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The world just redefined the kilogram

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Forty feet underground in Gaithersburg, Maryland, in a bright white laboratory that requires three separate keys to enter, the United States stores a precious collection of small, shiny metal cylinders that literally define the mass of everything in this country.

They are beautiful, with mirror finishes, and I have to resist the urge to touch them. If I did touch them, I could contaminate them with oil from my skin and potentially increase their weight. Patrick Abbott, the “keeper of the kilogram” here at the National Institutes of Standards and Technology (NIST), tells me this would be very bad.

Currently, the kilogram has a very simple definition: It’s 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 seven at NIST in Gaithersburg** —** that are used to calibrate scales and make sure the whole world is on one system of measurement.

Here is one of the copies at NIST, called K4, forged from the same piece of metal from which Big K was created in the 19th century.

Take a good look at it. Because very soon, this 129-year-old standard for the kilogram will change.

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.

This will be more than a scientific victory. It’s a philosophical one too, as I learned from the NIST scientists who have been working for years on the redefinition and call this moment the most exciting time of their entire careers.

With the new definition , the General Conference on Weights an Measures completes the original dream of the metric system, which was embraced amid the French Revolution. The metric system — which evolved into the International System of Units, or SI — was designed to be “for all times, for all people.”

“Objects always change,” says Stephan Schlamminger, a NIST scientist involved with the redefinition. With the new definition, he says, “we go from an object” on Earth “to the stuff that’s in the heavens.”

And that’s something worth celebrating. In a world where everything always seems to be in flux, these scientists have now made sure the kilogram will never change.

### A brief history of the kilogram

How do you know what something weighs? I know, there’s an obvious answer: You put it on a scale.

But when you go to a grocery store and weigh a bundle of apples, how does that scale know what a pound of fruit feels like?

For mass measurements to make sense, we need a fixed point of comparison. Those apples need to weigh more or less than something. To avoid chaos, and to allow our economy to function, that something has to be universally recognized.

The scale at your grocery store was calibrated with a weight that was calibrated with a weight that was calibrated with a weight, and so on. And all those calibrations trace back to right here, in the bowels of NIST. Consistent weights and measures matters for more than groceries: Imagine if Boeing couldn’t figure out precisely what an airplane weighs, or if the pharmaceutical industry couldn’t determine the exact mass of a tiny, potentially lethal, dose of medicine.

In the United States, we still use imperial units: pounds and ounces. But really, all our measurements are derived from the International System of Units, or SI, which uses meters and kilograms as the fundamental units of length and mass.

When it comes to mass in the US, everything traces back to these puck-shaped cylinders, which are precisely machined to weigh 1 kilogram. Officially, in the US, 1 pound is defined as 0.45359237 kilograms. Officially, a foot is defined as 1200⁄3937 meters.

But the system wasn’t always so orderly. Before the French Revolution and the invention of the metric system, the systems of weights and measures the world over were a chaotic, unruly mess.

“Imagine a world where every time you travelled you had to use different conversions for measurements, as we do for currency,” Madhvi Ramani of the BBC explains. “This was the case before the French Revolution in the late 18th Century, where weights and measures varied not only from nation to nation, but also within nations.”

The French Revolution was about toppling old, archaic, chaotic hierarchies left over from the feudal era and remaking society with egalitarian principals in mind.

Inspired by the revolution, scientists at the time wanted to start fresh on a new, consistent system of measurement, basing units not on arbitrary mandates from kings, but on nature. The goal was to create a system of measurement “for all time, for all people.”

Thus, when the International Bureau of Weights and Measures was founded in France in the late 1800s, the meter — the standard unit of length — was created to be one ten-millionth of the distance from the North Pole to the equator. The gram takes inspiration from the density of water: It’s roughly equal to the mass of 1 cubic centimeter of water held at 4°C.

To disseminate these new units — to make sure that everyone in the world understood them — the inventors of the metric system decided to create physical objects to embody and define them. They crafted a metal bar to be exactly 1 meter long. They created Big K to represent the mass of 1 kilogram, or 1,000 grams.

Since the 19th century, all the physical relics of the old metric system have been replaced by measurements affixed to constant forces of nature. The meter was originally defined as a proportion of the size of the Earth. But even the shape of the world isn’t permanent. Heck, the Earth might not even be permanent. So today, the meter is defined by the speed of light. The second is affixed to the motion of the atoms of the element cesium.

Only the kilogram is still defined by a physical object, for now.

So what is this new definition of the kilogram? Prepare yourself, because it’s a bit of a doozy.

### The science of redefining the kilogram in terms of the Planck constant, explained

Friday’s vote at the General Conference on Weights and Measures passed unanimously. But the changes won’t take effect until May 2019. When the change comes, here’s how the kilogram will be defined in the International System of Units:

What the heck?

It’s a lot harder to explain than a lump of metal in France. But let’s try.

Basically, the General Conference on Weights and Measures will be fixing the value of the Planck constant, which describes how the tiniest bits of matter release energy in discrete steps or chunks (called quanta).

With the vote Friday, the Planck constant will now and forever be set as 6.62607015 × 10-34 m2 kg/s. And from this fixed value of the Planck constant, scientists can derive the mass of a kilogram.

This redefinition effort has taken decades because the Planck constant is tiny (it starts with a decimal point and is followed by 34 zeros) and had to be calculated down to a super-tiny margin of error. The work required careful measurements with an incredibly complicated machine called the Kibble balance (more on that below), as well as observations of an extremely round sphere of silicon.

That explanation might seems wonky. And it is. But to better appreciate it, 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