A New Definition for the Kilogram

[ehbowen]

It looks like after this week the accepted SI unit of mass, the last fundamental unit of measure to be defined by a physical artifact, will be redefined in terms of fundamental constants of physical theory.

The kilogram is dead; long live the kilogram

Nearly every measurement of weight you’ve ever made, from peeking at your bathroom scale to measuring out flour for a recipe, can be traced back to just a single object: a metal kilogram made of platinum and iridium that resides under lock and key in an underground vault in Paris. It’s called the International Prototype Kilogram, or IPK, and since its creation in 1889 it has been the standard by which the world’s weights are defined. But not for much longer.

That one chunk of platinum and iridium has defined the kilogram for over a century. This is not an event that happens every day....

[RandallS]

It looks like after this week the accepted SI unit of mass, the last fundamental unit of measure to be defined by a physical artifact, will be redefined in terms of fundamental constants of physical theory.

I (think) I understand how the Kibble balance works -- barely understand, that is. .

[Sefiru]

It looks like after this week the accepted SI unit of mass, the last fundamental unit of measure to be defined by a physical artifact, will be redefined in terms of fundamental constants of physical theory.

Huh. I remember learning in school that 1 kg was defined as the mass of 1 L of water, which in turn is defined as 1000 cubic centimeters.

[ehbowen]

Huh. I remember learning in school that 1 kg was defined as the mass of 1 L of water, which in turn is defined as 1000 cubic centimeters.

Actually, that was the original (1789) definition. It turned out not to be precise enough; difficult to measure pure water (how pure?) with the same precision as a cylinder of metal (protected from oxidation) can be weighed on a balance beam scale.

The unit has not changed, but now the state-of-the-art has advanced enough to make the Kibble balance even more precise and, on a fundamental physical level, more universally repeatable.

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[ehbowen]

It looks like after this week the accepted SI unit of mass, the last fundamental unit of measure to be defined by a physical artifact, will be redefined in terms of fundamental constants of physical theory.

The vote has passed—unanimously—although the new definition doesn't officially take effect until May 2019.

On Friday, scientists from around the world met at the General Conference on Weights and Measures in Versailles, France, and voted to change the definition of a kilogram, tying it to a universal constant in nature. The change will go into effect on May 20, 2019.

One important reason for the change is that Big K is not constant. It has lost around 50 micrograms (about the mass of an eyelash) since it was created. But, frustratingly, when Big K loses mass, it’s still exactly one kilogram, per the current definition.

When Big K changes, everything else has to adjust. Or even worse: If Big K were stolen, our world’s system of mass measurement would be thrown into chaos.

With the vote Friday, the world’s top measurement scientists chose to affix the kilogram to the Planck constant, a fundamental concept in quantum mechanics that can never, ever change — here on Earth or in the deep reaches of the universe.

(Much of this VOX article covers the same ground as the Verge article quoted in the OP, but there's some new material.)