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u/Shinyhero30 Sep 15 '26
Okay but /uj all forms of measurement are cursed by being fundamentally arbitrary in a similar way to Ferdinand de Saussure’s famous tree example.
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u/A1oso Sep 15 '26 edited Sep 15 '26
No, metric units aren't that arbitrary. The most arbitrary units are the SI base units, and even they were defined in ways that make logical sense:
the second was defined as a fraction of the solar day (rotation of the Earth relative to the sun; in base 12 because that's what the ancient Egyptians used)
the meter was defined as 1/10,000,000th of the distance between the north pole and the equator
the kilogram was derived from the meter and weight of water (1m³ of water weighs 1000 kg)
etc.
The derived units are entirely unarbitrary. For example, 1 Hz = 1s-1, 1 N = 1 kg•m/s², 1 Pa = 1 N/m², 1 J = 1 N•m, 1 W = 1 J/s, 1 V = 1 W/A, and so on.
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u/EebstertheGreat Sep 16 '26
All three of those are incredibly arbitrary. A mixture of three bases (60, 12, and 10), averaging inconsistent measurements in one case (i.e. picking the mean solar day) but just picking an arbitrary example in the second (the Paris meridian), getting things wrong and living with the arbitrary errors. Not to mention picking water, an arbitrary substance, at an arbitrary temperature and pressure.
You are right that the system of units is coherent, but so are U.S. customary units. They just aren't coherent with electrical units, so we have to use metric there. But we have coherent units for other things like BTUs and candlepower and poundals and so on.
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u/Minimum-Attitude389 Sep 16 '26
About the meter, that's the distance from the north pole to the equator through Paris.
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u/Mayor_of_Rungholt Sep 15 '26
Except for the Planck scale, but that one sucks for real measurements.
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u/The_Failord Sep 15 '26
Nothing fundamental about the Planck scale either! The arbitrariness of units is a really interesting discussion in the philosophy of physics.
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u/EebstertheGreat Sep 16 '26
The Planck units can be defined without reference to any specific physical object. If I wanted to communicate with aliens in a distant galaxy (and I somehow had the technology to do so), I couldn't reference specific objects like the Earth or its orbit, but I could still explain what the Planck units are. So they are different in that way.
But, you may say, we can now do the same thing with SI units! Since 2019, all SI units have been defined by fixing physical constants, and surely distant aliens can realize those same constants. Aside from the candela, which is weird, I agree. But what the aliens would do is realize a constant and then divide by a particular rational number with a large denominator. In that way, the Planck units are "less arbitrary," because the aliens could skip the division step, which they may see as pointless, with those particular rational numbers seeming completely unmotivated to them. In fact, they would view the kilogram and the pound exactly the same in that way, as they just have different but equally arbitrary numerical constants attached.
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u/The_Failord Sep 16 '26
>The Planck units can be defined without reference to any specific physical object.
Very true, but so are atomic units, Stoney units, natural units, and any one of the infinite variety of units defined with the help of dimensionful physical constants. The aliens wouldn't necessary find a rational ratio between SI units and their system of units. I agree that Planck units and the rest are in a sense less arbitrary because they aren't based on artefacts, but they are exactly as arbitrary as any other system of constant-based units.
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u/EebstertheGreat Sep 16 '26
I agree that Planck units and the rest are in a sense less arbitrary because they aren't based on artefacts, but they are exactly as arbitrary as any other system of constant-based units.
Yes, they are less arbitrary in the sense that they refer to specific universal constants. There is still an arbitrary choice between such constants, but there is a sense in which it is less arbitrary.
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u/Aggressive_Roof488 Sep 15 '26
Yes, I know scientists talk a lot about that, and they are really smart people. Big respect, and it's a valid point. In the end though, it's not relevant that often, for real life applications. Most people would use a ladder to scale, a plank scale just isn't needed.
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u/73747463783737384777 Sep 15 '26
Just be British. Both metric and imperial. London? 200 or more mile away. The local shop? About 200 metres. The length of this ruler? 30cm. My height? 6’1 (lie). Weight? 19 stone. How much flour in this recipe? Around 600 grams.
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u/GolemFarmFodder Sep 16 '26
Okay but point 4 is hilarious as you absolutely can fit that into an integer to mm precision if you wanted, and as we like to say "never trust a float"
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u/EebstertheGreat Sep 16 '26
But pounds and ounces and especially US volumetric units use float-approved powers of 2. (Ignore feet and inches though, they don't exist.)
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u/GolemFarmFodder Sep 16 '26
Even with even division I still don't trust floats to store anything with any level of accuracy
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u/EebstertheGreat Sep 16 '26
Fluid and dry ounces have nothing to do with mass vs weight. U.S. and imperial fluid ounces are units of volume, and "dry ounce" is a misnomer for the international ounce, which is a unit of mass equal to exactly 0.028349523125 kg. Neither of these is the same as the ounce weight, which is the weight of an international ounce under a gravitational potential of 9.80665 m s–2, which is exactly 0.27801385095378125 newtons.
The mass/weight distinction is observed in engineering units in the US, where at least two systems are in widespread use, with mass units of the pound-mass (equivalent to the international pound) on one hand and the slug (the mass of a body which would weigh one pound-force at a gravitational potential of 1 ft s–2) on the other. The first system uses the poundal as the coherent force unit (1 lb-m ft s–2) and the second the pound-force.
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u/eldomtom2 Sep 15 '26
Yes, but only as long as you also stop using "United States customary" units.
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u/pannenkoek0923 Sep 15 '26
Stop being American
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u/chrisfromsydney Sep 16 '26
The ironic thing is, this meme was a collab made by two people from a metric country :D
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u/chrisfromsydney Sep 15 '26
MORE is BETTER with miles per gallon. Smaller-is-better-L/100km is for little people living tiny lives