"In the SI, the standard metre is defined as exactly 1/299,792,458 of the distance that light travels in a second."
"The kilogram was originally defined as the mass of one cubic decimetre of water at 4 °C, standardized as the mass of a man-made artefact of platinum-iridium held in a laboratory in France, which was used until a new definition was introduced in May 2019. Replicas made in 1879 at the time of the artefact's fabrication and distributed to signatories of the Metre Convention serve as de facto standards of mass in those countries. Additional replicas have been fabricated since as additional countries have joined the convention. The replicas were subject to periodic validation by comparison to the original, called the IPK. It became apparent that either the IPK or the replicas or both were deteriorating, and are no longer comparable: they had diverged by 50 μg since fabrication, so figuratively, the accuracy of the kilogram was no better than 5 parts in a hundred million or a proportion of 5x10−8:1. The accepted redefinition of SI base units replaced the IPK with an exact definition of the Planck constant, which defines the kilogram in terms of the second and metre."
There was an issue with the standard for the Kilogram, which was recognised then corrected by introducing a definition which is based on physically measurable phenomenon. The new approach allows independent experiments to derive the value of the Kilogram.
That seems to me like a process that works, and I struggle to think of a better outcome.
That the metric system is just as arbitrary as the customary units. Things like the meter and (kilo)gram were based on arbitrary objects rather than anything objective. They've since been redefined using physical constants to come close enough to the old reference objects. (And the US customary units are officially defined as exact fractions from the SI units -- making the whole world happy to have exact measurements regardless of the system you use.)
At least the customary units have nice divisors. Just saying.
There was some debate at the time about using a pendulum with a 1s period to define the metre, but it was known to be less accurately reproducible than the geodetic version.
Their aim was to define a system that could in principle be recreated from scratch anywhere on Earth without necessarily relying on access to the physical prototypes. In practice the relative imprecision of the original measurements compared to the later demands of science and engineering meant that the prototypes became the definition.
It’s interesting to compare with the British Imperial units reform in the 1820s. Part of that act specifies how to recreate the prototypes from physical properties of the units. When the Palace of Westminster burned down in the 1830s the ancient prototypes were destroyed. But the replacements were recreated from copies, not from the specification in the act. And not much later, the official definition was based on the metric system because the metric standards were manufactured to much greater quality.
No-one claims it isn't arbitrary. Any choice of units is arbitrary. "Not being arbitrary" just isn't a desideratum when considering what makes a good system of units.
The main desirable property is coherence [0], which SI has.
But I don't think anyone would claim SI is perfect. It's not; it's got at least a couple of warts. One is that "kg" is a base unit and is prefixed. But that's about the worst of it.
CGS is also coherent, but has a similar problem with a prefixed base unit (in this case, cm).
US Customary units is not only incoherent, but does things like conflate mass and force (lb is a technically a measure of force, so is the long/short ton, and slug is the proper unit for mass) and has far too many derived units that measure the same thing (e.g. ft lb/s, hp and BTU/h, vs just W).
[0]: https://en.wikipedia.org/wiki/Coherence_(units_of_measuremen...
So let's call it 150 million oscillations, and refer to it simply as "time", since the word second only makes sense in context of an analog clock anyways. Then we can start rounding the other weights and measures accordingly until everything is nice and clean. It appeals to the OCD in me, despite the societal chaos that would ensure.
But then the universe will undoubtedly throw random numbers at us like π or the fine-structure constant and mess it all up.
But at least we still have horsepower! My PSU is a 3/4-horse unit. My toaster oven is a full horsepower.
I like it.
My 70kWh battery holds only 95 horsepower-hours. But I use the British Thermal Unit when talking about energy.
My BEV has 240 kilo-BTUs stored in it.
Decimal is a system best suited for written math.
You can count to 12 that way, too, e.g. by folding the thumb first to indicate 1, and then unfolding it with the rest of the fingers folded when you get to 6. Or you can turn the fist upside down etc. Again, there are many techniques, but counting by folding or unfolding fingers is the simplest and the most obvious, which is why base-10 is the most common.
But the good craft beer places have a sign saying what their pints are in millilitres to prevent unpleasant surprises when you were expecting 568mL but got 425mL.
* height is normally distributed for men.
* The average height of a man is 5'9".
* The standard deviation for men's height is 3".
So six foot is one standard deviation above average. I am sure one or more of the above does not hold for NZ. I just think this is neat, that the commonly stated preference happens to be for one standard deviation above the average.
They'll probably tell you if they know it's not a true pint, but I expect most bartenders have no idea.
The German system where there's a line on the glass that needs to be reached by beer under penalty of law puts a smile on my face.
Some US bars will serve imperial pints on request and/or offer them for British or Irish beers.
Typically in a British pub you will be served beer in a glass that is calibrated as “pint to brim”, and any head on the beer is taken out of the nominal pint. So usually a pint as served is much closer to half a litre.
Except nobody remembers what a 'pottle' is.
The problem is getting that design adopted by everyone.
When we actually have this well designed system where everything fits beautifully together...