| Past LightconeA
cone-shaped portion of <!g>spacetime containing all (past) locations from which
light could arrive at a particular location within spacetime (the tip of the
cone). The tip can be reached from points inside the cone at the speed of light
or slower, and from points on the surface of the cone only at the speed of
light. Since nothing can travel faster than light, points outside the cone
cannot in any way affect the tip. Contributed by: <!g>CTNS 
  
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