This is the founding premise of science fiction "The Three Body Problem"

It starts with two axioms

  1. Survival is the most important goal of every civilization
  2. Every civilization will continue to expand and grow , but resource in the universe is limited.

With two assumption

  1. Suspicion Chain
  2. Technology Explosion

Lets start with a thought experiment. We assume that civilization A discovered civilization B.

Civilization A have two primary choice

  1. Do nothing
  2. Contact in a certain way

Now we note that we simplify this problem by categorizing two kinds of civilization in the universe.

Hostile and Friendly

Hostile civilization will attack another civilization when another civilization is discovered. Friendly civilization will only attack when is threatened.

Now here comes suspicion chain

  1. Assuming A is friendly, however A has no way of knowing if B is friendly and vice versa.
  2. Even if A knows B is friendly, and B knows A is friendly. How does A knows that B think A is friendly and vice versa?
  3. Now if A knows B think A is friendly, how does A knows B knows A knows B think A is friendly and vice versa?

This is an endless cycle. And although in Earth we can eliminate the suspicion by communicating. In space, the communication is limited by the speed of light, and therefore you cannot be sure if the attack is under way while you are communicating. It is optimal not to contact. And let us be pedantic here, we note it is optimal not to contact when B is more technological advance than A as A has no way of knowing B's true intention due to suspicion chain. It is noteworthy to say that if A is more technologically advance there are no risk.

However here comes another issue. Going back to our thought experiment's second assumption. If A does nothing or proceed to contact, we note that there is a chance that technology explosion might occur in B's civilization. B might be able to surpass A technologically. Now since A has no way of knowing B's true intention, to ensure the first axiom A has only one optimal move: destroy any civilization when discovered.

This is logical.

Consider the following payoff matrix in game theory

B\A Contact(Destroy) Not Contact

Contact(Destroy) (0,0) (0, -infinity)

Not Contact (-infinity, 0) (-K, -L)

for which K is the probability that A surpass B and that A is hostile and L is the probability that B surpass A and that B is hostile.

We can see that the only clear Nash Equilibrium is (Destroy, Destroy)

Note: Obviously this is a crude way of approximating, but we can deduct from Natural Selection that there is probably only three kind of surviving civilization: Not yet detected, very good in hiding oneself, always destroy.

This give rise to the Dark Forest Postulate

The universe is a dark forest. Every civilization is a hunter with a gun. They pass quietly through the forest like a ghost. They must be extremely cautious and try their best to keep silence, because they know there are a number of hunters out there. And if a hunter discover another, no matter if he is an angel or a demon, an old or a young ...... the only thing he can do is to kill it. In this forest, others are the eternal threat. Anyone who reveals its location will be destroyed.

  • Can't we assume communication is always faster and easier (cheaper) than attack? I don't see why the situation would be inherently different on earth (say with tribes in a real forest) but nowhere did we find such simplistic outcomes in real forests. Also: axiom 2 is dubious. Is it always rational to continue growing? Commented Nov 16, 2014 at 22:47
  • Given that the cost of communicating with a hostile race is the possible extinction of one's race due to suspicion chain, one would not communicate. Imagine you found a technologically more advanced civilization, you cannot know if they are friendly or hostile, why would you contact them? Furthermore lets imagine you found a less technologically advanced civilization, if the distance between the two civilization requires travel time greater than many technological cycle, it is possible that they can be dangerous due to both technological explosion and suspicion chain Commented Nov 16, 2014 at 22:58
  • Regarding axiom 2, if you can expand by colonizing other star system, what limits you not to grow? Commented Nov 16, 2014 at 23:00
  • My point is that the suspicion chain would undermine any kind of contact if it were valid, but practically, this is not how it works. Regarding axiom 2: you're saying a civilisation will expand if it can, axiom 2 says it will always expand. There's a difference: under limited (or too expansive) resources, a civilisation will not expand and might be happy with that. Commented Nov 16, 2014 at 23:56
  • In any case, this type of a priori argument seems irrelevant to me. Only empirical observation will settle the issue of how interstellar diplomacy actually works. Simplistic models of this kind are usually poor guides (including in economics for example) they do not correspond to actual psychology. Commented Nov 17, 2014 at 0:04

6 Answers 6


There are a ridiculous number of "solutions" to the Fermi "paradox". Here's a sample.

  1. Space is enormous. Given the seeding rate of intelligent life via abiogenesis + evolution, we may not expect to find anything.

  2. Intelligent life is trying to contact us all the time, but we're not listening because we don't realize what the most effective communications method is.

  3. Galaxies are incredibly far apart, and the seeding rate gives an expectation of less than one civilization per galaxy. Nobody's bothered trying to make it to ours from theirs because the cost/benefit just isn't there (payoff in a hundred million years??).

  4. Everyone realizes how to switch over to a dark energy existence where you can (potentially) have perpetual motion machines and leaves the universe we know behind.

  5. Intraspecies warfare is a bigger problem that interspecies conflict. Intelligent civilizations stay small or get bogged down fighting themselves.

  6. The other civilizations know about the Nash equilibrium result, and that to get around it you need plenty of time for reciprocal interactions. There is some minimum size of a civilization needed for that to be practical. They don't bother with anyone smaller.

  7. They're fighting each other, not particularly secretly, but in the open. They're too busy to bother with us.

And on and on.

In any case, it is simply not true that the dark forest solution is the only one. The Federation of Planets model (Star Trek) is a good alternative: mutual agreements for defence, so if anyone tries to be a dark hunter, the entire federation is alerted and can either evade or squash the hunter. (Space is big, so it will take a really long time for this to play out.)

Big predators on earth have an alternate "agreement": packs of wolves don't attack bears much (or vice versa) because both sides stand to lose a lot in the conflict. Two powers will damage each other badly if they fight; it's far better to work out a way to avoid fighting.

Also, physics don't really work out for dark hunters to exist. If a civilization is spacefaring, the chance that you can catch everyone is just about exactly zero. Earth is vulnerable to an asteroid or something, but escaping is incredibly easy because you simply accelerate and wait, and you're pretty much invisible for arbitrarily long. Unless you as a dark hunter can move at the speed of light, your prey is going to get away.

So I just don't think there's any deep problem here. (Not a deep philosophical problem, anyway.)

  • The Fermi "paradox" isn't really a paradox. That's why all the "solutions" to it are absurd.
    – user18800
    Commented Jan 4, 2016 at 20:25
  • Federation of Planets / agreement between predators wouldn't work if A) It was easy to destroy a star system, B) It was possible to do so anonymously. If those conditions were true, the galaxy would become populated by species prone to shooting first. Systems of mutually assured destruction and defensive pacts are difficult to setup and are nullified by a 3rd party.
    – TechnoCore
    Commented Feb 20, 2017 at 17:47

I think that sophon-based communication somewhat invalidates the dark theory, because it is a FTL communication device (whereas the theory makes an assumption that light speed limits the speed of communication). Trisolarians could use that to turn off the droplet 2+ light years away in real-time, as well as to do realtime monitoring on earth, so its application is not limited to a phone line. If humans had sophons, they could have done the same thing to observe Trisolarians, and chains of suspicion wouldn't have any greater effect than what exists between countries of earth today. Bad, but not fatal.


If the scenario is reframed without the science fiction bit, it becomes clear that the suspicion chain is a bad model for interacting agents. Consider that every single relationship in any human's entire life is subject to this issue, and yet we don't seem to get hung up on it all that often (except perhaps in highschool as puberty kicks in). Clearly humans have come up with an approach which is not well modeled by these assumptions.

There are several flaws. The number one flaw is that it shouldn't come as a surprise when there is no advantage for anyone to making contact, that the best answer does not involve making contact. As written, the quad-chart you wrote doesn't even have a prisoner's dilemma in it! It very clearly starts from the assumption that you should never send a signal because there's no benefit from sending a signal.

I think the big issue is assumption 1: "Survival is the most important goal of every civilization." It does not appear to model real civilizations very well. First off, it assumes that a civilization is a well defined concept. If you make contact, and the other race is friendly, you may be able to spread the parts of your civilization which are most essential. That way, when someone comes to your doorstep ready to destroy, the most important parts are free. Also, since we are on a philosophy forum, consider that Nietzsche claimed that this sort of thinking is valueless for a civilization (he called this the mindset of The Last Man). Alternatively, the other species could be far more advanced and friendly, in which case you can be assimilated. Your lives become better, at the expense of losing the identity of your civilization.

Finally, what you describe is a game theory system. Its a good model if we only have two options: do nothing or blare signal as loud as humanly possible for as long as possible. If you enter drama theory, it starts to be useful to send out messages which are hard to tell if they are signal or just noise, and then observe how others react to it. Over many cycles, you may change your approach as you identify places that might contain friendly intelligent life, and send more signal to them. (I'm assuming we're talking on a galactic timescale, so that the transmission times are shorter than the survival of the civilizatino)


There are two points in answer to your question:

  • you provide an a priori possible explanation for the Fermi paradoxe. However there is a number of ways each of your assumptions could be defeated, there is much uncertainty, and such a pure a-priori logical reasonning might not be a very good guide in this context.
  • there are many alternative explanations and for the same reasons (uncertainty) we are not in a situation to judge which one is plausible or not. We lack independent evidential support for these alternatives.

The second point was addressed by Rex Kerr's answer. Let me address the first one. Note that the plausibility of all the arguments I give is debatable. The point is not that your explanation is strictly impossible, rather that it rests on a number of assumptions that could possibly turn out to be false.

About axiom 2: "Every civilization will continue to expand and grow, but resource in the universe is limited." It is not necessary true that every civilization will continue to expand.

  • Expansion might be desirable in certain circumstances, but there might be a tradeoff between the cost of expansion and its benefits. Possibly, every advanced civilization finds it more rational to stop its expansion.
  • We make less children in modern society than before, here on earth, which seems to undermine the axiom. This trend could be general to all civilisations.
  • Or perhaps, every civilization with strong expansion desires in their genotype end up destroying itself.
  • or perhaps advanced civilizations all live in a configuration space to which we have yet no access rather than in physical space, and have no desire to expand in physical space

Regarding the second aspect of the axiom:

  • resources in the universe might be virtually unlimited. For example, advanced civilizations could be able to produce virtually unlimited energy from nuclear fusion, and they might all be very efficient and use very little material resource for their survival (or be able to synthesize them with their unlimited energy resources).

Regarding the suspicion chain: it assumes that a civilization has no way of knowing if another is friendly or not prior to a contact, or to know that the other knows that, etc. There is a number of ways this assumption could turn out to be false.

  • For example, attacking another civilization would require having sufficient knowledge of the other, which would require minimum communication first, and there could be no actual situation where a civilization does not know at all the risk of hostile intention of another one. Advanced civilizations would all be aware of this fact, and diplomacy would become a pragmatic balance between friendly communication and threat (pretty much as it is on earth).
  • Or all advanced civilizations might be inherently friendly and aware of that fact (for a reason or another: maybe cooperation is always beneficial, or communication costs less than attack, or moral progress is inevitable when a civilization grows).
  • the rational premisses on which the suspicion chain rests (which seems inspired by game theory) might turn out to be false. Psychological research shows that human agents do not follow the axioms of game theory. There might be good reasons for this, yet to be discovered.

An argument against the suspicion chain is this: it seems to render any form of communication impossible. Yet we know, for sure, that communication is possible (for example we established first contact with tribes in the rainforest.) I don't see why huge distances make a crucial difference: in any case it seems prima facie rational to communicate before attack, and if everyone thinks this way, it is not rational to hide systematically.

Also note that we are no longer interested in contacting isolated tribes in the rainforest, despite our alledged appetite for expansion. We think it's better to leave them alone, for moral reasons, such as cultural diversity protection. Maybe alien civilizations thinks this way with regard to us.

Another argument is that if this whole line of reasonning were true we would have disappeared already (or there are no civilizations in our close neighbourhood, which is a smaller Fermi paradox).

All this makes the whole argument rather tenuous in my view. All steps are more or less plausible but that does not mean the conclusion is (if ten assumptions have a .9 degree of credence, there combination is still lower than a half). You need some sort of fine tuning for the argument to go through. So while this rationale is great for a science fiction scenario, personnally I would tend to give more credit to historical reasonning based on actual diplomacy, extended to this new type of situation, rather than pure a priori reasonning of this kind. There are too many unknowns to reason logically (or even for bayesian reasonning) and I think empirical data based on earth diplomacy is a more robust input than abstract game theory.


Doens't this hold only if the two civilsations are 'similarly' advanced ?

What if civilisation A is WAY more advanced than civilasation B ? like humans and termites ?

The likelihood of civilisation B having a technological advance to overtake A would be very low, and so from A's point of view, there would be not a lot of gain in contacting a civilsation which represents a slightly tedious cusiosity - after initial discovery.

We see this happening: Say we humans discover a new kind of frog... nice frog. Perhaps a few experiments and cataloguing, then we leave them alone. No need to contact further as we can't communicate eaily with someting lika frog. By the time they've evolved enough to communicate, so will we, and may not even be on Earth anymore.

My point being that where there's a significant difference in technology because of "less evolution" (ie less brain power available), there's no point in spending out destrying a civilisation because the chance of them becoming a threat is so low.

Someting else missing is, I think : kindness. It would feel awful to destroy a civilisation 'just in case it becomes a threat', and something of an assumption to considerthat of all entities in the universe, humans are the only ones to feel compassion ?


Why are people saying "Your argument rests on assumptions that could be proven untrue." Like no crap, that's what an assumption is - I'm just surprised people would waste paragraphs and page space on that. It's almost as redundant as this comment.

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