to do this while minimizing redundancy and maximizing continuity and readibility.

I believe strongly, that the physics community should not call c 'the speed of light'. (I try to call it simply c.) In most of the cases where it is used the true meaning is something on the order of 'speed limit of the universe'. The fact that c is also the speed at which photons (since they are massless) happen to go at is in most cases irrelevant. Worse it misleads people with respect to cause and effect. That being said, the article needs to, from the very beginning (perhaps the first paragraph), separate the term 'speed of light' into 'speed limit of the universe, c ' and 'speed of light v ' The last term can have many different meanings because no wave moves as a whole. TStein (talk) 14:04, 21 April 2009 (UTC)


 * I did the reorganizing I mentioned above, taking into consideration everything all of you suggested. The current sections will help identify repetition in the article by grouping some things closer together than they were before, while hopefully still being an improvement on the previous version.  I am shying away from adding new information to the article, or doing any substantial rewrite, until the article gets pared down a bit, which will take some time.  One section that probably needs to be added is "The speed of light in materials" or something to that effect. Pecos Joe (talk) 18:31, 22 April 2009 (UTC)


 * In response to User:68.114.14.80, the "defined" speed of light has only an oblique relationship to the physical speed of light because the defined speed applies only in the hypothetical and unrealizable medium of free space. The physical speed of light occurs in real materials, of which vacuum is one, and outer space and ultra-high vacuum are examples. If anything should be dropped from the lead, it is not the "trapdoor" of speed of light in materials, but the undue emphasis upon the "exact" speed of light, which is a red herring. And, as TStein has observed, the speed of photons is distinct (in principle) from the concept of "speed limit" for information transfer, which underlies the notion of spacetime and simultaneity, and is presently under serious scrutiny in the arena of quantum gravity. Brews ohare (talk) 13:11, 16 May 2009 (UTC)

Infinite speed and perception
Copied from the science reference desk. Jay (talk) 09:14, 30 April 2009 (UTC) 

I first thought of posting this on the Language ref desk, then changed my mind. Isn't something grammatically wrong with this line from Speed of light: "Francis Bacon argued that the speed of light was not necessarily infinite, since something can travel too fast to be perceived."Jay (talk) 10:43, 20 April 2009 (UTC)
 * I can't see anything wrong with it. What do you have in mind? Algebraist 11:03, 20 April 2009 (UTC)
 * The grammar is ok but the logic is questionable because the underlying hypothesis about "something" is not explicit. The OP could bring this up on the article discussion page to see what others think. Would the following sentence be better? "Francis Bacon argued that the speed of light was not necessarily infinite, since it could just be travelling too fast to be perceived."Cuddlyable3 (talk) 11:12, 20 April 2009 (UTC)
 * Yes, sorry about that. I should have said logically wrong instead of grammatically wrong. Cuddlyable3's version is making sense to me now. The meanings I had in mind were: "Francis Bacon argued that the speed of light was not necessarily infinite, since nothing can travel too fast to be perceived.", and "Francis Bacon argued that the speed of light was not necessarily infinite, since something cannot travel too fast to be perceived.", both meanings are opposite of the original line. Jay (talk) 11:29, 20 April 2009 (UTC)


 * I think it's vague about what is being perceived; I mean, light is definitely perceived so he can't be referring to that. I assume he's referring to the finite speed being perceived i.e it's really fast and we can't tell the difference between that and infinite speed.  How about
 * "Francis Bacon argued that the speed of light was not necessarily infinite, since it could be travelling at a very large but finite speed which was beyond the scope of contemporary experimental accuracy."
 * Then again, I could have misinterpreted his point. Martlet1215 (talk) 11:37, 20 April 2009 (UTC)
 * I think you nailed it. Light is perceived as something instantaneous in all "normal" situations, but that is just an artifact of imperfect observation - and that was the point Bacon was making. --Stephan Schulz (talk) 11:49, 20 April 2009 (UTC)
 * User:Wile E. Heresiarch who added the line in 2004, has since left Wikipedia, and it is hard to confirm the source. This reference however says that Francis Bacon believed the speed of light to be infinite! Jay (talk) 12:25, 20 April 2009 (UTC)
 * It says that Roger Bacon believed it to be finite, though. Perhaps Heresiarch confused the two. Algebraist 12:29, 20 April 2009 (UTC)
 * Guys you should take your content discussion to this talk page Cuddlyable3 (talk) 13:30, 20 April 2009 (UTC)


 * It seems like the original quote implies that Bacon argued that it's possible to imagine a thing that travels imperceptibly fast but not infinitely fast, and that light might be one of those things.  User:cuddlyable3's version could be less clear depending on what Bacon actually said. Cuddlyable3's version implies that he specifically theorized that light could travel very fast, but doesn't specify whether Bacon thought this was a special case or whether other items could also attain imperceptible speeds. I have no idea which is correct, but they don't mean exactly the same thing.
 * This distinction might not matter at all in context. I dunno. APL (talk) 13:46, 20 April 2009 (UTC)
 * I'd can "can" and replace it with "could". Clarityfiend (talk) 15:53, 20 April 2009 (UTC)
 * This guy at least included a direct quotation from (Francis) Bacon and a citation: "'Even in sight, whereof the action is most rapid, it appears that there are required certain moments of time for its accomplishment...things which by reason of the velocity of their motion cannot be seen—as when a ball is discharged from a musket' (Philosophical Works of Francis Bacon; J.M. Robertson, Ed.; London, 1905; p. 363)." The ellipsis is awkwardly placed, though, and I don't happen to have a copy of the Philosophical Works lying around in which to examine Bacon's full argument. Deor (talk) 17:01, 20 April 2009 (UTC)
 * I should have guessed that Google Books would have snapped this up. If one reads the whole chapter, one can say only that Bacon seems to be rather unclear whether he's discussing physics or physiology. Sometimes he seems to be maintaining that light has a finite speed; at other times, he seems to argue that it's our visual impressions that require time for their formation. And I admit to not being able to make much sense of his discussion of the idea that starlight takes time to reach us: he seems to say that he has discarded his initial suspicions that this is so, and now believes that our perception of stars is instantaneous. Deor (talk) 18:00, 20 April 2009 (UTC)
 * I don't think it's the light that's being not-perceived it's the speed of light. If you have a friend standing on a far distant hilltop with a mirror and you light a candle - the time it takes for the light from the candle to travel to the mirror and back again is too fast to perceive.  Bacon argues that the time might not be zero - but merely so short that you can't perceive the delay...smart guy...he turned out to be correct!  SteveBaker (talk) 16:58, 20 April 2009 (UTC)

Is anyone interested in getting this article back into shape?
This article has now been degraded into a shadow of its former self. Is anyone interested in getting it back into shape? Martin Hogbin (talk) 22:33, 25 May 2009 (UTC)
 * I have restructured and trimmed the article of much of its extra content recently, because it seemed to me to go into far too much detail about things unrelated to the speed of light. I hope it is not this effort that has made this article into a "shadow of its former self."  I have my own ideas about what should be done to make this article better; do you have any specific improvements in mind?  Also, see my question in the topic below this one.  Thanks, Pecos Joe (talk) 20:50, 27 May 2009 (UTC)
 * I have many, but it is no use mentioning them all without the consensus to improve the quality of the article. Martin Hogbin (talk) 09:29, 31 May 2009 (UTC)
 * I don't think anyone here wants to make the article worse, so I'll go first and offer that I think the article could use a lot of effort focused on copy editing: making smooth transitions and clear and grammatically correct prose especially. Perhaps someone will come along and help make the article better, even though I only have one very vague suggestion. Pecos Joe (talk) 20:08, 1 June 2009 (UTC)
 * I was really talking about the science. It seems to now consist mainly of amateurish interpretations with much of the fundamental and philosophical nature of the subject missing. Martin Hogbin (talk) 22:25, 2 June 2009 (UTC)
 * I agree that the article is scattered and incoherent. I think we need to organize it into sections, perhaps
 * Introduction (non technical, per Wikipedia standards)
 * Notation (needs to be near the front)
 * History
 * Use in physics
 * Practical impact (should be linked from the intro, since it will be a ways down the article)
 * I've started with the introduction. What do others think?   LouScheffer (talk) 01:28, 3 June 2009 (UTC)


 * I like the new introduction; it is a better length and covers more of the material properly. I felt I had to undo the organization because it seemed to cause more problems than it helped.  Several of the pictures ended up in unrelated sections, which I think would be very confusing for a first-time reader, and one section was just the section heading.  As for the suggestions above, I think that the practical impact should be close to the top of the article, while leaving more technical discussions further down.  Also, I will note that I think having a section devoted to discussing the speed of light in media (plural of medium) would be good because it would be useful for people trying to understand the concept. Pecos Joe (talk) 21:02, 3 June 2009 (UTC)

Content addition to cosmology section
One of my recent efforts to trim this article down to size was undone recently. I would first like to say I am glad to see it, because it allows specific discussion on the content that should stay in the article. The content that was reinserted was in the section about cosmology, with the reason that it establishes knowledge needed to understand subsequent remarks. I, however, didn't think this was true, and I wonder if someone could explain to me why they think this material should be here.

The material seems to mostly concern the time-dependence of the the speed of light: I think that the treatment here is far too detailed, especially considering the experimental results are not precise enough to differ from 0. Therefore, I thought a mention of the theory (with a link to the page explaining that theory) and a summary of the experimental results was all that should be in this section. Anything more seems to give undue weight to this section. Thanks, Pecos Joe (talk) 20:49, 27 May 2009 (UTC)


 * The importance of this topic is due to the very fundamental questions raised. At the moment experiment is not much help in settling these questions due to the large errors and the small predicted effects. However, I don't think we can suggest that a topic that interested Dirac, Weyl, Eddington has little importance just because the effect is small. That is how much of physics proceeds - small effects with big logical consequences. Moreover, I suspect that the popular interest in these matters is great, exactly because of the fundamental issues involved. Brews ohare (talk) 05:38, 29 May 2009 (UTC)


 * I'm mostly with PJ. Yes, it's interesting whether or not physical constants change in time. A general discussion of this should be (and is) in the physical constant article. If that's not enough, you can make a new article, time dependence of physical constants. But no need to go on and on -- particularly in an article with a broad readership. The average reader of this article doesn't care exactly why quantum gravity predicts optical nonlinearities, or how the Planck length is defined, or how the fine-structure constant is defined, etc.


 * Also, you need to realize that there are loads of physicists who make a career by exploring "what if" scenarios that no one thinks is likely...What if photons have a mass? What if space is anisotropic? What if electrons have quadrupole moments? What if charge quantization isn't exact? Blah blah blah. For every possibility you could dream of: At least one theorist has dreamed up some model where there's an infinitesimal correction; and at least one experimentalist has quantified how small the correction must be. Now, I'm not opposed to this pursuit, but in Wikipedia you need to be discriminating...not all these explorations are important, not all deserve to be described in any level of detail (or at all), particularly in less-technical articles like this. Some are extremely important (e.g. "What if there were magnetic monopoles", which is the subject of thousands of articles and textbooks). But a lot more of these (perhaps most?) are not.


 * I'd say this section should be only 3 sentences:
 * Some scientists have speculated that the speed of light (and related constants such as the fine-structure constant) may have varied over time in the history of the universe [insert references]. However, direct measurements, based on either astronomical observation or laboratory experiments, have thus far failed to find any evidence for this hypothesis. [insert references] For more information, see the article: Time dependence of physical constants.
 * :-) --Steve (talk) 08:13, 29 May 2009 (UTC)

This denigration of this work as another "blah, blah, blah ... what if" is not supportable. There are many accessible articles cited in the article that refute this view. The questions have a long and very reputable history (also cited), and are under active experimental and theoretical exploration. They are at the heart of such matters as expansion of the universe (a very big problem :-) ) and the testing of relativity's notions of causality and simultaneity on small scales, which is extremely relevant to the speed of light article. They illustrate the way science progresses, and that it is not just parroting odd facts and dusty ideas. I just don't see how a turf war over this paragraph is warranted. Wiki can afford this space, it is not a distraction, and this is the correct place to put it. That is not to say that this section couldn't be shorter. Definitions of fine structure constant and Planck length could be removed at some loss of intelligibility. Some verbiage could be consolidated. But reduction to three sentences and removal of most citations and all historical context is overkill. Brews ohare (talk) 13:57, 29 May 2009 (UTC)


 * I think we are not too far from agreement here. I agree that historical context would be valuable to this section.  Currently, the article states the names of some people who looked at the temporal change; I'd like to see the article say precisely what they did with respect to the change of the speed of light.  I think that's probably why I left the experimental results in; it showed a clear result, and I happen to think those specific results should stay in the article.  I don't think that naming three or four scientists (whom the general reader may or may not know) without much extra context helps us to understand why the speed of light may change over time.


 * Much of the material here should be in a different article, though it should still be in Wikipedia. An important consideration is 'if I were a reader looking for this information, where would I look for it?', combined with the desire to not have too much duplicate content so that the prose is easier to maintain.  The bit about quantum gravity theories should be in the quantum gravity article, as that article doesn't currently mention the word 'light'.  The bit about the fine structure constant should probably be in the physical constant article, with links from here and the fine structure constant articles.  The bit about photon mass should be in the photon article, and if I remember correctly, it was already there when I removed it from here.  Do you agree with those points?  Thanks, Pecos Joe (talk) 20:01, 1 June 2009 (UTC)

Consisting of
a light pulse consisting of multiple frequencies - just because it's describable using fourier transforms, does that mean it "consists of" them? 78.86.37.93 (talk) 00:02, 31 May 2009 (UTC)


 * My answer would be "yes, as long as the transformed pulse has non-negligible width," but there are certainly better ways to word it for the article. I just can't think of one right now. Pecos Joe (talk) 20:11, 1 June 2009 (UTC)

the speed of light can be measured so accurately
The article has a split personality, on the one hand stating the speed is measured and on the other hand that it is defined. These metrology issues have to be resolved. The reference in the Introduction to the "accuracy of measurement" of a defined value for 'c' is beyond comprehension and illustrates a confused mentality. The point really is that the overall error assessment of measurements has led to the view that accuracy is better served when the speed of light is removed from measurement altogether and replaced by measurement of time and wavelength with a defined value for 'c'. Brews ohare (talk) 18:49, 8 June 2009 (UTC)


 * It makes sense to me - the redefinition of the metre is quite a recent thing in historical terms, but even if it had always been defined that way, we would still need to measure it to work out how long the metre was. CrispMuncher (talk) 19:41, 8 June 2009 (UTC).

I'm left unsure what it is that makes sense to you: do you believe that it makes sense to "measure" a defined quantity? Bear in mind, a definition has no error bars, while a measurement always is subject to experimental uncertainty or confidence interval. In a perfect vacuum, 'c' = 299 792 458 m/s exactly, regardless of how long a meter is. Brews ohare (talk) 20:00, 8 June 2009 (UTC)


 * More substantially, the confusion that arises is over the logic behind the definitions. Up to 1983 the metre was defined as "The metre is the length equal to 1 650 763,73 wavelengths in vacuum of the radiation corresponding to the transition between the levels 2p10 and 5d5 of the krypton 86 atom." If c = λ f, 'c' could be calculated using the second.


 * After 1983 the second became "The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom." "This definition refers to a caesium atom at rest at a temperature of 0 K."


 * That change in definition of the second does not require any change in the status of the speed of light, which could still have been left a derived quantity. Instead, it was decided to make the meter a derived quantity: λ = c / f. "The meter should be realized by means of the wavelength in vacuum λ of a plane electromagnetic wave of frequency f; this wavelength is obtained from the measured frequency f using the relation λ = c0/f and the value of the speed of light in vacuum c0 = 299 792 458 m/s." That is, an additional step was taken: 'c' is not to be measured, but is simply a number.


 * I do not have the official explanation for this changed status of 'c' from measured to defined quantity. BIPM suggests that "that these various forms, making reference either to the path travelled by light in a specified time interval or to the wavelength of a radiation of measured or specified frequency, have been the object of consultations and deep discussions, have been recognized as being equivalent and that a consensus has emerged in favour of the first form". Some web sites of the government agencies suggest that the change is a consequence of better measurements of the second, but, as I have outlined above, I see no reason why a better second should have this consequence: c = λ f still works with the 1960 definition of the metre (or a revision in terms of some wavelength more stable than the krypton transition). I assume that it is an issue of measurement accuracy, not an issue of theoretical physics or basic principles.


 * In any event, the context is poorly explained in the article, and various viewpoints are expressed about the present-day stance that are not mutually consistent. Brews ohare (talk) 20:06, 8 June 2009 (UTC)


 * Yes, measurement is still needed - a definition changes nothing. Indeed, it actually makes precise measurement more critical since it is now the basis of our system of measurement.  The speed of light is by definition 299,792,458 ms-1.  In order for us to be able to derive the metre, we need to know how far light travels in one second.  Without that we have some mathematical slight of hand that describes a metre, but nothing that tells us how far it is.  If we had no knowledge of the speed of light then the unit called a "metre" could be an inch long (if light travelled at 299,792,458" per second) or a mile long (if light travelled at 299,792,458 mi per second).  Therefore the accurate measurement is fundamental to to the practicality of using c in our measurement system.


 * As for your question about where it is defined, I do not see the problem. The very source you cite states:


 * ... that there is an advantage, notably for astronomy and geodesy, in maintaining unchanged the value of the speed of light recommended in 1975 by the 15th CGPM in its Resolution 2 (c = 299 792 458 m/s)


 * that a new definition of the metre ...


 * CrispMuncher (talk) 21:54, 8 June 2009 (UTC).

I'm lost. What do we mean about "how far is a meter?" Apparently it is how far light goes in 1/299,792,458 s in perfect vacuum. The real issue is: how perfect is my vacuum, and how accurate is my clock. Not "how far is a metre?" Redefining the meter as "how far light goes in air in 1s" doesn't change anything, except a massive retooling cost and the need to determine how standardized "air" is to be arranged. Do we agree about that? Brews ohare (talk) 22:19, 8 June 2009 (UTC)


 * You're still not getting the central point - the difference between definition and using that definition to determine an exact length, which intrinsically requires measurement. Let us consider an artificial unit, the CrispMuncherHour, which is the distance I walk in an hour (we'll assume that my walking speed is constant and it therefore makes sense to use it as a basis of measurement).  Now we can tell from the definition how far I will walk in an hour, four years or thirty seconds (1, 35064 or 1/120 CrispMuncherHours).  What we can't do from the definition is tell how far those lengths actually are. How can we convert those lengths into another unit?  It is actually more fundamental than that, since it is about the physical length itself rather than how it is expressed other units.


 * So far we have no idea how long the CrispMuncherHour is. We can make educated guesses based on how fast typical people walk but we have no exact measure.  To do that we need to measure how fast I walk - say 5.1 miles an hour.  Now we know that a CrispMuncherHour is 5.1 miles and we can scale that conversion factor to determine what the other quantities are in terms of actual length.  However, until we have an actual measurement the definition does not allow us to say that a CrispMuncherHour is 5.1 miles and not 3 or 10. CrispMuncher (talk) 18:14, 9 June 2009 (UTC)

Sorry, CrispMuncher, I am a bit slow here. Saying a meter is the distance light travels in 1/299,792,458 s seems pretty definite to me, provided I have a vacuum and an accurate clock. Converting to other units seems pretty straightforward, given a definition. For example, if my definition of the unit of length is 1 650 763.73 wavelengths in vacuum of the radiation corresponding to the transition between the levels 2p10 and 5d5 of the krypton 86 atom, I can find out what time light takes to travel this far and make the conversion to meters. I might have questions as to the accuracy to which I have physically realized the krypton transition (line widths and all that) and how good a vacuum I've got, and there is consequent uncertainty in whether I really have 1 650 763.73 wavelengths to measure, but that doesn't impact the definitions. It impacts the "best good practices" necessary for realization or practical embodiment of the definitions.

None of this seems to bear upon the point that one cannot measure a definition. You can measure a specific object to within some accuracy using the definition of the unit of length, but that cannot impact the definition itself. Brews ohare (talk) 19:14, 9 June 2009 (UTC)

"Best good practices"
I suspect that what you really are driving at is the determination of "best good practices" for embodiment of the standard, which is a significant problem not very well documented by the standards agencies. For example, how do you determine whether you have a "perfect vacuum"? I believe the standards organizations largely focus upon reproducibility: if practical measurements require a certain accuracy, reproducibility of the standard must be better than the measurement requirement. If it isn't, you fiddle about with the "best good practices" until you find what is the perturbing factor you missed -  two recent examples: failure to correct for gravitational time dilation and failure to correct for T ≠ 0 K. Sometime in the future, standards will be forced to grapple with the fact that all real vacuums are nonlinear, anisotropic and dichroic. Brews ohare (talk) 16:03, 10 June 2009 (UTC)

All standardized measurements must be made in "vacuum", which is an unobtainable ideal medium. Corrections must be made to measurements done in a real medium to relate them to the "vacuum". One way to handle these corrections for imperfections in the vacuum is simply to define the "vacuum" as the idealized medium where c has a defined numerical value. In other words, defining c in vacuum actually has zero content without a definition of "vacuum". The practical result is to define vacuum as an ideal medium where the speed of light is c, and propose a set of corrections that bring measurements in any real medium back to this ideal. For example, if the lab vacuum has partial pressures of constituents, the contributions of these constituents to the refractive index are estimated and subtracted from the measured results. If the corrections don't work out (e.g. are not reproducible, indicating an unidentified source of variation), then revised corrections are sought. For example, see. Brews ohare (talk) 18:09, 10 June 2009 (UTC)

Vacuum
I have made some changes in wording that I feel more comfortable with. The modern vacuum is not the same as the classical vacuum, even in principle, and it seems to me that some distance should be placed between the various vacuums: the now archaic "vacuum" of classical electrodynamics, the BIPM official "vacuum", the realizable vacuum of interstellar space, and theoretical vacuums like the QCD vacuum.

Another issue is the introduction by BIPM of the "defined value" of c. This definition is tantamount to a definition of BIPM "vacuum", and is a bit unsatisfactory as there is no theoretical definition of this "vacuum". Rather, it is a moving target that is actually defined in an empirical manner through a library of BIPM corrections that are not completely specified but left up to "standard best practices". About all one can say in its favor is that it is a "reduction to practice" of "vacuum". However, having no theoretical basis outside of this (ever changing) operational definition, it is subject to definition by committee based upon criteria some of which have no physical basis, but are related (ultimately) to convenience, politics, and economics (which is, after all, what metrology is all about) as well as reproducibility and accuracy.

It would be preferable (IMO), to define the speed of light in this article on a physics basis (for example, as the maximum rate of information transfer) and leave its precise value as something that can be measured, whether or not BIPM defines the value. In any case, its value should not be predicated on metrology issues. (An analogy might be the article on matter, where the BIPM definition of the mole is only one way to go.) Perhaps (I'm pretty tentative on this) the entire connection to electromagnetism should be left as a matter for experiment in the sense that (so far) light appears to be the fastest form of information transfer, but the connection is ultimately a matter for experiment. The BIPM defined value is then in the nature of a modern consensus, and not to be confused with some reality of nature. Brews ohare (talk) 18:56, 17 June 2009 (UTC)


 * Hi! The main purpose of the lead paragraphs is to introduce the topic - see Lead section.  In particular, it should avoid jargon and words not obvious to an intelligent but non-specialist reader.  I think all readers, within reason, would be familiar with vacuum, but free space is more of a concept for specialists.  In fact, I think that anyone who even might distinguish free space from vacuum probably does not need the intro to a Wikipedia article.  Also note that even the BIPM, sticklers for accuracy that they are, just says "the distance light travels in vacuum", without further specification, as do almost all elementary physics books.  So whether or not this is the best possible theory, it's probably the right level of abstraction for a Wikipedia introduction.  Other's views on this are of course welcome.  LouScheffer (talk) 20:05, 17 June 2009 (UTC)

A major obstacle to understanding "the" speed of light is that this term may well be taken differently than is meant due to two confusing aspects: (i) the "vacuum" is not so clear cut as normally might be assumed, corresponding not to the "absence of everything" but rather to at least three or four possible meanings and (ii) the "defined" speed of light is at the least a confusing idea (as this talk page testifies) and leads to the idea that one can "measure" a "defined value" and that "vacuum" is a measurable and attainable medium. In the interest of avoiding these quagmires, a bit more careful introduction than you would like might prove to be very useful. Brews ohare (talk) 21:47, 17 June 2009 (UTC)
 * One way forward might be to treat c0 as a limiting value: it's the fastest that information can travel, because it's the fastest that light can travel (and nothing else can travel faster). I agree that c0 isn't defined as 299792458 m/s: that's getting the definitions the wrong way round! Physchim62 (talk) 00:51, 18 June 2009 (UTC)
 * Actually, c0 is defined by NIST and BIPM as 299792458 m/s, and is said to apply in "vacuum" (whatever that may be). I take it that "vacuum" is where the speed of light is c0; that is, if you measure c and c=c0, that is a necessary requirement for your sample to be a "vacuum". There are also the requirements of no dispersion, no dichroism, complete linearity and perfect isotropy. Oh yes, and c must have been c0 forever in the past and forever in the future (that's a bit tougher to substantiate). Brews ohare (talk) 01:16, 18 June 2009 (UTC)

No, the BIPM defines the metre in terms of the speed of light in a vacuum, not the speed of light in a vacuum in terms of the metre: that's what I meant by getting the definitions the wrong way round. The BIPM "vacuum" is self-evidently a practical vacuum, not a theoretical concept, as the SI is a system of practical units. Physchim62 (talk) 01:34, 18 June 2009 (UTC)
 * Right, the meter is found at metre as the distance traveled by light in "vacuum" in 1 / 299 792 458 s. However, that results in the speed of light being 299 792 458 m/s exactly, as the NIST link says. (It's a tautology.) That is, there is no measurement uncertainty. No measurement uncertainty is possible only if there is no measurement; that is, "vacuum" is a medium that has this property, this c-value. In short, the definition defines "vacuum" by implication as the medium referred to if measurements are corrected using the "standard best practices" corrections. As such the defined value of c0 never changes, but what we call "vacuum" will change if more or different corrections are added to the list.


 * Please notice that the definition does not mean that there is no error in measuring any actual physical length, and so we cannot verify a meter was traveled in 1 / 299 792 458 s with zero error. However, this error is not a reflection on the exactitude of the definition itself, only upon its implementation. So for example, our atomic clock has a line width, and we can't know exactly and reproducibly 1 / 299 792 458 s elapsed, and we cannot count fringes to an exact fractional fringe, so we don't know exactly and reproducibly the number of fringes between the arrival and origination points of the light. We also don't know if we have a "vacuum", but we trust the "standard best practices" to fix that. The wavelength of the light is an estimate, its speed is not. Brews ohare (talk) 01:46, 18 June 2009 (UTC)

Imagine you're measuring the wavelength of light of a known frequency (which if how these things are really done). You measure the wavelength at atmospheric pressure, then you pump on your apparatus to take the gas out, taking readings as you go. You will find that the wavelength increases, corresponding to an increase in the speed of light in your medium of study. You wont be able to get to zero pressure, but you can extrapolate your results to deduce what the wavelength would have been a zero pressure using standard statistical procedures: hence you have your practical measure of the speed of light in a vacuum. Under the current definition of the metre, what you're really interested in is the wavelength, which you can then use for optical interference measurements of other objects, but if you have another standard for the unit of length, you will have measured the speed of light in a vacuum for that system of units. Physchim62 (talk) 02:16, 18 June 2009 (UTC)

Then comes the point I think you were making at the start of this section: is this measured speed of light in a vacuum the same as the theoretical speed of light in free space? This is a little bit like the question of inertial mass vs. gravitation mass: we don't know that they're the same, but every experiment we do indicates that they're the same. Physchim62 (talk) 02:31, 18 June 2009 (UTC)


 * Note that the standard does not define the speed of light, and for good reason. Suppose the speed of light is defined to be X, and you measure and get X'.  If only the speed of light is defined, you can fix this by changing the second, the meter, or both, until you get the correct speed.  But what the measurement bureaus want is just what they said - the meter is the one that is modified to make this true.  LouScheffer (talk) 02:45, 18 June 2009 (UTC)

Indeed. Let's imagine that there was a superbly precise test of Special Relativity that indicated that the speed of light as it appears in relativistic equations is very slightly higher than the measured speed of light in a vacuum. The definition of the metre in terms of the speed of light in a vacuum probably wouldn't change (because that's what we measure practically to use as a length standard), but the speed of light in free space (or at least the tiny correction factor) would become a measured physical constant. Physchim62 (talk) 02:58, 18 June 2009 (UTC)

Speed of light and the defintion of the metre
The statement in the article: "'In modern times, the speed of light can be measured extremely precisely, to the point where the metre is now defined in terms of a known time interval.'" does not appear to be consistent with the view of BIPM and NIST that the speed of light has a definite numerical value and not a measured value. (The speed of light is 299 792 458 m/s exactly according to NIST and the BIPM. The measurement uncertainty is zero.)

Thus, this portion of the introduction simply ducks the question: how can the value of the speed of light have a definite value, one that cannot be changed by measurement? Instead, the text suggests incorrectly (albeit indirectly) that 299 792 458 m/s is a measured value when it is not.

One approach to fix this issue is to say "The metre is defined by NIST & BIPM as 'The meter is the length of the path traveled by light in vacuum during a time interval of 1/299 792 458 of a second.'" The correct follow-on statement is: "According to NIST: 'Note that the effect of this definition is to fix the speed of light in vacuum at exactly 299 792 458 m/s.'"

This definition solves the problem about the speed of light, because its value is now a tautological result of the definition of the meter.

This definition of the metre is satisfactory only supposing that the speed of light in "vacuum" is truly a universal constant. If it isn't, then reproducibility of the meter is a flop. As part of this assumption we need to know what "vacuum" is. We know what it is not: it is not the absence of everything, it is not quantum vacuum, it is not ultra-high vacuum, it is not interstellar space. Rather it is a list of corrections to be made to any measurement in any real medium; corrections specific to the chosen medium that will bring the measurement back to the "vacuum". Specifically addressing the "vacuum" the CIPM cautions that:CIPM adopted Recommendation 1 (CI-1983) Appendix 1, p. 77 “provided that the given specifications and accepted good practice are followed; • that in all cases any necessary corrections be applied to take account of actual conditions such as diffraction, gravitation or imperfection in the vacuum; … ”

Less specifically, in regard to the second, the idea of "best practice" is again raised: BIPM suggests in the case of the second: "'It should also be noted that to achieve the uncertainties given here it is not sufficient just to meet the specifications for the listed parameters. In addition, it is necessary to follow the best good practice concerning methods of stabilization as described in numerous scientific and technical publications.'"

In sum, the statement in the article identified at the beginning of this comment is insufficient. Some of the ground that needs to be covered has been presented elsewhere in the article, but it all has to be drawn into a unified stance. Brews ohare (talk) 11:53, 18 June 2009 (UTC)

Time to reform this article.
From once being a Featured Article this article has now become a rambling essay on different types of vacuum, with much of the important conceptual physical significance of the speed of light being removed.

This was once a good physics article and it is time to get it back that way. Any offers of help? Martin Hogbin (talk) 17:31, 10 July 2009 (UTC)


 * I agree. I do not wish to be uncivil but the reason for this can be summed up simply: Brews Ohare.


 * The same thing is happening all over the physics articles. A single disruptive editor attempts to clarify every single point in an article (even the ones that do not need clarifying) with no real understanding of the subject matter.  As a result the article loses accuracy and focus and drifts well away from the nominal subject.


 * Many editors have complained of exactly the same thing but it quickly becomes tiresome when faced with a barrage of ill-considered rapid-fire edits. I recall particular my previous encounter with him above.  Despite explaining the situation in terms even a child should understand, he completely missed the point.  He then attempted to start exactly the same discussion a few days later completely ignoring what had already been said.


 * If you feel this article needs pruning personally I will support you and I suspect many other editors will, but it is not an undertaking to be accepted lightly. CrispMuncher (talk) 18:15, 10 July 2009 (UTC)


 * I suggest we leave the lead section until last as this is meant to be a summary of the article. I have made some changes.  Let us see what others think. Martin Hogbin (talk) 20:09, 10 July 2009 (UTC)


 * I fully support reforming the article, but suggest re-writing the lead section first. I think it's easier to re-write the rest when the outline of ideas to be covered is already in place.  Thanks for the effort,   LouScheffer (talk) 10:30, 12 July 2009 (UTC)


 * If we work that way, I suggest that it would be best to prepare a new lead section elsewhere rather than mess up the version here. I have suggested something here, based on the current version and an earlier one.  See what you think. Martin Hogbin (talk) 20:50, 12 July 2009 (UTC)

Where to start?
I think that the article main text should start with some kind of description of what the 'speed of light' is. The lead currently says that the 'speed of light' is the actual speed of light in free space. This is fine but we also need to say something along the lines of, 'according to currently accepted and verified theories of physics, the speed of light is believed to be a fundamental physical constant'.

But first, some questions. Do we need to say 'believed to be' and 'according to currently accepted and verified theories of physics'? Is this not normally understood to be the case? Should we just say, as the old lead use to, that the speed of light is a fundamental physical constant. The only problem with the simple statement is that, according to some respectable but speculative theories, the actual speed at which light travels may not be exactly the same as the fundamental constant linking the units of space and time.

Can anyone suggest any good sources on this somewhat philosophical subject. I think it is very important to get this question sorted at the start to avoid the confusion that we currently have in the article between currently accepted theory, experiments designed to verify that theory, and valid theoretical speculation. Martin Hogbin (talk) 10:48, 11 July 2009 (UTC)


 * If anything, might I suggest Scharnhorst effect ?

69.140.12.180 (talk) 19:03, 11 July 2009 (UTC)Nightvid


 * I am not sure how that answers the question that I asked. Martin Hogbin (talk) 01:04, 12 July 2009 (UTC)


 * As no one else has, I have answered my own question. In common with other articles we should just use the wording 'is' rather than 'believed to be' when referring to current, established and verified theories of physics. Martin Hogbin (talk) 16:20, 13 July 2009 (UTC)


 * Seems reasonable. Tons of super-reliable sources would support the claim that c is a "physical constant". --Steve (talk) 18:56, 13 July 2009 (UTC)


 * Not sure what you mean here Steve. Martin Hogbin (talk) 21:12, 13 July 2009 (UTC)
 * Do you want a link such as this []? and would you like to add that it is a dimensionful constant? Martin Hogbin (talk) 21:27, 13 July 2009 (UTC)


 * I just meant, "is a physical constant" is a fine and justified thing to say. No need to be ultra-cautious by saying "is believed to be a physical constant". I suppose a reference like that link is good, since the statement is evidently controversial.


 * I don't think the word "dimensionful" is important or even helpful. The average reader would already think of c as having dimensions...stating something that's so obvious would only be confusing. --Steve (talk) 22:07, 13 July 2009 (UTC)


 * Fine, I only suggested 'dimensionful' because dimensionless constants are considered more fundamental. Martin Hogbin (talk) 22:29, 13 July 2009 (UTC)

Delbrück scattering
The article says "Quantum electrodynamic theory predicts deviations from a unitary refractive index in the vacuum state for extremely strong electromagnetic fields.[15] To date, there has been no experimental confirmation of that effect." However, this is not quite true, as Delbrück scattering (which is the scattering of gamma rays by the electric field of an atomic nucleus) is experimental evidence that the speed of light is altered (locally). Shouldn't we mention that? 69.140.12.180 (talk) 18:59, 11 July 2009 (UTC)Nightvid

US or Brit spelling?
I do not care much which we decide on but we should choose one and stick to it. We currently have both 'color' and 'colour' and 'travelling' has just been changed to 'traveling'. Martin Hogbin (talk) 18:54, 12 July 2009 (UTC)

Fundamental importance in physics
I have attempted to collect all the information about the fundamental importance of the speed of light in physics into one section. The article in its current state seemed to miss much of this information. The material has mainly come from the current article and earlier versions. Martin Hogbin (talk) 11:54, 13 July 2009 (UTC)

My recent changes
I have recently made quite a few changes to the article and no one has reverted any of them or commented here. Do I take that as tacit approval or is it just that no one cares or is watching this page? Martin Hogbin (talk) 21:48, 14 July 2009 (UTC)


 * Well Martin, as is your wont, you are confusing defined behavior in unrealizable free space with actual observation on real media. You are proposing that definition is confirmable by experimental observation, while all that can be done is to confirm models approximate reality. Please get these differences straight in the article. Brews ohare (talk) 11:20, 15 July 2009 (UTC)


 * This distinction is made quite clear in the article, but only once. Martin Hogbin (talk) 11:34, 15 July 2009 (UTC)


 * Actually, it is still mentioned twice; that is plenty. Please do not keep adding references to this distinction throughout the article.  I agree that it is an important point that needs to be made, but it is made.  To many people it is a detail that does not need to be mentioned at every opportunity. Martin Hogbin (talk) 11:41, 15 July 2009 (UTC)


 * A statement of principle does not compensate for misstatements that contradict the principle elsewhere in the article. Brews ohare (talk) 12:11, 15 July 2009 (UTC)


 * I have changed to wording to use 'verify'. This is standard terminology.  As I am sure you know, an experiment can never prove a theory or model.  The model in question predicts that the velocity of EM radiation in (the hypothetical) free space does not depend on frequency.  The actual experiment, which took place in (the real medium of) interstellar space does not produce evidence contrary to the model.  The model, or theory, is therefore said to be verified by the experiment. Martin Hogbin (talk) 12:17, 15 July 2009 (UTC)

Martin: you cannot "verify" or "confirm" a definition. You statement is " According to classical electromagnetism, the speed of electromagnetic radiation in free space is the same for all frequencies. This has been verified to a high degree of accuracy by experiment." Apparently you are unaware that "free space" is not a real medium and is not measurable. Therefore, one cannot "verify" its properties: they are given. One can measure outer space however, and establish to within some accuracy that it behaves like free space. Hence the difference in meaning. The test is the comparison between the model and reality, which establishes that reality behaves like the model. One does not test to see if free space is dispersionless (it is, by definition), but whether outer space is dispersionless. Brews ohare (talk) 12:20, 15 July 2009 (UTC)


 * Brews. This is how physics works.  You develop a theory, in this case classical electromagnetism, which predicts no dispersion in (hypothetical) free space.  You then do experiments to test the theory.  In this case an experiment in (the real medium of) interstellar space.  You then ask yourself if the results of the experiment are contrary to the predictions of your theory.  If this is not the case the experiment is said to verify the theory.  I can explain in more detail if you like.


 * Please also note the actual title wording of two of the quoted sources: 'Severe limits on variations of the speed of light with frequency' and 'Probing the Speed of Light with Radio Waves at Extremely Low Frequencies'. They both just say 'speed of light'. Martin Hogbin (talk) 12:31, 15 July 2009 (UTC)

Martin: Here's the statement: "According to classical electromagnetism, the speed of electromagnetic radiation in free space is the same for all frequencies. This has been verified to a high degree of accuracy by experiment." I understand this to mean that experiment has verified that a property of free space is that the speed of light is the same for all frequencies. That is simply not correct, neither in fact nor in principle. This is not a question about experimental method. The simple fact is that the statement that free space is dispersionless is true independent of any experimental test whatsoever. Do you understand that? Brews ohare (talk) 12:40, 15 July 2009 (UTC)

As for the sources saying "speed of light", that has nothing to do with the question of whether they measured "free space". Of course, they did not. Brews ohare (talk) 12:42, 15 July 2009 (UTC)


 * Experimental results are always the product of lots of assumptions. An assumption here is that real vacuums behave like the hypothetical free space *to the accuracy needed by the experiment*.  The general scientific concensus is that this assumption is true, and the exceptions where it may be expected to fail (super high fields, tiny scales, etc) are well known.  Of course it is philosphically possible that the speed of light *does* vary with frequency, but the dispersion of 'real vacuum' cancels it out in these specific measurements, but in general the simplest (and only accepted) explanation is that *both* the speed of light is constant with frequency, *and* real vacuums behave like free space to the accuracy of the measurement.  So in practice the experiments verify *both* assumptions. Since the article is about the speed of light, saying the speed of light is verified seems reasonable.  At the very most, a footnote saying something like "Technically, this also assumes interstellar vacuum can be modelled to sufficient accuracy as free space, but this is widely accepted." could be added.  LouScheffer (talk) 12:53, 15 July 2009 (UTC)

There is no "true" or false consensus. There is agreement that "such and such" media have the same properties as "free space" to within such and such accuracy. Attributing "measured" properties to the idealized model of free space is simply a logical error or a misuse of language. What one does is confirm to within some accuracy that the model approximates the behavior of a realizable system. One does not verify the model properties in the abstract, but that the model properties apply to some real system to some accuracy. And you must specify which system you claim they apply to. Brews ohare (talk) 13:12, 15 July 2009 (UTC)


 * Brews, you seem to have a unique view on this subject that I am happy to discuss with you but, in the meantime, please leave the article alone. Martin Hogbin (talk) 13:30, 15 July 2009 (UTC)


 * It is the classical theory of electromagnetism (and maybe other theories) that is being put to the test by these experiments. Theory predicts no dispersion in free space. It is not space itself that is being tested but our theoretical description of it.Martin Hogbin (talk) 13:30, 15 July 2009 (UTC)

Martin: "Theory" does not predict anything about free space. It is a hypothetical medium with permittivity εo and permeability μo. With these postulated values, of course it has no dispersion. Where is the "theory" for this medium? There isn't any: not in say, continuum mechanics, nor quantum field theory. It's an unrealizable, ideal medium, untouchable and unreachable by experiment.

Measuring some medium like outer space does not "test" electromagnetic theory. What it does is determine whether the measured medium has ε ≈ εo and μ ≈ μo. If it does not, if, say, the discrepancy in outer space is in accord with quantum gravity, then we have some fundamental changes to make, like failure of causality at Planck lengths. Free space and its properties do not come into it; it is the measured properties of outer space that matter. Free space is just a reference point - we could use any other standard to compare to.

What is this "space itself" that you believe in? It isn't quantum vacuum. It isn't QCD vacuum. What is it? Brews ohare (talk) 14:56, 15 July 2009 (UTC)

Why aren't the imperial units listed as exact values ?
miles per second = 186282.39705122

miles per hour = 670616629.384395

Computations: 1/.0254/12/5280*299792458

1/.0254/12/5280*299792458*3600

Seriously this is a scientific article, values should be exact not approximated. —Preceding unsigned comment added by 69.232.221.200 (talk • contribs) 19:08, 14 July 2009


 * 1. Those aren't exact values. It's a repeating decimal, for example 186282.3970512208701185079137835043346854370476417720512208701185079137835043346854370476417720512209...


 * 2. Who would ever want to know the speed of light in miles per hour to more than 0.1% accuracy? No one. Almost no one ever wants better than 0.1% accuracy for anything. Those who do are professional scientists who are capable of calculating it themselves, and moreover wouldn't do so in imperial units.


 * 3. It clutters and distracts. If I look for 0.1 seconds at the number 186282.39705122, it's a meaningless mess of digits. If I look for 0.1 seconds at the number 186,000, I can understand it immediately. Plus, the extra digits add to the width of the whole table. --Steve (talk) 00:36, 15 July 2009 (UTC)

"Measuring" free space
From the article:"'According to classical electromagnetism, the speed of electromagnetic radiation in free space is the same for all frequencies. This has been verified to a high degree of accuracy by experiment.'" The statement that free space is dispersion free is a defined property of the unrealizable, ideal medium of free space. To claim that this defined property is experimentally verified is a logical error, as one cannot measure a defined property: its value is what the definition says it is. All that experiment can do is confirm whether some realizable medium, like outer space say, has this property. Such confirmation serves to support the notion that "free space" is a useful model, but it cannot change the model. It can only support its utility.

These statements in the article should be replaced with something like:"'According to classical electromagnetism, the speed of electromagnetic radiation in free space is the same for all frequencies. This behavior has been verified by experiment to a high degree of accuracy for media such as outer space or ultra-high vacuum, showing that in this respect these media are good approximations to free space.'"

The present statement is logically ridiculous. Brews ohare (talk) 13:02, 15 July 2009 (UTC)


 * J.D. Jackson (Classical Electrodynamics) says the definition "c=299792458 m/s, exactly...assume[s] that the speed of light is a universal constant, consistent with evidence". According to Brews, c=299792458 m/s exactly is a defined property of free space and therefore does not require any evidence or assumptions. So Jackson supports what's in the article, and not what Brews believes.


 * Brews, I've said it many times: It must be possible to measure free space, or else there is no way to measure any length in metres. --Steve (talk) 15:34, 15 July 2009 (UTC)

Steve: The BIPM and NIST have defined the meter so that it is not possible for the speed of light to differ from c=299792458 m/s, exactly. The meter just changes to make it so. The citations are listed in the intro. As evidence, aside from logic, and aside from the word by word quoting of NIST and BIPM, one cannot have an experimentally determined quantity that is exact. There is always an error involved.

To measure some length in meters, one shoots a photon down the path, and determines how many seconds it took from point a to point b. Then one multiplies that time by 299792458 m/s and that is the length in meters between a and b. It is cut and dried. You would like to say that there is more to it, but there isn't.

You can ask some fundamental questions, like how do we know that the speed of light hasn't changed since the last time we did this? That could be answered by repeating the measurement. One might ask whether we got the number wrong for c? The answer is: that is a dumb question, we've made up our minds. We could have picked 100,000,000 m/s to make the math easier, but we didn't because there are so many bolts and nuts out there with standardized sizes it just would cost too much in dollars and in nuisance. We could ask if it wouldn't be more fundamental to say the meter is x wavelengths of some atomic transition? The answer is, been there done that. The error analysis is better at present doing it this way, but that might change if some other measurement becomes more accurate.

More basically, one might ask if the speed of light is "really" the limiting speed in relativity? Or does the limiting speed of light apply at a Planck length? Those are interesting questions. They don't depend upon free space, or what definition is taken for the numerical value of c. Brews ohare (talk) 16:02, 15 July 2009 (UTC)


 * You say: "To measure some length in meters, one shoots a photon down the path, and determines how many seconds it took from point a to point b. Then one multiplies that time by 299792458 m/s." OK, well what medium is that photon traveling through during this measurement? Air? High-vacuum? --Steve (talk) 16:55, 15 July 2009 (UTC)


 * Good point. The answer is any medium, say air to be definite, but you must employ the BIPM accepted list of corrections and follow corrections recommended in the technical literature. Do these corrections impact the defined properties of free space? No. Do these corrections impact the actual measurement? Certainly. From a practical standpoint, one simply applies the corrections by adopting the tabulated refractive index for air prepared as you have done it.


 * How are the corrections decided upon? Well, of course, that is a very technical matter, and involves metrology, not just physics. (For instance, an evaluation of whether such and such microwave set-up is well understood, compared to another one.) But in principle one has a theory of the medium, e.g. air, and its ε & μ and this theory is shaken down by making measurements of functional dependence upon partial pressure, field strengths, polarization etc. Then a judgment call is made based upon those dependencies how to extrapolate to free space. Does that affect the definition of free space? No. The refractive index of free space is 1. Does it affect the determination of length? Yes. The speed of light is reduced if the refractive index > 1.


 * If the corrections for air are incorrect, I believe the view is that this error will show up as poor reproducibility of the measurement. So, for example, if the wavelength of a particular transition is measured using different media and the corrections (peculiar to each medium) do not lead to the same number for the wavelength, then some correction is inaccurate. Does that mean free space will change? No. Does that mean the theory of ε & μ will change? You bet. For example, if measurements are made in quantum vacuum using two polarizations and different results are obtained, does that mean free space is dichroic? No. It means the quantum vacuum is dichroic.  Brews ohare (talk) 17:52, 15 July 2009 (UTC)


 * OK. So I measure the time for a photon to travel from a to b in high-vacuum, then correct that time to take into account gas molecules, gravitons, etc. according to best practices in the technical literature. The result is "t". Then, I can say for sure that a and b are "299792458 m/s × t" apart. Do you agree? --Steve (talk) 18:48, 15 July 2009 (UTC)

No, Steve. I do not correct the time. I correct the refractive index. Then ℓ = ct/n. Brews ohare (talk) 20:20, 15 July 2009 (UTC)


 * Great, let's be perfectly clear. I measure the time for a photon to travel from a to b in high-vacuum, then figure out what n is by taking into account gas molecules, gravitons, etc. according to best practices in the technical literature. Then, I can say for sure that a and b are "299792458 m/s × t/n" apart. Correct? --Steve (talk) 22:47, 15 July 2009 (UTC)
 * I gather you are setting me up, but yes, I think that's it unless I forgot something. Of course, the determination of n has some experimental error, as does the determination of the time. Brews ohare (talk) 01:00, 16 July 2009 (UTC)


 * OK, now I do this measurement procedure to find the distance from a to b, using a red photon and the red index of refraction, ℓred = 299792458 m/s × tred/nred. Then I repeat this procedure using a blue photon and the blue index of refraction, ℓblue = 299792458 m/s × tblue/nblue. I compare ℓred to ℓblue. Are they the same or different (within measurement error)? If they're different, then free space has a dispersion. This procedure measures experimentally the dispersion of free space. If you disagree, please tell me where I went wrong. --Steve (talk) 04:22, 16 July 2009 (UTC)


 * If they are different, your measured "high vacuum" in fact has an index that varies with wavelength. Free space has an index of 1 by definition at all frequencies. Therefore, the corrections of "good practice" must be changed. Your theory behind the corrections is wrong, or you have not fitted the extrapolation curve correctly, or you have a source of measurement error. Notice that free space really does not enter into the discrepancy, because you are really comparing your blue and red measurements, its an red-FS cf blue-FS comparison, FS doesn't matter. Brews ohare (talk) 04:52, 16 July 2009 (UTC)


 * Of course my high vacuum has an index that varies with wavelength. I already figured out nred and nblue separately, by reading technical literature and following best practices. I never assumed they were equal; they're not. There is no measurement error or theory error, I'm very systematic. This is an experimental measurement. You have no right to say how this experiment will come out. They might be the same and they might not.


 * What does this have to do with free space? Whenever the number 299792458 m/s comes up, free space must be involved.


 * I'm trying to show you that your understanding of free space is not self-consistent. You believe in an "operational" definition which is the true basis for practical metrology ("Follow best practices"), and you simultaneously believe in an a priori definition which cannot ever be used in the real world ("Free space is the medium with c0,μ0,ε0"). These two definitions contradict each other, and I'm trying to walk you through one example of that. If free space can be the basis for measuring a real-world length (and it obviously is), then free space is basically an experimentally accessible thing, and then it must be possible to experimentally measure whether or not it has dispersion, as described above. --Steve (talk) 07:16, 16 July 2009 (UTC)
 * Steve: I see no contradiction here. The best practices simply establish a method to relate measurements on real media to a common baseline. The baseline can be fictitious. The theory of any real medium for ε, say, explains how various contributions to ε affect its value. For instance, theory predicts the dispersion of ε. One could take some real medium and specify that its ε at some ω was a particular value. Then you could relate all other ε-values to this standard value. Any evolution of physical theory would focus upon relating a measured ε to the standard. It introduces nothing to this work to say now that the standard value has been changed to some other material at some other frequency. It introduces nothing to this work to say we will adopt a fictitious ε = εo that is defined by taking all fields to zero amplitude, all partial pressures to zero, etc. etc. The exact same theory already developed is used and the appropriate limits are found. This "limiting" medium is of course, not realizable. Brews ohare (talk) 15:07, 16 July 2009 (UTC)
 * Free space is not "the basis for measuring a real world length". You can take the length as time of flight divided by n and multiplied by any constant value whatsoever. All that does is change the length you call a meter (and screw up existing nuts and bolts which now have 40 decimal-point diameters and pitches). So now free space has a different c, so what? Does the value of c tell us anything about nature? No. Does the lack of dispersion tell us anything about nature? No. We can measure any real medium and find its dispersion. That tells us about nature. Brews ohare (talk) 15:07, 16 July 2009 (UTC)
 * Brews' assertion that all EM radiation is defined to travel at the same speed in free space is absurd. You cannot define physical quantities in this way.  The only arbiter in physics is experiment, and it could be that a future experiment will show different frequencies of light to travel at different speed in free space (although this is presently not the case).Martin Hogbin (talk) 08:37, 16 July 2009 (UTC)
 * Martin: experiment may show that dispersion exists in any real medium, like outer space, and has already done so for less exotic media. However, you can't measure free space. No matter what experiment tells us about outer space, or QCD vacuum, or iron filings, free space has | c0 independent of frequency because that is what free space is. If you measure something different, you are not measuring a medium well described by free space. That's all. Brews ohare (talk) 15:07, 16 July 2009 (UTC)


 * On the other hand it is possible to define light to travel at a fixed speed because, as new experimental evidence is obtained, it merely refines the length of the metre. In the event that the velocity of of EM radiation were found to vary with frequency it would be necessary to specify a frequency of light to define the metre. Martin Hogbin (talk) 08:37, 16 July 2009 (UTC)
 * Martin: New experimental evidence cannot be obtained about free space. New evidence can be found for outer space, or QCD vacuum, or any real medium. The point is you have to prepare a real medium to measure the speed of light. Speed of light in that medium is any value that you measure. The theory of that medium has to predict values that agree with your measurement for it to be valid. But you cannot prepare free space to do your measurements in. You can prepare only some realizable medium. Please answer these three questions: how you expect to measure free space? How do you prepare a sample of free space? How do you establish that it is free space? My answer to these questions is that it cannot be done. Brews ohare (talk) 15:07, 16 July 2009 (UTC)


 * Martin: A major arbiter of a theory is experiment (although some allow some other criteria, like simplicity or other aesthetic issues to decide between compatible theories). However, experiment is not involved here: it is definition that is involved. You just haven't come to grips with the posting of | c0 and this remark: "The effect of this definition [of the meter] is to fix the speed of light in vacuum at exactly 299 792 458 m/s." There is no room for dispersion here. This is not about measurement. This is about definition. The experimental finding of dispersion in any real medium would impose a correction to the meter as measured in that medium. It would have no effect upon the defined zero dispersion of free space, because your measured medium was not free space. Brews ohare (talk) 15:07, 16 July 2009 (UTC)

Brews, I didn't expect you to see the condradiction between your two definitions of free space. That's why I walked you through it, step by step. I described above the exact experimental procedure that will test whether or not free space has dispersion. How do you respond to it? --Steve (talk) 15:52, 16 July 2009 (UTC)

Reversion of edit by LouScheffer
I reverted your edit because it seemed to be an explanation of the situation for the benefit of Brews. I think it is a good idea to explain the situation to him but not in the article. To most people it is quite simple. Also the words that I have used are the same as the reliable sources that we have quoted.

The article got in a terrible mess before because Brews wanted to put his comments about free space and actual vacua throughout the article. This is not necessary, we have it twice, that is enough.

I am happy to discuss this topic on the talk page but unless there is a consensus good reason not to, we should stick to the wording in the quoted sources. Martin Hogbin (talk) 19:14, 15 July 2009 (UTC)


 * Martin: I don't see much discussion here on your part. Just your assertion that you have the best idea of what is going on. LouScheffer made a sensible correction, and your position is contrary to logic. What is your argument? How do you measure a defined property?? You can say only the measured medium has approximately the same properties as the defined properties of the ideal reference system. You cannot verify the properties of free space; you can verify only that some real medium has approximately the same properties as free space. Measured values are not the same as defined values. A dollar is 100 cents, not 100±ε cents. Brews ohare (talk) 20:22, 15 July 2009 (UTC)


 * I added this back in. This appears to be the first place it's mentioned, and in a physics section the idea of a classical linear theory being the limit of a more comprehensive quantum theory is important.  Also, for this article in particular, it's important to point out that the speed of light is completely independent of energy and amplitude in classical EM, but may not be in QM.  This is not a fringe view - the very first sentence of the first reference states "Explosive astrophysical events at high red shift can be used to place severe limits on the fractional variation in the speed of light (∆c/c), the photon mass (mγ ), and the energy scale of quantum gravity (EQG )."  On the other hand, this is a good place to point out that the linear approximation is very good indeed, and then elsewhere in the article we can drop the distinction between hypothetical free space and a real vacuum, since the observations indicate they differ by at most a tiny amount.   LouScheffer (talk) 21:23, 15 July 2009 (UTC)


 * You are simply adding a new level of confusion to this subject. Your quote, 'Explosive astrophysical events at high red shift can be used to place severe limits on the fractional variation in the speed of light' is pretty well exactly what I said, which was, 'According to classical electromagnetism, the speed of electromagnetic radiation in free space is the same for all frequencies. This has been verified to a high degree of accuracy by experiment'. The point simply is that theory has been verified by experiment. Martin Hogbin (talk) 21:55, 15 July 2009 (UTC)

Martin, you do not understand the meaning of the sentences you have written. They do not say what Lou says at all. He says measurements confirm various properties of real media. You say measurements confirm properties of free space. Free space ≠ real media. Brews ohare (talk) 22:29, 15 July 2009 (UTC)


 * Brews is right to object about Martin's implication that experiment verifies the speed of light in free space, however Martin is right when he says the current text adds a new level of confusion. It may be more clear to dedicate one section to electromagnetic theory and one to experiment.  In addition, I have been unable to find a modern source that explicitly or implicitly states what 'free space' is with any clarity.  It seems the modern term (per BIPM in 1983) should be 'vacuum'. Pecos Joe (talk) 23:26, 15 July 2009 (UTC)


 * Pecos Joe: The properties of vacuum are specified by NIST as μ0 ε0, and | c0 all of which have exact values. Accordingly, "vacuum" has no dispersion. As the article Free space points out, "vacuum" refers to no known real medium. Brews ohare (talk) 00:35, 16 July 2009 (UTC)


 * But 'free space' is not exactly the same as 'vacuum', and that is why terminology changed. I couldn't see that any of the references of the free space article gave a clear definition of free space, and I think using the more precise term 'vacuum' will help resolve the argument. Pecos Joe (talk) 01:38, 16 July 2009 (UTC)

Both free space and "vacuum" have μ0 ε0, and | c0. I haven't traced the history carefully, but I believe you will find that permittivity of free space = permittivity of the vacuum of free space = electric constant = ε0 and similarly for the other two parameters. So there is no distinction here between "vacuum" and "free space". Brews ohare (talk) 04:46, 16 July 2009 (UTC)


 * Lou, I much prefer what you have written now. I have made a few small changes.  I do not think that saying EM is a linear theory adds anything for the benefit of the reader.  Your comment about QM needs a citation, also do you mean QM, QED or SM, we must get this right.


 * There are many attempts at a theory of quantum gravity, none with any success, so I have changed the wording to 'some theories'. Also I think it is very important to separate established and tested theories like relativity, QED, and QM, from research theories, which currently have no experimental basis. Martin Hogbin (talk) 15:50, 16 July 2009 (UTC)

Lou, I have removed your comment about relativity and dispersion because classical EM was relativistic from the start and relativity has nothing to say about dispersion.

I am not convinced about your comment about QM and dispersion. None of my QM books mention the speed of light in free space. What does your reference actually say? Martin Hogbin (talk) 21:44, 16 July 2009 (UTC)


 * The standard theories of the quantum vacuum and QCD vacuum are cited in those articles. They predict a number of departures from μ0 ε0, and | c0, most of which are still too small to measure given experimental accuracy. However, one cannot claim these theories have anything to say about free space itself. They have to do with quantum vacuum and QCD vacuum, which are real media in principle, although their realization is hard to verify experimentally. Brews ohare (talk) 00:14, 17 July 2009 (UTC)

Slowing light down (is light speed really constant?)
I think light can be slowed down (I'm not sure). Therefor, the speed of light is not constant in an absolute sense. Is this explicitly mentioned in the article? —Preceding unsigned comment added by 76.179.212.205 (talk) 01:15, 16 July 2009 (UTC)
 * There is a section in the article about this: Speed_of_light. Brews ohare (talk) 01:22, 16 July 2009 (UTC)
 * It is not as explicit as it should be, IMO, but the term speed of light is used for two separate quantities. The first is the speed that light travels in a given medium.  It is usually denoted by v and depends on the frequency of the light and the medium through which it travels.  Light waves have a complex structure with different 'parts' moving at different speeds so that a given wave is describable in terms of many different 'speeds of light' including group velocity, phase velocity, and front velocity, all of which can differ from the speed that the photons of that wave travel.  The second meaning of speed of light is as an important property of space time which is a constant, usually denoted by c, corresponding to a 'speed limit' for which information can travel. It is sometimes called the speed of light in a vacuum or the speed of light in free space to avoid confusion with the first.  I prefer to call it simply c because it has nothing to due with light other than the fact that all mass-less particles, including the photon, travel at c.  It is this constant for which you refer above.  It, c, truly is a constant (as far as we know); it is also independent of the speed that light travels.  The confusion between v and c arises because the speed of light, v, of a given volume of material approaches c as the volume approaches a vacuum when material is removed.  This is true for all frequencies, materials, and types of measurements (phase, group, and front). TStein (talk) 17:31, 16 July 2009 (UTC)
 * Nice discussion. I'd separate speed of information travel from "speed of light in vacuum" or "speed of light in free space", because the latter is a defined value characteristic of a hypothetical medium, not necessarily related to real properties of the universe, but definitely related to man's convenience in comparing measurements upon real media. The former, speed of information travel, is presumably a real property of the universe and, as you have pointed out, may or may not be the same thing as the speed of light propagation in any real media. Brews ohare (talk) 17:39, 16 July 2009 (UTC)
 * The distinction between free space and real media is made quite clear in the article already. TStein, have you read the article?  It makes quite clear that the term 'speed of light' is generally used for the speed in free space and that it is this value that is an important spacetime constant. Martin Hogbin (talk) 17:49, 16 July 2009 (UTC)
 * Brews: In my above discussion I did not say that 'speed of information travel' is the same as the 'speed of light in free space'; rather I said that c is the 'speed of information travel' and it is called the 'speed of light in free space'. Whether or not c is the 'speed of light in free space' it is still called that term quite commonly.
 * Martin:I have read the article. I agree, there is nothing new in my statement that is not covered in the article.  76.179.212.205 was confused by a common misconception after reading this article.  I tried to address his problem directly with a summary of the important point that will answer his question.  In doing so, it was my hope that if the user was helped by this summary that something similar can be added to the lead.  If not then no harm done.  I still believe that the fact that there are two different meanings for the term speed of light needs to be addressed 'explicitly' in the lede.  Read the five paragraphs in the lede from the perspective of someone who does not understand that they are two separate concepts, for instance.TStein (talk) 23:02, 16 July 2009 (UTC)


 * TStein, thanks for your reply. The lead currently contains, 'The term "speed of light" generally refers to the speed of light in free space' and 'The speed of light when it passes through a transparent or translucent material medium, like glass or air, is less than its speed in free space'. Would it be better to combine these two into, say,'The term speed of light generally refers to the speed of light in free space. When light passes through a transparent or translucent material medium, like glass or air, it travels more slowly than in free space.'


 * Should we say that, unless otherwise specified, the term 'speed of light' is used in this article exclusively to mean the speed in free space? I think this might be too cumbersome for the lead.  It is hard to cover every possible misconception about light in the body of the article, I do not think we can do this in the lead.


 * I might add some more to Cerenkov effect as this is an example where the distinction is clear as matter actually outruns light. Martin Hogbin (talk) 10:36, 17 July 2009 (UTC)

The article is clear, but mistaken. The important spacetime constant is not necessarily the defined number c0 used for the hypothetical medium free space, although it may be so. See the Request for Comment. Brews ohare (talk) 18:08, 16 July 2009 (UTC)


 * It is according to the currently accepted and verified theories of physics. According to some theories that have not yet been completed or verified it might not be. Martin Hogbin (talk) 20:36, 16 July 2009 (UTC)

Speed of light set by definition
Why do we need to be so long winded with 'According to NIST' twice

According to NIST "Note that the effect of this definition is to fix the speed of light in vacuum at exactly c = 299 792 458 m/s." According to NIST "A consequence of this definition [adopted by CGPM in 1983] is that the speed of light is now a defined constant, not to be measured again."

Surely this would be neater as one combined statement

The effect of this definition is to fix the speed of light in vacuum at exactly c = 299 792 458 m/s, not to be measured again.

With the same two references. Martin Hogbin (talk) 21:32, 16 July 2009 (UTC)


 * A direct quote demonstrates that this is not a flaky comment by some editor. That avoids future challenge. Editors don't read footnoted sources, or cannot find relevant sentences. Brews ohare (talk) 21:51, 16 July 2009 (UTC)


 * This is not necessary, neither is it the way Wikipedia works, otherwise it would consist almost entirely of quotes. Editors should write in their own words citing reliable sources. This is all that is needed to show it is not some 'flaky comment'.Martin Hogbin (talk) 09:11, 17 July 2009 (UTC)

So you say Martin. It's not my experience with you or Dicklyon or Fugal or Paolo.dL: an absolute verbatim quote is the only way to avoid endless debate. And as a reader, flaky paraphrases of sources are commonplace. Brews ohare (talk) 18:35, 17 July 2009 (UTC)

Brews's view of free space
This is primarily for Martin, but Brews you can correct me if I get anything wrong. Martin, you seem mystified by Brews's statements about free space, as was I at first, and you're both talking past each other. I think I have a pretty good idea now, so here goes:

First, what is Brews's free space?

(1) Free space = an imaginary universe where Maxwell's equations are exactly true.

Next, how does that relate to the real universe? Brews's view is that there is a unique and well-defined decomposition:

(2) True laws of electromagnetism = Maxwell's equations + "corrections"

The thing called "best practices" is finding the "corrections" and subtracting them off. Whatever you're left with (the "free space result") must be in accordance with Maxwell's equations -- otherwise your original decomposition-and-subtraction procedure was incorrect.

I don't think Brews is correct, but it's not totally and immediately ridiculous. I hope this helps. :-) --Steve (talk) 00:47, 17 July 2009 (UTC)


 * That's an extrapolation of my views I think. I see no need for an imaginary universe. I just solve QED to find ε & μ of my medium, say the quantum vacuum. By following "good practices" I can relate this result to free space, if I want. That comparison has no power to change my theory. The theory is compared directly to experiment, not to free space. However, if I want to measure a meter, I have to follow "good practices" and extrapolate my ε & μ to the limiting case of free space. Being a limiting case, it is not "real", it is not "realizable", it is not directly measurable. Realizability doesn't matter; one adopts the convention to follow "good practice" in the extrapolation so everybody can agree on the meter, like everybody on Earth agrees on how to convert their time to GMT. The practices are readily implemented, so I don't have to go visit a specially housed standard to set things up, as with the kg. Brews ohare (talk) 03:09, 17 July 2009 (UTC)

Firstly, I am happy with the form of words that we currently have in the article so, if you and Brews are also happy with this, we can leave this subject and get on with improving the article. I would like to try to get back its FA status. One thing which will not help with this and which I am determined to avoid is to have the same point being made countless times throughout the article. We all agree that free space is a hypothetical state not a real medium and that point is made in the article. I lost count of how many times I removed that same point from the article in my recent edits. It is a good point that needs to be made - once.

Thanks for your attempt to mediate. I can understand what you are saying but after reading Brews' reply I have no idea what he is getting at. The only thing that is clear is that the speed of EM radiation is not constant at all frequencies by definition as Brews claims. The speed of light in free space is constant by definition and we have a link to that definition in this article. Brews has consistently failed to show me a definition that the speed of EM radiation in free space is the same at all frequencies.Martin Hogbin (talk) 09:42, 17 July 2009 (UTC)


 * I do not understand why you think the statement by BIPM, NIST etc., that the speed of light is a specified exact constant "that never need be measured", does not in itself rule out dispersion in free space. If dispersion were important, wouldn't they specify c in some frequency range? Is BIPM "light" different from EM radiation? What is the distinction you are making here? Brews ohare (talk) 15:15, 17 July 2009 (UTC)

Brews, are you happy with what we have now? If so let us move on. I am quite happy to continue to discuss the subject with you but let us leave the article as it is. Martin Hogbin (talk) 09:42, 17 July 2009 (UTC)


 * It appears the logical confusion has been avoided, which satisfies me provided it does not recur later in the article. I don't agree with the statement: "Quantum mechanics maintains this prediction, holding that all photons travel at c, and adds that all massless particles must travel at c as well.[14]" The source is about the theory of relativity and the limiting speed of massless particles; it is not about quantum mechanics nor about dispersion.


 * In addition, the c that occurs here is the standard c of free space, and stems from using Maxwell's equations in free space (uses the defined co). Therefore, it cannot be construed as commenting upon the behavior of bodies in any real medium. That would require a much more complex analysis based upon QED.


 * This sentence should be removed. Brews ohare (talk) 14:29, 17 July 2009 (UTC)

I removed it. Brews ohare (talk) 18:01, 17 July 2009 (UTC)

Light is measured to travel at the same speed in all inertial frames
This statement is inaccurate. A light measurement in an anisotropic medium can be made in an inertial frame. Then its speed would not be isotropic and would not be c. This wording has to be changed.

The measurements in support of isotropy are based upon experiments like M-M, which are measurements in air or terrestrial vacuum, I don't remember. I'm unclear about whatever corrections may have been made for anisotropy; probably none. In any event, the proposal is that c is isotropic in free space, not in any real medium.

Perhaps a disclaimer could be placed at the top of the Spacetime section saying this discussion is restricted to free space? Brews ohare (talk) 16:37, 17 July 2009 (UTC)


 * I take the statement 'The term speed of light generally refers to the speed of light in free space' at the start of the article to be a disclaimer. Perhaps we should add, like we do with c, that unless specified otherwise we always use 'speed of light' with this meaning throughout the article.  Personally I think this is too cumbersome,especially for the opening sentence.  Martin Hogbin (talk) 00:33, 18 July 2009 (UTC)

Vacuum or free space
There seems to be a consensus forming that we should use the term 'vacuum' rather than 'free space' throughout this article to mean the hypothetical medium in which light travels at c. Does anyone disagree? Obviously, we will still define exactly what we mean by the term in the article. Martin Hogbin (talk) 14:40, 18 July 2009 (UTC)


 * As with most commonly used terms "vacuum" has many meanings. The "casual reader" is quite possibly left unaware of there being any distinctions involved here, while "free space" is a commonly used term in texts, and gives the reader a heads up that there is something a bit different here than "he felt an emotional vacuum", or "his death left a vacuum." Of course, if you really don't care much about the subject, anything works, but the standard here should be higher. "Classical vacuum" is another term now becoming more prevalent in the literature.  Brews ohare (talk) 15:13, 18 July 2009 (UTC)


 * Are you suggestion the we should use 'free space'?


 * I have a slight preference for 'vacuum' do not care that much. It is essential that we explain what we mean (as we now do) and preferable to be consistent. Martin Hogbin (talk) 16:50, 18 July 2009 (UTC)

Free space is more specific, widely used, has an article that explains it carefully: free space, and is consistent with usage in other WP articles. Brews ohare (talk) 17:03, 18 July 2009 (UTC)


 * I guess you will have to argue it out with Lou. It would be good to try and reach some kind of agreement.  Please not 'vacuum of free space' it is just too long. Martin Hogbin (talk) 17:18, 18 July 2009 (UTC)

Brews' latest edits
Despite attempting to discuss the relevant issues with him, Brews has decided to scatter the article with edits representing his own idiosyncratic view on the subject.

Other editors need to decide whether we want this article to be a potential FA representing the generally accepted views on the subject or Brews' personal soapbox in which to push his own opinions.

Please let me have your views on the matter. Martin Hogbin (talk) 02:55, 18 July 2009 (UTC)


 * Perhaps you could say specifically what you object to and why? Brews ohare (talk) 03:42, 18 July 2009 (UTC)


 * Let me give you one example. In the section in light as electromagnetic radiation you changed a simple statement that the speed of light can be calculated from Maxwell's equations to a much more complicated (in fact completely incomprehensible to non-technical reader) statement that essentially refers to light in anisotropic media.Martin Hogbin (talk) 10:21, 18 July 2009 (UTC)
 * The speed of light cannot be calculated from Maxwell's equations. These equations can take one of two forms. In empty space, they are homogeneous, and all currents and charges are zero. In this form the parameters εo μo appear. Their numerical values are not an essential part of the equations per se and cannot be deduced from Maxwell's equations. The second form has source currents and charges. Inasmuch as the initial statement in the text separates free space out for special attention, one might gather that the lead sentence refers to this inhomogeneous case. In this case, the wave equation cannot be derived without some form for the source terms, and such from is provided by the constitutive equations. I am sorry you don't understand this. Brews ohare (talk) 15:13, 18 July 2009 (UTC)
 * Is this true? I learned in school that the values of epsilon and mu were known from experiments.  When Maxwell formulated his equations, he realized they supported wave solutions, and calculated their speed of propagation of these solutions, determined by eps and mu.  He got a speed close to that of the measured speed of light.  Of course all of these numbers were obtained in air, so he got the speed of light in air, but it was close enough for him to realize light was probably EM radation.  Now not everything you learn in school is true, but if epsilon and mu were known, then the speed of light can be calculated from Maxwell's equations, just as we find the speed of oscillation in a rope under tension.   LouScheffer (talk) 15:36, 18 July 2009 (UTC)
 * In fact, looking at Maxwell's paper "A Dynamical Theory of the Electromagnetic Field", this is exactly what he did. He knew the value of mu (1 in his units), he refers the measured value of epsilon, then he finds what speed of waves his equation predicts, and compares it to the measured speed of light.  See page 499 of the paper, which is available on-line. at [[File:A Dynamical Theory of the Electromagnetic Field.pdf]].   LouScheffer (talk) 15:52, 18 July 2009 (UTC)
 * All of what you say Lou is entirely consistent with what I have said, and I agree with it. The issue is that the values are not calculated from Maxwell's equations,; they are input data. From where did the values come? Elsewhere. Brews ohare (talk) 17:13, 18 July 2009 (UTC)
 * The original statement was correct. Given values for the electric and magnetic constants (or the ratio between electrostatic and electromagnetic units) it is a simple matter to calculate the speed of light.  That is what Maxwell did when he first concluded that light was EM radiation.  It is true that the constants he used were probably based on measurements in air rather than free space and it is also true that today the speed of light is defined, so the equation is used in reverse. But these are minor complications that are covered elsewhere and do not to be added throughout the article.Martin Hogbin (talk) 10:21, 18 July 2009 (UTC)
 * As per my above remarks, the original statement is not correct. It does not say "given the values of εo  μo" it says the speed "can be calculated" from Maxwell's equations. If you include the caveat: "given the values of  εo  μo", you will then be forced to say where these values came from. The answer is that they are posited (not calculated) values for free space. Maxwell's contribution is not the calculation of a value for  c, but the recognition that c was connected to εo  μo. The numbers provided circumstantial support that the connection was there. Brews ohare (talk) 15:13, 18 July 2009 (UTC)


 * This is just the same old stuff again. You can can calculate the speed of light in any medium in which you can measure μ and ε, so the situation is exactly the same as measuring the speed of light (or delineating the meter).  You cannot actually make measurements in  the hypothetical free space but you can make measurements in a real vacuum and then correct for its imperfections.  This is an important point but it is one that we have already covered.  We do not need to include again it at every possible opportunity. Martin Hogbin (talk) 16:39, 18 July 2009 (UTC)
 * It's just a matter of logic, again. One does not calculate the speed of light from Maxwell's equations, whatever the medium . To say it is calculated implies somehow that the Maxwell equations allow a first-principles evaluation of c based upon some basic understanding of the mysteries of physics, not just taking a square root of some provided numbers. Brews ohare (talk) 17:13, 18 July 2009 (UTC)
 * The simplest resolution is to delete this sentence, especially as the following sentences say all that is necessary. I have done this. Brews ohare (talk) 17:23, 18 July 2009 (UTC)
 * The mention of anisotropic propagation of light in certain media is an interesting one that should be in the article, but the obvious place for this is in the 'Transparent media' section.Martin Hogbin (talk) 10:21, 18 July 2009 (UTC)
 * The use of constitutive equations may include anisotropy, dispersion, dichroism etc. but it also includes the simpler isotropic, nondispersive, nondichroic media as special cases. Brews ohare (talk) 15:13, 18 July 2009 (UTC)


 * What I propose to do is restore that section to something like its original wording (trying to make it as correct as possible whilst retaining a level of simplicity) and move your addition to a new section 'Anisotropic media', starting with a simple non-technical description of the topic. I hope you can live with this. Martin Hogbin (talk) 10:21, 18 July 2009 (UTC)


 * Also, you have attempted to summarize the content of two research papers on QED and the quantum vacuum.  Neither you nor I nor anyone else  that I am aware of who is active on this page at the moment is qualified to do this.  QED is a very complicated subject and in my opinion should only be summarized by real experts.  That is why I have restricted myself to very broad comments along the lines that QED provides a more complete description.  A cut-and-paste approach from research papers that you have come across in subject searches is not satisfactory. Anything that we add on thus subject should be based on clear statements from secondary sources. Martin Hogbin (talk)
 * It was you who brought up QED. Why? Is that dangling sentence that QED exists useful in isolation? No summary of papers by me is attempted. The quoted excerpt cannot be misconstrued: it just isn't ambiguous. The Rev Mod Phys is not a flaky journal, but very well respected in the physics community. Perhaps your view is that the vetted literature is too uncertain for use in WP? Any reader has the opportunity to read the Rev Mod Phys article on line and decide for themselves what they think about it, or consult the cited sources further if they are so inclined. There is no requirement in WP that no sources be used except high school text books.Brews ohare (talk) 15:13, 18 July 2009 (UTC)


 * Agreed that a distinction that is not important to NIST does not belong in a general purpose article. We should just use vacuum everywhere (more familar to most folks than free space) and point out that QM is a more complete theory in one place.   LouScheffer (talk) 12:06, 18 July 2009 (UTC)
 * Since when this topic is not important to NIST? Brews ohare (talk) 15:13, 18 July 2009 (UTC)

Firstly we need to decide whether to use the term 'vacuum' or 'free space'. Generally physicists seem to use 'vacuum' and engineers 'free space'. I slightly prefer 'vacuum' for the same reason that you do but I would accept either. If no one has a strong opinion I suggest that we change the term used throughout the article to be consistent. We have been through all permutations in the past of 'vacuum', 'free space' and 'vacuum of free space'. Let us try to pick one and stick to it.Martin Hogbin (talk) 12:35, 18 July 2009 (UTC)
 * Another choice that seems to work in the literature is classical vacuum to distinguish it from quantum vacuum. However, this issue came up a long time ago and "free space" was settled upon as making a clear distinction from other vacuums and as a term used in many texts. Brews ohare (talk) 15:13, 18 July 2009 (UTC)

Secondly the well established and verified quantum theory that deals with light is QED. As nobody here understands it properly, we should just stick to my original statement that it is a more complete theory, which I think you agree with. Martin Hogbin (talk) 12:35, 18 July 2009 (UTC)
 * That is your opinion. It isn't mine. How is QED more complete? What's missing? What is the bottom line here? I've provided answers to these questions that provide a glimmer of where things are going.Brews ohare (talk) 15:13, 18 July 2009 (UTC)


 * I strongly prefer vacuum in this case, since it's much more intuitive to a casual reader (which is most readers).  Free space has too many other meanings in everyday life, as in the forward scored easily since the defender gave him too much free space.   Also 'vacuum' is more inclusive, and can refer to free space, quantum vacuum, a practical (non-ideal) vacuum, etc.  This is an *advantage* in this context since *for the purposes of this article* the distinctions are largely un-important (no matter how important they may be on a philosophical level).  LouScheffer (talk) 13:05, 18 July 2009 (UTC)


 * As with most commonly used terms "vacuum" has many meanings. The "casual reader" is quite possibly left unaware of there being any distinctions involved here, while "free space" is a commonly used term in texts, and gives the reader a heads up that there is something a bit different here than "he felt an emotional vacuum", or "his death left a vacuum." Of course, if you really don't care much about the subject, anything works, but the standard here should be higher. Brews ohare (talk) 15:13, 18 July 2009 (UTC)


 * 'Vacuum' it is then, although I do not think that being inclusive is an advantage, but we should make clear exactly what we mean in the article. I will suggest here that we standardise on vacuum.  If no one objects I suggest we do it. Martin Hogbin (talk) 14:37, 18 July 2009 (UTC)

Inasmuch as the basic article here is free space, are you proposing to say: "vacuum" in this speed-of-light article means the same thing as free space in all other WP articles? Brews ohare (talk) 16:43, 18 July 2009 (UTC)


 * Just a point of clarification. The speed of light cannot be calculated using Maxwell's equations. Maxwell calculated the speed of light using Newton's equation for the speed of sound in a solid, along with input data from an experiment that was conducted in 1856 by Weber and Kohlrausch. The principle behind all this was that Newton's equation contains the ratio of transverse elasticity to density. Maxwell linked that ratio with the ratio of the dielectric constant to the density of his vortex sea. In 1856, Weber and Kohlrausch had experimentally determined this ratio, and so Maxwell was able to show that magnetic disturbances propagated at a speed which was equal to the speed of light, as experimentally determined by Fizeau.


 * The modern equivalent to Maxwell's approach above is simply to insert the measured values for the electric permittivity and the magnetic permeability into the equation c^2 = 1/εμ. This modern equation is a skeleton remnant of Newton's equation for the speed of sound as appeared at equation (132) in Maxwell's 1861 paper.


 * Maxwell's equations as such don't come into it. Maxwell's equations can be combined to produce the EM wave equation. But we can only know the speed of EM waves by knowing the values of ε and μ. David Tombe (talk) 01:04, 20 July 2009 (UTC)

Red shift
When frequency changes are under discussion, why is red shift not relevant? Brews ohare (talk) 20:51, 18 July 2009 (UTC)


 * I am trying to get some kind of logical structure to this article. The section in question refers to the constant speed of light in inertial frames, even the comment on Doppler shift is not particularly relevant.  I left it there because there is sometimes a misconception that light is completely unchanged by relative motion.


 * Cosmological red shift is a much more advanced topic which does not belong in this section. I was thinking of moving your words to a more appropriate section but I am not sure that there really is one, the article is about the speed of light.  Martin Hogbin (talk) 09:27, 19 July 2009 (UTC)

Light in transparent media
I think this section could do with some rewriting for the following reasons:

1) It should start with a simple statement that light goes slower in media, rather than the current technical description about permittivity etc.

2) The current explanation seems an unclear mixture of classical EM and QED.

Any comments? Martin Hogbin (talk) 10:56, 17 July 2009 (UTC)


 * I'd say a preliminary statement like you suggest is a good idea. I don't understand your second point. Classical EM uses currents and charges, but does not calculate them. Relating the currents and charges to the fields is the role of condensed matter physics and constitutive equations, which is outside of classical EM, could be classical (e.g. Boltzmann equation) and could involve QED or QCD, and also has a statistical physics aspect. Are you suggesting a digression to point out the theoretical origins of ε & μ, or are you suggesting that all reference to refractive index should be dropped? I think it important to keep the reference to refractive index.Brews ohare (talk) 14:39, 17 July 2009 (UTC)

The offending paragraph seems to be this.

"When light enters materials its energy is absorbed. In the case of transparent materials (dielectrics) this energy is quickly re-radiated. However, this absorption and re-radiation introduces a delay. As light propagates through dielectric material it undergoes continuous absorption and re-radiation. Therefore when the speed of light in a medium is said to be less than c, this should be read as the speed of energy propagation at the macroscopic level. At an atomic level, electromagnetic waves always travel at c in the empty space between atoms. Two factors influence this slowing; stronger absorption leading to shorter path length between each re-radiation cycle and longer delays. The slowing is therefore the product of these two factors. This reduction in speed is also responsible for bending of light at an interface between two materials with different refractive indices, a phenomenon known as refraction."

First the general concept is found in Hewitt (Conceptual Physical Science 11.2 p. 261 in 2nd edition), although there is no reference to where Hewitt derived his understanding. My reading is that he was trying to conceptualize the phase shift that occurs in the classical E&M model of the index of refraction that approximates atoms as driven-damped harmonic oscillators.

Second, Hewitt was careful not to say that this speed is the speed of energy propagation. The speed that seems to be described here is most likely the phase velocity. Energy typically does not travel at the phase velocity. (The phase velocity can be greater then c and if I recall correctly is often greater than c in the X-ray. The larger than c phase velocity allows total external reflection to focus X-rays.) I am not 100% sure what the best speed to say that energy travels. I think that in many cases it corresponds to the group velocity. Even the group velocity can be made greater than c though.


 * Surely if you are using a driven-damped harmonic oscillators model, there is no change in speed of the incoming wave - phase or group? The point is that the original wave is absorbed by the material. The material than radiates a new wave, which travels at c. Effect must follow cause and so the new wave must always arise after original. If we look at the problem as photons travelling through a material, then basic concept of absorption and re-radiation is the same. This is a model of how individual photons/wave-packages propagates through material. The energy of the orginal wave is absorbed and then re-radiated; how is this not the speed of energy propagation? Surely phase and group velocities are emergent from the interference of multiple waves at a detector? Special:Contributions/131.227.79.25|131.227.79.25]] (talk) 15:00, 22 July 2009 (UTC)

In short, it may make a decent very conceptual overview, but it has many flaws. It doesn't distinguish between phase, group, and front velocities. It does not allow for the faster then c phase and group velocities that are seen. It way over simplifies energy propagation velocities. I wish I can help you find a better alternative, though.TStein (talk) 22:49, 17 July 2009 (UTC)


 * I have moved some of Brews comments about dispersion etc here. It seems more natural to list what the speed of light in a medium can depend on rather that to try to list all the things it does not depend on in free space. I still think the section needs reorganizing. It should start with the simple stuff (refractive index) and then go on to the more complicated stuff.  I have added a basic intro Martin Hogbin (talk) 17:22, 18 July 2009 (UTC)


 * Suggested order for this section would be, refractive index, refraction, dispersion, other effects, slow light. Martin Hogbin (talk) 19:31, 18 July 2009 (UTC)

Intro
I've put in a new formulation of the first two paragraphs. I hope it will be considered objectively, and if changes are needed some minor edits are made, rather than a rapid revert. What I particularly like about this proposal is its treatment of relativity. Brews ohare (talk) 17:16, 21 July 2009 (UTC)


 * Your attempt a a potted history of relativity in the lead is misguided. You cannot summarize the theory or its origin as simply as you have done.  Even if this could be done, the lead of this article is not the place for it. Martin Hogbin (talk) 18:34, 21 July 2009 (UTC)

"Potted" history, eh? Is that like "potty" history? The advantage of the eradicated history is that it makes sense of the connection between c, relativity, and electromagnetism, which the replacement leaves as unconnected items. Why should c be the speed of light and also the c of the spacetime metric?? Brews ohare (talk) 21:44, 21 July 2009 (UTC)

The "potted" history is pretty self-contained in the bare sentences, even without the quotes. BTW, the usefulness of the references is not limited to the verbatim quotations. The sources provide a much fuller discussion of the topics alluded to in the quotes. Those quotes are included primarily for those who don't like to look up the sources, even though they are on line. Brews ohare (talk) 23:32, 21 July 2009 (UTC)


 * I didn't know what it meant either - it appears we would use "canned" instead of potted (as in a canned response, meaning prepared in advance). I would suggest not detailing a difficult or controversial subject in the lead, as the reader's eyes would just glaze over. Pecos Joe (talk) 07:28, 22 July 2009 (UTC)


 * That is one of the things that I meant, Brews. If you are trying to make some new and important point, just putting into the lead section is not the best way to do it.  The lead is meant to be brief summary of the subject, which should be covered in more detail on the main body of the article.


 * Secondly, it is not at all clear what you are trying to say. For example, what do you mean by, 'The theory of special relativity originated in the requirement that all the laws of physics have the same spacetime structure as the laws of classical electromagnetism'? Martin Hogbin (talk) 08:41, 22 July 2009 (UTC)

The spacetime structure of the Maxwell equations is governed by the Lorentz transformations, which also are the basis of relativity, whose mathematical objective was to make all physics Lorentz invariant, and to provide a physical interpretation leading to that mathematical result. That is what the last of the three quoted sources points out. Brews ohare (talk) 12:29, 22 July 2009 (UTC) I've beefed this up a bit. Brews ohare (talk) 13:40, 22 July 2009 (UTC)

"Obvious" statement
Martin: If this is so obvious, why is it made a particular point by the source? And what is wrong with pointing out a consequence, obvious to you, but maybe not immediately evident to all, and certainly important. What is your objective here? Worried about a sentence longer? Worth the cost. Brews ohare (talk) 15:46, 22 July 2009 (UTC)
 * My point is that according to relativity light travels at the same speed in all inertial frames, whatever. Mentioning just the motion of the source as a possible reason for the speed to change dilutes this clear and strong statement.


 * Please do not try to amplify the above statement as this will do nothing to clarify what is a simple postulate on which relativity is based. Martin Hogbin (talk) 15:58, 22 July 2009 (UTC)

It is not clarification of the postulate, it is a corollary of the postulate. Brews ohare (talk) 16:02, 22 July 2009 (UTC)

Actually, this problem arises because of inaccurate statement of the laws of relativity. I fixed this problem. Brews ohare (talk) 16:35, 22 July 2009 (UTC)


 * Not really, see above. Martin Hogbin (talk) 19:10, 22 July 2009 (UTC)

Free space
For reasons outlined above, free space should be introduced somewhere. Where?

My solution was to introduce both in the intro. Lou has reverted this suggestion.

It is arguable, as I have tried to argue above, that despite the use of "vacuum" by NIST, this usage of "vacuum' is technical in this context, so the word "vacuum" is no less "jargon" than is "free space". It's just less obviously jargon, with the problem of running a mistaken identity.

In short, Lou, your approach raises the problem of explaining that "vacuum" has a special meaning, as already discussed at great length in the article free space, and use of "vacuum" conflicts with the use of "free space"" in many other WP articles.

Please address the issues, rather than reverting the solution. Brews ohare (talk) 03:53, 20 July 2009 (UTC)


 * We already discussed this. If you go up to an average Wikipedia reader, and ask what 'vacuum' is, you'll get a reasonable answer (perhaps not what a physicist would say, but a reasonable guess).  This won't happen with 'free space', where only someone who already knew what was going on could give the correct answer.  Also, the NIST/BIPM statement is in terms of 'vacuum', so that is what is used for defining the meter.  I suspect this is deliberate; from a NIST/BIPM point of view measuring the propagation of light in vacuum is at least theoretically possible - just remove every last atom from a volume.  To measure the speed of light in free space, you would need to turn off quantum mechanics, which seems quite a bit harder.  LouScheffer (talk) 04:13, 20 July 2009 (UTC)

Lou, your statement "measuring the propagation of light in vacuum is at least theoretically possible - just remove every last atom from a volume" is correct, but it is not what you do to measure c. Your procedure measures c in a real medium, and of course would have an error bar associated with it. Also, your statement " To measure the speed of light in free space, you would need to turn off quantum mechanics" is nearly correct; actually to do it you measure light in partial vacuum and implement "corrections in accord with good practice" to get the value in free space. That is what c means, not the measurement in your idea of vacuum. Brews ohare (talk) 04:19, 20 July 2009 (UTC)


 * This is not the way I read NIST publication 330, Appendix I, page 77, which talks about how to measure the meter in practice. To me it looks like any corrections applied are in terms of imperfect vacuum, not to get the free space value.  Can you point to an official NIST/BIPM document to support your conclusion?  LouScheffer (talk) 04:28, 20 July 2009 (UTC)

Lou, I assume the text you refer to on p. 77 is "that in all cases any necessary corrections be applied to take account of actual conditions such as diffraction, gravitation or imperfection in the vacuum".

The corrections applied take the actual measurements back to free space, i.e. ideal "vacuum". What do you think they are correcting for? They are correcting for the "imperfections in the vacuum". If there are no imperfections, the vacuum is ideal. If there are imperfections, the corrections fix the errors to make the measurements closer to those that would result in an ideal vacuum. What do you think all this means?

Apparently you feel that this document does not support my interpretation? Please explain how you can interpret this quote differently. Brews ohare (talk) 04:34, 20 July 2009 (UTC)


 * The part you quoted does not contain the words "free space." I have done a cursory search of the BIPM and NIST web sites, and have found no relevant uses of the term "free space."  So, the part that is troubling is where you equate "free space, i.e. ideal "vacuum"," to which I will say that you need a reliable source to confirm that assertion; also note that BIPM does not use the word "ideal" to qualify the vacuum.  Even if you could find that source, I think "vacuum" should still be exclusively used, because BIPM exclusively uses "vacuum."  I can think of no good reason to use different nomenclature than the sole recognized international authority.Pecos Joe (talk) 07:27, 20 July 2009 (UTC)

Pecos Joe: This quote refers to "imperfection of the vacuum", which I'd say implies there is a comparison implied to a "perfect vacuum", which, being perfect, also is not realizable. To separate "ideal" from "perfect" is a quibble, and to suggest that "ideal vacuum" is not relevant to this NIST text is a misunderstanding of the text. Brews ohare (talk) 11:23, 20 July 2009 (UTC)


 * As to the original question (where should free space be introduced?), I would like to see it in the "History" section, because it is no longer the preferred term, but it once was. Pecos Joe (talk) 07:27, 20 July 2009 (UTC)


 * Pecos Joe: Well, you call it history, but how about this: Google book search turning up 647 recent books (published since 1995) using the term. More can be found with other key words. The point is that the term "free space" is widely used (even today) and means the same thing as the NIST "vacuum" (as evidenced by its having the same electric constant, magnetic constant and exact speed of light). The advantage in using this term is that it is very specific, and without involved explanation (which may be found at the article free space) says what NIST means in saying the speed of light is exactly 299 792 458 m/s in "vacuum". Brews ohare (talk) 11:23, 20 July 2009 (UTC)


 * You forgot to note that vacuum is used at least as much as free space. Here are some searches comparing frequencies of the phrases "speed of light in free space (vacuum)": free space in books (621), vacuum in books (723), free space in scholar (2630), vacuum in scholar (14000).  So the reasons to use vacuum are: 1)It is a scientific concept, and vacuum is overwhelmingly preferred by the scientific community; and 2) vacuum is widely used whenever one refers to the speed of light.  I think any reasonable editor would agree, based on those points, that vacuum should be used throughout the article.  Perhaps this wording would be a good compromise: "The speed of light in vacuum (sometimes called the speed of light in free space) ... ", with all following text referring to vacuum. Pecos Joe (talk) 17:01, 20 July 2009 (UTC)

Pecos Joe: Yes, vacuum also is used. Your wording might be a good compromise. My point, of course, is not that "free space" is the only term used, or even the most commonly used term, but that it is in use and that (unlike "vacuum") it is very specific and untrammeled by other usages that could lead to confusion or imprecision in thinking about what c refers to. Brews ohare (talk) 23:31, 20 July 2009 (UTC)


 * So how about this: the lead says vacuum but with a link to free space. In the notation and units section (just the once) we say something like "The speed of light in vacuum (or more specifically free space) ... ".  Throughout the rest of the article we use 'vacuum'. Martin Hogbin (talk) 08:56, 21 July 2009 (UTC)


 * The point about what we mean by vacuum/free space is made in the article thus: 'A perfect vacuum or free space is a reference state. Like absolute zero, it is an idealized state that only can be approximated in the physical world. Measurements in any real-world medium, such as air or a medium perturbed by gravity must be corrected so as to relate to free space.' 'We really do not need the same point made at every opportunity throughout the article, especially in the lead.


 * Regarding which term to use can I suggest that we leave the wording in the article as it is rather than start and edit war and continue to discuss the subject here. Martin Hogbin (talk) 07:38, 20 July 2009 (UTC)

That's all fine Martin, but the lead uses the link to vacuum, which article describes a number of different concepts in its lead, beginning with realizable vacuum. Thus, at best, the reader does not know which meaning is intended in speed of light unless they jump to the very last section of the article (which jump is nowhere suggested in the lead). It is most probable that they will interpret the lead to speed of light as saying the speed of light in partial vacuum is "exactly 299 792 458 m/s". I do not believe that it is good WP practice to deliberately employ wording that is misleading on the grounds that common usage should be preferred to accuracy. It isn't likely that you will restore the status of this page in this manner. Brews ohare (talk) 11:23, 20 July 2009 (UTC)


 * I agree that the link should go to free space as this article better describes the concept of interest. My slight preference for using the word 'vacuum' is just because it is shorter, more commonly understood (roughly speaking) and it happens also to be the term used by NIST, but I would be happy with either. Martin Hogbin (talk) 12:57, 20 July 2009 (UTC)


 *  I do not think free space should be mentioned at all, except in connection with classical EM. By analogy, suppose we create an idea of NewtonSpace, where Newton's laws hold exactly, velocities add linearly, and so on.  When would this be useful?  For examples, and classical mechanics, it's just right.  But for measurements, it seems much less useful since all measurements occur in the real universe, not NewtonSpace.  Of course it would be possible to "correct" measurements to the values they would have in NewtonSpace, but it's seldom helpful.  Likewise, think about creating a new meter stick.  You measure how far light goes in the specified time in a good vacuum, you correct for partial gas pressure, and gravity, and that's your result.  You do NOT make any further changes to compute the corresponding value in free space.  That's WHY the standard says vacuum, and not free space.  You can extrapolate from real, imperfect vacuums to what would happen in a perfect vacuum in the universe as it really exists.  There is no need to map the result to a known-wrong approximation of reality called 'free space'.   Free space exists for the computational convenience of classical EM buffs, and for almost nothing else.  It adds nothing here.   LouScheffer(talk) 13:55, 21 July 2009

Not much of a change. The subject c refers to free space in classical EM. The limiting speed of relativity is c because special relativity is cooked up to agree with classical EM. So there is not much in the article except stuff directly related to classical EM. The only things (possibly) outside classical EM are the parts that find it hard to survive deletion, like the Scharnhorst effect and the whole cosmology section. Brews ohare (talk) 14:14, 21 July 2009 (UTC) In fact, here is a better intro to the article:"In classical electromagnetism, the term speed of light generally refers to the speed of light in free space, a fundamental physical constant usually denoted by the symbol c. It is the speed of not just visible light, but of all electromagnetic radiation. The theory of special relativity originated in the requirement that all the laws of physics have the same space-time structure as the laws of classical electromagnetism. Therefore, in special relativity, c is found to be the speed of anything having zero rest mass,[2] and is believed to be the speed of gravitational waves. The speed of light in free space is exactly 299,792,458 metres per second (m/s)[3] because of the way the metre is defined." A section or two could be added under the title "the speed of light outside classical EM". A few more sections under the title "comparison of classical EM with experiment". Brews ohare (talk) 14:21, 21 July 2009 (UTC)


 * Lou, Classical EM is a fully relativistic theory and is thus in full agreement with Einstein's theory relativity and spacetime. The only other currently accepted and verified theory on the subject is QED which really comes into its own for very small scale phenomena, as far as I know it reduces to classical EM at a larger scale. So the idealized free space of classical EM is a very good 'reference medium' for defining the speed of light in.  I just slightly prefer the word 'vacuum' just because it is simpler and more common in physics.  Have you seen the suggestion that I made today in this section above?  It may suit everyone.


 * Brews, We already have a few things about the speed of light outside classical EM, but remember that stuff is still the subject of current theoretical research has not been generally accepted or verified. Martin Hogbin (talk) 14:39, 21 July 2009 (UTC)


 * This is primarily in response to the proposed wording "The speed of light in vacuum (or more specifically free space) ... ". I have no objections to changing the article as you describe there, because it is a clear improvement over the current text which switches between vacuum and free space.  I (and I think Lou) would still have a minor problem because the proposed wording seems to favor free space; as a reader, that wording would make me think that I should use the term free space when I talked about the concept, when BIPM prefers vacuum.  I side with BIPM because its recommendations (which are made by trusted scientists in various related fields) are basically dedicated to removing the need for systematic uncertainty or imprecision in measurement systems, and they use vacuum as the most precise representation of what they mean (basically, I trust the smart, pedantic people on this).  Free space is not as cleanly defined as you may hope it to be, however.  Scientists working in optical communications tend to use free space as a substitute for a medium with a line-of-sight (often air, sometimes space).  See, for example Free-space_optical_communication and Free Space Optical Communications as examples.   I think radio frequency/microwave communications also uses that meaning, but couldn't find a reference for that quickly. Pecos Joe (talk) 17:30, 21 July 2009 (UTC)

Thanks for those links, which I'll incorporate in free space. They fit in with the section on patent office definitions. I don't think these usages are a real problem because they are narrowly used and are not used in scientific circles. Brews ohare (talk) 17:57, 21 July 2009 (UTC)


 * Free space is also used that way in scientific circles; the examples below were found using this search. They are in well-established peer-reviewed scientific journals.


 * In Physical Review Letters: Practical free-space quantum key distribution over 1 km.


 * In Applied Physics Letters: Transmission properties of composite metamaterials in free space: "The transmission measurements performed in free space exhibit a passband..."


 * and (from the looks of the search) several thousand others, but I think the above should be sufficient to show that free space does not always refer to some impossible-to-realize medium in scientific circles. So, because free space can mean atmosphere, free space should not be equated to vacuum in this article.  Also, because the SOL in FS is numerically equal to the SOL in vacuum, and both terms are used, those two should be equated.  I think of it as SOL in FS is just another name for SOL in vacuum.  I hope this clarifies what I am trying to say. Pecos Joe (talk) 20:22, 21 July 2009 (UTC)

Hi Pecos Joe: There is no argument that NIST "vacuum" and "free space" are one and the same as far as c is concerned. There also is no doubt that "free space" is used in some contexts to mean propagation in a medium like air, as distinct from a waveguide or the like. There also is no doubt that "vacuum" has a very, very much wider range of meanings, many non-technical, and many meaning again a medium, like terrestrial vacuum. So, I think your argument really comes down to this: “it doesn't matter what a general audience might conclude from its use; if "vacuum" is NIST's selection of term, who are others to argue, a large number of textbooks and papers notwithstanding?” Brews ohare (talk) 02:45, 22 July 2009 (UTC)


 * Thanks for responding. I have previously shown that the general audience uses the term vacuum slightly more than free space, and that scientists vastly prefer vacuum; thus vacuum is the most common usage.  It is also true that vacuum is the term used by standards organizations, thus vacuum should be preferred in the article.  I have begun discussion on your talk page about some points not directly related to this article; please see that. As for this article, I hope we are very close to a solution.  Does anyone object to the wording (as the first sentence) "The term speed of light generally refers to the speed of light in vacuum (also called the speed of light in free space), a fundamental physical constant usually denoted by the symbol c."  I think it is stylistically best to avoid the footnote and allow a curious reader to click the link.  After that mention, the article would refer to c as the speed of light in vacuum.  I don't want to potentially confuse the reader by stating that vacuum is always the same as free space, nor do I want to favor free space over vacuum (nor to disfavor free space).  Would this wording need any changes to be acceptable to you? Pecos Joe (talk) 07:24, 22 July 2009 (UTC)


 * I understand your point that "vacuum" is used by NIST and by many scientists. What I don't like about "vacuum" is that these scientists seldom define "vacuum", because they assume you know what "vacuum" is. An exception is the area of quantum vacuum and QCD vacuum, where careful distinction is made from classical vacuum. One would have to say that scientists in this area are really the ones that are thinking about the matter, not the EM people or the radio engineers.


 * However, definitions of "free space" (or classical vacuum) can be found, and two are presently cited. You will notice that these definitions are not the same: one says "absence of conductors, dielectric and magnetic materials" and definitely allows free charges and currents; the other says "void of matter", which under the usual definition of matter excludes free charges and currents.


 * I'd say NIST avoids the subject of what is vacuum in two ways: (i) by suggesting one make corrections for imperfection in the vacuum in accord with good practice, which really amounts to an (unspecified) algorithm, and isn't very helpful. The closest one can come is to read about measurements in air (say) and what corrections are actually used in that arena; and (ii) by defining the electric constant and magnetic constant of vacuum, which in effect means that any real medium can be taken to be vacuum within experimental error if its ε ≈ εo and μ ≈ μo within experimental error. This second definition is the most specific, and in fact, constitutes how "best practices" are discovered and updated.


 * So without knowing exactly what wording to use, this is the tenor of what should be conveyed. Brews ohare (talk) 12:57, 22 July 2009 (UTC)

I notice, Joe, that you have decided to edit the article to show 'vacuum'. I hope that this does not lead to the insertion of a series of lectures on the subject by Brews into the article. Martin Hogbin (talk) 18:05, 24 July 2009 (UTC)


 * I read the last comment by Brews in this section as acknowledging that vacuum should be used, but that it should be defined in this article. I happen to think that it should not be defined in this article (much the same way "speed" and "light" should not be defined here, my preference would be to link to definitions in their own articles), but I will not remove any of the current explanations, as long as they can be adapted to say vacuum in the main article text.  I don't expect any further explanation of vacuum will be necessary in my mind or anyone else's. Pecos Joe (talk) 18:50, 24 July 2009 (UTC)


 * I am happy with that, I was just keen not to re-start a war on the subject. Martin Hogbin (talk) 22:27, 24 July 2009 (UTC)

Speed constant in inertial frames
Brew, you have made a number of edits to this section but it is not clear what you are trying to achieve or what point you are trying to make.

What was wrong with the section as it was before? Was there some conceptual error or are you just trying to clarify something? If so, what?

The second paragraph was intended to give a clear picture of the precise experimental evidence that we have on the subject of the speed of light to date, followed by Einstein's postulate. Your addition of 'theoretically' simply confused the issue. Martin Hogbin (talk) 09:28, 22 July 2009 (UTC)


 * Martin: The claim to provide sources that were really about experimental confirmation was unsuccessful. The statements about c being experimentally confirmed to be the same in all inertial frames, for example, was not the subject of the sources. The claims that c was experimentally confirmed to be a constant actually referred to SOL over times comparable to the age of the universe, which is not exactly what relativity was about. So I switched to theoretical predictions because these were documentable claims. Several of the sources are not available on line at all, so I could not check them out, but the paucity of other sources makes me suspicious that they are only tangential to the claims. Brews ohare (talk) 12:35, 22 July 2009 (UTC)


 * If you read the article and understood the subject you would know that the constant speed of light in any inertial frame cannot be verified experimentally. As the article now states this is a result of the theory of relativity, which is based on Einstein's two postulates. Martin Hogbin (talk) 15:39, 22 July 2009 (UTC)

The article previous to my revisions claimed to have sources verifying this point. I removed that claim. You need to calm down and cool off. Brews ohare (talk) 15:47, 22 July 2009 (UTC)


 * I will be glad to do so when you stop trying to make points by endless editing of perfectly good text.


 * Once again you have shown your lack of understanding of the subject by the way you now claim in the article that the first postulate is necessary to make the statemment that the speed of light is constant in all inertial frames. Many modern, well respected, text books, such as d'Inverno, have the second postulate as saying that the speed of light is the same in all inertial frames.  The way to learn physics is not by endlessly editing the article to reflect your personal misunderstandings. Martin Hogbin (talk) 18:57, 22 July 2009 (UTC)


 * What you have added now is at least correct, but there is one problem. The article is entitled 'Speed of light' and most of your addition is about other issues.  The relevant point about relativity is that it says that the speed of light is constant in all inertial frames.  The rest of the information that you have added is best accessed by the reader following the link to the relativity article.  Adding more and more irrelevant facts to an article does not improve it, it just tries the readers patience.  I will restore it to just stating the relevant facts about relativity. Martin Hogbin (talk) 21:22, 23 July 2009 (UTC)

The speed of light has been shown experimentally to be independent of the motion of the source.
The sources cited to support this statement are not available. They should be replaced. I cannot find substitutes. Brews ohare (talk) 16:12, 22 July 2009 (UTC)


 * I have corrected the main source, which is the book by Zhang and removed the other source. Martin Hogbin (talk) 18:47, 22 July 2009 (UTC)

Unfortunately, Zhang has no preview on-line. Maybe it could be supplemented? Brews ohare (talk) 14:53, 23 July 2009 (UTC)

Zhang is recognized by physicists as the definitive work on this subject and the page quoted is a table of experiments and their results, but I agree that an online reference would be good. Feel free to add one. Martin Hogbin (talk) 21:13, 23 July 2009 (UTC)

Spacetime constant & Speed of light in relativity
I simply copied some material from Special relativity which has the advantage of uniformity with that article and a better sourced presentation. Brews ohare (talk) 14:16, 23 July 2009 (UTC)

Cosmology and quantum gravity
In this section we have simple statement, 'These questions remain an interest of on-going research', with five references!! This does not add credibility it just looks silly. Somebody is trying to make a point and the place to do that is here on the talk page, not in the article. Martin Hogbin (talk) 22:11, 23 July 2009 (UTC)
 * It only looks silly because some editors will not allow a proper discussion of the topic. Each of these articles is of interest in its own way. Brews ohare (talk) 22:21, 23 July 2009 (UTC)
 * By,'a proper discussion of the topic' do you mean inserting a personal interpretation of a few, randomly selected, areas of current theoretical research into the article? Martin Hogbin (talk) 22:27, 23 July 2009 (UTC)
 * In the original text Brews described a lot of speculative theories...things that are legitimate and interesting ongoing research, but pretty obscure, not widely accepted and widely taught, and for the most part will probably eventually be proven false. Things like that don't belong in this article. Since then, the section was cut down (rightly), but the references remain. I suggest we replace all the references with a footnote: "More information and references can be found in the article variable speed of light." --Steve (talk) 22:41, 23 July 2009 (UTC)


 * Why not just a 'see also'? My objection was to the continued use of editing the article to make a point. Martin Hogbin (talk) 22:53, 23 July 2009 (UTC)

Brews, the way to deal with this is not to try to make your point yet again by aggressive editing of the article. Martin Hogbin (talk) 22:58, 23 July 2009 (UTC)


 * Martin, that's the pot calling the kettle black, eh? It is very poor idea to merge the topics of quantum gravity and cosmology. They have some overlap, but it simply muddies up the treatment to lump them together. Brews ohare (talk) 23:01, 23 July 2009 (UTC)


 * I merged them, not Martin. . This is an article about the speed of light. Why is this section in here? (1) There are speculative theories in both quantum gravity and cosmology in which the speed of light varies in time. (2) There are speculative theories in both quantum gravity and cosmology in which the speed of light varies with frequency. Yes there are differences between quantum gravity and cosmology, but insofar as they relate to the speed of light, there are really only these two little things to say, and there they overlap completely. (I'd also be happy for this section to be removed altogether.) --Steve (talk) 03:16, 24 July 2009 (UTC)

The cosmology issues relate to the change over time and the evolution of the universe. The quantum gravity theory is about unifying quantum mechanics and general relativity. Yes, physics is involved in both. That doesn't mean they are the same. Yes, some of the same issues arise; that doesn't make them the same either. The impact of a valid quantum gravity theory upon cosmology probably will not be to resolve the cosmology issues, and vice versa.

As for why they are here? They reflect the importance of the speed of light and its behavior in modern physics. It's not all cut and dried. It has impact. It involves tons of experimental and theoretical activity. Are we ashamed that not all physics is settled??? Do we believe that all Wiki readers want is the cut & dried stuff??? I don't think so. Brews ohare (talk) 05:46, 24 July 2009 (UTC)


 * No, we're not ashamed that not all physics is settled. But it doesn't make sense to waste people's time teaching them about physics ideas that will almost certainly turn out to be incorrect. Let's be clear: There are at least 20 theories of quantum gravity that all contradict each other, which means that if you pick a random theory of gravity it has at least a 95% chance of being totally wrong. Meanwhile, there's already way too much to say about the speed of light that's definitely true. We can easily fill the whole article with definitely-true stuff, and then we can avoid spending valuable time and space explaining theories that are (almost certainly) totally false.


 * This is an encyclopedia article on the speed of light. It should be engaging, but this isn't a middle-school science class where the real goal is to show people how science is great and it's fun to be a scientist and science is still going on and whatever. It's an encyclopedia article, the real goal is to communicate knowledge. :-) --Steve (talk) 07:05, 24 July 2009 (UTC)


 * I agree entirely. Please see my comments and explanation for what might seem like heavy-handed editing at times in the 'We do not want to duplicate the 'Light' article here' section below. Martin Hogbin (talk) 10:47, 24 July 2009 (UTC)

QED
The following reverted edit is a perfectly accurate statement that is almost a verbatim quote from an established text. It also has a link to photon that this article needs. The present reference to matter in connection with QED is overstated and inaccurate.

A more complete theory of light that describes the interaction of photons with electrons and positrons is given by quantum electrodynamics (QED) in which c plays the role of a fundamental constant. Brews ohare (talk) 22:19, 23 July 2009 (UTC)


 * Yes, that quote is quite correct, but it is not exclusive. QED also describes many other interactions. Martin Hogbin (talk)


 * The other interactions are the strong force (maybe the weak force) and gravity. None of these is relevant to the subject of the article which is about the way that light interacts with matter.  The current theory covering that subject is QED and it does it very well (apart from being very complicated).  Martin Hogbin (talk) 22:37, 23 July 2009 (UTC)


 * Martin, I don't understand how you could say that "QED is the current theory of light and matter, which has little to do with the standard model." in this edit summary. Surely you know that the standard model includes QED as a special restricted case? And that the standard model includes aspects of the light-matter interaction that are not included in QED? For example, I can draw a multi-loop Feynman diagram describing the interaction of a photon and an electron, that also includes virtual quarks and gluons and neutrinos. This is a legitimate and measureable part of the interaction, but not included in QED. --Steve (talk) 22:30, 23 July 2009 (UTC)


 * As far as I know the electromagnetic interaction is completely covered by QED. The standard model is normally relevant to particle physics rather than the interaction of light with matter, however I am not an expert.  I will ask someone who is. Martin Hogbin (talk) 22:37, 23 July 2009 (UTC)


 * I'm saying you can't completely isolate the electromagnetic interaction from the other ones. A photon and electron may interact via a complicated Feynman diagram that involves not only electromagnetism but also strong and/or weak force. If we say "electromagnetism", QED is sufficient. If we say "light-matter interaction", you need the whole standard model. :-) --Steve (talk) 22:44, 23 July 2009 (UTC)


 * Maybe this is just semantics but I would say the non-EM forces are the reaction of matter with matter. As far as I know the photon does not 'feel' the strong force. Are you an expert on the subject? Martin Hogbin (talk) 22:50, 23 July 2009 (UTC)

Steve may be an expert (more than the two of us), but regardless of that, Martin how can you argue with the source, which is extremely clear on the subject? QED is involved in the interaction with matter, but as the source and as Steve says, with the exception of electrons and positrons the other forces enter in. That is all the source says, nothing more. Brews ohare (talk) 22:56, 23 July 2009 (UTC)


 * Steve has not yet confirmed that he is an expert. I explain about the source above. Martin Hogbin (talk) 23:18, 23 July 2009 (UTC)

You "explained" it incorrectly using factual errors: you said :"QED also describes many other interactions." Why do you insist on overriding a perfectly satisfactory sourced statement?? Brews ohare (talk) 00:49, 24 July 2009 (UTC)


 * As Steve says below, QED does much more than tell us about interactions between photon, electron and positron.


 * QED describes (extremely well) the electromagnetic interaction (or electroweak interaction if you prefer), between any particles that interact in this way. That includes many more particles that just the electron and positron.  This does not conflict with your source, which simply says that other particles also undergo other types of interaction.


 * In reality, pretty well all interaction of light with ordinary matter is with the electrons, positrons tend not to stick around for long and the protons in the nucleus are hardly ever involved. QED provides an excellent description of this process even though it might not be 100% complete. Martin Hogbin (talk) 14:31, 24 July 2009 (UTC)


 * Martin, I took a two-semester graduate course in Quantum Field Theory at Harvard University and got an A. That doesn't make me an expert, but maybe it makes me "knowledgeable" or something. :-) The photon does not directly feel the strong or weak force, but the photon feels quarks and quarks feel the strong force. The way Feynman diagrams work means that that's enough for the photons to feel the strong interaction a little bit. Here's an example: The question "What is the electron magnetic moment" appears to be a purely electromagnetic question. And it can be answered to 99.9999999999% accuracy within QED. But to get to the parts-per-trillion level, you need to include strong interactions (see Figure 2 of, "hadronic" means strong force) despite the fact that neither the electron nor photon feels the strong force. :-) --Steve (talk) 03:55, 24 July 2009 (UTC)


 * You certainly seem to know about the subject. Has this strong interaction with the photon been experimentally verified yet? I understand that the theory that describes this kind of thing is QCD which still has some way to go. An experimentally verified accuracy of 99.9999999999% seems pretty good to me and I still think your wording was a bit strong.  Perhaps we could come up with some reasonable compromise. Martin Hogbin (talk) 09:49, 24 July 2009 (UTC)


 * Having looked at what I wrote, I would say that is is completely correct. I wrote, 'A more complete theory of the interaction of light and matter is given by quantum electrodynamics'.  This compared to classical EM, which gives an excellent description of light on a macroscopic scale but does not cover the quantum aspect at all.  This exactly what QED does cover.  Note also that I said more complete rather that just complete.  According to what you have just said the incompleteness of QED in dealing with the interaction of light with matter is tiny, so small, I would say, as hardly be be worth a mention.  I certainly think that to say that the standard model is the best description of the interaction of light with matter is something of an overkill, and even that may not be complete, there is still the interaction of light with gravity. Martin Hogbin (talk) 14:13, 24 July 2009 (UTC)


 * Yea that's fine. Here's my edit, I was trying to avoid going into details about what is or isn't in QED by just saying "Standard Model" from the start. (I guess those details were put in by Brews not you?) Not a big deal or anything. :-) --Steve (talk) 17:02, 24 July 2009 (UTC)


 * Are you happy then for me to restore my original statement of, ''A more complete theory of the interaction of light and matter is given by quantum electrodynamics'? Martin Hogbin (talk) 22:24, 24 July 2009 (UTC)


 * Yes. Although as Brews points out, it would be appropriate to have photons mentioned somewhere around here. Photon-theory-of-light is an even more basic and important idea than QED-theory-of-light...the word and link photon should be present somewhere if it's not already. Maybe it already is. --Steve (talk) 23:04, 24 July 2009 (UTC)

We do not want to duplicate the 'Light' article here.
We should not try in one line to define in this article what light is. There is an article called Light to do that. All we want to mention here are the properties of light relevant to its speed. Martin Hogbin (talk) 23:25, 23 July 2009 (UTC)


 * So why did you bring up electromagnetic radiation?? Maybe to place light in the EM spectrum?? Maybe to say c isn't just for "visible" light??? Maybe to explore a bit what "light" refers to in connection with "speed of light"?? Brews ohare (talk) 00:48, 24 July 2009 (UTC)


 * The fact that light is EM radiation is very relevant to its speed. Trying define here what 'light' means is confusing since there are several possible definitions (from visible light to all EM radiation) which are much better discussed elsewhere. Also, we make the point in the article that the speed of all EM radiation in free space is the same.  Appearing to restrict ourselves to only a limited part of the EM spectrum at the start of the article confuses this issue.


 * I am sorry if my editing appears heavy handed at times but I am trying to prevent the following from happening;


 * Putting everything we know about physics in this article, especially more than once. The article is about the speed of light, we do not want to define 'speed' or 'light' or even 'free space' in great detail here.  One big advantage of WP is that it is easy to follow links to relevant articles if more details or clarification of a term is required.


 * Arguing and making points about the various points of interest in the article itself. By this I mean, for example, giving undue prominence within the article to a recently made point or trying to make your point by starting a citation war.


 * Putting too much about current valid theoretical research on the subject into the article. The article should be primarily about what generally accepted and experimentally verified theories of physics say about the speed of light.  As Steve has pointed out there are many speculative theories on the subject and, statistically speaking, most of them will be proved wrong.  I suggest that they are briefly mentioned here with links to pages that can be edited by those who know more on the subject.  For example, it is quite obvious that nobody here understands theories of quantum gravity and we should not pretend that we do.


 * I think that the content has already reached the standard required for a FA, we have more here now that was in the article when it got FA status before. We now want to try to keep the content stable except, of course, for dealing with errors and important omissions.

Martin Hogbin (talk) 09:29, 24 July 2009 (UTC)

Martin: You are self-appointed guardian and will rabidly revert anything that doesn't immediately appeal to you, regardless of the merits. Among your personal criteria, you actually fear putting anything in the article you do not know yourself from your own present knowledge, even if sourced and irrefutable. You insist upon a pedestrian content, and will allow zero life into the article. You do not see the article as part of WP, but as a stand-alone, and remove links to the rest of WP that could be helpful to the reader. You will not accept assistance or commentary.

A single example is your reversion of the info about relativity, which was sensibly ordered with the correct title and a number of related subsections. Instead, you renamed the main heading, eliminated the necessary statement of the principles, which very directly mention the speed of light, left in its place an emasculated follow up paragraph with no context. Because of the renaming, the subsections are no longer related to a main heading, and are subsections without any thread to string them on. Bad moves, poorly executed, no cooperation at all. Brews ohare (talk) 14:57, 24 July 2009 (UTC)


 * Brews, you're not exactly seeking out and following consensus either. I don't know what you mean by "pedestrian content" and "life into the article", but it seems to me like the tone and content of this article, as it stands after Martin's edits, is about right for a good wikipedia article. This article has plenty of links...as I recall it used to have some links to totally unrelated articles like Green's function (many-body theory), so good for whoever deleted that. --Steve (talk) 17:21, 24 July 2009 (UTC)


 * Brews, my tone above was intended to be conciliatory as I apologized for appearing heavy handed and tried to explain what my motives were. Do you disagree with any of the points above or are you saying that my edits were not directed at achieving them?


 * I certainly am cautious about adding anything on a subject on which I have no background knowledge and I suggest that you should be also. For example, subjects like quantum gravity are extremely mathematical and complicated and it is simply not possible for a non-expert to add a summary of a published paper on the subject to the article. To cite a paper and then quote a couple of lines out of context from it, with no understanding of the subject matter, is not the same as adding well-sourced information.  You have done much the same sort of thing on other articles.


 * Regarding links to other articles, as you will see from my comments above I am generally in favour of them; this is how WP works. What I am against is contriving sentences to add to the article just to get a couple of unnecessary links in.


 * As Steve has said above, WP is intended to be an encyclopedia promoting knowledge of currently accepted and verified theories. It is not a medium for research or discussion on the subject.  Your lack of understanding of the difference between these two approaches is another reason that you should be editing cautiously. Martin Hogbin (talk) 17:46, 24 July 2009 (UTC)

I don't think WP has to be restricted to currently accepted and verified theories, especially as that is a judgment call, and the distinctions are a gray area. For example, MOND is certainly not an accepted and verified theory, and neither is dark matter. The account of these developments is at least as interesting as Sarah Palin. Brews ohare (talk) 18:35, 24 July 2009 (UTC)


 * Dark matter is a bad example, but MOND, variable speed of light, loop quantum gravity, etc., all prove the point. Yes, a theory of physics that's most physicists think is false is not a priori out of bounds for wikipedia. But it's almost certainly going to be less notable than the theories that everyone thinks is true. (For example, textbooks are a really great way to check notability in physics, and these will generally leave out or de-emphasize speculative theories.) So such a theory should mostly show up in more specialized and specific articles. For example, loop quantum gravity should certainly be discussed at length in the article loop quantum gravity, but should not be discussed at length, or better yet at all, in the article gravity.


 * Speed of light is an article of interest to non-specialists with lots of non-speculative things to say about it, so it shouldn't discuss speculative theories much or at all. Quantum gravity, on the other hand, should discuss speculative theories because every theory of quantum gravity is (to some extent) speculative...there's nothing else to discuss but speculative theories! Anyway, that's my view on this. :-) --Steve (talk) 19:05, 24 July 2009 (UTC)

There's no doubt that specialized articles should deal with detail. The general article can, however, present a sufficient discussion to alert the reader that something is going on, and possibly what the main issues are. That provides some vitality to the article. The presence in the general article is not a "seal of approval" and that should be made clear. However, underemphasis of physics having some growing pains is as bad as overemphasis.

Somewhat unrelated, the restriction of discussion in a general article to the narrowest interpretation of the subject of that article is a major disservice to the reader. If there is any agreement about the value of WP, it is that readers look to WP to get a broad view of the topic and its connections to other topics that they may not have thought of as related. Curtailing this breadth to imitate a paper encyclopaedia is not supporting this major usage of WP. Brews ohare (talk) 19:20, 24 July 2009 (UTC)


 * Yes. WP should when possible illuminate connections between subjects that are actually related. And at the same time, every two topics in physics are in some marginal way related. If we describe each of these relations, every physics article would be hundreds of pages, and totally inaccessible and unreadable and useless. There is such a thing as too much information in a wikipedia article. (Likewise, too many links, etc.) There is also such a thing as too little. I think we can all agree on this general idea.


 * One thing you seem to be missing, Brews, is a sense for what's really interesting new research that's going on, and what's not. You acquire this after doing enough physics, going to enough seminars, reading enough press releases, etc. You get a little jaded, and realize you shouldn't immediately believe everything you read. Through this process, you get some amount of judgment...you can see something and say, this thing is an implausible idea that one or two random theorists thought up, no good journal has published the research, no one is following up, and no one cares. Except sometimes the theorists will issue a press release and it will get unwarranted mainstream press. Anyway, this happens so often it's ridiculous. If Wikipedia put all of these ideas in its articles it would be an overwhelming amount of information. But with good judgment you can recognize some other research that's a plausible idea, physicists regard it as promising, it gets published by more and more good physicists in good journals, and is worth paying attention to, and maybe briefly mentioning in certain articles. Anyway, when you write about cosmology or whatever, it is a problem that you don't know any of the details of the field...but not for the reason Martin says. You can't follow the details of the paper? Fine. I trust that you can read the abstract and get the gist. The problem is that you've show poor ability to judge for what is good and interesting early-stage research, and what's a dime-a-dozen dead-end idea that some random guy thought up. --Steve (talk) 21:04, 24 July 2009 (UTC)

Well thanks for that nuanced appraisal of my judgment. I doubt that it is a general consensus that quantum gravity in all its forms is crackpot. I suspect you of making wild extrapolations to suit your snap judgment of me as well. All this gratuitous slander doesn't help me to work with you. Brews ohare (talk) 21:30, 24 July 2009 (UTC)


 * Hmm, that came across meaner than I intended. I'm sorry. I believe that some of your edits show a modest lack of sense for notability within physics. Not all of your edits, and not a total lack of sense...far from it. --Steve (talk) 22:02, 24 July 2009 (UTC)

Making a point in the article
This is the kind of thing that I am trying to prevent. After a discussion with Softvision, Brews has added this line to the article:

“It is important to separate the definition of a quantity from the definition of the unit in which it is measured.”[90]

As it happens this is a point that I entirely agree with and it is one that I agree needs to be made in the article, but not in this way, as a direct quotation, on its own surrounded by white space to make it prominent. It looks as though it has been done just to make a point to Softvision rather than to improve the article.

I agree that we do need a sound discussion of the fact that the speed of light has been set by definition and suggest that we work together on providing this. Martin Hogbin (talk) 10:57, 25 July 2009 (UTC)

Speed of light set by definition - invalid conception leading to problems and possibly confusion
Speed of light is natural fundamental cause and effect, that exist independently of any theory, measurement units and consciousness. Speed of light is fundamental physical reality, that essentially and detrminicticaly influences all physical phenomena. Definition of metre as speed-time is absolutely unphysical, uncausal and invalid, because value of speed depends on the distance unit. It is hard to understand how this can be accepted by physics scientifical authorities. If $$r_u$$ is metric unit and $$t_u$$ is time unit, than definition of metre : "The metre is the length of the path travelled by light in vacuum during a time interval of 1 ⁄ 299 792 458 of a second" means, that $$r_u = 299792458\ \frac{r_u}{t_u} \cdot \frac{1}{299792458} t_u = \frac{r_u t_u}{t_u} = r_u$$. That means, definition states, that $$r_u = r_u$$ (metre is metre), or $$1 = 1$$. If someone will declare, that speed of light is c = 299792457 m/s, how will you prove, that your value is valid ? By definition ? You cannot prove validity of the speed of light value without ignoring the SI definition of metre. If you measure the distance by speed of light, you cannot measure the speed of light, because you will measure your own assumption. I think it is clear. Definition of unit using the same unit is deeply invalid. I strongly recomend to cancel this invalid quasi-definition, and to perform more accurate measurements of the speed of light, based on measurements of propagation speed of electromagnetic field on various possible frequencies, and precise definitions of metric and time units. If we can measure micrometers and nanoseconds, or more, this is sufficient to extend precission of "speed of light" measurement. It is strange, that physics scientific authorities are cool and satisfied with integer value of fundamental constant, based on arbitrary and historical definitions of metric and time units. Interesting alchemistic result, syntheticaly fixed in International System of Units. Softvision (talk) 19:39, 24 July 2009 (UTC)


 * This is the talk page for discussing improvements to the Speed of light article. This is not a forum for general discussion about the article's subject. If you have a problem with how SI units are defined, write a letter to BIPM. --Steve (talk) 20:35, 24 July 2009 (UTC)


 * This is a difficult topic, which accounts for how much it gets discussed on this talk page. However, supposing one can measure time accurately, a definition of the metre as how far light travels in vacuum in a specified time is a workable definition, wouldn't you agree? You don't even have to know what the speed of light is in order to measure things in metres. You do have to know what a vacuum is, which is rather a tougher issue. However, if we suppose the measurement is done in air, and everyone accepts a list of corrections to account for the air not being a true vacuum, everything works out. So we don't really need to know anything about the speed of light. We can say it's 1 m/s, 100000000 m/s or whatever proves the least expensive when it comes to retooling machine shops. Brews ohare (talk) 20:38, 24 July 2009 (UTC)
 * Why this topic is germane to the "speed of light" article is another question. Maybe because "speed of light" is a phrase that enters the definition of the metre? I'd say its something of a marginal topic except for the great confusion it engenders. Brews ohare (talk) 20:41, 24 July 2009 (UTC)
 * What I suspect bothers you is that the speed of light seems a valuable concept, and to say that it could be 1 m/s or 10000000 m/s, it's our choice, seems to fly in the face of its having some particular value. Probably that concern can be addressed by asking how long it takes for light to travel from point A to point B. We have a notion of how long it takes to drive or walk or for sound to travel the distance, and we'd like to know how that compares with how fast light takes. We still do not have to know what units we measure the A to B distance in; we could pick metres or parsecs or light years, whatever. Brews ohare (talk) 20:51, 24 July 2009 (UTC)


 * If someone will declare, that speed of light is c = 299792457 m/s, how will you prove, that your value is valid ? By definition ? You cannot prove validity of the speed of light value without ignoring the SI definition of metre. If you measure the distance by speed of light, you cannot measure the speed of light, because you will measure your own assumption. But if you ignore the SI definition of metre, the results of your experimental provement can be rejected as not compliant with SI. This is the root of the problem. SI definition is invalid and fixes this serious error. Steve : I suppose that correcting, removing or supplementing the contradictive information is improvement of the article. Softvision (talk) 21:02, 24 July 2009 (UTC


 * Softvision, find a reliable source that shares your concern about this issue. Then we can talk. --Steve (talk) 21:06, 24 July 2009 (UTC)


 * The truth and reality is the best reliable source. My intension is not to waste your time with discussion. I am not editor of this article. Editors are responsible for the content in the article. I want to help editors to improve this article in public behalf. Softvision (talk) 21:22, 24 July 2009 (UTC)

Softvision: It is perfectly true that the speed of light is a tautology if the metre is defined as how far the light travels in a specified time. You set c (in m/s) when you specify the time interval. (Of course, that has nothing to do with how fast light travels, say compared to other speeds.) If you accept my discussion above (which you have not addressed) it makes no difference what your metre is. Hence, you cannot "prove" anything about the validity of your choice. All you can say about this is that it is a great convenience if we all make the same choice, and the convenient choice is the distance light travels in 1/ 299,792,458 s. Brews ohare (talk) 21:16, 24 July 2009 (UTC)


 * I don't want to make some odd discussion, and I understand the intension of the article discussion pages. The basis of my submission is above. The concept of the speed requires independent and valid measures of space and time. If space and time are dependent measures, there must exist the function of this dependence, and this dependence must be distinct. Consider this more seriously. Whatever you do, you will allways come back to space and time, because it is manifestation of reality. You can choose your space and time measures, whatever you want. Only requirement is the stability of theese measures. Speed than is dependent physical property, distinctly defined by your space and time measures. That means, you cannot choose the measure of the speed. But if you think, that speed of light is more important for you, than space or time :-), and if you think that it will help you in your physical description of reality, you can choose some (299792458) speed value as the fundamental distinct (constant) measure of your choice. But than you must abandon the measure of space or the measure of time, because it will emerge from your measure of speed. You cannot have all three measures independent. SI definition of the metre, abandoned the independence of meter measure. If speed of light is fixed SI constant, now we must consider, if the second is realy the second, and if it is constant. I realy do not think, that this is the right way how to improve physical knowledge. Softvision (talk) 21:53, 24 July 2009 (UTC)

Addition to the article
As a result of this discussion, the following has been added to the article (with my slight rewording of the first sentence):

Defining the metre as the distance light travels in a specified time has the effect of setting the speed of light to a definite numerical value when measured in the SI units of m/s. However, that doesn't make a judgment about the physical value of the actual speed of light, say compared to the velocity of sound, or the speed of an electron with a given energy, because all these speeds are scaled by the same factor when the metre is set.

When talking about the philosophical aspects of the decision to fix the speed of light it would be good to have the views of a good secondary source on the subject. Martin Hogbin (talk) 22:22, 24 July 2009 (UTC)


 * I am sorry, but I think this is not true. If speed of light is fixed and space unit is dependent on the speed of light value, than space measure is also fixed. If the space-time ratio - c [m/s] will be experimentaly measured, it can be measured only by setting the time measure as free parameter. I am shure, that speed of light is not integer value, based on arbitrary historical measures, and that there is serious requirement to make more precise measurements of the speed of light, which means, in current SI definitions, to measure the value of second. These measurements will produce the valid space time ratio, wich will express the speed of light in SI units, and in backward compatibility it will show, that speed of light was not integer value, like it is now. I think, it is minimaly good, to add to the article, that fixing the speed of light as integer value, and fixing the metre as speed of light dependent measure, is making the time unit a free parameter, that can be measured, in process of the theories improvement and fundamental constants validation, which is certainly not finished. That means, that it is not true, that the speed of light cannot be measured in the future. Claiming that is absurd and unscientific. SI units must be and will be reconsidered. Softvision (talk) 22:34, 24 July 2009 (UTC)

The underlying requirement is that the speed of light be a reproducible speed under recognizable circumstances. The reproducibility and the circumstances are established by experiment. With that established, we do not need to know how fast the speed of light is, only that we can obtain a signal with that speed when we want it. Then all other speeds are in effect related to this one. Given a measure of time, we don't need to define the length independently, because length = c t. The unit of length is set by choosing an interval of time: ℓu = c tu. Changes in the accuracy of time measurement result in some change in the assessed values of particular lengths (e.g. wavelengths) in units of metres, but do not affect the numerical value of c in m/s. For example, the wavelength of Ytterbium 2S1/2 (F = 0, mF = 0) – 2F7/2 (F = 3, mF = 0) transition is 466878090.067 fm based upon the measured frequency f=642121496772.3 kHz (with its associated uncertainty) and the relation λ = c/f. Brews ohare (talk) 23:27, 24 July 2009 (UTC)


 * The point is that the current definition of the meter is part of the SI system which defines of units of measurement. The question of what the word 'length' means conceptually is an important one and although there are reasons to prefer a definition based on light rather than a piece of metal, there are also reasons to prefer a definition based on a piece of metal.  No doubt the the BIPM would have considered such philosophical issues when deciding on how to define the metre but their primary function is to, 'provide the basis for a single, coherent system of measurements throughout the world, traceable to the International System of Units'.  For maybe some philosophical reasons, but mainly for practical reasons connected with metrology, the BIPM have decided to base the unit of length called the 'meter' on the distance travelled by light in a defined time.  The SI system thus becomes more like the natural system of units in which the speed of light is set to be 1.  There is no point in asking what if the speed of light turns out not to be 1 the system defined such that it is.  The SI system of units is now, in part, set up in a similar way; the speed of light is 299,792,458 m/s, by definition. This cannot change the time it takes for light to travel the length of a piece of metal but it does define the length of the piece of metal as measured in metres.


 * However, the above was not the main point that I am trying to make. The article currently contains Brews understanding of the situation, above is my understanding.  What we ideally need in the article is reliable secondary source that makes the situation clear.  If we cannot get that then I think we should discuss a form of words here, supported by sources, that we can all agree on. Martin Hogbin (talk) 10:19, 25 July 2009 (UTC)


 * I notice that Brews has added some good references. Martin Hogbin (talk) 10:39, 25 July 2009 (UTC)


 * I consider this discussion as useful and leading to valid solution. The concept of stability and reproducibility of measures is fundamental and valid. Both, piece of metal and speed of light, are not absolutely stable and reproducible. Both depend on the technical and environmental conditions. But that is reality, not a mistake. From the point of view, that speed of light can be slowered to fraction of metre per second, the piece of metal is more distinct. From the point of view, that speed of light in vacuum is fundamental constant and real physical limit, that affects all physical effects, the speed of light in vacuum, when measured with highest possible precission, can be theoreticaly used for redefinition of metre. Practical usage of such definition is controversial, beacuse environment significantly affects the speed of light. Air - 0,997c = c-0.3%c, water - 0,75c = c-33.3%c. However refractive indexes can be used for measurement corrections, but precision of measurement must include nonlinearity and inhomogeneity of refractive indexes, along with the precission of refractive index measurement. It woud be useful, to add these facts to the article, and to stress the precision limits of the metric measurements using current definition of metre. Definitely the assumption of constancy of time flow in various environments is required, beacuse definition of the metre is bound to the unit of time. As I wrote in previous submissions in this discussion, the speed of light cannot be now numerically changed, because it is fixed by SI definition, but that does not mean, that the physical meaning of this value cannot change. Speed of light in vacuum is physical effect with highest singnificance, independent on theory and consciousness. Therefore it can and must be measured in highest possible precission. Current SI definitions allow this only by changing the time unit. Current SI definition states, that the speed of light is exact value. That means, that there is exact real and physical ratio between the metric unit and time unit. That means, that the precission of metric unit is dependent on the precission of the time unit. If metric unit is bound to the speed of light definitive constant, the real physical meaning of metric unit and speed of light constant are fully dependent on the precission and real physical meaning of the time unit. Therefore exactnes of the speed of light constant is not physical exactness, but only numerical exactness. SI definitions should stress such types of "exact" "physical" constants. Speed of light in SI definition is not physical constant, but anthropomorphic constant. One of the metre/time/speed units must not be anthropomorphic, to make valid interconnection between SI unit system and physical reality. Please incorporate these facts to apropriate articles, including this, and if it is possible, initiate apropriate processes. Softvision (talk) 12:27, 25 July 2009 (UTC)

One point relevant to this topic that might be mentioned somewhere in the article is that cat that the speed of light is not a dimensionless constant, thus its numerical value is entirely dependent on the system of units used. Martin Hogbin (talk) 12:30, 25 July 2009 (UTC)


 * Softvision, see my point above. The speed of light is not a dimensionless constant thus its numerical value can be anything that we want depending on how we set up our system of units. If this is what you mean by anthropomorphic constant then you are correct.  The SI system is based on a time standard set by the natural radiation of the caesium atom, and the fundamental property of our spacetime that we know as the speed of light.  From these two they define the unit of length called the metre.


 * Are you familiar with the systems of natural units?  In many of these the speed of light is defined to be 1. Martin Hogbin (talk) 12:47, 25 July 2009 (UTC)


 * Softvision: As noted above, as an example, the wavelength of Ytterbium 2S1/2 (F = 0, mF = 0) – 2F7/2 (F = 3, mF = 0) transition is 466878090.067 fm based upon the measured frequency f=642121496772.3 kHz (with its associated uncertainty) and the relation λ = c/f. The uncertainties of this and other wavelengths are stated at the html link.
 * It seems to me that we've come to agreement (I'm guessing Steve and Martin are on the same page).


 * 1) We agree that the speed of light as a physical entity is not expressed by 299,792,458 m/s. This number is merely a conversion factor from time to length. See Wheeler;Kuokkanen; Sachs; Russell.
 * 2) We agree that measurement of any physical length using, for example, the relation λ = c/f (or ℓ = c t) is subject to the experimental uncertainty in the frequency (or time) measurement
 * 3) We agree that the speed of light as a physical entity will vary according to the medium in which it is measured, and possibly will vary over time, for example, due to changes in the universe. Such matters are a subject of experimental examination, in the form of tests of reproducibility of the speed itself, of the measurement conditions, and establishment of the properties of the medium. (Measurements of variation over time are on-going via monitoring of the fine structure constant using atomic clocks).
 * Do points of disagreement remain? Brews ohare (talk) 13:00, 25 July 2009 (UTC)


 * The current definition of the time unit is : "The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom." The number of 9,192,631,770 periods is not measured, but defined. Intension of this definition is to make SI time unit definition consistent with previous time unit definitions, as best as possible, that means not to invalidate existing time measuring devices. This action is anthropic action, human intension, based on assumption of existing time measuring devices standards. Again integer number. That means, that the time unit is also anthropic. That means, that all three units metre/time/speed are now anthropic and disconnected from the physical reality. The level of the disconnection is given by the precission of last valid physical experiments, and can be exactly quantified. Softvision (talk) 13:17, 25 July 2009 (UTC)

Do you agree with the numbered points above? Is the speed of light issue settled?

The time issue may be a digression. The physical embodiment of time as the frequency of a particular atomic transition is simply a choice of which transition to use, and is a selection of a physical entity (an atomic clock). The relation of the second to this entity by a conversion factor is convention. The issues then resemble those for the speed of light: is the physical embodiment stable, reproducible, dependent upon the environment in which the transition takes place (linewidth), subject to gravitational corrections, constant over long intervals of time, etc. etc.? All these matters are subject to experimental observations. The reproducibility issues lead to the uncertainties attached to any particular realization of an atomic clock. Brews ohare (talk) 13:19, 25 July 2009 (UTC)


 * 1) One of the meter/time/speed units must not be anthropic. If time unit is free parameter, that means open for measurement and validation, the required condition for interconnection of SI units with physical reality is satisfied. In this sense I agree.
 * 2) Uncertainities of measurements are not subject of my submissions. The main subject is interconnection of SI units with physical reality and balanced selection of free parameter - unit - that interconects the SI unit system with physical reality, on the level available by technical resources and scientific knowledge. Uncertainities of measurement have fundamental origin, not only technical. Fundamental uncertainities originate from the quantitative state on fundamental level - scale, and theese quantitative relations are allways components of measuring device and system beeing measured. That does not imply that fundamental interactions are uncertain. The origin of uncomplete determinizm of reality is not in uncertainity of fundamental interactions, but in locality of systems. Both, space and time, manifest locality. The causal speed and 1/r^2 laws represent qantitative expression of this phenomenon.
 * 3) The subject of the causal speed variation is open. Causal speed is the limit of spacetime interaction, and today it is called speed of light. Maximal causal speed is not bound to the speed of light. I do not think that the maximal causal speed varies, in the sense of the fundamental limit, and I have serious reasons for that. Maximal causal speed should be subject of measurements using new and more precize methods, examining various wave lengths. These methods could contain converging processes, in the sense of opening one or other meter/time/speed parameter - unit, but never two of them.


 * Valid experiments and measurements should be based on two anthropic units and one open unit of meter/time/speed units. Interaction, synergy, transformation, are fundamentaly based on space distance and time duration. Space distance can be expressed as time distance using causal speed parameter, but time duration (impulse) cannot be expressed meaningfuly by metric units. There is no way to constitute the meter/time/speed unit system only on one anthropic unit. Constitution of unit system on anthropic time and space units leads to synthetic errors on corresponding scales, that means subscales and hyperscales.


 * Softvision (talk) 15:03, 25 July 2009 (UTC)


 * Brews : Choice is an anthropic action. Choice means free will. Free will is not physical measurement. Physical measurements cannot be based on free will. Physical measurements must fit into unit system and scientific context. Measurement is observation of reality. If I take any stable cyclic system, I can derive a coefficient of one second. Derivation of coefficient is mathematical reexpression - formal operation. That does not mean the time unit derivation is invalid. The time is certainly one of the units, which can be anthropic most of all. Softvision (talk) 15:22, 25 July 2009 (UTC)

Softvision: It appears you are approaching the matter from some philosophical stance. IMO your concerns are met. The entities "speed of light" and "atomic transition" are operational in the real universe and have nothing to do with man's conventions. The suitability of these entities for standards of measurement is subject to experimental observation as to their reproducibility, stability over time, and so forth. These qualities are again a property of nature, not convention. So I'd say we have here two standards, one of time and one of speed, that stand outside of convention, that are monitored for their suitability, and therefore are not suspects in some Eddington-like conspiracy to isolate the physicist from nature. The relation of these entities to the measurement units is, of course, a matter of convention, but that is not a reflection upon the chosen standards themselves. Brews ohare (talk) 16:09, 25 July 2009 (UTC)


 * Concepts "anthropic" and "convention" are equivalent in the context of this discussion. Let us forget about the old conventional metre. Let's try to define space distance unit [sdu] based on natural phenomena of speed of light [sol] and atomic transition time [att]. Let us try not to use any numerical values. Now we can define space distance unit [sdu] as x=ct, that means, sdu=sol*att. In this equation sdu, sol and att does not represent any exact numbers. They represent natural entities or phenomena, and we just know, that they represent distinct physical quantities, distinctly manifested by the natural effects of sol and att. Until now our views must agree. Now let us differentiate between sdu,att and sol. sol is fundamental physical cause, physical phenomenon, probably constant. att is some atomic effect, that means selected by convention, stable, reproducible, and this effect (not the cause of this effect) is independent of sol and sdu. Now, how to enclose the equation in valid manner ? If we assume, that sol is constant, we can define sol as arbitrary integer value = 1. The precission of sdu (d) is the same as precission of att (d), and we have exact value for sol. It seems that everything is fine. But, sol is now $$sol = \frac{1 \left[sdu\right] \pm d \left[sdu\right]}{1 \left[att\right] \pm d \left[att\right]}$$. That means $$sol = \frac{1 \left[sdu\right] - d \left[sdu\right]}{1 \left[att\right] + d \left[att\right]} ... \frac{1 \left[sdu\right] + d \left[sdu\right]}{1 \left[att\right] - d \left[att\right]}$$. This is in contradiction, that sol is exact value. What does this mean ? How to solve this contradiction ? What is the real value of sol ? -- sol is defined, fixed, and that means value of sol is expressing real physical quantity in context of sdu and att. The number of sol in this case acts just like a symbol that represents real physical quantity. Without sdu and att this symbol has no physical meaning. If att is sol independent unit, sdu is open unit, that gives the quantity of sol real meaning. Refinment of sol (physical phenomenon) measurements will result in change of sdu unit, or more exactly - in change of the physical meaning of the sdu unit. This is not philosophical construction. This is manifestation of reality. If SI unit system will preserve exact and fixed constant of the speed of light, and time unit, physical meaning of the metre unit value will change in the future (not definition) according to the new more precise measurements. Theese changes will be probably not significant in the context of one meter scale. If the precission of current speed of light constant value is not sufficient, extended precission of a future measuring devices will create necessity of the physical meter unit value meaning refinment, to preserve (match) the consistency of the space/time/speed of light realities. However, that means, that physical meaning of the speed of light constant will change, and this means, that speed of light - physical phenomenon - was measured. If speed of light is synthetic constant, and time unit is valid, the only way how to implement the new space/time ratio is to change the physical meaning of the metre unit value (the piece of metal). I think it is clear. I have tried to do my best to explain these relations. I do not consider current definitions as a full error. I understand the intension of such units definitions. But I think, it is necessary to make curent definitions of speed of light constant and metre unit more exact and consistent with reality, and to make new more precise measurements of the maximal causal speed. I think, that using convergent processes, it is possible to minimize errors in unit system physical meaning. Current definition of the metre unit can be a little bit confusing, and this may lead to invalid implementation of the metre unit in metric devices or systems. I think, that some basic experimental quantities can be obtained from the GPS and similar systems, by analysing the error cumulation. Valid space-time ratio in context of signal propagation is basic requirement for consistency of apropriate measurements with reality. Softvision (talk) 21:32, 25 July 2009 (UTC)

Brews, Please keep all discussion to this page and do not use the article itself to emphasize any points being made. Martin Hogbin (talk) 09:52, 26 July 2009 (UTC)

According to the current definition of metre, using atomic clock the precission of the metric measurement cannot be better than 2.99 micrometers - c dt=dx => 299792458 m/s * 1E-14 s = 2.99792458 10-6 m. That means, that the precission of current meter unit definition is bound to the precission of the time measuring devices, and today cannot be better than 2.99 micrometers. Using 1GHz sampling osciloscope the precission of the direct metric measurement cannot be better than 29.9 centimenters. This type of measurement is suitable mostly for long distance measurements. When deriving physical meaning of shorter distances (piece of metal) from longer distances, the precission of physical division of longer distance must be considered. Softvision (talk) 12:52, 27 July 2009 (UTC)

The speed of light can be physicaly realized exactly, because it is fundamental, stable, reproducible natural phenomenon, but only under specific conditions - vacuum (maybe others). The time unit and space unit can be physicaly realized only with limited precission, because any physical representation of the space unit and time unit is dependent on the structure of physical representation. This represents the fundamental nature of the space and time phenomena. Space and time are fundamental realities that do not manifest independently as physical objects - spacetime content, but they manifest distinctly in context of content. Light is spacetime content. Light is causal component. Softvision (talk) 13:03, 27 July 2009 (UTC)


 * Softvision, I am not clear what point you are trying to make. This page is about improving this article, which is about the speed of light as understood and used by mainstream science.  It is a fact that, in SI units, the speed of light is fixed by definition.  That is because the organizations that decide such matters have chosen do do things that way.  This article must therefore reflect that fact; whether you or I agree with it is of no importance.


 * Apart from philosophical considerations, one of the main reasons that the metre has been defined the way that it has is that the experts in metrology in the world's various standards organizations have considered it to be the most suitable method for this purpose from the point of view of the precise, stable, and portable realization of the metre. Martin Hogbin (talk) 15:59, 27 July 2009 (UTC)

If :

The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.

And :

The metre is the length of the path travelled by light in vacuum during a time interval of 1⁄299 792 458 of a second.

Than :

The metre is the length of the path travelled by light in vacuum during a time interval of $$\frac{9 192 631 770}{299 792 458}$$ (30,66332...) periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.

Or :

The metre is the $$\frac{9 192 631 770}{299 792 458}$$ (30,66332...) x wavelength of the radiation in vacuum corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.

According to this, meter is defined by atomic radiation in vacuum. The definition of metre unit is distinctly connected to the reality. Error in radiation is proportional to the background noise and transition stability. In this sense the meter unit is defined exactly. Precission of the physical meter implementation is mostly dependent on the technical realization.

The current model of space/time/speed of light definitions in SI unit system is valid.

The main error in my consideration was, that I did not understood the stop of measurement of the speed of light - physical phenomenon. On the other hand, according to the current definition of metre unit, the speed of light is allways measured, when performing the measurements based on current definition, because the speed of light is fundamental component of the measurement. That means, the speed of light is used for meter unit calibration. Whenever the physical calibration of the meter unit changes, this leads to the change of the speed of light physical meaning. In this sense (the speed of light physical meaning) the speed of light can be used for meter unit calibration. From this point of view, the SI speed of light constant is not exact physical quantity. Its physical meaning can change. However it is exact numerical constant consistent with metre definition.

Please incorporate apropriate explanations to the article, to avoid possible misconceptions in the future. Thank you. Softvision (talk) 20:07, 27 July 2009 (UTC)

Speed of light constant and other fundamental constants
I am sorry, and I hope, that this is not a start of some other misunderstanding. But during this discussion I have considered a wide area of relations. According to my conclusions above, the SI speed of light constant is consistent with SI metre unit definition. Now I think, that it would be usefull, to consider also consistency of the speed of light constant with other fundamental constants, that are related to the speed of light constant, but not proportionaly to the metre unit. But this is probably not related to this article. Softvision (talk) 22:45, 27 July 2009 (UTC)