The updated constants include
the Boltzmann constant (which relates temperature to energy), and
the Planck constant (which can relate mass to electromagnetic energy),
the charge of the electron and
the Avogadro constant (the quantity that defines one mole of a substance).

“There are no dramatic changes. The Boltzmann constant is very consistent with earlier values. The temperature experts requested 8 digits for the constant and the last digit happened to be 0”.

The Planck constant has shifted downward by 15 parts per billion from its earlier value, due to new data collected since 2014. The Planck constant was determined by 2 experimental techniques, known as the Kibble balance and the Avogadro method. All of the measurements that were used for determining the new Planck value met previously agreed-upon international guidelines for levels of accuracy and consistency with one another.

The Planck constant can be used to define the kilogram, and using a fundamental constant for defining mass will solve many problems. Mass must be measured over a very large scale, from an atom to a pharmaceutical to a skyscraper. “At the low end, you currently use 1 type of physics to determine mass; at the high end, you use another type of physics”.

But the Planck constant will provide a consistent way for defining mass across all of these scales, with whatever laboratory method is used to measure mass.

The volt will change as well, since the Planck constant will also help to define it in the revised SI. A volt based purely on the fundamental constants will be very slightly smaller, ~100 parts per billion, than the current scientific realization of the volt, established in 1990. The top-level metrology labs will have to recalibrate their high-precision voltage measurements.

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