ELI5-What is entropy?
In: 1763
Entropy is a concept that initially was just something physicists cooked up for 2 reasons:
1. To have some benchmark for heat engine efficiency. (See Ideal/Carnot Heat Engines)
2. To definitively falsify the possibility of machines that could be in perpetual motion. (Lots of charlatans would claim they invented free energy systems and cheat people out of their money)
It was then later crowned as the “2nd law of thermodynamics” (i.e we recognized it as fundamental as energy conservation) and we have been noticing that although entropy (just like energy conservation) is a classical description, in some form it appears all over nature. (There is a very recent paper from L. Susskind et al., where they show that even complex systems could theoretically exhibit something analogous to entropy)
Entropy has many definitions, but the most common you’ll see is: the quantitative measure of a systems order / disorder and the most common definition for order/disorder is the number of states available on the microscopic level for a given macroscopic state. The less microscopic states available, the lower the entropy and as these states increase the entropy increases until it hits a maximum. We define this maximum entropy as thermal equilibrium (where things get very boring).
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