Talk:Speed of light

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 3.844×10⁵ km, so one sidereal lunar orbit is 2π×3.844×10⁸ m = 2.415×10⁹ 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)×2.415×10⁹ m = 2.611×10⁹ m. 12000 synodic lunar orbits is thus 3.133×10¹³ m. Dividing by 86400 seconds in one solar day yields 3.626×10⁸ m/s for the speed of light in the Qur'an vs the modern value of 2.998×10⁸ 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 = mc₀² 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 Wikipedia talk:WikiProject_Physics#Question of Wikipedia policy consensus was to switch back to c. I'll be doing this (if someone doesn't beat me) at some point. --Falcorian ^(talk) 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 ^(talk) 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)

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 3.844×10⁵ 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 3.564×10⁵ km while its extreme apogee is 4.067×10⁵ 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 Wikipedia 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/wiki/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 Wikipedia'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. [This requirement] come[s] 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 Wikipedia 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 Wikipedia 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)

Length of time for light to travel...
1 foot	1.0 nanoseconds
1 metre	3.3 nanoseconds
1 km	3.3 microseconds
1 mile	5.4 microseconds
Around Earth's equator	0.13 seconds
From Earth to the moon	1.3 seconds
From Earth to the sun	8.3 minutes

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 Wikipedia'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 Wikipedia's use of "c" an "anachronism"; maybe you mean that it "fails to be ahead of its time". Anyway, the spirit of Wikipedia'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 [2] 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: Wikipedia talk:WikiProject Physics/Archive March 2008#Question of Wikipedia 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 Wikipedia 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 Wikipedia'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 Wikipedia 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)

But you were the one who reverted on the basis of WP:Mosnum. Martin Hogbin (talk) 19:31, 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 Wikipedia hodgepodge. Greg L (talk) 05:43, 24 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)

You all 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 all of you seemed only too anxious to try to make your point by editwarring over one single number in the article without any apparent concern for the rest of the damned thing and trying to achieve a consistent practice within the article. Further, if someone wants to try to say that MOSNUM and MOS guidelines have a preference for not using commas, the above words don’t support that position.

“Awe, shucky darn! I’ll show Greg L and convert all those other comma-delimited numbers too!” you might say. Gee, I hope someone doesn’t act like a baby and 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. 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. 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. Americans on the other hand, are not so accustomed to other methods besides comma delimiting. 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)

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)