November 16, 2018
The kilogram was redefined in terms of the Planck constant, changing the way this unit of mass is measured in the International System of Units (SI).
Versailles, France | International Bureau of Weights and Measures
On November 16, 2018, a significant milestone in the field of metrology was achieved when the General Conference on Weights and Measures (CGPM) voted to redefine the kilogram. This decision marked a pivotal shift in how the unit of mass is measured within the International System of Units (SI).
Prior to this redefinition, the kilogram was defined by a physical object: the International Prototype of the Kilogram (IPK), a platinum-iridium alloy cylinder stored at the International Bureau of Weights and Measures (BIPM) in Sèvres, France. Since 1889, this artifact served as the standard for the kilogram. However, reliance on a physical object posed challenges, including potential changes in mass due to surface contamination or material degradation over time.
The 2018 redefinition of the kilogram was based on fixing the numerical value of the Planck constant (h), a fundamental constant in quantum mechanics. The Planck constant was set at exactly 6.62607015 × 10^-34 joule-seconds. By anchoring the kilogram to this constant, the unit is now defined through fundamental physical constants rather than a physical object.
The Planck constant is integral to the equation E = hf, which relates the energy (E) of a photon to its frequency (f) with h as the proportionality constant. By using the Planck constant, the kilogram is now linked to the second and the meter, which are defined by the speed of light and the cesium frequency, respectively.
The redefinition officially took effect on May 20, 2019, World Metrology Day. This change ensures that the kilogram is stable, universally accessible, and can be realized in laboratories worldwide using advanced technologies such as the Kibble balance, which measures the Planck constant with high precision.
This redefinition was part of a broader effort to base all SI units on fundamental constants, enhancing the precision and reliability of scientific measurements. It reflects the ongoing evolution of metrology and the pursuit of greater accuracy in scientific research and industry.
The redefinition of the kilogram represents a landmark in scientific history, transitioning from a tangible artifact to a constant of nature. This change underscores the importance of precision in measurement and the role of fundamental constants in defining our understanding of the physical world.
Source: www.nature.com