Anniversary of the SI Units Redefinition
By Max Lin '24
In 2019, four of the seven SI base units specified in the International System of Quantities were redefined in terms of natural physical constants, rather than human measurements such as the kilogram. A physical constant is both universal in nature and has constant value in time. Effective May 20th, 2019, which was also the 144th anniversary of the Meter Convention, the kilogram, ampere, kelvin, and mole are now defined by setting exact numerical values, when expressed in SI units, for the Planck constant (h), the elementary electric charge (e), the Boltzmann constant (k_B), and the Avogadro constant (N_A), respectively.
In November 2018, the 26th General Conference on Weights and Measures (CGPM) unanimously approved these changes, which the International Committee for Weights and Measures (CIPM) had proposed earlier that year after determining that previously agreed conditions for the change had been met.  These conditions were satisfied by a series of experiments that measured the constants to high accuracy compared to the old SI definitions, and was made possible through decades of research.
The four new definitions sought to improve the SI without changing the value of any units, ensuring continuity with existing measurements. The definitions of some units were defined before by measurements that are difficult to precisely realize in a laboratory, such as the kelvin, which was defined in terms of the triple point of water. Now, with the 2019 redefinition, the SI became entirely derivable from natural phenomena with most units being based on fundamental physical constants.
An example of one of the redefinitions is the Kelvin one. Rather than using the triple point of water to fix the temperature scale, the new definition uses the energy equivalent as given by Boltzmann's equation.
Previous definition: The kelvin, unit of thermodynamic temperature, is 1/273.16 of the thermodynamic temperature of the triple point of water.
2019 definition: The kelvin, symbol K, is the SI unit of thermodynamic temperature. It is defined by taking the fixed numerical value of the Boltzmann constant k to be 1.380649×10^{−23} when expressed in the unit J⋅K^{−1}, which is equal to kg⋅m^2⋅s^{−2}⋅K^{−1}, where the kilogram, meter and second are defined in terms of h, c and Δν_{Cs}.
The kelvin may be expressed directly in terms of the defining constants as: