A fundamental lesson of general relativity is that there is no fixed spacetime background, as found inNewtonian mechanicsandspecial relativity; the spacetime geometry is dynamic. While easy to grasp in principle, this is the hardest idea to understand about general relativity, and its consequences are profound and not fully explored, even at the classical level. To a certain extent, general relativity can be seen to be arelational theory,in which the only physically relevant information is the relationship between different events in space-time.On the other hand, quantum mechanics has depended since its inception on a fixed background (non-dynamic) structure. In the case of quantum mechanics, it is time that is given and not dynamic, just as in Newtonian classical mechanics. In relativistic quantum field theory, just as in classical field theory,Minkowski spacetime is the fixed background of the theory.