
Physical kilogram is officially dead - orpheum
https://www.nature.com/articles/d41586-019-01614-8
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sctb
Previous discussion:
[https://news.ycombinator.com/item?id=19936454](https://news.ycombinator.com/item?id=19936454)

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MikeTaylor
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.

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falcrist
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!

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tomrod
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](https://en.wikipedia.org/wiki/Uniformitarianism)

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

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gmueckl
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.

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gmueckl
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.

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

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rawrmaan
> "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.

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pkulak
> "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?

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gmueckl
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.

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

