I will discuss the notion of symmetries in physics in a non-technical manner and how some are actual symmetries, and some are mere redundancies in our description. The central part of the talk will be about a simple financial toy model that demonstrates a symmetry physicists call “local gauge invariance.” This type of symmetry is ubiquitous in physics; e.g. it is found in both the standard model and general relativity. Intriguingly, the dynamics of this toy model is surprisingly determined by a physics equation famous enough to appear on T-shirts…