Platinum Cylinder to be Retired

[Recusant]

For a very long time, the kilogram has been defined by a cylinder of platinum alloy kept in pristine condition under a triple bell jar in France. It's the last of the physical standards used in the metric system--for instance the mètre des Archives (a physical bar of platinum alloy) was retired as long ago as 1960. Now even the kilogram will be defined by a scientific constant, relegating the platinum cylinder to history.

"Kilogram, ampere, kelvin and mole redefined" | ScienceDaily

The International Prototype of the Kilogram (IPK) is held in an underground vault at the International Bureau of Weights and Measures (BIPM), located in Sèvres, near Paris. Following the decision to redefine the SI kilogram starting 20th May 2019, the definition of the kilogram will no longer be based on the physical artefact called the International Prototype but rather on a constant of physics, as are the other seven base units of the SI. The redefinition of the kilogram marks the end of artefacts used to define our measurement units.
Image Credit: © BIPM

Today, in a landmark decision, representatives from 60 countries voted to redefine the International System of Units (SI), changing the world's definition of the kilogram, the ampere, the kelvin and the mole, forever.

The decision, made at the General Conference on Weights and Measures in Versailles, France, which is organised by the International Bureau of Weights and Measures (BIPM), means that all SI units will now be defined in terms of constants that describe the natural world. This will assure the future stability of the SI and open the opportunity for the use of new technologies, including quantum technologies, to implement the definitions.

The changes, which will come into force on 20 May 2019, will bring an end to the use of physical objects to define measurement units.

The definition of the kilogram for more than 130 years, the International Prototype of the Kilogram (IPK), a cylinder of a platinum alloy stored at the BIPM in France, will now be retired. It will be replaced by the Planck constant -- the fundamental constant of quantum physics. While the stability of the IPK could only be confirmed by comparisons with identical copies, a difficult and potentially inaccurate process, the Planck constant is ready for use everywhere and always.

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[Ecurb Noselrub]

I've read several articles and do not understand how you get to a kilo by Planck's constant.  I think there is a formula in which Planck's constant is an element, but I don't understand how, given Planck, you can get a kilo.  Anyone have any insight?

[jumbojak]

I've read several articles and do not understand how you get to a kilo by Planck's constant.  I think there is a formula in which Planck's constant is an element, but I don't understand how, given Planck, you can get a kilo.  Anyone have any insight?

Think about it, it's a planck. The only more appropriate use would be as a basis for length.

[Dark Lightning]

The current mass standard is changing mass due to nuclear decay, since even platinum has radioactive isotopes. I'm interested myself about the use of the Planck constant. I got my BSc degree in physics many years (35) ago. I'll go look it up and see if there is a layman's explanation. A lot of what I did is so mathematical that only brilliant people can make them understandable to others...and I ain't brilliant.

[hermes2015]

It comes from the unit of the Planck constant (kg⋅m2⋅s−1), since the meter and second have been defined before. This quote is from an article I found (https://www.vox.com/science-and-health/2018/11/14/18072368/kilogram-kibble-redefine-weight-science):

The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be 6.626 070 15 × 10-34 when expressed in the unit J s, which is equal to kg m2 s -1 , where the meter and the second are defined in terms of c and ∆νCs.

[hermes2015]

[Tank]

No coffee yet. Will read later.

[Tank]

The current mass standard is changing mass due to nuclear decay, since even platinum has radioactive isotopes. I'm interested myself about the use of the Planck constant. I got my BSc degree in physics many years (35) ago. I'll go look it up and see if there is a layman's explanation. A lot of what I did is so mathematical that only brilliant people can make them understandable to others...and I ain't brilliant.

I knew it was getting lighter, but not that that was the cause.

[Ecurb Noselrub]

It comes from the unit of the Planck constant (kg⋅m2⋅s−1), since the meter and second have been defined before. This quote is from an article I found (https://www.vox.com/science-and-health/2018/11/14/18072368/kilogram-kibble-redefine-weight-science):

The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be 6.626 070 15 × 10-34 when expressed in the unit J s, which is equal to kg m2 s -1 , where the meter and the second are defined in terms of c and ∆νCs.

That's a bit better than what I found. I'll just go with that. Thanks.

[Dark Lightning]

Beat me to it. Nice explanation!

[Caliasseia]

A full exposition of the reasoning (and an elegant one at that) can be found here.

[Tank]

A full exposition of the reasoning (and an elegant one at that) can be found here.

A Kibble balance sounds like a dog owner would need

[Sandra Craft]

Man, I just saw the picture of it and thought "that's so pretty".