Light cone
In special and general relativity, a light cone (or null cone) is the path that a flash of light, emanating from a single event — localized to a single point in space and a single moment in time — and traveling in all directions, would take through spacetime. == Details == If one imagines the light confined to a two-dimensional plane, the light from the flash spreads out in a circle after the event E occurs, and if the growing circle with the vertical axis representing time is graphed, the result is a cone, known as the future light cone.