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 of the universe.
In the context of the novels, this is a great plot device. It is, however, not a plausible theory about how the life in the universe works. I don’t know if Liu really thinks it is, or if he was was using it purely for entertainment value. But anyway, it might be interesting to explain why this isn’t likely.
Why debate the plausibility of a science fiction story? Well, some people might find it a plausible solution to the Fermi Paradox, which I guess some people think is a real puzzle. Also, it tells us interesting things about the nature of civilization and intelligent life.
2. Problems with the Dark Forest Theory
a. Undetectability + first-strike capability
I see three reasons why the Dark Forest Theory is unlikely. First, the theory requires that civilizations (at least, some of them?) have the ability to destroy one another with a first strike, without significant risk of retaliation. This requires each civilization to have amazing weapons, which one can deploy without tipping off one’s own location. Perhaps this is reasonable. But it must also be that the civilizations that have this amazingly advanced weapons technology somehow cannot locate each other unless one of them broadcasts their location to the rest of the universe (?).
If there were an incredibly advanced alien species such as Liu imagines, they could easily send out probes to the thousand nearest star systems—or even all 100 billion stars in the galaxy—to find out which ones have intelligent life, thus ending the “dark forest” state. Liu, by the way, imagines a species creating probes the size of a proton, so it’s not clear what would stop them from making 100 billion of them. For comparison, humans manufacture about 1.8 billion cell phones per year. Seriously.
But even without that technology, a species capable of destroying another entire civilization in a first strike should also be capable of sending out a large number of probes to locate other civilizations—say, to all the nearby stars that look like reasonable candidates for life.