Microstate (statistical mechanics)

In statistical mechanics, a microstate is a specific configuration of a system that describes the precise positions and momenta of all the individual particles or components that make up the system. Each microstate has a certain probability of occurring during the course of the system's thermal fluctuations. In a quantum system, a microstate is a pure state, which is specified by a wave function.

In contrast, a macrostate of a system is a subset of its microstates; the microstates in a macrostate often share macroscopic properties, such as temperature, pressure, volume and density. In this description, microstates appear as different possible ways the system can achieve a particular macrostate. Thus, a macrostate is an equivalence class of microstates whose differences are ignored for a given analysis.

In the thermodynamic limit, the microstates visited by a macroscopic system during its fluctuations all have the same macroscopic properties.

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