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 special relativity, a lightcone is the pattern describing the temporal evolution of a flash of light in Minkowski spacetime.
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 lightcones.
The backward lightcone of event E is the imaginary cone-shaped surface of spacetime points formed by the paths of all light rays reaching E from the past.
Their age difference isn't caused by anything, just as light's going at the speed of light instead of at some other speed isn't caused by something but is just the way nature behaves, at least insofar as the theory of relativity is concerned.
Any measurement that produced a different value for the speed of light would be presumed initially to have an error in, say, its measurements of lengths and durations, or in its assumptions about the influence of gravitation and acceleration, or in its assumption that the light was moving in a vacuum.
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