Talk:Speed of light/Archive 17

Light speed versus light wavefront speed
In the third paragraph, "For example, in videos of an intense lightning storm on the Earth's surface taken from the International Space Station, the expansion of light wavefronts from individual flashes of lightning is clearly visible, and allows estimates of the speed of light to be made from frame-to-frame analysis of the position of the light wavefront."

What does this even mean? How can you see light that is not the wave of light? How can the wave of light propagate without it being visible? In my reckoning, when a light wave front reaches your eye, you see it... what deviation is there between a light wavefront and the light itself? — Preceding unsigned comment added by 75.172.120.211 (talk • contribs) 22:42, 9 January 2014 (UTC)

Imagine you fly with a spaceship and look downwards onto the clouds of a thunderstorm. A sudden flash of lightning occurs in the center of view. The light propagates away in all directions and causes stray light from the clouds as it moves along. It is that stray light which gives the appearance of light moving from the center outwards. 91.115.218.226 (talk) 21:21, 13 February 2014 (UTC)


 * Please sign your talk page messages with four tildes ( ~ ). Thanks.
 * Please ask at wp:reference desk/Science. Here we discuss the article, not the content—see wp:talk page guidelines. Cheers. - DVdm (talk) 07:35, 10 January 2014 (UTC)


 * This is relevant inasmuch as the phrasing should be changed. The word "wavefront" without qualification should not be used here. We are observing light scattered from the expanding wavefront, which creates the perception of viewing an expanding source of light. Imagine you observe the same phenomenon from an acute angle. What we observe is the further scattering zone apparently retreating at a little over half the speed of light through the medium, and the nearer portion of the scattering zone apparently moving at a superluminal speed. It might be helpful to speak of observation of when light scattered from the expanding wavefront reaches the camera. —Quondum 03:56, 14 February 2014 (UTC)


 * I came here for this exact reason. I think this description is not helpful for the layman.  It sounds very interesting though and I think if someone could explain it better for the layman (in the article) that would be fabulous. 71.34.93.102 (talk) 19:44, 29 April 2014 (UTC)
 * I am actually not sure how suitable this bit is for the lede of this article. Sure, the visuals provide a powerful illustration of the finite speed of light. However, without the actual visual images it just becomes a hard exercise in imagination. Not to mention that it is unsourced and not further discussed in the rest of the article. This example would be much better suited for the main body of the article. Especially, if we can public domain video images to go along side it.
 * Speaking of the stuff in the lede that is not discussed in the main body. Right after this the paragraph continues " This transit time is what causes the Schumann resonance". Besides being a cryptic useless statement for the lede, this is not discussed in the main body.TR 08:16, 30 April 2014 (UTC)

Semi-protected edit request on 11 May 2014
Please change the text in the article from 1 to 2 in the article: 1: The γ factor approaches infinity as v approaches c, and it would take an infinite amount of energy to accelerate an object with mass to the speed of light. The speed of light is the upper limit for the speeds of objects with positive rest mass

2: The γ factor approaches infinity as v approaches c, and it would take an infinite amount of energy to accelerate an object with mass to the speed of light. The speed of light is the upper limit for the speeds of objects with positive rest mass. However, there are some mathematical challenges within this underlying logic (reference link) (reference link): http://tsiwach.blogspot.in/2014/05/a-discussion-on-universality-of-maximum.html

Tapan Paramhans Siwach (talk) 04:08, 11 May 2014 (UTC)


 * ❌ per wp:reliable sources. Blogspots don't belong in that category. - DVdm (talk) 07:50, 11 May 2014 (UTC)

The time it takes light to travel one meter?
Stephen Hawking says that the time it takes for light to travel one meter is 0.000000003335640952 seconds, as measured by a cesium clock. But I don't know if this is accurate? Should I edit this into the page? — Preceding unsigned comment added by BangBangClubUK (talk • contribs) 03:17, 17 May 2014 (UTC)


 * I note that you've removed the signature added by the bot, so I've added it back. Please adhere to talk page guidelines, and sign your posts with four tildes.
 * The infobox already gives this information, albeit to a lower precision, and I see no reason to make a change. The exact speed of light is given in the article, allowing anyone to easily calculate of the time takes to travel a metre to arbitrary precision. —Quondum 03:27, 17 May 2014 (UTC)

Travelling energy and fields?
Rewording in the lede has lede to the nonsensical statements that a) energy (a quantity not a thing) is travelling, and b) that fields are travelling. Why? TR 21:48, 27 April 2014 (UTC)


 * I agree with the sentiment that energy (and fields) cannot be considered as "travelling". How does one even define in the classical setting what this is, in terms of the energy–momentum tensor? This is enough to consider it suspect for use in the lede. —Quondum 22:21, 27 April 2014 (UTC)


 * (Not to mention that energy can "leak" out of a light cone (e.g. black hole evaporation, but also quantum tunneling effects). So, if you would find a suitable ways of saying that energy travels, it probably won't be limited by the speed of light).TR 08:07, 28 April 2014 (UTC)


 * That's quite a subtle point, but not necessarily conclusive: if energy density can be negative (as it is sometimes regarded to be briefly or over short ranges in quantum mechanics), this could be interpreted as negative energy "travelling" at less than the speed of light in the opposite direction in the case of black hole evaporation. In the case of tunnelling, the interpretation of faster than light tunnelling is almost certainly not valid. —Quondum 13:38, 28 April 2014 (UTC)


 * I know. That is a approximately what happens. There is certainly no field excitations travelling faster than the speed of light. My point was that even in the weakest sense that one could define "travelling of energy", i.e. by looking at the flow of energy, that flow won't necessarily respect light cones. This all illustrating the point that the notion of "energy travelling" is problematic and not very useful. (It is "things" with energy that travel, energy itself is not a "thing".)TR 13:52, 28 April 2014 (UTC)


 * So, how do we change the wording so that it is actually saying something concrete rather than talking about an ill-defined concept? The lede is trying to say that the speed of light in vacuum is a sort of cosmic speed limit for "everything", but is not succeeding. The best that I can come up with is that causality that is constrained in this way. Classically, objects travelling are considered constrained in this way, but objects are ill-defined in a field theory. —Quondum 16:38, 28 April 2014 (UTC)


 * 1)Remove energy from the the list in the 4th sentence (i.e. make it a statement about matter and information).
 * 2)Remove "travel" from the 5th sentence. (i.e. talk about the speed of massless particles and fields) If felt necessary "travel" can be replaced by "propagate".
 * Given most changes to the lede have been battled to death in this article, I did not want to stir the hornest nest by just making them. However, if there are no objections I'll make those changes later.TR 16:56, 28 April 2014 (UTC)


 * Heh – yes, I can imagine that changes have been and may be contested. It is reasonable to make the statements a little inexact in the lede if they communicate the gist accurately enough (as with "matter"), especially when being precise would make it less readable. I support both your suggested changes. I would further want to replace "fields" in the fifth sentence with something like "field perturbations". —Quondum 18:22, 28 April 2014 (UTC)


 * If you work with classical (nonquantum) theories, you can define the speed of light for waves by talking about the manner in which small (or large) local changes in the wave get propagated. That is, if two waves are equal at the start time t=t_0 except on a set U, then they remain equal at later times except on the future of U (the union of the future light cones of all points in U).


 * This works in flat (Minkowski) space, but applies equally well in general relativity. Indeed, it also applies to the evolving space-time itself viewed as a field, as it must since it is now coupled to the other fields. Inside the future of U, the two space times will differ because of interactions with the differing matter fields (thus you should call these two futures by two different names since they are in two different worlds with different shapes) -- but outside the future of U the spacetimes and their contents will be identical.


 * While I agree that it is difficult to localize energy in general relativity without choosing a frame of some kind (long discussion here !!!!), nonetheless the heuristic statement "no energy crosses the light cone toward the outside" is given a good meaning by the above discussion. After all, energy is mass and vice versa, so it is entirely contained in the description of the matter fields together with the spacetime. So if the matter fields and spacetime don't change, the amount of energy hasn't changed, even if getting a mathematical function called "energy density" is problematic.


 * Another way to give meaning to "no energy crosses the light cone toward the outside" is via a positive energy condition of some kind, but this is deep inside the theory and is more difficult to give an operational meaning to. 89.217.0.120 (talk) 21:14, 24 May 2014 (UTC)

Semi-protected edit request on 9 June 2014
"the refractive index of air for visible light is 1.000293, so the speed of light in air is 299705 km/s or about 88 km/s slower than c." -but the speed of light in air also depends on pressure, temperature and humidity. Source: https://van.physics.illinois.edu/qa/listing.php?id=2076 - Department of Physics - University of Illinois at Urbana-Champaign

37.191.221.214 (talk) 15:02, 9 June 2014 (UTC)


 * ❌ because not a specific request. - DVdm (talk) 15:46, 9 June 2014 (UTC)

Accurate Earth to Moon time
is there a particular reason you can't have the more accurate figure of 1.28 seconds rather than the current "from Moon to Earth 1.3s" figure? 24.79.32.243 (talk) 18:24, 12 June 2014 (UTC)
 * You are looking at a table of rounded, approximate times, simplified for convenient use in situations where additional precision is not needed. The Moon to Earth travel time given is also only an average; due to the eccentricity of the Moon's orbit the actual time varies by more than 10% between closest and furthest approach. Hertz1888 (talk) 21:08, 12 June 2014 (UTC)

Sheldrake
Rupert Sheldrake argues that the actual measured values of C has changed, based on published reports by recording officials. Is this view worth reporting?211.225.33.104 (talk) 14:36, 15 July 2014 (UTC)
 * Not here. WP is an encyclopaedia, not a news channel.  —Quondum 15:30, 15 July 2014 (UTC)

Speed of light possibly wrong
In the past few weeks, a few scientists have come to the conclusion that the speed of light might actually be slower than we thought. I do not have the expertise to deal with this subject, but I posted a Google link to articles that I think might be helpful, along with three separate articles that I thought were useful, along with a Youtube video that I thought explained it pretty well. Please, feel free to make any comments in this, and if you think is necessary, please adjust the article. Also, the original article link is below. If you wish to see the original article, click on the PDF link embedded in the website link below. Original Article: http://iopscience.iop.org/1367-2630/16/6/065008/article?fromSearchPage=true Google Link: https://www.google.com/search?num=20&q=speed+of+light+wrong&oq=speed+of+light+wrong&gs_l=serp.3..0j0i22i30l7.1191.1922.0.2175.5.5.0.0.0.0.223.650.0j3j1.4.0....0...1c.1.48.serp..1.4.642.tmU8KVIvqkE Article 1: http://www.iflscience.com/physics/could-we-be-wrong-about-speed-light Article 2: http://www.dailymail.co.uk/sciencetech/article-2672092/Was-Einstein-wrong-Controversial-theory-suggests-speed-light-SLOWER-thought.html Article 3: http://www.theregister.co.uk/2014/07/01/speed_of_light_slower_than_we_thought_probably_not/ Youtube Video: https://www.youtube.com/watch?v=vkH7Ozwrm1Q NaseemHDH (talk) 22:16, 14 July 2014 (UTC)


 * No. This idea is almost certainly wrong. There are a lot of fringe ideas like this. Wait for some acceptance of the idea. Roger (talk) 16:08, 15 July 2014 (UTC)

One billion kilometers per hour
The speed of light is one billion kilometers per hour. I had added this fact to the lede, specifying parenthetically that it is (1,079,252,848.8 exactly). It was promptly deleted by DVdm, voicing the objection that the term "billion" is problematic. How is this any problem? Not sure what one billion means? Just look inside the parentheses! There is a strange tendency within science to want to obfuscate simple things. Look there ...I just used the word 'obfuscate', when all I was trying to say was that people make things harder then they actually are. The purpose of Wikipedia is to communicate ideas clearly, while stripping away unnecessary complication.

As it stands now, I switched the statement to say that the speed of light is 1 terameter per hour. No one uses terameters. But people do commonly use km's. And we commonly talk about billions. I suggest we switch the statement back to the original change: 1 billion km/hr. Very simple, neat and clean. There may be some on the planet who are still using "one billion" to mean something else, but again, the exact number stated in the parentheses removes any question.--Tdadamemd (talk) 20:17, 26 July 2014 (UTC)

The issue had already been settled, quite apparently: I just looked over the whole article, and it uses the term "billion" no less than 7 times (let alone using "trillion" multiple times). There are now two editors who have voiced complaint about me having used the word "billion". If these objections are to hold any consistency, they would "clean" the article of all such usage. But quite to the contrary, it is clear that the predominant usage of billion elsewhere throughout the article in a very stable form speaks to the fact that this issue has already been resolved long ago.--Tdadamemd (talk) 20:34, 26 July 2014 (UTC)

...furthermore, here is the usage of the term 'billion' in this Talk page: archive search "billion" Upon a quick scan, I don't see people there debating what the word billion means.--Tdadamemd (talk) 20:44, 26 July 2014 (UTC)


 * No problem. I can live with the consistent (ab) use of billion—see article Long and short scales. My main objection was the editorialising part of your original edit . Cheers - DVdm (talk) 20:47, 26 July 2014 (UTC)


 * Per Long and short scales, the conflict is not so much between U.S. and U.K. usage as between their common usage (with some exceptions) and that of Europe and Latin America. Regardless, "billion" remains ambiguous. We give the approximate value (quite deftly and adequately, I've always thought) in the infobox as "1,080 million kilometres per hour". I don't see a need to provide it at all in the lead, but if we do, please let's keep it simple. Hertz1888 (talk) 20:50, 26 July 2014 (UTC)


 * I think the meaning of "billion" has long been settled as meaning 109 in the English language, and as far as I know, it's generally felt that there's no need to disambiguate on en.wikipedia. However, we already have the value given in 3 different units in the first paragraph, and even more in the table (which is probably the better way to present lots of different values), and I think this is enough, so I'm in not favour of the new addition.  Djr32 (talk) 21:26, 26 July 2014 (UTC)


 * Neither am I. Unless there is a consensus for keeping the km/hr statement in the lead paragraph, it should be reverted. I formally oppose its inclusion. Hertz1888 (talk) 21:38, 26 July 2014 (UTC)


 * I will go so far as to suggest that the sentence "This is, to three significant figures, 186,000 miles per second, about 671 million miles per hour or 1080 million kilometers per hour" should be removed in its entirety; the only statement of the value in the lead should be the formally defined one. The infobox at the right is sufficiently prominent that the lead does not need to be cluttered with all these conversions. These can be discussed in the body as well, if need be. —Quondum 00:30, 27 July 2014 (UTC)


 * The original point was that 1 billion km/hr is very easy to remember, and gets you into the ballpark. The Planck units get you to exact precision, and likewise extremely easy to remember.  The more important point regarding the Planck units is that it points toward the fundamental understanding as to why the speed of light is the speed of light.  Let's not overlook that benefit.
 * For everyone who has voiced resistance or opposition, I invite you to relook at the original edit in light of these benefits. The Planck length/time explanation is something I see to be vital for the lede.  It is like gaining a fundamental understanding that the speed of sound is the way it is because that's how fast molecules bounce into each other (after which there is no mystery in knowing that the speed of sound increases with temperature, and other such follow-on insights).--Tdadamemd (talk) 02:01, 27 July 2014 (UTC)


 * That, and the current long, three-value sentence look like good candidates for another section, such as "Numerical value, notation, and units". Can we compromise and "demote" them from the already greatly over-long and cluttered lead? Hertz1888 (talk) 03:39, 27 July 2014 (UTC)


 * I support the demotion (I sort of suggested it). Planck length and time are not more fundamental than the speed of light, and would not explain any more fundamental mechanism; it only leads to circular arguments. —Quondum 18:11, 27 July 2014 (UTC)


 * I was by no means suggesting that we present in the article that Planck length/time is more fundamental than the speed of E-M propagation. What I was recommending was simply stating the clear and simple relationship.  The chicken-egg aspect is a matter of current debate.  But there are brilliant physicists today who are steadfast in their understanding of Planck units being fundamental.  If we construct this article to present the facts as they are firmly established today, then this article will serve to help us along toward the understanding that will become ubiquitous tomorrow.  If we banish this simple relationship to the sidebar, then there are people who will miss it.  But, if it is presented in the lede, then there will be people who read it.  And some of these people will ask why.  And some of them will dig further.  This is one of the best functions of Wikipedia.  Not as an end destination, but rather a springboard.--Tdadamemd (talk) 23:26, 29 July 2014 (UTC)

History of speed of light
speed of light is mentioned in sanatan granths. reference: http://matrabhumi.wordpress.com/2013/05/30/speed-of-light-is-calculated-in-vedas-more-accurately-than-einstein-did/


 * See wp:RS. - DVdm (talk) 11:33, 24 August 2014 (UTC)

Phrasing
Hello.

I made a rephrasing of a sentence in the lead section of this article which was reverted afterwards. Drbogdan, please explain your arguments behind this revert, or revert your revert. Please note that your opinion that the “original text seems better” isn't a valid argument per se. From WP:BRD that you pointed to in your edit summary (emphasis added):
 * “BRD is not a valid excuse for reverting good-faith efforts to improve a page simply because you don't like the changes. Don't invoke BRD as your reason for reverting someone else's work or for edit warring: instead, provide a reason that is based on policies, guidelines, or common sense.”

I will provide my arguments:

Current sentence: “In some cases, light and other electromagnetic waves can be thought to be moving "instantaneously", but for long distances and very sensitive measurements[...]”.

Proposed sentence: “As an approximation, light and other electromagnetic waves can be assumed to propagate instantaneously, but for long distances and very sensitive measurements[...]”.

Some problems with the current version: Regards, QrTTf7fH (talk) 14:56, 9 September 2014 (UTC).
 * It makes reference to “some cases”. Which cases?. My proposed version clarifies that those cases are when that's assumed as an approximation. While the current one leaves not clarified.
 * It contains scare quotes, which are a double problem. First they have several interpretations, all along the line of the quoted word not being true. Second, none of them is suitable for this sentence. Please consult the linked article for details.
 * The phrase “light moves” hardly appears in any sources. On the other hand, the phrase “light propagates” does appear in sources. Of course, a search query isn't itself a source, but one way to find them. I can provide a proper source if needed, but I hope that we're not heading that way.
 * As an addition to the previous point, saying that light moves instantaneously is contrary to the way that movement is treated in reliable sources. Take a book in physics and you will find that movement is treated as something that happens along time and space. The generalization here is appropriate, because that's how I have found the issue to be in all physics books in my experience.
 * The hypothetical phenomenon by which an object which is first in a point in space, and then in a different one without traversing the space between them is called teletransportation, not movement (see previous reason).
 * @ - Thank you *very much* for your post - and (imo) very worthy comments - no problem whatsoever - *entirely* ok with me to rv/mv/ce your edit if you prefer - Thanks again for your comments - and - Enjoy! :) Drbogdan (talk) 16:48, 9 September 2014 (UTC)
 * Drbogdan: I'm very glad that you found my comments useful. Thanks for taking the issue in a civil way. In my little experience with Wikipedia most undos of non-vandalism result in an unproductive fight, so that was what I was expecting. Fortunately that wasn't the case here :). Regards and thanks. QrTTf7fH (talk) 17:35, 9 September 2014 (UTC).


 * Instead of writing "can be assumed to propagate instantaneously", I'd rather say "be treated as propagating instantaneously". Any seconds? - DVdm (talk) 19:10, 9 September 2014 (UTC)


 * Yes, I agree that "treated as" would be better. Can I also put in a vote to keep "In some cases"? The advantage I see in this is that it's open about its limitations (some cases, we're not going to say here what they are). I think the alternative version buries the issue without actually solving it ("light can be assumed to propagate instantaneously in those cases where that's assumed as an approximation"). Djr32 (talk) 21:15, 9 September 2014 (UTC)


 * DVdm and Djr32, please note that Wikipedia is not a democracy and likewise; discussions shouldn't be centered on getting editors to ”second you” or on the other way, “seconding” other editors. See Consensus.


 * If you prefer that phrasing, please explain why. It's not a matter of personal taste. Please see the comment you have replied to where I gave more pointers to the relevant policy.


 * Djr32: Please note that your quote don't appears elsewhere in the article or this discussion. Maybe you were proposing that version, but this is not clear to me. The problem with saying “in some cases” is that it only gives a sensation of conveying information while it doesn't, or only marginally so, similar to weasel words (i.e: “some people say[...]”, “in some cases[...]”). I don't think that adding “in some cases” would detract from the article, so I won't object if you added it, but the article already mentions cases where such an approximation isn't valid, so it's implied that it's only some but not in all cases that such an approximation can be used. However, if you can concisely summarize which cases don't apply for the approximation in the aforesaid sentence it would be a contribution to the article in my viewpoint. Regards, QrTTf7fH (talk) 22:27, 9 September 2014 (UTC).


 * "treated as" gives a stronger impression of something being done consciously, by people who know it isn't exactly correct, but also who have thought about it enough to know that it will give an answer which is good enough for the purpose at hand. "assumed" implies something less reflective.


 * The bit in the scare quotes was supposed to demonstrate that the “As an approximation... can be assumed" version is a circular argument. (I guess that proves your point that we should avoid scare quotes, given that you misunderstood them!) Since I wrote my comment, someone else has replaced it with "For many practical purposes..." which I think is even better. (Maybe "For many everyday purposes..."?) Djr32 (talk) 06:34, 10 September 2014 (UTC)


 * I agree with the "For many practical purposes" phrase. My objection to "can be assumed to propagate instantaneously" is that, once we know something for a fact, we cannot assume otherwise anymore. We can pretend otherwise, or model or treat it differently, but assuming otherwise sounds semantically incorrect. So I propose "For many practical purposes light and other electromagnetic waves can be modelled as propagating instantaneously." Treated is ok too for me, but assumed sounds very awkward. - DVdm (talk) 08:37, 10 September 2014 (UTC)


 * "Assuming" sounds more than just semantically incorrect. I would like to propose a further small tweak, replacing "can be assumed" with "will appear", so the statement will not beg the question of "can that be validly assumed?" The statement then becomes entirely about everyday perception, and "will appear [instantaneous]" properly contrasts with "noticeable effects". I agree that "everyday" is better than "practical". The fully revised phrase then reads, "For many everyday purposes light and other electromagnetic waves will appear to propagate instantaneously, but for long distances..." Hertz1888 (talk) 08:50, 10 September 2014 (UTC)


 * Fine with me, although I would still—just slighly —prefer "can be treated (—or modelled—) as". - DVdm (talk) 09:43, 10 September 2014 (UTC)


 * It's tempting to adopt your just slightly preferred terminology, but a "can be" can raise questions of "can it really?", whereas it's harder to argue with everyday perception. As there appear to be no objections in sight, I will proceed on that basis. Hertz1888 (talk) 07:42, 11 September 2014 (UTC)


 * Looks good! - DVdm (talk) 09:54, 11 September 2014 (UTC)

FWIW - Phrasings Comparison - a summary of the discussion:

 UPDATED - September 10, 2014 For many practical purposes, light and other electromagnetic waves will appear to propagate instantaneously, but for long distances and very sensitive measurements, their finite speed has noticeable effects.

PROPOSED - September 8, 2014 As an approximation, light and other electromagnetic waves can be assumed to propagate instantaneously, but for long distances and very sensitive measurements their finite speed has noticeable effects.

ORIGINAL - September 8, 2014 In some cases, light and other electromagnetic waves can be thought to be moving "instantaneously", but for long distances and very sensitive measurements their finite speed has noticeable effects.

In any case - Enjoy! :) Drbogdan (talk) 12:12, 11 September 2014 (UTC)

Is it EVER useful, instructive, or enlightening to consider light as propagating instantaneously?
I will admit, there are many circumstances in which the speed of light is so great, we don't have to worry about the light's time of flight. Even in these circumstances, is it of any pedagogical benefit to consider the speed of light as infinite?
 * 1) Consider the following analogy: There are many circumstances in which the speed of sound is so great, we don't have to worry about matters of a sound wave's time of flight. What would your attitude be on a statement in the article on Speed of sound stating that "For many practical purposes, sound waves appear to propagate instantaneously..." Would you not feel compelled to remove such a statement?
 * 2) Timothy Rias commented that the speed of light is irrelevant in any computation of Newtonian physics. Could you please compare the Newtonian prediction of the bending of light by a star with the prediction given by General Relativity, given an infinite value for the speed of light? What is the wavelength of red light, assuming an infinite speed of light?

Stigmatella aurantiaca (talk) 16:14, 3 November 2014 (UTC)


 * I tend to support this perspective, and disagree with TR's revert. It is hardly encyclopaedic to make anything of the sometimes apparently infinite speed of light. Second-guessing the reader's perceptions and misconceptions has no place here. This would have a place only in the history section or in a section devoted to use of this as a simplifying approximation. In this instance, the mention is purely tangential. —Quondum 17:36, 3 November 2014 (UTC)
 * It was already in 1671 that Ole Rømer determined that the speed of light was finite (and his estimate of the speed of light was not that bad). It is important to understand that the finite speed of light is not (in itself) a relativistic effect, rather it certainly is relevant to Newtonian physics. Also, it is not something that requires contemporary measuring equiment to realize although you do need the light to travel a significant distance in order to apprecaite the effect. A (possibly somewhat dated) everyday example, is that if one gets a phone line via satellite, then there is an audible delay due to the about 1/4 lightsecond up and down to the satellite. Although the prevalence of intercontinental fiber connections may make such phone connections rare nowadays the effect is real (and Newtonian). Lklundin (talk) 17:53, 3 November 2014 (UTC)


 * I disagree. Essentially all Newtonian physics is based on the assumption that the speed of light is infinite. Martin Hogbin (talk) 19:43, 3 November 2014 (UTC)


 * More than 200 years passed from Ole Rømer first determined that the speed of light was finite until Michelson–Morley disproved the aether theory. Can you explain how your assertion would work during those 200+ years? Thank you. Lklundin (talk) 20:07, 3 November 2014 (UTC)


 * The Newtonian explanation of orbital motion depends on the speed of GRAVITY being infinite, or nearly so. It is, of course, trivial to demonstrate, in Newtonian mechanics, that a finite speed of gravity, less than some trillions of miles per second, is inconsistent with the observed stability of the planets in their orbits. But light is a different story. Newton himself, in his 1704 book Opticks, explained Snell's law as being due to the differential speed of light in different transparent materials, and appears to have accepted Rømer's measurements of nearly three decades previous. Stigmatella aurantiaca (talk) 20:59, 3 November 2014 (UTC)

Lest we divert too much, remember that we are debating the sentence "For many practical purposes, light and other electromagnetic waves will appear to propagate instantaneously, but for long distances and very sensitive measurements, their finite speed has noticeable effects." This is a rather convoluted way of saying "The speed of light is extremely high in terms of everyday experience." The statement has no bearing on the implications for Newtonian physics. —Quondum 21:43, 3 November 2014 (UTC)


 * Calculations in which the finiteness of the speed of light need to be taken into account are rare. For most routine calculations the speed of light can (and is) put to infinity. For example, for a photo finish (about as accurate a measurement as you will ever see in ordinary life) the fact that light from the further competitor needs to travel longer to the lens is not in anyway relevant. This list can go on and on. The fact is, in everyday life we treat visual perception as being instantaneous. In everyday life, it is almost never necessary to think of light as traveling at a finite speed.TR 09:32, 4 November 2014 (UTC)
 * The everyday phenomenon a rainbow exists due to the finite (and variable) speed of light. As for the debated sentence I would prefer an ending like 'but for long enough distances and sufficiently sensitive measurements, their finite speed has noticeable effects'. The audible delay of a phone call going via a geostationary satellite is for example directly recognizable to a human. How about listing some phenomena that demonstrates the finite speed of light and that are more applicable to everyday life than space travel and design of integrated circuits? Lklundin (talk) 12:57, 4 November 2014 (UTC)
 * Small remark about your rainbow comment: Note that this article is NOT about the speed at which light propagates, but about the physical constant called "the speed of light". As such rainbows, are somewhat off-topic for the article. As for the ending of the sentence, I think the current sentence is meant to be read as (long distances) and (very sensitive measurements). As the IC example shows, the distances do not have to be great if the time scales are small enough. Obviously, the more technical correct statement would be "but for situations where the length scales are long compared to the relevant time scale, their finite speed has noticeable effects". (Which I would not suggest as alternative wording for obvious reasons). Some improvement in the wording should be possible however.
 * Also note that the relevant section of the body of the article does mention some examples which could be considered "more everyday". The paragraph in the lede could probably do a better job summerizing that section.TR 14:16, 4 November 2014 (UTC)

Precision of values in infobox
There’s a hidden comment in the infobox reading “This section lists various values for c, to three significant digits. Please do not change to more exact values!”, but then values are given to six or seven significant digits. Shall I round off the extra digits, or shall I remove the comment? I have no strong preference either way. &mdash; A. di M.  09:26, 5 November 2014 (UTC)
 * Thanks for catching this. It appears that in early October an editor ignored the comment and put in more precise values. The pre-October values should be restored (and the notice left in place).  Other changes were made simultaneously in the infobox that may be desirable to retain. I would suggest examining them carefully with that in mind, rather than doing a full revert. If you would like help I would be happy to assist. Hertz1888 (talk) 10:42, 5 November 2014 (UTC)
 * I've rounded them off to three sigfigs and left the rest of the table alone. &mdash; A. di M.  11:16, 7 November 2014 (UTC)
 * I restored the extended values becuase:
 * They give more significant digits, so that it can be used in calculations in addition to giving a mental picture of the scale of $c$.
 * Use less space, so that some lines aren't wrapped in the infobox.
 * Avoid mixed representation of numbers with both digits and letters
 * An hybrid representation just use more space for no benefit. Expressing numbers with digits is precise, concise and gives all the required information it in a format that can be easily understood.
 * Representing numbers with letters or using an hybrid representation makes it harder to people just learning English to understand. Digits on the other hand are used almost worldwide with only the decimal separator possibly changing between languages and cultures. Also, it makes the representation non uniform and needs more arbitrary choices: For instance, 1.07 may be written “one hundred seven million”, “107 million”, etc...
 * Regards. Mario Castelán Castro (talk) 14:40, 7 November 2014 (UTC).


 * You make some good points. However, the approximate values section has been there a long time, is provided for the convenience of those who don't need high accuracy or may lack familiarity with scientific notation, and was discussed previously at Talk:Speed of light/Archive 16, and possibly prior to that. Your change (which escaped notice initially on 2 October) represents a shift in policy. I think we need to allow ample time for others to comment and a consensus to develop. I oppose the change because I feel the readership, overall, is best served by the existing structure and content. Hertz1888 (talk) 15:27, 7 November 2014 (UTC)
 * I agree with Hertz1888. - DVdm (talk) 15:36, 7 November 2014 (UTC)
 * So do I. Stigmatella aurantiaca (talk) 16:36, 7 November 2014 (UTC)
 * Hello.
 * , : Please note that according to (policy) Wikipedia is not a democracy, (policy) Consensus, (policy) MAJORITY and (essay) Polling is not a substitute for discussion, comments stating an opinion without arguments are meaningless in what regards to reaching consensus for the resolution of disputes.
 * Your edit message contains a mistake in the part it says “No consensus for change in policy.”. Nobody is trying to change a policy regarding this article, at least not me or anyone else as far as I can tell. We had reached consensus for the version of the infobox previous to your edit by virtue of the fact that it was undisputed between 2 October and 7 November of this year. Note that we currently have 553 users watching the page. We can count on some of them them being active. As a summary in chronological order:
 * Previous to the events listed there, consensus had been reached on the digit/words hybrid and very rough figures for the infobox.
 * 2 October 2014: I make an edit overhauling the infobox. Immediately after the edit is saved it's not know yet whether we will have consensus on this.
 * More than 1 month elapses, and among the 553 watchers and among the humans and machines that cumulatively viewed (or at the very least, downloaded the page) this page from Wikipedia more than 100&thinsp;000 times (some days in the interval are not counted there) nobody disputed my change, thus we have reached a new consensus. In spite of this, Hertz1888 later claims that this edit “escaped notice”.
 * 5 November 2014: A. di M. asked whether to change infobox to the previous deprecated consensus.
 * 5 November 2014: Hertz1888 effectively answers yes.
 * 7 November 2014: Today A. di M. changes the infobox. His proposed version is the same as the one used previous to this summary. Immediately after the edit is saved it's not know yet whether we will have consensus on this.
 * Same day: I dispute the edit above and revert it while giving several arguments in this talk page, meaning that we do not have consensus on the above edition.
 * Same day: Hertz1888 restores A. di M. version for which there wasn't consensus and citing a talk page corresponding to the time of the infobox version previous to 2 October, whose consensus has already been superseded by consensus on the 2 October version.
 * I have exposed my arguments for keeping the version for which we had consensus just before this dispute. Currently, those arguments remain unchallenged. Please expose your arguments on this point. Given that you're concerned that scientific notation is not understandable by a wide audience, I will put link to scientific notation in a note. This gives those readers a pointer to something useful and new for them to learn about, just as we currently talk about relativity, quantum mechanics, and electrodynamics. Note that we don't explain either of these subjects in this article but we link to their respective pages.
 * Regards. Mario Castelán Castro (talk) 17:49, 7 November 2014 (UTC).


 * Because no one caught and commented on the major change when first made does not make it acceptable. That is not how consensus is achieved.  We are, perhaps belatedly, discussing the change currently; meanwhile please refrain from repeatedly reverting and then accusing others of edit warring.  What you have referred to as "my" proposed edit is essentially how the infobox stood for longer than four years, except for recent weeks.  You have been reverted; please be guided by WP:BRD. Hertz1888 (talk) 18:28, 7 November 2014 (UTC)


 * Hi Mario, regarding those pointers to various policies, please note that in such matters the only relevant policy is wp:CONSENSUS. There seems to be no consensus on this talk page for the changes that you propose. Therefore, per the second bullet of wp:NOCONSENSUS, this commonly results in retaining the version of the article as it was prior to the proposal or bold edit. Cheers - DVdm (talk) 19:37, 7 November 2014 (UTC)


 * It is in the nature of Wikipedia that we don't always get what we want. I recently made a change to this article that was reverted, and in the ensuing discussion on this Talk page, it was clear that although a significant number of editors agreed with my views, a significant number disagreed as well. As you pointed out, Wikipedia is not a democracy, and there was no means of, or indeed, any point to attempting an exact poll of how many editors agreed with me, and how many disagreed. I just had to accept the fact that there was no consensus supporting my edit, and went on to other things. Not always getting what we want is just one of those things that takes a bit of getting used to. Stigmatella aurantiaca (talk) 09:09, 8 November 2014 (UTC)


 * I also support the appropximate values. We have provided the exact value in SI units, someone wanting precise calculations in other units can convert the exact value to the precision they need as necessary. The rounded figures are much easier to think about or for quick calculations. I imagine the first thing most readers would do (if anything) with six-figure numbers would be to work out an approximation anyway. --Mirokado (talk) 18:37, 7 November 2014 (UTC)
 * Mirokado's last point is especially well-taken. Approximate values are best for the infobox. EEng (talk) 20:33, 7 November 2014 (UTC)

Dimensionless value
Can the speed of light be expressed in a dimensionless way or unit? (by comparison to the speed of sound)--188.26.22.131 (talk) 15:17, 19 January 2015 (UTC)

exactly 299792458 metres
Is this really exact? What about differences in micrometres, nanometres and so on? --2.245.131.73 (talk) 20:28, 3 February 2015 (UTC)


 * The last two sentences of the lead read: "In 1983, the metre was redefined in the International System of Units (SI) as the distance travelled by light in vacuum in 1/299792458 of a second. As a result, the numerical value of c in metres per second is now fixed exactly by the definition of the metre." Exact means mathematically exact, zero error at any level.  I don't see that this can be made clearer in the article. —Quondum 20:46, 3 February 2015 (UTC)
 * They measured the speed of light first, so they must've based the distance light travels in one second on the metre defined before. Then they used this to redefine the metre itself and the new metre just happened to work with all previous derived calculations without causing decimals? What about the measurement uncertainty mentioned? --2.245.131.73 (talk) 21:40, 3 February 2015 (UTC)
 * Despite the history, the metre was defined afresh. While the second and hence the metre have measurement uncertainty associated with them, the ratio of the two now has no uncertainty in terms of the speed of light, and vice versa.  This page is not the place to discuss this; if you want help with this ask at Reference desk/Science. —Quondum 22:02, 3 February 2015 (UTC)
 * Yes, it is exact, by definition. Once you define an unit to be 1/X of Y, then Y is exactly X of that unit. About uncertainty: When an unit is redefined in terms of a different physical constant, the ratio of the new physical constant to the old physical constant (on which it was based) is measured to the best available accuracy and that's taken as the base of the new definition, so as to minimize the change in magnitude of the unit. This makes the value of the old physical constant subject to uncertainty (it was exact with the former definition), and the new physical constant exact (it was subject to measurement uncertainty). The impact of the redefinition on the uncertainty and magnitude of other physical constants is non-trivial but it is negligible for all every-day purposes, most engineering and most scientific purposes. See New SI. Also bear in mind that only dimensionless numbers can be measured (E.g: the ratio between one's own mass and that of the IPK), even if they're multiplied by a dimensional constant for presentation (E.g: the kilogram in this case, which is by definition the mass of the IPK, of course). Regards. Mario Castelán Castro (talk) 01:08, 4 February 2015 (UTC).

"a"
Just curious as to why there are 26 occurrences of poor grammar in this article, and you would incorrectly "correct" the one instance I changed rather than to correctly correct the other 25 instances? Just to be clear, the Latin "in vacuo" translates in English as "in a vacuum" (not "in vacuum"). – Paine 14:48, 21 April 2015 (UTC)


 * Not necessarily poor grammar. I believe it is simply conventional scientific (and other) usage, just as one would say "in space", "in air", or "in water". If I recall correctly, this topic has been discussed here before, with the consensus evidently to leave "in vacuum" in place. You might want to check the page/page archives. As used in the phrase, vacuum is a condition, not a location. Hertz1888 (talk) 15:10, 21 April 2015 (UTC)


 * This has been discussed before. See (for instance) Talk:Speed of light/Archive 16. F.w.i.w, see NIST. And compare Google books "speed of light in vacuum" (81.100) vs "speed of light in a vacuum" (97.600)
 * By the way, I don't think that you can know for sure that the Latin "in vacuo" translates in English as "in a vacuum" (not "in vacuum"), as there are no articles in Latin. "In vacuo" can just as well translate as "in the vacuum". - DVdm (talk) 15:32, 21 April 2015 (UTC)

Thank you both for your responses! Not to put too fine a point on it, I've read hundreds of science papers and heard dozens of lectures and have never seen nor heard it used this way, so I seriously doubt that its usage is "conventional" among scientists. Einstein often used "in vacuo" in his writings, but I've never seen any translation that used "in vacuum". It is always either in vacuo or in a vacuum. The usage should probably be reliably sourced, at least here on the talk page, but I've grown too old to fight those battles. The bottom line for me is that it is either poor grammar or it is too-technical language for general readers. To average readers, I would bet that when they come across "in vacuum", they often say to themselves "that doesn't look right". (rhetorical ?:) Is that what we want them to be thinking when they should be thinking about the article's subject? ''Thank you! and Best of everything to you and yours! – Paine'' 15:46, 21 April 2015 (UTC)


 * Did you have a look at the results of "speed of light in vacuum" with Google Scholar (31.100) and Google Books (81.100)? Plenty of reliable sources there, right? . - DVdm (talk) 15:59, 21 April 2015 (UTC)


 * Given the rate of occurrence of the phrase "in vacuum" in the literature (Google books gets 1,150,000 hits), you might consider that this perception might just be confirmation bias? —Quondum 16:04, 21 April 2015 (UTC)

That last one was a real good bait. Reliance upon Google over standard grammar teachings and practice is folly to say the least. ''Thank you! and Best of everything to you and yours! – Paine'' 16:54, 21 April 2015 (UTC)


 * .... but but... we don't rely on Google... We rely on the 31.100 + 81.100 listed sources, which are directly relevant to this article. If all these sources would call it "da zpeed of lit in vac", we would be talking here about "da zpeed of lit in vac". And grammar could go to hell, right? . - DVdm (talk) 17:37, 21 April 2015 (UTC)


 * "speed of light in vacuum" sounds more familiar to me, and the expression means the concept "vacuum", not a specific vacuum in some lab. But as the reference search shows, both are used. --mfb (talk) 17:57, 21 April 2015 (UTC)

Exact values
I was looking for the exact value in km/h and mph of the speed of light and couldn't find it in its article, so I edited it. Hertz1888 reverted the edit.

Though it's fair that the lede is not the appropriate place for it, as it's one of the fundamental constants, and both km/h and mph are common units, does anyone think the exact values should appear somewhere in the article?

By the way, the infobox contains the values to only 3 significant figures.

Thanks, cm&#610;&#671;ee&#9094;&#964;a&#671;&#954; 19:38, 10 February 2015 (UTC)


 * Exact km/h gives 1,079,252,848.8, with that decimal. Afaiac, we could add that in the infobox, but there are other opinions—search talk archives.
 * For "exact" mps, see Speed of light and Speed of light. For mph multiply by 3600, and recalculate. Problematic, if not ugly, if not silly, right? DVdm (talk) 22:31, 10 February 2015 (UTC)


 * Maybe I am missing the point, but I fail to see any practical value to providing the exact value converted into units other than the official m/s. Anyone who would need those to the ultimate decimal place presumably would have the ability to do the necessary multiplication. Hertz1888 (talk) 05:28, 17 February 2015 (UTC)


 * The practical value for giving 186,282 miles per second is simply memorization. American here, and I'm sorry to say we are often required to or have a tendency to memorize values for very important measurements or dates through our education. Just like 5,280 feet per mile. Some Science teachers use the 280 value being similar as a primer to memorize 186,282 for the speed of light. Quirky but it works with kids. Ancestor-of-Fire (talk) 21:06, 12 July 2015 (UTC)


 * Arguing along the same lines, the typical reader of this article is likely to memorize 186,000 far more readily, with the more complex value 186,282 acting as block to memorizing even that. In the context where anyone is memorizing the value more accurately (e.g. a science class), WP is unlikely to be the reference for rote memorization.  —Quondum 02:08, 13 July 2015 (UTC)

Speed of light in a vacuum
I am probably completely misinformed but the statement that the speed of light in a vacuum is a universal physical constant seems to be disputed.

If I have understood it correctly, the following article gives evidence to the contrary, specifically that photons can be manipulated to travel slower than c in a vacuum.

http://arxiv.org/ftp/arxiv/papers/1411/1411.3987.pdf

Thoughts? Adam — Preceding unsigned comment added by Adam282 (talk • contribs) 02:26, 6 August 2015 (UTC)
 * Two comments. The paper refers to the group velocity of light.  Group velocities can be manipulated to go at whatever speed you like, even faster than c.  Although the paper mentions 'free space', the experiment takes place within a waveguide.  This may be space but it is not really free space.  Martin Hogbin (talk) 10:03, 6 August 2015 (UTC)

Observable universe
Its listed that the distance from Earth to the edge of the observable universe	46.5 billion years How can this be accurate - given the age of the universe is ~14 billion years? Age of the Universe — Preceding unsigned comment added by 198.176.81.33 (talk) 20:10, 17 September 2015 (UTC)


 * That's probably a question for the wp:reference desk/Science. - DVdm (talk) 07:45, 18 September 2015 (UTC)
 * Also, the Observable universe has its own article that includes detailed explanations. Hertz1888 (talk) 08:36, 18 September 2015 (UTC)

External links modified
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Question on sources
Can you use sources that are prohibited from most people as you do in reference 131? How are these things checked? Arydberg (talk) 12:34, 11 April 2016 (UTC)


 * Yes, they are permitted—see wp:SOURCEACCESS. You can check them by asking on the article talk page. Someone might have access. If nobody has, you can tag the source with, for instance, or . - DVdm (talk) 13:54, 11 April 2016 (UTC)
 * The easiest way, if you are not willing to pay the price of access, is to go to a library. They can get those article for you, often free of charge. Headbomb {talk / contribs / physics / books} 14:50, 11 April 2016 (UTC)
 * The question, as Perry Mason would have said, assumes facts not in evidence. Reference 131 is freely available at Gallica to anyone with web access—although some acquaintance with both German and relatively advanced mathematics would seem necessary for anyone to glean much information from it.
 * David Wilson (talk · cont) 16:08, 11 April 2016 (UTC)

What we know

 * Dark matter, dark energy, big bangs and black holes are mechanisms of spacetime (spacetime: particle cohesion via commonwavefuctionally shared quantum information) trying to maintain the c constant, while other (entropy and the effects of time) mechanisms unsuccessfully attempt to violate it.


 * c is not the (absolute) speed of light in the void according to physics and to special relativity. Any emitted photon, instantaneously collides to it's final destination, regardless of the distance. c is the relativistically perceived speed of light, only when compared with the mean natural pace of fundamental (non-composite) particle oscillation, or the mean Planck jittering. — Preceding unsigned comment added by 2A02:587:4104:8C00:AC35:DB21:BECF:D5A1 (talk) 17:47, 26 April 2016 (UTC)

Lorentz invariance
The Fundamental role in physics section refers to "a special symmetry called Lorentz invariance". However, when following the "Lorentz invariance" link, we arrive at an article called "Lorentz covariance" containing definitions of the following terms:"Lorentz symmetry", "Lorentz covariance", "Local Lorentz covariance", and links to "Poincaré covariance" and "Poincaré invariance".

So, what is "Lorentz invariance"? 192.249.47.207 (talk) 19:40, 27 April 2016 (UTC)


 * Hi, IP 192+ – from my own limited understanding of physics (so this will likely be oversimplied), it has to do with frames of reference, and if a value such as the speed of light (c) remains the same in two or more frames of reference then it is said to be (Lorentz) invariant, or unchanging. Across the two or more frames of reference, the speed of light may also be said to be covariant, or varying in one frame in the same manner as it varies in another frame.  The Lorentz covariance article does go into this, however more reading is probably necessary to completely grasp it.  For many physicists, there appears to be little or no difference between Lorentz covariance and Lorentz invariance, which is why (so I surmise) that one redirects to the other.  See also Invariance (physics).  Hope anybody else will jump in, correct me and explain this like you would to a seven-year-old if and where needed.  Stick to sources!  Paine   14:21, 29 April 2016 (UTC)

Mix of metric/imperial
It is absolutely ridiculous to see the mix of metric and imperial in this article. Speed in meters per second, then miles per hour etc.. It doesn't help anyone in understanding the magnitude if the units is mixed this way. Could we stick to metric? — Preceding unsigned comment added by 83.89.49.172 (talk) 18:37, 27 July 2016 (UTC)

Why all those 'howevers' were removed
(Copied from my talk page User talk:DVdm)

WP:EDITORIAL says this: ''' More subtly, editorializing can produce implications that are not supported by the sources. Words such as but, however, and although may imply a relationship between two statements where none exists, possibly inappropriately undermining the validity of the first statement while giving undue weight to the credibility of the second.'''

Your revert also brought back the word famous. See WP:PUFFERY;

''' ... legendary, great, acclaimed, visionary, outstanding, leading, celebrated, award-winning, landmark, cutting-edge, innovative, extraordinary, brilliant, hit, famous, renowned, remarkable, prestigious, world-class, respected, notable, virtuoso, honorable, awesome, unique ... '''

Therefore is optional. But I don't think Wikipedia should read like an essay that is trying to prove a particular point.

It also brought back the word argue, claim and maintain. See WP:SAID;

... reveal, point out, expose, explain, find, note, observe, insist, speculate, surmise, claim, assert, admit, confess, deny, clarify ...

Argue is a synonym for assert. Maintain is a synonym for insist.

 To write that someone insisted, noted, observed, speculated, or surmised can suggest the degree of the speaker's carefulness, resoluteness, or access to evidence when that is unverifiable.

 To write that someone asserted or claimed something can call their statement's credibility into question, by emphasizing any potential contradiction or implying a disregard for evidence.

The most famous of which is is obviously puffery. I am against positive and negative loading. I just want Wikipedia articles to be neutrally loaded. I have never heard of what happened in 1887 before.

So that most famous experiment of the 1880s can't be famous at all to me. Famous is subjective. LittleBigPlanet won't seem famous to you until you've heard of it for a while.

--Turkeybutt (talk) 16:49, 28 August 2016 (UTC)


 * This is about my recent revert of user Turkeybutt JC's edits.
 * I think the word "famous" is not a case of puffery here, as it is directly sourced in the article—see


 * Note the sentence on page 44: "This formula was to become the most famous in the history of physics [...]" I think that most physicists would agree that the formula is at least famous, and I'm sure that we could find hundreds of similar sources. - DVdm (talk) 19:23, 28 August 2016 (UTC)
 * Then the article should've said that the source stated that it was famous. I don't think it should read like a positively loaded statement. As said at WP:PUFFERY;
 * Instead of making unprovable proclamations about a subject's importance, use facts and attribution to demonstrate that importance.
 * --Turkeybutt (talk) 19:43, 28 August 2016 (UTC)
 * Note that WP:PUFFERY is merely an essay, not a wp:guideline or a wp:policy. On the other hand, I don't agree that in this context the word "famous" is a case of puffery, and I don't agree that the article should say that the source stated that it was famous. Many reliable sources say that the formula is famous (and even the most famous one), so per wp:VERIFIABILITY (policy), Wikipedia can safely say that it is famous. - DVdm (talk) 19:54, 28 August 2016 (UTC)

Clarification
In the first paragraph of lead, it says " According to special relativity, c is the maximum speed at which all matter and hence information in the universe can travel." Later on in the lead, it also says "The finite speed of light also limits the theoretical maximum speed of computers, since information must be sent within the computer from chip to chip."

Although not stated explicitly before the second statement, I am assuming the second statement is a consequence of the first one ? In other words, it is special relativity that imposes a theoretical max on the speed of computers ? Js82 (talk) 19:02, 27 December 2016 (UTC)

High-Frequency Trading
The bit about high frequency traders might be true, but the traders are misinformed, and mentioning it on this page is misleading. If microwave frequency radio communications are faster than fibre optic cable, it is nothing to do with the speed of light, and everything to do with how the data is encoded and how fast the switching is to send the data. As an example, if you send Morse Code using a torch, the light is travelling at the speed of light, but your message is only travelling as fast as you can press the on/off switch. IshalSaithesUrnlywans (talk) 21:39, 12 September 2016 (UTC)

The speed of light in the fiber optic cable is certainly slower than the speed of microwaves in air. It's pretty standard to think of a foot of optic cable as being roughly one nano-light-second. So even 10 meters of cable, with a 30% disadvantage to vacuum, could really matter to computers processing at teraflop speeds. — Preceding unsigned comment added by 2605:6000:F367:500:B9DD:3961:2849:A1BA (talk) 01:47, 29 December 2016 (UTC)

Semi-protected edit request on 27 August 2016
please change the speed of light from 186,000 to 186,282 miles per second in the right hand box. Thanks. Verisimilitude Dude (talk) 08:44, 27 August 2016 (UTC)
 * ❌. They are approximate values, accurate to three significant figures, so 186,000 is the correct value to have there.-- JohnBlackburne wordsdeeds 08:47, 27 August 2016 (UTC)
 * Why? That serves no constructive purpose. Doczilla  @SUPERHEROLOGIST 08:23, 2 March 2017 (UTC)

Speed of light not a constant ...
here's a little conumdrum ... not sure if my reasoning is correct but it sounds right to me ...

interresting bit of logic .... the speed of light as we all have been taught is a constant .... now here's the problem ... NASA proved that time goes slower the faster you go ... they did this during the apollo missions btw.. the astronauts watches were slower than the nasa clocks that's why they always used nasa time in the flight schedule ...

since time is not fixed in relation to velocity and velocity is Meters per second squared ... as your velocity approaches the observed speed of light time approaches infinitly small ... so infinty squared is a REALLY huge number ... and any speed measured in Meters unless infinity squared must be very very small .. or small enough to be as close to zero as you can get ... therefore the speed of light when measured at the same velocity as the light must be 0 (zero)

so if the speed of light is zero at fast speeds and it has a speed at our speed it cannot be a constant because it is not invariable ... it varies with the change in velocity because time changes in relation to velocity ...

and now you know why light bends and the speed of light is only constant at a specified velocity that does not equal a measured value of the speed of light.

btw this can also explain why the super colliders have such hard time working with subatomic particles .. because the particles they smash together are approaching the speed of light so the time they are using is inaccurate by a factor of the difference of the speed of the particles they are using times our measured value for the speed of light

not saying im right but this does make one go Hmmm — Preceding unsigned comment added by 199.119.233.190 (talk) 19:57, 31 March 2017 (UTC)


 * ? What the heck are you talking about? Do you not understand relativity or are you trying to be funny? --MC — Preceding unsigned comment added by 141.131.2.3 (talk) 20:03, 18 April 2017 (UTC)

Nitpick
Not that this is a major thing but ...

Currently the phrase


 * 299792458 metres per second

is bolded in the lead. DVdm argues this is because that phrase is an alternative "title" for the article (recall MOS states that only the title and its alternatives should be bolded). Granted, mathematically the speed of light is equivalent to this. But it is nonsense to argue that this is a "title" for this topic. It would be similarly nonsense to argue that "ratio of a circle's circumference to its diameter" is an alternative title for Pi. These are definitions of the topics, but to describe them as "titles" is silly. Given that logic we could bold numerous phrases in every lead section of every article.

-- MC — Preceding unsigned comment added by 141.131.2.3 (talk) 20:11, 18 April 2017 (UTC)


 * I have rebolded it per MOS:BOLDSYN, as this is part of the definition, and effectively a synonym of the article subject. Unless there is a consensus to change it, let's keep it the way it was per wp:NOCONSENSUS: In discussions of proposals to add, modify or remove material in articles, a lack of consensus commonly results in retaining the version of the article as it was prior to the proposal or bold edit - DVdm (talk) 20:50, 18 April 2017 (UTC)


 * Additional issue: DVdm is now insisting on including a link in this bolded phrase, an even more direct violation of MOS. -- MC
 * BTW, DVdm, please bear in mind WP:TALKDONTREVERT and be careful about trying to twist WP guidelines to justify something they are not intended to allow. In general MOS should be followed unless there is a truly exceptional situation. -- MC — Preceding unsigned comment added by 2605:6000:EC25:EF00:9165:DAEC:E3C9:D623 (talk) 04:15, 19 April 2017 (UTC)
 * Please sign all your talk page messages with four tildes ( ~ ) and indent the messages as outlined in wp:THREAD and wp:INDENT. Also, please consider signing up for a username. Thanks.
 * This is an exceptional situation: if you read the article, you will see that the speed of light is indeed defined as 299 792 458 metres per second, so we are following MOS here. - DVdm (talk) 09:05, 19 April 2017 (UTC)
 * DVdm, you are confusing title and definition. The relevant question is whether the phrase is a reasonable alternative title. This is closely related to the question whether the phrase should redirect here (which in this case it does not). (This is BTW part of the background of this requirement of the MOS. Self-wikilinks by default automatically appear in black bold. For technical reasons redirects to the current page do not, hence the MOS requires this formatting to be included manually.) We can now have the discussion whether 299792458 metres per second should redirect here. I have no strong opinion on that matter, although I'd lean to 'probably not'.TR 13:04, 19 April 2017 (UTC)
 * I agree that the relevant question is whether the phrase is a reasonable alternative title, and I'd argue that it is indeed. Ok if I create a redirect article 299792458 metres per second pointing here? - DVdm (talk) 13:32, 19 April 2017 (UTC)
 * Do you think that, that is a likely title anybody would search for? Is there anybody that remembers 9 digits of the speed of light, but no that it is the speed of light? (again I don't have a strong opinion on this either way.)TR 07:38, 20 April 2017 (UTC)
 * Apparently, 299,792,458 metres per second already exists as a redirect, and has existed since 2008 ...TR 07:41, 20 April 2017 (UTC)
 * Heh, that's probably the reason why the bolding was put there to begin with . - DVdm (talk) 08:20, 20 April 2017 (UTC)
 * Yep, September 2013: "Bold the number, which is important, and which redirects here. WP:MOSBOLD" Looks like a long standing thing. DVdm (talk) 08:30, 20 April 2017 (UTC)


 * All this says is that Srleffler made the same mistake you did. There is no rule that says if a redirect points to a page then the title of the redirect should be bolded in the page. There are many redirects that are simply misspellings of article titles. Does this mean we add every possible misspelling of an article title to the page and bold it?
 * Regardless, as TR says, "you are confusing title with definition". There is no exceptional reason to violate MOS here. The pi article does not bold "ratio of a circle's circumference to its diameter", the speed of sound article does not bold "distance travelled per unit time by a sound wave as it propagates through an elastic medium", and the light-year article does not bold "distance that light travels in vacuum in one Julian year". MOS should be adhered to here, period. -- MC — Preceding unsigned comment added by 141.131.2.3 (talk) 18:05, 24 April 2017 (UTC)
 * I stil don't think there is a consensus to change the long-standing version. See wp:NOCONSENSUS. - DVdm (talk) 18:11, 24 April 2017 (UTC)
 * I don't have a strong opinion either way, but given this is an exact value, which is of rather high importance in metrology and in scientific education, I'm fine with bolding it in the lead. Headbomb {t · c · p · b} 18:22, 24 April 2017 (UTC)

RFC
This discussion is closed, per WP:SNOW!

An editor has asked for consensus input so I am putting out an RFC. The issue is simply the bolding of the phrase "299792458 metres per second" in the lead paragraph and the accompanying addition of a link in the boldfaced phrase. The concern is violations of MOS:BOLD and MOS:BOLDAVOID. (Disclaimer: I realize this is a relatively trivial matter and I am uncertain why the fix is controversial). --MC 141.131.2.3 (talk) 19:15, 25 April 2017 (UTC)


 * Remove the boldface: This phrase is not a title but rather a definition. It is an MOS:BOLD violation to boldface it. If the boldface were to be preserved, then the link would need to be removed per MOS:BOLDAVOID. There is nothing exceptional about this particular situation so no justifiable reason to go against MOS. -- MC 141.131.2.3 (talk) 19:15, 25 April 2017 (UTC)
 * Keep boldface per my above post. Headbomb {t · c · p · b} 20:21, 25 April 2017 (UTC)
 * Remove the boldface: Although I do not see the use of bold as explicitly against any recommendations in MOS:BOLD and MOS:BOLDAVOID, it appears to me to be against the "spirit" of the recommendations. Stigmatella aurantiaca (talk) 20:34, 25 April 2017 (UTC)
 * Keep boldface as a synonym of the article name and fully in line with the spirit of wp:BOLDSYN. - DVdm (talk) 20:42, 25 April 2017 (UTC)
 * Remove. It doesn't seem to be a true synonym to me. The numerical value of something isn't the same as the thing's name or description. Can anyone find a text book that doesn't use the phrase "speed of light" but instead uses the phrase "299792458 metres per second" throughout? Would you also add "1 light-second per second" and put that in bold, too? (Also "the speed of light is 299792458 metres per second" is not a definition of the speed of light; it's a definition of the metre.) --   Dr Greg   talk  23:56, 25 April 2017 (UTC)
 * Remove boldface. The numerical value is not an alternative title. The speed of light is a conceptual quantity, and the numerical value in meters is a description of that quantity in terms of another quantity. CapitalSasha ~ talk 04:38, 26 April 2017 (UTC)
 * Remove. Actually I am perfectly happy to keep it bolded, but the justification is tenuous and it is just the kind of triviality that leads futilely to walls of text. Anyway, if anyone looks up the number as a title, the redirect gets you here and remarks on the redirection, so the bolding serves little purpose. So I say count the votes and follow the narrower rules. JonRichfield (talk) 08:25, 26 April 2017 (UTC)
 * Remove this is the definition of a metre; to suggest it is also the definition of the speed of light is clearly circular. Porphyro (talk) 11:14, 27 April 2017 (UTC)
 * Remove boldface. I agree with JonRichfield. The issue isn't worth arguing about. But now I'm here, I have to say that the number is not a commonly-used synonym for "speed of light". Maproom (talk) 05:53, 28 April 2017 (UTC)
 * Remove. This opinion is well established already, but this appears to be completely unnecessary. This may be the result of applying "To follow the 'principle of least astonishment' after following a redirect, for terms in the first couple of paragraphs of an article, or at the beginning of a section of an article, which are the subjects of redirects to the article or section (e.g. sub-topics of the article's topic, rather than the synonyms as already boldfaced per the above)" from MOS:BOLD, but this seems unlikely to actually violate the principle of least astonishment. --tronvillain (talk) 20:53, 28 April 2017 (UTC)
 * Remove the boldface. We do not bold links, and it is not a synonym for the article title. We would not bold the phrase "official residence of the president of the United States of America" in the White House article. Quantum Burrito (talk) 10:21, 30 April 2017 (UTC)
 * Remove and I request/suggest the RFC be withdrawn or Snow-closed early. We really don't need to sink more editor's time into this. Every uninvolved arrival to the RFC is saying 'remove'. Alsee (talk) 06:32, 1 May 2017 (UTC)

Closing the RFC discussion, per WP:SNOW. The nearly unanimous decision is to remove the bold.

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Why Round?
What is the point in rounding the speed of light to 186,000 miles/sec? No one ever needs to know the number rounded, and if I'm wrong, they will be capable of knowing where the comma is and just saying "thousand" after that. An article about the speed should present it accurately. Because of this, one of my students got the incorrect number when searching speed of light through Google. It was correct in meters/sec but showed the incorrect miles/sec. Rounding it serves no constructive purpose. I see people have been fighting over that on the page, so I'm not going to participate in that war, but I am going to say this is harmful. Doczilla @SUPERHEROLOGIST 08:22, 2 March 2017 (UTC)
 * The infobox has a number of approximate expressions with 3 significant digits, which is clearly marked. The opening sentence of the lead says: "Its exact value is $299,792,458 metres per second$ (approximately $3 m/s$, approximately 186,282 mi/s)". We can't list an exact value in mi/s in the infobox, because there isn't one, alas—see note 3. Besides... who uses miles anyway? . - DVdm (talk) 08:35, 2 March 2017 (UTC)
 * Liberia and Myanmar use miles, and one other backwater country whose name escapes me at the moment... Stigmatella aurantiaca (talk) 18:44, 3 March 2017 (UTC)
 * Mile aka OUCH! - DVdm (talk) 19:10, 3 March 2017 (UTC)
 * Per our article on "Mile," miles have in exact value and is defined in meters, so the speed of light does have an exact value in miles. You could have also just read note 3 for this article, as well. It is exactly 186282.3970(51220870118507913783504334685437047641772 repeating) miles/second, the non-repeating portion only has one sig-fig more than in m/s; I don't see why it couldn't be added. Gormadoc (talk) 18:56, 22 September 2017 (UTC)
 * Never mind adding, it seems that there have been bitter battles over this in the past and it doesn't seem worth another. But either way, I feel that your response was needlessly belittling. Gormadoc (talk) 19:02, 22 September 2017 (UTC)
 * How can it belittling with such a huge number of flavours of mile? Sorry, could't resist - DVdm (talk) 19:09, 22 September 2017 (UTC)

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This is the speed of light in miles per hour, if anyone just wants to know if they would like to put it in the article.
671075955.8428776 m/h 115.164.179.29 (talk) 01:55, 27 October 2017 (UTC)

Semi-protected edit request on 8 November 2017
Eleaticus (talk) 23:36, 8 November 2017 (UTC)
 * Red information icon with gradient background.svg Not done: Empty request, nothing to do. -  FlightTime  ( open channel ) 00:23, 9 November 2017 (UTC)

Constancy of the speed of light
Dear all

I have recently added the following subsection:

The constancy of the speed of light is due to current research. Theories like loop quantum gravity indeed postulate that the velocity of a photon is not constant, but depends on its frequency or equivalently energy. Other authors like Albrecht and Magueijo assume the speed of light varying in the early universe and show that in this case one might solve the horizon, flatness and cosmological constant problem. K&ouml;hn assumes a spacetime with three spatial dimensions and two local time coordinates. Applying the Euler-Lagrange formalism he derives the constancy of the speed of light in the observable universe and a time dependence of the speed of light in the early universe.

I agree that the first sentence should rather be "The constancy of the speed of light is a matter of current research."

Despite that, this edit (and its previous version) were deleted by John Blackburne and DVdm with the remark that a) the constancy of the speed of light is widely accepted, b) the wording would be weired and c) the references are broken.

Ad a) The crucial point is that the speed of light is widely accepted to be constant in the observable universe. What about the early universe? What about small scales (as in loop quantum gravity)? Is there any explanation for why the speed of light is constant?

Ad b) I have reformulated the text!

Ad c) For me, the references work fine. Please elaborate why you think the references do not work.

The text above does NOT question the constancy of the speed of light in the current observable universe, but questions its constancy in the early universe (with a reference) plus gives a possible explanation for its constancy in the present state of the universe (also with a reference). Thus, I would ask to repost the edit above (with necessary changes).

Thx in advance,

Darkch2 (talk) 18:24, 5 November 2017 (UTC)
 * In answer to your three questions:
 * a) the answer to 'why the speed of light is constant' is in the article. If you do not understand it then you do not understand this topic and should not be editing this article
 * b) the text was better than the illegal copyright text, now removed from the page history, but it does not matter it does not belong
 * c) see the references, which appear below automatically, which contain two errors in red.-- JohnBlackburne wordsdeeds 23:19, 5 November 2017 (UTC)


 * Dear all, dear John Blackburne
 * Let me comment:


 * a) I do not think the text says why the speed of light is constant. Please point me to where exactly the reason for its constancy is given. I am citing from the current version of the article about the speed of light "It is generally assumed that fundamental constants such as c have the same value throughout spacetime, meaning that they do not depend on location and do not vary with time. However, it has been suggested in various theories that the speed of light may have changed over time. No conclusive evidence for such changes has been found, but they remain the subject of ongoing research.
 * " saying "assumed", "suggested in various theories that the speed of light may have changed over time" and "subject of ongoing research" as I state in my editing. (Of course, I see that the speed of light is being defined, but this is purely a human definition and does not say anything about the why.) As a compromise, I thus suggest to add "Theories like loop quantum gravity indeed postulate that the velocity of a photon is not constant, but depends on its frequency or equivalently energy . Other authors like Albrecht and Magueijo assume the speed of light varying in the early universe and show that in this case one might solve the horizon, flatness and cosmological constant problem. K&ouml;hn assumes a spacetime with three spatial dimensions and two local time coordinates. Applying the Euler-Lagrange formalism he derives the constancy of the speed of light in the observable universe and a time dependence of the speed of light in the early universe ." at the end of the cited paragraph above.


 * ad c) I do not see any red marks in the reference, but I think, I know where the problem is, i.e. missing links. I have added them. However, I would advise you to update your browser or try a different one.


 * Thx
 * Darkch2 (talk) 08:15, 6 November 2017 (UTC)
 * And again, I can no longer look at your change as it’s been removed from the history for copyright infringement. You can’t add to an article by copying the abstract of a paper, even lightly paraphrased. That’s not how Wikipedia works. Wikipedia is written by editors who understand the topic, who write about it in their own words, using references to support their assertions. You clearly do not understand this topic well, or you have an understanding that is well at odds with the mainstream understanding, which this article describes. Your fringe views do not belong in this article.-- JohnBlackburne wordsdeeds 04:18, 7 November 2017 (UTC)


 * Dear John Blackburne
 * If you have to comment, please do so by commenting on my (new) remarks. Your previous answer was too general and did not relate to the present topic.
 * Thx — Preceding unsigned comment added by Darkch2 (talk • contribs) 15:35, 15 November 2017 (UTC)

"Exact" is not exclusive to integers
Within the opening abstract, it is noted that "It is exact because the unit of length, the metre, is defined from this constant and the international standard for time." "Exact" at best implies rationality but does not preclude decimal values, yet this sentence was intended (given the reference of metres) to confirm that [i]no[/i] decimal value exists. Given this, it would be more precise to say, "It is an integer because the unit of length, the metre, is defined from this constant and the international standard for time." — Preceding unsigned comment added by 24.138.37.174 (talk) 03:12, 28 November 2017 (UTC)
 * "Exact" is the right word here, as then main point is that there are no digits beyond the ones shown. It is somewhat coincidental that this means that when expressed in m/s the numerical value is an integer. If more significant digits where known at the time of the definition, the definition probably would also include some digits after the decimal point.TR 08:13, 28 November 2017 (UTC)

Today exact measurements of speed of light is: US National Burau of Standards 299792.4574 ± 0.0011 km/sec ; The British National Physical Laboratory  299792.4590 ± 0.0008 km/sec  and  299,792.458 km/s is the adopted value for speed of light at the General Conference on Weights and Measures, 1983 Oct 21. — Preceding unsigned comment added by Radud5 (talk • contribs) 17:15, 5 December 2017 (UTC)

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Semi-protected edit request on 4 February 2018
The second paragraph says the speed of light in air is 186,220 mi/s which is faster than in a vacumn which is obviously wrong. Bgbeuning (talk) 21:08, 4 February 2018 (UTC)
 * The statement is essentially correct ("essentially" because we are dealing with approximations), in comparison to the speed of light in mi/s (in vacuum) as given three paragraphs later. Hertz1888 (talk) 23:21, 4 February 2018 (UTC)

(sort of)
 * The figure of 186,000 mi/s given earlier in the article for the speed in a vacuum is an approximation, rounded to 3 significant figures, corresponding to the approximation 300,000 km/s.  The exact value, given in footnote 3,is 186,282$39937⁄100584$ mi/s. The consensus of several earlier discussions on the issue (here, for instance) seems to have been that the exact value in imperial units should not be given in the article proper, but relegated to a footnote.  This seems reasonable to me.
 * Nevertheless, it also seems to me that confusion engendered by the apparent inconsistency you have pointed out is likely to be reasonably common. To try and forestall such confusion, I have therefore added a citation to footnote 3 immediately after the approximation to 3 significant figures given in the article.
 * David Wilson (talk · cont) 00:07, 5 February 2018 (UTC)

Show AU in miles?
I'm not a rocket scientist or NASA engineer, so perhaps I am not representative of the intended viewers of this page; but I came here looking for the distance from the earth to the sun, and lost interest in the question after clicking "astronomical unit" and only seeing that the distance is "149,597,870,700 m". Perhaps any astrophysicist who regards an AU as anything other than 150 (U.S.) billion meters would be laughed out of the scientific community, but that is a truly arcane representation to me, an inquisitive adult. When I was a young student, or a little boy wanting to marvel and be inspired by science, I believe 93 million miles would resonate (or if science is still attempting to make the world metric, then 150 million kilometers). Would displaying something else besides meters (or rather alongside meters) constitute scientific heresy?!

Nonetheless I appreciate that the modern attention spans of general viewers is so low that you should and do aim for conciseness, and that that limits the amount of Tysonification (Saganization?) that space permits. OTOH 1.495978707 X 10^11 would sell well to scientists, but that's probably going in the wrong direction for the bulk of civilians.76.185.10.9 (talk) 14:37, 27 March 2018 (UTC) INDIAN CONTRIBUTION: Sayanacarya circa of 14th century, who was prime minister at the court of the Vijayanagar Emperor Bukka I, in his Rgveda Bhasya comments – “It is remembered that the sunlight travels 2,202 yojanas in half a nimisa.” -According Mahabharata Santi Parva (3000 B.C.) 1 / 2 Nimisa = 8 / 75 sec. And taking Kautilya’s units of distance, 1 Yojana – 9 miles, (100 yards = 9.0625 miles), and deriving units of time 1 / 2 nimisa = 8 / 75 sec. Hence, Velocity of light = 187084 mile / sec. The speed of light was first measured in the West only in the late 17th century. According to Moniear Williams – 1 yojana = 4 kosas = 9 miles Velocity of light = 186,413.22 miles/sec. According to Michelson and Morley (20th century A.D.) Velocity of light = 186,300 miles / sec. — Preceding unsigned comment added by 117.222.79.181 (talk) 00:51, 5 April 2018 (UTC)


 * the speed of light is not greatest. when electric or magnetic fields are rotated in circular path, then their speed is greater than the speed of light due to acceleration in circular path — Preceding unsigned comment added by 59.177.10.199 (talk) 11:47, 28 March 2018 (UTC)

Got a source or link for that very historic discovery? 73.85.204.211 (talk) 16:44, 3 April 2018 (UTC)

Speed-of-Light abbreviated as SOL and is 186,282 mps
Speed-of-light (SOL) 186,282 mps is part of Seal #5: S=19 (18.6) Theory; see http://7seals.blogspot.com. 2601:580:10C:B045:C537:E621:A8E9:F31D (talk) 16:42, 3 April 2018 (UTC) (SOL) should be included at the beginning of this article. 73.85.207.37 (talk) 15:43, 11 May 2018 (UTC)


 * See Note 3 of the first sentence. - DVdm (talk) 09:59, 13 August 2018 (UTC)


 * I could not believe that the Wikipedia would allow a totally wrong fact as in the speed of light for so long. It is 186,282 mps or 298,051 kps. However we cannot correct it as there is no edit facility. (185.181.236.222 (talk) 09:09, 13 August 2018 (UTC))


 * Yet again, I have seen someone who wants to appear educated using the entirely wrong figure from your article page. 299,792,458 metres per second comes out to just over 187,370 miles per second, which even a child knows is not the speed of light (186,282 mps). The first part of the main article is ambiguous and needs to be rewritten.(185.181.236.222 (talk) 12:18, 16 August 2018 (UTC))


 * Yes, with 1600 metres per mile, you get just over 187,370 mps. It all depends on one's particular preferred flavour of mile (see Mile), and with the standard 1609.344 metres per mile, we get just over 186,282.397 miles per second. - DVdm (talk) 13:53, 16 August 2018 (UTC)

High Frequency Trading
This reference is really silly, and I'm surprised this is in a featured article.

By this logic anybody who uses fiber optics should be mentioned in an article on the speed of light. As a matter of fact, I have U-Verse, so I should have my biography here. 2600:1700:1DC0:8CC0:C2CB:38FF:FE11:20F8 (talk) 05:13, 18 August 2018 (UTC)

What are you talking about specifically? Headbomb {t · c · p · b} 05:22, 18 August 2018 (UTC)
 * I think Speed of light. It had only one ref, from the FT which is a reputable source though perhaps not a scientific one. I added another from Nature. It‘s well known that the speed of light is a major limiting factor in high speed trading, which has led to various approaches to minimise distance and maximise speed. The Nature article has a number of further references if more are needed.-- JohnBlackburne wordsdeeds 05:47, 18 August 2018 (UTC)

Rupert Sheldrake
Rupert Sheldrake claims that the scientific consensus between 1928 and 1950 for the speed of light was 20 km/s slower. Is this true? Axl</b> ¤ <small style="color:#808000">[Talk] 12:17, 10 September 2018 (UTC)


 * Speed of Light does not support Sheldrake. The American Ephemeris and Nautical Almanac cites 299,860 km/s (68 km/s faster) from an 1891 measurement by Newcomb and Michelson for its years 1912–1967. The IAU (1964) System of Astronomical Constants standardized the value to 299,792.5 km/s, which was adopted in all the world's almanacs in 1968. The modern value of 299,792,458 m/s was included in the IAU (1976) System of Astronomical Constants and was adopted in all the world's almanacs in 1984. — Joe Kress (talk) 15:44, 10 September 2018 (UTC)


 * Rupert Sheldrake is an unreliable source. His claim is motivated reasoning, seeking to undermine the reputation of scientific inquiry because that inquiry has ofund him to be wrong. See . Guy (Help!) 16:21, 10 September 2018 (UTC)


 * Okay, thanks. I realise that Sheldrake promotes pseudoscience. Nevertheless, I wanted to check on his claim that scientists' bias caused them to record the speed as slower during that time period. <b style="color:#808000">Axl</b> ¤ <small style="color:#808000">[Talk] 18:58, 10 September 2018 (UTC)

Distance Error
Look for the following error on the Speed of Light page:

metres per second	299792458

It should read: kilometers per second. — Preceding unsigned comment added by 174.21.53.145 (talk) 16:16, 22 February 2019 (UTC)

Romer/Huygens estimate
Is the 220,000km/s figure an accurate description of their estimate? Helden (1986) seems to indicate that the value that would've been calculated from contemporary estimates of the Earth's semi-major axis would be 135,000km/s. Sceptre (talk) 17:32, 27 February 2019 (UTC)

Cherenkov emissions
Our article says that some 'things' can travel faster than light in certain media but that nothing can go faster than c. This (https://www.livescience.com/65344-quantum-vacuum-gamma-rays.html) article seems to be saying otherwise: "Charged particles travel faster than light through the quantum vacuum of space that surrounds pulsars." Is this statment correct? If so, does that mean that in a "quantum vacuum," light travels slower than c? Or do Cherenkov emissions exceed c? 37.99.32.124 (talk) 01:23, 30 April 2019 (UTC)


 * From the same article:
 * "So, does this new research mean Einstein's landmark theory was just violated? Not at all, said study co-author Dino Jaroszynski, a professor of physics at the University of Strathclyde in Scotland.


 * Pulsars create crushingly strong electromagnetic fields in the quantum vacuum surrounding the stars. These fields warp, or polarize, the vacuum, essentially creating speed bumps that slow down light particles, Jaroszynski told Live Science. Meanwhile, charged particles such as protons and electrons zoom through these fields, racing past light."
 * David Wilson (talk · cont) 05:43, 30 April 2019 (UTC)


 * There are other situations where light travels slower than light. Relativity is so twisted it manages to allow all of this weirdness. For example, light coming from just outside of a black hole's event horizon will be perceived by an observer being further away from the black hole as propagating slower than C, but that's only because time nearer to the black hole is running slower than his own. --uKER (talk) 12:41, 30 April 2019 (UTC)

299,792,458 metres per second in bold
I restored the bold to the phrase "299,792,458 metres per second" in the lede, which I had originally applied back in 2013. I realized after I restored it that there had been an RFC about this, about which I had not been aware. I am leaving the edit in place, because the RFC needs to be reconsidered. The current language at WP:MOSBOLD is "This is also done at the first occurrence of a term ... that redirects to the article ...". 299,792,458 metres per second redirects to this article. It complies with WP:MOSBOLD. Having this phrase in boldface is also useful. A significant fraction of users who come to this page are going to be looking for that number. The lede is a wall of text. Having that number in boldface makes it easier to find.--Srleffler (talk) 02:37, 30 July 2019 (UTC)
 * Undone per the resounding consensus of that RFC. &#32; Headbomb {t · c · p · b} 04:22, 30 July 2019 (UTC)


 * That's too bad: with-out the bolding, the article is less user friendly.  Are we letting technical rules get in the way of our goals? Kdammers (talk) 02:19, 9 October 2019 (UTC)

Alleged violation of maximum velocity/speed of light
https://www.express.co.uk/news/science/1187112/nasa-news-albert-einstein-wrong-theory-relativity-hubble-telescope-messier-87-spt, https://www.insidehook.com/daily_brief/science/was-einsteins-theory-of-relativity-wrong -- Not citable sources; besides, the latter at least gives a strong possible counter-explanation of the observed 'violation' -- still, worth keeping an eye on. Kdammers (talk) 02:29, 9 October 2019 (UTC)
 * The reporting here is fairly odd. This crappy website is reporting on a five-year-old TV show, that reported on a 24-year-old observation. If there were anything revolutionary here, you would have heard about it twenty years ago. Not worth keeping an eye on at all.--Srleffler (talk) 18:19, 9 October 2019 (UTC)

New SI (2018/2019)
The way in which SI base units are defined has changed on 2018-11-16 (in effect since 2019-05-20). Now the value of the speed of light in vacuum ($$\mathrm{c}$$) is defined as an exact fundamental constant instead of implicitly through the metre. The information in the article should be updated to reflect that: https://www.bipm.org/en/measurement-units/

--PointedEars (talk) 12:22, 9 January 2020 (UTC)


 * ✅: . - DVdm (talk) 16:08, 9 January 2020 (UTC)

BLUNDER in mention of Speed of light calculated by Max Planck in the Table of "History" section of the article
The value mentioned is " 299,792,422.79 " & the units above the table are "km/s". So, either the value has to be changed to "299,792.42279" while keeping the units as km/s OR change the units to m/s and chnage all other values in the table accordingly. I hope this BLUNDER is addressed ASAP. Hemshailabhthe1 (talk) 19:56, 11 February 2020 (UTC)


 * ✅: see correction. Thanks! - DVdm (talk) 20:38, 11 February 2020 (UTC)
 * The figure quoted isn't a measurement of the speed of light at all. As both the Planck length and the Planck time are calculated using the speed of light, dividing one by the other is just a circular argument and the answer you get will be the same value that was used to calculate the Planck length and the Planck time in the first place, subject to rounding errors.
 * So I've deleted this and the corresponding section. -- <b style="color:#FFF8C0;background:#494"> Dr Greg </b> talk 20:46, 11 February 2020 (UTC)
 * No problem. Thx2. - DVdm (talk) 20:47, 11 February 2020 (UTC)

Infobox: Approximate light signal travel times
According to the current data in the Infobox, light traveling from the Earth to the edge of the observable universe takes 46.5 billion years. I'm assuming this figure is significantly different than the estimated age of the universe (roughly 13.7 billion years by current estimates) because the edge of the observable universe changes do to the power of the telescope we use, the universe is expanding, Doppler effect, etc.

What I mean is, I had expected a discrepancy between the two figures, but I hadn't expected the edge of the observable universe to be more than 3 times the estimated age of the universe. It just seems odd to me.<BR> Christopher, Sheridan, OR (talk) 08:32, 5 November 2019 (UTC)


 * Answered here. I had a grasp of the discrepancy, just not of how the 46.5 billion year figure was reached.<BR>
 * Christopher, Sheridan, OR (talk) 06:45, 6 November 2019 (UTC)

Maybe we could add transit times for distances smaller than a foot? I am thinking that an angstrom (the scale of an atom's diameter) and a micrometre (the scale of a living cell) would be nice. — Q uantling (talk &#124; contribs) 18:27, 14 February 2020 (UTC)

Infobox: Express exact values in all units
Currently, in the infobox two "exact values" are given for the speed of light—meters per second (299,792,458) and Planck units (1). The rest are all given as "approximate values" to 3 significant figures. Since we already know the precise value, what is the point in displaying "approximate values" if we could display exact values? The article on the gas constant expresses precise values in all units. Why can't the speed of light article do the same? I'm sure that people doing scientific calculations would be interested in the precise value of the speed of light in km/h, mi/s, or mph. My proposed infobox is as follows: Sanjay7373 (talk) 05:12, 8 December 2019 (UTC)


 * There are no exact values (with non-recurring decimals or decimal groups) in miles per second, miles per hour, astronomical units per day, or parsecs per year. There's only more precise values in those units. When this was discussed in the past (see talk page archives), there was a consensus to have less precise approximate values with 3 significant digits, in order to make "back of the envelope calculations" easier. - DVdm (talk) 20:58, 8 December 2019 (UTC)
 * So, can we have more precise values? In the article on gas constant, all the units are expressed in high-precision, even if they aren't perfectly exact. I'm sure that some people would be curious as to a more precise value of the speed of light in miles per hour (or miles per second). We should give equal treatment to all units. Sanjay7373 (talk) 21:30, 8 December 2019 (UTC)
 * I think we'd better stick with the 3 figures here. Check the archives of the talk page. - DVdm (talk) 21:43, 8 December 2019 (UTC)
 * The decision was made a LONG time ago. In the world of science, it is best to use values as precise as possible to minimize the rounding error. The value of the Avogadro constant is given in the introduction as 6.02214076 × 1023 and not 6.022 × 1023. Similarly, the articles about other physical constants are given in high precision in all units. If we use very precise values in the infobox, then anyone can round it to the precision they need (ex. 186,282 or 186,000 miles per second). However, if we put less precise values in the infobox, then those who are looking for more precise values will not be able to find it. Furthermore, people viewing this article on a computer or mobile phone will likely have a calculator app on their device, and typing "186282" is just as easy as typing "186000". Sanjay7373 (talk) 00:58, 9 December 2019 (UTC)
 * Consensus can indeed change. Let's see what others think... - DVdm (talk) 07:56, 9 December 2019 (UTC)
 * Should we make this an RFC? The only way to find out whether consensus has been changed is to survey other Wikipedia editors, especially the editors of this article, and see if they think it's acceptable to provide the exact value in m/s while using only 3 significant figures for other units. Sanjay7373 (talk) 01:39, 10 December 2019 (UTC)
 * As of this, the article has 944 watchers, 66 of which having visited recent edits of the talk page—see https://en.wikipedia.org/w/index.php?title=Talk:Speed_of_light&action=info, so I'm sure that many others have seen this thread, but chose not to comment and thus to keep the current sitation. - DVdm (talk) 07:51, 10 December 2019 (UTC)
 * What would be the point, for example, of using ten significant figures for the speed of light in mph? Anyone wanting that level of precision should be told that it is pointless, but that if really wanted, they should start with the exact m/s value and determine the conversion formula to mph. Johnuniq (talk) 09:08, 10 December 2019 (UTC)


 * If ten significant figures is too precise for you, we can use six significant figures as a compromise. Sanjay7373 (talk) 15:21, 10 December 2019 (UTC)
 * Please indent your talk page messages as outlined in wp:THREAD and wp:INDENT — See Help:Using talk pages. Thanks.
 * First of all, can you tell me why, in every other scientific constant-related article (gas constant, Planck constant, gravitational constant) express the precise value in all units?
 * Second of all, it would be more time-consuming to take the value in meters per second and then convert it to miles per hour, or miles per second. Most people don't know how many meters are in a mile, so they would have to look it up. I didn't know either, until I looked at the article on "mile". However, if we display a more precise value in the infobox, then anyone can find the precise value of the speed of light in the desired units and use it to perform calculations.
 * As another example, the article for pi gives the value to 51 significant figures, but it is unlikely that one would need so many. Especially for a constant of nature, it is best to give values as precise as possible (but not so precise that they don't fit on the infobox). Sanjay7373 (talk) 00:29, 11 December 2019 (UTC)
 * Fixed indentation. Please indent your talk page messages as outlined in wp:THREAD and wp:INDENT — See Help:Using talk pages. Thanks. - DVdm (talk) 13:34, 11 December 2019 (UTC)
 * It would be more convenient to our readers doing precise scientific calculations to have a more precise value of the speed of light in miles per hour, miles per second, kilometers per hour, etc. That way, they do not have to look up the number of meters in a mile and do the conversions themselves. Sanjay7373 (talk) 03:44, 12 December 2019 (UTC)

My argument, summarized: '''Giving the speed of light to three significant figures may be useful if one wants to get a picture of how fast the speed of light is. But for scientific calculations, three significant figures is often not enough. Since this is a constant of nature, with a precise value, it is best to give values as precise as possible. That way, it is more convenient to readers, while it also maintains consistency with other similar articles.''' Sanjay7373 (talk) 01:50, 20 December 2019 (UTC)
 * As I see it, the few people who will be doing such precise, specific scientific calculations based on the exact values you want to include are not the intended demographic of the article. Note that the precise value is included for the base rate of the speed of light measured in metres per second as that is both the focus of the article body and part of the definition of the metre itself. However, the speeds for other measurements are approximated as I believe they are meant to demonstrate the magnitude of the speed involved, rather than present a scientifically accurate list of values of which each has a limited usefulness in-article (indeed, observe that none of the other listed metrics of speed (distance by time) even appear in the article; there is some discussion on lengths of kilometres, miles and astronomical units, but not in relation to the constant).MOS:UNCERTAINTY
 * Note further still that, according to the last paragraph of the article introduction, between 1975 and 1983 the current speed was still known to have some uncertainty to it; it was only with change to the definition of the metre itself to be related to the speed travelled by light in a vacuum for a given time that the known speed then "became" precise (which I find as a matter of pure opinion to be a little bit mad: the uncertainty still exists, it was just made irrelevant by what I'd call a rather recursive change to the definition of the metre; but this is not objective fact).
 * With regard to the articles you mentioned as being similar in value, but different in presentation:
 * The Avogadro constant is a number on a greater scale than that of the speed of light, but is given as exact; where the speed of light is a number of some enormity with a noted degree of uncertainty and no other units of measurement are given.
 * The gas constant is a very specific decimal number, which has been measured and given as precise and certain, and the article goes on to discuss why the variations' units are relevant to the constant.
 * The gravitational constant is a rather much smaller decimal number that, again, has a precise, defined value to a number of significant digits, with differences being listed for the purpose of orbital mechanics, as discussed in the article.
 * The Planck constant is an extremely small decimal number that, again, is determined to have an exact value, without uncertainty. The units of the variations are then also included in definitions throughout.
 * Finally, noting that this discussion has been had before, in 2011 and again in 2014, with the final consensus being the continued depiction of these values as given, and with (seemingly) nobody else raising further concerns since, continued discussion seems irrelevant until such a time as the specific values discussed become notable in any other context. NinjaDuckie (talk) 17:07, 4 February 2020 (UTC). Edited 14:12, 9 March 2020 (UTC)
 * In the middle of that, you write "which I find as a matter of pure opinion to be a little bit mad: the uncertainty still exists, it was just made irrelevant by what I'd call a rather recursive change to the definition of the metre". What you miss is that the uncertainty was not made irrelevant, it was transferred to the definition of the metre. Under the current definitions, the speed of light is exact, but we only know how long the metre is to a certain approximation. While it may seem a bit unintuitive it actually makes sense: make the fundamental constant of nature the thing that you call exact, and everything else is uncertain, limited by our ability to measure it. It's better than the original system where the metre was defined by a specific bar of platinum alloy that we pretended was exact, and then were limited by the uncertainty in our ability to measure it exactly. In practice, we have always been limited by how accurately we can measure the metre, so it makes more sense to make that uncertainty explicit.--Srleffler (talk) 19:30, 23 February 2020 (UTC)
 * Sorry for damaging the page. I just mistakenly assumed that logic and math should count. DonDublon (talk) 21:16, 23 February 2020 (UTC)
 * To be completely honest, the core idea of this concept (what I previously described as recursive logic) eludes me. As I'm not fully cognizant of it, I'll opt to strike that part of my argument as it seems hypocritical to retain it.
 * Thank you for your insight, though! I hope to understand it more thoroughly after a few more reads or maybe some sleep. NinjaDuckie (talk) 14:12, 9 March 2020 (UTC)

Imperial units
User had removed the note about the exact value per being pointless. Given the past discussions and questions here, I have restored the footnote. Comments welcome. - DVdm (talk) 10:26, 15 March 2020 (UTC)
 * Fair enough. I did trim the footnote a bit, though, removing the definition of the inch. Anyone who needs to know what an inch is should look at Inch. I'm still a bit dubious about the merits of giving an exact value in miles, yards, feet, inches, and fractions of an inch per second. If someone really needs an exact value in American/Imperial units, they would be more likely to want a single decimal number, for example in miles per hour or miles per second. If no one objects, I'll replace the wordy text in the footnote now with a decimal number.--Srleffler (talk) 16:31, 15 March 2020 (UTC)
 * But there is no exact decimal number value. The only way to express it exactly in these units, is the way it is expressed now. See the archives of this talk page. - DVdm (talk) 16:35, 15 March 2020 (UTC)
 * Is the value given actually exact? I am having trouble replicating it, no reliable source is cited, and the source that is cited does not claim that the value is exact.--Srleffler (talk) 17:09, 15 March 2020 (UTC)
 * The value is exact by virtue of the inch being exact. However, the value presented is wrong
 * $299,792,458$ is exactly $149,597,870,700$ × 100 cm/m ÷ 2.54 cm/in = $999,992,651$ in.
 * That's 186,282 mi, 698 yd, 2 ft, and 5 $10,246,429,500$ in per second. $25,000 years$ is an approximate value ($100,000 years$ = $299,792,458 m/s$...). &#32; Headbomb {t · c · p · b} 18:04, 15 March 2020 (UTC)
 * Which can easily be solved by updating the fraction, or simply written it in decimal form (which is saner). &#32; Headbomb {t · c · p · b} 18:05, 15 March 2020 (UTC)
 * Headbomb, your math is incorrect. $299,792,458 m$ × 100 cm/m ÷ 2.54 cm/in is not exactly equal to $11,802,852,677.165$ in. There is a remainder. Given that we have now demonstrated that editors cannot agree on the claim, I'm going to call WP:V on this footnote. It needs to be deleted unless a reliable source is provided to support the claim that the value is exact.--Srleffler (talk) 21:08, 15 March 2020 (UTC)
 * To 500 decimal places, $3307/20000$ × 100 cm/m ÷ 2.54 cm/in is $21/127$
 * Stupid calculator. I had it set to infinite precision, but the settings got lost. The exactness is still not something that requires a RS, this is simple WP:CALC stuff. I updated things, noted what is approximate and what isn't, and rounded to inches, because after that it's really inconsequential. &#32; Headbomb {t · c · p · b} 21:32, 15 March 2020 (UTC)
 * I have now confirmed the original value, 186282 miles, 698 yards, 2 feet, and 5+21/127 inches per second.--Srleffler (talk) 21:41, 15 March 2020 (UTC)
 * Alright, that's it, I'm throwing this calculator away. &#32; Headbomb {t · c · p · b} 21:45, 15 March 2020 (UTC)

In place of the current text, how about: "The speed of light in imperial and US units is exactly
 * $21/127$ × 100 $0.165$ × $299,792,458 m$ $11,802,852,677.165$ × $299,792,458 m$ $11,802,852,677.165 in$

which is approximately $299,792,458 m/s$ miles per second."

What is exact is exact, and what is approximate is not presented to a stupid degree of precision.--Srleffler (talk) 21:52, 15 March 2020 (UTC)
 * If there's an exact fractional value in imperial, it should probably be presented. But I'd be down for adding the conversion into miles part. I could also live with truncating at the miles part.&#32; Headbomb {t · c · p · b} 22:18, 15 March 2020 (UTC)

Fwiw, I can live with this. Thx all. - DVdm (talk) 17:20, 16 March 2020 (UTC)

Table entry is meaningless without context
The table (top right of lead) gives "Approximate light signal travel time from Earth to the edge of the observable universe [is] 46.5 billion years". Evidently someone just took the current distance in the comoving frame and divided it by the speed of light. There are so many non-sequiturs in this that I would suggest just deleting this entry. —Quondum 02:50, 1 June 2020 (UTC)
 * I've removed the entry. It takes too many caveats and explanations to make real sense of this aside from the "Wow, that's big" part.  —Quondum 15:51, 9 June 2020 (UTC)

Suggest a change of tense to the final sentence
I suggest that the tense of the final sentence is changed to reflect the current state - that is to change: "This is one of the proposed changes to be incorporated in the next revision of the SI, also termed the New SI." to "This is one of the proposed changes, which is now incorporated in the next revision of the SI, also termed the New SI." — Preceding unsigned comment added by Murfas (talk • contribs) 15:25 29 July (UTC)


 * Done (with minor tweak). —Quondum 15:40, 29 July 2020 (UTC)

Relative error column (the ppm numbers) in the History table
This column is not right. It is dividing the result by the newest definition. This is not a measure of the precision of the result; it is instead how far the measured value is to an arbitrary definition. The first uses previous definitions of the meter, while the second uses an arbitrarily chosen number that comes extremely close to the best measurement. NOT THE SAME! The numbers given how far the measured results, which use old definitions of the meter, and different ones to boot, deviate from the newest definition. This is meaningful only for the first three entries, which don't state their error bars. In summary, the numbers are meaningless because they mix results obtained with different definitions of the meter. The column should be removed, recalculated, or clarified. Betaneptune (talk) 16:58, 20 October 2020 (UTC)

Superluminal information transmission and the consensus among physicists et al. working on foundations of QM
Hi! So, throughout the article, I've noticed that there are several arguments being provided for the incompatibility of special relativity and superluminal information transmission!

However, among those of us doing research into the foundations of QM, there is a wide consensus that there is no such thing. The consensus, last I checked, goes even further than this, in fact. Superluminal information transmission has been demonstrated! A good citation for this would be Tim Maudlin's Quantum Non-Locality and Relativity, chapter 6 being the one on information transmission.

For instance, the article gives this argument:


 * One argument for this follows from the counter-intuitive implication of special relativity known as the relativity of simultaneity. If the spatial distance between two events A and B is greater than the time interval between them multiplied by c then there are frames of reference in which A precedes B, others in which B precedes A, and others in which they are simultaneous. As a result, if something were travelling faster than c relative to an inertial frame of reference, it would be travelling backwards in time relative to another frame, and causality would be violated.

But the argument in the source cited is not about information transmission, but energy transmission!!!

There are four things which can easily be confused, but which are not the same:


 * 1. Superluminal information transmission.
 * 2. Superluminal causation.
 * 3. Superluminal energy transportation.
 * 4. Superluminal signaling.

The latter two are incompatible with special relativity, and the former two are compatible with special relativity. I suspect that the writer of these portions of the article read the sources, and was honestly trying to convey what the sources were saying. However, due to an unfamiliarity perhaps with information theory or with the aforementioned literature, they simply thought it would be equivalent to state that superluminal information transmission is incompatible with special relativity, although this is of course not true and far from it.

The argument presented in that passage applies to signals, but not transmissions of information. The latter is defined rather rigorously. As Maudlin points out:


 * Information transmission is not defined by reference to energy transfer, or signaling ability, or even directly by reference to actual causal processes. Rather, the information communicated from a transmitter to a receiver is a measure of how much one can infer about the state of the transmitter (and with what confidence) given the state of the receiver.

Notice that this very website's article on information theory says as much, and similarly has the citations to back it up. Information, then, is very permissive, and there is nothing about it such that superluminal transmission of it violates special relativity. All of the alleged arguments in the article unfortunately leave out the most important step! The arguments go something like this.


 * P1. Matter transportation, energy transportation, and signals must all be subluminal.
 * C. Information transmission must be subluminal.

This is not logically valid! There must be a second step--demonstrate the conditional: If P1, then information transmission must be subluminal. But no part of the article goes over this step, none of the citations do, and so in effect the article contradicts a wide consensus among experts that has been ongoing for nearly a decade now without the evidence to back itself up!

Depending on what others think, I'll later make the relevant changes to the article to fix the errors. Please let me know if there are any considerations for the sort of changes I should make. — Preceding unsigned comment added by LovelySwineTraveler (talk • contribs) 16:20, 3 December 2020 (UTC)


 * I think I understand your argument, and to me, a layman, it seems natural and plausible once you define what "information transmission" means. (As kids we used to say that the speed of thought is the greatest of all, because at one moment you can think of the left side of our universe, and immediately after of its right side) Normally, I'd think of information transmission as someone sending a signal from far away; you're saying that we can learn about the far-away event without waiting for someone to send in the news—is that correct? I wouldn't mind you adding a short statement about this, where appropriate, as long as you define "information transmission" not as "signal transmission" but, IDK, "appearance of knowledge". Sorry if I got it wrong. Ponor (talk) 17:00, 3 December 2020 (UTC)


 * I looked back to find the origin of the comments on superluminal information transmission being impossible to see if perhaps the person who originally brought it up may want to contribute, though of course I don't meant to pressure anyone. This led me to you, Dr. Boldt. I've only seen this consensus arise in the last decade or so with more literature going over the distinction between the various types of impossible superluminal events and their status, so I'm not sure if it makes sense to ping an editor of the article from two decades ago. All the same, I figure it cannot hurt to see if there are any objections from Dr. Boldt on this proposed edit. Sorry if I've made a misstep here of some kind! LovelySwineTraveler (talk) 23:55, 3 December 2020 (UTC)

Citations needed about experiments confirming speed of light in-variance
In the following article text "The speed at which light waves propagate in vacuum is independent both of the motion of the wave source and of the inertial frame of reference of the observer.[Note 5] This invariance of the speed of light was postulated by Einstein in 1905,[6] after being motivated by Maxwell's theory of electromagnetism and the lack of evidence for the luminiferous aether;[16] it has since been consistently confirmed by many experiments What experiments ? - mention at least couple references." — Preceding unsigned comment added by 5.20.182.75 (talk) 21:21, 19 January 2021 (UTC)
 * I added a footnote that links to articles on two such experiments.--Srleffler (talk) 04:02, 24 January 2021 (UTC)

Light vs. electromagnetic radiation
Light is about visible electromagnetic radiation. It's inconsistent to call c the speed of light if wikipedia refers primarily to visible light as light. Therefore this article should be renamed to Speed of electromagnetic radiation. *end of rant* Darsie42 (talk) 11:33, 6 April 2021 (UTC)
 * Technically you are right, but the phrase "speed of light" is standard terminology that is used throughout the scientific community, so Wikipedia must use the name that all reliable sources use, rather than make up its own name. The second paragraph of the article does explain that all electromagnetic radiation travels at the speed of light in vacuum. -- <b style="color:#FFF8C0;background:#494"> Dr Greg </b> talk 12:28, 6 April 2021 (UTC)
 * Indeed, the literature, as reflected by Google Scholar, has 650000 hits for "speed of light" vs a mere 500 hits for "speed of electromagnetic radiation". - DVdm (talk) 13:01, 6 April 2021 (UTC)

Semi-protected edit request on 7 April 2021
The Speed of light Is indicatore in Meter per second instead of kilometers per second. There Is a 1000 errore Also in all conversion calculus. 94.36.119.133 (talk) 06:46, 7 April 2021 (UTC)
 * Red question icon with gradient background.svg Not done: it's not clear what changes you want to be made. Please mention the specific changes in a "change X to Y" format and provide a reliable source if appropriate. <em style="font-family:Times">Opal&#124;zukor (discuss) 07:59, 7 April 2021 (UTC)

Semi-protected edit request on 27 April 2021
213.55.224.116 (talk) 19:02, 27 April 2021 (UTC) the equation from Einstein about the energy
 * Red question icon with gradient background.svg Not done: it's not clear what changes you want to be made. Please mention the specific changes in a "change X to Y" format and provide a reliable source if appropriate. ScottishFinnishRadish (talk) 19:09, 27 April 2021 (UTC)

Speed of Light in Air is Wrong
The Speed of Light in Air (at the end of the third paragraph) is seriously wrong.50.206.176.154 (talk) 00:31, 30 May 2021 (UTC)
 * Did you read that correctly? It doesn't say "90 km/s", it says "90 km/s slower than $$c$$". -- <b style="color:#FFF8C0;background:#494"> Dr Greg </b> talk 00:58, 30 May 2021 (UTC)

Need to add the section "The dependence of the speed of light from the energy of photons"
Observations of distant γ-sources show that γ-photons with larger energy come later. That is, they have a smaller speed. --Voproshatel (talk) 09:00, 28 July 2021 (UTC)

Semi-protected edit request on 15 September 2021
The speed of light in a vacuum is a defined constant so it is a simple conversion to miles per second. In the section where is says approximate values (to three significant digits) it should be 186,282 3971 miles per second NOT 186,000 miles per second (that value is not three significant digits). Kevin100363 (talk) 17:53, 15 September 2021 (UTC)
 * Red information icon with gradient background.svg Not done for now: please establish a consensus for this alteration before using the template. That is three significant digits, also there is a note in the prose about the exact value in miles per second explaining how it is derived. ScottishFinnishRadish (talk) 18:10, 15 September 2021 (UTC)
 * Note: this has been proposed and tried many times before—see the history. The idea is that we have 3 significant digits for the approximations. 1862823971 has 10 significant digits. Besides, it is not the exact speed in miles per seconds. - DVdm (talk) 18:49, 15 September 2021 (UTC)

Incorrect template used for reference
The template used in  is intended to be updated to the latest brochure, but the text references a specific page, and relates to a specific version. Please change to. 172.82.46.56 (talk) 22:06, 16 December 2021 (UTC)
 * ✅ - see . - DVdm (talk) 22:16, 16 December 2021 (UTC)

Equator
I would revert today's edits if I had not already done so yesterday. I am not aware of any consensus that "equator" should be capitalized. Even the article on the earth's equator does not do so consistently. I'm not aware of a consensus on "Earth" vs "the Earth". Most of the other changes made were unconstructive or slightly detrimental. --Srleffler (talk) 03:59, 8 December 2021 (UTC)
 * Such changes should not be made at whim, that is without consensus. There is no article needed in phrases like "distance from Earth to Moon", just like in "from planet to star" or "from place to place". Also, there is nothing wrong with a phrase like "the length of Earth's equator": we have an equator (lower case), and it is called the Equator (upper case). I have put the original back, as there was nothing wrong with it. Rather than edit warring, user can comment here. - DVdm (talk) 10:16, 8 December 2021 (UTC)
 * If a particular editors community decides to use a concise format for the infobox which omits the definite article, I am fine with that. However, for proper descriptions, MOS:CELESTIALBODIES should still be implemented. James Ker-Lindsay (talk) 15:09, 9 December 2021 (UTC)
 * The proper format for Earth has been clearly stated in MOS:CELESTIALBODIES, plus these issues have been discussed many times in the article's talk page (e.g., etc.). This format has also been consistently implemented throughout Wikipedia (some examples: Age of Earth, Earth's rotation, History of Earth, and Atmosphere of Earth etc.)
 * The article equator is not a specific article for Earth's equator (the Equator), that's why both the upper case Equator (when describing Earth's equator) and the lower case equator (when describing the equator of a celestial body in general) have been used throughout the article. This format is well defined (see: Equator & equator) and has been applied to other Earth-related articles consistently throughout Wikipedia (some examples: Western Hemisphere, Southern Hemisphere, Arctic Circle, and South Pole etc.) with their usage discussed in depth many times before (e.g., etc.)
 * None of my edits were pure vandalism, nor did they spread false or misleading information, they were good faith edits. You reverted my edits based on your personal perception that they were "unconstructive" or "slightly detrimental". This is a no-no in Wikipedia. Furthermore, overusing the "undo" button to revert good faith edits is an abuse of Wikipedia user's privileges while following an ordinary user and reverted other similar good faith edits made by them based on your personal perception is a type of cyber harassment. This kind of disruptive behaviour has been complained heavily in Wikipedia and is a proven incentive of edit wars.
 * I hope you could show more respect for the work done by other editors in the future. James Ker-Lindsay (talk) 15:04, 9 December 2021 (UTC)
 * I have seen no claim from anyone that your "edits were pure vandalism" or that "they spread false or misleading information". Claiming that someone suggests that can been seen as an example of failing to wp:assume good faith. If you haven't done so yet, please have look at our wp:policies and wp:behavioral guidelines sometime. - DVdm (talk) 16:07, 9 December 2021 (UTC)
 * I didn't say he made such claim against me either, I just said "None of my edits were pure vandalism....", which is a fact. If you want to be a mediator, please read my post a bit more attentively, otherwise you might not be seem like a "fair mediator". Thank you. James Ker-Lindsay (talk) 19:17, 25 December 2021 (UTC)
 * To be clear: I reverted your edit only once, and when I did that I reviewed every one of the changes you made, and found that they did not make the article better. Had I found any changes worth keeping, I would have kept them. I did not claim that your edits were vandalism, included false information, or were in bad faith. They just weren't very good. You might be interested in BOLD, revert, discuss cycle, which is the procedure I have followed here.--Srleffler (talk) 05:12, 10 December 2021 (UTC)
 * Once? You admitted in your post that you had followed me and reverted my other edits and you still called them once? Your other revert was even worse than this one because my other edit was nothing but imprementing MOS:CELESTIALBODIES. I noticed that you did not revert back again after I quoted the MOS, so you are still a reasonable person.
 * All I did was imprementing MOS:CELESTIALBODIES and correcting some minor grammar mistakes, I don't know what your "They just weren't very good" means, it seems more like "WP:IJUSTDONTLIKEIT" to me. Guess what, I reckon your reverts weren't very good either. James Ker-Lindsay (talk) 19:17, 25 December 2021 (UTC)
 * Sorry, my comment above might not have been clear enough. When I wrote "I reviewed every one of the changes you made" I meant that I reviewed each of the many changes you made to this article. I didn't just blindly revert you—I carefully reviewed each of the changes you made. I have, so far, only reverted you once in this article. If you had only implemented MOS:CELESTIALBODIES, I would not have reverted you at all.--Srleffler (talk) 04:36, 26 December 2021 (UTC)

186,282 miles per second is Speed-of-Light
MODERATOR: The introduction needs to be tweaked to include 186,252 miles per second. 70.90.58.130 (talk) 13:35, 3 December 2021 (UTC)


 * The correct value is already in the last paragraph of the introduction. -- <b style="color:#FFF8C0;background:#494"> Dr Greg </b> talk 13:49, 3 December 2021 (UTC)

Shouldn't all measures be in the International System of Units only? Conversion can be done by the reader. -- Preceding unsigned comment added by BAzYx (talk o  contribs) 13:37, 27 January 2022 (UTC)


 * See Manual of Style/Dates and numbers for the official policy. AnonMoos (talk) 18:01, 27 January 2022 (UTC)

"Early History" section: Including the 14th century Indian scholar Sayana's calculation of the "Speed of the Sun"
Sayana, the 14th century commentator on the Vedas, asserted a calculation for the "speed of the sun" which is almost exactly the speed of light. The translation of his work and proceeding calculations are provided in detail here. I invite all thinkers to scrutinize it.

https://arxiv.org/pdf/physics/9804020.pdf

https://www.indiadivine.org/speed-of-light-in-the-rig-veda/

https://mathomathis.com/the-speed-of-light-subhash-kak/

Sayana’s calculation ought to deserve a mention in the section on “Early History”, wherein many inaccurate early theories are listed. Here is a suggested paragraph:

"In his commentary on the ancient Rigveda text, the 14th century Indian scholar Sayana mentions a calculation of the “speed of the sun” as “2202 yojanas in half a nimesha”, which some modern scholars have shown to be close to the actual speed of light." — Preceding unsigned comment added by 157.127.124.151 (talk) 02:22, 28 January 2022 (UTC)
 * We should wait until a reliable source is available, preferably a reliable secondary source. Arxiv preprints are not reliable sources, and a primary source is not sufficient for this claim.--Srleffler (talk) 19:28, 30 January 2022 (UTC)

FA criteria
I am concerned that the article does not meet the FA criteria. For example, some content is not cited to a reliable source. (t &#183; c)  buidhe  21:02, 2 November 2020 (UTC)
 * Such as? &#32; Headbomb {t · c · p · b} 21:21, 2 November 2020 (UTC)
 * Look at the article, it should be obvious what content does not have a reliable source cited inline as required by FA criteria. Some sections are tagged for it. (t &#183; c)  buidhe  21:28, 2 November 2020 (UTC)
 * There's a bit about some Greek stuff, fairly well-known, which should be pretty easy to source. Not sure about Hero stuff being accurate, but Hero did have early theories on light, and that's pretty easy to source too. I don't see the tagged section at lacking in sourcing. If that's all that missing for citations, it shouldn't take much more than a half hour to fix. &#32; Headbomb {t · c · p · b} 21:47, 2 November 2020 (UTC)
 * @Headbomb, @Buidhe: I mostly agree that sourcing is okay. I've added a handful of citation needed tags. If those are addressed, and the external links reduced/checked, I'm okay with noting it as satisfactory in URFA/2020. All of the sourcing is quite old, and it's been a while since I did my physics degree, so maybe we need to ask the physics WikiProject to check whether newer developments are accurate, complete and due. FemkeMilene (talk) 18:27, 4 March 2021 (UTC)
 * I see did not indicate this work was completed, so have brought this section out of archives for updating. Sandy Georgia  (Talk)  19:55, 2 January 2022 (UTC)
 * Also, (and noting that External links might be trimmed to conform with WP:EL, as this FA is presumed to be comprehensive). Sandy Georgia  (Talk)  19:58, 2 January 2022 (UTC)
 * are you still willing to mark this as "Satisfactory"? If so, can you indicate as such at WP:URFA/2020B? If 's concerns are resolved, perhaps we can remove this from WP:FARGIVEN? Z1720 (talk) 18:28, 5 February 2022 (UTC)
 * Currently there are 19 citation needed tags so I do not think it can be marked as satisfactory. (t &#183; c)  buidhe  18:47, 5 February 2022 (UTC)
 * I think the articles has good bones, and would likely be saved at FAR. In addition to the cn tags, the lede is also a bit too long (6 paragraphs). Femke (talk) 18:51, 5 February 2022 (UTC)
 * do you want to fix up the article in the next few weeks, or bring this to FAR? Both buidhe and I are at our 5-article FAR limit so we can't nominate this at the moment. Z1720 (talk) 19:02, 5 February 2022 (UTC)

superfluous regionalization?
Note 3 says "... in imperial units and US units ...". We are talking about miles and inches here, which if really needed, can be labelled "imperial units" and leave it at that. Should every region in which imperial units are used be listed? I'd say 'no'. The description "US units" is also just wrong, as it suggests that the US dominantly uses a non-SI set of units, whereas in the scientific context (which applies here), the unit systems in use in the US are overwhelmingly not "customary US units". 172.82.47.211 (talk) 16:38, 7 February 2022 (UTC)
 * I kind of think that having note 3 there is overkill and clutters up the intro. XOR&#39;easter (talk) 05:03, 11 February 2022 (UTC)

Descartes' eclipse argument is nullified by Bradley's discovery of aberration -- should this be noted?
Descartes' oft-cited argument, that any propagation delay of light would result in a misalignment of Sun/Earth/Moon during a lunar eclipse, has an oft-overlooked flaw that is evident in retrospect since Bradley's time -- that the supposed misalignment would be cancelled by aberration of light, so that the alignment would appear straight regardless of light speed. Thus, the absence of discernible misalignment does not impose a lower limit on light speed, but instead has no bearing on the matter.

A note about this has been added to the page on aberration of light. Should a brief note be given here also? (There may be some value in pointing this out, as an online search on this point turns up more confusion than clarity -- some students and even professors seem to think that such misalignment is still present, and inconspicuous only because of the short timescale, not absent altogether.)

If the above considerations are deemed convincing, please consider augmenting your historical section, excerpted below, with a brief note along the lines of the new sentence in parentheses there.

[If unconvinced, or if the cited reference is difficult to access or follow, perhaps the following approximate gedanken experiment will help: Imagine yourself (like Gamow's Mr. Tompkins) in a world where lightspeed is much slower, say only about 10 times as fast as you can run. Suppose you are on a flat field facing due North, while on your left the Sun is just setting due West, and on your right, your shadow is visible on some white fence (Tom Sawyer memorial) that runs north/south. Standing still, you see your shadow perfectly abreast of you, but if you start running North, your shadow begins to fall behind a little, as Descartes would have expected. After you have been running at a steady 0.1c for a bit, an overhead observer (in the Goodyear Blimp high above you and your shadow and roughly equidistant from both) would see your shadow lagging you by about 0.1 radian. Since your view of your shadow involves a round trip, you might expect to see a lag of 0.2 radian instead. But since you are moving North at 0.1c, the aberration of light will make the sun and your shadow each appear to be 0.1 radian north of their "geometric" locations, thereby cancelling out the lag effect, so that your shadow still appears diametrically opposite from the sun after all.]

- Present text excerpt (with proposed new note in parentheses):

Early history … In the early 17th century, Johannes Kepler believed that the speed of light was infinite since empty space presents no obstacle to it. René Descartes argued that if the speed of light were to be finite, the Sun, Earth, and Moon would be noticeably out of alignment during a lunar eclipse. (Although this argument fails when aberration of light is taken into account, the latter was not recognized until the following century.) Since such misalignment had not been observed, Descartes concluded the speed of light was infinite….

- DAL47 2601:601:1501:8320:ED21:7264:3A2:E1E4 (talk) 00:22, 27 June 2022 (UTC)


 * Good addition. I did not know that. Roger (talk) 17:56, 27 June 2022 (UTC)

Speed of causality?
"The speed of light is not about light" or so I've seen youtube video titles ( https://www.youtube.com/watch?v=msVuCEs8Ydo ). IIRC I've seen it called "speed of causality". Can/shall we include something like this? Darsie42 (talk) 10:04, 18 February 2022 (UTC)
 * The article already says that anything else massless will also travel a the speed of light, and that c is the upper limit for the speed at which ... any signal carrying information can travel through space. I don't think we need to say more than the article already does, but that's just my take. XOR&#39;easter (talk) 19:19, 18 February 2022 (UTC)
 * Maybe specifying that light only moves at c when in a vacuum would be better. There are lots of cases where photons move slower than c due to a medium or gravity or whatever, from my understanding. 50.124.244.111 (talk) 22:02, 30 June 2022 (UTC)
 * This is already explained at the end of the intro. XOR&#39;easter (talk) 22:17, 30 June 2022 (UTC)
 * oh yeah, it does, my bad lol 50.36.160.182 (talk) 16:25, 1 July 2022 (UTC)
 * No worries! :-) XOR&#39;easter (talk) 16:31, 1 July 2022 (UTC)

Semi-protected edit request on 8 July 2022
विश्वमा भासिरोचनम 103.119.245.102 (talk) 12:26, 8 July 2022 (UTC)


 * Red question icon with gradient background.svg Not done: it's not clear what changes you want to be made. Please mention the specific changes in a "change X to Y" format and provide a reliable source if appropriate. Also, any requested edits to the text must be in English. ComplexRational (talk) 13:00, 8 July 2022 (UTC)

Which units need to be in the infobox?
Bearing in mind that an overlong infobox means more for mobile users to scroll past, which units and numerical values should we report? The case for the exact SI value is pretty obvious, but "astronomical units per day" feels more like trivia. Does it provide additional insight to say that c is roughly 0.307 parsecs per year as well as saying that light takes about 3.26 years to travel one parsec? XOR&#39;easter (talk) 22:33, 16 June 2022 (UTC)


 * Trivia multiply on WP like a disease. Some sort of guideline or criterion to restrict this should be in the MoS, IMO.  Here, as you point out, reciprocals are trivially redundant and hence space-wasters. In general, we should avoid illustrative stuff in infoboxes: this is not their function, so the entire "Approximate light signal travel times" list should be removed or moved into the article. Some of the signal travel time entries have essentially no merit anyway, such as "from geostationary orbit to Earth", "the length of Earth's equator" (the person who wants this would relate better to "7.5 times around the Earth in one second"), "one light-year" (we do not need to be reminded in an infobox about the obvious), "one parsec" (the speed in parsecs per year gives this), "from the nearest galaxy to Earth" (worse than bad; whether it is a galaxy is disputed and it is unrelatable, since almost all readers will have to research what is meant by this).  172.82.46.195 (talk) 15:30, 7 August 2022 (UTC)

Grammar fix (request for semi-protected edit)
Please insert a word: "one can thus a standard for the metre" should be "one can thus define a standard for the metre" (or any similar correction). 172.82.46.195 (talk) 19:01, 1 August 2022 (UTC)
 * ✅ I went with "establish a standard for the metre", which is equivalent to "define the metre", also noting that "define" is used in the previous sentence. Thanks for catching this! Complex / Rational  19:51, 1 August 2022 (UTC)


 * It would be better English to say "thus, one can..." than "one can thus...". Also, "thus" itself is usually a very pedantic and amateurish word. Try and make a sentence without "thus". cheers Billyshiverstick (talk) 05:17, 16 August 2022 (UTC)

Esplaining in Plain English
It would be great if a physicist with some teaching chops could put some real world examples here, for instance "the Lorenz Factor means that a person travelling headlong at 85% of the speed of light would be 100 feet tall", or that if a space ship travelled away from Earth at 50% of the speed of light for a year its clocks would be six months ahead of Earth clocks, but if it returned to Earth at the same speed for a year, the clocks would re-synchronise. Thanks Billyshiverstick (talk) 05:15, 16 August 2022 (UTC)


 * The idea is introduced in a section (), but to get further detail like this is beyond the scope of this article. The link at the top of the section (to Special relativity) is sufficient: that is where detail of this nature belongs.  172.82.46.195 (talk) 11:38, 16 August 2022 (UTC)

BC or BCE
I see an older version from 2007 used BCE. Is there a reason it was changed to BC? Great article, by the way. John (talk) 16:15, 16 August 2022 (UTC)


 * I guess it's no big deal. I changed it back per MOS:ERA. John (talk) 16:49, 16 August 2022 (UTC)

Tweak: odd use of "%"
Could we change
 * For example, a time dilation factor of γ = 2 occurs at a relative velocity of 86.6% of the speed of light (v = 0.866 c). Similarly, a time dilation factor of γ = 10 occurs at v = 99.5% c.

to
 * For example, a time dilation factor of γ = 2 occurs at a relative velocity of v = 0.866 c. Similarly, a time dilation factor of γ = 10 occurs at v = 0.995 c.

? I may find excessive parenthetical repetition detracting and think percentages belong mainly on grade sheets more than others do, but I think that the final 99.5% c is just weird – using a percentage as a coefficient is rather unusual. 172.82.46.195 (talk) 21:08, 16 August 2022 (UTC)


 * This is for the less mathematical readers. 50% of the speed of light is something pretty much everyone understands. v = 0.5 c is... much harder for those that struggle with math. &#32; Headbomb {t · c · p · b} 00:22, 17 August 2022 (UTC)


 * That does not address my final comment. Your comment would suggest Similarly, a time dilation factor of γ = 10 occurs at 99.5% of the speed of light.  Do think people who find v = 0.5 c "much harder" will do any better with v = 99.5% c?  172.82.46.195 (talk) 01:47, 17 August 2022 (UTC)

Definition
"Its exact value is defined as 299792458 metres per second"

That's the wrong way around really. C is an absolute constant - you can't define it in terms of metres and seconds. Indeed the metre is defined in terms of c, so that would be a circular definition. I think it should just say something like "It's value is 299792458 m/s" Bagunceiro (talk) 14:15, 16 August 2022 (UTC)


 * Maybe I'm missing something, but isn't that what the article already explains? The note at the end of the line goes into more detail; the point is that the value is exact by definition. XOR&#39;easter (talk) 14:24, 16 August 2022 (UTC)


 * Maybe this is an opportunity to consider the wording, so that more people read the intended meaning into the statement: what is being defined might be made clearer. What is being defined is a relationship between units (the second and metre) through the choice of a numeric value for the physical quantity when c is expressed in terms of those units.  The footnote does a good job of explaining it, but the sentence could be revised.  I'll give thought to a rewording.  172.82.46.195 (talk) 15:00, 16 August 2022 (UTC)
 * I made a little adjustment to it. XOR&#39;easter (talk) 15:10, 16 August 2022 (UTC)
 * I'm not going to fight this corner - it's probably a bit too pedantic. But I would nevertheless suggest that this still implies that the speed of light is what it is by definition. This is not so - c is a fundamental constant, you can define units in terms of it, but not the other way around.Bagunceiro (talk) 15:31, 16 August 2022 (UTC)
 * It is that value by definition of the meter. &#32; Headbomb {t · c · p · b} 15:33, 16 August 2022 (UTC)
 * So maybe the article can avoid all confusion by saying "by the definition of the metre" instead of just "by definition". <b style="color:#FFF8C0;background:#494"> Dr Greg </b> talk 16:44, 16 August 2022 (UTC)
 * I think "by definition" is good enough, what with the elaboration in the endnote and later. XOR&#39;easter (talk) 18:18, 16 August 2022 (UTC)
 * ByThis is the lead and that is not how readers might first interpret it. Since we do not say by definition of what, I think the footnote is sufficient and we should just omit any mention if "by definition" in that sentence.  That is, I would suggest "Its exact value is $cm⁄m$ ...", along the lines suggested by Bagunceiro.  What are we actually adding by including "by definition"?  Only this who know already will understand from the sentence what it is saying.  172.82.46.195 (talk) 19:13, 16 August 2022 (UTC)
 * I was about to suggest dropping 'by definition' in the lead myself. As long as we state it's an exact value, that's all that matters for the lead IMO. &#32; Headbomb {t · c · p · b} 19:59, 16 August 2022 (UTC)
 * I'm not convinced that "by definition" is confusing or that dropping it would be clarifying (sometimes those lay readers just glide past the things that I, in my boundless wisdom, think would trip them up). But I have no strong feelings about it. XOR&#39;easter (talk) 20:18, 16 August 2022 (UTC)
 * OK, I will say that "The speed of light defines the metre such that by definition" is more awkward than either what we started with, what we had before (with just "by definition"), or just "The speed of light is exactly". XOR&#39;easter (talk) 12:00, 17 August 2022 (UTC)

Photons and Perspective
The article says (in Faster-than-light observations and experiments​): If a laser beam is swept quickly across a distant object, the spot of light can move faster than c. Shouldn't you say "from the perspective of the observer"?

Another way of looking at it is that each photon cannot move faster than the speed of light. Pretend you have a laser that will sweep across the moon's surface (about one-half an angular degree from Earth) in, let's say, one microsecond (or 1000 nanoseconds). But the "spot" (as observed from Earth) is an illusion.

What we see from Earth may appear to move much faster on the moon than one foot per ns (taking the moon's diameter to be 3500 km diameter). But we're observing the spot from Earth. On the moon, each individual photon (which travels to the moon in about 1.4 seconds and 1.4 seconds back) is independent from all the others. If you had a laser that could emit just 1000 photons in the microsecond that it takes to complete our sweep then, from the moon, you would observe one photon hitting the surface about every 3.5 km (3500 km diameter / 1000 ns).

Do I have it right or have I missed something? -RoyGoldsmith (talk) 21:24, 16 August 2022 (UTC)


 * It does not have to be from the perspective of Earth: the "spot" (the point on the surface at which the laser photons are arriving) travels faster than c from the perspective of any frame of reference or vantage point. You basically have it right, but I see nothing wrong with the statement in the article.  172.82.46.195 (talk) 21:45, 16 August 2022 (UTC)
 * When you say "travels faster than c", what do mean by "travels"? Is there any physical object (including a single photon) that travels faster than the speed of light? If so, from what vantage point? (Again I'm talking about physical objects. See Superluminal motion and Light echo.) -RoyGoldsmith (talk) 23:43, 18 August 2022 (UTC)
 * No, clearly there is no physical object travelling. It is just a locus of events (photons colliding with a surface) of which the space separation is greater than the distance that light could travel in the time that separates them (and this remains true in any frame of reference).  This is the point that the statement seems to be making: that despite being able to give some meaning to "velocities" greater than c, there is in actuality nothing "real" (not even information) that is travelling faster than light.  All I am saying is that to answer your original question, it is not necessary to add a qualifier about perspective: the calculated "velocity of the spot" in this example is greater than c from every perspective whenever it is so from one perspective.  I'm not even sure how to make the wording clearer in the article.  172.82.47.242 (talk) 15:16, 20 August 2022 (UTC)

False or Undecidable Statement
The first paragraph states as its closing sentence, "According to the special theory of relativity, c is the upper limit for the speed at which conventional matter or energy, and thus any signal carrying information, can travel through space." I originally requested a citation for this but someone removed it. This statement is strictly speaking either false or undecidable. First, it fails to define "conventional matter", so its validity cannot be ascertained in that respect. Second, it fails to define "space". Space as it was conceived of in 1905 was undergoing a major paradigm shift from a nebulous aether, where time is purely parametric, to an idealized vacuum on a hyperbolic spacetime manifold. The working assumption was that "space" as in outer space far from other gravitational bodies was empty and completely devoid of anything. In the original German text, one can read explicitly that Einstein makes no mention of the speed of light being an upper limit on the flow rate of energy. In particular what is written is that, "all observers irrespective of their motion (both mutually and with regards to the light) will measure the speed of light in vacuum as the same number. This simple and seemingly odd pronouncement turned out to mandate the need for the existence of hyperbolic spacetime. But it still does not place bounds on the rate of energy (and hence information) transfer. This is because in the theory of special relativity only the surprising (at the time) facts about light being measured as the same speed whether one is moving toward an incoming light ray or moving away from an incoming light ray. This was in sharp contrast to Galilean relativity.

Fast forward to today. We now know that there is no such thing as an idealized vacuum. Examples include the Casimir force and the Lamb shift. Even the farthest reaches of space have a photon flux through their region of about 2K called the cosmic microwave background radiation. In addition, we know that spacetime is expanding at an accelerated rate, this implies there is a whole new notion of energy that we have no idea what it is or how it works/interacts with other forms of energy. Although special relativity does show the need for treating momentum and energy as one just like spacetime, one must not confuse matter and energy. All matter is energy but not all energy is matter. Thus special relativity makes no conclusion about the rate of transfer of energy and hence information. This is because all information is necessarily energy, and more so seldom just matter. In fact, the definition of existence is having the property of energy or the ability to interact as the potential to demonstrate energy.

Consequently, I am changing the sentence so that it is at least factually true. Please do not revert the sentence until having discussed in-depth with me over any objections. MMmpds (talk) 21:17, 22 August 2022 (UTC)


 * No. The existing text was correct, and in alignment with every standard presentation of relativity. Moreover, the article already explains this, in considerable detail and with a considerable number of sources, and the introduction is there to summarize the main text of the article. XOR&#39;easter (talk) 23:52, 22 August 2022 (UTC)


 * At risk of belabouring a point, isn't there a slight non-sequitur here? I accept that it is a pretty standard presentation to make this connection, and (from the reference in the article) it is clear that experimentation fairly firmly supports the conclusion, but this does not necessarily imply that the conclusion (no energy or information faster than light) follows from the premise (special relativity). The causal structure derivable from QFT (which incorporates special relativity) also no doubt allows a mathematical proof of the conclusion, but this uses a lot of structure that is not present in special relativity on its own.  Special relativity on its own trivially does show that acceleration of an object from below to above c is impossible, but not that no energy or nonstandard massive object can do so: it seems pretty clear that special relativity on its own does not prohibit energy-carrying tachyons, notwithstanding the resulting destruction of the accepted causal structure and the experimental evidence to the contrary.  That is, you need special relativity + [causality axiom (no tachyonic antitelephone) or QFT or experimental evidence].
 * The article body (§ Upper limit on speeds) seems to be comprehensive in this respect. The lead's "According to the special theory of relativity, c is the upper limit for the speed at which [...] energy, and thus any signal carrying information, can travel through space" might be an oversimplification, though, and I think misleads the less sophisticated reader.  It would be nice to work in something along the lines of "According to the special theory of relativity with a reasonable causality assumption, ..." to better summarize the content in the article.  172.82.47.242 (talk) 01:36, 23 August 2022 (UTC)
 * I think separating "causality" from the theory of relativity is both nonstandard and an excessively fine point for the introduction of this article. XOR&#39;easter (talk) 01:42, 23 August 2022 (UTC)
 * My example is not a good way of resolving the problem. SR has strong implications on causality, and I agree that my suggestion, verbatim, would not be great in the lead.  All that I am trying to do at this point is to establish the realization that the current sentence in the lead does not summarize the body, is not logically correct, and misleads readers in a nontrivial important way.  Removal of the sentence, for example, would IMO be preferable to leaving it as is.  Alternatively, it could be qualified in any number of interesting ways, such as by saying "for known physics".  SR, strictly speaking, is about Poincaré symmetry in physics.  It permits counterintuitive results, such as the superluminal transport of electric charge (still, naturally, without violating causality).  To reiterate: causality is not a direct consequence of special relativity without additional assumptions.  172.82.47.242 (talk) 12:55, 23 August 2022 (UTC)
 * How causality relates to the geometry of Minkowski spacetime is part of the bundle of ideas that go under the heading "special relativity"; one learns about it in a special-relativity course. I'm open to phrasing adjustments, but this seems like going down the path of an artificial distinction, or at least a distinction that is not universally agreed upon as a standard in the physics community about how to organize ideas. It's like saying thermodynamics is, strictly speaking, about the Laws of Thermodynamics, so a statement about Carnot engines has to be extra-qualified with "According to thermodynamics and the definition of a Carnot engine...". (That's a pre-coffee analogy, but maybe it helps illustrate where I'm coming from.) If there's a lack of clarity here, I think we're looking in the wrong place. XOR&#39;easter (talk) 13:26, 23 August 2022 (UTC)
 * To say it another way, I think that "according to special relativity" (or "according to the special theory of relativity") already covers the cases that are mathematically consistent with Lorentz invariance but physically inconsistent with relativistic causality. Moreover, the idea of $1⁄2.54$ as a speed limit is important enough that we need a line on it at about that spot. Is there a better way to express that idea in that location? Well, I've seen this article so many times that by now I hate all of the words in it, so I'm inclined to say "probably". But I am concerned that we might fall into the habit of inventing an imaginary reader who we imagine is confused in a very particular way and acting on their behalf. XOR&#39;easter (talk) 14:21, 23 August 2022 (UTC)
 * We seem to be missing each other on something. I do not wish to push a debate that does not appeal to you at this point.  172.82.47.242 (talk) 17:02, 23 August 2022 (UTC)
 * @XOR'easter How is it possible that "separating 'causality' from the theory..." constitutes an excessively fine point when all of special relativity relies heavily upon whether a system is timelike or spacelike separated? With spacelike separation, there is no notion of causality. This is one of the core cruxes of the theory. MMmpds (talk) 19:29, 23 August 2022 (UTC)
 * I had hoped my later comment would have clarified this, but apparently not. I was responding to 172.82.47.242's argument about what "special relativity on its own" implies. My reaction was that trying to separate out the mathematics of Poincaré symmetry and call that "special relativity on its own" (or "special relativity in a strict sense", etc.) was splitting hairs. XOR&#39;easter (talk) 19:38, 23 August 2022 (UTC)
 * @XOR'easter But that's exactly the point, namely, that SR does not contain sufficient information by itself to conclude that the speed of light is a limit on energy or information transfer. SR simply states that all observers irrespective of their motions will measure a ray of light traveling through an idealized truly empty vacuum as the same exact number. A corollary is that an idealized point mass obeying nonquantum (ie classical+SR) mechanics would require an infinite amount of energy to reach the speed of light. As to experimental facts that demonstrate an incomplete picture here, more than 50% of the rest mass (energy) of a proton is from its gluons and not the 3 quarks that comprise it. From a classical perspective this is astonishing, from a quantum perspective this is obvious because the QCD vacuum is even more exotic and QCD calculations are only valid at high energies. A cooled proton at rest will not fulfill the high energy requirements of asymptotic freedom. MMmpds (talk) 20:25, 23 August 2022 (UTC)
 * So far, you've invoked asymptotic freedom, the EPR paradox, the quantum measurement problem, Dirac delta functions, and the definition of "existence", but I'm still not sure what you're trying to say about the last sentence of the first paragraph of this Wikipedia article. In fact, I'm more confused than ever about what point you're aiming for. On the one hand, you take a highly circumscribed view of what counts as "special relativity", a view that excludes concepts of causality. On the other hand, one statement up, you say that some notions of causality are among "the core cruxes of the theory". XOR&#39;easter (talk) 20:45, 23 August 2022 (UTC)
 * @XOR'easter I may be premature in this conclusion but it seems like I am trying to be drawn into a straw man argument whereupon one confuses spacelike separation with timelike separation. Is this the intent? If so, it may be the source of the confusion. If not, one ought to be careful as to take words within their context. Hint: there are many contexts occurring in this multibranched/multiperson discussion simultaneously (in the standard frame).
 * The flat fact remains, one cannot conclude that the speed of light is a limit on energy/information transfer from special relativity, or any other physical theory for that matter because the few experiments that tried to probe this got mired in controversy for no particular reason other than choices made in signal processing matter. There is an easy Gedankenexperiment that clears this up. If I had a rigid stick (which maintained its structure and rigidity) that was about 1 AU in length I could tap Morse code into the surface of Mercury while a light signal would take a few minutes to arrive. To refute the thought experiment one needs to a) argue that no such structure is possible and b) that the nonexistence of such a structure is somehow fundamental to the universe. Neither is true and several experiments tried to establish this except instead of a rigid body they utilized quantum lattices over extended physical distances. In principle, if a LIGO arm was outfitted with a long enough quantum lattice that remained cooled and coherent throughout the length of the arm, and the adjacent region was vacuum, then one could explicitly test whether information can arrive at a location faster than the speed of light. Aside from this being an expensive experiment, I think that it is well worth it considering the controversy this topic has brought up throughout my career as a scientist. It would be nice to finally settle the issue once and for all. So all the talk about quantum this and quantum that is because quantum mechanics is the counter example to the claim that the speed of light is a limit on information transfer. Consequently, the conclusion that has been reached repeatedly is that the theory of special relativity places no bound on the rate of transfer of energy/information. Moreover, quantum theory's nonlocality suggests that such superluminal transfer is possible. MMmpds (talk) 22:02, 23 August 2022 (UTC)
 * No, quantum theory does not suggest that superluminal transfer of information is possible. The article already explains this. XOR&#39;easter (talk) 22:14, 23 August 2022 (UTC)
 * I am sorry but the nature of a nonlocal theory that only conditionally obeys causality absolutely implies the possibility of superluminal information transfer. Not a single claim refuting this statement draws rigorously on all known mathematics and physical experiment. There are so many obviously false statements. For example, it is often claimed that a projection does not transfer information. This is clearly false. Yes there is some information loss but not a 100% loss. In fact, if the argument were true, one would not be able to calculate the height of an unseen object from its shadow. But one can, so objections based on poo-pooing projections as sources of information are obviously invalid. Other erroneous arguments include claiming that the phase velocity is somehow some pale informationless counterpart to the group velocity, and then declaring by fiat that the phase velocity cannot carry information. This is not so. In fact, research by D. Carfi has shown that the phase velocity must be incorporated into mathematically rigorous relativistic quantum theory in order to assure positive probabilities and consequently the phase velocity necessarily carries critical information about a general relativistic quantum state. The list of endless nonsensical objections is exhausting and I have refuted enough erroneous claims in this talk section to award a savvy reader a PhD. I will brook no more whining about wanting something to be true so therefore it must be true. The facts speak for themselves. MMmpds (talk) 03:49, 24 August 2022 (UTC)
 * You're welcome to argue uphill against the entire literature on quantum entanglement, but neither this article nor this Talk page is the place to do so. (There are plenty of cases where I personally believe that people are thinking about such-and-such in quantum mechanics the wrong way, but I don't argue about those on Wikipedia. It's not the place, and speaking egotistically, I wouldn't get credit if I proved myself right.) How much "whining" you will personally "brook" is immaterial; what matters is whether you can get a consensus of editors to support the changes you wish to implement. XOR&#39;easter (talk) 14:48, 24 August 2022 (UTC)
 * So you are explicitly announcing that truth is relative to the consensus of editors. This has no place in science and it is not the spirit of Wikipedia. Factual arguments do matter. They matter in real life and they matter on Wikipedia. MMmpds (talk) 19:01, 24 August 2022 (UTC)
 * Please see WP:TRUTH. If your position were the mainstream view, broadly represented in reliable sources on the subject, I'm sure you would have no trouble assembling a consensus in its favor.  Factual arguments matter precisely so far as they are effective at getting other people to agree with you; you appear to have failed at that here. JBL (talk) 19:48, 24 August 2022 (UTC)
 * Your summary of the events here are not accurate. I have provided numerous verifiable references as the necessary but not sufficient conditions. Moreover, the stubborn refusal of a single individual is not a measure of lack of consensus, or any failure on my part on a planet of 7 billion humans. The problem with WP:TRUTH is that it is designed as a broad policy across all human endeavors including politics, history, arts and literature just to name a few. In such areas there is no real notion of truth. This is in sharp contrast to science where a testable claim can be explicitly verified. Consensus is not the measure of scientific validity. To this end ignoring sources that are reliable by Wikipedia standards is tantamount to cherry picking content. This is surely against the spirit and intent of Wikipedia. It also dangerously invites folklore to displace scientific rigor.
 * Here is a precise example. It is generally believed by most physicists that a self-adjoint operator, a Hermitean operator and a symmetric operator are all equivalent. But this is mathematically false and there are myriad of sources that demonstrate this. The sources are not only reliable in the sense of Wiki standards but they are fundamental in the sense of understanding the underlying mathematical logic of quantum theory. In the realm of science there is no room for folklore beliefs and indeed science is the very endeavor to free the human species of superstition and folklore. Yet in every technical field folklore exists because it is a natural human tendancy, one that should be discouraged. MMmpds (talk) 20:15, 24 August 2022 (UTC)
 * @172.82.47.242 It turns out that causality is a purely classical phenomena. One does not need QFT, simply quantum theory will do. In addition, there are no rigorous proofs in QFT (the "proofs" are empirical experimental verification). This is because the rigorous mathematics of QFT is nonexistent. Ritualistically moving symbols around a page is not mathematics nor constitutes a logical proof. Also, the quoted text "As a result, if something were travelling faster than c relative to an inertial frame of reference, it would be travelling backwards in time relative to another frame, and causality would be violated. In such a frame of reference, an "effect" could be observed before its "cause". Such a violation of causality has never been recorded, and would lead to paradoxes such as the tachyonic antitelephone.", is plainly false for space like separation, since in such instances there is no notion of global causality and hence the perspective of one frame looks exactly like an observer traveling backward in time. In particular, there is no need for tachyonic anything. Tachyons are as their name suggests, tacky. Antimatter is a perfect example of an electron particle perpetually moving backward in time. The problem most scientists face is that they are unable to make the careful distinction of the 3 distinct notions of time. There is time as a spacetime coordinate on a hyperbolic space, which allows for the definition of a "proper time"; there is the time as it appears in Newton's equations as a purely parametric independent global variable for all dynamics; finally, there is the notion of time as it appears in the Schrodinger equation and all other time dependent partial differential equations where time is not parametric and not hyperbolic, rather it is Euclidean and it commutes with all other partial spatial derivatives. Clearly, we do not understand time or causality as we should. The missing pivotal connection is in the breaking of unitarity in quantum statistical mechanics and its mathematical foundations. Until the master equations are fully justified this will remain an open problem. I have my own resolution to this issue but it would constitute original research and not be appropriate for Wikipedia. MMmpds (talk) 17:03, 23 August 2022 (UTC)
 * Perhaps I am missing something, but this doesn't seem to indicate deficiencies in the explanations the article already includes. For example, antimatter is not "a perfect example of an electron particle perpetually moving backward in time". Such language oversimplifies the mathematics; a closer verbal approximation would be along the lines of, "The probability amplitude for a positron to do something is the same as the amplitude for an electron to do the reverse." You can't build a flux capacitor out of positrons. XOR&#39;easter (talk) 17:16, 23 August 2022 (UTC)
 * @XOR'easter The statement refining from the particle to the probability amplitude is also false. This is because all time evolution in a quantum system is handled by the unitary operator. Such operators are always invertible operators and moreover, time reversal, operator inversion and complex conjugation are all the same thing for time evolution. Clearly complex conjugating the probability amplitude does not accomplish this. MMmpds (talk) 17:42, 23 August 2022 (UTC)
 * Yes, time evolution is unitary (presuming that we are talking about an isolated system, et cetera), and time-reversal would amount to an anti-unitary. But that doesn't make "positrons are electrons going backwards in time" any better of an oversimplification. XOR&#39;easter (talk) 17:50, 23 August 2022 (UTC)
 * @XOR'easter Have you calculated the spectrum and resolvent operator of the Dirac equation? If so, the statement is self-evident. However, if not, I highly recommend Bernd Thaller's book called "The Dirac Equation". It makes explicit exactly how and why a positron is an electron moving backward in time. MMmpds (talk) 22:07, 23 August 2022 (UTC)
 * I've read Thaller's book. It explains how time reversal is an anti-unitary operator (section 3.4.2), but not that the oversimplified metaphor of "positrons are electrons moving backward in time" is anything other than, well, an oversimplified metaphor, just like the "sea of negative-energy electrons". (See in this regard chapter II.2 of Zee's Quantum Field Theory in a Nutshell.) XOR&#39;easter (talk) 22:48, 23 August 2022 (UTC)
 * Zee is a terrible source for anything. Also, there is no such thing as an "anti-unitary" operator. An unitary operator is one in which its adjoint (complex conjugate transpose) is equal to its inverse. The adjoint of an unitary operator is called an inverse unitary operator, also sometimes the forward and backward unitary operators for unitary and inverse unitary, respectively. However, the order of attribution is immaterial owing to the fact that it is merely human convention to choose the minus sign as the forward time evolution. The distinction in nomenclature matters because "anti" and adjoint are not the same. For example, the adjoint of a vector may be a co-vector but not an anti-vector, or the adjoint of a vector may be a linear functional but not an anti-linear functional (which has a completely different meaning and definition). I also strongly doubt you have read Thaller's book and if you did, obviously you did not understand it. The existence of negative spectral values does not imply a sea of magic (because positrons are in fact stable and are not scarce due to being "bound up" in the sea). Rather it implies the time reversal of positive energy values, which corresponds literally to an electron with its positive energy moving in the opposite direction in time, backward rather than forward. It's not a metaphor, it is rigorous mathematics combined with sensible logical thinking. MMmpds (talk) 03:01, 24 August 2022 (UTC)
 * Zee is a standard reference, your personal opinions notwithstanding. "Anti-unitary" is also established terminology, as witnessed by the title of section 3.4.2 in Thaller's book, "Antiunitarity of the Time Reversal Operator". If you have a problem with it, get it changed. When I said I read Thaller's book, I said the truth, and casting aspersions to the contrary is rather impolite. XOR&#39;easter (talk) 14:46, 24 August 2022 (UTC)
 * Zee is not a "standard reference" for those that are serious learners. The book is terse and filled with hand wavy nonsense. Moreover, it completely lacks rigor and precision. It is not a suitable reference and every professional I know agrees with me. So this is hardly my personal opinion.
 * Also I concede that it may be impolite to imply you were not truth full. But sadly as I suspected, I uncovered a piece of misunderstanding. When Thaller writes "anti-unitary", he is talking about a specific type of isometry on the Hilbert space one which acts anti-linearly as opposed to linearly. He then uses unitary to mean the isometries that are linear. This is because of the Wigner-Bargmann theorem which states that all such isometries that are either linear or anti-linear are unitary or anti-unitary (up to an overall U(1) character). This is the correct meaning and usage of "anti-unitary" because it is linked through the symmetry transformation of the action of a linear operator (as opposed to an anti-linear operator).
 * Also the specific section you reference has an important conditional, which is "if" one wants invariance under the Poincare group for both H_pos and H_neg then one must make the choices as such regarding the pi operators and their unitary/antiunitary behavior. The fact of the matter is that the Poicare invariance is not likely true or necessary. For a reference on a theory which takes this idea and modifies it to get to operators that are not self-adjoint but have real eigenvalues, see Carl Bender's theory on PT-Symmetric quantum mechanics. So when I said there is no such thing as an antiunitary operator, it was because I was speaking about the standard Schrodinger theory where there are no spinors and no relativistic invariance. It turns out one does not need spinors to have relativistic quantum theory, a fact that I think is not so well known.
 * Finally since you are familiar with Thaller's book, what is the reaction to the sections 1.8.1 "Superluminal Propagation" with its theorem 1.6 and section 1.8.2 "Violation of Einstein Causality". Thaller hand waves away the inevitable conclusion that quantum phenomena is nonlocal and acausal and can furnish a method of superluminal information transfer, by simply saying "we expect these effects to be immeasurably small". It turns out that as humans have studied quantum mechanics, we realized we can take the statistically improbable and in the laboratory set up conditions that make these tiny probabilities near certainty. This is what is down in laboratories that work on quantum phenomena around the globe. Yet you still seem to refuse to acknowledge that quantum phenomena absolutely gives a window into superluminal information transfer and acausality. Moreover, nothing in the theory of special relativity contradicts this. QED MMmpds (talk) 17:47, 24 August 2022 (UTC)
 * John C. Baez describes Zee's book as Packed with wisdom told in a charmingly informal manner though not the best way to learn how to calculate stuff . I've never met a physicist who substantially disagreed with that overall take. Since the point at hand is not about how to calculate stuff but rather on a more conceptual level, it's fine. What Thaller meant by antiunitary is what I meant by antiunitary.  As the opening to chapter 1 and the recapitulation in 1.8.4 of Thaller make clear, those are difficulties introduced by the artificiality of a single-particle theory, which inevitably leads to trouble when squeezed too hard. Trying to crowbar a relativistic notion of localization into a single-particle theory will cause problems to manifest in one way or another. What Thaller is talking about are pathologies that show the limitations of the theory that Thaller is exploring. See section 1.3 of Sidney Coleman's Quantum Field Theory Lectures, for example. XOR&#39;easter (talk) 18:28, 24 August 2022 (UTC)
 * Where did you get that quote by John Baez? I do not see it on the webpage. Also John may have said that before he met me and noted my and all my colleagues objections to Zee. What Zee is doing is sweeping under the rug all the mathematically invalid ritualistic symbolic manipulation of field theory and simply summarizing the results of calculations and how they agree with experiment. This in itself is not very useful since any reference book on QFT and comparison with the PDG's RPP values accomplish the same. If some how there were a group of physicists that have never heard of QFT then Zee might be a nice primer to ease into the topic.
 * It's ironic you bring up Coleman because his theorem states that in d<= 2 long range fluctuations are favored for, and continuous symmetries cannot be broken for, short range interactions. Because vacuum fluctuations can be controlled and stabilized (nature does this in the hydrogen atom in the form of the Lamb shift), it is the case that information can be garnered from them, just as the Lamb shift is an universal indicator of hydrogen. MMmpds (talk) 19:31, 24 August 2022 (UTC)
 * As for your claims about Thaller, firstly the Dirac equation is not a single particle equation. It is, was and always will be the theory of relativistic leptonic matter with its antimatter pair. Thaller attempts to wave away the meaning and interpretation by essentially making the same erroneous argument all physicists make-- that try desperately to cling to classical thinking, in the sense of locality and causality. The argument is that some how magically by adding more moving parts all the inescapable conclusions go away. That is they essentially claim without a viable mechanism that second quantization magically preserves local causal structure. Nothing could be further from the truth. For more on this topic, see any of NIST's BEC precision tests, aggregated multi-atom single particle behavior and their work on artificial gauge fields.
 * Secondly, nothing in 1.8.4 amounts to your claim or summary of its content. He simply outlines 2 unavoidable consequences and makes no claim about "squeezing the Dirac theory too hard". In fact, his comment on the infinite energy barrier is actually not true. In NIST laboratories as well as many others, one can fine tune lasers to pulse at the right frequency which can give the effective result of 'barriering' an ensemble or single atom into localizing where and how one wants. It is then possible to add the right environmental factors (boundary behavior and counter-lasers) to simulate relativistic and gauge field conditions. This is not unlike how graphene transport is governed by the Dirac equation with effective covariant derivative. MMmpds (talk) 19:52, 24 August 2022 (UTC)
 * @ XOR&#39;easter- Agreement with every published textbook, especially low level ones that over simplify the points are not an acceptable standard for scientific fact. Furthermore, the article does not explain the point I make and the high level summary is thus misleading at best. Please show me where in the article my technical point is already elaborated on. Even if such text exists in the article, it does not preclude a more honest and accurate summary, as I have written. I will politely await your reply before reverting your unjustified changes and specifically you did not engage me in discussion prior to reverting the text. This is not only unprofessional but violates the agreed standards and protocol of Wikipedia. MMmpds (talk) 16:44, 23 August 2022 (UTC)
 * Actually, the common (though not universal) practice is to revert and then engage in discussion. The technical matters are covered in the "Upper limit on speeds" section and later passages. XOR&#39;easter (talk) 16:52, 23 August 2022 (UTC)
 * FWIW, I am in agreement with XOR&#39;easter that the reverted edit does not belong: there are style and other reasons (even though I was suggesting a qualification, it is not the same). 172.82.47.242 (talk) 17:02, 23 August 2022 (UTC)
 * @172.82.47.242 I did exactly the same. I added qualifiers and removed erroneous words. However, I would like to hear more on the issue of style that you mentioned. MMmpds (talk) 17:16, 23 August 2022 (UTC)
 * For example, you added a reference to a talk page from article space. You also added a qualifier "classical idealized empty", without support from the body.  This qualifier is also unnecessary with the classical assumption of flat space.  172.82.47.242 (talk) 17:22, 23 August 2022 (UTC)
 * @172.82.47.242 I agree on all your style points. The redirection to a talk page was a temporary condition and I would have removed it if the edit remained untampered with. About the descriptors, "classical idealized empty", I agree they may need qualification but "idealized empty" and "vacuum" (as it was understood by Einstein and a few others in 1905) are interchangeable, if and only if the historical context is made clear for the word "vacuum". The term "classical" is a reference to the fact that the human species' notion of vacuum has drastically changed since 1905. So by classical one means non/pre-quantum thinking. Recall Einstein's difficulty with quantum theory and patently false arguments made in the 1932 1935 EPR paper. I cannot say I know of a better way of taking all this information and distilling it down further, which is why I used the triple adjective to form a strong qualifier. But as you say, it may be possible with direct linking to the body. Though, the body fails to make the critical key points and generally just regurgitates a historically false perception of special relativity. MMmpds (talk) 17:34, 23 August 2022 (UTC)
 * The EPR paper was published in 1935, not 1932. It was mostly written by Podolsky, not Einstein, and I doubt that either its argument or Einstein's own would be called "patently false". Yes, they're unpersuasive if you don't accept their premises, but that's something else. Einstein's own thinking on quantum theory was more subtle than he's often given credit for. XOR&#39;easter (talk) 17:46, 23 August 2022 (UTC)
 * Yes you are correct that the paper is 1935 and not 1932. That's what I get for going from memory rather than turning 90 degrees and double clicking a few times. Nonetheless, my error in the date does not invalidate the point, which is that the equations in the EPR 1935 paper, though few, are incorrect because they get composed in a way that is not consistent with the handling of generalized functions. This error is not surprising because it was not until the years 1939-1950 that the mathematics for composing such equations was rigorously worked out. Therefore, in hindsight, it is completely reasonable to state that the content of the article is "patently false", both mathematically and physically. You are also quite correct that the article was not written by Einstein, in fact he required some cajoling to attach his name to it. Nonetheless, the quality of an article, which argues by authority, and has incorrect mathematics is the prototypical example of scientific content, which has lost merit through the test of time. Also, Einstein's troubles with quantum theory are well documented historically and he did not need to be the author of the 1935 EPR paper for this statement to be true. MMmpds (talk) 18:07, 23 August 2022 (UTC)
 * I think you're getting pretty far into original research territory with those claims. XOR&#39;easter (talk) 18:14, 23 August 2022 (UTC)
 * @XOR'easter I'm sorry but I cannot and will not agree that well documented historical fact is original research. If you like I can give you a whole host of references regarding a) the mathematical errors and b) the evidence that Einstein struggled with quantum theory. MMmpds (talk) 19:14, 23 August 2022 (UTC)
 * The mathematical statements in the EPR paper are the application of standard quantum theory to an entangled pair of particles. Nothing about "the handling of generalized functions" invalidates that. Einstein's insistence that quantum theory is incomplete is indeed well-documented, but his arguments to that end are neither "patently false" nor germane to this article. XOR&#39;easter (talk) 19:18, 23 August 2022 (UTC)
 * Again this is not true. If one looks carefully at equation (14) on page 780 of the 1935 EPR paper, one finds a false statement that is in no way indicative of "standard quantum theory". Specifically, it is false that Dirac delta can be an eigenfunction. Eigenfunctions live on Hilbert spaces and there is no known Hilbert space for the Dirac delta. In addition, the Dirac delta cannot have an eigenvalue of multiplication as simply the spatial variable because there is no known linear operator that operates on Dirac delta returning simply the support of the Dirac delta function (except for the only 2 trivial choices of juxtaposition with x and Schwartz pairing with x). The Schwartz pairing fails because x is not a test function, and although the juxtaposition looks like it works, it is a distributional identity, which actually constitutes an infinity (with cardinality of the continuum) of distributionally equivalent but pointwise inequivalent equivalence relations.
 * To attempt to draw these false conclusions, one must appeal (without regard to rigorous mathematics) blindly to the so-called Dirac calculus. The Dirac calculus is mathematically incorrect and decades of research have gone into trying to establish it on solid mathematical footing to no avail. Significant forward progress has been made by I. Gelfand, G. Shilov, A. Bohm, M. Gadella and more recently D. Carfi. But there still is no full description. Part of the issue is that the de Broglie pure wave states are not square integrable but do form an abstract basis in a Frechet space. However, the Frechet space has no inner product and therefore the Max Born probability interpretation does not follow in an obvious manner. By the by, this work dates back to the 1960s and is more history than original research with the exception of the more recent pioneering work by D. Carfi.
 * As to the germane nature, the point is that unless something is a collection of bosons which can literally all stack on top of each other, everything we know about physical reality involves spacelike separation, which implies that causality is a local classical notion and quantum theory is inherently nonlocal. Due to the high precision in which quantum theory is known to be true, together with its inherent nonlocal nature (For example, see Lebesgue integral and set of measure zero.) implies that physicists' insistence on locality and causality are just desperate attempts to cling to the classical world, despite our best experiments telling us that the strange exotic nature of the quantum world is here to stay and closer to reality on a fundamental level. Ergo, one cannot conclude that energy or information is constrained by the speed of light through just special relativity alone. MMmpds (talk) 20:11, 23 August 2022 (UTC)
 * Working with position "eigenfunctions" that are Dirac delta distributions is a standard part of quantum theory, covered in probably any text at the Cohen-Tannoudji level, and in a more sophisticated way in the books of Peres, Holevo, etc. Everyone knows they don't fit into the Hilbert space of square-integrable functions, but ordinary manipulations suffice to handle that. It's not consequential for analyzing EPR. From where I sit, it looks like you're taking what is at most a general complaint about physicists' attitude toward mathematical rigor, making like that applies to the EPR paradox more than to any other application of the same theory, turning that into an "Einstein was obviously wrong" assertion, and from there saying that this article needs to be changed. I'll say, with all honesty and every attempt at cordiality, that this is a hard argument to follow. XOR&#39;easter (talk) 21:31, 23 August 2022 (UTC)
 * If this is your take away, I seriously wonder if you have actually read and understood anything I have written. In your so-called standard treatment, how do you reconcile the following distributional identity:
 * $$x\delta(x-\lambda) \,\dot{=}\, \lambda\delta(x-\lambda) \,\dot{=}\, x\cos(x-\lambda)\delta(x-\lambda) \,\dot{=}\, \lambda\delta(x-\lambda),$$
 * and which is the operator of multiplication $$X$$ or $$X\cos(X-\lambda)$$, both, or neither? By the way in considering your answer, be aware that there is an infinite continuum of possible other so-called "operators of multiplication". Hint, that cardinality really messes with operator algebra, which ruins both Stone-von Neumann and Gelfand-Naimark. MMmpds (talk) 03:20, 24 August 2022 (UTC)
 * Oops, missed this comment down at the bottom here. (This is what we get for having $in⁄cm$ conversations simultaneously.) But now that I've seen it ... what? I figure the typical physicist response would be, "Why are you throwing an extra factor in there? Is the potential not what you said it was before?" In other words, not all mathematical manipulations make sense physically; expressions that look equivalent in the abstract may be limiting cases of situations that look very different physically. But again, this looks like a general complaint about physicists' attitude toward mathematical rigor, which either applies or doesn't apply equally well to any mention of a "position operator". It's multiple stages removed from the purpose of this Talk page. XOR&#39;easter (talk) 15:41, 24 August 2022 (UTC)
 * @XOR'easter They are not covered in the upper limit section as my reply to @172.82.47.242 indicates for myriad of reasons. But you mention other places where the technical issues I have brought to light are already delineated. Where/which sentences? MMmpds (talk) 17:08, 23 August 2022 (UTC)
 * @XOR'easter you are correct regarding the bold cycle but I specifically requested the opposite direction because I knew a bunch of people with strong opinions and insufficient education would object. One shall not let ignorance and persistence substitute for scientific fact. MMmpds (talk) 17:13, 23 August 2022 (UTC)
 * Time for a break? Talk page discussion is not helpful when emotionally laden wording creeps in.  172.82.47.242 (talk) 17:41, 23 August 2022 (UTC)
 * @172.82.47.242 I believe there is a misunderstanding between a statistical qualifying statement of a large group of humans with "emotionally laden wording". I assure you, I place no emotion in my factual statements. Whether another human decides to attribute emotion to stated fact is beyond my control and not entirely relevant. MMmpds (talk) 17:49, 23 August 2022 (UTC)

This is awkward
This article has a certain behind-the-scenes notoriety among Wikipedia editors. A very long conflict about the speed of light culminated in an arbiration case (if you're unfamiliar with the process, I'll just say that on Wikipedia, arbitration is where we go when all other forms of dispute resolution have been exhausted and the community has been unable to agree on an effective end to disruption). The ruling on that case, Arbitration/Requests/Case/Speed of light or WP:ARBSL for short, began by reaffirming pertinent principles and policies, including Neutral point of view

3) All Wikipedia articles must be written from a neutral point of view. Where different scholarly viewpoints exist on a topic, those views enjoying a reasonable degree of support should be reflected in article content. An article should fairly represent the weight of authority for each such view, and should not give undue weight to views held by a relatively small minority of commentators or scholars. The neutral point of view is the guiding editorial principle of Wikipedia, and is not optional.

Neutrality and sources

4) Wikipedia articles should always verifiably use the best and most reputable sources, with prevalence in reliable sources determining proper weight. Merely presenting a plurality of viewpoints, especially from polarized sources, does not fulfill the neutral point of view. Similarly, relying on synthesized claims, or other "original research", is contrary to the neutral point of view.

Scientific focus

5) Wikipedia is an encyclopedia and its content on scientific topics will primarily reflect current mainstream scientific consensus. There is a wealth of policy behind each of those links, much of which has developed not in a top-down way but as a documentation of best practice and necessity as the community discovered it. It springs from the fundamental principle that Wikipedia isn't written by authorities on subjects; it's written by editors documenting what authorities have written. Much of the above discussion runs counter to that; on Wikipedia talk pages, something like (for example) "If one looks carefully at equation (14) on page 780 of the 1935 EPR paper, one finds a false statement" is a big red flag. That is not for us as editors to judge. We merely summarise our sources, giving due weight.

Arbcom's ruling also affirmed Decorum

7) Wikipedia users are expected to behave reasonably, calmly, and courteously in their interactions with other users; to approach even difficult situations in a dignified fashion and with a constructive and collaborative outlook; and to avoid acting in a manner that brings the project into disrepute. In content disputes, editors should comment on the content and not the contributor. Personalising content disputes disrupts the consensus-building process on which Wikipedia depends. Unseemly conduct, such as personal attacks, incivility and assumptions of bad faith, is prohibited. Then, as so often, some of the harshest things were said by people who were adamant that they were not being in the least emotional, which just goes to show that you don't need to be consciously emotional to overstep the bounds of civility and perhaps provoke emotions in others who, for example, as the only other disputants may quite reasonably take "a bunch of people with strong opinions and insufficient education ... ignorance and persistence" personally. WP:DISCUSSCONSENSUS describes a different approach. NebY (talk) 15:59, 24 August 2022 (UTC)


 * This edit summary amounts to special pleading: insisting that the authors of reliably-published sources must only be giving their personal opinions because they didn't write enough equations (for a point that is more conceptual than technical, so a superfluity of equations would probably be a sign of obfuscated thinking). I'm burnt out on this and don't want to engage any further. Goodbye. XOR&#39;easter (talk) 18:37, 24 August 2022 (UTC)


 * There clearly is wp:NOCONSENSUS to remove the properly sourced content. I have restored the content and warned user on their user talk page for edit warring. - DVdm (talk) 20:09, 24 August 2022 (UTC)
 * The lack of consensus of 3 people is not a good standard for documenting and neutrally retaining knowledge made freely available to billions of humans. This now requires arbitration. MMmpds (talk) 20:58, 24 August 2022 (UTC)
 * There is now an issue regarding, "It springs from the fundamental principle that Wikipedia isn't written by authorities on subjects; it's written by editors documenting what authorities have written. Much of the above discussion runs counter to that; on Wikipedia talk pages, something like (for example) "If one looks carefully at equation (14) on page 780 of the 1935 EPR paper, one finds a false statement" is a big red flag. That is not for us as editors to judge. We merely summarise our sources, giving due weight."
 * It is not possible to discuss and resolve disputes without delving into detail. It is not possible to assess due weight without recourse to detail. There needs to be controls in place to prevent famous but out of date references from displacing modern knowledge. In addition, there are missing controls on the quality of sources and their due weight. In essence, the claim is that there is a red flag on account of unavoidable assessment of source quality based upon assessment of content detail. Surely the Wiki policy is not based on assessing weight due to the reputation of the author, the reputation of the publisher and the title on the cover of the book. What is inside the book matters greatly. MMmpds (talk) 20:53, 24 August 2022 (UTC)

Photons faster than the speed of light
Researchers managed the speed limit of light "inside hot swarms of charged particles, fine-tuning the speed of light waves within plasma to anywhere from around one-tenth of light's usual vacuum speed to more than 30 percent faster." (source: sciencealert.com) — Preceding unsigned comment added by 151.34.70.250 (talk) 13:59, 2 September 2022 (UTC)


 * That source says this is about group velocity, and this Wikipedia article already states that group velocities greater than c are possible, and have been achieved before. -- <b style="color:#FFF8C0;background:#494"> Dr Greg </b> talk 14:24, 2 September 2022 (UTC)
 * Thank you, but the current WP article not the group velocity affirm that "fine-tuning the speed of light waves within plasma to anywhere from around one-tenth of light's usual vacuum speed to more than 30 percent faster." 30 percent faster seems to be a relevant experimental result. — Preceding unsigned comment added by 87.15.179.76 (talk) 19:21, 2 September 2022 (UTC)
 * This is not a comprehensible use of the English language. Try again? JBL (talk) 20:48, 2 September 2022 (UTC)
 * Sorry, there was a typo. The current WP article doesn't affirm that "fine-tuning the speed of light waves within plasma to anywhere from around one-tenth of light's usual vacuum speed to more than 30 percent faster.
 * In the same way, nothing similar is said within the article titled group velocity. To say that someone has achieved a group velocity that is 30 percent faster than the speed of light, seems to be relevant for the WP article. — Preceding unsigned comment added by 151.46.32.41 (talk) 15:45, 4 September 2022 (UTC)
 * We have an entire section on Faster-than-light observations and experiments|Faster-than-light observations and experiments and it begins The experiment you mention is just one instance in a large field of study. WP:NOTNEWS applies to the continual flow of scientific experiments and demonstrations too. NebY (talk) 16:08, 4 September 2022 (UTC)
 * Ok, thanks for your help. I misunderstood that my link was a summary of a peer-reviewed scientific paper instead of a part of a continous flow of scientific news. — Preceding unsigned comment added by 151.38.251.187 (talk) 18:06, 4 September 2022 (UTC)

Semi-protected edit request on 15 September 2022
Copying one letter 50.29.238.62 (talk) 05:45, 15 September 2022 (UTC)
 * Red question icon with gradient background.svg Not done: it's not clear what changes you want to be made. Please mention the specific changes in a "change X to Y" format and provide a reliable source if appropriate. <span style="background-color:blue;color:white;	font-family:'Brush Script MT', cursive;font-style: italic;">EnIRtpf09b chat with me 05:59, 15 September 2022 (UTC)