Author Greg Egan has a new book out, and wrote up an explainer behind the math: “A space that is connected (in the sense that there is always a path between any two points), but not simply connected, is known as a multiply connected space, because there are multiple, fundamentally different ways to get from A to B. … The purpose of this page is to discuss the way that Newtonian gravity can be adapted to a multiply connected space like this.”