Physical kilogram is officially dead • Hacker News Board

Re: Physical kilogram is officially dead

#11

Post by pkulak » Tue, May 21, 2019, 10:44 PM UTC

> "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?

Re: Physical kilogram is officially dead

#12

Post by gmueckl » Tue, May 21, 2019, 10:48 PM UTC

mehrdadn wrote: post #5

tomrod wrote: post #2

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? :-)

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.

Re: Physical kilogram is officially dead

#13

Post by gmueckl » Tue, May 21, 2019, 10:54 PM UTC

pkulak wrote: post #11

> "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 hav…

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.

Re: Physical kilogram is officially dead

#14

Post by falcrist » Tue, May 21, 2019, 11:38 PM UTC

MikeTaylor wrote: post #6

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!