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The kilogram now de-artifacted - defined using a fundamental physical quantity

lpetrich

lpetrich

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Over the previous year: De-artifacting the kilogram and Scientists vote on new way to measure a kilogram

It's happened: The kilogram just got a revamp. A unit of time might be next | Science News and Adieu, Le Grand K: The kilogram to be redefined for the first time in 130 years and BIPM - About the BIPM - the International Bureau of Weights and Measures.

On May 20, these new SI definitions went into effect. The International Prototype Kilogram platinum-iridium cylinder is now officially retired as humanity's primary mass standard, though I think that it will join some similar artifacts as a secondary standard.

It represents an end to this departure from an important design feature of the original metric system: universality. The meter was originally defined in terms of the circumference of the Earth along a certain meridian, and the (kilo)gram in terms of the mass density of water. These two quantities are universally accessible, even if difficult to measure very accurately, and it turned out to be more practical to use artifacts for them. The meter was redefined in terms of physical quantities in 1960, but the kilogram was more difficult.

That redefinition depended on precisely measuring Planck's constant, the constant of quantization in quantum mechanics. When it was done well enough, it could be officially fixed, as was earlier done with the speed of light in a vacuum.

Likewise fixed are Boltzmann's constant, Avogadro's number, and the elementary charge.


That Science News article mentioned an effort to redefine the second by using a visible-light frequency, something that should get 100 times the precision of the current cesium-133 clocks.
 
lpetrich said:
Over the previous year: De-artifacting the kilogram and Scientists vote on new way to measure a kilogram

It's happened: The kilogram just got a revamp. A unit of time might be next | Science News and Adieu, Le Grand K: The kilogram to be redefined for the first time in 130 years and BIPM - About the BIPM - the International Bureau of Weights and Measures.

On May 20, these new SI definitions went into effect. The International Prototype Kilogram platinum-iridium cylinder is now officially retired as humanity's primary mass standard, though I think that it will join some similar artifacts as a secondary standard.

It represents an end to this departure from an important design feature of the original metric system: universality. The meter was originally defined in terms of the circumference of the Earth along a certain meridian, and the (kilo)gram in terms of the mass density of water. These two quantities are universally accessible, even if difficult to measure very accurately, and it turned out to be more practical to use artifacts for them. The meter was redefined in terms of physical quantities in 1960, but the kilogram was more difficult.

That redefinition depended on precisely measuring Planck's constant, the constant of quantization in quantum mechanics. When it was done well enough, it could be officially fixed, as was earlier done with the speed of light in a vacuum.

Likewise fixed are Boltzmann's constant, Avogadro's number, and the elementary charge.


That Science News article mentioned an effort to redefine the second by using a visible-light frequency, something that should get 100 times the precision of the current cesium-133 clocks.
Click to expand...

That redefinition of the second wouldn't really be a fundamental change though - it's more akin to switching from a cast iron mass standard to a platinum/iridium standard - it's potentially more precise, but still based on a physical object, rather than a mathematical relationship between physical constants. The 'new' kg represents a fundamental change.

Of course, a fixed value for the second would also imply a fixed value for the metre, as the metre is defined in terms of the second (and the fixed constant, c). Currently we have a fixed value of c, a measured value for the second (based on photon frequency using the 133Cs hyperfine transition as the standard), and the length of the metre 'floats' based on the most accurate current estimate of the duration of a second. Moving to a visible light frequency as the standard for the second may narrow down that estimate, and therefore allow a more precise value for the length of the metre to be calculated; But it still leaves the duration of the second (and hence the length of the metre) uncertain - albeit within a very narrow range indeed.

Of course, unlike lumps of metal, atoms and the photons they emit are at least universally accessible, so this is probably a distinction that's only of interest to metrologists.
 
A massive achievement. There was an earlier thread that linked to the techique and instrument. I thought the new technique mandated a change in the time standard.

Now there is no threat of terrorists holding the world hostage by threatening to destroy the standard....
 
Last edited:
steve_bank said:
A massive achievement. There was an earlier thread that linked to the techique and instrument. I thought the new technique mandated a change in the time standard.

Now there is no threat of terrorists holding the world hostage by threatening to destroy the standard....
Click to expand...
And in destroy, it could be just dumping some gravy on it. The new measure for the kg is just crazy.
 
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