In special relativity, a light cone is the pattern describing the temporal evolution of a flash of light in Minkowski spacetime. This can be visualized in 3-space if the two horizontal axes are chosen to be spatial dimensions, while the vertical axis is time. If a flash of light happens at an event at time t=0, only points within the light cone will be reached by this light for a given positive time t. The other, symmetric half of the light cone where t<0 then is the region from which light could have reached the event at t=0 from all the events occurring at the negative time t.
If space is measured in light seconds and time is measured in seconds, the cone will obviously have a slope of 45°, because light travels a distance of one light second in a vacuum during one second. Since special relativity requires the speed of light to be equal in every inertial frame, all observers must arrive at the same angle of 45° for their light cones. This is ensured by the Lorentz transformation.
In general relativity, the future light cone is the boundary of the causal future of a point and the past light cone is the boundary of its causal past.
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