light cone

The light cone is the answer to a simple question: standing here and now, which parts of the universe can I possibly affect, and which parts could possibly have affected me? Because no signal — no push, no message, no influence of any kind — can travel faster than light, there is a hard boundary around every event, and that boundary, drawn on a spacetime diagram, has the shape of a cone. It sorts all of spacetime into 'reachable' and 'forever out of reach'.

Precisely: from a chosen event, imagine a flash of light spreading outward in all directions; in a spacetime diagram those light rays trace the surface of a cone opening upward into the future (the future light cone) and a mirror cone opening downward into the past (the past light cone). Events inside the future cone can be reached from here by something moving slower than light — they are your causal future. Events inside the past cone could have sent a signal to here — your causal past. Everything outside both cones is 'elsewhere': too far away in space, in too little time, for any influence to bridge the gap in either direction.

The light cone is the geometric guardian of cause and effect. Every observer, however they move, agrees on which events lie inside your light cone and which lie outside, so all observers agree on what can cause what — causality is absolute even though simultaneity is not. Two events outside each other's light cones can appear in either time order depending on the frame, which is fine precisely because neither can influence the other.