Squeezing Chaos Into a Single Number
Try packing a suitcase for a trip when you have no idea what you will need. If there is a lot of stuff and it is all different, you will spend ages rearranging it and still have no clue how neatly everything fits. That is roughly how physicists long looked at entropy, the measure of disorder, in systems that never settle down: bacterial swarms, granular materials, swarms of active particles.