Major Revision of the International System of Units (SI) - May 20, 2019

On May 20, 2019, the International System of Units (SI) underwent a significant revision, marking a pivotal moment in the history of measurement science. This revision redefined four of the seven base units: the kilogram, ampere, kelvin, and mole. These changes were based on fundamental physical constants, ensuring greater precision and stability in scientific measurements.

Background

The SI system, established in 1960, is the most widely used system of measurement worldwide. It is essential for scientific research, industry, and commerce. Prior to the 2019 revision, some of the SI units were defined by physical artifacts or empirical properties, which posed limitations in terms of precision and reproducibility.

Key Changes

Kilogram

• Old Definition: The kilogram was defined by the mass of the International Prototype of the Kilogram (IPK), a platinum-iridium alloy cylinder stored in France.
• New Definition: The kilogram is now defined by the Planck constant ((h)), fixed at (6.62607015 \times 10^{-34}) joule seconds. This change allows the kilogram to be realized using a Kibble balance, which measures the Planck constant in terms of electrical measurements.

Ampere

• Old Definition: The ampere was defined by the force between two parallel conductors carrying an electric current.
• New Definition: The ampere is now defined by the elementary charge ((e)), fixed at (1.602176634 \times 10^{-19}) coulombs. This provides a more stable and precise definition based on the charge of a single electron.

Kelvin

• Old Definition: The kelvin was defined by the triple point of water.
• New Definition: The kelvin is now defined by the Boltzmann constant ((k)), fixed at (1.380649 \times 10^{-23}) joules per kelvin. This links temperature to energy at the molecular level, independent of a specific substance.

Mole

• Old Definition: The mole was defined by the number of atoms in 12 grams of carbon-12.
• New Definition: The mole is now defined by the Avogadro constant ((N_A)), fixed at (6.02214076 \times 10^{23}) entities per mole. This change provides a universal definition based on a fixed number of entities.

Significance

The 2019 revision of the SI units represents a shift from definitions based on physical artifacts and empirical properties to those based on unchanging fundamental constants of nature. This ensures that the units are universally accessible and can be realized with high precision anywhere in the world. The redefinition enhances the accuracy of measurements in scientific research and technological development, supporting advancements in fields such as quantum physics, chemistry, and materials science.

Conclusion

The redefinition of the SI units on May 20, 2019, was a landmark event that reflects the progress in our understanding of fundamental physics. By anchoring the units to constants of nature, the scientific community has ensured that the SI system remains robust and relevant for future generations.