SI units: because 9,192,631,770 is such a round number

December 6, 2009 at 10:38 pm 4 comments

Base units.
(Click for full 1024×768 image)

Note: The kilogram is technically defined only from the ridiculous lump of metal in a vault in France. The thing with water is the traditional, imprecise definition, but I included it for the sake of explanation (maybe the second’s definition as a fraction of a day should be in there, but ah well.) I used the American spelling of
“meter”, but the quotes (both from Wikipedia) use the official international spelling.

Interestingly, the main complaint with the SI system seems to be that “amount of substance” isn’t a proper physical quantity.
What’s the matter, guys? I thought you liked arbitrary big numbers?

Hmm. I should note, too, that it’s still a far better system than anything else out there, before the powers-that-be force me to use Furlong/Firkin/Fortnight instead.

I want a t-shirt with “Fuck the International Prototype Kilogram”.


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4 Comments Add your own

  • 1. Jake Barnes  |  December 7, 2009 at 12:56 am

    The elegance and simplicity is in using the units, not the definitions, but I’m sure you know that. All definitions will be arbitrary, really.

    The definition of the ampere is pretty damn fucked up, I’d like to know how you could do that practically.

  • 2. Daedalus  |  December 7, 2009 at 4:17 pm

    It’s helpful to separate the actual SI definitions from the very useful SI system as a concept: exact, physical definitions together with derived units and decimal prefixes.

    My main gripe is with the number of digits in the arbitrary numbers; it would have been very easy to tweak the meter, and make c = 300,000km/s exactly. Of course, there’s the issue of backwards-compatibility… it would be pretty damn annoying to get an email from Switzerland asking you to divide all your measurements by a factor of 1.00007…

    Most of the EM-related units have the simplest and best definitions, but they sort of outsource the awkwardness to the ampere (as you mentioned). I might define the coulomb from the electron’s charge, and the ampere from the coulomb… if they don’t mind using cesium as a reference, electrons should be fine.

  • 3. dubistkomisch  |  December 7, 2009 at 7:40 pm

    Yeah, I agree completely. It’s always hard or near impossible to change something after it’s been around for so long and as widely used as SI. The best they can do is to have precise definitions for the sake of accuracy and consistency.

    Defining the coulomb like that would make much more sense, but I guess there’s no real difference once you’re actually using the units. And after all, natural units are the only sensible ones. :D

  • 4. Daedalus  |  December 7, 2009 at 9:10 pm

    Oh, of course. And actually, it wouldn’t require huge numbers as long as we used the Planck hectogigayottalength and the Planck decaexayottatime.


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