Entropy

Entropy measures how widely energy is spread and how many possible arrangements a system can have, helping explain why many changes naturally proceed in one direction.

A drop of food coloring spreads through a glass of water instead of gathering itself back into one tiny drop. Entropy helps explain why energy and particles naturally spread among more possible arrangements.

Entropy is a central idea in thermodynamics and statistical physics. It describes how energy and particles can be arranged within a system. The second law of thermodynamics says that the total entropy of an isolated system tends to increase, so energy becomes more spread out and less available for useful work. This helps explain why heat flows from hotter objects to cooler ones, why broken objects do not spontaneously rebuild themselves, and why many natural processes have a direction in time. Local order can still form—as in stars, living things, and snowflakes—provided that even more entropy is produced elsewhere.