Is entropy truly a measure of disorder or is it something else entirely?

[LEELA PRATHAP KUMAR]

How much correct to say entropy is always measure of disorder
Is disorder always related to entropy

 

[phinds]

How much correct to say entropy is always measure of disorder
Is disorder always related to entropy

That topic is often discussed here. I suggest a forum search (always a good idea on basic questions). A good place to start is with the links at the bottom of the page (the forum does a brief search for you, based on your subject line)

 

[DrClaude]

How much correct to say entropy is always measure of disorder
Is disorder always related to entropy

Certainly not "always." How do you define disorder?

 

[Dr. Courtney]

Most scientific areas use common words and give them a more narrow technical meaning. There are lots of common uses and broader understandings of physics words that may differ with the strict physics definitions: energy, force, momentum, revolution, period, etc. Disorder is one of those words also.

Entropy is one measure of disorder. There are other uses and non-technical understandings of the word disorder which do not exactly correspond with the physics definition of entropy.

 

[vanhees71]

Well, this you often read, but to be honest I never understood it. Also the tradiational definition of introducing temperature as an integrating factor to make $\delta Q=T \mathrm{d} S$, which introduces entropy into the game, didn't help me much.

What I find most convincing is to use Shannon's reinterpretation of entropy as a measure for missing information (relative to a prior state defining complete information) for a given probability distribution, leading to the Shannon-Jaynes-von-Neumann entropy in (quantum-)statistical physics. The idea of course goes back to Szilard's famous paper on the Maxwell demon of 1928.

A good introduction to statistical physics (both classical and quantum) using the information-theoretical approach is

A. Katz, Principles of Statistical Mechanics, W. H. Freeman and Company, San Francisco and London, 1967.

I also think that with the recent experimental work on "quantum Maxwell demon" it's almost empirically proven that the information-theoretical interpretation of entropy is the most comprehensive one.