“At the heart of this book, both physically and metaphorically, is the "principle of least action" ("action" being a technical term that refers, basically, to the overall energy in a system over time). The most important idea in physics, it can be summed up in the phrase "the universe is lazy." Among many other things, the principle underpins why light moves in straight lines and how light "knows" which angle to turn through when it moves from one medium (such as air) to another (such as glass). The principle can be used to describe the parabola traced by a thrown baseball and, indeed, to derive Newton's laws. ”
