Entropy

The entropy, S, is defined by

$S=k_{B}\ln \Omega$

where $k_{B}$ is the Boltzmann constant and $\Omega$ (sometimes written as $W$) is the number of microscopic configurations that result in the observed macroscopic description of the thermodynamic system.

Arrow of time

See Refs. 1 and 3.

See also:

• Entropy of a glass
• Shannon entropy
• Tsallis entropy

References

1. Milan M. Ćirković "The Thermodynamical Arrow of Time: Reinterpreting the Boltzmann–Schuetz Argument", Foundations of Physics 33 pp. 467-490 (2003)
2. William G. Hoover "Entropy for Small Classical Crystals", Journal of Chemical Physics 49 pp. 1981-1982 (1968)
3. Joel L. Lebowitz "Boltzmann's Entropy and Time's Arrow", Physics Today 46 pp. 32-38 (1993)