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In special as in general relativity, the speed of light sets the upper limit for the transmission of influences and signals. Thus, studying the propagation of light, one can find out for an event A which other events can influence A, which can be influenced by A, and where any influence is impossible (because such influence would have had to traval faster than light - cf. causal structure). Graphically, the boundary between the two sets of events where influence is possible/impossible has the form of a double cone (for a sketch, see the page Spacetime in the chapter Special relativity of Elementary Einstein). It is formed by all world-lines of hypothetical light signals that would be emitted at the event A or, coming from an arbitrary direction, absorbed there.