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{{Spoken article requested|] (])ScientistBuilder] (]) 17:18, 29 January 2022 (UTC)|The speed of light is central to physics fields including the Big Bang Theory, special relativity, general relativity, spectroscopy, optics, as well as real world applications such as signal processing and GPS networks}}
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== Speed of light in a medium == == Is this part accurate in History? ==


Quote:
The current opening paragraph in this section seems rather confused to me and surely does not represent Feynman's words on the subject as the reference suggests. We currently have:


'''Connections with electromagnetism'''
''When light enters materials, its energy is absorbed. In the case of transparent materials, this energy is quickly re-radiated. However, this absorption and re-radiation introduces a delay. As light propagates through dielectric material it undergoes continuous absorption and re-radiation. Therefore the speed of light in a medium is said to be less than c, which should be read as the speed of energy propagation at the macroscopic level. At an atomic level, electromagnetic waves always travel at c in the empty space between atoms. Two factors influence this slowing: stronger absorption leading to shorter path length between each re-radiation cycle, and longer delays. The slowing is therefore the result of these two factors''


In the 19th century ''Hippolyte Fizeau'' developed a method to determine the speed of light based on time-of-flight measurements on Earth and reported a value of 315000 km/s (''704,634,932 m/h'').
This seems to mix up two models into one confused explanation. In classical electromagnetic theory, the original wave is extinguished by interaction with the bound electrons in the atoms of the substance and replaced with another wave moving at a lower velocity. This is a bulk phenomenon, representing the interaction of an EM wave with the huge number of electrons in the material.


His method was improved upon by ''Léon Foucault'' who obtained a value of 298000 km/s (''666,607,015 m/h'') in 1862. ] (]) 01:06, 18 November 2023 (UTC)
::Where is the confusion? The EM-wave encounters a field of bound-charges. You then have a simple case of a field of driven oscillators. The new wave, travelling at c, is in turn extinguished like the original. As with any driven oscillation (with a mass) each generation undergoes a phase-delay - thus the phase-velocity of the propagation is reduced.


:Are you suggesting our article may not be correct or proposing that it include conversions to km/h at that point, and in either case, why? ] (]) 11:07, 18 November 2023 (UTC)
::You suggest that the 2nd generation wave propogates through the field of bound electrons without further interaction - which is not possible.


::There's a definite discrepancy in number of significant digits between the quoted metric and traditional measurements... ] (]) 13:10, 18 November 2023 (UTC)
:::I think your interpretation of the text above is somewhat charitable. I think that whoever wrote it was trying to explain the quantum behaviour of light in a medium in layman's terms, as ] says below. The text has been changed now anyway. ] (]) 23:10, 15 June 2010 (UTC)
:::Indeed, but the values in parentheses aren't in the article. If we wanted to include them, we could use {{tl|Convert}}, which would probably round them appropriately automatically, and wouldn't abbreviate miles to "m" either, but I don't see why we'd want to include such conversions in that part of the article anyway. ] (]) 13:46, 18 November 2023 (UTC)
::::I added the parenthesis. It's just a conversion to m/h that I made, just to show how different they are & to convert it into U.S. terms. ] (]) 00:15, 24 December 2023 (UTC)
::I'm not sure if 315000 or 298000 km/s is correct. I feel it's 315000 km/s, but I'm not sure. ] (]) 18:04, 25 November 2023 (UTC)


== Why not also include an accurate description of c in miles per second? ==
In QED photons are repeatedly absorbed and emitted by atoms resulting in a delay in the signal propagation through the material. As photons are quantum entities, I would stop short of saying that they travel at ''c'' between the atoms although this may be the mental model that many people have. ] (]) 14:08, 13 May 2010 (UTC)


186282.3970512 mi/s, to be fairly accurate.
::Stopping short implies that photons might be travelling at some other velocity...


== Speed of light in vacuum ==
:I think the text is trying to explain the quantum behaviour of light in a medium in laymen's terms. Talking about virtual processes as if they were real always makes me cringe a little, but it is exactly the sort of thing Feynman does all the time. Someone with quick access to the Feynman lectures should check that this paragraph accurately reflects his explanation, though. ] (]) 08:34, 17 May 2010 (UTC)
::No, IIRC, Feynman gives a purely classical explanation in that section (treating each electron as a harmonic oscillator, essentially; he only mentions at the end that for quantum reasons each one actually has several frequencies). He assumes an incoming wave on the left of a plane layer of transparent material, and describes the outgoing wave at the right a superposition of the incoming one and a wave generated by oscillating electrons, and the result has (at sufficiently low densities) almost the same amplitude but different phase than the original wave. If it's exp(ikx - i\omega t + i\phi_1) on the left, it's exp(ikx - i \omega t + i\phi_2) on the right, then he shows that \phi_2 - \phi_1 is proportional to the thickness, so that inside the material it can be written as (i(k+k')x - i\omega t + i\phi_1) and calls \omega/(k+k') + c/n. (When I get back home, I'll check whether it's actually as I remember it.) ] (]) 09:35, 17 May 2010 (UTC)
:::Yes, I checked this out and Feynman is discussing the classical model in which the original wave is extinguished and a new one created.
:::As I suggest above, I am not against giving a layman's QED answer but we must be careful not to describe photons as classical particles. ] (]) 22:19, 18 May 2010 (UTC)
::::I definitely agree on the last bit. Any ideas for a clear exposition? ] (]) 07:46, 19 May 2010 (UTC)


Misplaced Pages should get rid of all occurrences of the phrase "speed of light in vacuum". There is only one speed of light, which is a universal constant. Also the speed of light doesn't change if not in vacuum. ] represents the real speed of a photon, and that doesn't change. Only ] is changing, causing the optical effects that mislead people. But this very article is explaining the same in the section ]. ] (]) 13:24, 20 May 2024 (UTC)
I wish I had more insight. I don't know if I like any qualitative description of how the speed of light varies in a material before the article tells me which speed of light it is talking about. The mechanism for the phase speed is different then for the group speed and the front speed. My first instinct is that description fits the description of the front velocity and not the phase velocity since phase velocity can be and is for certain circumstances greater than c. The explanation for phase velocity almost certainly needs to involve waves since it is a wave phenomenon.


:If you have a reference for your point of view please share it. ] (]) 15:15, 20 May 2024 (UTC)
This article has progressed a lot since I last saw it, but this section is still its Achilles's heal. (The only other significant item is the amount of physics speak in the relativity section.) I will see if I can find something better somewhere. ] (]) 06:37, 22 May 2010 (UTC)
::@] shouldn't this work the other way around? I don't want to add anything. I want something to be removed which has no reference. ] (]) 15:52, 20 May 2024 (UTC)

:::The article has rather a lot of mentions of the speed of light in vacuum that are supported by references to ]. Merely in ], we have {{tqb|Sometimes {{Math|''c''}} is used for the speed of waves in any material medium, and {{Math|''c''}}<sub>0</sub> for the speed of light in vacuum.<ref name=handbook>See, for example:
:As this article is not about light, it is somewhat out of the scope of the article to provide an explanation of why light travels at a different speed in a medium. I've been bold and rework the section to omit this, and focus on relaying the facts about light propagation in a medium. I've also added a good source discussing in detail the bussiness of group and phase velocities being superluminal or negative, and showing that the front velocity is equal to ''c''. ] (]) 15:48, 25 May 2010 (UTC)
* {{Cite book
::Looks good. I've copy-edited it a little. ] (]) 18:21, 27 May 2010 (UTC)
|last=Lide |first=D. R.

|year=2004
== Am I interpreting this wrong? ==
|title=CRC Handbook of Chemistry and Physics

|url=https://books.google.com/books?id=WDll8hA006AC&q=speed+of+light+%22c0+OR+%22&pg=PT76
'From the observation that the periods of Jupiter's innermost moon Io appeared to be shorter when the earth was approaching Jupiter than when receding from it, he concluded that light travels at a finite speed, and was able to estimate that would take light 22 minutes to cross the diameter of Earth's orbit.'
|pages=2–9

|publisher=]
Light would take about 1/23 of a second to cross the diameter of the earth. What's this 22 minutes all about? <span style="font-size: smaller;" class="autosigned">—Preceding ] comment added by ] (]) 14:02, 29 May 2010 (UTC)</span><!-- Template:UnsignedIP --> <!--Autosigned by SineBot-->
|isbn=978-0-8493-0485-9
:The time needed to cross the diameter of the Earth's orbit. Not the time needed to cross the diameter of the Earth.] (]) 20:04, 29 May 2010 (UTC)
}}

* {{Cite book
== Misusing of refs ==
|last=Harris |first=J. W. |year=2002

|title=Handbook of Physics
{{User|Jagged 85}} is one of the main contributors to Misplaced Pages (over 67,000 edits; he's ranked ]), and practically all of his edits have to do with Islamic science, technology and philosophy. This editor has persistently ]. This editor's contributions are always well provided with citations, but examination of these sources often reveals either a blatant misrepresentation of those sources or a selective interpretation, going beyond any reasonable interpretation of the authors' intent. Please see: ]. The damage is so extensive that it is undermining Misplaced Pages's credibility as a source. I searched the page history, and found 39 edits by Jagged 85 (for example, see of ). ] (]) 20:45, 14 June 2010 (UTC)
|url=https://books.google.com/books?id=c60mCxGRMR8C&q=speed+of+light+%22c0+OR+%22+date:2000-2009&pg=PA499
:From the edits above, the information that needs to be checked is the following text that is currently in the ] section:
|page=499
::] initially agreed with the ] that light had no speed of travel. In 1021, ] ] (Ibn al-Haytham) published the '']'', in which he used experiments related to the ] to support the now accepted intromission theory of ], in which light moves from an object into the eye.
|publisher=Springer
::This led Alhazen to propose that light must therefore have a finite speed, and that the speed of light is variable, decreasing in denser bodies.
|isbn=978-0-387-95269-7
::Also in the 11th century, ] agreed that light has a finite speed, and observed that the speed of light is much faster than the speed of sound.
|display-authors=etal}}
:I have seen the discussion of this editor's work, and there is wide consensus that enormous misrepresentations of sources have occurred, so this text needs to be confirmed. ] (]) 23:55, 14 June 2010 (UTC)
* {{Cite book
::Well, the MacTutor and other sources quoted support the claims made almost verbatim. Since those sources are generally reliable the facts seem OK. The only issue remaining is the somewhat undue emphasis of the Islamic nature of these philosophers. ] (]) 08:24, 15 June 2010 (UTC)
|last=Whitaker |first=J. C.
:::Agreed. Are there any reliable sources suggesting that the religion of the philosophers was relevant to their scientific theories. If not the reference religion should be removed. ] (]) 23:14, 15 June 2010 (UTC)
|year=2005

|title=The Electronics Handbook
::::The point is the culture, not the religion. People of this culture are often referred to as Islamic (or Muslim) or Arabic (that is, by religion or by language) to distinguish them from other cultures that existed at about the same time in the same region. This should not be taken as an indication that the religion had much to do with the science. The ] article refers to Alhazen as an "Arab polymath"; some sources call him "Persian"; he was born in what's now Iraq and lived mostly in Egypt. ] (]) 02:46, 16 June 2010 (UTC)
|url=https://books.google.com/books?id=FdSQSAC3_EwC&q=speed+of+light+c0+handbook&pg=PA235
|page=235
|publisher=CRC Press
|isbn=978-0-8493-1889-4
}}
* {{Cite book
|last=Cohen |first=E. R. |year=2007
|title=Quantities, Units and Symbols in Physical Chemistry
|url=https://books.google.com/books?id=TElmhULQoeIC&q=speed+of+light+c0+handbook&pg=PA143
|page=184
|edition=3rd
|publisher=]
|isbn=978-0-85404-433-7
|display-authors=etal}}</ref> This subscripted notation, which is endorsed in official SI literature<ref name=BIPM_SI_units>{{SIbrochure8th|page=112}}</ref> ....}} I find the idea that we would deny the current definition of the metre rather disturbing. ] (]) 16:20, 20 May 2024 (UTC)
::::This is what I was talking about from the beginning. You are confusing two different concepts as well. ''c''<sub>0</sub> has a place in physics. In one place. Optics. In case of refraction the phase velocity is used for calculations, because the phase of light is shifting constantly if travelling in a medium which is not vacuum. Every other area of physics is using the universal constant ''c'', which can be calculated using ]. By the way the ] article also says "Photons are massless particles that always move at the speed of light when in vacuum." which is plain wrong. Photons are unable to travel slower then ''c''.
::::If you think that Misplaced Pages is correct in its current state, than I won't say anything more. ] (]) 16:58, 20 May 2024 (UTC)
:::::I fixed the ] article thanks. ] (]) 20:20, 20 May 2024 (UTC)
:::@] You asked that all occurences across Misplaced Pages to be changed. I think we better discuss a reference for your claim first. You pointed to one section, ], but it has sources so you need to explain why they should be removed. ] (]) 16:58, 20 May 2024 (UTC)
::::Understanding should be the key here. Please look at this . After watching it you will have the urge searching for references, too. ] (]) 17:10, 20 May 2024 (UTC)
:::::That's a great video. But it's not news, sorry. It explains the atomic model of the index of refraction. Based on this video I recommend no changes. ] (]) 17:58, 20 May 2024 (UTC)


* In the literature:
== Are we ready for FA? ==
::{| class="wikitable" style="text-align: center"
|-
! Google Search !! Scholar !! Books
|-
| "Speed of light in vacuum"
|
|
|-
|}
: Getting rid of the term would be spectacularly against Misplaced Pages's mission. - ] (]) 19:41, 20 May 2024 (UTC)


{{reflist-talk}}
When I returned to this article last week, after about a month I hadn't significantly edited it, I found it even better than I remembered. I am quite positive that it is ready for FA status, but can anyone find any issue with it before I nominate it again? ―&nbsp;<i style="background: white; color: blue; font-weight:600; font-family: monospace">]_di_M.</i><sup style="font-family: fantasy">]</sup> (formerly Army1987) 18:41, 10 July 2010 (UTC)


:I have an issue with the 'Fundamental role in physics section. In this section there important facts about the subject of the article have been relegated to footnotes. Why are they not in the main text? ] (]) 23:40, 10 July 2010 (UTC) :If the speed of light is always the same, then ] shouldn't exist. ] (]) 09:24, 21 May 2024 (UTC)
::The Doppler effect and the Terrell rotation are quite irrelevant to the point being made, so I would keep them where they are. The Scharnhorst effect is so small that it might well be never observed during the lifetime of anyone around here, so it was agreed that it was undue weight to put it in the main text.
::Anyway, I'm going to move the reference to the relativity of simultaneity and the tachyonic antitelephone back into the main text (being sent to a footnote only comprising one link distracts more than seeing it in the text, IMO). As for the note on one-way vs two-way speed, I think it should be in the main text too, but I seem to remember there once was an opposition to that and was moved to the footnote as a compromise. ―&nbsp;<i style="background: white; color: blue; font-weight:600; font-family: monospace">]_di_M.</i><sup style="font-family: fantasy">]</sup> (formerly Army1987) 13:56, 11 July 2010 (UTC)


:The existence of Cherenkov radiation is already mentioned at the end of {{section link|Speed of light#In a medium}}. <span style="box-shadow:2px 2px 6px #999">]]</span> 10:16, 21 May 2024 (UTC)
== Measurement ==


== Speed of light in literature ==
The introduction says about the speed of light: “Its value is <u>exactly</u> 299,792,458 metres per second”. is cited in this connection, as is “turning ''c'' into a conversion factor whose value is fixed and arbitrary” (p. 280). I'd suggest that some reconciliation of this viewpoint be attempted in the section which explains many methods for determining the speed of light and cites values like 299,710±22 km/s with error bars, as is appropriate only for an uncertain quantity, not an exact value.


I think there needs to be a section about speed of light in popular culture somewhere, namely the teleportation gimmick used areas like in Star Wars and Kingdom Hearts. The disambiguation mentions a few examples but not this article. ] (]) 00:00, 15 October 2024 (UTC)
I do not feel comfortable in pursuing a discussion of these matters, which caused me a great deal of difficulty with ArbCom in the past, and brought the most extreme invective and vituperation upon me that I have experienced in my 71 odd years of life.


:We already have a page on ] and one on ] and on ]. ] (]) 01:32, 15 October 2024 (UTC)
However, it still seems to me that a naive reader is likely to wonder what is going on here. I hope that some enterprising soul can brave this wilderness to bring some clarity to the Measurement section. That involves at most the addition of a few sentences of reconciliation. ] (]) 20:49, 15 July 2010 (UTC)
:As for the introduction, the reader is going to understand what is going on by the time they get to the end of the third paragraph. Maybe a paragraph could be added after the first paragraph of "Measurement" stating that today measuring ''c'' in metres serves no purpose (other than verifying that your measuring instruments are properly calibrated and properly working) and that measuring it in some other unit is equivalent to measuring the length of other unit in metres ... but I'm not sure of how to word it. What would you propose? <small><span class="autosigned">—Preceding ] comment added by ] (] • ]) 08:06, 16 July 2010 (UTC)</span></small><!-- Template:Unsigned --> <!--Autosigned by SineBot-->


:A fairly well-known old science-fiction story where the speed of light plays a prominent role is "]" by Philip Latham. The speed of light actually remains the same, but other things change, resulting in the doom of the universe... ] (]) 07:41, 15 October 2024 (UTC)
::A. di M.: I wouldn't hazard an attempt at wording given the history of conduct among editors of this article.
::The underlying difficulty with the ] article is short shrift given to how a system of units based upon replacing ‘distance’ with ‘time-of-flight’ compares with a different system where distance and time are kept separate. That comparison could be done in a general manner for any speed standard, explaining the need for reassurance that the "standard" speed has been realized in any given measurement, and the role of definitions in making that speed "exact". That presentation could then be narrowed to describe why light-speed is a good choice. ] (]) 11:36, 16 July 2010 (UTC)

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Is this part accurate in History?

Quote:

Connections with electromagnetism

In the 19th century Hippolyte Fizeau developed a method to determine the speed of light based on time-of-flight measurements on Earth and reported a value of 315000 km/s (704,634,932 m/h).

His method was improved upon by Léon Foucault who obtained a value of 298000 km/s (666,607,015 m/h) in 1862. Kailandosk (talk) 01:06, 18 November 2023 (UTC)

Are you suggesting our article may not be correct or proposing that it include conversions to km/h at that point, and in either case, why? NebY (talk) 11:07, 18 November 2023 (UTC)
There's a definite discrepancy in number of significant digits between the quoted metric and traditional measurements... AnonMoos (talk) 13:10, 18 November 2023 (UTC)
Indeed, but the values in parentheses aren't in the article. If we wanted to include them, we could use {{Convert}}, which would probably round them appropriately automatically, and wouldn't abbreviate miles to "m" either, but I don't see why we'd want to include such conversions in that part of the article anyway. NebY (talk) 13:46, 18 November 2023 (UTC)
I added the parenthesis. It's just a conversion to m/h that I made, just to show how different they are & to convert it into U.S. terms. Kailandosk (talk) 00:15, 24 December 2023 (UTC)
I'm not sure if 315000 or 298000 km/s is correct. I feel it's 315000 km/s, but I'm not sure. Kailandosk (talk) 18:04, 25 November 2023 (UTC)

Why not also include an accurate description of c in miles per second?

186282.3970512 mi/s, to be fairly accurate.

Speed of light in vacuum

Misplaced Pages should get rid of all occurrences of the phrase "speed of light in vacuum". There is only one speed of light, which is a universal constant. Also the speed of light doesn't change if not in vacuum. Group velocity represents the real speed of a photon, and that doesn't change. Only phase velocity is changing, causing the optical effects that mislead people. But this very article is explaining the same in the section Speed of light#In a medium. Lustakutya (talk) 13:24, 20 May 2024 (UTC)

If you have a reference for your point of view please share it. Johnjbarton (talk) 15:15, 20 May 2024 (UTC)
@Johnjbarton shouldn't this work the other way around? I don't want to add anything. I want something to be removed which has no reference. Lustakutya (talk) 15:52, 20 May 2024 (UTC)
The article has rather a lot of mentions of the speed of light in vacuum that are supported by references to reliable sources. Merely in Speed of light#Numerical value, notation, and units, we have

Sometimes c is used for the speed of waves in any material medium, and c0 for the speed of light in vacuum. This subscripted notation, which is endorsed in official SI literature ....

I find the idea that we would deny the current definition of the metre rather disturbing. NebY (talk) 16:20, 20 May 2024 (UTC)
This is what I was talking about from the beginning. You are confusing two different concepts as well. c0 has a place in physics. In one place. Optics. In case of refraction the phase velocity is used for calculations, because the phase of light is shifting constantly if travelling in a medium which is not vacuum. Every other area of physics is using the universal constant c, which can be calculated using Maxwell's equations. By the way the Photon article also says "Photons are massless particles that always move at the speed of light when in vacuum." which is plain wrong. Photons are unable to travel slower then c.
If you think that Misplaced Pages is correct in its current state, than I won't say anything more. Lustakutya (talk) 16:58, 20 May 2024 (UTC)
I fixed the Photon article thanks. Johnjbarton (talk) 20:20, 20 May 2024 (UTC)
@Lustakutya You asked that all occurences across Misplaced Pages to be changed. I think we better discuss a reference for your claim first. You pointed to one section, Speed of light#In a medium, but it has sources so you need to explain why they should be removed. Johnjbarton (talk) 16:58, 20 May 2024 (UTC)
Understanding should be the key here. Please look at this video. After watching it you will have the urge searching for references, too. Lustakutya (talk) 17:10, 20 May 2024 (UTC)
That's a great video. But it's not news, sorry. It explains the atomic model of the index of refraction. Based on this video I recommend no changes. Johnjbarton (talk) 17:58, 20 May 2024 (UTC)
  • In the literature:
Google Search Scholar Books
"Speed of light in vacuum" 67,400 68,200
Getting rid of the term would be spectacularly against Misplaced Pages's mission. - DVdm (talk) 19:41, 20 May 2024 (UTC)

References

  1. See, for example:
  2. International Bureau of Weights and Measures (2006), The International System of Units (SI) (PDF) (8th ed.), p. 112, ISBN 92-822-2213-6, archived (PDF) from the original on 2021-06-04, retrieved 2021-12-16
If the speed of light is always the same, then Cherenkov radiation shouldn't exist. AnonMoos (talk) 09:24, 21 May 2024 (UTC)
The existence of Cherenkov radiation is already mentioned at the end of Speed of light § In a medium.  Dr Greg  talk  10:16, 21 May 2024 (UTC)

Speed of light in literature

I think there needs to be a section about speed of light in popular culture somewhere, namely the teleportation gimmick used areas like in Star Wars and Kingdom Hearts. The disambiguation mentions a few examples but not this article. Jordf32123 (talk) 00:00, 15 October 2024 (UTC)

We already have a page on Teleportation and one on Teleportation in fiction and on warp drive. Johnjbarton (talk) 01:32, 15 October 2024 (UTC)
A fairly well-known old science-fiction story where the speed of light plays a prominent role is "The Xi Effect" by Philip Latham. The speed of light actually remains the same, but other things change, resulting in the doom of the universe... AnonMoos (talk) 07:41, 15 October 2024 (UTC)
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