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

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.

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.

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.

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)

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)

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)

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)

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)

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

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.

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.