Macrostates and Microstates

A macrostate of a system is said to be specified when the number of particles in each phase space volume dV is specified. That is, if the phase density is specified everywhere, then the macrostate of the system has been specified. Later we shall see how the phase density can be used to specify all the physical properties of the system.

To discuss the notion of a microstate, it must be assumed that there is a perceptible difference between particles, because in a microstate, in addition to the number of particles in each volume, it makes a difference which particles are in which volumes. If the specification of individual particles can be accomplished, then it can be said that a microstate has been specified. Clearly one macrostate could consist of many microstates. For example, the number of balls on a pool table might be said to be a macrostate, whereas the specification of which balls they are would denote a specific microstate. In a similar manner, the distribution of suits of playing cards in a bridge hand might be said to represent a macrostate, but the specific cards in each suit would specify the microstate.