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A stupid question. I see metre is officially defined based on the speed of light:

The meter is the length of the path travelled by light in vacuum during a time interval of 1 / 299 792 458 of a second.

My question is why don't we update it with a sharp number. Why couldn't we first define c=100,000,000 m/s, then define metre as a length

... length of the path travelled by light in vacuum during a time interval of 1 / 100 000 000 of a second.

Wouldn't that be better? Is that just too troublesome to change or is there any other reason I neglect? Thanks

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closed as off-topic by ACuriousMind, Martin, JamalS, David Z Mar 18 '15 at 11:52

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    $\begingroup$ Well, your definition would make all existing rulers and measurements useless, and just create confusion when reading stuff written before the change, I guess. $\endgroup$ – ACuriousMind Mar 18 '15 at 2:54
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    $\begingroup$ I'm voting to close this question as off-topic because questions about the reason for notation/terminology are off-topic. $\endgroup$ – ACuriousMind Mar 18 '15 at 2:55
  • $\begingroup$ OP's proposed length unit is already known as 10 nanolightseconds. $\endgroup$ – Qmechanic Mar 18 '15 at 9:16
  • $\begingroup$ If it's off topic here, perhaps we should migrate it to History of Science and Mathematics $\endgroup$ – David Z Mar 18 '15 at 9:25
  • $\begingroup$ Just a note from HSM: We rejected the migration (with one dissenter, so it was not unanimous) because the question isn't about the history behind the definition. It's more about the concept itself. $\endgroup$ – HDE 226868 Mar 21 '15 at 13:09
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You could do that, but if you did, you'd best not call the new unit "meter" in order to avoid confusion. That is why we don't define the meter as the length traveled during 1/300 000 000 th of a second, for instance: such a definition would cause confusion with older precision measurements that used previous definitions of the meter.

You can only meaningfully "redefine" a unit as long as your new definition is still consistent with every measurement that's been done with past definitions. That's why we feel free to define the meter such that the speed of light in m/s is an integer, but we don't feel free to define the meter such that the speed of light is 300 000 000 m/s. If we did define a length unit that way, we'd best call it something else.

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