kelvin
Temperatureexact by definition
The kelvin is the SI base unit of thermodynamic temperature, defined since 2019 by fixing the Boltzmann constant at exactly 1.380649e-23 joules per kelvin. It shares its size with the degree Celsius, so a change of one kelvin and a change of one degree Celsius are the same interval, but the kelvin scale starts at absolute zero rather than near the freezing point of water. As the base unit it needs no conversion factor, and the factor of 1 is exact by definition.
Watch out: Write 300 K, not 300 °K. The kelvin is not a degree and takes no degree sign. It is also always positive on any real body: temperatures below absolute zero do not exist in the ordinary sense.
| 1 K | -272.15 °C |
Named for William Thomson, Lord Kelvin, who argued in 1848 for an absolute scale whose zero is the point at which a body has no thermal energy left to give up. The unit was called the degree Kelvin until 1967, when the General Conference on Weights and Measures dropped the word degree and the symbol became a bare K.
About the kelvin
Every other temperature scale in common use starts somewhere convenient for humans. Fahrenheit starts near the coldest brine a Dutch instrument maker could mix, Celsius near the freezing point of water. The kelvin starts at the one temperature that is not a matter of taste: absolute zero, the point below which no body can be cooled because there is no thermal energy left to remove. That is why the gas laws, the Stefan-Boltzmann law, and every equation in statistical mechanics take kelvin and nothing else. Halving the absolute temperature of a fixed volume of gas halves its pressure, and the arithmetic only works if zero on the scale means zero energy. Try it in Celsius and you get nonsense.
Since May 2019 the kelvin has been defined by fixing a constant of nature rather than by a property of water. The Boltzmann constant \(k_B\) is now exactly \(1.380649 \ imes 10^{-23}\) J/K, which pins the kelvin to the joule through the relation between energy and temperature, \(E = k_B T\) per degree of freedom. Before 2019 the kelvin was defined as 1/273.16 of the temperature of the triple point of water, which sounds tidier but tied the world's temperature scale to the isotopic composition of one sample of water. The redefinition changed the size of the kelvin by nothing measurable, and it deliberately kept the old value of the triple point at 273.16 K, now a measured quantity rather than a defined one.
Room temperature is about 293 K, water boils near 373 K, liquid nitrogen sits at 77 K, and the cosmic microwave background is 2.7 K. The lowest temperatures ever produced in a laboratory are a few billionths of a kelvin, and they are produced by removing energy from atoms one photon at a time.