Dark Forest Cosmology

1. Liu’s Theory

The title is a reference to Cixin Liu’s science fiction novel, The Dark Forest, from the Three Body Problem trilogy. (That trilogy, by the way, is among the greatest works of science fiction.)

[ Spoiler alert: the following discussion reveals some plot elements from the novels. ]

Liu gives a metaphor, that the universe is like a dark forest that has some people hiding in it, in unknown (to each other) locations. These people are in a Hobbesian dilemma: each of them is afraid that any of the others might try to kill him, and so each will kill any of the others if he gets the chance, to prevent that other from later killing him. Every once in a while, some foolish person might light a fire, thereby making his location known to nearby others. That person will then quickly get shot.

You can imagine conditions in which this evil behavior would be rational, indeed almost forced. Suppose you know (a) that there is a decisive combat advantage for whoever strikes first, (b) there are at least some hostile others who will strike first, perhaps due to evil or craziness, (c) you can’t tell which those people are, (d) there is little or no benefit from peaceful interaction with other people in the forest (e.g., they don’t have anything interesting to trade with you). In these conditions, it’s pretty clear that the game theoretic prediction is that anyone immediately kills anyone whom they discover.

Liu posits, in his fictional world, that the universe contains many civilizations, hidden among the vast number of stars, and that they all exist in this “dark forest” state. They generally don’t know where each other are, but periodically a civilization reveals its location to the universe—say, by emitting electromagnetic radiation with signs of intelligence—whereupon another civilization will quickly destroy that civilization.

This offers a solution to the Fermi “Paradox”—the question of why we don’t see evidence of intelligent life elsewhere in the universe. (This really isn’t paradoxical, but I guess some people find it surprising because there is a very large number of other stars, so these people think there should be many other intelligent species on other planets.) We don’t see the other civilizations because almost all of them, being smarter than us, are aware of the dark forest state, and so they wisely hide their locations from the rest