For over a century, the kilogram, the base unit of mass, was defined by a physical artifact: a platinum-iridium cylinder stored in a vault in France. However, a recent international agreement redefined the kilogram, linking it to fundamental constants of nature. This marked a significant shift in how we define and measure mass.
The U.S. National Institute of Standards and Technology hailed the change as a pivotal moment, even though an object weighing a kilogram remains unchanged. The crucial difference lies in how we define that kilogram.
The Old Kilogram: Le Grand K
For 130 years, the kilogram was literally set in stone (or rather, platinum-iridium). The International Prototype Kilogram (IPK), affectionately known as “Le Grand K,” was the definitive kilogram. Every measurement of mass on Earth was ultimately traceable to this single artifact locked away in a triple-locked vault in Paris. Metrologists, the experts in the science of measurement, relied on this physical standard.
The Redefinition: A New Era of Measurement
However, on a historic Friday, representatives from 60 countries decided to redefine the kilogram. The new definition sets the kilogram’s mass in terms of electric current, connected to fundamental constants, rather than comparing it to a physical object. This means all seven base units in the International System of Units (SI) are now defined by these unchanging constants:
- Meter: Distance
- Second: Time
- Kilogram: Mass
- Mole: Amount of substance
- Ampere: Electrical current
- Kelvin: Temperature
- Candela: Luminous intensity
This transition, which took effect in May 2019, signifies the end of using physical objects to define measurement units and marks a new chapter in the history of science.
Why Redefine the Kilogram? The Problem with Physical Standards
Historically, weights and measures were inconsistent and varied locally. The metric system and the kilogram emerged from efforts to standardize these measurements in the late 18th century. In 1791, the French National Assembly aimed to create a system “for all time, for all people”. The new unit of length, the meter, was defined as one ten-millionth of the distance from the North Pole to the equator along the meridian passing through Paris. The liter, used to measure capacity, was defined as one-thousandth of a cubic meter. The kilogram was defined as the mass of one liter of water.
The Treaty of the Meter in 1875 harmonized measurement systems across the world, and the IPK was created fourteen years later. But problems emerged over time.
By 1989, scientists discovered that the IPK was 50 micrograms lighter than its replicas distributed globally. This discrepancy, though seemingly small, highlighted the limitations of relying on a physical artifact. Since the IPK is the kilogram by definition, any change to its mass alters the definition itself. This raised concerns about the accuracy and reliability of measurements for advanced research and technological applications.
The New Definition: Constants of Nature
The new definition of the kilogram relies on three fundamental constants:
- Speed of light: A fundamental constant in physics.
- Cesium atom’s natural microwave radiation: Used to define the second.
- Planck constant: Describes the quantization of energy at the atomic level.
The old definition related the kilogram to the force exerted by gravity. The revised definition uses an electromagnetic measurement tied to the Planck constant and based on electrical current and voltage.
The Kibble Balance: A Tool for the New Kilogram
The redefinition utilizes an instrument called a Kibble balance (formerly known as a watt balance), named after its inventor Bryan Kibble. This device generates an electric current in a coil to produce a magnetic field strong enough to offset a mass of one kilogram. By precisely measuring the electrical current and voltage, the Kibble balance links the kilogram to the Planck constant.
The Impact of the Redefinition
Martin Milton of the International Bureau of Weights and Measures (BIPM) described the redefinition as a “landmark moment in scientific progress”. Barry Inglis, Director of BIPM, added that the new definition frees us from the limitations of physical objects and paves the way for greater accuracy and scientific advancement. The move ensures a stable foundation for scientific understanding, technological development, and addressing societal challenges.
