Units in the changing times – on the occasion of the 2019 redefinition of the SI base units

Units in the changing times Ralf Greve Institute of Low Temperature Science, Hokkaido University, Sapporo, Japan Salvador Dalí: The Soft Watches On the occasion of the 2019 redefinition of the SI base units Ralf Greve: Units in the changing times 2/17 “These Britons are crazy” (~ 50 BCE) Goscinny/Uderzo (1966): Asterix in Britain Ralf Greve: Units in the changing times 3/17 Mars Climate Orbiter lost in translation (1999) Newspaper cartoon. Source: Slideplayer.com Ralf Greve: Units in the changing times 4/17 Metric system introduced in Revolutionary France (1790s) Metre, for length 1/10,000,000 of the distance between the North Pole and the Equator through Paris Are (100 m2), for area of land Stère (1 m3), for volume of firewood Litre (1 dm3), for volumes of liquid Gram, for mass Mass of one cubic centimetre of water Franc, for currency Six new decimal units: Decimal multiples by Greek prefixes (kilo-, centi-, milli-, etc.) Ralf Greve: Units in the changing times 5/17 SI base units Second (s) for time Metre (m) for length Kilogram (kg) for mass Ampere (A) for electric current Kelvin (K) for temperature Mole (mol) for amount of substance (since 1971) Candela (cd) for luminous intensity SI (Système International d’unités): Modern form of the metric system, established in 1960 Ralf Greve: Units in the changing times 6/17 Second Historically defined as 1/86400 of a mean solar day. Ephemeris second (1956): Fraction 1/31,556,925.9747 of the tropical year for 1900 January 0 at 12 hours ephemeris time. Atomic second (1967–today): Duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom (at a temperature of 0 K). Can be interpreted as fixing the value of a physical constant: Caesium frequency ∆νCs = 9,192,631,770 s–1. Ralf Greve: Units in the changing times 7/17 Metre Historically: 1/10,000,000 of the distance ... Wavelength definition (1960): 1,650,763.73 wavelengths of the orange-red emission line in the electromagnetic spectrum of the krypton-86 atom in a vacuum. Fixed value of the speed of light (1983–today): c= 299,792,458 m s–1. International prototype metre (1889, 1927): Distance, at 0°C, between the two lines on the standard bar of platinum–iridium kept at the International Bureau of Weights and Measures (BIPM) in Paris. Credit: BIPM Ralf Greve: Units in the changing times 8/17 Kilogram Historically: Mass of one litre of distilled water at 0°C. Fixed value of the Planck constant (2019): h= 6.62607015×10−34 J s [kg m2s−1]. International prototype kilogram (1889): Mass of the platinum–iridium cylinder kept at the International Bureau of Weights and Measures in Paris. Credit: AFP/Getty Images Energy and equivalent mass of a photon: Ralf Greve: Units in the changing times 9/17 Ampere Credit: User:Danmichaelo/ Wikimedia Commons Force between two parallel, current-carrying wires of infinite length 1 metre apart in a vacuum (1946): The constant current that will produce an attractive force of 2×10–7 N per metre of length. Fixed value of the elementary charge (2019): e= 1.602176634×10−19 C [A s]. Ralf Greve: Units in the changing times 16/17 Some more thoughts Using physical constants for defining the base units could set them all to 1! 1 time unit = 1/(caesium frequency ∆νCs)≈ 1.088×10–10 s 1 length unit = c× 1 time unit ≈0.0326 m 1 mass unit = (h∆νCs)/c2≈6.777×10–41 kg etc… Very common in theoretical physics! Thank you THANK YOU Credit: wallpapercave.com