Spacetime diagram
This is a spacetime diagram
of Vermilion emitting a flash of light.
In a spacetime diagram, time goes upward,
while space dimensions are horizontal.
Really there should be 3 space dimensions,
but I've drawn 2 for simplicity.
In a spacetime diagram, the units of space and time are chosen
so that light goes one unit of distance in one unit of time,
i.e. the units are such that the speed of light is one,
\(c = 1\).
Thus light moves upward and outward at 45° from vertical
in the spacetime diagram.
The lines along which Vermilion and Cerulean move through spacetime
are called their worldlines.
Each point in 4-dimensional spacetime is called an
event.
Light signals converging to or expanding from an event
follow a 3-dimensional hypersurface called the
lightcone.
In the diagram,
the sphere of light expanding from the emission event is following the
future lightcone.
There is also a
past lightcone,
not shown here.
Consider again the challenge problem.
The problem is to arrange that both Vermilion and Cerulean
are at the centre of the lightcone, from their own points of view.
Here's a clue.
Cerulean's concept of space and time may not be the same as Vermilion's.
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