 Second, we will need a clear notion of the kind of geometry that allows for escape, or makes such escape impossible. For this, we need the notion of the causal structure of spacetime. At any event in the spacetime, the possible trajectories of all light signals form a cone (or, more precisely, the four-dimensional analogue of the boundary of a cone). Since light travels at the fastest speed allowed in the spacetime, these cones map out the boundaries of the propagation of possible causal processes in the spacetime. If an occurrence at an event A is able to causally affect another occurrence at event B, there must be a continuous trajectory in spacetime from event A to event B such that the trajectory lies in or on the light cones of every event along it. (For more discussion, see the Supplementary Document:  Light Cones and Causal Structure.)  