SIdefinitioner

SIdefinitioner refers to the definitions of the base units of the International System of Units (SI). The SI is the modern form of the metric system and is the world's most widely used system of measurement. It comprises a coherent set of seven base units: the meter for length, the kilogram for mass, the second for time, the ampere for electric current, the kelvin for thermodynamic temperature, the mole for amount of substance, and the candela for luminous intensity.

The definitions of these base units have evolved over time. Historically, they were based on physical artifacts or specific physical phenomena. However, since 2019, all SI base units are defined by fixing their numerical values in terms of fundamental physical constants. For instance, the meter is defined by fixing the speed of light in vacuum to exactly 299,792,458 meters per second. The kilogram is defined by fixing the Planck constant to 6.62607015 × 10⁻³⁴ joule-second. The second is defined by the frequency of the cesium-133 atom's ground-state hyperfine transition. The ampere is defined by fixing the elementary charge to 1.602176634 × 10⁻¹⁹ coulomb. The kelvin is defined by fixing the Boltzmann constant to 1.380649 × 10⁻²³ joule per kelvin. The mole is defined by fixing the Avogadro constant to 6.02214076 × 10²³ elementary entities per mole. Finally, the candela is defined by fixing the luminous efficacy of monochromatic radiation of frequency 540 × 10¹² Hz to 683 lumens per watt.

These definitions ensure that the SI units are realized independently of any experiment or object, providing a stable and universally accessible foundation for measurement science. The consistent redefinition of base units in terms of fundamental constants is a testament to the ongoing progress in metrology.