# Time-like Intervals and Causality

by *FaerieLight*
Tags: causality, intervals, timelike
 P: 43 Hi If two events are separated by a time-like interval, then all inertial reference frames will agree on the order of the two events. For example, if event A occurred before event B in one inertial reference frame, then event A occurred before event B in all inertial reference frames. The invariant interval, given by I = -(c$\Delta$t)2+($\Delta$x)2+($\Delta$y)2+($\Delta$z)2, for two events separated by a time-like interval is always less than 0. My question is, for two events that are causally connected, can the invariant interval be less than or equal to 0? Or does I have to be strictly less than 0 for such events? I'm not sure because as far as I'm aware, there can only be a causal connection between two events if it is possible for a signal to travel between the two events, so that means that two events can be causally connected if they are separated by a light-like interval too, ie, I = 0. But if this is the case, then in the reference frame of the light, doesn't event A and event B occur at the same time then, in which case there isn't any causal connection between event A and event B? Thanks
Physics