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Comment by rtpg | Hacker News Reader

Slightly off topic, but I have a coworker who is not as much of a fan of the metric system as others I know.

His main issue was that the metric system is based off of things heavily linked to our planet (like involving water at atmospheric pressure). How hard would it be to reproduce the metric measurements on the moon, for example?

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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’t require the context of Earth or a specific object or anything like that.

A meter, for example, is defined as “the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second”.

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.

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.

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.

It was originally defined that way, but the definition was changed to use physical constants for this very reason.

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".

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.

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.

> And yet base 10 is pretty arbitrary.

I think that you are missing my point. I'm saying that not being arbitrary is too much to ask for from a practically useful system of measure, but what the metric system has going for it is that it is at least consistent with itself.

> but it's hard to escape the impression that metric is an arbitrary system trying to retrofit claims of non-arbitrariness.

I'd like to see any of these claims of non-arbitrariness. I have managed to escape them. The point of redefining the unit in terms of some universal constant is not to avoid arbitrariness but to avoid the inherent volatility of defining it in terms of something which is not constant and not easily reproducible outside earth. No one is arguing that 1/29979245 isn't a completely arbitrary number.

Fantastic video by Vertiasium on this: 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)

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.

Interestingly, the Fahrenheit temperature scale was set up because it's easy to calibrate independently of the boiling/freezing points of things at specific atmospheric pressure. A 1:1:1 mixture of ice, water and ammonium chloride will self-stabilize at a consistent temperature; that temperature is the zero point of the Fahrenheit scale. Fahrenheit also wanted the freezing point of water at 32 and human body temperature at 96 for easy marking on a thermometer (64 degrees between the two, easy to mark off halves and get the full scale).

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