On Monday, May 20 — World Metrology Day — humanity will have a new definition of the kilogram, the standard unit of mass used the world over.
The change will occur more silently than when a leap second was added to a year. It will disrupt nothing. The number on your bathroom scale isn’t going to change, for better or worse.
But still, it represents an impressive achievement: a victory of humankind against the chaos that pervades the universe.
When scientists met at the General Conference on Weights and Measures in Versailles in November, and voted for the change, they were realizing the founding dream of the metric system. The metric system — which evolved into the International System of Units, or SI — was designed to be “for all times, for all people.”
Until now, the kilogram hasn’t been for all time. It’s been imperfect. But starting on Monday, May, 20, it will be.
Until now, the definition of a kilogram has been a hunk of metal in Paris
For more than a century, the kilogram had a very simple definition: It was the mass of a hunk of platinum-iridium alloy that’s been housed at the International Bureau of Weights and Measures in Sèvres, France since 1889.
It’s called the International Prototype Kilogram (a.k.a. Big K, or Le Grand K), and it has many copies around the world — including several at NIST in Gaithersburg, Maryland — that are used to calibrate scales and make sure the whole world is on one system of measurement.
These copies make sure a kilogram is a kilogram is a kilogram — whether its being measured on the factory floor of an airplane maker, or on the digital scale at your grocer’s checkout counter. And for those thinking the kilogram doesn’t matter in the US, which uses imperial units like pounds, feet, and gallons, our measurements are derived from SI units. Officially, in the US, 1 pound is defined as 0.45359237 kilograms.
The problem is that Big K is a manmade object, and therefore, it is imperfect. If Big K changes, everything else has to adjust. And this has happened. 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 old definition.
That’s no good. So what’s better?
The new definition anchors the value of the kilogram to a constant in nature, which can never, ever change
Starting Monday, the kilogram will be defined by the Planck constant.
The Planck constant is a concept in quantum mechanics (i.e. the study of how the tiniest components of the universe works), which describes how the tiniest bits of matter release energy in discrete steps or chunks (called quanta). Basically, you can think of the Planck constant as the smallest action an electron can take.
What’s important about the Planck constant is that it can never, ever change. And that makes it a worthy concept to anchor the definition of the kilogram to.
But first, to appreciate why the Planck constant can define the kilogram, it’s helpful to look at how the meter — the world’s standard unit of length — was redefined in terms of the speed of light as an example of why this was necessary.
The meter was originally defined as the length of a bar at the International Bureau of Weights and Measures in France. (It was then redefined to be equal to a certain wavelength of radiation.) Again, the problem with this definition was its imprecision. It was not based on unchanging properties of the universe.
Light speed, on the other hand, is unchanging. By 1983, physicists had gotten really good at measuring the speed of light. So they used it to fix the length of the meter forever, to make it permanent.
Here’s how: They redefined the meter to be equal to the distance light travels in a vacuum in 1/299,792,458 of a second. Essentially, the definition of the meter is now baked into the definition of the speed of light.
So let’s get back to the Planck constant.
To understand, let’s take a look at it. Written out, the Planck constant is 6.62607015 × 10-34 m2 kg/s.
You can treat the units of this number — meters squared per kilograms divided by seconds — as an equation. Kilograms are in the units, and that’s important. Because once know the value of the Planck constant, and fix it forever, the value of the kilogram also doesn’t change.
Every unit in the Planck constant is defined by an unchanging force of nature. The meter is defined by the speed of light. The second is defined by the frequency of atoms in the element cesium. And once the value of the Planck constant was measured and agreed upon, it means the kilogram can be set too.
And we know the value of the Planck constant, because of years of work in measuring it to a precise degree (read more about that here). This was an enormously hard task: to measure, basically, the smallest thing there is to measure. And it took decades, involving a beautiful and complicated machine called the Kibble balance.
Again, don’t worry: The new kilogram has the same mass as the old kilogram. That’s because scientists used the old kilogram to measure the value of the Planck constant.
In doing so, they essentially anchored the old kilogram to the Planck constant. Just like the length of the meter was anchored to the speed of light. (Also debuting Monday, new definitions for electric current, temperature, and amount of substance. These are also now tied to constants in nature. But unlike the kilogram, their definitions were not based on a physical object. Big K was the last one.)
If you glossed over it all, here’s what all this change boils down to: We’ll no longer need a government — the US, France, whoever — or an international governing body to tell us what a kilogram is. It will be a fundamental truth of the universe, available to anyone with the proper equipment to realize it.