Talk:Speed of light

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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 ([1]). 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:

• 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).

Regards, QrTTf7fH (talk) 14:56, 9 September 2014 (UTC).

@QrTTf7fH - 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 Wikipedia: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. — 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. — 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, 1.07×10¹¹ 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#Rounded speeds, 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.

DVdm, Stigmatella aurantiaca: Please note that according to (policy) Wikipedia is not a democracy, (policy) Wikipedia:Consensus, (policy) Wikipedia:MAJORITY and (essay) Wikipedia: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.

Hertz1888: 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 100 000 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)

Dimensionnless 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)