An important part of SR is explaining which events may cause other events (e.g., a signal cannot get from A to B when they are space-like separated). Moreover, I vaguely remember that at least some minor arguments in relativity actually rely on the assumption that causality must not be violated (I might be wrong on this, though). Can someone explain to me why we are so worried about preservation of causality? Let me clarify my question, referring to either the classical relativity theory, or the quantum relativity with only unitary equations (it's just too confusing for me to discuss wave function collapse). In either case, the world is completely deterministic: what will happen in the future is fixed and unchangeable just as much as what happened in the past. In this situation, it makes no sense to talk about an information signal from A to B, since what happens at both events is known in advance. An information signal would only be meaningful if an observer at A could change something there, and then notify an observer at B about that change. It is as if the whole world's past and future in written down in a special book. Would it be meaningful to ask whether a signal from page 100 of the book can reach its page 200? No, because the book's pages aren't going to change, and so the signal can carry no information. Thanks!