A couple of weeks ago, I was writing up a description of Einstein’s general theory of relativity, and I thought I’d compare the warping of spacetime to the motion of Earth’s tectonic plates. Nothing on Earth’s surface has fixed coordinates, because the surface is ever-shifting. Same goes for spacetime. But then it struck me: if nothing has fixed coordinates, then how do Google Maps, car nav systems, and all the other mapping services get you where you’re going? Presumably they must keep updating the coordinates of places, but how?