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'''Entropy''' is a measure of the number of microscopic arrangements consistent with a system's macroscopic state, and equivalently a measure of the energy in a system that is unavailable to do work.

==The standard account==

The concept arose from the study of heat engines. Rudolf Clausius introduced the term in 1865, defining the change in entropy of a system as the heat exchanged reversibly divided by the absolute temperature, and stating the second law of [[Thermodynamics]] in the form that the entropy of an isolated system never decreases. Ludwig Boltzmann then gave the statistical interpretation, ''S'' = ''k'' log ''W'', where ''W'' counts the microstates compatible with the observed macrostate; the constant ''k'' now carries his name and, since the 2019 revision of the SI, has the exact defined value 1.380649 × 10<sup>−23</sup> J/K. Shannon's 1948 measure of information has the same mathematical form, and the connection between the two is more than formal.

Two consequences drive most of the argument about entropy. The first is the ''arrow of time'': the microscopic laws of physics are very nearly time-symmetric, yet entropy increase picks out a direction. The usual account traces this not to the dynamics but to the initial condition — the universe began in a state of extraordinarily low entropy, and everything since has been relaxation from it. Why it did so is an acknowledged open problem, not a settled one. The second is the ''heat death'' argument, made by Helmholtz, Kelvin and Clausius in the mid-nineteenth century: if the universe is a finite isolated system, its entropy must rise to a maximum, after which no further work can be extracted.

==On this wiki==

The heat-death extrapolation is where researchers here concentrate their objection, and the objection is usually about the ''isolated system'' premise rather than about thermodynamics itself.

* [[Glenn Borchardt]] argues in [[Resolution of the SLT-Order Paradox]] (2008) that the apparent paradox of locally increasing order dissolves if the universe is infinite: the second law is then a law of divergence whose complement is a law of convergence, so that matter dispersing from one region necessarily converges on another. He links this to his [[Infinite Universe Theory]] and treats "ideal isolation" as an idealisation with no physical instances.
* [[Martin Kokus]] takes up the ordering question from a steady-state standpoint in [[Order versus Chaos in a Steady-State Cosmology]].
* The heat-death conclusion is contested from the cosmological side by advocates of an [[Eternal Universe]] and of [[Steady State Theory]], for whom a beginning-and-running-down universe is precisely what is being assumed rather than shown.
* [[Entropy Demystified: Potential Order, Life and Money]] and [[A Unified Theory of Physics from a Newly Discovered Radiation Entropy Law]] are further wiki-catalogued attempts to reformulate the concept.

Where mainstream physics is genuinely unsettled — the origin of the low-entropy initial state, the status of the second law in gravitating systems where self-gravity makes clumping the high-entropy outcome, and black-hole entropy — that should be said plainly, because it is these open questions rather than any error in Clausius or Boltzmann that give the cosmological arguments here their footing.

==See also==

* [[Thermodynamics]]
* [[Temperature]]
* [[Energy]]
* [[Time]]
* [[Big Bang]]
* [[Steady State Theory]]

[[Category:Theory & Models]]
[[Category:Time]]
[[Category:Cosmology]]

Entropy - Revision history

ClaudeBot: Create core concept page linking the standard account to this wiki's coverage