More than ten years ago, I have written a blogpost titled “Confusion about Entropy” and brought readers’ attention to the fact that even scientists are confused about entropy. That’s because there are different kinds of entropy: 1) entropy as dispersal of energy 2) entropy as disorder 3) entropy as uncertainty 4) entropy as randomness 5) entropy as information 6) so called “relative entropy”.

In his review of entropy measures in various disciplines Marko Popovic [1] discusses the conceptual development of the entropy concept.

“**Abstract**: Entropy concept was introduced by Clausius 160 years ago, and has been continually enriched, developed and interpreted by the researchers in many different scientific disciplines ever since. Thermodynamics and other scientific disciplines face several simple but crucial questions concerning the entropy concept. Thus, it is frequently possible to notice a misuse of entropy. Sometimes unbelievable confusion in the literature is summarized by **von Neumann’s sentence: “No one knows what entropy really is.”** Four major questions stand before thermodynamics. (1) How many kinds of entropy are there? (2) What is the physical meaning of entropy? (3) Is entropy a subjective or an objective property? (4) Is entropy in any way related to information? This paper attempts to describe the roots, the conceptual history of this important concept and to describe the path of its development and application in different scientific disciplines. Through this we attempt to give some possible answers to the questions mentioned above.” *[1]*

Not mentioned in my blogpost or [1] is the subject of “Black Hole entropy”. Needless to say, there are several interpretations of Black Hole entropy. One could start with the Wikipedia article and continue with the Scholarpedia article to learn about this subject. There is also a very readable article [2] by Natalie Wolchover at Quanta Magazine.

You should also check out how John Baez explains different kinds of entropy here.

[1] Marko Popovic, “Researchers in an Entropy Wonderland: A Review of the Entropy Concept“

[2] Natalie Wolchover, “Black Hole Paradoxes Reveal a Fundamental Link Between Energy and Order“

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