Doug Bonderud

Aug 30th 2019

The Weight Is Over: New Kilogram Moves From Platinum Pucks to the Planck Constant


On May 20th, 2019, the General Conference on Weights and Measures tossed the old definition of a kilogram and replaced it with a brand-new measurement. But why the change? What’s the benefit? How is the new standard calculated? And what’s the impact at scale? Here’s why the new kilogram was worth the weight.

For All Time, For All People

America remains one of the few global holdouts still using the Imperial System (feet, pounds, miles per hour) for measurements of distance, weight and speed. Most of the world uses the metric system, also called the International System of Units (SI), which emerged after the French tossed the monarchy and decided the most rebellious thing they could do was codify measurements worldwide — and to be fair, they weren’t wrong.

As noted by Vox, the newly-formed Bureau of Weights and Measures created a system “for all times, for all people” and decided that the best way to get everyone on board was to link units of measurement with physical representations:

  • The Meter — One ten-millionth of the distance from the North Pole to the equator. A metal bar was crafted to exactly match this length for easy comparison.
  • The Gram — Linked to the mass of 1 cubic centimeter of water at four degrees Celsius.
  • The Kilogram — Weighing in at 1,000 grams, the kilogram was embodied by the International Prototype Kilogram (also called Big K), which is a piece of platinum-iridium alloy originally created in 1889 and stored in Sèvres, France. Copies exist across the globe, including seven at the National Institutes of Standards and Technology (NIST), to help standardize weights.

Let’s (Not) Get Physical

Weighing apples at the grocery store? Measuring your kids’ height on the wall? Even in the U.S., you’re using rulers and scales defined by the metric system. Every piece of measuring equipment can trace its accuracy back to the meter sticks and platinum pucks stored around the world.

The problem: Physical constants aren’t constant. The Earth isn’t a uniform, immutable shape, which means the meter measurement calculated using the North Pole-to-equator distance isn’t consistent. And despite incredible care, Big K is slowly losing mass — approximately 50 micrograms over the last 100 years — which CNN notes is “roughly equivalent to the weight of an eyelash.” For day-to-day applications, this isn’t a problem, but for scientific endeavors — such as weighing out tiny doses of pharmaceuticals or accurately measuring radioactive compounds — this small loss could have big consequences.

The solution? Create a new kilogram based on the fundamental properties of the universe.

Beyond Big K

Replacing Big K with something better didn’t happen overnight. For decades, scientists have been improving the ability to accurately measure key universal constants. According to NIST, this led to a 2018 vote which would redefine four of the seven measurements used by the International System of Units. The vote passed, and on World Metrology Day 2019, the new definitions went into effect.

For time, this meant tying a second to the amount of time it takes one atom of cesium 133 to complete 9,192,631,770 cycles of microwave radiation release, as LiveScience explains. For length, it meant linking a meter to the speed of light itself — specifically, the distance light travels in 1/299,792,458th of a second. Because these constants don’t change (at least from the perspective of human beings) these measurements are accurate and reliable whenever, wherever. But how do you redefine a kilogram?

Turns out, you don’t use weight — you use a constant that has mass as one of its parameters. Specifically, the Planck constant.

It looks like this: 6.62607015 x 10-34m2kg/s. The constant describes how matter releases energy in discrete amounts — known as quanta — and thanks to a tool known as the Kibble Balance, scientists have been able to calculate the Planck constant down to an extremely tiny margin of error. By accurately equating the mechanical energy of an object’s mass with its electrical energy, the balance has increased Planck constant confidence to the point that it can serve as the non-physical representation of the SI kilogram.

Here’s why: The super-accurate Plank constant contains three units — meters, seconds and kilograms. Both the meter and second are now defined using universal constants, making them incredibly reliable. With a near-perfect Planck in hand, scientists have agreed to define the new kilogram in terms of the constant, allowing more precise — and unchanging — measurements.

Killing the Kilogram

So, what does this mean for metric (and Imperial) users across the world? From a practical standpoint, almost nothing. A pound is still a pound at the grocery store or on your bathroom scale. But we’re no longer tied to the slowly-shrinking mass of not-so-big Big K, meaning increased accuracy for super-small weights and measures and a standardization of units that relies on the fundamental properties of the universe — rather than the frail physicality of protected platinum pucks.