Figure 3.

Perhaps hypothetical Type III civilizations are in an inaccessible region of biotechnological–population size state space, where civilization trajectories are bounded by a ‘burnout horizon’, and long-lived civilizations have consciously reoriented their trajectories away from growth in population size and length scales to explore other dimensions of biotechnological state space. Note that we do not rule out the possibility of some kind of channel that might allow a transition to the Type III region.