Complete SI Units Reference (2019 redefinition)

The Metre Convention (Convention du Metre) was signed in Paris on 20 May 1875 by representatives of 17 nations. The treaty created the International Bureau of Weights and Measures (BIPM) and committed signatories to a common scientific measurement system. The motivation was practical: industrial trade and scientific collaboration were being strangled by every nation maintaining its own length, mass, and time standards. The original International Prototype of the Metre (a platinum-iridium bar) and International Prototype of the Kilogram (a platinum-iridium cylinder) were manufactured by Johnson Matthey in London and deposited at the BIPM in Sevres. The convention chose 20 May as 'World Metrology Day' - a date the BIPM still celebrates annually 150 years later.

The early 20th century saw the slow accumulation of additional units. The General Conference on Weights and Measures (CGPM) - the political body created by the Metre Convention - met every 4-6 years to ratify changes. The ampere was added in 1948 to join the existing metre, kilogram, and second, creating the "MKSA" system. The kelvin (then called "degree Kelvin") was added in 1954 along with the candela for luminous intensity. By the late 1950s scientists had six base units but no unified name for the system. At the 11th CGPM in October 1960 the assembled delegates voted to christen the system "Le Systeme international d'unites" - the International System of Units - and to abbreviate it SI. The name stuck and has been the official designation for 66 years.

The mole joined SI as the seventh base unit at the 14th CGPM in 1971. The CGPM chose to define it as the amount of substance containing the same number of elementary entities as 12 grams of carbon-12 - a definition that tied the mole to the kilogram. This was practical chemistry, not abstract physics: working with mass ratios was how chemists actually used the mole. Avogadro's number became a derived constant rather than a defining one. This arrangement worked for 48 years until the 2019 redefinition reversed the relationship and made NA the defining constant.

The need for the 2019 redefinition had been brewing for decades. The platinum-iridium International Prototype Kilogram - a single physical artefact in a vault in Sevres, France - was drifting by about 50 micrograms per century relative to its official copies, for reasons that were never fully understood (possibly surface contamination, possibly atomic-scale erosion during cleaning). Worse, the entire SI mass scale was anchored to a single chunk of metal: if the prototype was lost or damaged, the kilogram itself was lost. The metrology community spent over 30 years developing two independent methods - the Kibble balance (formerly called watt balance) and the X-ray crystal density method - that could realise the kilogram from fundamental constants. By 2017 both methods agreed to about 1 part in 10^8, triggering the redefinition vote.

The redefinition came into force on 20 May 2019 - exactly the 144th anniversary of the Metre Convention. The 26th CGPM had voted unanimously the previous November in Versailles. Four base units (kilogram, ampere, kelvin, mole) were redefined to depend on exact values of the Planck constant h, the elementary charge e, the Boltzmann constant k, and the Avogadro constant NA. The metre had already been defined via the speed of light c since 1983, the second via caesium since 1967, and the candela via the luminous efficacy Kcd since 1979. After 2019, every SI base unit traces back to one of seven defining constants - a complete and coherent system, finally free of physical artefacts.

The 27th CGPM in November 2022 added four new prefixes to the SI system: quetta (Q, 10^30), ronna (R, 10^27), ronto (r, 10^-27), and quecto (q, 10^-30). The motivation came partly from data: by 2030, projected global data storage would exceed yotta (10^24) and need new headroom. Partly from cosmology: the mass of the Sun is conveniently expressed as 2 quettagrams. And partly from particle physics: an electron mass of 0.911 rontograms is more pleasant to write than 9.11 x 10^-28 grams. The new prefixes were chosen to start with the same letters as the corresponding numbers (q = quattuor, r near octo) while avoiding any conflict with existing physics symbols. This was the first prefix expansion since 1991 (yotta and yocto).

Modern metrology continues to refine the realisations of SI. Caesium fountain clocks currently define the second to about 1 part in 10^16; optical lattice clocks using strontium-87 or ytterbium-171 are pushing toward 10^-18 and may redefine the second in the 2030s. The Kibble balance and X-ray crystal density method realise the kilogram to about 2 parts in 10^8. Quantum Hall and Josephson effect devices realise the ohm and volt with comparable precision. The system has reached a remarkable maturity: the SI definitions themselves are now exact by fiat, and the measurement uncertainty has migrated entirely to the realisations - the lab experiments that connect the platonic constants to the messy real world. Future SI revisions will likely focus on the second (via optical clocks) and on extending precision metrology to chemistry and biology.