Today, there is the kilogram, which is THE kilogram, against which all other kilograms measure their own fidelity. Across political divides, languages, cultures, beliefs, we all agree on how much a kilogram weighs. Because of that one kilogram.
But progress onward marches, our tools sharpen, and we find ourselves, occasionally, doubting the veracity of our Canonical Kilogram, currently resting in Paris under the nom de plume of "International Prototype Kilogram."
What if this reference kilogram sheds or loses a bit of weight? Or what if it amasses a wee bit more, in some mysterious way that middle-aged men attest to be entirely possible, somehow? What then is the "official" weight? Sure, there are backups, around the globe, copies of The One in Paris, but these are, even now, starting to diverge, ever so-slightly, in their weights from one another.
"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. Between 2010 to 2017, Schlamminger led the efforts to measure the Planck constant h, the basic unit of all quantum mechanics.
He is a good person for the job, because now the kilogram is being redefined entirely in terms of Planck's constant.
This doing away with the physical kilogram is part of a larger, mostly completed, shift towards redefining measurement using fundamental constants of nature rather than "artefacts," as the reference kilogram. We know that the measure of any quantity is a multiple of some known unit. "In the end, units that rely on artefacts are only as stable as the artefacts themselves," Schlamminger 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." No artefact, no confusion. Just pure science coming at ya. In addition to the kilogram, the ampere, the kelvin, and the mole are also getting a complete makeover.
How do you measure something against Planck's constant? With a Kibble Balance, which I've heard is some truly brilliant engineering, even as it is a bit difficult to describe. In short, the weight of an object can be read as an electromagnetic force. In effect, this device offers a balance between mechanical and electrical forces, providing a steady voltage commiserate with Planck's constant.
The actual weight of the kilogram was set by International System of Units (SI) in 1875. Next month, the Member States of the Metre Convention will vote in a “proposed revision” of the SI. If accepted, it will come into effect on 20 May 2019, 144 years after the Metre Convention.
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)