≤ kg

Summary

Radiolab – "≤ kg" (June 13, 2014)
WNYC Studios
Hosts: Jad Abumrad and Robert Krulwich

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Episode Overview

This episode of Radiolab explores the surprisingly dramatic and philosophical world of measurement standards, homing in on the kilogram—the last of the International System's base units still defined by a physical object. Through historical narrative, expert interviews, and trademark Radiolab wit, the show investigates how and why humanity established universal standards for measurement, how the "grand K" kilogram came to embody mass, and what happens when that standard starts to shift. Ultimately, it’s a story about our quest for certainty in a seemingly uncertain world.

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Key Discussion Points & Insights

1. What is a Kilogram? (02:00–03:57)

• Introduction to SI Base Units:
  Andrew Moretz brings a list of official measurement definitions, highlighting the complexity of most (like the meter and the second, defined by the speed of light and atomic vibrations respectively). The kilogram stands out for its elegant simplicity—it’s an actual physical object:

  "The definition of the standard unit of measurement, that is a kilogram, is no math, no numbers. It is a thing." (Andrew Moretz, 03:57)

• Contrast with Other Standards: Most other SI units are tied to physical constants or atomic properties. The kilogram, however, is a particular platinum-iridium cylinder—“le grand K”—housed in France.

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2. A Historical Mess: How We Got Here (04:22–08:44)

• Early Measurement Systems: Latif Nasser explains that ancient standards were intuitive and body-based (hands, feet, "throw a rock" distances), but unpredictably variable.

  "If you ask Achilles, it could be two throw-a-rocks away. But if you ask me, it would be like 78." (Robert Krulwich, 05:43)

• Enlightenment France and Measurement Chaos (06:00–07:51): Pre-Revolution France had around 250,000 different measurement units, crippling commerce and clarity. Famine and riots put pressure on authorities to act, as inconsistent bread weights triggered unrest.

• The Revolution and a New Standard: Revolutionaries decided to base standards on the constancy of nature. The meter was defined as a fraction of the Earth’s circumference; the kilogram was set as the mass of a certain volume of water—a standard then materialized as a platinum cylinder.

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3. The Sacred Kilogram and Its Mystique (08:44–12:41)

• A Visit to the Standard: The team descends into the bowels of the US National Institute of Standards and Technology, viewing a copy of the kilogram through layers of glass and security akin to Russian dolls.

• The Chain of Calibration (10:41–11:41): Every weight in the world can be “bloodlined” back to the international kilogram via a system of official duplicates (called "witnesses") that calibrate national standards down the line.

  "Every time somebody loses a pound on that TV show Biggest Loser, you can actually trace that... back to the international prototype kilogram, to a single object in a basement in France." (Robert Krulwich, 11:15)

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4. The Problem: Kilogram Losing Weight? (11:47–13:51)

• The 1989 Crisis: Scientists discovered the official kilogram had become lighter than its copies—by about the mass of a grain of sugar.

  "Which led them to the troubling possibility that the international standard for weight was losing weight." (Robert Krulwich, 12:58)

• Potential Causes: Theories ranged from over-cleaning to "outgassing" (hydrogen slowly escaping the metal), or even, half-jokingly, espionage.

  "One thing I read said foul play cannot be ruled out." (Andrew Moretz, 13:25)

• Philosophical Quandary: Since the kilogram is defined by "the thing," it’s impossible for it to be wrong—even if it changes:

  "If the definition of a kilogram is the mass of the International Prototype kilogram, whatever happens when you put that thing on the scale, that's a kilogram...and then everything else in the world is wrong." (Andrew Moretz, 13:40)

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5. Fixing the Standard: The Watt Balance (16:14–21:33)

• Toward a Universal Constant (16:27–20:23): John Pratt, from the National Institute of Standards and Technology, introduces the Watt balance: an instrument that can equate a mass to electrical and quantum constants, “wrenching” the kilogram from a physical artifact to fundamental constants.

• Demonstration with the Lego Watt Balance (18:48–19:43): Pratt uses a small model to show how balancing a known mass with a precisely measured magnetic force can, in theory, define mass as a function of the Planck constant.

• Global Effort for Agreement: Multiple labs worldwide are working to match measurements to unprecedented precision—agreement needed to eight decimal places. When (and only when) that’s achieved, the kilogram will be defined by Planck’s constant.

  "If they can do that, then and only then will the grand K be no more." (Robert Krulwich, 21:06)

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6. Afterlife of the Grand K (21:47–22:58)

• Museum of Retired Standards: Radiolab visits the Musée des Arts et Métiers in Paris, a "church" for decommissioned measurement artifacts like the original liter and meter, illustrating the end of an era for physical standards.

  "To imagine, like the thing, the grand thing, being in this place, sort of like seeing the Pope in shorts or something. It makes me a little uncomfortable." (Jad Abumrad, 22:58)

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Notable Quotes & Memorable Moments

• On the Insecurity of Measurement:

  "We may have a slightly trippy situation here. We got a hunk of metal losing weight, and yet because it is these things, it still weighs exactly a kilogram." (Robert Krulwich, 13:37)

• On the Beauty (and Absurdity) of Standards:

  "It's ridiculous... That's like something that like the North Korean government would do, just be like, no more cash like that." (Andrew Moretz, 13:51)

• Defining the New Kilogram:

  "The kilogram is the SI unit of mass. Its magnitude is set by fixing the numerical value of the Planck constant to be exactly 6.626069... times 10 to the minus 34." (John Pratt, 21:17)

• Philosophical Reflection:

  "Yet another meditation on what things or unthings are all about." (Robert Krulwich, 24:03)

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Timestamps of Key Segments

• Why SI Units Matter & The Kilogram’s Uniqueness: 02:00–04:14
• Early Measurement Chaos & Need for Standards: 04:39–08:44
• Inside the US Standard Kilogram Vault: 09:15–10:44
• The “Bloodline” of Mass: 11:13–11:41
• Grand K Losing Weight – Existential Crisis: 11:47–13:51
• Quantum Redefinition & Watt Balance: 16:27–21:33
• Museum of Retired Standards: 21:47–22:58
• Philosophical Wrap-up: 24:03

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Tone & Language

As always, Radiolab’s tone is curious, lively, slightly irreverent, and philosophically playful. The hosts use humor ("hangry," "the Pope in shorts") and metaphor to humanize abstract scientific concepts and keep listeners both entertained and enlightened.

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Summary
"≤ kg" turns the kilogram—a usually overlooked artifact—into a star and a lens through which deeper questions about certainty, knowledge, and progress are explored. The episode unpacks the surprisingly human problems and solutions behind our standards, showing how objects, ideas, and measurement are tangled with history, politics, and the aspiration for universality.

Transcript

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[Robert Krulwich]

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My cousin Freddy showed up to surprise us.

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[Jad Abumrad]

Okay.

[Robert Krulwich]

All right. Okay. All right.

[Fidelity Ad Voice]

You're listening to Radiolab.

[Andrew Moretz]

Radio Lab shorts from wnyc.

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[Jad Abumrad]

Hey, I'm Jad Abumrad.

[Robert Krulwich]

I'm Robert Krulwich.

[Jad Abumrad]

This is Radiolab, the podcast.

[Andrew Moretz]

And this actually brought a list.

[Robert Krulwich]

Okay, why don't you share with me your list?

[Andrew Moretz]

Where is this thing?

[Robert Krulwich]

This is Andrew Moretz. He's a writer and editor at the New Yorker magazine.

[Andrew Moretz]

Oh, I might have gotten lost.

[Robert Krulwich]

Who occasionally pops onto our show? Maybe you were mugged by a. Ah, here it is. And he recently got obsessed with a. It's a list of measurements.

[Andrew Moretz]

Base units, they're called. They're SI Base units. The Systeme Internationale, you know.

[Robert Krulwich]

So let me do it this way. Have you ever wondered how long an inch is? Exactly how long?

[Jad Abumrad]

I know, I just look at a ruler.

[Robert Krulwich]

Well, but how do you know that your ruler and my ruler do have the same amount of inch space? Or that someone in China that their inch is our inch is your inch is my inch.

[Jad Abumrad]

I haven't really thought about it, but I just assume that there's like a master inch somewhere.

[Robert Krulwich]

Bien sur. I say it in French for a reason, which you'll feel in a moment. That is what was on this list that Andrew was looking at. It's a list of standard measures for everything we have around how big something is, how far something is, how hot something is. It's all on this list.

[Andrew Moretz]

Okay, so when you go down the list of the systeme international DES units.

[Robert Krulwich]

Here'S what you get.

[Andrew Moretz]

A meter. A meter is a fraction of a second of the distance traveled by light in a vacuum.

[Robert Krulwich]

Okay, what?

[Andrew Moretz]

A second is how much radiation corresponds to the transition between two hyperfine levels of the ground state of the cesium 133 atom.

[Jad Abumrad]

That's the definition of a second.

[Robert Krulwich]

How many times does a particular atom jiggle?

[Andrew Moretz]

Yeah.

[Jad Abumrad]

Wow.

[Andrew Moretz]

An ampere which measures electric current. You know, an amp is a constant current which if maintained in two straight parallel conductors of infinite length, would produce between these conductors a force equal to 2 times 10 to the negative 7th Newtons per meter of length. I have no idea what that means.

[Robert Krulwich]

See, that's the thing. If you look at the actual definitions of any of these things, amp, meter, second, whatever you go. But there is one standard on the list that is unique for its simplicity.

[Andrew Moretz]

The definition of the standard unit of measurement, that is a kilogram is no math, no numbers. It is a thing.

[Robert Krulwich]

A particular thing.

[Andrew Moretz]

A plum sized thing.

[Robert Krulwich]

It is the only thing we use to measure things. It's the last one standing, the only physical standard left.

[Planet Visionaries Narrator]

Why is it the last?

[Jad Abumrad]

And why were there? Is it.

[Planet Visionaries Narrator]

What?

[Jad Abumrad]

Wait, what?

[Robert Krulwich]

Let me just take you back to the beginning of the story.

[Latif Nasser]

Like, I must admit that I expected this story to be a lot more boring than I found. It's like an epic story. It's really.

[Robert Krulwich]

That is Latif Nasser, science historian, regular on our show. And he says if you go all the way back to the very first farmers back in Mesopotamia, all of the.

[Latif Nasser]

Earliest measurements were super intuitive.

[Robert Krulwich]

And he says a lot of them.

[Latif Nasser]

Came from the body.

[Robert Krulwich]

As in that bunny is coming close to the net. How close, dad?

[Latif Nasser]

Two hands. But it's not just like, because we think of like hands and feet, but it was also there so many other kinds of measurements. Like you would say, oh, something is as far as, you know, my voice can carry. Something is as far as I can see sitting on the top of a camel or something is as far as I can throw a stone.

[Robert Krulwich]

So that would mean like say, okay, I'm going to build a farm here and I'm going to do it 3. Throw rocks across.

[Latif Nasser]

Yeah, yeah. The way I read about it was like travelers. Like if you're a Saharan traveler and you need to know where the next watering hole is, that's kind of a life and death measurement. They Would say it's, you know, three throw rocks away, or it's 10 throw rocks away.

[Robert Krulwich]

But you know that there might be some built in uncertainty there, because if you ask Achilles, it could be two throwerocks away. But if you ask me, it would be like 78.

[Latif Nasser]

You have nailed exactly the problem with the throw a rock system.

[Robert Krulwich]

And these problems kind of came to a head in the 1700s.

[Latif Nasser]

It's the Eve of the French Revolution.

[Robert Krulwich]

In a little town called Paris.

[Latif Nasser]

It's a pretty cosmopolitan place, which means that people are coming from different places, and they all have their own measures. Approximately 250,000 different units of measurement in regular use.

[Robert Krulwich]

250,000.

[Latif Nasser]

Every commodity has its own measure. So you have grain, wine, oil, salt, hay, coal, wood, fabric, everything.

[Robert Krulwich]

And it's extraordinarily confusing, not to mention it's extraordinarily bad for trade. So if I came to you and I said, monsieur, I have here a bit of cloth, you would say, how much cloth you got? And I'd say, I have two yards. And you say, what's a yard? I said, it's this much. And the other guy would say, no, no, it's this much. And he said, no, no, it's this much. And he would, no, no, it's this much. And you could see that.

[Jad Abumrad]

Frustrating.

[Robert Krulwich]

It was frustrating, yeah.

[Latif Nasser]

And making matters worse, in the 1780s, there was a famine. So there was a shortage of grain, and people were hungry, and people were angry, which I am gonna call they were hangry.

[Robert Krulwich]

They were hangry.

[Latif Nasser]

They were very hangry. So the bakers at the time, they knew that if they raised the price of bread, like an angry mob would basically come and kill them.

[Robert Krulwich]

But they also knew that with no absolute standard, there was no way to be sure that what you were getting is what you were getting.

[Latif Nasser]

And so what they started doing was they started just lightening their bread loaves by just a little. So as the famine got worse, people would be waiting in longer and longer lines to pay the same amount of money for. For smaller and smaller loaves. So they were getting hangrier and hangrier. And so one of the things that people are like, crying out for is that they want standardized weights and measures. If I go to the bakery and I buy a loaf of bread, I want a whole loaf of bread. Don't short me on this. This is serious.

[Robert Krulwich]

Well, you know what happens next.

[Latif Nasser]

The Bastille is stormed and the king is under house arrest. And then under the guillotine.

[Robert Krulwich]

And as soon as the revolutionary government takes over, they say, all right.

[Latif Nasser]

Okay. This is one of our first priorities. We are going to make a new.

[Robert Krulwich]

Standard, but not based on something arbitrary, like a king. This is the Enlightenment.

[Latif Nasser]

Why don't we draw on some kind of totally different authority? The authority of nature.

[Robert Krulwich]

Of nature.

[Latif Nasser]

Of nature.

[Robert Krulwich]

So, long story short, they took the circumference of the Earth, they took a quarter of that circumference, divided that by 10 million, and they got the meter. The meter. They then divided by 10 cubed it, filled the cube with water, took the mass of the water, minted a cylinder of metal with that mass, and voila, they created the world's first kilogram.

[Latif Nasser]

The idea of this was, if we make this thing that is so beautiful and perfect and everybody can see it that way, then not only will France use it, but the whole world will use it. Then goods and ideas can be exchanged everywhere by all people, and it will be beautifully glorious.

[Robert Krulwich]

Fraternite egalite.

[Latif Nasser]

Exactly. They wanted something that would be eternal and unchanging for everybody, for all time.

[Robert Krulwich]

So now, I guess you want to see it. No. Yeah. Okay. Okay. So it's in here. We ended up visiting the National Institute of Standards and Technology in Maryland. And this is where we'll be going in, but we're going to go into this guy. Patrick Abbott, assistant, was our guide. They took us three stories down into the bedrock of the state of Maryland because they want things down here to be totally still. We've just gone through one double door. Here comes another double door. Then we stepped into this vault of a room, and there it was. What we're looking at then is a glass jar with a little handle on top. And then inside, inside that is another glass jar with a little handle on top. And inside that is. Is the thing. The thing. It's kind of gorgeous, really. The shiniest little cylinder you've ever seen. Very small. And it looks very clean, doesn't it, to you?

[Andrew Moretz]

Yeah. It's almost hard to tell where the, like, Russian doll glass jar stops because it's reflective.

[Latif Nasser]

This might be a crazy question, but can we hold a kilogram?

[Jad Abumrad]

That's our producer, Lynn Levy.

[Robert Krulwich]

No, I'm just curious to know what it feels like.

[Latif Nasser]

We've been talking about it so much.

[Robert Krulwich]

They are very careful with the kilogram. And this isn't even really the real one. The original of the original of the original of the original.

[Andrew Moretz]

Le Grand Ca, as they call it.

[Robert Krulwich]

Lives in a basement in France. You can't get anywhere near that one.

[Jad Abumrad]

I could.

[Robert Krulwich]

No, you couldn't.

[Jad Abumrad]

I could get all Tom Cruise on.

[Robert Krulwich]

That, you die trying. Here's how it works. The international prototype is the big gahuna. That's the one used to calibrate six identical platinum cylinders, what they call witnesses, or tamois in French. Those witnesses are then used to calibrate another set of cylinders, which are then used to calibrate the US Standards, which is what we saw. And that one is used to calibrate all kinds of things. The weight of your lemons, the scale in your bathroom.

[Latif Nasser]

Green team, you lost 34 pounds.

[Robert Krulwich]

Every time somebody loses a pound on that TV show Biggest Loser, 5.5. You can actually trace that, like a bloodline, if you will, or an unbroken chain, back to the international prototype kilogram, to a single object in a basement in France, the holy of holies, that is the kilogram.

[Jad Abumrad]

But you're telling me that when something is weighed in the world, often it goes all the way back to this one hunk of metal?

[Robert Krulwich]

That's what I'm saying. Which was why the next part of the story is so disconcerting.

[Andrew Moretz]

What happened in 1989 is that according.

[Robert Krulwich]

To Andrew, the folks who take care of the official kilogram, the big K, they took it out of its jars.

[Andrew Moretz]

They put it in a steam bath.

[Robert Krulwich]

Hit it with the steam that rinses.

[Andrew Moretz]

Everything, wait for it to dry, then.

[Robert Krulwich]

They commence a ceremonial weighing. Right. How do you weigh the thing that is the standard of weight?

[Andrew Moretz]

Well, you weigh it against the copies.

[Robert Krulwich]

Like the US Copy, for example. So they get one of those, and they put it on one side of the scale, and then they put the grand K on the other, and the.

[Andrew Moretz]

Ipk, the Le Grand Ca. The one is light.

[Jad Abumrad]

What?

[Robert Krulwich]

It's light.

[Andrew Moretz]

It doesn't.

[Robert Krulwich]

How many. How many? How much lighter is it than its sisters?

[Andrew Moretz]

Roughly the mass of. Of a grain of sugar. Oh, yeah.

[Robert Krulwich]

Is that gigantic?

[Andrew Moretz]

It's measurable.

[Jad Abumrad]

Wait, how did they know that it was light and not that the other ones were heavier?

[Andrew Moretz]

Right? Well, they didn't. So they used the second sister copy, still light, and the third sister copy, still light. In the fourth and fifth and sixth.

[Robert Krulwich]

In comes the man from Germany.

[Andrew Moretz]

Light.

[Robert Krulwich]

In comes the man from Canada.

[Andrew Moretz]

Light.

[Robert Krulwich]

In comes the man from Spain, Light. Which led them to the troubling possibility that the international standard for weight was losing weight. Well, we think that. We think the big guy's the problem.

[Andrew Moretz]

As far as how it lost that weight.

[Latif Nasser]

Really?

[Andrew Moretz]

No one knows.

[Robert Krulwich]

One possibility is it got cleaned too much, and maybe some of it got scraped away.

[Andrew Moretz]

Although it's disputed whether cleaning it more would make it lose weight or gain weight. The other theory is outgassing.

[Robert Krulwich]

Like maybe a little hydrogen is seeping out of the metal.

[Andrew Moretz]

And then there was one thing I read that said foul play cannot be ruled out.

[Robert Krulwich]

Well, see, I was thinking maybe the Taliban. What's clear is we may have a slightly trippy situation here. We got a hunk of metal losing weight, and yet because it is these.

[Andrew Moretz]

Things, it still weighs exactly a kilogram.

[Latif Nasser]

Right?

[Andrew Moretz]

If the definition of a kilogram is the mass of the International Prototype kilogram, whatever happens when you put that thing on the scale, that's a kilogram, you can't do that. And then everything else in the world is wrong.

[Robert Krulwich]

No, you can't do that.

[Andrew Moretz]

It's ridiculous. It's like that doesn't sit right. That's like something that like the North Korean government would do just be like, no more cash like that. We can't just go around capriciously doing stuff like that.

[Planet Visionaries Narrator]

Radiolab is supported by Planet Visionaries, the podcast created in partnership with the Rolex Perpetual Planet Initiative. The show is hosted by Alex Honnold, who you may recognize from Free Solo, where he climbed El Capitan without ropes. Now he's turning his focus to the biggest challenge of protecting the only planet we've got. Every episode brings you stories that prove climate optimism and isn't naive. It's a strategy. The episodes span the globe, from Arctic scientists and Amazon Forest Guardians to entrepreneurs reimagining fashion and food systems. You'll hear from explorers, scientists, activists and storytellers who are working to reshape the future in practical human ways. In one episode, Alex sits down with wildlife photographer Bertie Gregory to discuss how animals can teach humans resiliency and empathy and hope in partnership with the Rolex Perpetual Planet Initiative. Check out Planet Visionaries Listen or watch on Apple, Spotify, YouTube, or wherever you get your podcasts.

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[Robert Krulwich]

Should tell the people who we are and what our new show is. I'm Robert Smith and this is Jacob Goldstein and we used to host a show called Planet Money. And now we're back making this new podcast about the best ideas and people and business in history and some of the worst people, horrible ideas, and destructive companies in the history of business. We struggled to come up with a name, decided to call it Business History.

[John Pratt]

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Why? Because it's a show about the history of business. Available everywhere you get your podcasts.

[Jad Abumrad]

All right, so if the standard of weight is, as you're saying, losing weight, so how do you fix that?

[Andrew Moretz]

Well, I'm getting zero cell phone reception down here. It means we're really deep.

[Robert Krulwich]

When we were down in that underground room in Maryland, we met a guy who has some thoughts about this.

[John Pratt]

Oh, there he is.

[Andrew Moretz]

Okay.

[Robert Krulwich]

His name's John Pratt.

[John Pratt]

I'm the leader of the Fundamental Electrical Measurements Group at the National Institute of Standards and Technology.

[Latif Nasser]

Hi, John.

[Robert Krulwich]

John walked us through even more high security doors, and then we walked into this. Oh, my God. Amazing room.

[Latif Nasser]

It's big.

[Robert Krulwich]

It is big. About three stories tall. Yeah, it's. And it's made of. It's like a silver room. It has a silver gray floor. It has silver shiny walls. And your hair is on the silvery side.

[John Pratt]

Very much so.

[Robert Krulwich]

You probably wouldn't be allowed in here if you were a redhead. No, no. I don't even know how to describe it. It looks like a wheel turned on.

[Andrew Moretz]

Its side with the thing itself looked sort of just like a. Like a massive round metal cauldron or like a big metal pot. But then there are all these weird little gizmos and parts and then all.

[Robert Krulwich]

These coiled up wires, and it's just a stunning machine.

[Andrew Moretz]

But it's all just for the benefit of the one.

[John Pratt]

The one measurement.

[Andrew Moretz]

The one kilogram.

[John Pratt]

Yep.

[Robert Krulwich]

Because inside that giant cauldron, there is an extremely, extremely sensitive balance, an equal arm balance, which is basically like a.

[John Pratt]

Seesaw or a teeter totter. And usually you would set that up so that you would literally put kid on one side of the teeter totter, kid on the other side of the teeter totter.

[Robert Krulwich]

Now, you've been in a playground, so you know how this goes. But what they've done here is on one side of the teeter totter, they've got the kilogram, like the ground K. That's kid number one. On the other side, instead of another.

[John Pratt]

Kilogram or kid two, we'll have a highly variable magnet.

[Robert Krulwich]

Now, here's the thing. The magnet won't be touching that side of the scale. It's not be exerting a force. An Invisible force on that side, it'll.

[John Pratt]

Produce a force and we could use that to hold the balance still.

[Robert Krulwich]

And the force it takes to hold up the balance. That of course is the same as the weight of the Goncat sitting on the other side. And if you can convert that force into a number that everybody agrees to.

[John Pratt]

Voila.

[Robert Krulwich]

You have just redefined the kilogram. You have wrenched it from, from the world of things and it's become attached to the fundamental forces of the universe.

[John Pratt]

Yep, you've grasped the gist of it. You want to see that happen right now I can show you this with our Lego version of the Watt balance if I can fire it up.

[Jad Abumrad]

Lego Lego 1.

[Robert Krulwich]

Well, see, the big one was being tested or something, so they took us over to look at the little one.

[John Pratt]

Okay, so we have, have a little scale and everything. You can see I just disturbed the balance and it's, you know, jiggling around a little. It's free floating.

[Robert Krulwich]

Okay, so you're now going with your tweezers and you're plucking a itty bitty.

[John Pratt]

Yep, two gram mass.

[Robert Krulwich]

He puts this tiny little thimble thing on the balance and now it's going to, he says levitate.

[John Pratt]

Now it's, it prompts me. Mass on.

[Robert Krulwich]

Mass on.

[John Pratt]

Yeah, I'm going to put the mass on.

[Robert Krulwich]

He pushes a button.

[John Pratt]

All right, and.

[Andrew Moretz]

Wait, but when do we see the levitation?

[John Pratt]

That was it.

[Robert Krulwich]

I didn't, I missed it. Do it again. It was floating.

[John Pratt]

It is floating. Sitting on the balance.

[Robert Krulwich]

Okay, it's not floating.

[John Pratt]

That is floating. Does it fall to Earth?

[Robert Krulwich]

That's a different idea, levitation. Now the truth is that once I finally figured out what this guy was doing, it was actually sort of cool. He had taken a little metal weight, he put it on one side of the scale, and on the other side of the scale it was just empty. But yet the thing didn't tip over because the empty side actually had a magnetic force equivalent to the metal holding it just perfectly still.

[Jad Abumrad]

So if they're able to do that, does that mean that the Grand K's reign is done?

[Robert Krulwich]

Not yet, no. Because first of all, you have to get straight with a lot of math.

[John Pratt]

MC squared equals H nu. Work backwards.

[Robert Krulwich]

You gotta divide by E and then by M measure the B field.

[Jad Abumrad]

Woo.

[John Pratt]

Let's go.

[Robert Krulwich]

And then you get your amperes and your watts and your Planck's constant classical.

[John Pratt]

Bohr model of atoms and stuff.

[Robert Krulwich]

Anyway, it is actually way more complicated, this whole thing, than I frankly will ever understand. But here's where we are at, you got all these different teams around the world. You got John's team in Maryland with his seesaw. You got another lab, actually a couple of them that have their seesaws. You got a third lab. It's literally counting the atoms. They're all doing experiments, comparing numbers, trying to get the numbers to agree so that by whatever route, everybody agrees on exactly what a kilogram is. Right now they're close. They're in agreement out to about six decimal places. And that's not good enough. They want the numbers to agree out to eight decimal places. But if they can do that, then and only then will the grand K be no more. Yeah, because instead of defining the kilogram as whatever is equal to the grand K, now you have a new definition.

[John Pratt]

The new definition of the kilogram. The kilogram is the SI unit of mass. Its magnitude is set by fixing the numerical value of the Planck constant to be equal to exactly 6.626069. And we have X's because we haven't all agreed what the final.

[Andrew Moretz]

Those are the missing decimals.

[John Pratt]

Those are the missing decimal places times 10 to the minus 34 when it's expressed in the unit for action joule seconds, which is meter squared kilogram per second.

[Robert Krulwich]

Whew.

[Andrew Moretz]

That'll be such a simpler definition.

[John Pratt]

Oh, yeah. No, you.

[Jad Abumrad]

And what will happen to the Grand Cay when the new definition goes into effect?

[Robert Krulwich]

Well, so this is the sad part. Looks like a church. You will see after the end, the church where the. The Grand Cay may eventually end up. In a place like this, that's a big deal where so many standards have gone to die. This is the Musee des Arts et Metiers in Paris. So this is the beginning. Cyril Fasseau was our tour guide. What is this? He showed us the original litre zero.

[Andrew Moretz]

Virgil.

[Robert Krulwich]

Wow. Some early thermometers.

[Fidelity Ad Voice]

There's one funny object here, one room.

[Robert Krulwich]

He showed us the original. I think it was the Parisian meter. So in Paris, this was the infallible, the absolute standard from 1801. I think it's in a wooden box with a velvet packing and it's got silk ribbons at either end. And it's just a very beautiful looking silver rod.

[Jad Abumrad]

Oh, to imagine, like the thing, the grand thing, being in this place, sort of like seeing the Pope in shorts or something. It makes me a little uncomfortable.

[Robert Krulwich]

Special thanks to Ari Adland and Eric Perlmutter and also to Terry Quinn. We don't want to forget Richard Davis.

[Andrew Moretz]

Ken Alder, Bob Waters, Michael Baum, Michael.

[Jad Abumrad]

Newman and Finally, thank you to our math angel soprano, Melissa Hughes.

[Fidelity Ad Voice]

Very weird to say my own name.

[Jad Abumrad]

Also, big props to reporter Andrew Morantz, Latif Nasser, and our producer Lynn Levy. And oh oh, oh, oh, oh, oh oh. Also, you should go to Radiolab.org not only to support the show by clicking the support button, but also to check out a collaboration video that we did with Henry Reich from Minutephysics. You can see it at radiolab. Org or also YouTube.com minutephysics.

[Robert Krulwich]

Yet another meditation on what things or unthings are all about.

[Jad Abumrad]

I'm Jad Abumrad.

[Robert Krulwich]

I'm Robert Krolwich.

[Jad Abumrad]

Thanks for listening.