On May 20th, the 26th General Conference on Weights and Measures instituted a new standard for the kilogram mass. A metal block kept in a hermetically sealed vault in Paris had been the mass standard for 130 years. New standards were also introduced for the unit of current (Ampere), temperature (Kelvin), and the mole.
It used to be the case that standards for the basic units used in science had been defined in terms of macroscopic objects like this and thus could be easily understood. But the need for increasingly precise and unvarying standards meant that these were no longer suitable and standards have increasingly shifted to using the fundamental constants of nature and getting from those to the familiar quantities involves quite a long chain of reasoning. The kilogram is the last remaining physical object to be so displaced.
This article recounts the history of those gradual changes in the standards.
Starting Monday, the kilogram will be redefined not by another object, but by a fundamental property of nature known as Planck’s constant. Like the speed of light, the value of Planck’s constant cannot fluctuate — it is built with exquisite precision into the very fabric of the universe.
“Unlike a physical object, a fundamental constant doesn’t change,” said Stephan Schlamminger, a physicist at the National Institute of Standards and Technology (NIST) in Gaithersburg, Md. “Now a kilogram will have the same mass whether you are on Earth, on Mars or in the Andromeda galaxy.”
A similar philosophy of using fixed constants underlies the new definitions of the mole, the kelvin and the ampere. After Monday, the mole will be defined by the value of Avogadro’s constant, the kelvin by the value of the Boltzmann constant (which relates temperature to energy), and the ampere by the value of the elementary charge, the smallest observable charge in the universe.
Understanding how the new mass standard is used is not easy as can be seen from this article. As another article explains, the new kilogram standard depends upon the Planck constant because the value of that constant influences the value of Avogadro’s number that in turn determines the number of atoms in a given mass of an element. Arriving at agreement on the standard required a convergence of values obtained by different methods. This article gives a more detailed explanation. For yet more details, see here.
Here’s a video explainer.
The figure from this article summarizes the state of affairs of the standards just before these new standards were decided upon.