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# Ensembles in thermostatistics

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Thermostatistical ensembles are collectives whose average behaviour reflects the actual behaviour of a physical system. Ensembles are fundamental in the study of statistical mechanics. From one point of view they can be seen as mathematical labour-saving devices, given the intractability of following the trajectory of a macroscopic system in its journey through phase space. For example, celestial mechanics is hard enough, however, the Avogadro constant is on a par with the total number of stars in the universe. Clearly one needs a device that adequately represents the system of interest. Ensembles provide a link between the expectation value of physical observables (such as temperature, internal energy etc.) and the individual motions of the plethora of constituent atoms and/or molecules.

## History

The concept of thermostatistical ensembles was introduced by Josiah Willard Gibbs (Ref. 1-3). Albert Einstein also made contributions (Refs. 4 and 5).