Physical kilogram is officially dead | Hacker News Reader

Physical kilogram is officially dead

nature.com

48 pointsorpheum13 comments

Previous discussion: https://news.ycombinator.com/item?id=19936454

This is obviously the right thing.

Still, I can't help lamenting it. There was something delightfully baroque about knowing that the kilogram was in a vault in France. I'm going to miss it.

It's also a little surprising when you think about it. Look how far science has come despite a very shaky foundation in the system of measurements.

I'm sure most people know, but the mass of the official kilo actually drifted by tens of micrograms since it was made. When you're talking about very large weights, that can be an enormous difference!

May our fundamental constants maintain uniformity! [And they should, I hope, and this change is a _good_ thing!]

[0] https://en.wikipedia.org/wiki/Uniformitarianism

I hope so too, but I think the kilogram is just a base unit, not a fundamental constant? :-)

Now it is.

It is defined in terms of the Planck constant. Now if that constant is not actually constant over time or space, the definition would be in trouble.

For example, it is unclear whether the Hubble constant is, in fact, constant over time. Different measurements give different values for different time periods in the past and these have yet to be reconciled.

This is huge, but subtle. For the first time, the SI units are no longer relying on unit prototypes that can only be copied. Given the new definition of the SI units, decent enough lab equipment and enough time, any lab can now theoretically manufacture its own references to arbitrary precision, no matter where they are in the known universe. There is no longer an insurmountable limited incurred by the precision of the local copies of these references.

Who gets it, I wonder? The actual old physical kilogram. What are the plans for it?

> "it also means that scientists can work at the highest level of precisionon any scale, without losing accuracy"

Still waiting on scientific journalists to reach the same precisionon.

> "The change also means that the constants on which the new definitions rely — the charge of the electron for the ampere, Avogadro’s constant for the mole, Boltzmann’s constant for the kelvin and Planck’s constant for the kilogram — are from today fixed values with zero uncertainty."

Does anyone know what that last paragraph means? Why would defining new constants in terms of existing ones make the existing ones have zero uncertainty?

The constants on which the SI units rely have been assigned fixed numerical values by definition. This is so that you can now build a certain type of experiment and calibrate it so that it measures this defined numerical value for that constant. And in doing the calibration, you actually produce a reference for an SI unit.

Good. Finally something that doesn’t depend on some weird artifact lying around somewhere losing mass mysteriously

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