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Comment by dhimes | Hacker News Reader

The second is defined as the time it takes for two hyperfine levels of the ground state of the 133 Ce atom to make 9,192,631,770 transitions. How is that not special?

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Because that definition was grandfathered in such that it matched the previous definition of a second up to the previously specified precision. You've just moved the arbitrary part to the number 9,192,631,770. The same thing was done for the meter (defined such that the speed of light is precisely 299,792,458 m/s), ampere (current which produces 2 × 10−7 N/m of force between two parallel conductors). There's plans to do the same thing for the kilogram using Plank's constant, but for now we're stuck with the prototype kilogram whose mass unfortunately changes more than we'd like.

I could see making a case that Planck time is special enough for 440 vs 432 to matter, but it is so far off the scale of human hearing that its even integer points would form an effectively continuous scale when it comes to audio.

I suspect the poster's tongue was firmly in his or her cheek.

Indeed. But damn, this is a tough crowd :)

I LOLed

> current which produces 2 × 10−7 N/m of force between two parallel conductors

Hate to be a stickler, but: "...conductors of negligible cross-section and of infinite length." It's still one of my least favorite SI definitions.

If anybody wants to read lots of commentary on units and their definitions, take a look at the data sheets for the Frink language. The ampere and the candela are two of the more irksome ones, as described by the author.

https://frinklang.org/frinkdata/units.txt

Fascinating. Thanks! At one point I wrote a unit conversion program before such things were ubiquitous and I included a field that had information about the units involved. In service of this, I copied a number of older books on measurement units though I never got around to folding most of this into my program.

Couldn't the Kilogram be derived from the Liter, which in turn can be derived from the Meter? From what I recall:

- 1 Liter of pure water at 4 Celsius is 1 Kg.

- 1 Liter is a cube with 10 cm on each side.

I always thought it was amusing that the basic definition of the liter was taken at 4C, but that lab equipment was calibrated at 20C...

I think that's because 4C is the point where water is the densest, whereas 20C is a comfortable room temperature.

Interestingly enough it started out as being defined as 1 dm³ in the 18th century, then in the early 20th century was defined through water at highest density, then in the 60s back to 1 dm³.

Generating a perfectly homogenous 4C over an environment large enough to contain a kilogram of any useful substance is going to be incredibly difficult and mearusing it even moreso. There's a reason every unit of measurement aside from mass is derived from an electrical property; we can measure electrical properties down to ridiculous precision, measurements small enough that all measurement uncertainty is removed.

That is good to know.

Werent they trying to count the atoms in a perfect sphere of silicon?

You could do that, but if you're trying to set standards, you have to take into account your ability to measure these things reliably and accurately. (It doesn't do any good to set standards that nobody can conform to because their equipment is never sensitive enough to use them.) Can you measure a liter more accurately than you can measure a kilogram of some reference material?

Imagine if it had been defined as the time it takes to make 9,192,631,771 transitions.

All our music would sound wrong.

It's less special and more pretty much arbitrary. It's a slightly fudged mean[0] of the number of transitions matching the ephemeral second according to Markowitz, Hall, Essen and Parry ("Frequency of Cesium in terms of ephemeris time", 1957), with the original paper adopting "a probable error of ±20".

[0] of 4 measurements (761, 767, 771, 780) — the exact mean being 769.75

It's not a power of 2 or 10 :-)

9,192,631,770 is divisible by 441, why are we not using 441Hz?!

I thought the definition of one second had been updated to the time it takes light to travel c meters? Although when I think about it, it does seem very circular.

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