Geroch's theorem

Geroch's theorem states that in the framework of general relativity, if the universe is both complete (no edges or boundaries) and follows certain smoothness conditions, then it cannot contain 'closed timelike curves'—paths in spacetime that loop back in time, potentially allowing time travel. Essentially, the theorem shows that, under these assumptions, the laws of physics prevent the possibility of time loops, maintaining a consistent cause-and-effect order. It highlights how the structure of spacetime constrains the kinds of time travel scenarios that could exist within our universe.