causal structure

Because the speed of light is a maximum speed at which energy and information can be transmitted, events in the history of the universe can be organized in terms of their possible causal relations. To do this one indicates, for every pair of events, whether the first is in the causal future of the second, or vice versa, or whether there is no possible causal relation between them because no signal could have travelled between them. Such a complete description defines the causal structure of the universe.