What is entropy?

Entropy is a measure of disorder more precisely, the number of possible arrangements a system can be in.

Simple example: imagine all air molecules in a box packed into one corner. That is one very specific arrangement. Release them and they spread through the whole box there are astronomically more ways for molecules to be spread out. The spread-out state has higher entropy.

Second Law of Thermodynamics: in any isolated system, entropy naturally increases over time. Things move from ordered to disordered states. This is why:

• Ice melts in a warm room (not the reverse)
• A cup falls and shatters (it does not reassemble itself)
• Smoke disperses into a room (it does not reconcentrate itself)

Entropy is also related to the arrow of time why the past and future feel different. Processes that increase entropy are irreversible.

In information theory, entropy measures uncertainty or unpredictability in data. A coin flip has high entropy (50/50). A coin that always lands heads has zero entropy.