 Light cones exist at each point of the spacetime and are invariant structures. Since the speed of light is an invariant quantity, all “observers” agree as to which points of the spacetime are illuminated by the popping of the flashbulb at \(\mathbf{O}\). Furthermore, as special relativity is standardly understood, the speed of light is a limiting speed. No material particle can be accelerated from a speed less than \(c\) to a speed equal to or greater than \(c\). Electromagnetic radiation (including light) always propagates in a vacuum at speed \(c\). (To see why \(c\) is held to be limiting speed, see Mermin (1968, Chapter 15) and Nahin (1999, pp. 342–353 and Tech Note 7.) Given these suppositions, the light cone structure divides all spacetime into three distinct sorts of regions relative to each spacetime point \(\mathbf{O}\). (See chapters 5 and 6 of Geroch (1978) for a thorough discussion.)