Definition
A light cone is the set of all events in spacetime that a flash of light emitted from (or arriving at) a given event would sweep through.
Each event has a future light cone — everything light it emits could possibly reach — and a past light cone — everything that could possibly have sent it a signal. Events outside both are causally disconnected from it, the region Minkowski called the elsewhere. Causal structure is the geometry of these cones.
Why it matters
How it works
In flat (Minkowski) spacetime, plot time vertically and one spatial dimension horizontally. A light flash from the origin moves along the diagonals x = ±ct — two lines at 45° if the units are chosen so c = 1. In two spatial dimensions the diagonals become a cone; in three, a 3-sphere. The interior of the future cone contains timelike world-lines (slower than light, accessible to massive observers); the cone surface itself contains null world-lines (light rays); the exterior region is spacelike — separated from the origin in a way no signal can bridge.
Special relativity reorganizes spacetime around this structure. The Lorentz transformations are exactly the linear transformations that preserve light cones. Two observers in relative motion disagree on which events are simultaneous, but agree on which events are inside, on, or outside each other's light cones. The invariant interval s² = −(ct)² + x² + y² + z² is negative inside the cone, zero on it, positive outside.
In general relativity, the metric varies from point to point and the light cones tilt with the curvature. Near a black-hole event horizon they tilt inward; at the horizon, the future cone points entirely inward; inside, every future direction leads to the singularity. In an expanding cosmology, the past light cone of an observer today bends — events from beyond the particle horizon (the comoving distance corresponding to the age of the universe) lie outside our past cone and have never been observable.