> If you can identify all material objects that all observers can feasibly see, then you subtract the material objects that one person sees, from the material o

> If you can identify all material objects that all observers can feasibly see, then you subtract the material objects that one person sees, from the material objects the other person sees in their reference frame... Generally speaking an event is "observable" if some signal (usually light) could reach you from that event. It is generally uninteresting that different observers can see different sets of events at different points in their world-line. If you are lightyears apart, the signals will take time to reach you. This is what "spacelike separation" is all about. But if you just sit around in one spot, signals can reach you from billions of light-years away. A certain event in your history might have been separated by space, but a point in your future may be separated from it in a timelike manner. You can't get away from events by simply going fast, either: the velocity addition makes signals impossible to outrun. What you need is to employ acceleration. Consider a modified Zeno's paradox: you are 1 light year away from a signal, and begin accelerating. By the time it has covered 0.5ly, you are now 0.75 ly away. Unlike the original paradox, the math on this one works out: it is the acceleration equivalent of a black hole. (Most gravity and acceleration effects are interchangeable like that). We could draw this out with a Minkowski diagrams, and find some line beyond which events become truly unobservable. We could call this line the "event horizon" (which, after all, is what the term is about in other contexts, too.)