Talk:Speed of light

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[edit] 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)

[edit] 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)

[edit] 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)

[edit] 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)

[edit] 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)

[edit] 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)

[edit] 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)

[edit] 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)

[edit] '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)

[edit] 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)

[edit] 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)

[edit] '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)

[edit] 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)

[edit] 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)

[edit] 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)

[edit] 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)

[edit] 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)


[edit] 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)

[edit] 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)

[edit] 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)