How was the Boltzmann constant used in the 2019 SI revision?

In the 2019 revision of the International System of Units, the Boltzmann constant was designated as one of seven defining constants. By fixing its numerical value to an exact figure, the revision redefined the kelvin, replacing the old definition based on the triple point of water. This change allowed the entire system of units to be derived directly from universal physical constants rather than physical artefacts or laboratory conditions.

What we know

• The Boltzmann constant is one of the seven defining constants in the 2019 revision of the SI, assigned an exact finite decimal value in SI units. [3]
• The Boltzmann constant was given the exact value of 1.380649×10^−23 joules per kelvin to define the SI unit kelvin. [3]
• The 2019 redefinition replaced the previous definition of the kelvin, which had relied on the difficult-to-realise triple point of water. [2]
• Because both the Avogadro constant and the Boltzmann constant now have exact numerical values, the SI value of the molar gas constant has also become exact. [5]

Where to go next

• the bigger pictureWhat are the seven defining constants of the revised SI?
• compared withHow did the triple point of water previously define the kelvin?
• how it workedWhat role does the Avogadro constant play in the molar gas constant?
• whenWhen did the 26th CGPM approve the SI unit redefinitions?
• what followedWhat criticisms arose regarding the 2019 SI revision?
• an unexpected connectionHow does the molar gas constant link to Ludwig Boltzmann's constant?The molar gas constant is mathematically the product of the Avogadro constant and the Boltzmann constant, meaning fixing Boltzmann's constant directly locked the gas constant into an exact value.

Sources

• [1]redefinition of SI base units (Wikidata Q3076763) · CC0 1.0
• [2]2019 revision of the SI · CC BY-SA 4.0
• [3]Boltzmann constant · CC BY-SA 4.0
• [4]SI base unit · CC BY-SA 4.0
• [5]Gas constant · CC BY-SA 4.0