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This article is within the scope of WikiProject Physics, a collaborative effort to improve the coverage of Physics on Misplaced Pages. If you would like to participate, please visit the project page, where you can join the discussion and see a list of open tasks.PhysicsWikipedia:WikiProject PhysicsTemplate:WikiProject Physicsphysics B This article has been rated as B-class on Misplaced Pages's content assessment scale. Top This article has been rated as Top-importance on the project's importance scale.

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Archives

Talk:Speed of light/Archive 1 (Up to end of 2004) Talk:Speed of light/Archive 2 (2005-July 2006) Talk:Speed of light/Archive 3 (July-2006-end of 2006) Talk:Speed of light/Archive 4 (2007)

Muslim Claims

Muslims claim that the speed of light as it is currently known was predicted in the islamic holy book using the following calculation - see video at http://www.speed-light.info but I think its pretty misguided. Can anyone help ? —Preceding unsigned comment added by Basilthehorse (talk • contribs) 22:01, 30 December 2007 (UTC)

The Qur'anic verse 32:5 in the Speed of light video site is translated differently at his Speed of light text site. Unlike the video verse, the English version in the text site clearly refers to a distance in a day. I'm sure that many other translations of the Arabic text are available which may or may not refer to a distance. Additionally, the equation in the video is explained in much more detail in the text site. I assume for the purpose of calculation that his interpretation of the verse is correct—that the speed of light in the Qur'an is 12000 lunar orbits in one day. However, he uses some esoteric assumptions in order to arrive at a figure that agrees with the modern value of the speed of light to seven significant digits. I reject that, preferring to use for distance the length travelled by the Moon in 12000 synodic months and to use one solar day for time, both of which are used by the present Islamic calendar (it uses neither the sidereal day nor the sidereal month).

The average distance to the Moon given in the Explanatory Supplement to the Astronomical Almanac (U.S. Naval Observatory) is 3.844×10 km, so one sidereal lunar orbit is 2π×3.844×10 m = 2.415×10 m. The ratio of the orbital length travelled by the moon in one synodic month is longer than the sidereal lunar orbit via the ratio 29.53059 days in one synodic month to 27.32166 days in one sidereal month. So the length of one synodic lunar orbit is (29.53059/27.32166)×2.415×10 m = 2.611×10 m. 12000 synodic lunar orbits is thus 3.133×10 m. Dividing by 86400 seconds in one solar day yields 3.626×10 m/s for the speed of light in the Qur'an vs the modern value of 2.998×10 m/s.

If it can be shown that the verse really does refer to a distance, interpreting it as the speed of light is just as tenuous or just as firm (depending on your point of view) as the Hindu verse already in the article. — Joe Kress (talk) 09:39, 2 January 2008 (UTC)

why a speed limit

what happens at 186,000 mps that makes the speed of light stop at? —Preceding unsigned comment added by 70.240.146.45 (talk • contribs)

No one knows what happens. DVdm (talk) 10:09, 4 January 2008 (UTC)

That speed is what the universe considers "instantaneous". Consider the photon: it experiences zero time from its emission to its absorption. --BlueNight (talk) 08:14, 9 February 2008 (UTC)

In other words, what happens is that you can be everywhere at the same time by traveling at the speed of light. John (talk) 06:32, 16 February 2008 (UTC)

I'm sorry, but that is false. By your definition, you can be here AND at the sun at the same time if you are going at light speed, which would in reality take about 8 seconds if I do remember correctly. —Preceding unsigned comment added by 72.145.38.159 (talk) 21:14, 22 March 2008 (UTC)

Firstly, please note the talk page guidelines that the talk page is for discussion about the article, not about the topic. Secondly, you're taking into account one aspect of special relativity (the speed limit) without considering the rest of it. The reason that things are "instantaneous" and you are "everywhere" is that as you speed up, your perception of time slows down (everything speeds up around you). At the speed of light, time "stops" (everything around you happens at once). So whilst from our point of view light takes 8 minutes to get from the Sun to the Earth, from the perspective of a light ray it is instantaneous, it it at the Sun and the Earth and everywhere in between at the same time. (Not that light rays necessarily can have a perspective, as as far as they are concerned they do not exist for any time at all.) Stannered (talk) 23:43, 22 March 2008 (UTC)

If you want to continue this discussion, I suggest that you do so on sci.physics.relativity Martin Hogbin (talk) 10:50, 23 March 2008 (UTC)

Overview statement too strong

The overview states that the speed of light may be logically deduced to be constant in any frame of reference from Maxwell's equation and the principle of relativity. Historically and logically this is too strong a statement. The above deduction is only valid if we take Maxwell's equations to be part of the 'laws of physics'. Although this may seem eminently reasonable today I believe that this assumption should be stated explicitly if the statement is to be made in this way.

Alternatively it should be made clear, as Einstein did, that the constant speed of light is a convenient convention. Martin Hogbin (talk) 13:35, 16 January 2008 (UTC)

I'm just wondering what you meant by the last sentence. Could you elaborate? Thanks. MarkWayne (talk) 03:12, 28 April 2008 (UTC)

I would be happy to do so but this is probably not the right place. I suggest either the sci.physics.relativity newsgroup or by private email (martin001@hogbin.org).Martin Hogbin (talk) 21:53, 30 April 2008 (UTC)

Alternative Suggested

As there has been no response to the above, I am going to propose an alternative to the first paragraph:

Experimental evidence has shown that the speed of light is independent of the motion of the source. It has also been confirmed experimentally that the two-way speed of light (for example from a source, to a mirror, and back again) is constant. It is not, however, possible to measure the one-way speed of light (for example from a source to a distant detector) without some convention as to how clocks at the source and receiver should be synchronized(3). Einstein (who was aware of this fact) postulated that the speed of light should be taken as constant in all cases, one-way and two-way.

The speed of light (c) is now viewed as a fundamental physical constant. This postulate, together with the principle of relativity that all inertial frames are equivalent, forms the basis of Einstein's theory of special relativity.

3. Zhang, Yuan Zhong, "Special Relativity and its Experimental Foundations" p171 Martin Hogbin (talk) 19:44, 17 January 2008 (UTC)

Definition of The Speed of Light

The article says "In 1983, the 17th Conférence Générale des Poids et Mesures adopted a standard value, 299,792,458 m/s for the speed of light. This in turn defines the length of a metre in terms of the speed of light". This is not enough to define the length of a metre. We also need the definition of a second (which I believe is defined in terms of Cesium). Does anyone want to add this? 142.162.15.11 (talk) 04:04, 30 January 2008 (UTC)

Done - DVdm (talk) 07:36, 30 January 2008 (UTC)

Suggested deletion from Overview

I suggest that the paragraph beginning, 'Due to special relativity's time dilation...' is deleted. Martin Hogbin (talk) 23:45, 1 February 2008 (UTC)

Yes, no problem, get rid of it - and from the opening phrase of the next paragraph: "To put it another way,". DVdm (talk) 22:03, 2 February 2008 (UTC)

I had several objections to that paragraph, firstly it has no conclusion (such as 'this is why ftl travel is impossible), secondly it breaks up a section on causality, and thirdly it reports just one aspect of a more general phenomenon. I wonder if it is best to delete the paragraph and replace it with something (in a different place) along the lines of: 'The equations of relativity show that, for an object travelling faster than the speed of light, several physical quantities would be not represnted by real numbers. Many physicists take this to indicate that travel faster than light is not possible'?

As for the next paragraph, I think something on light cones should be there (maybe not in the overview) although, with my proposed deletion, the light cone section follows on more logically from the causality bit.

The question is, 'who will be reading the overview and what will they expect to get out of it?' I think the formulae should be moved to a different section.Martin Hogbin (talk) 11:12, 3 February 2008 (UTC)

Go ahead, be bold :-) DVdm (talk) 11:44, 3 February 2008 (UTC)

faster-than-light

I intend to try and reorganise the FTL section into various categories of FTL motion and maybe add a bit. Does anyone have any objections? Martin Hogbin (talk) 20:35, 20 February 2008 (UTC)

I'm pretty sure the "Closing speeds" section is just flat wrong; it ignores special relativity. It should probably be rephrased to specify a frame. Is it speaking about the stationary frame in which in both spaceships are approaching at .8c? If so, that should be clarified. From either ship's perspective, the other's velocity is still less than c. 128.12.119.76 (talk) 09:59, 10 March 2008 (UTC)

I agree - the statement is incorrect. First, both frames are inertial - so the last statement is misleading. And due to SR, the closing speed is still less than c using relativistic velocity addition. If someone disagrees then speak up. Otherwise, I think the section should be removed.PhySusie (talk) 14:50, 10 March 2008 (UTC)

The section is not wrong. We are talking about just one inertial frame, and in that frame the two objects are both measured to be travelling at 0.8c, in opposite directions (if that is not clear then it should be made so). They therefore have a closing speed of 1.6 c in that same frame. Have a look at 'Third Party Observers' in http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/FTL.html#2 Martin Hogbin (talk) 19:41, 10 March 2008 (UTC)

Is the problem with this section that you believe that closing speeds cannot exceed c, or is it that the description given of a closing speed is not clear? You can easily verify for yourself that closing speeds can exceed c from many web and written references. If, on the other hand, you think my description is not clear, I would welcome any help help in rewording the section to make it quite clear what is being described. Martin Hogbin (talk) 09:42, 12 March 2008 (UTC)

Thanks for the clarification - it was not clear that you were talking about a third party observer measuring the closing velocity. It sounded as if the people in the spaceships were measuring approach speeds greater than c. PhySusie (talk) 10:59, 12 March 2008 (UTC)

If it was not clear to you then there is a case for wording it more clearly. Any suggestions? Martin Hogbin (talk) 11:11, 12 March 2008 (UTC)

c or c₀

I note that c has been changed to c₀, in accordance with ISO31, except for the first time it is show. Should some explanatory note be added? Maybe something like, 'ISO31 recommends the symbol c₀ is used for the speed of light in a vacuum and this symbol us used throughout this article, however most older publications use just c and many physicists may continue to do this in cases where there is no ambiguity'.Martin Hogbin (talk) 11:45, 24 February 2008 (UTC)

Your explanation is present as footnote 1. Brews ohare changed all instances of c to c₀, including the first occurrence, and also added that footnote. However, the phrase "c for constant" is a historical usage which must not include a 0. — Joe Kress (talk) 08:36, 26 February 2008 (UTC)

So e = mc₀ now? How common is this? I think c₀ should probably be changed back to just c. 71.102.172.130 (talk) 11:42, 2 March 2008 (UTC)

On Misplaced Pages talk:WikiProject_Physics#Question of Misplaced Pages policy consensus was to switch back to c. I'll be doing this (if someone doesn't beat me) at some point. --Falcorian  00:22, 3 March 2008 (UTC)

Further, we may want to add a note about the adoption of this (i.e. not very wide spread) next to the note saying c_0 is the standard. --Falcorian  00:28, 3 March 2008 (UTC)

I missed the discussion on this as, for some reason, it took place on another page. There is a case for using c₀ on this page because this page is specifically about the speed of light and it is important to make the distinction between its speed in a medium and its speed in a vacuum (see section on headings below) . I agree that this notation is not widely used and that this should be pointed out to the reader, along the lines of my original suggestion. Alternatively, if we agree to use c, I would suggest adding, 'ISO31 recommends the symbol c₀ is used for the speed of light in a vacuum, however older publications use just c and many physicists will continue to do this in cases where there is no ambiguity. In accordance with common practice, c is used throughout this article exclusively to denote the speed of light in a vacuum'. Martin Hogbin (talk) 10:35, 5 March 2008 (UTC)

'Dubious' and 'citation' tags

I have removed the two recent 'dubious' tags but left the 'citation' tags. Perhaps 88.105.78.216 could say why they have marked the items as dubious. —Preceding unsigned comment added by Martin Hogbin (talk • contribs) 09:36, 28 February 2008 (UTC)

Birefringence of vacuum

Re the hypothesis, mentioned via footnote in Speed_of_light#Speed_of_light_set_by_definition -- the article referenced says that there is no experimental evidence for this theory. In any case, if there were, the SI definition would be adjusted to avoid it. For example, if the permittivity of vacuum changes, say, in strong electric fields, the definition would say "in the absence of electric fields" or something like that. You can see this sort of approach at work in the note about "at 0 K" for the definition of the second. Paul Koning (talk) 22:36, 3 March 2008 (UTC)

I agree, the effect has not yet been observed and I am sure that the definition would be modified as suggested if this became necessary . Is there anywhere else the footnote could be put? Martin Hogbin (talk) 22:55, 3 March 2008 (UTC)

I'm not sure if WP:NOR applies. Maybe a way to cover the point is to have the text say that the current definition treats the speed of light in vacuum as constant, so for example it doesn't address the hypothesized birefringence of vacuum (which has not been observed experimentally). In other SI units, the practice for dealing with perturbing factors is to write the definition in a way that excludes them, as is done for example in the definition of the second.

The implication is that, if it came to that, the definition of the meter would do likewise, but it avoids the need to cite an authority for such a claim, which may be hard to come by because it's a hypothetical event.

I'd suggest making it part of the article, not a footnote. Paul Koning (talk) 22:45, 4 March 2008 (UTC)

If it is made part of the article it is better put under 'Other theories concerning the speed of light'. It has no real relevance to the definition of the metre. Martin Hogbin (talk) 22:55, 4 March 2008 (UTC)

I believe that Paul's discussion is the real meat of the hypothetical question, and the real issue is how such an eventuality would be handled. Paul's observations seem completely correct, with the caveat that somewhere long down the road when measurement errors are much smaller than now, a troublesome birefringence or dichroism would simply lead to a revised definition that deals with the issue, for example, by specifying the polarization of light necessary for a standard meter. A few sentences à la Paul would cover the matter very nicely. Brews ohare (talk) 03:55, 10 March 2008 (UTC)

That point applies to any theory that might affect the speed of light. In all cases there would have to be a discussion about how the new effect would be handled regarding the definition of the metre. To put all possibilities under that heading detracts from the important point being made. Martin Hogbin (talk) 09:23, 10 March 2008 (UTC)

True. Perhaps it needs a line or two saying that the current meter definition effectively treats the speed of light in vacuum as a constant, since that is the current state of experimental physics. If at some point it is found that the speed of light in vacuum is affected by experimental conditions (for example, the hypothetical birefringence) then the definition would change to cover that. Paul Koning (talk) 17:04, 10 March 2008 (UTC)

I do not think that it is a good idea to put hypothetical possibilities in the definition of the metre section. If light is found not to travel at c the situation would get complicated. It is really c, the constant of spacetime, that is used in the definition of the metre. I suggest that the vacuum birefringence is put in the 'other theories' section with a note to the effect that, if some of these theories prove to be correct, some aspects of physics and metrology would need to change. Martin Hogbin (talk) 20:21, 10 March 2008 (UTC)

Use of words rather than symbols in headings

Although I agree that it is generally better to use words rather than symbols in headings, in two headings that have recently been changed it is hard to convey the correct meaning without the use of either a symbol or cumbersome punctuation. In the heading 'Things which only appear to travel faster than light' it is important to make clear that it is the speed of light in a vacuum that is being referred to. Changing the heading to 'Things which only appear to travel faster than light in a vacuum' is ambiguous as it is not clear whether it is the things or the light that are in a vacuum. The best way round this problem seems to me to use the symbol 'c' (or c₀). If suitable wording cannot be found, I propose to change the headings back to the use of symbols. Martin Hogbin (talk) 23:19, 4 March 2008 (UTC)

I'd say that a heading doesn't have to say everything. It is simply a guide to help to find what interests the reader, and the text rapidly explains the details. Brews ohare (talk) 03:45, 10 March 2008 (UTC)

Agreed, but the heading should not mislead. Better to use a symbol than to mislead, in my opinion. Martin Hogbin (talk) 09:19, 10 March 2008 (UTC)

'Free space' vs 'vacuum'

The current wording seem cumbersome to me. Should we just change it to 'speed of light in free space'? Alternatively, what about 'vacuum of free space'?Martin Hogbin (talk) 00:01, 9 March 2008 (UTC)

Free space sounds good to me, particularly as there is a very complete article free space. "Vacuum of free space" is more wordy, although often used, and says the same thing. There is no article with that name. Brews ohare (talk) 03:47, 10 March 2008 (UTC)

I would be happy with that, although 'vacuum' is more common when talking about the speed of light, yet 'free space' is much more common when referring to permittivity. Both can be misunderstood but, as you say, there is a good article on 'free space'. Martin Hogbin (talk) 09:19, 10 March 2008 (UTC)

I have changed 'vacuum' to 'free space' in the top paragraph only, to see what people think. I do not think that it is practical or necessary to do this throughout. Martin Hogbin (talk) 13:48, 12 March 2008 (UTC)

c for constant?

I note the comment about the symbol c standing for constant has been removed. The reference cited, however, goes into some detail as to why c does stand for constant. The comment should be reinstated or a more authoritative reference found to support the current text. Martin Hogbin (talk) 11:28, 12 March 2008 (UTC)

Gravity is instant

There are no verifiable sources to this statement in the wikipedia entry. It is true that Newton stated that gravity is instant. Einstein said that the speed of gravity is equal to the speed of light. Here is a link that shows Einstein was correct.

The Speed of Gravity: Einstein Was Right!

The statement "Gravity travels faster than the speed of light. Gravity is instant." should be changed.

196.192.105.155 (talk) 22:50, 19 March 2008 (UTC)

I cannot see the section that you refer to. Martin Hogbin (talk) 23:56, 19 March 2008 (UTC)

I see that it has already been removed. Martin Hogbin (talk) 00:00, 20 March 2008 (UTC)

Laboratory vs Optical measurements

The "Laboratory Measurements" section states: "In 1946, Louis Essen in collaboration with A.C. Gordon-Smith used a microwave cavity of precisely known dimensions to establish the frequency for a variety of normal modes of microwaves—which, in common with all electromagnetic radiation, travels at the speed of light in vacuum. As the wavelength of the modes was known from the geometry of the cavity and from electromagnetic theory, knowledge of the associated frequencies enabled a calculation of the speed of light. Their result, 299,792±3 km/s, was substantially greater than those found by optical techniques, and prompted much controversy."

But the final, and presumably best, "optical method" in the previous section was:"Michelson began his lengthy career by replicating and improving on Foucault's method... The precise measurements yielded a speed of 186,285 miles per second (299,796 kilometres per second)."

Thus, the article seems to be inconsistant: the laboratory-method result was in fact less than the value found by optical techniques, not "substantially greater than". Geoffrey.landis (talk) 19:20, 21 March 2008 (UTC)

Distracting Animation

The animation on the top right showing the light traverse time from Earth to Moon is quite distracting while reading the passage. It should either be removed or should be made a still photograph (animation can be triggered by a click of mouse or hovering of mouse...).Ahirwav (talk) 09:24, 21 April 2008 (UTC)

I agree, it needs a switch on it. Also the timing is wrong except on the very fastest computers. SpinningSpark 23:28, 15 June 2008 (UTC)

speed of light gif

on the page for "sped of light" the gif image says that it is a scale model of earth and the moon and it shows lght traveling between it. the text states that it takes about 1.5 secs althought the pictur take 4-5. either change the picture or the text to make the match (i havent calculated h actual time it would take light to travel that distance so im not sure which on is correct). —Preceding unsigned comment added by 210.8.54.104 (talk) 04:29, 5 May 2008 (UTC)

     That is just an issue with your internet speed.  —Preceding unsigned comment added by 216.228.198.151 (talk) 17:26, 23 January 2009 (UTC)

Speed of light through a vacuum

The third paragraph states:

The speed of light when it passes through a transparent or translucent material medium, like glass or air, is less than its speed in a vacuum.

But the text accompanying the illustration states:

Light traveling through a medium such as air (for example, this laser) travels slower than light through a vacuum.

While these two statements agree, I think the first statement can be more easily misconstrued to mean "less time" which, of course would be the opposite of the intended meaning. I think the second statement is clearer and less ambiguous. Should there be either be increased parity between these two statements, or maybe a restating after the first along the lines of the second? —Preceding unsigned comment added by 69.19.14.26 (talk) 19:00, 6 May 2008 (UTC)

I can't see the problem with the third paragraph. I would have thought that it was clear to anyone who understands speed. Why would anyone think it meant less time? There used to be an illustrative example here. We can reinstate this if anyone else thinks that there is a possibility of misunderstanding. Dbfirs 19:41, 6 May 2008 (UTC)

Optokinetics

I reverted recent edits by Optokinetics because they appear to be a conflict of interest since that editor appears to be either the author or editor of the cited book by H. H. Marks, Introduction to Optokinetics, The amazing speeds of lights. New York, 2008, iUniverse. Furthermore, it posits a controversial theory the the speed of light varies depending on the observing conditions. — Joe Kress (talk) 01:19, 8 May 2008 (UTC)

discrepancy with the german article

I just noticed that the diagram of light going moon -> earth is said to be about 1.3 seconds on the english page but only 1.2 seconds on the german page. Which is which? —Preceding unsigned comment added by 68.229.203.103 (talk) 08:51, 14 May 2008 (UTC)

On average the moon is closer to Germany than to the U.S.

Sorry, couldn't resist. DVdm (talk) 09:37, 14 May 2008 (UTC)

The average distance from the Earth to the Moon is 3.844×10 km according to the Explanatory supplement to the Astronomical Almanac. Dividing by the speed of light 299,792.458 km/s yields 1.282 s. Its extreme perigee is 3.564×10 km while its extreme apogee is 4.067×10 km according to Astronomical algorithms, hence 1.189 s to 1.357 s. Thus 1.2 s would be correct for its perigee while 1⅓ s would be correct for its apogee. I'm changing the caption of the English version to the average, 1.28 s (without changing the graphic itself). — Joe Kress (talk) 07:57, 15 May 2008 (UTC)

Foreshortening and dilation

Observers traveling at large velocities will find that distances and times are distorted in accordance with the Lorentz transforms

has been changed to this;

By observing inertial systems traveling at large velocities in rapport to us, we will find that their distances and times are also distorted in rapport to ours in accordance with the Lorentz transforms

Don't know about anyone else but to my mind that obscures the explanation rather than illuminating (even if it is more accurate). SpinningSpark 17:57, 15 June 2008 (UTC)

I agree: the original was clearer, though we could make it even clearer by combining the two. Dbfirs 19:44, 15 June 2008 (UTC)

I think that it would be best to drop this paragraph from the overview. Length contraction is better covered elsewhere. The Lorentz transformations cannot easily be explained in one sentence. Martin Hogbin (talk) 22:13, 27 June 2008 (UTC)

Interaction with transparent materials

this artical nor any other i can find on wiki explains y light slows down when passing through a transparent medium, nor does it provide a discription of what makes a medium transparent or other wise (219.89.95.106 (talk) 08:45, 26 June 2008 (UTC))will n

This is explained in refractive index#speed of light. However, we do seem to be lacking a mathematical treatment of this phenomena. SpinningSpark 10:38, 28 June 2008 (UTC)

A mathematical treatment of the subject is complex and beyond the scope of the article. Martin Hogbin (talk) 22:13, 28 June 2008 (UTC)

I never said it wasn't complex, nor did I suggest that it belonged in this article. I merely pointed out that Misplaced Pages lacks any mathematical treatment of this, which it certainly should have. SpinningSpark 18:27, 29 June 2008 (UTC)

Accelerated frames of reference and general relativity

Apart from the first sentence, this section does not make much sense and should be reviewed. Martin Hogbin (talk) 22:17, 27 June 2008 (UTC)

I suggest deleting the whole section. Martin Hogbin (talk) 16:00, 29 June 2008 (UTC)

This should not be deleted. It is valid, see for instance Friedmann-Lemaître-Robertson-Walker metric#General metric for a solution of the GM field equations with variable speed of light, also gravitational time dilation#Important things to stress which makes the same point as this article. Possibly it could be cleaned up, but there is no good cause for deleting it. SpinningSpark 18:41, 29 June 2008 (UTC)

I see little connection between the reference you quote and the validity of the article. In relativity the concept of an extended accelerated reference frame is not a good one. How, for example, are the clocks to be synchronized in such a frame?Martin Hogbin (talk) 21:30, 29 June 2008 (UTC)

You don't? Gravitational time dilation in the place linked addresses exactly the question of which clocks are being observed. There is no question of needing to synchronise clocks, the clock used is in the lcoal frame, but it is measuring time between events occuring in a different place and different gravitational field. It does not seem surprising to me that the result of this measurement is a speed other than c. "Extended accelerated frame . . not good" why not? If you are observing from an accelerated frame then the whole universe is accelerated from the point of view of your observations. What fact exactly are you disputing? SpinningSpark 23:06, 29 June 2008 (UTC)

You seem to be confusing an accelerated reference frame with an accelerated observer. An accelerated observer is fine, but a reference frame is a conceptual means to assign space and time coordinates to any event, far or near. In other words there must be clocks at every point in the frame (in principle) that must be synchronized in some way so that a time coordinate can be assign to any distant event.Martin Hogbin (talk) 21:54, 30 June 2008 (UTC)

I propose to delete this section as the first part is a duplicate of a similar statement in the overview and the second part makes little sense.Martin Hogbin (talk) 16:36, 6 July 2008 (UTC)

History section

I notice that the history section in this article is at the end. In many other technical and , for example the ones on the atom and strong interaction, the history section is the first section after the introduction. Why is this article's history section at the end? Oneforlogic (talk) 17:36, 9 July 2008 (UTC)

I think that the history section is better at the end for most scientific and technical articles. Many people will be more interested in the current state of knowledge then the history of a technical subject.Martin Hogbin (talk) 21:48, 9 July 2008 (UTC)

Generally, I'd agree. Is there a policy on this? Oneforlogic (talk) 21:05, 11 July 2008 (UTC)

Medieval and early modern theories

The alleged experiment using a camera obscura showing finite light speed seems somewhat dubious to me. Unless anyone can give an explanation, I propose to delete it.Martin Hogbin (talk) 16:45, 12 July 2008 (UTC)

Some of the theories in this section need to be restated to show that they are theories and not actual experiments (or if in fact the text does refer to actual experiments then details should be given and the items moved to a more appropriate section). Also the comment from a verse in the Rigveda makes claims of undue accuracy and gives no indication of the basis on which the claims were made. I propose to rewrite these sections and would welcome comments from anyone who can give any information about the subject.Martin Hogbin (talk) 22:56, 13 July 2008 (UTC)

The camera obscura experiment that has been reinstated does not prove or demonstrate that light has a finite speed, neither does the intromission theory of vision. This information is therefore not relevant to an article on the speed of light and should be removed.Martin Hogbin (talk) 19:48, 16 July 2008 (UTC)

I consider it extremely unlikely that any measurement of the speed of light could possibly have been made with a camera obscura and I have therefore deleted that sentence. If indeed such a measurement was made, then some details should be given in the appropriate section. References to other experiments with light should not be in this article; maybe they have a place in articles about theories of vision. Martin Hogbin (talk) 19:46, 17 July 2008 (UTC)

Of course the speed of light was never measured using experiments in medieval times, but I think it is important to give the reasons behind why a scientist would claim that the speed of light is finite in the first place. In Alhazen's case, it was on the basis of his intromission theory that made it possible for him to make such a claim. The idea of the speed of light being finite is not compatible with the emission theory but it is compatible with the intromission model, therefore I think it is relevant to this article. Jagged 85 (talk) 22:16, 17 July 2008 (UTC)

Are you claiming that Alhazen was the first to propose the intromission theory of vision (essentially the existence of light)?Martin Hogbin (talk) 08:26, 18 July 2008 (UTC)

Jagged, your response to my edits seems to have been to write much more on the same subject. I do not doubt the good work done by the scientists that you mention but you make far too much of it. The article is about the speed of light but most of what you discuss is the theory of light and vision rather than the speed of light although I do accept that it is necessary for light to exist for it to have a speed. I prefer you latest wording on the subject but I propose to make various edits to reduce the volume of the section and remove some repetition. I will make the edits in sections, with comments so that you can consider each individually.Martin Hogbin (talk) 17:50, 18 July 2008 (UTC)

The second paragraph of this section basically gives a list of scientists who agreed with, or otherwise, with the emission theory of vision which adds little to our understanding of the speed of light. I suggest that it is replaced with a single sentence.Martin Hogbin (talk) 10:41, 23 July 2008 (UTC)

I suggest something like, 'This debate continued in Europe and the Middle East throughout the Middle Ages'.Martin Hogbin (talk) 10:18, 26 July 2008 (UTC)

Jagged has just added more text about the above debate. In my opinion this is too much for this section. There were really only two theories: infinite speed, and finite speed. How much do we need to write on this subject? Martin Hogbin (talk) 20:51, 2 August 2008 (UTC)

Help with citations

Help! I am trying to add some references to the overview but the system demands a title for papers. I do not have titles but I do have all other relevant information. Any suggestions?Martin Hogbin (talk) 19:25, 1 August 2008 (UTC)

Don't use the template. Just put it between ref tags and format it manually. SpinningSpark 20:14, 1 August 2008 (UTC)

Done. Thanks.Martin Hogbin (talk) 20:37, 1 August 2008 (UTC)

Speed of light, new approach

Wouldn't it be more understandable to approach the concept of time and distance, commonly called velocity or speed, recoprocally, say the other way round. In such case we would write s/m, seconds per metre. Causality dictates that the maximum speed cannot be less than 0 seconds per metre. This concept automatically deals with the classical example of two spaceships going in opposite direction, both travelling at almost lightspeed. Their combined speed (relative to each other) would then simply be represented by almost zero seconds per metre, devided by 2. This results in a speed still being less than the maximum speed at which can be travelled. Note that the reciprocal of the classical 300000000 m/s for the speed of light is not apliccable, as the speed of light would be (almost) equal to the ultimate speed of 0 s/m.

Your ultimate 'speed' of 0 s/m represents the reciprocal of what is known as a proper speed (see 'faster than light' section). It is the elapsed proper time of the object considered to be moving divided by the distance travelled in some other reference frame. This cannot be strictly applied to light, but for other things it bears little relation to the way speed is normally understood. Our normal concept of speed is that of a coordinate speed, that is to say, a distance travelled in a particular reference frame divided by the time it takes to do that, measured in the same frame. Martin Hogbin (talk) 11:25, 2 August 2008 (UTC)

Is the sum of 60 hm/h and 40 km/h still 100 km/h? In this approach we would have to write 0.06 s/m and 0.09 s/m. The sum of these velocities would have to be calculated as follows: 1/((1/0.06)+(1/0.09)). The answer being 0.036 s/m. This equals 100 km/h exactly. This only holds for relatively low speeds, as can be concluded from the above.

Alternative concepts of speed have already been considered - have a look at 'rapidity' and 'celerity'.Martin Hogbin (talk) 11:25, 2 August 2008 (UTC)

Jay2U: Jay2U (talk) 10:19, 2 August 2008 (UTC)

Question about reference

Greetings. This article uses the following reference:

P Beckman and P Mandies Radio. Sci 69D (1965) 623.

However, the only place I'm seeing this citation is on wikipedia mirrors. I am finding ghits for the following:

P. Beckman, Radio Sci. 69D, 629 (1965).

Is there a way to confirm which is correct? Thank you.—RJH (talk) 17:07, 20 August 2008 (UTC)

Faster-than-light: observations from a rotating Earth

Is it worth adding that most stars appear to be moving faster than the speed of light when observed from the surface of the Earth. I know it is a non-inertial frame of reference, but if a star is k light years away and its declination is δ then I would have thought its apparent speed is about 365×2π k cos(δ) times the speed of light, usually mcuch more than 1 unless it is above a pole.--Rumping (talk) 13:06, 9 September 2008 (UTC)

Yes, I would say so, perhaps under the heading of measurements made in non-inertial frames, although the subject is rather complicated.Martin Hogbin (talk) 17:07, 9 September 2008 (UTC)

Perhaps something along the lines of:

Non-local measurements of time and distance in non-inertial frames are dependent on the way the frame is defined and are therefore ambiguous. This can result in the observation of faster-than-light speeds. For example, in the the non-inertial frame of the rotating Earth, stars can be measured to be moving round the Earth faster than light. Martin Hogbin (talk) 11:44, 10 September 2008 (UTC)

How long does sunlight take to reach the earth?

As light from the Sun takes 8.4 minutes to travel 1.93 millions miles to Earth. But

E=mc^2, where c is the speed of light in vacuum (where gravity still exist), often expressed distance travel in time. Now gravitational time dilation is the effect of time passing at different rates in regions of different gravitational potential which was tested/ confirmed with difference of nanosecond recorded by atomic clocks at different altitudes.

Sun gravity decreases as we move away from it. Similarly eath's gravity has the same effect on light due to different gravitational potential. This means that speed = distance/time of light is not constant because of the time difference due to time dilation.

Similarly,

Second is currently defined as

http://en.wikipedia.org/Second#Inte...

the duration of 9 19 631 770 periods of the radiation corresponding to the transition between the two hype levels of the ground state of caesium 133 atom.

This definition refer to the caesium atom at rest at a temp of 0 K(absolute zero). The ground state is defined at zero magnetic field. The second thus defined is equivalent to the ephemeies second which was based on astronomical measurements.

From the above time dilation and definition of second, the high magnetic filed and temperature of sun should effect the length (duration) of time ? If yes, then how long does sunlight take to reach the earth? Myktk (talk) 03:23, 19 September 2008 (UTC) Khattak

Since time is defined as what you measure on your clock (at rest with respect to you!), it depends on where you (and your clock) are:

• If you are here on Earth: 8.4 minutes.
• If you are somewhere in the vicinity of Sun: slightly less than 8.4 minutes (due to gravitational time dilation of Sun)
• If you are far away from Sun, Earth, or any other mass: slightly more than 8.4 minutes (due to absence of gravitational time dilation of Earth and Sun)
• If you are moving at 86.6% of lightspeed w.r.t. the Earth/System, very close to 4.2 minutes (due to special relativistic time dilation, ignoring gravitational dilation of Earth and Sun)

DVdm (talk) 10:11, 19 September 2008 (UTC)

My Dear Friend

Time passes or clock is ticking at slower rate at higher altitude (low gravity) than lower altitude (high gravity). Further, acceleration is the rate of change of instantaneous speed. This means that light changes or reduces its speed at every gravitational potential due to time dilation and should decelerating till it diminish. While C^2 (Speed which should not be constant) was used in the equation instead of deceleration.

Similarly I don’t understand how astronauts on the moon were communicating to their colleagues on earth due to time dilation. Note: Present time on moon wrt earth is past. How they able to communicate in their future (earth’s present) which is impossible at any present time.Myktk (talk) 02:10, 29 September 2008 (UTC) Khattak

An article's talk page (see guidelines) is not a place to discuss one's (mis)conceptions and (mis)understandings of the article's subject. It is a place where the content and the form of the article is discussed. Questions like yours are formulated and answered on Usenet forums like for instance sci.physics.relativity on a daily basis. You will meet other people with similar questions and/or motives there - warmly recommendend. I know, I should not have answered your original questions to begin with, and I apologise for that. Good luck on Usenet. DVdm (talk) 13:30, 29 September 2008 (UTC)

You can also post questions on Misplaced Pages's Science Reference Desk where volunteers will try and answer any science related question. SpinningSpark 18:52, 29 September 2008 (UTC)

Time Travel

Does the speed of light make you go back in time? —Preceding unsigned comment added by Kombinatorik (talk • contribs) 03:36, 5 October 2008 (UTC)

No, but this is not the place to discuss this. Please see comments just above your question. Dbfirs 07:58, 5 October 2008 (UTC)

Causality and Relativity.

I am not sure that the lead section has things quite right regarding, causality, relativity, and going faster than light. In my view matter or information cannot travel faster than light because, according to the theory of relativity, that would violate causality and such a violation has never been observed. In particle physics, causality is simply defined in terms of light cones - the cause is whichever happened 'first'. Martin Hogbin (talk) 08:42, 27 October 2008 (UTC)

I agree with what this statement. To me, this seems consistent with the lead's description though

"The principle of causality in physics requires that no matter or information can travel faster than the speed of light. come from the theories of special relativity and general relativity..." Could you be more specific where you're disagreeing in content or emphasis? --Steve (talk) 14:29, 27 October 2008 (UTC)

I guess it is just the way it is put: "The principle of causality in physics requires that no matter or information can travel faster than the speed of light" suggests to me that the principle of causality on its own prevents anything from going faster that light. Perhaps we should say something like "Einstein's theory of relativity together with the principle of causality require that ..." Martin Hogbin (talk) 18:57, 27 October 2008 (UTC)

I agree, that would be an improvement. :-) --Steve (talk) 04:00, 28 October 2008 (UTC)

Overview?

The section heading and the content do not seem to match. Should the section be called something else? Martin Hogbin (talk) 22:42, 21 November 2008 (UTC)

There are a number of areas where the article could use better organization, IMO. The random assortment of information in that section is one symptom of that. --Steve (talk) 05:21, 25 November 2008 (UTC)

I am thinking of deleting the overview section, we already have a lead section, and moving most of the content to a new section called 'Constant speed of light' to be positioned just above 'Speed of light set by definition'. The remainder could be absorbed into the article elsewhere. Any thoughts? Martin Hogbin (talk) 10:29, 7 December 2008 (UTC)

Hard to know precisely what you have in mind, but it sounds to me like it would be an improvement. Certainly the symbol "c" discussion doesn't need to be so early in the article, and would be fine somewhere in the "History" section for example. The diagram and text about space-like and time-like intervals would seem to me to more logically go in the "Physics" section, as would any mention of blueshift and redshift. "Light years" would actually fit pretty well into the text of the "Communications and GPS" section, although the section would have to be re-titled. The thing about one-way and two-way speeds of light is unnecessarily pedantic and should probably be deleted...no one to my knowledge has ever made a serious argument that the one-way and two-way speeds of light should be different, it's like if someone points to a bird and says "that bird is blue" and someone responds, "well, we only know for sure that the side of the bird facing us is blue"! I like the table (obviously since I made it), but it would be fine either at the very top (now that there's no picture there) or in whatever Section 1 ends up being.

The bit about one-way and two-way speed of light is not pedantic but maybe a bit esoteric. The constancy of the two-way speed of light is an experimental fact but that of the one-way speed remains a postulate (axiom or convention if you prefer). The theory of relativity is based on the constancy of light's speed and the speed of light's place as a fundamental constant of nature is related to the theory or relativity. It is therefore an important philosophical point that need to be made clear. Martin Hogbin (talk) 20:40, 7 December 2008 (UTC)

On a different note, I think the "faster than light" section is far longer than it should be, particularly when there's already a main article Faster-than-light dedicated to this. --Steve (talk) 19:53, 7 December 2008 (UTC)

I agree with you although I rewrote much of this section to make clear and categorise the different kinds of FTL phenomena. The problem is, how to reduced it: miss out some of the types, reduce the explanation on each, not have headings and put all as one text paragraph? Maybe just have a very short not to the effect that FTL phenomena exist with a link to the main article? Martin Hogbin (talk) 20:40, 7 December 2008 (UTC)

For one-way vs two-way speed of light, maybe there was a time that it was an axiom, but now it appears to be an experimental fact. See this Phys. Rev. D paper from 1992, which explicitly describes some recent experiments as proving the constancy of the "one-way velocity of light", unlike Michaelson-Morley where the light goes in a closed loop. Unless there's more to the literature than portrayed in that article, or I'm misunderstanding something. :-)

I believe the paper by Will was something of a mistake. This experiment, and several others like it, were re-analysed in 1997 by Zhang who came to the conclusion that the constancy of one-way light speed cannot be confirmed by experiment. His book 'Special Relativity and its Experimental Foundations' is now considered the standard work on the subject. I believe (but cannot present any evidence at this stage) that Will, a very reputable scientist, has since accepted his error. One thing this discussion does prove is the importance of retaining the section under discussion. Martin Hogbin (talk) 09:31, 8 December 2008 (UTC)

For shortening the faster-than-light section, I can't offer any specific ideas right now, but I'll discuss it with you further if I get a chance at some point to take a closer look. :-) --Steve (talk) 01:27, 8 December 2008 (UTC)

I will have go myself some time. Martin Hogbin (talk) 09:31, 8 December 2008 (UTC)

Feet per ns???

Hello, the speed of light is written also in "feet per ns". I ask: what for? I guess there isn't anybody, who can handle with nanoseconds and either not yet has a feeling for the speed of light or isn't able to convert the other values on the fly. If someone still wants to have a more visible description rather choose one based on frequencies (eg 30kHz on 1km loop) or frequency for rounding the earth. I will delete it with respect to this commentary. —Preceding unsigned comment added by 192.116.220.91 (talk) 10:15, 23 November 2008 (UTC)

One foot per nanosecond is a rule of thumb used commonly by electronic engineers and I tend to think that it should be included in the article somewhere. Martin Hogbin (talk) 10:52, 23 November 2008 (UTC)

I'm not an electronic engineer, but I quite liked the "one foot per nanosecond". It has an everyday feel, gives a quick comparison with the speed of sound (that I know in feet per second), and makes a lot more sense than the suggested frequency which I don't understand. Dbfirs 13:19, 23 November 2008 (UTC)

Would you say it would be better where it was, in the info-box, or in the section on communications somewhere? Martin Hogbin (talk) 13:43, 23 November 2008 (UTC)

I am an electronics engineer and confirm that that rule of thumb is used in practice, for cutting cable lengths to match sync pulses from different locations for instance. I think it should stay in the table (why put it somewhere else?) but maybe the table should not be in the first para of the overview - put in "light years" section maybe? I really don't think we need to be strict about using SI units here, we are not writing an exam textbook and measurements in feet are still common in many countries - albeit unofficially. SpinningSpark 16:46, 23 November 2008 (UTC)

Hello, I'm also an electronic engineer but as I'm from Europe continent I never calculate anything feet. If it is common, well, than take it, but anyway I guess, that most people don't have a feeling for nanoseconds. This people which have a use for this approximation, I guess, know it anyway. Others won't use it. --192.116.220.91 (talk) 17:34, 23 November 2008 (UTC)

I'm reverting your deletion because the consensus per the discussion above was to keep it, which I agree with. — Joe Kress (talk) 20:04, 23 November 2008 (UTC)

Thanks. There seems to be a tendency amongst some Europeans (not 192.116.220.91 who has been generous) to purge Misplaced Pages of any units that are not standard SI. Some of us in the off-shore part of the continent still think in Imperial (though we no longer have an empire), so it is good to see that Misplaced Pages caters for all. Dbfirs 20:23, 23 November 2008 (UTC)

I put that chart in a few weeks ago. Before, all those numbers were listed in an endless stream of text in the second paragraph of the article; I figured the table would be an improvement. I kept it near the top because of its importance for the article. I kept the feet per nanosecond figure for the reasons mentioned above; the nanosecond does come up in electrical engineering (EE), physics, and other places, so there are a number of people who have a feel for a nanosecond but haven't gone to the effort of approximating how far light travels in that time. I would be strongly in favor of adding "0.3 meters/nanosecond" into the table, for the many EE people who don't know what a foot is. Does anyone object? --Steve (talk) 00:50, 24 November 2008 (UTC)

Another possibility is to move the feet/ns out of that table and put in this new, second table: --Steve (talk) 03:36, 24 November 2008 (UTC)

(outdent) Yes, I like Steve's table. Just right for the less-technical reader. Dbfirs 08:34, 24 November 2008 (UTC)

Use of the symbol 'c' for the speed of light

As a supporter of the c₀ notation on the basis that it is becoming more and more common in textbooks, and on the basis of international agreement ISO 31-5, and on the basis that every other parameter of free space has a subscript, viz Z₀, μ₀, ε₀, I am opposed to any and all attempts to downplay the fact that Misplaced Pages's position on this matter is an anachronism, sustained because a bunch of traditionalists want to continue a notation whose time has past and whose logic has evaporated. In particular, I oppose the idea of relegating the discussion of symbol c to a remote location. Brews ohare (talk) 01:28, 8 December 2008 (UTC)

I did a survey of the books on my shelf here. Of the 13 books that mention the speed of light, 12 (92%) use "c", and one (8%) uses c_0. (The one that uses c_0 is a contributed chapter to a technical handbook on nanoelectronics; the other 12 include four modern widely-used electromagnetism textbooks, among others.) In such circumstances, I don't think it's appropriate to call Misplaced Pages's use of "c" an "anachronism"; maybe you mean that it "fails to be ahead of its time". Anyway, the spirit of Misplaced Pages's philosophy is not to be ahead of its time, it's to be with its time, for example in how NPOV is applied to downplay newly-emerging scientific theories.

I 100% agree that it would be a better world if everyone used c_0. It would also be a better world if English spelling were rationalized, and better still if we all spoke Esperanto. :-) It might be true that c_0 is becoming "more and more common in textbooks"...it used to be 0%, and now it might even be as high as 8%, although I would guess that that's an overestimate. (Is your experience different?) But that doesn't mean it's "common", at least at this point in time. if it is common in the future, we can change it then. :-) --Steve (talk) 05:50, 8 December 2008 (UTC)

I'd agree that the bulk of texts use c, although I'd say that is because the bulk of texts are just updates of old texts. It's a bit difficult to get an appraisal from Google books. I tried "speed of light" + "SI units" + "c0" and found a few examples, I didn't pursue it very diligently because a lot of weeding has to be done. However, certainly there are over a dozen examples.

I am resigned to waiting until the next decade for Wiki to come up to speed. I just don't want the article to look like it is incognizant of change afoot - that adds inattention to Wiki's other faults. Brews ohare (talk) 06:13, 8 December 2008 (UTC)

What this debate fails to realise is that there's a difference between the use of c₀ (note that the zero is not italicized) and c to refer to the speed of light. The former refers to the speed of light in a vacuum and is a physical constant, the latter refers to the speed of light in a given medium and is a variable. Given that the article has a long section on faster-than-light paradoxes, I would have thought that it would be important to point out the difference… By deciding on it's own particular usage of symbols, the article can't succinctly point out that the ratio of two speeds of light is equal to the refractive index, for example. Physchim62 (talk) 09:53, 8 December 2008 (UTC)

Just to make very clear, the twelve books I was referring to all used "c" specifically as the symbol for the "speed of light in vacuum". The speed of light in materials was explicitly discussed in at least six of these books, always using a different symbol besides "c". In fact, one of these books is entirely dedicated to the subject of light traveling through materials, and even that book very consistently and exclusively uses "c" for speed of light in vacuum.

As we speak, there are many physicists who have never heard of the use of "c_0", and yet are perfectly capable of clearly describing how the speed of light in vacuum compares to the speed of light in materials. There are many other symbols in the alphabet! For example, I just picked up a random textbook off of the shelf at work; it happens to be a widely-used introduction to materials science and engineering. It says "The index of refraction n of a material is defined as the ratio of the velocity in a vacuum c to the velocity in the medium v, or n=c/v." Seems like a pretty clear explanation to me, and that's without using c_0. "c'" is another very common symbol for the speed of light in a medium, and I would bet that far more textbooks and papers use (c' and c) than (c and c_0).

By the way, as much as I'm opposed to using c_0 to denote as the speed of light in vacuum in the article, I think it would even worse to use "c" anywhere in the article to mean something besides the speed of light in vacuum. Almost everyone in the world today assumes that when they see the symbol "c" it means speed of light in vacuum, and we don't want to mislead these readers. --Steve (talk) 20:30, 8 December 2008 (UTC)

I'm afraid that common usage supports Steve here, although there are contrary examples, such as Atomic Structure and Periodicity, Handbook of Physics and Chemistry, Handbook of Physics, Zoological Physics that use c0 for light and c for media, and many more that use c0 and c never comes up, for example, Thermal Physics, New Research on Astrophysics, Neutron Stars and Galaxy Clusters, Noncommutative Structures in Mathematics and Physics, Isodual Theory of Antimatter, Wave Motion, Computer Processing of Remotely Sensed Images.

The above is not an argument. It is a set of repeated jabs intended to soften up the opposition.:-) It also shows that newer books and books by those in engineering and science outside the staid physics community are catching up faster than you might think. Brews ohare (talk) 22:52, 8 December 2008 (UTC)

I hope you're right that c_0 may be gaining more traction. Like I said, I agree that if someday everyone universally used c_0, then the symbols of SI electromagnetism would have slightly more pleasing symmetry. However, I would caution you not to be over-optimistic, the world doesn't always follow an inexorable trend towards more-rational conventions, and c_0 could still fade away again or languish forever in obscurity. The same way that "Imperial units" have won out over SI in US manufacturing, QWERTY over DVORAK in keyboards, English over Esperanto in world-communication, SI over CGS-Gaussian in electromagnetism units for engineers, and so forth. :-) --Steve (talk) 03:48, 9 December 2008 (UTC)

We had this discussion some while back and I thought that we came to a reasonable compromise/consensus. The current version, 'usually denoted by the symbol c₀ or simply c ', is wrong as c₀ is rarely used in current practice. I agree that WP should educate, but it should not enforce. I think that we should work towards a form of wording that accurately reflects the current situation. Martin Hogbin (talk) 09:27, 9 December 2008 (UTC)

I have just had a look at the section on the use of c for the speed of light and it reads like promotional material for the use of c₀. We do not need to mention by name every learned body that recommends its use, or change the words 'speed of light' to c₀. Any persuasion should take place here, not in the article.

I also think that there may be a philosophical objection by some physicists to the use of c₀ as this puts it on a par with c_air for example, whereas one is a fundamental constant of physics and the other is a property of a particular form of condensed matter. Martin Hogbin (talk) 18:43, 9 December 2008 (UTC)

I have removed the long list of learned bodies that recommend the use of c₀. There in no reason to name them all in the article. If you are trying to make a point do it here. Also the reference cited clearly gives 'constant' as one possible reason for the use of 'c'. Martin Hogbin (talk) 23:29, 10 December 2008 (UTC)

Weber was writing in German; if had meant to use an abbreviation for "constant", he would have used 'k' for "konstante", just as Boltzmann did. There is every reason to name the learned bodies which recommend a symbol which, apparently, some editors have never heard of. We could, of course, just follow their learned advice… Physchim62 (talk) 00:08, 11 December 2008 (UTC)

We should of course mention that c₀ is recommended and we do so. Note that I have added the word 'most' to the text. But why should we list them all? Only to make a point. Martin Hogbin (talk) 09:44, 11 December 2008 (UTC)

I haven't been able to find a free copy of Weber's original paper, but allow me to quote from the reference Martin Hogbin find so dear:

So to understand why c became the symbol for the speed of light we now have to find out why Weber used it in his force law. In the paper of 1856 Weber's constant was introduced with these words "and the constant c represents that relative speed, that the electrical masses e and e must have and keep, if they are not to affect each other." So it appears that c originated as a letter standing for "constant" rather than "celeritas". However, it had nothing to do with the constancy of the speed of light until much later.

That's all there is to it, the basis for 'c' as an abbreviation of constant! The next paragraph of the same paper is interesting to this discussion all the same:

Despite this, there could still be some substance to Asimov's claim that c is the initial letter of "celeritas". It is true, after all, that c is also often used for the speed of sound, and it is commonly used as the velocity constant in the wave equation. Furthermore, this usage was around before relativity.

So much for c only ever being used for the speed of light in free space! Physchim62 (talk) 01:09, 11 December 2008 (UTC)

It is not my reference but it was the one quoted, and if you read it it seems to have been based on quite detailed research. Martin Hogbin (talk) 09:44, 11 December 2008 (UTC)

The German version of the cited sentence is "und die Constante c stellt dabei diejenige relative Geschwindigkeit vor, welche die elektrischen Massen e und e' haben und behalten müssen, wenn sie gar nicht mehr auf einander wirken sollen." on page 20 of R. Kohlrausch and W.E. Weber, "Ueber die Elektricitätsmenge, welche bei galvanischen Strömen durch den Querschnitt der Kette fliesst", Annalen der Physik, 99 (1856) 10. So Kohlrausch and Weber did indeed spell "Constante" with a "c" within the phrase "Constante c". — Joe Kress (talk) 04:23, 11 December 2008 (UTC)

I have just reverted a change from c₀ to c. From what I read above there is a consensus to use c. Martin Hogbin (talk) 09:44, 11 December 2008 (UTC)

Here is the discussion that settled on the consensus to not use c₀: Misplaced Pages talk:WikiProject Physics/Archive March 2008#Question of Misplaced Pages policy. The decision of whether c or c₀ should be used affects many many articles besides this one, so if anyone wants to disagree with this consensus, they should bring it back up at Misplaced Pages talk:WikiProject Physics. --Steve (talk) 18:32, 11 December 2008 (UTC)

And you call that consensus? All it says is that WP doesn't have to slavishly follow what the standards organisations do, It says very little about the case in point, ie c₀ vs. c. Instead of addressing that point, you have made ridiculous statements such as "there are many physicists who have never heard of the use of 'c_0'". That is certainly the case, and WP is here to educate them, given that c₀ is certainly used (and for good reasons) whether those people have heard of it or not. Physchim62 (talk) 20:00, 11 December 2008 (UTC)

Yes, my understanding of that conversation was that there was a consensus, albeit not unanimous, that the the speed of light in vacuum should be denoted c not c_0 in Misplaced Pages's many articles on electromagnetism, optics, etc. In particular, at the end of that discussion, this article and a few others were changed from c_0 (to which Brews had recently changed it) back to c. If you don't think that was the proper way for the discussion to have ended, or that it would have ended differently if you had been there to make the case for c_0, then feel free to post on Misplaced Pages talk:WikiProject Physics "I propose that all electromagnetism, optics, relativity, etc. articles that use a symbol the speed of light in vacuum, use c_0 instead of c." You can explain why you believe that would be best, and other people can agree or disagree. If c_0 really is better (and I don't think it is), then there are hundreds of articles that ought to be changed, and I'm sure you don't want to go through all that work yourself, arguing the case each time on each talk page.

Also, I'm perfectly happy for the article to tell readers that c_0 is a less-common symbol for the speed of light in vacuum, as it does now. I'm also perfectly happy for that information to be in a visible location near the top of the article, as it is now. I agree with Martin on the idea that the article shouldn't come across as POV-pushing, for example insinuating that c_0 should be used more commonly than it actually is used. But again, I don't see that as a problem the way it's written right now. --Steve (talk) 21:47, 11 December 2008 (UTC)

Excessive justification

Am I the only person that thinks that the mention by name of four standards bodes that recommend the use of c₀ is excessive and is an attempt to make a point that should be discussed here rather than a useful or desirable part of the article? Martin Hogbin (talk) 19:28, 11 December 2008 (UTC)

It appears so, yes. Given the dispute over terminology, in which you have played your own part, I think it's important that readers know that technical bodies recommend a different symbol than that use in the WP article. It's called giving people information, and letting them decide. Physchim62 (talk) 20:00, 11 December 2008 (UTC)

Of course it is important to let people know that most standards bodied recommend a different symbol, that is exactly what my version says. What is not required is to name several of these bodies in attempt to make a point in the article. Martin Hogbin (talk) 20:04, 11 December 2008 (UTC)

It appears to me that the proponents of c instead of c₀ have only one thing going for them and that is the long time constant of change. As for "excessive justification" - well, apparently no amount of justification is persuasive, regardless of its source or validity. It would never do to provide sufficient info to promote good judgment. Brews ohare (talk) 20:06, 11 December 2008 (UTC)

But I am not talking about here. You are entitled to make your point as strongly and clearly as you like on this page but the argument must not be carried over to the article itself. Martin Hogbin (talk) 20:20, 11 December 2008 (UTC)

You don't seem to have said that in respect of note 1 in the December 8 version, nor the "occasionally" PoV comment in the relevant section of the same version. There are people on this talk page who simply don't want to use c₀: fine, but they should actually find some arguments, rather than just shouting load and edit-warring. Physchim62 (talk) 21:49, 11 December 2008 (UTC)

Fine. I've modified the Speed of light#Use of the symbol 'c' for the speed of light section slightly to clarify just what was previously said there, and to add some example citations. I believe that is not beyond the pall. Brews ohare (talk) 20:30, 11 December 2008 (UTC)

I like it as it is right now, after Brews's changes. :-) --Steve (talk) 21:20, 11 December 2008 (UTC)

I am happy with it too although I have added a sentence to make clear the convention used throughout the article.Martin Hogbin (talk) 22:47, 11 December 2008 (UTC)

No, I don't like it as it is now. Of course c is widely used for the speed of waves of any sort, indeed for speed in general. The use of c for the speed of light in free space is a specific use, and c₀ is always there (and recommended by bodies far more learned than the average WP editor) when there is a risk of confusion. The paragraph should state that c₀ only means one thing, whereas c usually means exactly the same but might, on occasions, mean something different. We shouldn't run away from the dispute because, if we are arguing about it here, it probably means that our readers are confused as well! Physchim62 (talk) 22:03, 11 December 2008 (UTC)

The point you correctly make is made quite clear in the article as it is now. Martin Hogbin (talk) 22:49, 11 December 2008 (UTC)

C for 'constant'

In the section about the origin of c for the speed of light the quoted reference clearly states that c could stand for 'constant'. If anyone wants to change the text they must find a more authoritative reference that says otherwise. Martin Hogbin (talk) 22:41, 11 December 2008 (UTC)

You mean that we must find an authoritative referenec to say that c never' stood for constant, but you wish to continue to ignore the authoritative references that use c₀? and you expect other editors to take you seriously? Physchim62 (talk) 23:30, 11 December 2008 (UTC)

There are in fact tens of thousands of authoritative reference that say that the symbol for the speed of light in vacuum is c. That's why the article is justified in saying that c is the usual symbol for the speed of light in vacuum, but that c_0 is sometimes used instead. --Steve (talk) 05:01, 12 December 2008 (UTC)

My original point was that we should state that c might be considered to stand for 'constant'. The cited reference says, 'Weber apparently meant c to stand for "constant" in his force law...'. We cannot cite a reference and then not put what the reference clearly says. Martin Hogbin (talk) 19:03, 12 December 2008 (UTC)

Physchim62 links should be removed

I am unable to find any relevant info in:

Cohen, E. R.; Giacomo, P. (1987), "Symbols, Units, Nomenclature and Fundamental Constants in Physics (1987 Revision), Document IUPAP-25 (IUPAP–SUNAMCO

and the document

International Organization for Standardization (1993), ISO Standards Handbook: Quantities and units (3rd ed.), Geneva: ISO, ISBN 92-67-10185-4

is listed at Amazon as "out of print" and has no on-line version. The page listing in

Taylor, Barry N.; Thompson, Ambler, ed. (2008), The International System of Units (SI): NIST Special Publication 330, Washington, DC: NIST, p. 45

is incorrect: the only mention of c₀ for speed of light in this document is on p. 11.

I recommend removal of the first two links and correcting the page number on the third. I've implemented these recommendations. Brews ohare (talk) 15:25, 12 December 2008 (UTC)

Ho hum, we'll be looking at mediation soon. The references were taken from an earlier version of the article, when they were linked in with an editorial note. There is no requirement that references be in print, or that they have online versions. However there is a requirement that statement in articles be referenced, which many, many paragraphs of this article fail dismally at the moment. Couldn't you pass your time more constructively? Physchim62 (talk) 16:20, 12 December 2008 (UTC)

Yes, indeed. Couldn't you take the time to provide useful links? Adding garbage links just makes it look like you are using puffery to support your views. Brews ohare (talk) 17:29, 12 December 2008 (UTC)

I've already spent some eight hours or so going through every reference in the article, as per the FAR… If only obsession of other editors is the absence of a subscript zero, so be it: that sort of explains why the article lost its star (not that I think the FA star is worth very much on enwiki, but here you go, the quality of the article is far more important to me). Personally I've asked for mediation on the issue, as the editors involved are now aware. Physchim62 (talk) 19:29, 12 December 2008 (UTC)

Faster-than-light observations and experiments

I am trying to reduce the above section which some consider too long. I have removed the rather vague and unreferenced section on evanescent waves and also one of the two examples of group velocity. Martin Hogbin (talk) 22:31, 13 December 2008 (UTC)

In the section 'Things that really do travel faster than light' there are really only two types of FTL travel: synchronised events, which includes spots and shadows; group velocities, and people shouting; and quantum mechanics, which essentially refers to the EPR paradox. The current, perfectly good, explanation of synchronised events does not have any references. Does anyone know of a good, short, explanation of this effect from a reliable source that could be used here? Martin Hogbin (talk) 10:19, 14 December 2008 (UTC)

I have argued previously that the section referred to above should be entitled,'Things that really do travel faster than c' since this is the only reasonably short and unambiguous way of stating the required concept. I persuaded other editors at the time but I now see the title has been changed, presumably in response to the FA review. Martin Hogbin (talk) 10:26, 14 December 2008 (UTC)

Commas or spaces?

There seems to be an edit war over whether to use commas or spaces in the figure for the speed of light. As far as I can see WP:Mosnum prefers commas. Martin Hogbin (talk) 17:42, 23 December 2008 (UTC)

Hardly an edit war! As for WP:MOSNUM (hardly the most respected or highly regarded of the MOS sites), it specifically says that "In scientific context, thin spaces can also be used": is anyone suggesting that this is not a scientific context, the exact figure for the speed of light in free space in SI units? Physchim62 (talk) 18:28, 23 December 2008 (UTC) Get your facts and terminology straight. It’s not SI “units”, it’s per SI “writing style.” The SI writing style calls for a lot of crap, including a space between the number and the percent symbol, such as 75 % and we flout that guideline too (thankfully). So, your argument seems to be that if it is a “definition”, you want the number delimited differently from all the other values in the article! That makes no technical writing sense at all and makes the article look like just another Misplaced Pages hodgepodge. Greg L (talk) 05:43, 24 December 2008 (UTC)

But you were the one who reverted on the basis of WP:Mosnum. Martin Hogbin (talk) 19:31, 23 December 2008 (UTC)

Not at all, it was the edit I reverted that claimed to have MOSNUM on the editor's side. In any case, one edit, one revert hardly counts as an edit war. Let's hope that, in this season of goodwill to all humans and bots, things stay at that. Physchim62 (talk) 20:14, 23 December 2008 (UTC)

This is what you said: 'erm, no, let's stick to what WP:MOSNUM says rather than Greg's crusade against thinspaces'. Martin Hogbin (talk) 20:55, 23 December 2008 (UTC)

Physchim62 (Martin too), you missed the important issue here. MOSNUM states as follows:

In large numbers (i.e., in numbers greater than or equal to 10,000), commas are generally used to break the sequence every three places left of the decimal point, e.g. "8,274,527". In scientific context, thin spaces can also be used (but using {{{1}}} to prevent line-breaking within numbers), e.g. "8 274 527" (8 274 527, or using the thin space character instead of its HTML entity). Consistency within an article is desirable as always.

There are many, many instances of big numbers in this article—many that are the speed of light in kilometers per second—that are delimited with commas. Here are some of them:

9,300
10,100
1,000 Earth diameters
220,000 kilometres per second
136,000 miles per second
210,000 kilometres per second
298,000 kilometres per second
185,000 miles per second
313,000 kilometres per second
298,000
186,285 miles per second (299,796 kilometres per second)

Consistency within the article is important and yet you, Physchim62, seemed only too anxious to try to make your point by editwarring over one single number in the article without any apparent desire to achieve a consistent practice within the article. Further, if you want to try to say that MOSNUM and MOS guidelines have a preference for not using commas, the above words don’t support that position; Martin Hogbin’s observations are correct.

“Awe, shucky darn! I’ll show Greg L and convert all those other comma-delimited numbers too!” you might say. Gee, I hope you don’t pull that sort of stunt.

And Martin, stop personalizing this issue as one of “Greg's crusade against thinspaces.” You don’t know what the hell you are talking about and come across as only too anxious to turn everything in to a personal attack and bitch-fest on the talk pages. As for your past allegation, Physchim62, that I’ve had a “crusade against thinspaces”, please note that I’ve been the lead proponent of the {{val}} and {{delimitnum}} templates; the main purpose of which is to add thinspaces to the fractional side of the decimal marker so big numbers are easier to parse.

I also recognize the obvious reality that Europeans are extraordinarily accustomed to more than one system of delimiting numbers; Swedes are taught three of them in school—including the American system. Europeans are not generally confused by different methods for delimiting numbers on the integer side of the significand. Most Americans on the other hand, have no familiarity whatsoever with other methods besides comma delimiting; it is pretty much limited to American scientists and academics. It is not the job of an encyclopedia to “educate” readers on these things by confusing them with the very wording employed to communicate on topics. Good writing style is all about making reading fluid and natural and minimizing confusion.

Finally, I appreciate intellectual, uhm… *candor*. I just hate it when editors try to hide behind the apron strings of MOS or MOSNUM policy and beat around the bush in their arguments on talk pages in lame attempts to justify their actions. Like the Bible, a little bit of everything can be dug out of MOS and MOSNUM to justify pretty much anything one does. I would really much prefer it if you would just admit that “I think the SI method of using narrow-space delimiting on both sides of the decimal marker is so superior, I want to slip it into wherever I can on Misplaced Pages—even if doing so produces a retarded hodgepodge of delimiting styles within a single article and is confusing to many readers—in hopes that my efforts on Misplaced Pages’s general-interest articles will slowly educate some of those dumb-bastard Americans.” Though I would agree with your objectives and disagree with your tactics, I would at least respect and appreciate the candor. Greg L (talk) 05:32, 24 December 2008 (UTC)

Greg L, I think you are talking to the wrong person. I was reminding Physchim62 of what he had said. See the history of the article. Martin Hogbin (talk) 09:50, 24 December 2008 (UTC)

• Thank you Martin, I misunderstood. It was my fault and I apologize. Thanks. I’ve struck my main misstatement and revised and expanded the post to clarify. I also moved my in-line response to Physchim62 to its proper spot right after his post, not yours. Greg L (talk) 17:23, 24 December 2008 (UTC)

Some food for thought

Since photons still have mass, the speed of light technically isn't the fastest speed possible. If you were to reduce the mass of a photon you could reach higher speeds, but that would turn a photon into something different, which wouldn't be light. Also, it can generally be said that time is distorted by motion, greater speeds give greater distortions, as proven by the atomic clock experiment with the concord. So, I propose a question- if it were possible to accelerate humans in a rocket to a close enough speed to the speed of light, would it be possible to reduce the journey time of the rocket to a distant section of space by a considerable amount? say, at least 1,000,000 m/s? Theorycrafting like that could cut some costs somewhere. Napalm Flame (talk) 15:35, 24 December 2008 (UTC)

This is not really the place for discussion of speculative suggestions such as yours. Have a look at proper velocity which may answer your question. Martin Hogbin (talk) 15:45, 24 December 2008 (UTC)

• Napalm Flame: The issue is whether photons have rest mass, m₀, which they don’t. If you are noting that photons have energy and since energy and mass are equivalent per e = mc, and from that conclude that photons must have a “mass”, well, no; that isn’t a valid conclusion. Photons get their energy entirely from their frequency; a photon of infinite wavelength (zero frequency), has zero energy. If you want to create some ultra-low-energy photons, rub a balloon on your head to separate some charges and create some ions and quickly wave it back and forth. You will truly broadcast photons with wavelengths of about a hundred-thousand kilometers and a frequency of 3 Hz.

  As for your theory that the perceived journey time of humans in a rocket traveling at relativistic speeds would be reduced, that was the very point Einstein was making when he advanced his Theory of Special relativity. If you want to calculate for yourself the journey time (space ship time) for passengers onboard a craft traveling at relativistic speeds, see Lorentz factor. I’ve still got my spreadsheet for calculating the velocities of particles at different temperatures. It’s got a gamma factor built into it for ultra-high temperatures. (*pause why I go find the thing…*) There: if you want a space ship’s coast, or cruise time of 1000 days to be only one day for its passengers, it must cruise at 299,792,308.103 734 m/s, or 99.99995% the speed of light relative to their departure point, or inertial frame of reference (Earth). Really though, for high-accuracy work, the reference point would be the barycenter of the solar system; what is called Barycentric Coordinate Time (TCB). Greg L (talk) 17:19, 24 December 2008 (UTC)

Loss of Featured Article status

As editors here do doubt know, this used to be a Featured Article but is no more. I was about to ask who here was interested in trying to get that status back but it seems as though the work might have already started.

Having looked at the FA review, it would seem to me that most of the complaints were about fairly minor MOS issues which should not be too hard to fix. I am not an expert on formatting etc but I am glad to help out where I can. I would like to try to make to improvements to the content of the article and may make some bold edits. Please revert these if you think there are negative and we can discuss.

I have added a section below as a FA 'to do' list. Not all the items I have added relate directly to the FA review. Martin Hogbin (talk) 19:02, 24 December 2008 (UTC)

* Suggestion Martin, why not just revert to the version when the article achieved FA status? Then we can all look at what is in today’s version of the article and pick & choose the best additional material to bring forward? Greg L (talk) 19:21, 24 December 2008 (UTC)

• I see now, why that wouldn’t be practical. Never mind. Greg L (talk) 19:49, 24 December 2008 (UTC)

• Well, I’ve done my part for the day. The first step in FA is to have an attractive, topical decorative picture at that top. Not only does it improve the article for our readers, it rallies editors with some additional pride as they wade in to do their part to improve this article to FA status. No one likes an ugly-ass, dry article. That table of speed equivalencies was just atrocious. Why cite equivalent speeds to such low precision(?); no one is going to quote Misplaced Pages when speeds are expressed in scientific notation to a precision of one part in 108.

  While this is arguably a “scientific” article, ‘speed of light’ is clearly also a general-interest article. Editors should endeavor to avoid scientific notation where possible to keep this article accessible to the widest audience.

  Remember too, one uses a non-breaking space between values and unit symbols in numeric equivalencies. If the unit name is spelled out in full, such as 299,792,458 metres per second, then you use a regular space and allow the line-end word-wrap. Otherwise, you can have huge gaps at the end of a line as the entire numeric equivalency drops to the next line down. Using a non-breaking space between the numeric value and the first word of a spelled-out unit of measure is OK if the numeric value and the first word in the unit of measure doesn’t add up to something really long; 299,792,458 metres… is pushing it. Greg L (talk) 20:34, 24 December 2008 (UTC)

I think there might be a copyright problem with the picture, which is why it was removed. Martin Hogbin (talk) 21:05, 24 December 2008 (UTC)

• Interesting. I didn’t know it had been here before. I dug it up using Mayflower and used it on ‘Kilogram’. Regardless, the picture was taken at The Air Force Research Laboratory’s Directed Energy Directorate. Accordingly, the work is in the public domain in the United States because it is a work of the United States Federal Government under the terms of Title 17, Chapter 1, Section 105 of the US Code. You pay taxes, yes? Ergo, you get to benefit from the fact that this picture was made; the Air Force doesn’t collect royalties. Greg L (talk) 21:20, 24 December 2008 (UTC)

Looks like my mistake them. Martin Hogbin (talk) 22:01, 24 December 2008 (UTC)

• No, I don’t think you are mistaken. Looking at old versions of this article, it appears the same picture by a different name had been here and later deleted, probably for the perceived copyright problem you cited. No doubt, this new version was posted by an editor who properly cited its provenance and tagged it with the proper copyright notice. Greg L (talk) 00:50, 26 December 2008 (UTC)

FA 'to do' list

See above. Martin Hogbin (talk) 19:08, 24 December 2008 (UTC)

Rewrite lead section

It should be a summary of the article. Should it have references or should they be later in the main body?

Do something with 'overview'

Maybe it is just the title that is wrong but it is not really an overview, and should the lead section not be this?

Reduce the 'Faster than light' section

I am still not sure of the best way to do this but others have commented that it was too long (I have reduced it a bit).

Table

Does someone know how to specify a parameter to keep the body text from crowding so tight against the left-hand edge of the table? Isn’t there a way to do padding that isn’t visible? Greg L (talk) 21:57, 24 December 2008 (UTC)

P.S. I figured it out. I had to go off-Misplaced Pages to Web Design Group for the help. Specifying parameters of style="padding: 1.2em" FRAME= void did the trick. Misplaced Pages’s Help:Table blows. Someone with better table experience than I should go in and fix this article’s table. I really wanted a 2-pixel border (like this), but now that I’ve got a 1.2-em frame, the new parameter seems to interfere with the border width. What I’ve got at least no longer looks like an abomination. Greg L (talk) 22:46, 24 December 2008 (UTC)

Animation

I have removed the animated picture of light travelling to the moon. I do not think it adds anything as we have a table stating how long it takes light to travel various distances and everyone knows what travelling is. I do not have a particularly strong opinion on this. What do others think?

• A picture is worth a thousand words; an animation, even more. What better way to make the intangible (the universe’s fastest speed), extraordinarily tangible. Further, a visual image like this truly exploits the power of Misplaced Pages to its fullest; no ordinary print encyclopedia can avail itself of this technology. This animation has been in the Moon article for months and months. I can think of no more suitable place on all of Misplaced Pages for this animation than on this article on the speed of light; the two were made for each other. Greg L (talk) 07:24, 25 December 2008 (UTC)

Disambiguation page For 299,792,458?

The speed of light is 299,792,458 M/PS, But when the number 299,792,458 is searched it automatically comes up with this page.

So it should become a disambiguation page.

Thank You,

SaiyanEmperor2008 (talk) 16:06, 27 December 2008 (UTC)

I see no problem with that. The chances of 299,792,458 having any other significance are quite slim. Martin Hogbin (talk) 18:03, 27 December 2008 (UTC)

Deletion of light-years section

Here's the section, as written before its recent deletion:

Astronomical distances are sometimes measured in light-years, which is the distance light travels in one year. A light‑year is precisely equal to 9,460,730,472,580.8 km, or about 5,878,625,373,183.61 statute miles. Since astronomical distances are so great, light requires billions of years to travel across stretches of the universe. Thus, the light that is just now reaching Earth from distant objects in the universe had originally been emitted long ago; an observer on Earth sees distant objects in the universe as they appeared millions or billions of years in the distant past. Light from even objects in our own solar system requires quite a bit of time to reach us. Light from the sun takes around 8 1⁄3 minutes to reach the earth.

Here's the edit in which the section was deleted by User:DVdm: . Here was the edit summary: "Removed section about a *distance* listing examples of *time* scales (as this is an articale about a *speed*). Link to light-year (with many examples) already mentioned in intro".

I vote to reinstate this section. Certainly the section isn't perfect, but its topic is important to the article. The fact that telescopes see things that are millions of years old is a direct consequence of the fact that the speed of light is not instantaneous. I think that in order for a reader to have a good, encyclopedic understanding of the "speed of light", they should understand how it creates the distance-vs-time relationship in astronomy. What do other people think? :-) --Steve (talk) 17:35, 27 December 2008 (UTC)

I would prefer to have something on light-years. Martin Hogbin (talk) 18:04, 27 December 2008 (UTC)

Me too; I'd add mention and a link to parsec as well, as this unit is more common in astronomy. Brews ohare (talk) 19:05, 27 December 2008 (UTC)

Well, i.m.o. this section was very poorly conceived and formulated. It repeated a remark from the introduction ("In astronomy, distances are often measured in light-years, the distance light travels in a year (around ten trillion kilometers)"), and then gave a few examples of times light takes to reach us, in a section about a distance, in an article about a speed. By the way, adding a remark about parsecs would even be more off-topic, as they are 100% unrelated to the subject of the article.

I think the line in the introduction should be largely sufficient in an article about a speed. Anyway, I have taken the liberty to replace the phrase "around ten trillion kilometers" with the precise km-expression of the light-year. DVdm (talk) 19:59, 27 December 2008 (UTC)

I agree that parsecs should not be mentioned, they have nothing to do with the speed of light. Martin Hogbin (talk) 20:34, 27 December 2008 (UTC)

Speed = distance divided by time. Saying that light takes (blank) time to travel (blank) distance is a way of describing the speed of light, no?

Also, see WP:LEAD; the lead section is intended to repeat things that are elaborated elsewhere in the article. --Steve (talk) 22:57, 27 December 2008 (UTC)

Here's a proposed rewrite of this section:

In astronomy

The speed of light is particularly important in astronomy, where the vast distances involved mean that it can take a very long time for light to travel from its source to Earth. For example, it takes 13 billion years for light to travel to Earth from the faraway galaxies viewed in the Hubble Ultra Deep Field images; therefore, those photographs taken today capture images of the galaxies as they appeared 13 billion years ago (near the beginning of the universe). The fact that farther-away objects appear older (due to the finite speed of light) is crucial in astronomy, allowing astronomers to infer the evolution of stars, galaxies, and the universe itself without having to wait billions of years to watch these processes in real-time.

By the same token, astronomical distances are sometimes measured in light-years, the distance light travels in one year. A light‑year is around 10 trillion km, 5 trillion miles, or 0.3 parsecs. The closest star to Earth (besides the sun) is around 4.2 light‑years away.

Thoughts? :-) --Steve (talk) 23:51, 27 December 2008 (UTC)

• I agree with Martin Hogbin and Brews ohare here, Steve. It is too easy to delete stuff. I’ve restored the section and will be editing it to make it better tie to the concept of the finite speed of light. Greg L (talk) 00:14, 28 December 2008 (UTC)

I see the section has been restored. I.m.o. is has become 'much worse than it was:

• The clumsy and naive formulation is not even fit for a children's book:

• The phrase "across vast stretches" appears twice. That is, once as vast and once as fast.
• The phrase "just now reaching" appears twice.
• The phrase "millions and/or billions" appears three times.
• "...the light that is only just now reaching..."
• "...is old; many millions..."
• "The light from most galaxies in the universe that is just now reaching Earth..."
• "Light even from objects..."
• "...quite a bit of time..."
• "...1.255 seconds prior"

... I'm sorry, no offence, but was this meant as a joke?

• There is a link to "Earth was first formed" which has nothing to do with the subject of the article.
• The animation is downright annoying. Lucky thing it can be scrolled out of sight to allow readers to read the text.

I propose we replace the bad section with Steve's suggestion above - and, please waste that disturbing animation. DVdm (talk) 10:52, 28 December 2008 (UTC)

I agree that Steve's suggestion is better than the current section. Martin Hogbin (talk) 11:12, 28 December 2008 (UTC)

I have now done that. I think there is a reasonable consensus. Martin Hogbin (talk) 13:46, 28 December 2008 (UTC)

Yes Martin, much better. Made some additional tweaks to the formulation.

Still wish we could get rid of that animation though ;-) - Cheers, DVdm (talk) 14:09, 28 December 2008 (UTC)

A light year is precisely

I think you're guilty of overprecision here. What year are we talking about? 365 x 24 x 60 x 60 seconds? Sideral year? Headbomb {_{κοντριβς} – WP Physics} 00:41, 28 December 2008 (UTC)

• I assumed our Light-year article is correct, Headbomb. Bad assumption?? If it is not correct, please go correct it there (that should be a battle) before you correct it here. I wanted what is said here to match what is being linked to. Greg L (talk) 01:07, 28 December 2008 (UTC)

  P.S. I was skeptical about Light-year because it used 365.25 days and not the 365.24 days one would use for averaging across a mere century . Plus, there is a higher-precision number used for averaging across 400 years as I recall. I assumed that an overly precise number and false assumptions wouldn’t be so damned glaring in such a high-profile article without a tag being slapped on it. I get bogged down further than I often like when I get diverted trying to correct every single article I’m linking to during an editing session. That’s what happened on Kilogram: I linked to it once, took a look at the piece of shit, and stayed their for a God-damned year! Happy editing. Greg L (talk) 01:14, 28 December 2008 (UTC)

Well turns out the hunt wasn't hard. It's based on the Julian year, aka 365.25 x 24 x 60 x 60 seconds.Headbomb {_{κοντριβς} – WP Physics} 01:42, 28 December 2008 (UTC)

• Good. I see that it is based on this IAU recommendation on units rather than some Wikipedian’s arbitrary choice. Thanks. Greg L (talk) 02:23, 28 December 2008 (UTC)

Let's step back. I for one think that for this article, saying "9,460,730,472,580.8 km" is worse than saying "around 10 trillion km". The latter is easier to quickly read and understand for most readers, not to mention more concise and better at focusing attention on the important aspects of this information. The 5% difference in value offers no qualitative improvement in understanding how fast light travels. And finally the light-year article has all the details. I mean, even a professional astronomer doesn't ever need to know or use nearly this many digits of precision (nothing in astronomy is measured to 14 digits of accuracy!!), to say nothing of the audience for this (only-somewhat-related) article. --Steve (talk) 03:04, 28 December 2008 (UTC)

I agree. The exact value could be shoved in a footnote.Headbomb {_{κοντριβς} – WP Physics} 04:22, 28 December 2008 (UTC)

• I agree with you both. An approximate prose value is better reading. And, given that this is an encyclopedia, the exact value should be provided somewhere. A reference is perfect. Now done. Greg L (talk) 04:50, 28 December 2008 (UTC)
• Me too. Martin Hogbin (talk) 10:19, 28 December 2008 (UTC)

US/UK English

I have just changed all the UK 'travelling' to US 'traveling' as the article seems to mainly use US English, however, I notice that the earliest versions of the article appear to have been written in UK English. We need to chose one and stick to it. Martin Hogbin (talk) 09:32, 13 January 2009 (UTC)

Rash statement

Isn't it a bit bold to say that the speed of light "is the speed of not just visible light, but of all electromagnetic radiation, as well as gravitational waves"? After all, no one has ever detected a gravitational wave.Lestrade (talk) 00:01, 18 January 2009 (UTC)Lestrade

Yes, good point, perhaps we should say something like, '...and is the speed that gravitational waves are expected to travel at'. Martin Hogbin (talk) 00:22, 18 January 2009 (UTC)

I disagree, I think that's injecting doubt when really there is none. There's extremely strong indirect evidence that gravitational waves exist and behave the way we expect them to, and no serious expert in this field (to my knowledge) doubts this. I think the appropriate five-word summary in the introduction is "as well as gravitational waves". If it was a couple sentences in the body of the article, one could be more specific.

After all, it's also true that no one has ever measured EM radiation whose wavelength is one lightyear, let alone its speed, but the article rightly claims that this radiation travels at the speed of light. --Steve (talk) 00:57, 18 January 2009 (UTC)

That was my first thought, but there really is a considerable difference between EM and gravitational waves. EM waves are the most observed phenomenon in existence, and their speed (at several frequencies) has been directly measured. Gravitational waves are predicted by the well-established theory of GR but have only been indirectly observed and there are no direct measurements of their speed.

In the end it boils down to the question of the level of evidence that we choose to required to say 'is' rather than 'is believed to be'. I do not have a strong opinion but suggest that the matter is worthy of some thought. I guess we should look to see how reliable sources put it. Martin Hogbin (talk) 11:29, 18 January 2009 (UTC)

I have looked for some sources. Online, the VIRGO site says,'gravitational waves can travel through space...', and the LIGO site says, 'gravitational waves can travel through space...', but also ,'Actually, after 30 years of active research, we only have an indirect proof of their existence'. I cannot find mention of their speed in either site. Text books, such as 'Gravity' by Hartle, say that 'Linearized gravitational waves ..propagate at the speed of light', but this is clearly in theoretical context. I have found this paper ] which seems to set some experimental constraints on the speed of gravitational waves. Overall, with say the Hartle and the arxiv references, I agree that we can leave it as it is. Martin Hogbin (talk) 13:17, 18 January 2009 (UTC)

Direct detection is so passé. The luminiferous æther was real for a while, why can't gravitational waves and black holes be real for us, too?Lestrade (talk) 01:07, 18 January 2009 (UTC)Lestrade

I concur with those recommending change. Gravitational waves are undetected as yet. Although models suggest they will travel at the speed of light, detectors based on that model have yet to register anything. I don't think it would be out of line to say "...and the speed at which models predict gravitational waves travel." The web link added discusses "epsilon", the amount by which gravity waves vary from the speed of light. The observational data aren't strong enough to fix the value at the speed of light.Novangelis (talk) 14:08, 18 January 2009 (UTC)

It is a difficult call, we should really only say what reliable sources say on the subject rather than state our opinions. As I said above it boils down to what level of confidence is required to say 'is' rather than '...models predict...' bearing in mind that GR is a very well established and experimentally verified model.

I think it might be quite hard to find a really reliable source that literally says 'gravitational waves travel at the speed of light' in a context that is not obviously purely theoretical. I think we should look for one but, if one cannot be found, then we probably should change the wording along the lines that you suggest. Martin Hogbin (talk) 15:05, 18 January 2009 (UTC)

Well argued and researched, Martin. I concur. --Steve (talk) 17:10, 18 January 2009 (UTC)

Joseph H. Taylor and Joel M. Weisberg have observed the binary star system PSR B1913+16 for several decades. They claim that their observations indirectly prove the reality of gravitational waves and are within 0.2 % agreement with the prediction of general relativity. This claim is based on the logical conclusion that the orbits of the two stars should become smaller due to the emission of gravitational waves. Any other possible cause of the orbital decay is not considered. On the basis of this logical conclusion, they claim indirect proof of gravitational wave radiation. The orbits shrunk so there are gravitational waves. The scientific community has seized on this indirect proof and has been content to agree that gravitational waves are real. Due to the effect of the acceleration of the galaxy, Taylor and Weisberg do not think that the results of the test will improve. So, this is as close as we can get to proof of the existence of gravitational waves. Unlike electromagnetic waves, gravitational waves will have to remain a strong belief rather than an observed fact. Many sources of information, however, simply claim the reality of these waves and do not communicate their actual basis to the public. The Misplaced Pages article follows this trend.Lestrade (talk) 16:13, 19 January 2009 (UTC)Lestrade

There is no clear distinction between belief and fact in physics, it is just a matter of degree. The question is whether there is sufficient evidence or not to say that gravitational waves travel at the speed of light. This should be answered not by our own opinions but by reference to reliable sources. If we cannot find a reliable source to directly support the statement that gravitational waves travel at the speed of light then I agree that we should say something weaker. Martin Hogbin (talk) 22:16, 19 January 2009 (UTC)

The sentence "gravitational waves travel at the speed of light" is a categorical, unconditional, unqualified, explicit, declarative assertion. Can such a direct statement be made about something that has, and possible never will be, directly experienced?Lestrade (talk) 12:59, 20 January 2009 (UTC)Lestrade

There is no clear distinction between belief and fact in physics, it is just a matter of degree. There is a rash statement. I'd guess experimental physics people say measurements are facts, and the theoretical people would say that a theory that fits the known experiments is more than a "belief". The real issue here is whether one has to carefully state a theory to explicitly discount all areas where it is yet to be tested. I'd say that is an unlikely stance, particularly as it is hard to imagine every untested case, never mind list them all every time you state the theory. It suffices to state the theory (e.g. general relativity) and simply explore events that might disprove it. On that basis there is no call to invent caution over the speed of gravitational waves, or light waves of parsec wavelengths, that far exceeds any caution expressed in other areas of physics.

The speed of light is of fundamental importance in physics. It is the speed of not just visible light, but of all electromagnetic radiation, as well as gravitational waves and anything having zero rest mass. What has to be done is to rewrite this sentence to avoid the appearance of stating an observed measurement, when it really states a prediction based upon the best available theory. I've made a proposed change based upon this attitude. Brews ohare (talk) 15:15, 20 January 2009 (UTC)

There is a subtle point involved with the speed of light: it is a defined value not an observed one. See the discussion of meter. Brews ohare (talk) 16:23, 20 January 2009 (UTC)

The speed of light is defined, based on observations. All physics is constrained by the measurable speed of light. The meter is defined by the observation of light, not the reverse. The speed of light is not just a concern in theoretical physics. Relativistic aspects related to the very measurable speed of light have numerous applications including computer circuitry and particle colliders. There is no point restricting the scope to the theoretical. Novangelis (talk) 16:39, 20 January 2009 (UTC)

Novangelis: I see a storm brewing here. A bit of care in terminology might be a good idea. For example, the speed of light is "exactly 299,792,458 metres per second (m/s)". Obviously, this being a defined value, nothing can change it except international agreement; it is not changed by measurements. Do you agree? Brews ohare (talk) 18:18, 20 January 2009 (UTC)

A second observation: the word "theoretical" need not be pejorative, inasmuch as the theory agrees with the known observations. The theory is in effect a shorthand that encapsulates the observations. There is no implication that the predictions are "only a theory". Rather, the implication is that all considerations point to the prediction, including the experimental facts. Saying the theory predicts something thus does not imply a "restriction in scope". In fact, a prediction is weightier than "just the facts, ma'am." It implies that critique, correlation and sifting of all pertinent data has been exercised. Brews ohare (talk) 18:23, 20 January 2009 (UTC)

There is no storm brewing. We are both working towards the same goal. We are taking a clunky passage and trying to improve it. The fact that we are debating a few words does not indicate a conflict of goals. I am not suggesting theoretical has any pejorative meaning. Einstein was a theoretical physicist. There is a subdivision of physics between theoretical and experimental, and the use of the word theoretical creates an artificial constraint. More broadly, there is a contrast between the broader use of theoretical (which encompasses both) and applied physics which is also concerned with the speed of light (e.g. GPS). That should not be excluded, either.

We are working in territory that is all about technical definitions, and trying to write towards a lay audience while maintaining accuracy. This is not easy. I don't see any broad problems with what you wrote. It is better than where things started, but there is still vast room for improvement. We need to untangle observation from prediction in a readable manner. I've gone through more revisions with co-authors in the same room. The speed of light in vacuo has been measured over great expanses of the electromagnetic spectrum (technically, I admit, not all). Since photons and gluons are the only observed massless particles, and, to my knowledge, the speed of gluons has not been measured, there are a lot of fine details to cover. The limits to which we have pushed particles with mass provides a measurable asymptote. Since the article describes measurement, the lead should be congruent. The details of observed with speed measured, vs. observed directly but not clocked vs. observed indirectly in particle detectors vs. predicted by a model and inferred from astronomical phenomena is a lot to cover. Expect me to disagree with what you write, but not what you are doing. Keep it up. Novangelis (talk) 19:40, 20 January 2009 (UTC)

Thanks for your conciliatory tone. A useful set of guidelines is Misplaced Pages:Lead section. In particular, it suggests the lead should provide an overview of the article, not just a few points that might interest 80% of the readers. That objective might be a handful here. Brews ohare (talk) 20:31, 20 January 2009 (UTC)

I would say that there is sufficient evidence to say that all EM radiation travels at the speed of light and that this statement is supported by reliable sources. For example Feynman Vol2 18-5 pretty much says it. As always, it possible that one day somebody might discover that this is all wrong but at the moment I think we can state it as fact. On the other hand most reliable sources seem to use more cautious language about gravitational waves. There is no need to make a meal of it in the lead section so I suggest that we write something like, 'It is the speed of not just visible light, but of all electromagnetic radiation and it is also believed to be the speed of gravitational waves and anything having zero rest mass'. We can say more in the body of the article. Martin Hogbin (talk) 22:43, 20 January 2009 (UTC)

c

If c is the fastest speed in the universe, then c must be p r e t t y, p r e t t y fast.Lestrade (talk) 00:06, 18 January 2009 (UTC)Lestrade

c isn't a speed. Speeds have units of distance/time, while c has units of distance/time. --Steve (talk) 05:51, 18 January 2009 (UTC)

Speed is a quantity that has the dimension length/time. Isn't it true that when dimensions of quantity are multiplied by each other, the product is of the same dimension of quantity, squared? For example, three feet times two feet are six square feet, but it is still feet. So, speed times speed is speed squared, but it is still speed. What else could it be that makes sense mathematically and physically?Lestrade (talk) 14:17, 18 January 2009 (UTC)Lestrade

No, that is wrong. The speed of light squared in units where c is 1 is also 1, the same value as the speed itself, but in all other units this is not the case. What exactly is the point that you are trying to make? Martin Hogbin (talk) 15:11, 18 January 2009 (UTC)

So you're saying that six square feet is a distance? Have you ever met someone who's six square feet tall? --Steve (talk) 17:02, 18 January 2009 (UTC)

Height or distance is a one dimensional quantity, measured as a line. A square is a two dimensional quantity, measured as a surface or area. Some people say that c is to be considered as a proportionality or conversion factor. As such, only the absolute number of approximately 34,704,709,264 (if the English miles/second dimensions are used) or 89,875,543,056,250,000 (if the Metric meters/second dimensions are used) would be considered as the factor. In either case, it seems almost silly to say that we know that this number indicates precisely the ratio of energy to a unit of mass. Rather, it probably merely means "a really big quantity."Lestrade (talk) 20:51, 18 January 2009 (UTC)Lestrade

If Joule/amu units are used, then c has the value 0.0000000001492. Not such a big quantity now, eh? :-) --Steve (talk) 06:43, 19 January 2009 (UTC)

Using those units, and assuming the literal truth of the equation e = mc, one part of mass would convert to 0.0000000001492 parts of energy when a uranium nucleus is split. That's not much of a fission bomb.Lestrade (talk) 14:05, 19 January 2009 (UTC)Lestrade

That's right, 1 amu of mass converts to 0.0000000001492 joules of energy. In a big fission bomb, there's ~10000000000000000000000000 amu's of mass that get converted to ~1000000000000000 joules of energy. That's enough energy to vaporize a large city. --Steve (talk) 18:44, 19 January 2009 (UTC)

Proposed reorganisation

The section entitled 'overview' is not really an overview, and anyway this is the function of the lead section. I therefore propose that we merge the 'overview' section with the physics section and make it the first section of the article after the lead.

I also suggest that we remove references from the lead, which should be a brief overview of the article as a whole, and add them to the relevant parts of the body. Martin Hogbin (talk) 10:45, 21 January 2009 (UTC)

By WP:LEAD, the lead should be the intro, and a section called Introduction should not be present. Overview is about the same. There are three paragraphs in Overview; the first belongs in the lead, although the two sentences might be separated. The second belongs somewhere in the Measurement section. Following the Michaelson-Morley experiment seems appropriate. In that case, the third paragraph belongs with it. The lead should conform to verifiability. While references should remain, some of the details should move to the appropriate sections. Double citing may be overkill. That should reduce the density. When Overview is gone, perhaps History should come to the top. After that, Place a Terminology that differentiates in vacuo from in materials. The adoption of c can remain as a subsection. The last subsection of the article "Speed of light set by definition" might follow.

As the first paragraph of the overview says, the speed of light is a fundamental constant. While including everything that relates to it would mushroom it out of control, we need to decide what is incidental and what is fundamental. I think we should introduce the Electromagnetic wave equation into the article. Magnetic permeability and electrical permitivity are fundamental to the speed of light. This article has a cobbled together feel to it. A head to toe reorganization might be called for. Novangelis (talk) 13:00, 21 January 2009 (UTC)

I have just done a proposed rearrangement of the first half of the article, which you can find here. Have a look and see what you think Martin Hogbin (talk) 13:26, 21 January 2009 (UTC)

Martin: I think your lead looks good. I am glad it still has references in it, because there are so many Wiki articles that have unsubstantiated statements in them that I find it reassuring to see some citations indicating that the statements of the main points have some support beyond the whim of the author.

I think it is recommended that that there should not be references in the lead as this should be a summary of the body of article, which is where all the points in the lead are expanded upon and fully referenced. I will have a look at the MOS etc. Martin Hogbin (talk) 22:46, 21 January 2009 (UTC)

Somewhere in the article the various statements bearing upon using a definition for the speed instead of measuring it should be put together and clarified. That is a non-intuitive notion, and has to be made simple. Speed_of_light#Speed_of_light_set_by_definition and the tantalizing statement: "In branches of physics in which the speed of light plays an important part, such as in relativity, it is common to use a system of units known as natural units in which c is 1; thus no symbol for the speed of light is required." could be part of this discussion. Brews ohare (talk) 13:47, 21 January 2009 (UTC)

Very nice. You broke it up into units that can be managed. You introduced Doppler effect, which might serve as a segue to astronomy as we organize, and spacetime, which was mentioned in the lead and appeared nowhere else. A {{main|Doppler effect}} might be in order. Novangelis (talk) 13:55, 21 January 2009 (UTC)

Thanks. We should also add a section on EM radiation as you suggest. Martin Hogbin (talk) 22:46, 21 January 2009 (UTC)

If no one objects I will past the new bit in place. We can then work on improving it. Martin Hogbin (talk) 22:46, 21 January 2009 (UTC)

Light as EM radiation

I have added a section on light as EM radiation. At the end, it says that the speed of EM radiation is independent of frequency. I know this has been confirmed by observations on pulsars but I cannot find a good reference. Any offers? Martin Hogbin (talk) 10:35, 24 January 2009 (UTC)

Speed fixed by definition

Brews, you have raised some points about this subject, but I am not quite sure what you are saying. Are you suggesting that further explanation of this topic is in required in the article? Martin Hogbin (talk) 10:37, 24 January 2009 (UTC)

Yes, I find the notion of defining the speed of light rather than measuring it is counterintuitive. It seems circular without more explanation. Brews ohare (talk) 15:21, 24 January 2009 (UTC)

OK, I will try to write something on the subject. Martin Hogbin (talk) 16:47, 24 January 2009 (UTC)

Speed of gravitational waves

According to general relativity, gravitational waves travel at c, this is well known, as indicated by the reference from Hartle that I gave. There may be other, unconfirmed and not generally accepted theories, in which this is not the case.

The paper by Carlip that you quote refers to a recent attempt to measure the 'speed of gravity', and what is says is that the experiment did not actually measure the speed of gravity but the speed of light - there was some controversy over this at the time. It does not suggest in any way that they are different, just that the particular experiment failed in its objective to actually measure the speed of gravity.

It is true that the speed of gravity has not been confirmed experimentally but the only accepted, and well well-tested, theory of the subject (GR) predicts that it will travel at c. I believe that it is therefore justified to make the stronger statement that I made. Martin Hogbin (talk) 16:27, 24 January 2009 (UTC)

Hi Martin: I will leave you to your own devices. However, I find your overriding of my changes to be a bit muddy-minded, failing to clearly distinguish between observations (which necessarily refer to some physical medium, like outer space) and theoretical predictions, which in the case of classical electromagnetism have two aspects: behavior in free space (a hypothetical reference state, which is unobtainable like absolute zero) and predictions for a particular medium (like outer space) where constitutive relations for the relative permittivities and permeabilities are required. Brews ohare (talk) 16:55, 24 January 2009 (UTC)

There is nothing muddy-minded about my statement that gravitational waves are believed to travel at c. This is a clear prediction of GR, which the currently accepted theory of such matters. Thus 'believed to' is correct in he sense that it is what physicists expect to be the case.

My reason for removing you reference to outer space was as follows: The question is whether all EM radiation travels at the same speed in free space. Experiments have confirmed that a wide frequency range of EM radiation travels at the same speed in outer space. Now it would be startling if this same result did not occur in the theoretical free space. The reverse, of course is not true; a measurement made in free space would not necessarily predict the results of a measurement made in outer space, but the fact that the a speed has been confirmed in outer space very strongly implies that the result is true in free space. Thus the mention of outer space is somewhat superfluous. Martin Hogbin (talk) 17:24, 24 January 2009 (UTC)

I have just had a look and I agree that the statement was not clear as I left it, I hope it is now. Martin Hogbin (talk) 17:30, 24 January 2009 (UTC)

Distinction between theory and observation

Brews, are these quotes from the FA review or your comments? Martin Hogbin (talk) 17:00, 24 January 2009 (UTC)

The article frequently stumbles over whether statements like "all EM radiation travels at speed c" is (i) a definition, or (ii) a physical observation or (iii) a posit of some physical theories (e.g. relativity theory, quantum gravity theory etc.).

Agreed, this is now made clear in the EM section.Martin Hogbin (talk) 17:00, 24 January 2009 (UTC)

The article also frequently fails to point out what medium is under discussion: for example: is it the ideal free space which is the unobtainable reference state where c = c₀, or is it outer space where the speed of light may not be c₀ but possibly any measured value whatsoever?

Agreed also, I have left in your reference in the EM section.Martin Hogbin (talk) 17:00, 24 January 2009 (UTC)

I've changed some of these occurrences, but more are out there. Brews ohare (talk) 16:41, 24 January 2009 (UTC)

Yes, rather than always add 'in free space' everywhere , which may get rather boring and repetitive, we could use c in some cases to avoid that ambiguity. Martin Hogbin (talk) 17:00, 24 January 2009 (UTC)

The statement in the article reads:

According to classical electromagnetism, the speed of electromagnetic radiation in free space is the same for all frequencies. Observations confirm that any variation of the this speed with frequency is extremely small.

Here is the problem: free space is unobtainable; by definition no observation of the speed of light in free space is possible. By definition in free space this speed is c₀ and is (i)independent of frequency (ii)independent of direction (iii) independent of field strength, (iv) independent of polarization. Therefore, it is impossible for observations to "confirm" anything having to do with free space (e.g it's linearity, isotropy or dichroism). All that observations can do is establish to what degree a particular medium (like outer space or the QCD vacuum) behaves in a manner like free space. In particular, observations can determine whether the relative permittivity and permeability of outer space are very nearly unity, and very nearly independent of frequency. Brews ohare (talk) 17:54, 24 January 2009 (UTC)

That is an interesting point. The actual definition says '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', exactly what is meant by light in this context is not clear. However, the 17th CGPM also made the iodine-stabilised helium-neon laser 'a recommended radiation for realising the metre'. So it would seem that it is visible light that they have in mind.

Now if it transpires that there is some small variation in the speed of EM radiation in free space with frequency (and no definition could prevent this), the BIPM would need to make clear exactly what they meant by 'light' in the definition of the metre. They might well choose the iodine-stabilised helium-neon laser but they could in fact choose some other standard, possible based on some notional frequency of EM radiation. This would depend on the theory that was used to explain the speed variation.

At present, however, the standard says 'light' and recommends the use of a particular frequency of visible light so I would asy that it is theoretically possible for gamma rays to travel at a different speed from (visible)light. Martin Hogbin (talk) 18:11, 24 January 2009 (UTC)

Also see my comments in the section above. Martin Hogbin (talk) 18:11, 24 January 2009 (UTC)

Hi Martin: Your comment: "Now if it transpires that there is some small variation in the speed of EM radiation in free space with frequency (and no definition could prevent this)" is, in my opinion, based upon a misconception that measurement can impinge on the defined properties of "vacuum". In practice, any measurement must be in a medium. Hence, according to BIPM, the measurement must be corrected for its non-ideal behavior. Changes in accuracy of the measurement will affect the corrections to be made in this medium in order to refer it to the defined state of "vacuum", but do not change the defined "vacuum". It appears to me that you see free space as something real, while it is only an unobtainable reference state, like absolute zero. Brews ohare

Yes, I understand that free space is an ideal concept, and I agree that the speed of EM radiation in free space is not expected to vary with frequency, but experiment is the final arbiter of all things in physics. If it was found that there was a variation with frequency after all allowances had been made for any medium (such as interstellar space) then physics would have to account for this.

The point is that the experiment you quote does indeed set limits on the variation of the speed of EM radiation in outer space but this in turn sets experimental limits on its variation in free space. Martin Hogbin (talk) 18:41, 24 January 2009 (UTC)

Hi Martin: Our editing session overlapped. Where the "vacuum" of BIPM or "Free space" is concerned, experiment is not an arbiter; rather it is simply a matter of convention and in turn a matter of convenience. Just like any other standard.

In particular, if the meter established in air at two different light frequencies were to differ, the difference would be attributed to the medium "air" and not to the vacuum of free space. If some "universal dispersion correction" showed up in all media, and if the international community decided that making this correction all the time was a nuisance the definition of "vacuum" could be changed by international agreement to incorporate "the dispersion of the vacuum". The properties of the "vacuum" are not a reflection of nature, but of convention. The properties of nature are expressed in permeabilities and permittivities relative to the "vacuum". Instead, a 'standard medium" could be adopted, but that would require tests to see of the standard were met. Apparently, corrections to refer to a "standard medium" is viewed as a more cumbersome approach. Brews ohare (talk) 18:27, 24 January 2009 (UTC)

Of course if the speed of EM radiation were found to vary with frequency in air then the obvious explanation for this would be an effect of the air, and is exactly the case air is dispersive. If any variation of speed with frequency were found in the case of outer space the most likely explanation would be dispersion caused by the interstellar medium, this is not disputed. But, there is nothing in physics which says that the speed of EM radiation in free space cannot possibly vary with frequency. One might say that you can never prove that it does, but you can set limits on how much it doesn't. Martin Hogbin (talk) 18:56, 24 January 2009 (UTC)

Martin: We are a bit at cross purposes here. The idea of setting limits on properties of free space suggests that one can measure its properties. But that is not so. We say a dollar is 100 cents. You cannot observe dollars and say that the dollar is somewhere around 100 cents. Likewise you cannot observe free space and say the speed of light is around c₀. You also cannot measure its linearity, dichroism, dispersion or any other property. By definition, free space exhibits no dispersion, no anisotropy, no nonlinearity and no dichroism. Any measurement in any medium can be compared to this "ideal" vacuum, and may, of course, exhibit a different value for c, dichroism, nonlinearity, anisotropy and whatever. Brews ohare (talk) 19:02, 24 January 2009 (UTC)

The standard refers to light not EM radiation in general. If the standard did just refer to EM radiation the we could have the bizarre situation that the metre varied with frequency! Clearly this is not the object of the standard and such a situation would be absurd. For the moment at least let us take the standard to be The metre is the length of the path travelled by light from a iodine-stabilised helium-neon laser in vacuum during a time interval of 1/299 792 458 of a second. Are you happy to do that just to make discussion easier? Martin Hogbin (talk) 19:12, 24 January 2009 (UTC)

Hi Martin: "we could have the bizarre situation that the metre varied with frequency!" I assume you mean that it could happen that measured meters were different at different frequencies. I don't see that as resulting in a bizarre situation: it would be attributed to the medium in which the measurement was made, and a correction implemented. The selection of a particular frequency is more about getting the second right in the measurement than about dispersion.

The standard refers to a particular frequency, and to a distance in something called "vacuum". However, it is empty as regards identifying whether a "vacuum" is a physically realizable medium, and if it is, just how it might be identified as having been realized. I'd suggest that to insure a good realization of the vacuum is available in the lab (or in outer space), the properties of the lab medium would be measured (isotropy, dispersion, linearity etc.) and the departures from "ideal" would be be used as corrections. According to BIPM, measurements in any medium are "to be corrected for nonideality" according to "good practice". "Good practice" is a moving target, and various corrections have been added over the years, for example, gravitational dilation corrections to the second. The standard also is empty as regards how the measurement using a different frequency might be corrected to refer to the standard.

So where do we go from here? It appears that BIPM is not very helpful. Do you subscribe to free space? Brews ohare (talk) 19:45, 24 January 2009 (UTC)

So what you seem to be saying is that the speed of all frequencies of EM radiation through free space is the same by definition? Martin Hogbin (talk) 10:11, 25 January 2009 (UTC)

Hi Martin: Yes. By definition of "free space", rather than by definition of the speed of light. Brews ohare (talk) 14:09, 25 January 2009 (UTC)

When I look at the article you link to above I get this (my emphasis),'In the reference state of free space, according to Maxwell's equations, electromagnetic waves, such as radio waves and visible light (among other electromagnetic spectrum frequencies) propagate at the defined speed of light'. Is that what you are referring to? Martin Hogbin (talk) 14:55, 25 January 2009 (UTC)

I made some changes to Speed of light#Measurement of the speed of light. See what you think, Martin. Brews ohare (talk) 15:04, 25 January 2009 (UTC)

Firstly, can you confirm that the quote above is what you believe defines the speed of all EM radiation to be c. If not can you pleas refer me to where this definition is made clear.

Regarding your additions, although what you write is generally correct, most of it is, in my opinion, unnecessary. All physical measurements are approximations. When standards laboratories use the speed of light to delineate the metre they take account of all the possible sources of error, including the fact that the medium used is not precisely free space. Any uncertainty in measurement or any necessary corrections will be quoted in the result. There is no reason to pick on one particular potential source of error and not mention all the others. We should assume that when a standards laboratory determines the length of the metre that they do it properly. The medium used may be far from being the greatest source of uncertainty. Do you, for example, know at what pressure measurements are made and what the reduction in the speed of light would be expected at that pressure? Martin Hogbin (talk) 15:24, 25 January 2009 (UTC)

The gist of the changes is not that "All physical measurements are approximations", but to clarify that free space is not a real medium, but an idealization like absolute zero. This status for free space is a source of unending confusion throughout the article. Recognition of the role for free space makes it common sense that one can define the speed of light in free space to be any number you like (of course, the expense of converting to a value like 1 m/s would preclude using a nice round number). Otherwise it seems convoluted. Hence, your remark "There is no reason to pick on one particular potential source of error and not mention all the others" is not germane. It also is wide of the mark in suggesting that only one source of error has been raised: it refers to "all necessary corrections". If the meaning is this unclear, my additions should be rewritten to make the requisite points. Brews ohare (talk) 15:58, 25 January 2009 (UTC)

The sentence Observations confirm that any variation of the this speed with frequency is extremely small. referring to free space and not to a material medium, is indicative of a complete misconception and must be changed. Brews ohare (talk) 16:09, 25 January 2009 (UTC)

You seem to be avoiding the question of where you think that the speed of all frequencies of EM radiation is defined to be the same.

If you look here ] you will see how the refractive index of air varies with pressure. It is within 3 parts in 10^5 of unity at a pressure of 3 in of Hg. A UHV system will get down to around 10^-12 Torr (1 Torr is 1mm of Hg), so you can see that the effect of residual gas is going to be quite small, and that is before any correction is made.

Regarding the point that free space is not an achievable medium but an idealization this is well known and is made quite clear and discussed at some length in the article on the subject; we do not need to repeat it here. Can you explain why, as you put it, 'This status for free space is a source of unending confusion throughout the article'.Martin Hogbin (talk) 16:25, 25 January 2009 (UTC)

The answer to one question is: yes, I do believe all EM radiation travels at speed c in free space. The response to your observations on partial vacuum is: yes, departures from free space behavior may be small in some media. It also appears to be true that experimental error in terrestrial measurements are quite large, and resolving the distinctions between contenders for "best" realization of free space is beyond experiment at the moment. None of that bears upon the logical status of free space as an unobtainable reference state that cannot be measured. Brews ohare (talk) 16:32, 25 January 2009 (UTC)

Speed of all EM radiation is c

Firstly, can you confirm that the quote above is what you believe defines the speed of all EM radiation to be c?

I am unsure what quote is referred to here. However, the speed of all EM radiation in free space is c by definition, because, by definition, free space has no dispersion, no dichroism, no nonlinearity, and no anisotropy. The only issue in any actual measurement is whether it actually was done in a medium that approximates "free space", and just what corrections have to be made. Present theory on the QCD vacuum and the quantum vacuum suggest that realization of one of these vacuums is not tantamount to realization of free space, as these vaccua in theory are not isotropic, do exhibit dichroism and are nonlinear at large field strengths.Brews ohare (talk) 16:26, 25 January 2009 (UTC)

The statement I was referring to is from the article on free space where it says , 'In the reference state of free space, according to Maxwell's equations, electromagnetic waves, such as radio waves and visible light (among other electromagnetic spectrum frequencies) propagate at the defined speed of light'.

The speed of light is defined to be a certain value by international agreement, thus it is fixed by definition. Your statement is not a recognized definition but it is your opinion, based on what you think free space is. The two are not the same. Martin Hogbin (talk) 16:42, 25 January 2009 (UTC)

The speed of all EM radiation in free space is c by definition, because, by definition, free space has no dispersion, no dichroism, no nonlinearity, and no anisotropy. The proof of these statements is the defined values of electric constant, magnetic constant, impedance of free space and speed of light in free space. All these properties of free space by definition are dispersionless, field independent, and scalars (not tensors). If they are plugged into Maxwell's equations, they result in dispersionless, isotropic, field-independent propagation of EM waves in free space.

Of course, every realizable medium, be it outer space, partial terrestrial vacuum, QCD vacuum, or whatever, has a permittivity and permeability that can be measured to some level of accuracy and will exhibit (to a degree determined theoretically and/or experimentally) anisotropy, dichroism, nonlinearity and dispersion. Brews ohare (talk) 16:49, 25 January 2009 (UTC)

That is not what by definition means, you are talking about a calculation using Maxwell's equations. I fully agree that to the best of our knowledge Maxwell's equations are accurate (on a macroscopic scale) and no deviations from them have been observed, but, this does not mean that such deviations, and in particular a variation in the speed of light with frequency, could never be found to exist. Only experiment will tell. The speed of all frequencies of EM radiation is certainly not fixed by definition. Martin Hogbin (talk) 17:07, 25 January 2009 (UTC)

Martin: If you have the permittivity and permeability of free space established by definition, then Maxwell's equations inevitably lead to the dispersionless propagation of EM waves. So it appears that your quibble amounts to the suggestion that Maxwell's equations themselves may be incorrect. A modified Maxwell theory might predict that a free space with dispersionless isotropic field-independent permittivity and permeability does exhibit dispersion. I'm not sure that this hypothetical case clarifies anything. The status of free space as an unmeasurable ideal state remains unimpaired. The establishment of the correctness of the "new, modified" Maxwell theory would be based on observations in real media, not by observations of free space. Statements like Observations confirm that any variation of the this speed with frequency is extremely small. suggesting that experiment can be applied to free space should be purged. The most that can be said is that media exist (maybe partial vacuum or outer space) where the dispersion is very small. Brews ohare (talk) 17:16, 25 January 2009 (UTC)

What the hypothetical case I described does establish is that the the speed of all frequencies of EM radiation is not fixed by definition. It is however fixed according to our current best theory, but that is not the same thing. I am not sure how I can make this any clearer. Once we have established this point, we can go on to discuss the content of the article. Martin Hogbin (talk) 00:00, 26 January 2009 (UTC)

Granted; not by definition; also, "speed of all frequencies of EM radiation" is meaningless without stating whether "free space" is meant, or some real medium. If "free space" is meant, then experimental observation is impossible and irrelevant. Brews ohare (talk) 00:46, 26 January 2009 (UTC)

When I've seen the term "free space", it usually means the limit of a better and better physical vacuum. Obviously experiments can shed light on this. You seem to be using the term differently. What would you call the limit of a better and better physical vacuum? Whatever you call it, that's what we should be talking about, since that's what's actually relevant to our universe. :-) --Steve (talk) 06:09, 26 January 2009 (UTC)

Brews, as Steve has said, free space can be considered as a limit of a physical vacuum. As far as we can tell, the effects of a gas on EM radiation reduce with pressure, as would be expected. Based on the data I presented earlier, on refractive index vs pressure, the effect of the medium of outer space on the speed of light would be expected to be very small indeed. Experiment has shown that the speed of EM radiation in the medium of outer space is not affected by its frequency within the measurement uncertainty of the experiment. From everything we know about EM radiation, from both theory and experiment, we would expect any variation of speed with frequency to be less in the medium of ideal free space that it is in the medium of outer space. Thus it is justifiable to say that experimental evidence sets limits on the variation of speed with frequency in free space.

Of course it is always possible to imagine that in true free space something very strange would happen to light and the extrapolation from increasingly better physical vacua would break down, but physics is full of idealisations and, unless there is good reason not to, it is usual to assume that the ideal case can be calculated an extrapolation of real experiments. Martin Hogbin (talk) 09:38, 26 January 2009 (UTC)

"Free space" can be viewed a limiting case, I agree. It can be approached by some real media, but it cannot be reached. Theoretically, there is no "vacuum" with its properties. Experimentally, we can establish a medium resembles free space within experimental error, but of course there is always the possibility that further refinements in technique will reveal differences.

The EM properties of free space are defined by ε₀ and μ₀ (or by some pair of these two and Z₀ and c₀). These values constitute a reference case. It might be possible that a real medium comes close to the reference. Unfortunately, because of experimental error, it never can be established that any real medium is free space, only that it is close to free space.

That is why the logical impossibility of "measuring" free space has to be recognized. All we can do is measure a real medium and point out that it is as close to "free space" as we can tell within our experimental limitations.

Therefore, no statement in the article should suggest that we can measure free space. Nor, can we say measurement puts "limits" on the free space parameters: they are defined; they are not measured. They could be totally different, they could be tensors, etc etc. There is no underlying philosophical "meaning" to free space having the parameters it does. The values of ε₀, μ₀, Z₀ and c₀ are simply historical accidents.

We could choose instead a real standard medium like air that could be realized. That would not prove to be an advantage, because we could never be sure that our reference sample of air was exactly the same as the standard. We'd have to maintain a standard air sample for comparison and reassure ourselves that it was not changing with time because of improper storage, etc.

So choosing an arbitrary ideal reference is easier, especially if we can maintain a vigilant list of "best practices" to relate any measurement in real media to the standard. Hopefully, the standard has the merit that these "best practices" are not too difficult to apply in practice. (You tell me if preparation of an atomic clock is simple?) The BIPM and its associates decide by international agreement whether the reference is appropriate, and will change it if there appears to be a simpler or a more accurate standard that would make for "best practices" that were easier to use, or accessible to greater accuracy experimentally.

A statement like: we would expect any variation of speed with frequency to be less in the medium of ideal free space that it is in the medium of outer space. Thus it is justifiable to say that experimental evidence sets limits on the variation of speed with frequency in free space. is misleading: given Maxwell's equations we know that "free space" has absolutely zero dispersion. And we know that the validity of Maxwell's equations has nothing to do with the values of the parameters in free space. Maxwell's equations are established by measurements in the real world, on real media. Whatever real media behave like has no bearing upon the parameters of free space, neither upon the actual values nor their scalar dispersionless nature.

We could imagine a hypothetical Universe where every real medium we measured exhibited dispersion. These physical facts need not impact the parameters of free space. It might be decided that reference to free space was still the simplest methodology. Or, the BIPM might decide that free space would be modified. That is a decision based upon practicality and politics.

Again: because of its defined properties, no statement in the article should suggest that we can measure free space or that experiment "constrains" the parameters of free space to have particular values, or to be scalars, or to be dispersion free, or … .Brews ohare (talk) 13:00, 26 January 2009 (UTC)

Imagine: scientists discover a tiny dispersion in some yet-unexplored frequency range, and find that the better they make their vacuum, the strength of the dispersion stays constant. Someone comes up with a theory where Maxwell's equations have a small modification where the modified Maxwell's equations have a dispersion in an perfect vacuum, and further experiments confirm these modified equations until it is universally accepted by scientists. I would describe this as "it turns out there's dispersion in free space". How would you describe it? --Steve (talk) 15:59, 26 January 2009 (UTC)

The scientists' "vacuum" exhibits dispersion in a certain frequency range. Is this "vacuum" like free space? No, definitely not, because free space has no dispersion at any frequency (according to Maxwell's equations as we know them, coupled with ε₀, μ₀, Z₀ and c₀). Does that mean Maxwell's equations are wrong, or does it mean that this "vacuum" has some interesting properties (apparently previously unknown), and are these properties shared by other candidates for "vacuum"?? A great deal of thought will go into settling the matter.

Will this thought include modification of the properties of free space? No. When the dust settles, it might be decided that today's free space is more cumbersome to use than necessary (given the new version of Maxwell's equations), and should be changed. Or, it may be decided, like the decision to stick with electrons having negative charge instead of positive, that the change is just more trouble than it's worth. Or, it may be decided that "vacuum" as prepared by these scientists has led to discovery of some new fundamental particle, the "vacuum polaron"(?) that requires a new "best practices" correction when the metre is measured in this frequency range in the presence of "vacuum polarons".

In fact, a new "best practices" correction is exactly what will happen should experimental accuracy advance to the point that the properties of the quantum vacuum can be observed. In the case of the second, the addition of new "best practices" has included the addition of the caution that the clock has to be corrected to account for gravitational time dilation and to account for temperatures above T=0 kelvins. These corrections became a requirement as advances in technique made them necessary. Brews ohare (talk) 17:46, 26 January 2009 (UTC)

Brews, I am having increasing difficulty in understanding your problem with free space. It is an unobtainable ideal just like many other things in physics. As you say above, the time standard refers to caesium atoms at absolute zero - this is known to be impossible. School physics questions often referred to massless and frictionless pulleys - they do not exist. Free space is no different, it a conceptual medium whose properties we choose to extrapolate from real media. Martin Hogbin (talk) 18:27, 26 January 2009 (UTC)

Martin: I have no trouble with free space. I have trouble with statements in the article that suggest measurements have something to do with free space. Measurements are corrected to refer to free space, but do not impinge upon the defined properties of free space. Brews ohare (talk) 18:29, 26 January 2009 (UTC)

It is no different from the examples I gave above. Although they do not actually exist we can ascribe properties to massless pulleys and we can determine what we expect those properties to be from measurements made using real pulleys. Martin Hogbin (talk) 18:41, 26 January 2009 (UTC)

Inasmuch as your thinking has evolved to the point that these distinctions appear obvious, I have modified the article accordingly. Brews ohare (talk) 18:57, 26 January 2009 (UTC)

There has been no evolution in my thinking and, it would seem , there has been none in yours. Real measurements can be used to set limits for free space and the article should indicate this. I believe that your modifications should be reverted, but let us wait to see what others say. Martin Hogbin (talk) 19:12, 26 January 2009 (UTC)

Thank you, Martin. Brews ohare (talk) 19:30, 26 January 2009 (UTC)

Brews, you need to decide whether: (1) Free space is the extrapolated limit of a better and better vacuum, analogous to absolute zero, frictionless tabletops, etc.; or (2) Free space is where Maxwell's equations have (blah) form. You can't have it both ways, because (1) can be investigated by careful experimental and theoretical physics, while (2) cannot. You evidently have chosen (2): When I proposed a hypothetical case where (1) and (2) disagree, you responded by calling (2) free space and calling (1) a "vacuum" in scare-quotes. You agree so far? Questions: Is there literature support for saying that (1) is not actually the true definition of free space? In the context of this part of the article, I believe it makes more sense to discuss (1) than (2), since (1) is the basis for statements about the universe in which we live while (2) is not. Supposing that we wanted to discuss (1), what would you propose calling it? "Vacuum" in scare-quotes isn't a good option. :-) --Steve (talk) 19:44, 26 January 2009 (UTC)

Hi Steve: An interesting formulation of the discussion. Under (2), Maxwell's equation can be written for any medium: you just need the constitutive equations. The relevance of free space under (2) is that the constitutive equations are simply specification of ε₀ and μ₀. Evidently this specification does not need to apply to any real material (although it might). Free space is just this hypothetical material. As regards (1) there may be a sequence of steps (e.g. evacuation of a flask) that cause ε → ε₀ and μ → μ₀). Such a sequence might suggest free space is most nearly to be realized by pursuing this sequence. But so what? You can measure c in any medium and make corrections so your results are referred to free space, whether or not media approximating free space actually exist. There is zero information in free space and its constants.

Is there literature support for saying that is not actually the true definition of free space? I'd say the NIST web site where ε₀ and μ₀ a stated as defined values is the true definition, not the possibility of some limiting sequence. Maybe there is some historical commentary on this point, but it has nothing to do with our final definition. And, just to be nasty, is there any literature supporting the limiting sequence idea?

In the context of this part of the article, I believe it makes more sense to discuss (1) than (2), since (1) is the basis for statements about the universe. Free space is not about the universe. Measurements of real constitutive relations for real media, like outer space or terrestrial vacuum, is about the real universe, quite independent of what we adopt for free space.

Historically, the yard may be related to the arm-length of monarchs and the metre to the dimensions of the planet Earth, but neither has any relation to the Universe as observed in science. Brews ohare (talk) 20:40, 26 January 2009 (UTC)

To put things differently: there is a continuing effort experimentally to determine if the speed of light in space is isotropic, and to see if it varies with frequency. Evidently, Maxwell's equations using ε₀ and μ₀ predict no success. Would a success prove that space is not a good realization of "free space", but a medium with its own constitutive equations, or would it prove Maxwell's equations are wrong, or something else (maybe some weird aspects of general relativity: varying speed of light theories)? None of these three categories of conclusion would change ε₀ and μ₀. Brews ohare (talk) 20:57, 26 January 2009 (UTC)

You say: "Free space is not about the universe. Measurements of real constitutive relations for real media, like outer space or terrestrial vacuum, is about the real universe, quite independent of what we adopt for free space." Precisely. A statement about free space (as you understand the term) is not a statement about our universe or its laws. So why would we ever make a statement about free space in this article? I say we replace "free space" with "vacuum" everywhere in the article.

Also, here's a question: If you really believe NIST thinks of free space as something other than the limit of a better and better vacuum, then how do you propose that a careful scientist checks that her meter-stick is the right length? After all, if the meter is defined in relation to the speed of light in free space, and no real-universe measurement could ever give information regarding the speed of light in free space, then there isn't any way to confirm that a meter isn't the size of an atom, right? --Steve (talk) 22:01, 26 January 2009 (UTC)

Hi Steve: Changing terms to "vacuum" we still have c_O as the speed of light in a hypothetical medium. Only now "vacuum" is vague and subject to misinterpretation as a "real" medium.

if the meter is defined in relation to the speed of light in free space, and no real-universe measurement could ever give information regarding the speed of light in free space, then there isn't any way to confirm that a meter isn't the size of an atom

The first condition is met: the meter is defined in relation to the speed of light in free space

The second condition is met: no real-universe measurement could ever give information regarding the speed of light in free space

The speed of light in free space is c_O.

The conclusion is false: there isn't any way to confirm that a meter isn't the size of an atom

Here is how it could be done. Measure the meter in air, assuming the definition applies in air, not free space. Measure the refractive index of the air. Adjust the size of your meter according to the index. Brews ohare (talk) 22:46, 26 January 2009 (UTC)

The article you link talks about measuring the index of refraction of air using an interference refractometer. I've used one of these before. The way it works is you measure the ratio of the index of refraction of air, on the one hand, to the (literally) extrapolated limit of a better and better vacuum, on the other. Not the ratio to free space. --Steve (talk) 00:35, 27 January 2009 (UTC)

Hi Steve: The issue here is the usual one that occurs in setting up a standard, namely the "accumulated wisdom" effect. For example, how do we determine that one clock is better than another clock? It is partly accumulated experience on how well supposedly identical clocks agree, and partly a theoretical assessment as to what influences might disturb the clocks and how big these disturbances are expected to be. When experience shows the clocks don't all agree, we look around and find that it is gravitational dilation, an effect within theoretical estimation, one we should have seen coming, but one that simply was overlooked. Or, maybe its something else.

As technique improves, discrepancies in indices will show up. Sample vacuums that are supposedly identical turn out to be different. In accounting for the difference, maybe some of the observed discrepancy is due to quantum fluctuations of the vacuum, or whatever. In conjunction with theory we decide what these contributions are, learn how to avoid them, or account for their effects in the sample vacuums, and subtract them from measurements to get the "free space" value.

In dealing with air, present practice seems to be based upon some formulas that people feel have been well tested against different air samples. You just measure the partial pressures of water, CO₂ etc. in your sample and calculate the index for your case. Brews ohare (talk) 03:30, 27 January 2009 (UTC)

The accumulated wisdom of all the metrology experiments measuring a meter moves towards the goal of defining a meter based on what the speed of light is in an extrapolated better and better vacuum. This isn't always a direct measurement, for example as you suggest you can first measure the meter-using-air and then multiply it by the measured quantity (n_air/n_{extrapolated better and better vacuum}). But the extrapolated-better-and-better-vacuum is what's being measured, either way. On the other hand, there's no accumulated wisdom whatsoever for measuring a meter based on the speed of light in "free space" (as you define it), because no aspect of "free space" is measurable. If you're willing to accept that a legitimate measurement of the meter is to use refractive interferometry along with measurement of light in air, then you've in fact already agreed that the meter is actually defined in terms of the speed of light in the extrapolated limit of a better and better vacuum. --Steve (talk) 04:24, 27 January 2009 (UTC)

Light as electromagnetic radiation

Rather than continue the long discussion on the philosophical nature of free space, I would like to suggest the reversion of the wording of the section above from Brews' version:

'According to classical electromagnetism, the speed of electromagnetic radiation in free space is the same for all frequencies. Observations in outer space and in good terrestrial vacuums confirm that in such mediums any variation of the this speed with frequency is extremely small, suggesting that, in this regard, they are good approximations to free space'.

to my original version:

'According to classical electromagnetism, the speed of electromagnetic radiation in free space is the same for all frequencies. Observations confirm that any variation of the this speed with frequency is extremely small'.

I believe that my version is clearer, fully justified, and says all that needs to be said. I am not against adding more detail but I think that Brews' attempt to make some kind of philosophical point in the article is wrong and unhelpful to the reader. Martin Hogbin (talk) 09:37, 27 January 2009 (UTC)

Hi Martin: I agree that discussion strayed from topic.

You have inserted the sentence Observations confirm that any variation of the this speed with frequency is extremely small.

What observations are meant? Observations in outer space or terrestrial vacuum, perhaps, or observations in a medium with parameters ε₀ and μ₀? Probably you mean the former.

A medium with ε₀ and μ₀ necessarily exhibits no dispersion. (Unless we digress upon the validity of Maxwell's equations, which is an interpretation of your sentence that no reader would discover without further discussion.) By definition, free space has ε₀ and μ₀, and therefore exhibits no dispersion.. The sentence should be revised to indicate that observations meant are observations in outer space or terrestrial vacuum, not free space. Brews ohare (talk) 16:31, 27 January 2009 (UTC)

I suggest:

'According to classical electromagnetism, the speed of electromagnetic radiation in a perfect vacuum is the same for all frequencies. Observations confirm that any variation of this speed with frequency is extremely small'.

I think everyone will understand what a perfect vacuum is; it's the extrapolation of a better and better physical vacuum; while OTOH a lot of readers will not have heard of free space. The phrase "speed of light in vacuum" is an order-of-magnitude more common on Google than "speed of light in free space". NIST and CODATA both use "vacuum", not "free space". And moreover, to the extent that that "free space" means something different from "perfect vacuum" (and I'm still not sure it does in common usage), "perfect vacuum" is what we want to talk about, since "free space" (as Brews uses the term) has no necessary relationship to anything in our universe, while this is a physics article about our universe and its laws. --Steve (talk) 17:18, 27 January 2009 (UTC)

I am happy with that although I have a couple of minor points. I think we should decide on what term we are going to use (free space, vacuum, perfect vacuum) and use it consistently throughout the article; I have no strong opinion on which. My second point, I have just noticed, is actually with my own wording, '...any variation of this speed with frequency is extremely small', suggests that a small variation has been actually observed whereas all the experimental evidence shows no variation within the experimental uncertainty. Perhaps, '...observations and experiments to date confirm this'. Martin Hogbin (talk) 19:07, 27 January 2009 (UTC)

The terms "free space" or "vacuum of free space" are not uncommon. Another advantage of the term "free space" is that there is another article free space where considerable discussion of various meanings of the term "vacuum" and other links are provided. Brews ohare (talk) 19:33, 27 January 2009 (UTC)

How about,

'Measurements based on the arrival of electromagnetic radiation from distant astrophysical events puts severe limits on the possible variation in the speed of light with frequency'. This is much closer to what is actually stated in our reliable source and it shows the context of radiation travelling through outer space. Martin Hogbin (talk) 19:37, 27 January 2009 (UTC)

How about, 'Measurements based on the arrival of electromagnetic radiation from distant astrophysical events puts severe limits on the possible variation with frequency of the speed of light traveling through outer space'. Brews ohare (talk) 19:46, 27 January 2009 (UTC)

I prefer Martin's version. It's overly modest to say that they merely set limits on dispersion in outer space; with a bit of simple analysis, it actually sets quantitative limits on the dispersion in the "extrapolated better and better vacuum" (call it what you will). --Steve (talk) 22:00, 27 January 2009 (UTC)

My version is closer to what the source actually says, or at least the abstract. It does not mention, free space, vacuum, or outer space. I guess the reader can work out for themselves what the light travels through to reach us from a distant astrophysical event. Martin Hogbin (talk) 22:44, 27 January 2009 (UTC)

The so called "extrapolated better and better vacuum" is exactly free space and has exactly zero dispersion. There is no argument about the dispersion of free space and it has no limits or error bars placed upon it. The free space parameters (or those of the "extrapolated better and better vacuum" ) are defined to be the frequency independent scalars ε₀ and μ₀. This definition makes no reference to a limiting process.

Rather than say something clear, we will "leave it to the reader" or to "a bit of simple analysis". Great. Brews ohare (talk) 23:33, 27 January 2009 (UTC)

The sentence Measurements based on the arrival of electromagnetic radiation from distant astrophysical events puts severe limits on the possible variation with frequency of the speed of light traveling through outer space is not only perfectly clear, it obviously describes the observations and exactly what they mean. Brews ohare (talk) 23:46, 27 January 2009 (UTC)

Brews, it's an open experimental question what happens to light dispersion if you take a series of measurements in imperfect vacuums and then extrapolate them to zero pressure and zero temperature. Neither you, nor NIST, nor anyone, can define what the results of such experiments will be. This is what I mean by "extrapolated better and better vacuum". An "extrapolated better and better vacuum" is what experimentalists actually use to measure the meter, verify the isotropy of the speed of light, and so forth. These people are encouraged (and sometimes employed) by NIST, and when NIST talks about "the speed of light in free space" they're certainly talking about measurements taken in this way, since there's no other way to do it. So if you don't want to call these experiments "measurements of properties of free space", and you don't want to call them measurements of properties of an "extrapolated better and better vacuum", what do you want to call them? Name your term. Surely they're measuring something, right? --Steve (talk) 01:39, 28 January 2009 (UTC)

Steve: Here's the point, which your remarks do not address:

The sentence Measurements based on the arrival of electromagnetic radiation from distant astrophysical events puts severe limits on the possible variation with frequency of the speed of light traveling through outer space is not only perfectly clear, it obviously describes the observations and exactly what they mean. Brews ohare (talk) 02:10, 28 January 2009 (UTC)

The reason I dislike this, as I said above, is that it makes it sound like this is a measurement of a certain property of the interstellar medium. It's more than that, it's an experimental constraint on the basic laws that govern our universe. It's as if you were describing the Millikan oil-drop experiment as a "demonstration that small droplets of oil suspended in air have a quantized charge". It's accurate but gives readers the wrong idea. :-) --Steve (talk) 04:51, 28 January 2009 (UTC)

I would like to put, '...light traveling through free space ', Brews would like to put, '...light traveling through outer space '. We could argue over this for ever but the source itself says, '...light traveling through space', without specifying the kind of space. My proposal is therefore a compromise solution that is supported by a reliable source, which it is hard to argue against.

Personally, I believe that the authors meant 'free space'. In physics, the term 'speed of light' invariably means 'speed of light in free space'. The authors were not trying to measure the properties of the interstellar medium but to put limits on a fundamental property of EM radiation. However, they do not mention 'free space' explicitly so I am happy to compromise and leave this term out out. On the other hand they do not use the words 'speed of light through outer space' or similar so we should not say that either. Martin Hogbin (talk) 10:09, 28 January 2009 (UTC)

Brews, all you have written above and in the section below is your opinion, which you are entitled to, as I am entitled to mine, but articles in WP should be based on reliable sources, not opinion. Now I am not fan of quoting sources verbatim, I am happy with a degree of interpretation for the benefit of non-technical readers, but only when there is general agreement about the facts. In this case there is a difference of opinion, which I am happy to discuss, but until there is a consensus to do otherwise we should say what what our source says, in the words that it says it in. Martin Hogbin (talk) 13:57, 28 January 2009 (UTC)

Deeper meaning of the dispersionless nature of outer space

Steve and Martin: You both seem to believe that the measurement of of the properties of the interstellar medium has ramifications that go far beyond the properties of this particular medium. However, all the measurement actually does is establish the properties. Whatever you two want to read into it requires that this fact be placed within a theory where the theory provides a significance beyond the simple results of the observation. So bite the bullet and dig up the sources and write the summary paragraph that shows that the existence of some media with ε and μ as close to ε₀ and μ₀ as present accuracy can determine is a much more meaningful thing than the simple fact.

As far as Maxwell's equations go, they are totally indifferent to whether outer space approximates ε₀ and μ₀ or has some wild material properties. EM waves can propagate in any medium, and at any speed up to c₀. The measurement showing some media exhibit ε ≈ ε₀ and μ ≈ μ₀ doesn't do more than establish the existence of media with these properties.

It's like the M - M measurement of the speed of light as a function of the movement of the observer. : you need relativity to interpret the facts before the implications become clear. And it seems all special relativity demands is that an upper limit on speed of light exists, regardless of whether c₀ applies to outer space.

Maybe an approach to your discussion could be to show that if outer space exhibited dispersion it would have amazing repercussions according to some theory? Brews ohare (talk) 13:17, 28 January 2009 (UTC)

It may be possible to find a source that meets the requirements of your first paragraph but this will not be easy, nor in my opinion, is it necessary. There are many rarely discussed assumptions in the philosophy of physics and one is the essentially pragmatic view that, until proven otherwise, the world behaves 'sensibly'. In other words if we measure the properties of gases at reducing pressures we can extrapolate these properties to zero pressure to get the properties of free space. Of course there have been surprises before and there will, no doubt, be some in the future; they are dealt with as they arise.

Actual, philosophically pure, free space is of no interest to physicists because it can never be realized or measured. It exits only as an idealization, as the end point of a graph if you like. Free space is a concept only, it has no actual reality, thus we are able to treat it in any way that suits us, usually as the limit of a series of real measurements. Martin Hogbin (talk) 14:14, 28 January 2009 (UTC)

Martin: These ponderings of yours are a poor substitute for a documented argument. Brews ohare (talk) 17:18, 28 January 2009 (UTC)

It is the content of the articles that should be based on reliable sources. This is the page for discussion. Martin Hogbin (talk) 17:38, 28 January 2009 (UTC)

Brew, I still cannot work out what your point is. Perhaps it would help if we see what are the things that we agree on. Do we agree the following?

Free space is an idealization that is physically unobtainable.

The speed of light is defined to be a certain value in free space.

Actual measurements of the speed of light are made in media other than free space.

According to Maxwell's equations, all frequencies of EM radiation travel at the same speed in free space.

Martin Hogbin (talk) 18:11, 28 January 2009 (UTC)

Hi Martin: I agree on all these points. However, I believe no measurements can be made on free space, only upon media that are known to some accuracy to approximate free space. I do not find your sentence to agree with these points. It implies free space is amenable to measurement (has measurable properties, or is realizable), or that outer space (possibly, within the vagueness you advocate) is free space. Brews ohare (talk) 18:41, 28 January 2009 (UTC)

You say, '...no measurements can be made on free space, only upon media that are known to some accuracy to approximate free space' and I agree. But, accepting that we can never be absolutely sure about anything in physics, we can infer what we would expect the properties of free space to be based on real measurements, can't we? Martin Hogbin (talk) 18:54, 28 January 2009 (UTC)

We don't have to infer or speculate about the properties of free space: they are defined, and therefore certain. We can speculate whether outer space behaves like free space, and attempt to support these ideas by measuring outer space. Brews ohare (talk) 05:49, 29 January 2009 (UTC)

Newton's equation for the speed of sound

Maxwell used Newton's equation for the speed of sound in a long solid rod in order to obtain the speed of light on the basis of an elastic medium with a density related to the magnetic permeability, and a transverse elasticity related to the dielectric constant.

This suggests to me that light is a coherent dispersionless wave in an elastic solid.

Maxwell obtained the concept of displacement current from his own postulated elastic medium, and the concept was used in his derivation of the EM wave equation in 1864, three years after he did the analyis above with Newton's equation.

The modern derivation of displacement current bears no relationship to Maxwell's derivation of displacement current, and the modern derivation doesn't have the correct divergence properties to allow it to connect to the electromagnetic wave equation.

This is all telling us something about the nature of the so-called vacuum. David Tombe (talk) 16:07, 28 January 2009 (UTC)

The aether has no place in modern physics and should not be discussed in this article. Please do not add sections on aether to this article. There are several articles on the aether where your views might be relevant. Martin Hogbin (talk) 16:47, 28 January 2009 (UTC)

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