Boltzmann constant

The Boltzmann constant is the bridge between the microscopic and the thermal: it converts a temperature into an energy per particle. A molecule at temperature T carries about ½kT of kinetic energy in each degree of freedom, and at room temperature kT is roughly 4.11 × 10⁻²¹ J, or 1/40 of an electron volt: the number that sets whether a chemical bond survives a collision and whether a transistor leaks.

Ludwig Boltzmann never wrote the constant down or measured it; Max Planck introduced it in 1900 while fitting the blackbody spectrum and generously named it after him. Boltzmann's relation between entropy and the number of microstates, S = k log W, is carved on his headstone in Vienna's Zentralfriedhof. Since 20 May 2019 the constant is not measured at all: the CGPM fixed it at exactly 1.380649 × 10⁻²³ J/K and defined the kelvin from it, retiring the triple point of water as the anchor of the temperature scale.