5 ms·

Except for the kilogram (which still is an actual sphere in a vault in Paris somewhere, but hopefully not for long) all base units are defined so that they don’

• gsteinb88 12y ago
  Fantastic video by Vertiasium on this: https://www.youtube.com/watch?v=ZMByI4s-D-Y https://www.youtube.com/watch?v=ZMByI4s-D-Y (side note: that video is about a sphere, but the current standard kilogram and its copies are actually shaped like little bells)
  • avoid3d 12y ago
    To clarify: that sphere is not a 'standard' in a sense that it is going to definite a kilogram. It is a tool which is going to be used to measure the correct numbers to define the kilogram in terms of universal constants. In this sense it is very different from the 'bells' you refer to, and I think the way you stated the link was misleading.
• masukomi 12y ago
  to @rtpg's point though. The kilogram (until it ceases being tied to that object) would weigh a less on the moon. You could still call that a "kilogram" but there'd be an easy argument that on the moon a kilogram would have to be much more to weigh the same.
  • joshyeager 12y ago
    The kilogram is a unit of mass, not weight. Mass is independent of gravity, so the standard is the same on earth, on the moon, and in null gravity. The weight of a kilogram would change depending on your gravity. That happens on earth too, just at a smaller scale.
• rtpg 12y ago
  I remember hearing about how 1L of water weighs 1kg or something like that and that that was one of the links between SI units, but the definitions seem to be different.
  • joshyeager 12y ago
    It was originally defined that way, but the definition was changed to use physical constants for this very reason.
• ubernostrum 12y ago
  They are defined that way now, yes. But those are retrofitted universal-constant definitions chosen for their equivalence to the arbitrary things-on-earth definitions originally used. There is nothing intuitive or natural or universal about 1/299792458, for example, to suggest it as a good basis for a unit of distance, other than "that's really close to the original arbitrary length of the meter".
  • arrrg 12y ago
    Why would that be something you want of a base unit?! You make no sense. That would be a property of a unit without a benefit. That would be intellectual masturbation, nothing actually useful, and certainly not a worthwhile criticism of the metric system … People who say this about the metric system should get off their imperial addiction already. Or die of old age eventually and not teach their kids this bullshit so that we can live in peace.
  • boomlinde 12y ago
    Yes, but that criticism was not the basis of the argument he responded to, which he perfectly addressed. Try to think of a way to define a unit of measurement that isn't either volatile or completely arbitrary. Besides, I think that the really useful part of the metric system is that the metric prefixes intuitively make sense when expressed in decimal, and it's clearly expressed from, say, 100143.796 meters, how many millimeters, centimeters, decimeters, meters and kilometers it is. You could say the same thing about inches, feet, yards and miles, but usually I see a combination of those units of varying ratios rather than kiloyards or megainches.
    • ubernostrum 12y ago
      And yet base 10 is pretty arbitrary. It's not just tied to human anatomy, it's tied to a particular non-universal way of interpreting human anatomy. Granted, I don't really care much one way or the other -- I think metric is more useful for scientific/mathematical/engineering/etc. applications and some of the old imperial units are more useful for things like cooking, and tend to recommend people use whatever's most practical for their task -- but it's hard to escape the impression that metric is an arbitrary system trying to retrofit claims of non-arbitrariness.