Light cone

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.

Related Topics:
Special relativity - Light - Minkowski spacetime

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The light cone is constructed as follows. Taking as event p a flash of light (light pulse) at time t_0, all events that can be reached by this pulse from p form the future light cone of p, whilst those events that can send a light pulse to p form the past light cone of p.

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Given an event E, the light cone classifies all events in spacetime into 5 distinct categories:

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• Events on the future light cone of $E$.
• Events on the past light cone of $E$.
• Events inside the future light cone of $E$ are those which are affected by a material particle emitted at $E$.
• Events inside the past light cone of $E$ are those which can emit a material particle and affect what is happening at $E$.
• Events in the absolute elsewhere of $E$ are those that can never be affected by $E$.

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.

Related Topics:
Light second - Vacuum - Inertial frame - Lorentz transformation

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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.

Related Topics:
General relativity - Boundary - Causal future - Point - Causal past

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Light cone: See also ►