 Let’s work a bit more to make the idea precise. We say that a  half-curve  is a worldline which starts at some event and is extended as far as possible in the future direction. Next, the  observational past  of an event q, OP(q), is the collection of all events x such that there a is a worldline which starts at x and ends at q. Intuitively, a (slower than light) signal may be sent from an event x to an event q if and only if x is in the set OP(q). (See figure 2.2.1.) 