A means of illustrating the past and future of an event in spacetime. Spacetime diagrams in three dimensions are conventionally depicted with time as the vertical axis, and the other two axes representing dimensions in space. The axes are scaled such that one unit along the time axis is one second, and one unit along either of the spatial axes is 300 000 km (because the speed of light is approximately 300 000 km/s). On such a diagram, the paths of rays of light from an event slant upwards at 45° along the sides of a cone whose vertex is at the event and whose axis is vertical. This is the future light cone of the event. Any and all events occurring after the first event which are to be affected in any way by that first event must lie within the cone. Events outside the cone would require faster-than-light travel if they were to be affected by the initial event. Conversely, any earlier events which are to affect the event in question must be contained within a 45° cone going down from the event. This is the past light cone.
Subjects: Astronomy and Astrophysics.