# Meters

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Before 1983, the meter was a different size than the current meter. I read that the current meter is based on the distance time can travel in 1/299,792,458th of a second. Where did anybody come up with that number? Now we have to say that light travels at 299,792,458 meters per second. Wouldn't it make it easier on scientists if 1/300,000,000th of a second was used? That's just my opinion. Does anyone know why 299,792,458 was used?

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....

the length of a meter has not changed...

light travels approximatly 299,793,458 meters per second therefore thedistance light travels in 1/299,793,458 sec is one meter...

we don't use 300,000,000 because 299,793,458 is more accurate....

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We learned in Chemistry class that it has changed. The original meter was ten-millionth of the distance from the North Pole to the equator. Then they changed it to the wavelength of light emitted by an isotope of a Krypton. Then in 1983, it was changed to the distance light travels in 1/299,792,458th of a second.

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Never heard of either of those changes. But I don't know my science history very well.

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Presumably, we knew how big the world is before we knew how fast light is. Any subsequent adjustment to the defined length of a meter was therefore probably considered an increase in accuracy with a more universally generalizable unit of measure?

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Before 1983, the meter was a different size than the current meter. I read that the current meter is based on the distance time can travel in 1/299,792,458th of a second. Where did anybody come up with that number? Now we have to say that light travels at 299,792,458 meters per second. Wouldn't it make it easier on scientists if 1/300,000,000th of a second was used? That's just my opinion. Does anyone know why 299,792,458 was used?

Because that represents the speed of light. Once the decision was made to use c as the reference instead of a bar of iridium-platinum, they defined in terms of the best measurement of c, and then defined the value.

It is true that the oringinal meter was defined to make the distance from equator to pole equal to 10,000 km. But they incorrectly accounted for the oblateness of the earth, so that ended up being incorrect. The wavelength, and then c definitions were adopted because the meter could be determined more accurately and precisely, just as the definition of the second was shifted to an atomic definition from being some fraction of the earth's rotation under some set of conditions. You also have the advantage that anybody with sufficient technology can realize the standard themselves, without having to go to Paris to compare with the world standard, or have the BIPM manufacture a new one for you.

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Because that represents the speed of light.

How can you say that? Speed is how far something can travel in a given amount of time. The reason light travels 299,792,458 meters per second is because of the size of a meter. If a meter was only slightly smaller, the speed of light would change to 300,000,000 meters per second.

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The metre was defined before speed of light was measured. It wouldn't be re-defined only so that speed of light becomes exactly equalt to 300 00 00 00 m/s.

Only the defination of the metre was changed, prior to this definition it was defined with respect to a standard Platinum-Iridium rod kept at 273 K somewhere in France. The new definition ensures verifiability world-wide and is in some sense more general.

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How can you say that? Speed is how far something can travel in a given amount of time. The reason light travels 299,792,458 meters per second is because of the size of a meter. If a meter was only slightly smaller, the speed of light would change to 300,000,000 meters per second.

It's quite easy to say that. As pulkit said, we had already defined the meter. There is a tremendous amount of politics involved in defining standards, in part because there is a tremendous amount of commerce involved in standards.

Changing a standard involves retooling machinery and recertifying traceable standards. To suggest a change for a trivial reason such as making c a round number would get you laughed out of the room of whatever BIPM committee is involved. When the meter definition was changed, the length didn't. New odometers weren't required in cars. The cm3 still represented a gram of water at a given T and P. Just the official (and best) way of defining the length was changed. Same thing with the second's definition. We didn't all have to go out and buy new clocks (unless you worked in a national standards lab, perhaps) when the definition was changed to an atomic standard.

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Ok, I understand. Thank you for the replies.

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