What Is 2019 International System of Units

Overview

The International System of Units (SI) underwent a historic revision on May 20, 2019, marking the most significant transformation in measurement science in over a century. For the first time, all seven base units are defined by fixed numerical values of fundamental constants of nature, rather than physical artifacts or idealized experiments.

This redefinition enhances precision, universality, and long-term stability, enabling scientific and industrial progress across disciplines. By anchoring units to unchanging constants, the SI system becomes accessible anywhere in the universe, provided the necessary technology exists.

• Base units redefined: The second, meter, kilogram, ampere, kelvin, mole, and candela are now based on constants like the speed of light and Planck’s constant, eliminating reliance on physical objects.
• Kilogram shift: The International Prototype Kilogram (IPK), a platinum-iridium cylinder stored in France since 1889, was retired in favor of a definition using Planck’s constant.
• Universal access: Any laboratory with the proper equipment can realize the kilogram or ampere without needing access to a central artifact or standard.
• Improved accuracy: Quantum-based definitions reduce measurement uncertainty, supporting advancements in nanotechnology, quantum computing, and precision engineering.
• Global collaboration: The revision was approved by 60 countries at the General Conference on Weights and Measures (CGPM) in November 2018, reflecting international consensus.

How It Works

The 2019 SI redefinition replaces physical standards with invariant constants, linking units to reproducible quantum phenomena. Each base unit is now derived from a fixed value of a constant, ensuring precision and consistency across time and space.

• Second (s): Defined by the radiation frequency of cesium-133, specifically 9,192,631,770 cycles per second, enabling atomic clocks to maintain time with extreme accuracy.
• Meter (m): Based on the speed of light in vacuum (299,792,458 m/s), so one meter equals the distance light travels in 1/299,792,458 seconds.
• Kilogram (kg): Tied to the Planck constant (6.62607015×10⁻³⁴ J⋅s), realized through Kibble balances or silicon sphere experiments measuring Avogadro’s constant.
• Ampere (A): Defined using the elementary charge (1.602176634×10⁻¹⁹ C), allowing current to be measured via single-electron transport devices instead of force between wires.
• Kelvin (K): Based on the Boltzmann constant (1.380649×10⁻²³ J/K), enabling temperature measurement through acoustic gas thermometry or Johnson noise methods.
• Mole (mol): Now defined as exactly 6.02214076×10²³ elementary entities, fixing Avogadro’s number rather than relating to carbon-12 mass.

Comparison at a Glance

The following table compares old and new definitions of the SI base units:

This shift ensures definitions remain stable and reproducible regardless of location or time. Laboratories worldwide can now realize units independently, reducing dependency on centralized standards and supporting innovation in fields like quantum metrology and space exploration.

Why It Matters

The 2019 SI revision is more than a technical update—it’s a foundational shift enabling future scientific discovery and global equity in measurement. By removing reliance on physical artifacts, the system becomes more resilient and universally accessible.

• Scientific progress: Enables more precise experiments in particle physics, cosmology, and quantum mechanics by reducing measurement uncertainty.
• Industrial innovation: Supports advancements in semiconductor manufacturing, where nanometer-scale precision depends on stable unit definitions.
• Medical applications: Improves accuracy in radiation dosing, drug development, and diagnostic imaging technologies.
• Global trade: Ensures consistency in measurements across borders, reducing disputes in international commerce and compliance.
• Educational impact: Encourages teaching of measurement as a dynamic, evolving science rooted in fundamental physics.
• Long-term stability: Constants do not degrade, ensuring definitions remain valid for centuries, unlike physical prototypes that can change over time.

The 2019 SI redefinition marks a triumph of international cooperation and scientific foresight. As technology advances, these definitions will support discoveries we can only begin to imagine.