Why the world's most important measurements no longer rely on physical objects
For decades, the standard for mass was a physical cylinder of platinum-iridium kept in a French vault. But what happens to global science when your primary reference can be lost, damaged, or changed? The solution was a radical shift from physical artefacts to the immutable laws of physics.
The International System of Units (SI) is the world's most widely used measurement system, holding official status in nearly every nation. While it is the foundation for science, technology, and global trade, its modern architecture is far more sophisticated than a simple collection of scales and rulers. Historically, the system grew from the need to resolve the chaos of overlapping units, such as the inconsistencies between electrostatic and electromagnetic systems. This led to the establishment of the General Conference on Weights and Measures (CGPM) via the Metre Convention of 1875, eventually resulting in the formal publication of the SI in 1960.
The true revolution in the SI lies in its move away from 'artefacts'—physical objects that serve as standards. Relying on a physical prototype introduces uncertainties that cannot be reduced by better technology. To solve this, metrologists redefined the system's seven base units—the second, metre, kilogram, ampere, kelvin, mole, and candela—using seven defining constants of nature. By fixing the exact numerical values of constants like the speed of light (c) and the Planck constant (h), the units are now anchored to the fundamental fabric of the universe.
This shift ensures that as our measurement technology improves, we can refine our 'mises en pratique' (practical implementations) without needing to redefine the units themselves. For example, the kilogram is now defined by the Planck constant (6.62607015 × 10⁻³4 J⋅s). This means the definition remains stable even as our ability to measure the constant becomes more precise. While the system is incredibly robust, it remains a matter of convention; the choice of which quantities serve as base units is a human decision, even if the constants they rely on are universal.
Source: International System of Units
