Wikipedia:Featured article candidates/Speed of light

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[edit] Speed of light

Renomination from a couple of weeks ago - I didn't nominate last time, but supported (and made a few edits to deal with past objections). -- ALoan (Talk) 11:20, 11 Aug 2004 (UTC)

  • Comment:There is no picture in this article. [[User:Avala|Avala|★]] 12:38, 11 Aug 2004 (UTC)
    • What sort of pictures would you like? There are some equations. :) -- ALoan (Talk) 14:00, 11 Aug 2004 (UTC)
      • A laser beam (I've added one) is fine, but what about a diagram of some of the experiments to measure the SOL? [[User:Meelar|Meelar (talk)]] 14:21, 2004 Aug 11 (UTC)
      • Would the schematic diagram from Michelson-Morley experiment satisfy you? Not actually measuring the speed of light, per se, though. If someone wants to draw some more diagrams, I won't object. -- ALoan (Talk) 16:11, 11 Aug 2004 (UTC)
        • I've drawn a diagram of the Fizaeu method.theresa knott 18:04, 12 Aug 2004 (UTC)
  • I'm afraid I have to object again. The explanation of why nobody can travel faster than the speed of light is short and underdeveloped (in the "Overview" section), mentioning causality and special relativity without explaining either (and the link to causality doesn't mention speed of light at all, so I had no way of understanding). Basically, the history section is fine; but the explanation of why nothing can travel faster than light, and why it's a constant for all frames of reference, is poor, and demands far too much of the reader; I've got no problem reading other articles, but there should be a decent summary or explanation of thorny issues in this article). [[User:Meelar|Meelar (talk)]] 14:21, 2004 Aug 11 (UTC)
    • There was a brief explanation further down (in "Constant in all reference frames" where it was requested to be put last time) but I have moved it up, expanded it and added another diagram (taken from special relativity). Better? -- ALoan (Talk) 16:11, 11 Aug 2004 (UTC)
    • I'll change causality to causality (physics) (which is referred to causality and refers to special relativity). -- ALoan (Talk) 17:42, 11 Aug 2004 (UTC)
      • Support now, improved. [[User:Meelar|Meelar (talk)]] 19:15, 2004 Aug 11 (UTC)
  • Object: We really should make a better explanation of the constantness of the SOL: It is misleading to say, that this fact follows from Special Relativity. Historically, it was the opposite: The constantness was an empirical observation, and Einstein deduced his theory from this fact which allowed him to be confident in the principle of relativity. In other words: The Michelson-Morley experiment should be mentioned at a central place and not just briefly at the end, and the aether discussion should be mentioned as well. Simon A. 16:18, 11 Aug 2004 (UTC)
    • We are getting into deep waters here. The reason that speed of light is constant is not because it is observed to be so, but because the theory says it has to be. If someone makes a contrary measurement then we will have to tear up the theory and start again. As a historical fact, yes, the theory of special relativity was developed to fit the observations, otherwise it would not be a very useful theory, but the observations only tell you "what" not "why" (which is what Meelar had requested above). There is a short section on and link to the Michelson-Morley experiment - does we really need a longer explanation of a nil result and wrong theory here? However, I'll look at it again. -- ALoan (Talk) 17:24, 11 Aug 2004 (UTC)
    • Id'say the problem is condensed in this excerpt: "Einstein developed the theory of relativity by applying the consequences of the above to classical mechanics. Experimental confirmations of the theory of relativity directly and indirectly confirm that the velocity of light has a constant magnitude, independent of the motion of the observer." Now, this is clearly wrong: The addition formula is a consequence of the theory not a premise. I'll try to rewrite the paragraph. I rewrote the paragraph. Hope you like it. Simon A. 13:22, 12 Aug 2004 (UTC)
      • I'm happy if you are happy. Will you withdraw your objection? -- ALoan (Talk) 15:04, 12 Aug 2004 (UTC)
      • Yes, all fine now. ;-) Simon A. 15:09, 12 Aug 2004 (UTC)
  • I've removed my objection as I do not think it is significant enough to hold up featured status. anthony (see warning)
  • Support [[User:Sverdrup|❝Sverdrup❞]] 19:08, 11 Aug 2004 (UTC)
  • Support. (Not only fixed what I objected to before, but a bunch more.) Dandrake 17:57, Aug 12, 2004 (UTC)
  • Support. Ancheta Wis 14:50, 13 Aug 2004 (UTC)
  • Object. 1. The article uses the first person; it ought to remain in the third person throughout. 2. The article sometimes gives imperial units as the primary units, with the metric units merely in parantheses. Perhaps metric units should be employed throughout; I do not see the need for indicating imperial units in relation to a science article, except perhaps the figure 186,000 mi/s. 3. Should not the article explore the phenomena experienced by someone traveling at a speed approaching that of light? Time dilation is mentioned briefly; effects relating to length and colour could also be indicated. 4. The article mentions that velocities cannot be merely added when considering the speed of light. Would it be useful to mention the paradoxes that would arise if velocities could indeed be added? For example, I recall Carl Sagan writing something about an individual who observes a collision between one vehicle, oving towards the observer, and a horse, moving on another road perpendicular to the road of the former. If one would add velocities, the observer would notice the vehicle approaching him arrive well before the horse, whilst the rider of the vehicle would himself experience a collision. Perhaps a better elucidation may be made by someone more well-versed in science than myself.-- Emsworth 17:15, 13 Aug 2004 (UTC)
        1. Can you give some examples of where is the first person is used in the article?
        2. Has someone fixed this? I think metric almost always comes first (and often imperial is not used at all). There is one place where imperial is used without metric ("50+50=100") when the units are largely irrelevant, and imperial is used first once in the measurement section when I assume (I didn't add the facts, but apparently it was England in 1728) the measuremant was actually reported in imperial units. I may have missed some others - again, can you give examples?
        3. This is special relativity rather than speed of light, but I added a brief note.
        4. I think it is pretty clear from 4.1 that you cannot simply add velocities - unless anyone else would like to suggest an example.
      • -- ALoan (Talk) 02:09, 15 Aug 2004 (UTC)
        • The first person was used in section 4.1: "our usual intuitions," "We are accustomed," etc. But I have made the appropriate changes here.
          • Thanks for clarification and for fixing. -- ALoan (Talk) 22:43, 16 Aug 2004 (UTC)
    • I agree that metric units should be used throughout; however, the manual of style would seem to indicate that imperial units should be given in parenthesis. →Raul654 17:24, Aug 13, 2004 (UTC)
    • I didn't see 'first person'. There is a lot of passive tense. That is not first person. Natural units, which is what many theoretical physicists use, simply make the article obscure. Engineers like the ones in the illustration use imperial units and other non-metric units all the time, especially during experiments, as the standards usually haven't caught up with the science. The units of measure shouldn't be a show-stopper, in any event. Ancheta Wis 17:33, 13 Aug 2004 (UTC)
      • The manual of style is written by some individuals who choose to participate in the process. It only applies in some cases; it cannot be universal. It is, in my opinion, silly to include imperial units in science articles. They are never used, even in the U.S. (where they are most common), in relation to scientific matters. -- Emsworth 19:25, 13 Aug 2004 (UTC)
        • Agree that SI units should be used in scientific articles, but I don't think we should have too much on effects as to what happens when the speed of light is approached. That stuff id relativity related and should IMO go on special relativity not speed of light.theresa knott 19:42, 13 Aug 2004 (UTC)
    • I agree metric units should be used in the explaining text throughout, but not in history. For example, if Bradley calculated the speed of light as about 185,000 miles per second, it should be stated just so, just like if Ptolemy found the earth's diameter to be 50,000 stadia or what is was, we write just that. [[User:Sverdrup|❝Sverdrup❞]] 01:15, 14 Aug 2004 (UTC)
    • Emsworth you don't know what you are talking about. US standard units are very commonly used for scientific topics, esp when written by federal government employees. Both should be included and the measured units (in whatever system they were measured in) should be first with the other system in parenthesis. Sometimes that will mean metric (US standard) and other times it will be US standard (metric). --mav 03:26, 14 Aug 2004 (UTC)
      • Actually, given that I attend school in the U.S., I know that science classes (at least mine) use only metric units. As for the federal government employees, I would have imagined that they would use SI units, given that they are officially sanctioned by the U.S. government. -- Emsworth 18:51, 14 Aug 2004 (UTC)
          • And where do you think I went to school? Geography clases still use US standard often. Last time I checked that is considered to be a science. If you had stated physics classes, then I would have to have agreed with you. But you did not, so I disagreed with your blanket statement. ;) Either way, having both units will be helpful (see above for the order). --mav 23:40, 16 Aug 2004 (UTC)
  • Support but would like to see the refraction explained better, an explanation of how light is slowed in a BEC, and a brief explanation of what happens when something travels faster than the speed of light in a material. theresa knott 19:42, 13 Aug 2004 (UTC)
    • Better? -- ALoan (Talk) 02:09, 15 Aug 2004 (UTC)
  • Support. [[User:Avala|Avala|★]] 18:19, 14 Aug 2004 (UTC)
  • Object, all units except historical measurements should be consistent in metric units. The "50+50=100" example has no need to use imperial units and thus be inconsistent with the rest of the article. Also, in the "Faster than light" section, the two sentence paragraph on evanescent waves needs to be expanded to at least a summary of what that is and how it is involved in faster than light phenomena. If that can't be done, the section should be removed to the talk page until it can be expanded. In addition it notes - "it is not possible that information is transmitted faster than c." - what about the confirmed quantum mechanical experiments where measurement of the properties such as polarization of one photon instantaneously determines the polarization of the other particle which is too far away for that information to have traveled at the speed of light? My recollection of the physics of that is not perfect, but it does need an explanation in the article, as it is often brought up as a counterexample to the point I've quoted. - Taxman 19:57, Aug 16, 2004 (UTC)
    • Fixed the units of the simple example, expanded reference to evanescent waves, and added discussion of quantum entanglement (which, yes, permits quantum properties to be determined at a distance faster than light speed, but not information). -- ALoan (Talk) 22:43, 16 Aug 2004 (UTC)
      • Nice work, one last note, the section about defining the meter talks more about two other topics. Should the topic heading be more like "Measurement definitions based on the speed of light", or "Definition of the metre and other measurement standards"? I couldn't think of a perfect one or I would have done it myself. - Taxman 00:06, Aug 17, 2004 (UTC)
        • Thanks - have moved light year part as not entirely relevant to definition of the metre, but discussion of the definition of the second is vital to the definition of the metre - you need to know the length of one second so you can measure how far the light has gone in 1/299,792,458 of a second. I have clarified a little further. -- ALoan (Talk) 10:13, 17 Aug 2004 (UTC)
          • comment: Speed_of_light#Definition of the metre is getting equal to Metre.--Nabla 12:30, 2004 Aug 17 (UTC)
          • Ok good work. Now another note, the intro states - "At present, the speed of light in a vacuum is a definition" - which is not correct. The magnitude of the measurement of the speed of light that we assign is a definition, but the speed itself is an inherent quality, not a definition. Also I'm concerned with - "The constant speed of light derives from the Lorentz transforms" - that makes it sound like the speed of light is constant because of the Lorentz transforms. The equations do have a constant speed as a consequence, but they don't cause it. - Taxman 13:11, Aug 17, 2004 (UTC)
            • "The magnitude of the measurement of the speed of light that we assign is a definition, but the speed itself is an inherent quality, not a definition." That's a philosophical point which to some extent contradicts relativity. While you could say there is an inherent speed of light, there would be no way to measure such a speed, as it is essentially just a conversion factor. I'm not sure how
              • All I was referring to by inherent was that the speed is what it is, and is not affected by whether we give it a magnitude of 299,792,458 m/s or something like 23 parsecs/minute. The way it had been stated was that the speed was a definition. In any case, it is much better now. - Taxman 19:27, Aug 17, 2004 (UTC)
            • this can be simply explained. I had tried changing it to "the speed of light in a vacuum, in terms of meters per second, is a definition." But I think the truth is actually stronger than that. Also I'm concerned with - "The constant speed of light derives from the Lorentz transforms" - that makes it sound like the speed of light is constant because of the Lorentz transforms. I agree. This is poorly worded. anthony (see warning) 13:52, 17 Aug 2004 (UTC)
            • Reworded to: "The speed of light in a vacuum (denoted as c, reputedly from the Latin celeritas, "speed") is exactly equal to 299,792,458 metres per second, which is approximately 300,000 kilometres per second, or 186,000 miles per second. This exact speed is a definition, not a measurement, as the metre is defined in terms of the speed of light and not vice versa." anthony (see warning) 14:03, 17 Aug 2004 (UTC)

Moving back to the left margin... At Speed of light#Overview there is a factual error. There is stated that «observers in all such frames must observe the same speed of light» because «the speed of light in a vacuum can be derived from Maxwell's equations» and «laws of physics [...] are identical in all unaccelerated frames». The later statement already was part of Galilean Relativity and, by itself, is consistent with the Galilean transformation wich would lead to a variable speed of light when applied to the Maxwell's equations. The speed of light beeing a constant is a postulate of Einstein's Relativity. I wanted to fix that but I am having trouble finding a good phrasing for it. Help is appreciated.--Nabla 16:30, 2004 Aug 17 (UTC)

I've reworded the first one to "Observers travelling at large velocities will find that distances and times are distorted ("dilated") in accordance with the Lorentz transforms; however, the transforms distort times and distances in the same way so the speed of light remains constant. A person travelling near the speed of light would also find that colours of lights ahead were blue shifted and those of those behind were red shifted."
The section that Nabla refers to was added to deal with an objection by Meelar (see above) to explain "why nothing can travel faster than light, and why it's a constant for all frames of reference". If you can do better, I'd be grateful. -- ALoan (Talk) 16:38, 17 Aug 2004 (UTC)
The causality argument covers all that just fine I think, ALoan. Fully support now in any case. - Taxman 19:27, Aug 17, 2004 (UTC)
  • Phew - I think all the objections have been cleared up, but if anyone is still disatisfied, please comment below. -- ALoan (Talk) 20:53, 17 Aug 2004 (UTC)