An Etymological Dictionary of Astronomy and Astrophysics
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In → statistical thermodynamics, a probability equation relating the → entropy S of an → ideal gas to the quantity Ω, which is the number of → microstates corresponding to a given → macrostate: S = k. ln Ω. Same as → Boltzmann's relation.

→ Boltzmann's constant; → entropy; → formula.

1) A measure of the energy that is not available for work during a → thermodynamic process. It is defined by dS = dQ/T, where dS is the differential change in entropy, dQ is the differential amount of heat introduced to the system in a → reversible process, and T the → absolute temperature of the system. Entropy remains constant during → reversible processes and increases during → irreversible processes without ever decreasing. According to the → second law of thermodynamics, an → isolated system evolves toward a state of maximum entropy. See also → Maxwell's demon.
2) Statistical physics: A measure of → disorder of the configuration of → microstates which make up a → macrostate. → Boltzmann's relation, → Boltzmann's entropy formula. Highly disordered systems have a large entropy; highly ordered systems have low entropy.
3) Math.: A measure of information content. → information entropy.

From Ger. Entropie, coined 1865 by physicist Rudolf Clausius (1822-1888) from Gk. entropia "a turning toward," from en- "in" + trope "a turning, change," related to tropos "a turn, way, manner," from tropein "to turn," from PIE base *trep- "to turn" (cf. L. trepit "he turns").

Dargâšt, from dar "in" + gâšt present stem of gâštan "to cause to revolve, to turn," transitive of gaštan, variant gardidan "to turn, to change" (Mid.Pers. vartitan; Av. varət- "to turn, revolve;" cf. Skt. vartati; L. vertere; O.H.G. werden "to become;" PIE base *wer- "to turn, bend").

The measure of information, which is usually expressed by the average number of bits needed for storage or communication. In other words, the degree to which the values of a → random variable X are dispersed. If the → probability density function of X is P(x), the entropy is defined by: H(X) = -Σ P(x) log P(x). Also called → Shannon entropy.

→ information; → entropy.

A deconvolution algorithm which functions by minimizing a smoothness function in an image. The MEM seeks to extract as much information from a measurement as is justified by the data's signal-to-noise ratio.

→ maximum; → entropy; → method.

→ information entropy.

Claude Elwood Shannon (1916-2001), an American mathematician and pioneer of → information theory; → entropy.