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 2 months ago by [email protected] to c/mildlyinteresting
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Image source: https://facciamoungiroincentro.blogspot.com/2016/07/palazzo-della-mercanzia.html

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[–] subtext 19 points 2 months 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

[–] NegativeInf 7 points 2 months 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

[–] subtext 5 points 2 months ago* (last edited 2 months 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.