Hi there! I'm a third-grade physics student and I'm trying to understand the physics of topological insulators, I have found this thread, where somebody else has already asked this question, but in that thread it wasn't really answered in details, they concentrated on other issues. My question is: What does time-reversal invariance protection mean? As I understand intuitively, it means that if I apply magnetic field to my system or do anything with it, it doesn't change the topological phase, doesn't become simple insulator etc. But I don't really understand why. I know that some quantities are odd under t -> -t transformation and some are even, but I don't know what is the connection between this symmetry and stability of the system.