More precisely, in statistical mechanics, the Gibbs measure exp(− H / k B T) describes the relative probability of the system to be in any given state at temperature T, where H is a function describing the energy of each state and k B is the Boltzmann constant. In quantum mechanics, the transformation exp(− itH / ħ) describes time evolution, where H is an operator describing the energy (the Hamiltonian) and ħ is the reduced Planck constant. The former expression resembles the latter when we replace it / ħ with 1/ k B T, and this replacement is called Wick rotation.