Radiolab Episode Summary: "Less Than Kilogram"

Introduction

In the episode titled "Less Than Kilogram," Radiolab delves into the intricate world of measurement standards, focusing on the kilogram—the only SI unit still defined by a physical object. Hosted by Latif Nasser and featuring contributions from emeritus host Robert Krulwich and New Yorker writer Andrew Marantz, the episode explores the history, challenges, and eventual redefinition of the kilogram. Originally broadcast in 2014, the episode includes a dramatic update reflecting significant developments in the field.

Historical Context: The Birth of the Kilogram

Latif Nasser sets the stage by transporting listeners back to pre-revolutionary France, where the lack of standardized measurements hindered trade and caused widespread confusion. In Paris, approximately 250,000 different units of measurement were in use, varying by commodity and region. This inconsistency was particularly problematic during the 1780s famine, leading bakers to shortchange loaves of bread, exacerbating public unrest.

Latif Nasser (00:05:20):
"The earliest measurements were super intuitive. A lot of them came from the body—two hands, a foot, etc.—but there were also more abstract measures like throwing rocks to estimate distance."

The chaos of multiple measurement systems fueled the revolutionary desire for a unified, rational system. The newly formed revolutionary government sought to base measurements on natural phenomena, leading to the creation of the meter and, subsequently, the kilogram.

The Physical Kilogram: An Unchanging Standard

Robert Krulwich and Andrew Marantz introduce listeners to the International Prototype Kilogram (IPK), a platinum cylinder stored in France. This physical object became the universal standard for the kilogram, ensuring consistency in measurements worldwide. The IPK's role was pivotal in facilitating global trade and scientific research.

Andrew Marantz (00:04:00):
"A meter is a fraction of a second of the distance traveled by light in a vacuum. A kilogram is a particular thing, a plum-sized object."

The episode takes listeners on a virtual tour of the National Institute of Standards and Technology (NIST) in Maryland, where Robert Krulwich and Andrew Marantz examine the IPK and its meticulously maintained copies. These copies, known as "witnesses," are distributed globally to ensure uniformity in mass measurements.

Latif Nasser (00:09:15):
"The international prototype is the big kahuna. It's used to calibrate six identical platinum cylinders, the witnesses, which then calibrate another set of cylinders, and so on."

Challenges with the Prototype: Weight Loss and Uncertainty

The stability of the IPK has long been assumed, but in 1989, it was discovered that the prototype was losing mass. During a ceremonial weighing, the IPK was found to be slightly lighter than its witnesses, revealing the fragility of relying on a physical object for a universal standard.

Andrew Marantz (00:13:00):
"They put the IPK in a steam bath, dried it, and weighed it against the copies. It was lighter by about the mass of a grain of sugar."

This revelation sparked concern among metrologists, as even minor discrepancies could have significant implications for scientific research, manufacturing, and daily commerce. Theories ranged from physical degradation due to cleaning to potential outgassing of elements from the platinum cylinder.

Robert Krulwich (00:14:00):
"The international standard for weight was losing weight, and because it is the standard, everything else was unknowingly becoming inaccurate."

The Redefinition Process: From Physical to Abstract

Acknowledging the limitations of the IPK, the scientific community embarked on a mission to redefine the kilogram based on fundamental constants of nature. This shift aimed to eliminate reliance on a physical object, ensuring greater stability and universality.

John Pratt, NIST Physicist (00:21:38):
"The new definition of the kilogram is based on fixing the numerical value of the Planck constant to ensure consistency across all measurements."

The episode describes the complex process of achieving agreement among international laboratories using various methods, including the Watt balance and the counting of atoms. These techniques connect mass directly to quantum mechanical properties, anchoring the kilogram in the immutable laws of physics.

Latif Nasser (00:24:20):
"By fixing the Planck constant, we redefine the kilogram in terms of universal constants, detaching it from any single physical object."

Implications of the Redefinition: A New Era of Precision

The redefinition of the kilogram marks a significant milestone in metrology, providing unprecedented precision and stability. This change ensures that mass measurements remain consistent regardless of the physical condition of any prototype, fostering advancements in science and technology.

Latif Nasser (00:27:00):
"Replacing the physical kilogram with an abstract mathematical definition was a necessary evolution, ensuring that our measurements remain reliable and universal."

The episode also touches upon the cultural and symbolic aspects of this transition, highlighting the departure from historical artifacts to abstract concepts. This shift signifies humanity's progress towards more advanced and reliable systems of measurement.

Conclusion and Update

"Less Than Kilogram" concludes by celebrating the successful redefinition of the kilogram, rendering the IPK a historical relic. The new definition, grounded in fundamental constants, promises a future of precise and consistent measurements across all scientific and commercial fields.

Latif Nasser (00:27:48):
"The physical kilogram has been relegated to the dustbin of history, replaced by a definition that reflects the true nature of the universe."

Notable Quotes

• Latif Nasser (00:05:20):
  "The earliest measurements were super intuitive. A lot of them came from the body—two hands, a foot, etc."

• Andrew Marantz (00:04:00):
  "A meter is a fraction of a second of the distance traveled by light in a vacuum. A kilogram is a particular thing, a plum-sized object."

• Robert Krulwich (00:14:00):
  "The international standard for weight was losing weight, and because it is the standard, everything else was unknowingly becoming inaccurate."

• John Pratt (00:21:38):
  "The new definition of the kilogram is based on fixing the numerical value of the Planck constant to ensure consistency across all measurements."

• Latif Nasser (00:27:00):
  "Replacing the physical kilogram with an abstract mathematical definition was a necessary evolution, ensuring that our measurements remain reliable and universal."

Final Thoughts

"Less Than Kilogram" masterfully blends history, science, and storytelling to illuminate the profound impact of standardized measurements on society. Through engaging discussions and expert insights, Radiolab underscores the importance of precision in our daily lives and scientific endeavors, celebrating the harmonious marriage of tradition and innovation that has shaped our understanding of the world.