Talk:Metre

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From a previous revision:

The speed of light is believed to be constant everywhere, and thus a definition based on light is easier to maintain and more consistent than a measurement based on the circumference of the Earth or the length of a metal bar.

Good concept, but I can't see how to integrate it nicely in the History section. Anybody?

As far as I know (and forgive me if I'm wrong) but the speed of light is not constant everywhere. In Physics, (and I'm quoting from my Year 11 Physics text book, 'Physics Impact'), it states "When light travels fom one medium to another it may change its speed... [it] causes the change in the direction of the light ray". So you should refer specifically to the 'speed of light in a vacuum' or 'speed of light in air'. Don't assume it's always the same.

You are right -- speed of light in a vacuum is constant, but speed of light can vary from medium to medium. The article makes clear that it is talking about speed of light in a vacuum -- the above quote is from an earlier revision (I think). -- SJK

Perhaps a more precise refinement of this statement would be to say that the speed of light in a vacuum in the absense of external gravitational forces is believed to be a universal constant. Since it's the whole basis for a metre, it seems relevant to include in the first paragraph of the main article when the metre is being defined? --Abqwildcat 22:18, 23 Jul 2004 (UTC)

Thought about it a bit, and decided it's not key to include in the main definition at the top of the article, but some mention would seem important. Gravity affects the path and depending upon the theoretical framework in mind MAY affect the speed of the light. The fact that this constant may not be constant and thus our metre may not be constant seems important. --Abqwildcat 22:22, 23 Jul 2004 (UTC)

We should retain the current definition because a vacuum is a space entirely devoid of matter, so there are no gravitational forces. While it may be the case that this is an impossibility in the Real universe, it is the case in the theoretical, closed system of absolute vacuum to which the definition refers. The appropriate modification in this case may be to change "vacuum" to "absolute vacuum". Also, this is the definition established by the authoritative International Bureau of Weights and Measures and used worldwide for all purposes.

I'm not especially knowledgeable in physics but, as I understand it, gravity wouldn't change the scalar speed of light so much as it would change the relation between the local time and the reference time. That is, while light may seem to be going slower when measuring it from the reference frame, in the local frame it would be going at the same universal constant speed in terms of distance and duration. - Centrx 23:32, 23 Jul 2004 (UTC)

Exactly. After looking around a bit, I came to the same conclusion. It may warrant mentioning in the article, but the as I can tell - only the local definition of time would be changed. Absolutely correct. That also changes lengths, but I'm perfectly happy with changing vacuum to absolute vacuum. --Abqwildcat 23:37, 23 Jul 2004 (UTC)

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Perhaps we should have a page dedicated to the history of the metric system (itself rather fascinating). From there, one might explain how and why the different measurements were chosen. --Fleeb

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Article said:

Since the speed of light is believed to be constant everywhere, a definition based on light is easier to maintain and more consistent than a measurement based on the circumference of the Earth or the length of a specific metal bar. Thus, should either object be destroyed or lost, the standard meter can still be easily recreated in any laboratory.

LOL! Destorying or losing the circumfrence of the Earth! (Made it less humorous now.) -- SJK

drat, I shouldn't have drawn attention to the article again with that little edit. I was wondering how long that joke would last in there. :) -BD