Canonical Kilogram Dethroned by Invisible Forces

September 30, 2018

NIST is redefining the Kilogram

Today, there is the kilogram, which is THE kilogram, against which all other kilograms measure themselves in terms of their accuracy.

Across political divides, languages, cultures, beliefs, we all agree on how much a kilogram weighs.

But progress marches on and, as our tools sharpen, and as questions fester in scientist heads, we find ourselves, occasionally, doubting the veracity of our Canonical Kilogram. King Kilogram, the Kilogram Man, the Kilogram Mass Unit, ma'am. What if CK sheds, loses weight? Or, what if da King amasses more, in some mysterious way that middle-aged men would attest to be entirely possible, somehow.

Thing is: You just can't tell. And that can grow problematic, as the need for greater precision becomes an economic issue.

"The price per kilogram is meaningful only if the buyer and seller agree on what a kilogram is and that its definition is stable," writes Stephan Schlamminger, who is a scientist at the US National Institute of Standards and Technology, in an explanatory paper for NIST. Check: Between 2010 to 2017 he led the efforts to measure the Planck constant h. The individual knows measurement.

The actual weight of the kilogram was set by International System of Units (SI) in 1875, a year after the world agreement of the Metre Convention.

Well, next month, the Member States of the Metre Convention to vote in a “proposed revision” of the SI.

If accepted, it will come into effect on 20 May 2019, 144 years (precisely a dozen dozen, Sclamminger helpfully points out,) after the Metre Convention was signed on that same day in 1875!

Diabolical Designs

The doing away with a physical kilogram is part of a larger shift in how to think about measurement. In addition to the kilogram, the ampere, the kelvin, and the mole are also getting a complete makeover.

The SI group wants to "morph" the system of units from one "that rests on base units to one that is built on fundamental constants of nature," Schlamminger wrote.

"In the end, units that rely on artefacts are only as stable as the artefacts themselves," he writes. "Therefore, the stability of the artefact is the ultimate limit on how well a measurement can be linked to the definition of the unit."

"A unit based on a constant does not have this limitation."

OK, so we know that the measure of any quantity is a multiple of some known unit.

And SI is product of the fact that most physical quantities that can be measured out there can be written as some combination of SI's seven units. Behold, The Mighty Seven:

  • The metre (for measuring length),
  • The kilogram (mass)
  • The second (time)
  • The ampere (electric current)
  • The kelvin (temperature)
  • The candela ( luminous intensity)
  • The mole (amount of substance)

And the SI people, they want to keep it this way, after the revision.

Like like they virtualized the metre, they would like to do for the kilogram. It's part of a larger plan to define more base units through fundamental constants. In addition to the kilogram, current, thermodynamic temperature, and amount of substance are also going revised.

The meter was originally defined as a 10 millionth part of the quarter meridian.