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NIST to redefine the kilogram based on a fundamental universal constant

washingtonpost.com

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Re: NIST to redefine the kilogram based on a fundamental universal constant

#31

Post by zorked »

ZeljkoS wrote: post #13

Interesting fact: this is important for US too, because pound is defined as exactly 0.45359237 kg ( https://en.wikipedia.org/wiki/Pound_(mass)#Current_use )

It's kind of not important because nobody measures the kinds of things that require the level of precision applied to the kilogram definition using pounds.

Re: NIST to redefine the kilogram based on a fundamental universal constant

#34

Post by ramshorns »

kiernanmcgowan wrote: post #11

The kilogram is not the only unit that will be redefined based on universal constants. The seven base units[0] will transition to being based on elementary charge and the Planck, Boltzmann, and Avogadro constants[1]. [0] https://en.wikipedia.org/wiki/SI_base_unit#Seven_SI_base_uni... [1] https://en.wikipedia.org/wiki/Proposed_redefinition_of_SI_ba...

Four of the seven physical constants used to define the seven base SI units will change, so a bunch of physical constants that currently have exactly defined values will start being subject to measurement uncertainty. So μ_0 will no longer be exactly 4π × 10^-7 H/m, k_C = 1/4πε_0 will no longer be exactly 8,987,551,787.3681764 N m^2/C^2, and 1 mol of carbon-12 will no longer have a mass of exactly 12 g.

Re: NIST to redefine the kilogram based on a fundamental universal constant

#35

Post by theonemind »

slim wrote: post #30

It does not make sense, practically. So they'll be using a balance with multiple moving parts made of multiple minerals that have to be precisely calibrated with margins of error adding up, instead of a simple platinum cylinder? Although, it makes sense politically

Defining your fundamental unit of mass in terms of a single arbitrary physical object costs a lot of theoretical purity, though it might not cause a lot of problems in practice. You mention a potential loss of precision. But we should gain accuracy by defining it in terms of fundamental constants. When the object itself changes, do we simply have a standard that drifts more than the physical constants of the universe? Do textbook publishers need to update all examples and problems using micrograms because the unit drift at that scale became significant? Should anyone without access to a reference object not get a real, accurate value for the kilogram? Do we keep using the mass value we know the reference object had, or does our kilogram really change with the object? Do we need to update any equations with a constant in them involving mass in any unit or derived unit in the equation once a year for high precision applications?

Re: NIST to redefine the kilogram based on a fundamental universal constant

#37

Post by mintplant »

kazinator wrote: post #36

Ha, the irony! The USA 's NIST defines a SI unit to the rest of the world; meanwhile, most of citizens don't know what it is.

I would wager most US citizens are at least passingly familiar with the kilogram. Metric units are used in science courses, after all.

Re: NIST to redefine the kilogram based on a fundamental universal constant

#38

Post by oh_sigh »

slim wrote: post #30

It does not make sense, practically. So they'll be using a balance with multiple moving parts made of multiple minerals that have to be precisely calibrated with margins of error adding up, instead of a simple platinum cylinder? Although, it makes sense politically

The problem with a physical standard like that is that you can't (easily) ship it to labs all over the world so that they can calibrate to it. At least when you use fundamental constants, each lab can set up their own equipment to produce the correct measurement.

Also, anything physical will shed atoms, which will affect the mass.

Re: NIST to redefine the kilogram based on a fundamental universal constant

#39

Post by madengr »

There is an alternate definition using a sphere of silicon with N atoms:

https://www.nist.gov/physical-measurement-laboratory/silicon...

What's really need though is a universal, stable over eons, single standard for time, length, and mass. I believe time is N cycles of an excited sodium (light) emission. Length is N wavelengths of that same emission in a vacuum. Mass would be N atoms.

So why are they not using a single element to define everything? Is it a matter of finding the proper element that is easy to excite and stable enough (chemically and atomically) over the long term? Sodium is very reactive and easy to excite. Silicon is probably the opposite.

Re: NIST to redefine the kilogram based on a fundamental universal constant

#40

Post by zimpenfish »

oh_sigh wrote: post #38
slim wrote: post #30

It does not make sense, practically. So they'll be using a balance with multiple moving parts made of multiple minerals that have to be precisely calibrated with margins of error adding up, instead of a simple platinum cylinder? Although, it makes sense politically

The problem with a physical standard like that is that you can't (easily) ship it to labs all over the world so that they can calibrate to it. At least when you use fundamental constants, each lab can set up their own equipment to produce the correct measurement. Also, anything physical will shed atoms, which will affect the mass.

Or, indeed, get coated in atmospheric gunk.

https://www.wired.com/2013/01/keeping-kilogram-constant/

> Cumpson suspects that because the kilos living in national labs have been retrieved and handled more frequently than the international kilo, more carbon-containing contaminants have built up on them over time.

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