Entropy
Entropy measures how many microscopic arrangements of a system are consistent with what you can observe about it from outside. Clausius introduced the quantity in the 1860s to track the heat that becomes unavailable for doing work, and Boltzmann later showed it counts arrangements. The second law of thermodynamics says the total entropy of an isolated system never decreases, which is why heat flows from hot to cold and never spontaneously back. Entropy is not disorder in any everyday sense, and treating it as a synonym for mess gives the wrong answers.
Why it matters
It is the only fundamental quantity in physics that distinguishes the future from the past, since almost every other law works identically run backwards.
Where it comes up
Read about Entropy on Wikipedia
Sources
- Entropy English Wikipedia
- Second law of thermodynamics English Wikipedia
- Philosophy of Statistical Mechanics Stanford Encyclopedia of Philosophy
