That light always finds the best path to any destination has puzzled thinkers for centuries. Hero of Alexandria observed that light always takes the shortest path but how does it know what that is? It might seem obvious that it is a straight line but how, at each step, does light know what straight is? (Note 1) How light always finds the best path to any destination is still a mystery today.
QR3.6.1. A Wave Moves
QR3.6.2. The Law of Least Action
QR3.6.3. The Law of All Action
Note 1. By relativity, light doesn’t always travel in a straight line, so “straightness” is not self-evident.