Note 9 — Chapter IV · THE RUNNING-DOWN OF THE UNIVERSE

The law is so much disguised in the above enunciation that I must explain to the advanced reader that I am referring to “the Principle of Detailed Balancing.” This principle asserts that to every type of process (however minutely particularised) there is a converse process, and in thermodynamical equilibrium direct and converse processes occur with equal frequency. Thus every statistical enumeration of the processes is unaltered by reversing the time-direction, i.e. interchanging direct and converse processes. Hence there can be no statistical criterion for a direction of time when there is thermodynamical equilibrium, i.e. when entropy is steady and ceases to indicate time’s arrow.