Light cone

A light cone is the set of spacetime directions along which light can travel to or from an event. It separates events that can be causally connected from those that cannot.

Spacetime interval: A quantity combining spatial separation and time separation between events, invariant under Lorentz transformations. Its sign determines whether two events are timelike, spacelike, or lightlike separated.
Spacetime: The four-dimensional framework combining three spatial dimensions with time in relativity. A light cone is defined around an event in spacetime, not space alone.
Causal diamond: The intersection of the future light cone of one event and the past light cone of another. It identifies the region that can both receive signals from an event and influence a later one.
Euclidean geometry: The geometry of flat space governed by familiar distance and angle relations. Its distance-based geometry lacks the light-speed causal boundary that structures spacetime.
Null geodesic: A path through spacetime whose tangent has zero spacetime interval at every point. Light follows null geodesics, tracing the cone's surface in curved spacetime.
Event (relativity): A physical occurrence assigned a location and a time in spacetime. The cone's vertex is the event from which possible light paths begin or end.
Event horizon: A boundary beyond which events cannot send signals to a distant observer. A horizon marks where the global arrangement of light cones prevents escape.
Absolute simultaneity: The Newtonian idea that simultaneity is shared by all observers, regardless of motion. Relativity's tilted time axes preserve cones but reject a universal division into simultaneous events.
Causal structure: The pattern of which events in a spacetime can causally influence which others. The cones at every event together define spacetime's causal structure.
Speed of light: The invariant speed at which light travels in vacuum, approximately 299,792,458 meters per second. This speed fixes the cone's slope when space and time are plotted together.

Linked from 22 pages