From Physical Artifacts to Invariable Constants
For over a century, the global standard for mass was the International Prototype Kilogram (IPK)—a platinum-iridium cylinder housed beneath vacuum jars in Sèvres, France. Over decades, minute atomic contamination altered its mass by micrograms.
In 2019, the General Conference on Weights and Measures finalized a historic revision of the International System of Units (SI): all seven base units are now defined exclusively in terms of unchanging fundamental physical constants.
The Seven Fundamental Base Dimensions
The SI system anchors human measurement across seven independent physical dimensions: length (meter via speed of light c), mass (kilogram via Planck’s constant h), time (second via cesium-133 hyperfine transition frequency), electric current (ampere via elementary charge e), thermodynamic temperature (kelvin via Boltzmann constant k), amount of substance (mole via Avogadro constant NA), and luminous intensity (candela).
Every other scientific measurement—from pressure in pascals to force in newtons—is dimensionally derived from these invariant constants.