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 is 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. A foot is defined as 0.3048 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:

The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be 6.626 070 15 × 10

^{-34}when expressed in the unit J s, which is equal to kg m^{2}s^{-1}, where the meter and the second are defined in terms of c and ∆νCs.

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 }m^{2} 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 33 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 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 an unchanging 299,792,458 meters per second. No matter where you are, scientists believe, it stays the same. (At least, if it does change, that would upend most everything we know about physics.)

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.

There’s a poetry to this: Scientists took the meter — an arbitrary length measurement invented by humans — and affixed it to a universal constant. Our messy human measurements have transcended their messy humanness; they have been melded with an eternal truth.

The new, light-defined meter is the same length as the old meter standard in Paris. But unlike the old standard in Paris, now the definition of the meter can never, ever change.

The same thing is happening with the Planck constant. Like the speed of light, the Planck constant is a universal truth that will never change.

By setting a final value of the Planck constant — the units of which include the kilogram, much like the units of the speed of light include the meter — the size of a kilogram is forever stable. You can also think of it like this: The kilogram has been anchored to the Planck constant, where it will rest, forever.

(Perhaps if you’ve been reading closely, you’ve noticed there’s a bit of a chicken-and-egg problem here. How do you seek to define a meter in terms of the speed of light if your measurements of the speed of light also contain the unit “meter”? It’s the same thing for the Planck constant: It contains kilograms in its units. Short answer: This is why the people working on these problems have PhDs.)

### The Kibble balance is the machine that makes this all possible

Redefining the kilogram in terms of the Planck has been an immense challenge, one that’s taken decades to complete.

For one, scientists had to be able to measure the Planck constant to an extremely precise degree. If our estimate of the speed of light had a large margin of error, it wouldn’t be a reliable anchor to measure a meter. Same goes for the Planck.

For decades, the scientists at NIST, as well as a few other labs around the world, have been using a machine called the Kibble balance (sometimes referred to as the watt balance) to precisely measure the Planck constant to a careful enough degree that it can be used to redefine the kilogram.

Like the kilogram standards, the Kibble balance is housed deep underground at NIST. It’s built onto a concrete floor that can literally float above the building’s foundation to better isolate its sensitive equipment from any vibrations from the rest of the facility. I have to wear a plastic net over my hair and shoes to go see it because any bit of debris could throw it out of calibration.

If the Victorians had built a time machine and parked it in a brewery, I’d imagine it would look something like this.

The Kibble balance works somewhat like a simple mass balance. Picture the one Lady Justice holds in her hand: It has two pans that balance at a central point. A simple balance compares two weights on each of the pans, with the goal of equating them.

The Kibble balance — named after its late inventor, the British physicist Bryan Kibble — does something similar, but with a quantum mechanical twist. It equates the mechanical energy exerted by the mass of an object with an equivalent amount of electrical energy.

The formula the Kibble balance yields to equate mass and electrical power is complicated. (The NIST scientists brought me to the whiteboard shown below to explain.)

What’s important is that in that equation — among all the variables at play, which include mass, velocity, gravitational pull, magnetism, and electricity — lies the Planck constant. And using this machine, scientists were able to solve for Planck. The math works out because, as Albert Einstein taught us with his most famous equation E=mc^{2 }, mass and energy are essentially different expressions of the same thing.

Now you might be thinking: What does the Kibble balance do now that it’s defined the Planck constant?

Well, it replaces the need for Big K in France because it now knows the mass of a kilogram in terms of the Planck constant. And that will be a precise measurement, a way to keep ensuring a kilogram is still a kilogram, that can be used to weigh objects, and determine their mass according to the new standard.

“Right now our quality assurance on the stability of [Big K] is based on agreement,” Abbott says. “We say it’s not going to change. Our quality assurance on the Kibble balance is that it’s based on a constant of nature that has been measured, rigorously, by the entire world. and we know that it doesn’t change. It’s all the difference in the world.”

### The democratization of weights and measures is underway

Still with me?

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.

In theory, anyone can build a Kibble balance. (I’m told there are miniaturized ones on the way.) “They can build this experiment, and they can measure any mass they want, any material, just put it on the balance and you get the value of the mass, absolute, in terms of the Planck constant,” Darine El Haddad, who runs the Kibble balance experiment at NIST, says. The Kibble balance allows for an “absolute measurement” she says.

In the future, the manufacturing industry won’t need to send their weights and scales to NIST for calibration. They could have a Kibble balance on their factory floor. In that light, the new definition is more democratic — one that’s free to be used throughout all the world and not kept locked up in a case in France.

There are some big drawbacks to the change, however. “People don’t even understand the metric system,” Abbott says. “How are you going to explain a Kibble balance?” The complexity of the definition may be a turn-off to people who want to learn about science. An elementary school child can understand that a hunk of metal weighs a kilogram, but quantum mechanics?

Schlamminger argues that while the new definition is more technically complicated, “philosophically, it’s simpler.” The kilogram will soon be defined by the fundamental physics of the universe, not some human machination.

Schlamminger has the founding words of the metric system, “for all times, for all people,” tattooed on his arm, alongside the digits of the Planck constant. That’s how strongly he believes in the ideal. He sees this work as “finishing the arc that started with the French Revolution.” And it is complete: With Friday’s vote, the kilogram is now forever, for all time, and for all people.