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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)
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)
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 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)
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)
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)
I suggest that the paragraph beginning, 'Due to special relativity's time dilation...' is deleted. Martin Hogbin ( talk) 23:45, 1 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)
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)
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)
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)
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)
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)
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)
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)
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)
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)
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)
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)
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)
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)
on the page for "sped of light" the gif image says that it is a scale model of earth and the moon and it shows lght traveling between it. the text states that it takes about 1.5 secs althought the pictur take 4-5. either change the picture or the text to make the match (i havent calculated h actual time it would take light to travel that distance so im not sure which on is correct). —Preceding unsigned comment added by 210.8.54.104 ( talk) 04:29, 5 May 2008 (UTC)
That is just an issue with your internet speed. —Preceding
unsigned comment added by
216.228.198.151 (
talk)
17:26, 23 January 2009 (UTC)
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 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)
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)
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 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)
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)
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)
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)
I notice that the history section in this article is at the end. In many other technical and physics articles, 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)
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)
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! 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)
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.
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.
Jay2U: Jay2U ( talk) 10:19, 2 August 2008 (UTC)
Greetings. This article uses the following reference:
However, the only place I'm seeing this citation is on wikipedia mirrors. I am finding ghits for the following:
Is there a way to confirm which is correct? Thank you.— RJH ( talk) 17:07, 20 August 2008 (UTC)
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)
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
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
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)
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)
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)
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)
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 |
(outdent) Yes, I like Steve's table. Just right for the less-technical reader. Dbfirs 08:34, 24 November 2008 (UTC)
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 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)
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)
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)
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)
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)
I am unable to find any relevant info in:
and the document
is listed at Amazon as "out of print" and has no on-line version. The page listing in
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)
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)
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)
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)
Physchim62 (Martin too), you missed the important issue here. MOSNUM states as follows:
In large numbers (i.e., in numbers greater than or equal to 10,000), commas are generally used to break the sequence every three places left of the decimal point, e.g. "8,274,527". In scientific context, thin spaces can also be used (but using {{{1}}} to prevent line-breaking within numbers), e.g. "8 274 527" (8 274 527, or using the thin space character instead of its HTML entity). Consistency within an article is desirable as always.
There are many, many instances of big numbers in this article—many that are the speed of light in kilometers per second—that are delimited with commas. Here are some of them:
9,300
10,100
1,000 Earth diameters
220,000 kilometres per second
136,000 miles per second
210,000 kilometres per second
298,000 kilometres per second
185,000 miles per second
313,000 kilometres per second
298,000
186,285 miles per second (299,796 kilometres per second)
Consistency within the article is important and yet you, Physchim62, seemed only too anxious to try to make your point by editwarring over one single number in the article without any apparent desire to achieve a consistent practice within the article. Further, if you want to try to say that MOSNUM and MOS guidelines have a preference for not using commas, the above words don’t support that position; Martin Hogbin’s observations are correct.
“Awe, shucky darn! I’ll show Greg L and convert all those other comma-delimited numbers too!” you might say. Gee, I hope you don’t pull that sort of stunt.
And Martin, stop personalizing this issue as one of “Greg's crusade against thinspaces.” You don’t know what the hell you are talking about and come across as only too anxious to turn everything in to a personal attack and bitch-fest on the talk pages. As for your past allegation, Physchim62, that I’ve had a “crusade against thinspaces”, please note that I’ve been the lead proponent of the {{
val}} and {{
delimitnum}} templates; the main purpose of which is to add thinspaces to the fractional side of the decimal marker so big numbers are easier to parse.
I also recognize the obvious reality that Europeans are extraordinarily accustomed to more than one system of delimiting numbers; Swedes are taught three of them in school—including the American system. Europeans are not generally confused by different methods for delimiting numbers on the integer side of the significand. Most Americans on the other hand, have no familiarity whatsoever with other methods besides comma delimiting; it is pretty much limited to American scientists and academics. It is not the job of an encyclopedia to “educate” readers on these things by confusing them with the very wording employed to communicate on topics. Good writing style is all about making reading fluid and natural and minimizing confusion.
Finally, I appreciate intellectual, uhm… *candor*. I just hate it when editors try to hide behind the apron strings of MOS or MOSNUM policy and beat around the bush in their arguments on talk pages in lame attempts to justify their actions. Like the Bible, a little bit of everything can be dug out of MOS and MOSNUM to justify pretty much anything one does. I would really much prefer it if you would just admit that “I think the SI method of using narrow-space delimiting on both sides of the decimal marker is so superior, I want to slip it into wherever I can on Wikipedia—even if doing so produces a retarded hodgepodge of delimiting styles within a single article and is confusing to many readers—in hopes that my efforts on Wikipedia’s general-interest articles will slowly educate some of those dumb-bastard Americans.” Though I would agree with your objectives and disagree with your tactics, I would at least respect and appreciate the candor. Greg L ( talk) 05:32, 24 December 2008 (UTC)
Since photons still have mass citation needed, 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)
As for your theory that the perceived journey time of humans in a rocket traveling at relativistic speeds would be reduced, that was the very point Einstein was making when he advanced his Theory of Special relativity. If you want to calculate for yourself the journey time (space ship time) for passengers onboard a craft traveling at relativistic speeds, see Lorentz factor. I’ve still got my spreadsheet for calculating the velocities of particles at different temperatures. It’s got a gamma factor built into it for ultra-high temperatures. (*pause why I go find the thing…*) There: if you want a space ship’s coast, or cruise time of 1000 days to be only one day for its passengers, it must cruise at 299,792,308.103 734 m/s, or 99.99995% the speed of light relative to their departure point, or inertial frame of reference (Earth). Really though, for high-accuracy work, the reference point would be the barycenter of the solar system; what is called Barycentric Coordinate Time (TCB). Greg L ( talk) 17:19, 24 December 2008 (UTC)
As editors here do doubt know, this used to be a Featured Article but is no more. I was about to ask who here was interested in trying to get that status back but it seems as though the work might have already started.
Having looked at the FA review, it would seem to me that most of the complaints were about fairly minor MOS issues which should not be too hard to fix. I am not an expert on formatting etc but I am glad to help out where I can. I would like to try to make to improvements to the content of the article and may make some bold edits. Please revert these if you think there are negative and we can discuss.
I have added a section below as a FA 'to do' list. Not all the items I have added relate directly to the FA review. Martin Hogbin ( talk) 19:02, 24 December 2008 (UTC)
* Suggestion Martin, why not just revert to the version when the article achieved FA status? Then we can all look at what is in today’s version of the article and pick & choose the best additional material to bring forward?
Greg L (
talk)
19:21, 24 December 2008 (UTC)
While this is arguably a “scientific” article, ‘speed of light’ is clearly also a general-interest article. Editors should endeavor to avoid scientific notation where possible to keep this article accessible to the widest audience.
Remember too, one uses a non-breaking space between values and unit symbols in numeric equivalencies. If the unit name is spelled out in full, such as 299,792,458 metres per second, then you use a regular space and allow the line-end word-wrap. Otherwise, you can have huge gaps at the end of a line as the entire numeric equivalency drops to the next line down. Using a non-breaking space between the numeric value and the first word of a spelled-out unit of measure is OK if the numeric value and the first word in the unit of measure doesn’t add up to something really long; 299,792,458 metres… is pushing it. Greg L ( talk) 20:34, 24 December 2008 (UTC)
See above. Martin Hogbin ( talk) 19:08, 24 December 2008 (UTC)
Rewrite lead section
It should be a summary of the article. Should it have references or should they be later in the main body?
Do something with 'overview'
Maybe it is just the title that is wrong but it is not really an overview, and should the lead section not be this?
Reduce the 'Faster than light' section
I am still not sure of the best way to do this but others have commented that it was too long (I have reduced it a bit).
Does someone know how to specify a parameter to keep the body text from crowding so tight against the left-hand edge of the table? Isn’t there a way to do padding that isn’t visible? Greg L ( talk) 21:57, 24 December 2008 (UTC)
P.S. I figured it out. I had to go off-Wikipedia to Web Design Group for the help. Specifying parameters of style="padding: 1.2em" FRAME= void did the trick. Wikipedia’s Help:Table blows. Someone with better table experience than I should go in and fix this article’s table. I really wanted a 2-pixel border ( like this), but now that I’ve got a 1.2-em frame, the new parameter seems to interfere with the border width. What I’ve got at least no longer looks like an abomination. Greg L ( talk) 22:46, 24 December 2008 (UTC)
I have removed the animated picture of light travelling to the moon. I do not think it adds anything as we have a table stating how long it takes light to travel various distances and everyone knows what travelling is. I do not have a particularly strong opinion on this. What do others think?
The speed of light is 299,792,458 M/PS, But when the number 299,792,458 is searched it automatically comes up with this page.
So it should become a disambiguation page.
Thank You,
SaiyanEmperor2008 ( talk) 16:06, 27 December 2008 (UTC)
Here's the section, as written before its recent deletion:
Astronomical distances are sometimes measured in light-years, which is the distance light travels in one year. A light‑year is precisely equal to 9,460,730,472,580.8 km, or about 5,878,625,373,183.61 statute miles. Since astronomical distances are so great, light requires billions of years to travel across stretches of the universe. Thus, the light that is just now reaching Earth from distant objects in the universe had originally been emitted long ago; an observer on Earth sees distant objects in the universe as they appeared millions or billions of years in the distant past. Light from even objects in our own solar system requires quite a bit of time to reach us. Light from the sun takes around 8 1⁄3 minutes to reach the earth.
Here's the edit in which the section was deleted by User:DVdm: [1]. Here was the edit summary: "Removed section about a *distance* listing examples of *time* scales (as this is an articale about a *speed*). Link to light-year (with many examples) already mentioned in intro".
I vote to reinstate this section. Certainly the section isn't perfect, but its topic is important to the article. The fact that telescopes see things that are millions of years old is a direct consequence of the fact that the speed of light is not instantaneous. I think that in order for a reader to have a good, encyclopedic understanding of the "speed of light", they should understand how it creates the distance-vs-time relationship in astronomy. What do other people think? :-) -- Steve ( talk) 17:35, 27 December 2008 (UTC)
Well, i.m.o. this section was very poorly conceived and formulated. It repeated a remark from the introduction ("In astronomy, distances are often measured in light-years, the distance light travels in a year (around ten trillion kilometers)"), and then gave a few examples of times light takes to reach us, in a section about a distance, in an article about a speed. By the way, adding a remark about parsecs would even be more off-topic, as they are 100% unrelated to the subject of the article.
I think the line in the introduction should be largely sufficient in an article about a speed. Anyway, I have taken the liberty to replace the phrase "around ten trillion kilometers" with the precise km-expression of the light-year. DVdm ( talk) 19:59, 27 December 2008 (UTC)
Here's a proposed rewrite of this section:
- In astronomy
The speed of light is particularly important in astronomy, where the vast distances involved mean that it can take a very long time for light to travel from its source to Earth. For example, it takes 13 billion years for light to travel to Earth from the faraway galaxies viewed in the Hubble Ultra Deep Field images; therefore, those photographs taken today capture images of the galaxies as they appeared 13 billion years ago (near the beginning of the universe). The fact that farther-away objects appear older (due to the finite speed of light) is crucial in astronomy, allowing astronomers to infer the evolution of stars, galaxies, and the universe itself without having to wait billions of years to watch these processes in real-time.
By the same token, astronomical distances are sometimes measured in light-years, the distance light travels in one year. A light‑year is around 10 trillion km, 5 trillion miles, or 0.3 parsecs. The closest star to Earth (besides the sun) is around 4.2 light‑years away.
Thoughts? :-) -- Steve ( talk) 23:51, 27 December 2008 (UTC)
I see the section has been restored. I.m.o. is has become 'much worse than it was:
I propose we replace the bad section with Steve's suggestion above - and, please waste that disturbing animation. DVdm ( talk) 10:52, 28 December 2008 (UTC)
I think you're guilty of overprecision here. What year are we talking about? 365 x 24 x 60 x 60 seconds? Sideral year? Headbomb { ταλκ κοντριβς – WP Physics} 00:41, 28 December 2008 (UTC)
P.S. I was skeptical about Light-year because it used 365.25 days and not the 365.24 days one would use for averaging across a mere century . Plus, there is a higher-precision number used for averaging across 400 years as I recall. I assumed that an overly precise number and false assumptions wouldn’t be so damned glaring in such a high-profile article without a citation needed tag being slapped on it. I get bogged down further than I often like when I get diverted trying to correct every single article I’m linking to during an editing session. That’s what happened on Kilogram: I linked to it once, took a look at the piece of shit, and stayed their for a God-damned year! Happy editing. Greg L ( talk) 01:14, 28 December 2008 (UTC)
![]() | This is an archive of past discussions. Do not edit the contents of this page. If you wish to start a new discussion or revive an old one, please do so on the current talk page. |
Archive 1 | ← | Archive 3 | Archive 4 | Archive 5 | Archive 6 | Archive 7 | → | Archive 10 |
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)
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)
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 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)
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)
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)
I suggest that the paragraph beginning, 'Due to special relativity's time dilation...' is deleted. Martin Hogbin ( talk) 23:45, 1 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)
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)
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)
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)
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)
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)
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)
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)
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)
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)
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)
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)
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)
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)
on the page for "sped of light" the gif image says that it is a scale model of earth and the moon and it shows lght traveling between it. the text states that it takes about 1.5 secs althought the pictur take 4-5. either change the picture or the text to make the match (i havent calculated h actual time it would take light to travel that distance so im not sure which on is correct). —Preceding unsigned comment added by 210.8.54.104 ( talk) 04:29, 5 May 2008 (UTC)
That is just an issue with your internet speed. —Preceding
unsigned comment added by
216.228.198.151 (
talk)
17:26, 23 January 2009 (UTC)
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 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)
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)
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 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)
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)
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)
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)
I notice that the history section in this article is at the end. In many other technical and physics articles, 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)
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)
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! 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)
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.
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.
Jay2U: Jay2U ( talk) 10:19, 2 August 2008 (UTC)
Greetings. This article uses the following reference:
However, the only place I'm seeing this citation is on wikipedia mirrors. I am finding ghits for the following:
Is there a way to confirm which is correct? Thank you.— RJH ( talk) 17:07, 20 August 2008 (UTC)
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)
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
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
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)
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)
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)
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)
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 |
(outdent) Yes, I like Steve's table. Just right for the less-technical reader. Dbfirs 08:34, 24 November 2008 (UTC)
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 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)
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)
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)
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)
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)
I am unable to find any relevant info in:
and the document
is listed at Amazon as "out of print" and has no on-line version. The page listing in
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)
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)
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)
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)
Physchim62 (Martin too), you missed the important issue here. MOSNUM states as follows:
In large numbers (i.e., in numbers greater than or equal to 10,000), commas are generally used to break the sequence every three places left of the decimal point, e.g. "8,274,527". In scientific context, thin spaces can also be used (but using {{{1}}} to prevent line-breaking within numbers), e.g. "8 274 527" (8 274 527, or using the thin space character instead of its HTML entity). Consistency within an article is desirable as always.
There are many, many instances of big numbers in this article—many that are the speed of light in kilometers per second—that are delimited with commas. Here are some of them:
9,300
10,100
1,000 Earth diameters
220,000 kilometres per second
136,000 miles per second
210,000 kilometres per second
298,000 kilometres per second
185,000 miles per second
313,000 kilometres per second
298,000
186,285 miles per second (299,796 kilometres per second)
Consistency within the article is important and yet you, Physchim62, seemed only too anxious to try to make your point by editwarring over one single number in the article without any apparent desire to achieve a consistent practice within the article. Further, if you want to try to say that MOSNUM and MOS guidelines have a preference for not using commas, the above words don’t support that position; Martin Hogbin’s observations are correct.
“Awe, shucky darn! I’ll show Greg L and convert all those other comma-delimited numbers too!” you might say. Gee, I hope you don’t pull that sort of stunt.
And Martin, stop personalizing this issue as one of “Greg's crusade against thinspaces.” You don’t know what the hell you are talking about and come across as only too anxious to turn everything in to a personal attack and bitch-fest on the talk pages. As for your past allegation, Physchim62, that I’ve had a “crusade against thinspaces”, please note that I’ve been the lead proponent of the {{
val}} and {{
delimitnum}} templates; the main purpose of which is to add thinspaces to the fractional side of the decimal marker so big numbers are easier to parse.
I also recognize the obvious reality that Europeans are extraordinarily accustomed to more than one system of delimiting numbers; Swedes are taught three of them in school—including the American system. Europeans are not generally confused by different methods for delimiting numbers on the integer side of the significand. Most Americans on the other hand, have no familiarity whatsoever with other methods besides comma delimiting; it is pretty much limited to American scientists and academics. It is not the job of an encyclopedia to “educate” readers on these things by confusing them with the very wording employed to communicate on topics. Good writing style is all about making reading fluid and natural and minimizing confusion.
Finally, I appreciate intellectual, uhm… *candor*. I just hate it when editors try to hide behind the apron strings of MOS or MOSNUM policy and beat around the bush in their arguments on talk pages in lame attempts to justify their actions. Like the Bible, a little bit of everything can be dug out of MOS and MOSNUM to justify pretty much anything one does. I would really much prefer it if you would just admit that “I think the SI method of using narrow-space delimiting on both sides of the decimal marker is so superior, I want to slip it into wherever I can on Wikipedia—even if doing so produces a retarded hodgepodge of delimiting styles within a single article and is confusing to many readers—in hopes that my efforts on Wikipedia’s general-interest articles will slowly educate some of those dumb-bastard Americans.” Though I would agree with your objectives and disagree with your tactics, I would at least respect and appreciate the candor. Greg L ( talk) 05:32, 24 December 2008 (UTC)
Since photons still have mass citation needed, 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)
As for your theory that the perceived journey time of humans in a rocket traveling at relativistic speeds would be reduced, that was the very point Einstein was making when he advanced his Theory of Special relativity. If you want to calculate for yourself the journey time (space ship time) for passengers onboard a craft traveling at relativistic speeds, see Lorentz factor. I’ve still got my spreadsheet for calculating the velocities of particles at different temperatures. It’s got a gamma factor built into it for ultra-high temperatures. (*pause why I go find the thing…*) There: if you want a space ship’s coast, or cruise time of 1000 days to be only one day for its passengers, it must cruise at 299,792,308.103 734 m/s, or 99.99995% the speed of light relative to their departure point, or inertial frame of reference (Earth). Really though, for high-accuracy work, the reference point would be the barycenter of the solar system; what is called Barycentric Coordinate Time (TCB). Greg L ( talk) 17:19, 24 December 2008 (UTC)
As editors here do doubt know, this used to be a Featured Article but is no more. I was about to ask who here was interested in trying to get that status back but it seems as though the work might have already started.
Having looked at the FA review, it would seem to me that most of the complaints were about fairly minor MOS issues which should not be too hard to fix. I am not an expert on formatting etc but I am glad to help out where I can. I would like to try to make to improvements to the content of the article and may make some bold edits. Please revert these if you think there are negative and we can discuss.
I have added a section below as a FA 'to do' list. Not all the items I have added relate directly to the FA review. Martin Hogbin ( talk) 19:02, 24 December 2008 (UTC)
* Suggestion Martin, why not just revert to the version when the article achieved FA status? Then we can all look at what is in today’s version of the article and pick & choose the best additional material to bring forward?
Greg L (
talk)
19:21, 24 December 2008 (UTC)
While this is arguably a “scientific” article, ‘speed of light’ is clearly also a general-interest article. Editors should endeavor to avoid scientific notation where possible to keep this article accessible to the widest audience.
Remember too, one uses a non-breaking space between values and unit symbols in numeric equivalencies. If the unit name is spelled out in full, such as 299,792,458 metres per second, then you use a regular space and allow the line-end word-wrap. Otherwise, you can have huge gaps at the end of a line as the entire numeric equivalency drops to the next line down. Using a non-breaking space between the numeric value and the first word of a spelled-out unit of measure is OK if the numeric value and the first word in the unit of measure doesn’t add up to something really long; 299,792,458 metres… is pushing it. Greg L ( talk) 20:34, 24 December 2008 (UTC)
See above. Martin Hogbin ( talk) 19:08, 24 December 2008 (UTC)
Rewrite lead section
It should be a summary of the article. Should it have references or should they be later in the main body?
Do something with 'overview'
Maybe it is just the title that is wrong but it is not really an overview, and should the lead section not be this?
Reduce the 'Faster than light' section
I am still not sure of the best way to do this but others have commented that it was too long (I have reduced it a bit).
Does someone know how to specify a parameter to keep the body text from crowding so tight against the left-hand edge of the table? Isn’t there a way to do padding that isn’t visible? Greg L ( talk) 21:57, 24 December 2008 (UTC)
P.S. I figured it out. I had to go off-Wikipedia to Web Design Group for the help. Specifying parameters of style="padding: 1.2em" FRAME= void did the trick. Wikipedia’s Help:Table blows. Someone with better table experience than I should go in and fix this article’s table. I really wanted a 2-pixel border ( like this), but now that I’ve got a 1.2-em frame, the new parameter seems to interfere with the border width. What I’ve got at least no longer looks like an abomination. Greg L ( talk) 22:46, 24 December 2008 (UTC)
I have removed the animated picture of light travelling to the moon. I do not think it adds anything as we have a table stating how long it takes light to travel various distances and everyone knows what travelling is. I do not have a particularly strong opinion on this. What do others think?
The speed of light is 299,792,458 M/PS, But when the number 299,792,458 is searched it automatically comes up with this page.
So it should become a disambiguation page.
Thank You,
SaiyanEmperor2008 ( talk) 16:06, 27 December 2008 (UTC)
Here's the section, as written before its recent deletion:
Astronomical distances are sometimes measured in light-years, which is the distance light travels in one year. A light‑year is precisely equal to 9,460,730,472,580.8 km, or about 5,878,625,373,183.61 statute miles. Since astronomical distances are so great, light requires billions of years to travel across stretches of the universe. Thus, the light that is just now reaching Earth from distant objects in the universe had originally been emitted long ago; an observer on Earth sees distant objects in the universe as they appeared millions or billions of years in the distant past. Light from even objects in our own solar system requires quite a bit of time to reach us. Light from the sun takes around 8 1⁄3 minutes to reach the earth.
Here's the edit in which the section was deleted by User:DVdm: [1]. Here was the edit summary: "Removed section about a *distance* listing examples of *time* scales (as this is an articale about a *speed*). Link to light-year (with many examples) already mentioned in intro".
I vote to reinstate this section. Certainly the section isn't perfect, but its topic is important to the article. The fact that telescopes see things that are millions of years old is a direct consequence of the fact that the speed of light is not instantaneous. I think that in order for a reader to have a good, encyclopedic understanding of the "speed of light", they should understand how it creates the distance-vs-time relationship in astronomy. What do other people think? :-) -- Steve ( talk) 17:35, 27 December 2008 (UTC)
Well, i.m.o. this section was very poorly conceived and formulated. It repeated a remark from the introduction ("In astronomy, distances are often measured in light-years, the distance light travels in a year (around ten trillion kilometers)"), and then gave a few examples of times light takes to reach us, in a section about a distance, in an article about a speed. By the way, adding a remark about parsecs would even be more off-topic, as they are 100% unrelated to the subject of the article.
I think the line in the introduction should be largely sufficient in an article about a speed. Anyway, I have taken the liberty to replace the phrase "around ten trillion kilometers" with the precise km-expression of the light-year. DVdm ( talk) 19:59, 27 December 2008 (UTC)
Here's a proposed rewrite of this section:
- In astronomy
The speed of light is particularly important in astronomy, where the vast distances involved mean that it can take a very long time for light to travel from its source to Earth. For example, it takes 13 billion years for light to travel to Earth from the faraway galaxies viewed in the Hubble Ultra Deep Field images; therefore, those photographs taken today capture images of the galaxies as they appeared 13 billion years ago (near the beginning of the universe). The fact that farther-away objects appear older (due to the finite speed of light) is crucial in astronomy, allowing astronomers to infer the evolution of stars, galaxies, and the universe itself without having to wait billions of years to watch these processes in real-time.
By the same token, astronomical distances are sometimes measured in light-years, the distance light travels in one year. A light‑year is around 10 trillion km, 5 trillion miles, or 0.3 parsecs. The closest star to Earth (besides the sun) is around 4.2 light‑years away.
Thoughts? :-) -- Steve ( talk) 23:51, 27 December 2008 (UTC)
I see the section has been restored. I.m.o. is has become 'much worse than it was:
I propose we replace the bad section with Steve's suggestion above - and, please waste that disturbing animation. DVdm ( talk) 10:52, 28 December 2008 (UTC)
I think you're guilty of overprecision here. What year are we talking about? 365 x 24 x 60 x 60 seconds? Sideral year? Headbomb { ταλκ κοντριβς – WP Physics} 00:41, 28 December 2008 (UTC)
P.S. I was skeptical about Light-year because it used 365.25 days and not the 365.24 days one would use for averaging across a mere century . Plus, there is a higher-precision number used for averaging across 400 years as I recall. I assumed that an overly precise number and false assumptions wouldn’t be so damned glaring in such a high-profile article without a citation needed tag being slapped on it. I get bogged down further than I often like when I get diverted trying to correct every single article I’m linking to during an editing session. That’s what happened on Kilogram: I linked to it once, took a look at the piece of shit, and stayed their for a God-damned year! Happy editing. Greg L ( talk) 01:14, 28 December 2008 (UTC)