Microstate (statistical mechanics)

A diagram of the microstates and macrostates of flipping a coin twice. All microstates are equally probable, but the macrostate consisting of opposite microstates (with (H, T) equivalent to (T, H)) is twice as probable as the macrostates with equal microstates (H, H) and (T, T).

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 contrast, the macrostate of a system refers to its macroscopic properties, such as its temperature, pressure, volume and density.[1] Treatments on statistical mechanics[2][3] define a macrostate as follows: a particular set of values of energy, the number of particles, and the volume of an isolated thermodynamic system is said to specify a particular macrostate of it. In this description, microstates appear as different possible ways the system can achieve a particular macrostate.

A macrostate is characterized by a probability distribution of possible states across a certain statistical ensemble of all microstates. This distribution describes the probability of finding the system in a certain microstate. In the thermodynamic limit, the microstates visited by a macroscopic system during its fluctuations all have the same macroscopic properties.

In a quantum system, the microstate is simply the value of the wave function.[4]

  1. ^ Macrostates and Microstates Archived 2012-03-05 at the Wayback Machine
  2. ^ Reif, Frederick (1965). Fundamentals of Statistical and Thermal Physics. McGraw-Hill. pp. 66–70. ISBN 978-0-07-051800-1.
  3. ^ Pathria, R K (1965). Statistical Mechanics. Butterworth-Heinemann. p. 10. ISBN 0-7506-2469-8.
  4. ^ Eastman. "The Statistical Description of Physical Systems". Stanford. Retrieved 13 August 2023.