I am fascinated by the principle of least action. All current physics theories are built on this principle.
The principle of least action states that for any theory you can formulate a function (called the Lagrangian) that contains the entire theory. The integral of the Lagrangian is called the action.
By far the best description of this principle appears in The Feynman Lectures on Physics, Volume II, Chapter 19. Anyone interested in physics should read this chapter.
The strangest aspect of this principle is that it can be interpreted in the following manner: When you throw a ball from A to B, the ball takes all possible paths. However, except for the few paths that are near the ones of least action, the phase varies so rapidly from path to path that they interfere, and therefore don’t contribute. For the paths near the path of least action, the interference leads to diffraction and the uncertainty principle.
This interpretation is responsible, I believe, for what are called the “many worlds” interpretation of quantum physics. (But although I love the principle of least action, I regard the many worlds interpretation as useless; “not even wrong”.)
Intrigued? I’ll stop here because I am not able to match the glorious presentation by Feynman.