this post was submitted on 29 Sep 2024
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Mildly Interesting

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A tagliatelle made from gold that defines the official size of a tagliatelle, at the Palazzo della Mercanzia in Bologna (sh.itjust.works)
submitted 1 month ago by [email protected] to c/[email protected]
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Image source: https://facciamoungiroincentro.blogspot.com/2016/07/palazzo-della-mercanzia.html

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[–] [email protected] 54 points 1 month ago* (last edited 1 month ago) (3 children)

Right? Didn't they define the kilogram, make identical copies of the standard, sent them to different countries, then after years, reunited them and found they all diverged in mass?

And now they have made a perfect silicon sphere with the same mass as the standard kilogram, then counted all the atoms. So now we know the exact mass in silicon atoms of a kilo.

Let's just define tagliatelle in light nanoseconds and be done with it.

[–] [email protected] 19 points 1 month ago (1 children)

Since 2019, the kg is just defined in terms of the Plank constant and some math with the resonant frequency of cesium as well as the speed of light. There was too much variability in anything physical so they decided to just fix some constants at whatever value they were close to.

https://en.wikipedia.org/wiki/2019_revision_of_the_SI

[–] [email protected] 7 points 1 month ago (1 children)

It's part of the attempt to more accurately define Avogadro's number and the kilo.

https://www.nist.gov/si-redefinition/kilogram-silicon-spheres-and-international-avogadro-project

[–] [email protected] 5 points 1 month ago* (last edited 1 month ago)

The redefinition of the mole in 2019, as being the amount of substance containing exactly 6.02214076×10^23 particles

Since the 2019 SI redefinition, avogadro’s number is a constant.

https://en.wikipedia.org/wiki/Avogadro_constant

Edit: looks like we were both right! I was reading through your link and it seems the work reported by the NIST led to the exact definitions for Avogadro’s number and the Planck constant.

[–] [email protected] 5 points 1 month ago (2 children)

They counted the atoms?
Didn't they just took the mol mass and calculated it? (Not sure if mol mass is the right term... School chemistry is a long time ago...)
And I don't see how we even should be able to count them.
Would be really interested, if it happened that way, how they did it.

[–] [email protected] 2 points 1 month ago

They gave up on that plan. Defining Plank's constant happened first. It could still be done as a secondary confirmation, but it's less of a race now to get away from K