Talk:Speed of light: Difference between revisions

Browse history interactively ← Previous editContent deleted Content addedVisual WikitextInline

Revision as of 02:27, 15 August 2004 editAnthony (talk \| contribs)Extended confirmed users17,889 edits →begging the question ← Previous edit		Latest revision as of 08:22, 12 January 2025 edit undoRemsense (talk \| contribs)Extended confirmed users, Page movers, New page reviewers, Template editors63,000 edits Reverting edit(s) by 108.190.148.96 (talk) to rev. 1256166987 by DVdm: Non-constructive edit (UV 0.1.6)Tags: Ultraviolet Undo
Line 1:		Line 1:
	{{~~fac~~}}		{{skip to talk}}
			{{talk header\|noarchive=yes}}
			{{Article history
			\|action1=FAC
			\|action1date=20:53, 17 Aug 2004
			\|action1link=Misplaced Pages:Featured article candidates/Speed of light/archive1
			\|action1result=promoted
			\|action1oldid=5315343

			\|action2=FAR
	The following needs to be reworked to make it fit in the context of an encyclopedia article. As it is it is a bit too chatty.
			\|action2date=23:46, 7 December 2008
			\|action2link=Misplaced Pages:Featured article review/Speed of light/archive1
			\|action2result=removed
			\|action2oldid=256337011

			\|action3=PR
	:=== How Fast is the Speed of light ===
			\|action3date=10:00, 21 November 2009 (UTC)
			\|action3link=Misplaced Pages:Peer review/Speed of light/archive1
			\|action3result=Reviewed
			\|action3oldid=326969975

			\|action4=FAC
	:I like to use the vacation analogy to give people a feel for how fast the speed of light is. It goes like this. Let’s say I wanted to take a vacation on the moon. Fortunately there is a highway called Pretend that connects the earth to the moon. The speed limit on highway Pretend is 100 mph and I can only drive 10 hours a day. I had better pack a big trailer with plenty of food and pull it behind my SUV because under these conditions it is going to take me about 250 days to get from the earth to the moon. Light can travel the same distance in one and one forth seconds or about 5 beats of the drum at one-quarter time.
			\|action4date=21:05, 25 January 2010
			\|action4link=Misplaced Pages:Featured article candidates/Speed of light/archive2
			\|action4result=not promoted
			\|action4oldid=339898368

			\|action5=PR
	-----
			\|action5date=18:42, 12 October 2010
			\|action5link=Misplaced Pages:Peer review/Speed of light/archive2
			\|action5result=reviewed
			\|action5oldid=390277913

			\|action6=FAC
	As a layman, I don't understand how it's possible for something to travel faster than c but not carry information faster than c. Could someone explain this? -- ]
			\|action6date=04:35, 20 December 2010
	:A very rough explanation is that the "something' that travel faster than light doesn't carry energy. -- ] 00:43 Apr 19, 2003 (UTC)
			\|action6link=Misplaced Pages:Featured article candidates/Speed of light/archive3
	:Also, it is only in a vacuum than nothing (no information) can travel faster than light; in a medium thing can travel faster than light (see ]) -- ] 00:49 Apr 19, 2003 (UTC)
			\|action6result=promoted
			\|action6oldid=403246761

			\|action7 = FAR
	Irrelevant. The electrons involved in the ] are still going slower than c. (It is true that light travels even slower than those electrons in that medium).
			\|action7date = 2022-03-19
			\|action7link = Misplaced Pages:Featured article review/Speed of light/archive2
			\|action7result = kept
			\|action7oldid = 1077590852

			\|currentstatus=FA
	:The fact that ] is greater than c doesn't mean that there is really a particle moving (travelling) at that speed but rather that something is changing at this speed, in this case the waveform. (See also ]) -- ] 16:44, 18 Jul 2004 (UTC)
			\|maindate=October 29, 2004
			\|maindate2=16 August 2022
			}}
			{{WikiProject banner shell\|collapsed=yes\|class=FA\|vital=yes\|1=
			{{WikiProject Physics\|importance=Top \|relativity=yes }}
			}}
			{{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}}
			{{User:MiszaBot/config
			\|archiveheader = {{aan}}
			\|maxarchivesize = 250K
			\|counter = 18
			\|minthreadsleft = 4
			\|algo = old(90d)
			\|archive = Talk:Speed of light/Archive %(counter)d
			}}
			{{archive box \|search=yes \|bot=MiszaBot I \|age=3 \|units=months \|index=/Archive index\|
			* ] (Up to end of 2004)
			* ] (2005 – July 2006)
			* ] (July 2006 – end of 2006)
			* ] (2007)
			* ] (2008)
			* ] (Jan 2009 – Feb 2009)
			* ] (Feb 2009 — July 2009)
			* ] (July–Aug 2009)
			* ] (August 2009)
			* ] (Aug–Sept 2009)
			* ] (Sept–Oct 2009)
			* ] (Oct–Dec 2009)
			* ] (Nov 2009 – May 2010)
			* ] (May–Aug 2010)
			* ] (Aug–Dec 2010)
			* ] (Feb 2011 – Apr 2014)
			* ] (Jan 2014 – )
			}}
			{{DEFAULTSORT:Speed of light}}
			{{User:HBC Archive Indexerbot/OptIn
			\|target=/Archive index \|mask=/Archive <#> \|leading_zeros=0 \|indexhere=yes
			}}

			== Is this part accurate in History? ==
	Sure. Take a laser pointer. Make a spot on the moon. Then turn your wrist to make a spot on the earth. It takes over a second for light to travel from the moon to the earth, yet I can move that little spot the same distance in far less than one second.
	--] 03:54, 20 Jul 2004 (UTC)

			Quote:
	Similar, probably better, explanations:

			'''Connections with electromagnetism'''
	*http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/FTL.html#3
	*http://www.phy.duke.edu/research/photon/qelectron/proj/infv/fast_tut.ptml
	*http://www2.abc.net.au/science/k2/stn-archive1/posts/topic42526.shtm
	*http://answers.google.com/answers/threadview?id=73321
	*http://math.ucr.edu/home/baez/physics/Relativity/SR/scissors.html

			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. ] (]) 01:06, 18 November 2023 (UTC)
	"It is a solution to the ]"

			: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)
	:How is the speed of light be a solution to a vector equation? ] 08:08, 13 Nov 2003 (UTC)
	::As my electromagnetics professor explained it (and you'll have to bear with me - it's been almost 2 years), it's not that the speed is the solution, per se. It just doesn't have a solution for any other speed besides C. --] 08:28, 13 Nov 2003 (UTC)
	:::One more thing. Here is the exact derivation you are looking for: http://people.ccmr.cornell.edu/~muchomas/P214/Notes/OtherWaves/node18.html --] 08:36, 13 Nov 2003 (UTC)

			::There's a definite discrepancy in number of significant digits between the quoted metric and traditional measurements... ] (]) 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 {{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? ==
	If I understood correctly. <- This sentence was written by me. ] 21:30, 16 Nov 2003 (UTC) The rest wasn't. -> Faster than light transmission of information follows some uncertainty principals, it also sidesteps a couple rules. When information is transmitted at such speeds, it can never be proven that the light recieved is the light that was transmitted. Photons subjected to this process have their frequency changed, their overall energy content is different due to the processes that caused this feat. However, if those people in line were to shout in sequence, the information would have to be previously known, this caused it's own speculation. As with the noted experiment of 300c, the photons arrived faster than light accounts for, the arrival of the photons is information, it arrived at it's destination faster than C, there IS NO explanation. - GouRou

			186282.3970512 mi/s, to be fairly accurate.
	----

			== Speed of light in vacuum ==
	Wile, why are you removing the scientific notation approximation? In 99% of cases when I'm performing a calculation involving the speed of light, the number I'm looking for is 3 × 10<sup>8</sup>. I'm pretty sure this approximation, in this format, is useful to other people as well. ] ] 18:49, 8 Jun 2004 (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)
	:Well, it is cluttering, as it doesn't serve any obvious purpose. Anyone who is actually making use of ''c'' in computations is surely capable of approximating it as 3 times 10^8 or 0.2998 times 10^9 or whatever they please. The vast majority of the remainder of the readers will be much more at home with "thousands of somethings" instead of scientific notation. -- I feel pretty strongly that the introductory sentences of an article must get right straight to the point. Naturally it is quite possible that the introduction still isn't getting there, so let's work in that direction. Regards, ] 20:45, 8 Jun 2004 (UTC)

			:If you have a reference for your point of view please share it. ] (]) 15:15, 20 May 2024 (UTC)
	::How would you know that anyone actually making use of ''c'' is capable of approximating it? Every high school student? It isn't immediately obvious to everybody that 100,000 kilometers equals 10<sup>8</sup> meters. And sure, they might be able to figure it out, but our job is to make this information available as conveniently as possible. Perhaps the note should be placed elsewhere, but there's no reason not to provide it. ] ] 21:02, 8 Jun 2004 (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:
			* {{Cite book
			\|last=Lide \|first=D. R.
			\|year=2004
			\|title=CRC Handbook of Chemistry and Physics
			\|url=https://books.google.com/books?id=WDll8hA006AC&q=speed+of+light+%22c0+OR+%22&pg=PT76
			\|pages=2–9
			\|publisher=]
			\|isbn=978-0-8493-0485-9
			}}
			* {{Cite book
			\|last=Harris \|first=J. W. \|year=2002
			\|title=Handbook of Physics
			\|url=https://books.google.com/books?id=c60mCxGRMR8C&q=speed+of+light+%22c0+OR+%22+date:2000-2009&pg=PA499
			\|page=499
			\|publisher=Springer
			\|isbn=978-0-387-95269-7
			\|display-authors=etal}}
			* {{Cite book
			\|last=Whitaker \|first=J. C.
			\|year=2005
			\|title=The Electronics Handbook
			\|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:
	::: We don't seem to be discussing anything very fundamental here; the article is about the speed of light, not scientific notation. That 299,792,458 is close to 3 x 10^8 seems to be a footnote that is useful in some contexts. Maybe there is a place for it somewhere in the article (or maybe not) but it seems far from central. ] 21:24, 8 Jun 2004 (UTC)
			::{\| 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}}
	:::: This is not an article about scientific notation, but it is neither an article about obsolete imperial units (the mile). The question is indeed whether the information is central (useful to someone), and I hold that it is. It would be nice to get input from a few other people with regard to this matter. ] ] 07:36, 9 Jun 2004 (UTC)

			:If the speed of light is always the same, then ] shouldn't exist. ] (]) 09:24, 21 May 2024 (UTC)
	::::: I've put the 3 × 10^8 m/s just before 30 cm/ns (under the heading "Overview"). It is appropriate since both are convenient approximations. ] 16:21, 9 Jun 2004 (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)
	:::::: Thank you. ] ] 16:30, 9 Jun 2004 (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. ] (]) 00:00, 15 October 2024 (UTC)
	:::::: I'd like to support Fredrik on this point. It's far simpler to get a quick idea of the scale of ''c'' without having to count the number of digits in 300,000,000. ] 16:52, 9 Jun 2004 (UTC)

			:We already have a page on ] and one on ] and on ]. ] (]) 01:32, 15 October 2024 (UTC)
	----

			: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)
	I think that perhaps it should be clearly stated that by adding two velocities with the Einstein velocity addition formula we cannot get a speed greater than c. (Because the interval (-c,c) with that operation is an ].) I know that its said that c is a "speed limit" but what its not said is that the formula is consistent with that. -- ] 18:09, 18 Jul 2004 (UTC)


	----

	The article currently says:

	: '''Definition of the Metre'''

	: Since the speed of light in vacuum is constant, it is convenient to measure both time and distance in terms of c . Both the SI unit of length and SI unit of time have been defined in terms of wavelengths and cycles of light. In 1983 the metre was redefined in terms of c .

	I'm fairly certain that is correct for "distance", but incorrect for "time". If you read
	http://www.bldrdoc.gov/timefreq/general/precision.htm#Anchor-60273
	carefully,
	you see that "transitions of the cesium atom" are the reference for time, ''not'' the speed of that light.

	Suggested replacement:

	: '''Definition of the Metre'''
	: Since the speed of light in vacuum is constant, it is convenient to measure distance in terms of c and time. In 1983, the SI unit of length (the meter) was redefined in terms of c and time.

	:: Sounds good to me. Go ahead and change it if you want, I'll do it if you don't get around to it. ] 03:15, 21 Jul 2004 (UTC)

	For a Featured Article this should have some graphics. Yes, I know it it is hard to find a good graphic for speed of light, but perhapse will have something we can use? --] 21:37, 31 Jul 2004 (UTC)

	:A diagram of the Fizeau apparatus would fill the requirement, and might be useful in explaining that experiment. ] 08:25, Aug 6, 2004 (UTC)


	----

	In ] Galileo claimed to have performed his experiment, not just proposed it (though in a more ambiguous way than I'd realized); he also said explicitly, long before Hooke, that his experiment couldn't prove that the speed was infinite. Assuming he did perform it, this is the first known case of anyone's trying to make an actual measurement of the speed. The criticism by Descartes is interesting and looks as if it would be valid; does anyone know the argument in enough detail to say? ] 08:25, Aug 6, 2004 (UTC)

	==begging the question==
	''According to the theory of ], all observers will measure the speed of light as being the same, regardless of the ] of the observer or the velocity of the object emitting the light. A simple three-step analysis is sufficient to show that this is the case:
	# the speed of light in vacuum can be derived from ];
	# special relativity requires the laws of physics, such as Maxwell's equations, to be identical in all ] ], and so
	# observers in all such frames must observe the same speed of light.
	The constant speed of light derives from the ], which distort distances and times for observers travelling at large velocities in the same way.''

	Isn't 3 precisely what we are trying to show is the case? "all observers will measure the speed of light as being the same" because "observers in all such frames must observe the same speed of light"? Also, as the speed of light in a vaacum is a defined value, the fact that it can be derived from Maxwell's equations needs to be explained. ] ] 19:00, 12 Aug 2004 (UTC)

	:Er - to précis - "ToSR says SoL is constant. How? Because (i) SoL comes directly from Maxwell, (ii) Maxwell is a law of physics, (iii) laws of physics must be the same in all inertial frames; hence, (iv) SoL same in all inertial frames. Plus Lorentz gives you a way to work out how it all works." I don't think this is begging the question, is it? -- ] ] 19:14, 12 Aug 2004 (UTC)

	I guess I see what is being said. 3 is the conclusion from 1 and 2. This seems to be worded strangely, though. ] ] 19:41, 12 Aug 2004 (UTC)

	:Feel free to copyedit. :) -- ] ] 19:48, 12 Aug 2004 (UTC)

	As you've perhaps seen I've already made a number of copyedits. This one I'm not sure how to reword, though, in part because I'm not 100% sure what the point is. ] ] 20:12, 12 Aug 2004 (UTC)

	:I've tweaked slightly - is that better? -- ] ] 02:05, 15 Aug 2004 (UTC)

	::I guess. I'm still confused as to what we are showing to be the case. Are we showing that "all observers will measure the speed of light as being the same" or are we showing that this is a result of the theory of relativity? Because it's not really a result of the theory or relativity, it's a postulate of the theory or relativity (I think, I'll double-check that one). ] ] 02:21, 15 Aug 2004 (UTC)

	::Yes, "We will raise this conjecture (the purport of which will hereafter be called the ``Principle of Relativity'') to the status of a postulate, and also introduce another postulate, which is only apparently irreconcilable with the former, namely, that light is always propagated in empty space with a definite velocity c which is independent of the state of motion of the emitting body." Well, sort of. Einstein goes beyond saying that "all observers will measure the speed of light as being the same", and actually posits that the speed of light always '''is''' the same. ] ] 02:27, 15 Aug 2004 (UTC)

	== Some doubts and/or contradictions ==

	The article looks great! But... (ther's always a but):<br>
	(a) in 1: «The constant speed of light derives from the Lorentz transforms, which distort distances and times for observers travelling at large velocities in the same way» - I think it's the other way around. The Lorentz transforms derives from Maxwell's equations and Einstein's constant speed of light postulate.<br>

	(b) in 4.1: «Einstein took result as a given fact» contradicted by «It is uncertain whether Albert Einstein knew the results of the Michelson-Morley experiment» (in 5.4). - I don't know wich is true.<br>
	(c) I failed to understand the concept of "light slowing". Can someone elaborate on that?--] 00:12, 2004 Aug 13 (UTC)

	:re (a): Yes, that's the same point I argued. In a way, the very notion of "distance" requires some means of measuring it. Einstein's reasoning was to require everything to be done locally, at one point. So, he ''defined'' distance as the quantity that you can measure by sending a light pulse to a mirror and wait for it to come back, than halve the time it took and devide it by ''c''. You need this definition to derive the Lorentz transform. But the definition is only compatible to the usual notion of distance if you assume ''c'' to be constant.

	:re (b) I remember having read that Einstein said, that he was unaware of the works of Lorentz and Fitzgerald, who had found the Lorentz transformation and the relativistic length contraction before Einstein. (However, they only postulated them heuristically in order to explain the Michelson-Morley experiment and the anomalies of the Thompson parabolas, without being able to give a derivation.) But I doubt that Einstein was unaware of the whole aether discussion as otherwise he surely must have slept well during his university studies.

	:But let's check his origibnal 1905 paper which is translated : "''Examples of this sort, together with the unsuccessful attempts to discover any motion of the earth relatively to the "light medium," suggest that the phenomena of electrodynamics as well as of mechanics possess no properties corresponding to the idea of absolute rest.''" So what else could he have had in mind when referring to these "unsuccesful attempts" other than the Michelson-Morley experiment? => The second sentence is wrong.

	:re (c): It's not well written. First one should mention that of course light is always slower than ''c'' when travelling through media. But, usually, the light speed is perhaps 3/4 of it (in water), or maybe half of it (]) - but recently, people managed to reduce the speed down to only a few meters per second, using additional laser fields which modify the optical properties of the medium. In my opinion this should go in a seperate article - or maybe we have one already.

	: ] 08:57, 13 Aug 2004 (UTC)

	::Thanks.

	:: Re (a): of course, Lorentz formulated his transforms before Einstein thought up relativity, but I see what you mean. The intention here was simply to explain how a person travelling at 0.9''c'' could still measure a speed of light of ''c'' because distances and times are dilated. If you can think of a better way of doing it, please be my guest.

	:: Re (b): One of the sentences must be wrong. I've seen it reported a couple times that Einstein was not aware (or said he was not aware) of the Michelson-Morley result in 1905, so I've fixed the first passage (in any event, I think there were other, also unsuccessful, attempts to measure the velocity of the aether, apart from Michelson-Morley).

	:: Re (c): I've reworded substantially. -- ] ] 02:05, 15 Aug 2004 (UTC)

Latest revision as of 08:22, 12 January 2025

Skip to table of contents

This is the talk page for discussing improvements to the Speed of light article.
This is not a forum for general discussion of the article's subject.

Put new text under old text. Click here to start a new topic.
New to Misplaced Pages? Welcome! Learn to edit; get help.

Article policies

Find sources: Google (books · news · scholar · free images · WP refs) · FENS · JSTOR · TWL

Speed of light is a featured article; it (or a previous version of it) has been identified as one of the best articles produced by the Misplaced Pages community. Even so, if you can update or improve it, please do so.

This article appeared on Misplaced Pages's Main Page as Today's featured article on October 29, 2004, and on August 16, 2022.

Article milestones
Date	Process	Result
August 17, 2004	Featured article candidate	Promoted
December 7, 2008	Featured article review	Demoted
November 21, 2009	Peer review	Reviewed
January 25, 2010	Featured article candidate	Not promoted
October 12, 2010	Peer review	Reviewed
December 20, 2010	Featured article candidate	Promoted
March 19, 2022	Featured article review	Kept
Current status: Featured article

This

level-3 vital article is rated FA-class on Misplaced Pages's content assessment scale.
It is of interest to multiple WikiProjects.

Physics: Relativity Top‑importance

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

There is a request, submitted by ScientistBuilder (talk)ScientistBuilderScientistBuilder (talk) 17:18, 29 January 2022 (UTC), for an audio version of this article to be created. For further information, see WikiProject Spoken Misplaced Pages.

The rationale behind the request is: "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".

Archives

Archive 1 (Up to end of 2004)
Archive 2 (2005 – July 2006)
Archive 3 (July 2006 – end of 2006)
Archive 4 (2007)
Archive 5 (2008)
Archive 6 (Jan 2009 – Feb 2009)
Archive 7 (Feb 2009 — July 2009)
Archive 8 (July–Aug 2009)
Archive 9 (August 2009)
Archive 10 (Aug–Sept 2009)
Archive 11 (Sept–Oct 2009)
Archive 12 (Oct–Dec 2009)
Archive 13 (Nov 2009 – May 2010)
Archive 14 (May–Aug 2010)
Archive 15 (Aug–Dec 2010)
Archive 16 (Feb 2011 – Apr 2014)
Archive 17 (Jan 2014 – )

This page has archives. Sections older than 90 days may be automatically archived by Lowercase sigmabot III when more than 4 sections are present.

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

See, for example:
- Lide, D. R. (2004). CRC Handbook of Chemistry and Physics. CRC Press. pp. 2–9. ISBN 978-0-8493-0485-9.
- Harris, J. W.; et al. (2002). Handbook of Physics. Springer. p. 499. ISBN 978-0-387-95269-7.
- Whitaker, J. C. (2005). The Electronics Handbook. CRC Press. p. 235. ISBN 978-0-8493-1889-4.
- Cohen, E. R.; et al. (2007). Quantities, Units and Symbols in Physical Chemistry (3rd ed.). Royal Society of Chemistry. p. 184. ISBN 978-0-85404-433-7.
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)

Categories: