Absolute zero

Absolute zero is where a substance holds no thermal energy available to do work — not where all motion stops, since quantum zero-point motion remains. Its Celsius and Fahrenheit values are exact, because the Celsius scale is defined as t/°C = T/K − 273.15 and Fahrenheit follows from Celsius; −273.15 °C and −459.67 °F are definitions, not measurements.

Guillaume Amontons glimpsed it around 1702 by extrapolating the pressure of a gas held at constant volume down to zero, and got roughly −240 °C. William Thomson, later Lord Kelvin, put it on a firm footing in 1848 by defining a temperature scale from Carnot's engine efficiency rather than from any particular thermometric fluid. The third law of thermodynamics says you can approach it but never reach it in a finite number of steps; laboratories have got to a few hundred picokelvin, and the coldest thing in the observable universe is a dilution refrigerator, not deep space.