Preamble
Several decades ago, in 1950, scientific understanding of cosmology was different than that of today. Most pertinently for this discussion, it was believed that the universe had a steady-state cosmology, which entailed that the universe had always existed (and would always continue to exist) in a similar state as that of today.
During this year, in the Los Alamos National Laboratory, one Enrico Fermi was pondering the nature of alien life and recognized an apparent contradiction. I am unsure if a formal argument was ever made, but if it was, it might have looked something like this:
- The universe has existed for an infinite amount of time.
- Following 1, everything that is physically possible has already happened in the past.
- It is physically possible for intelligent life to form.
- Following 2 and 3, intelligent life has already formed in the past.
- It is physically possible for intelligent life to settle star systems neighboring their own.
- Following 4 and 5, intelligent life has already recursively settled neighboring star systems until it has occupied every star system in the universe, including our solar system and all of the stars we see in the sky.
- Seeing as the conclusions in 6 don't match our observations, there is an apparent contradiction (hence being called the Fermi Paradox). (also, the astute of you may have noticed that there isn't an actual contradiction here (but it still renders our observations as absurdly unlikely); I might outline why in the miniblog section later)
Now, as it turns out, this understanding of cosmology (point 1) was false; we live in a universe of finite age. The contradiction disappears. However, the question of why we don't see any alien life still remains and is worth answering. One that has gained a lot of attention on social media, particularly from those that have little knowledge regarding the Fermi Paradox and related subjects, in recent years is called...
The Dark Forest Hypothesis
The Dark Forest Hypothesis broadly suggests that no evidence of intelligent alien life exists because all alien civilizations inevitably go through the effort to hide their own existence, as not doing so would be too risky. The argument goes like this:
When your spacefaring civilization discovers the existence of another, it has three options: attempt contact, do nothing, or destroy it. Attempting contact makes it aware of your own existence, forcing it to make the same decision for you. Doing nothing allows for the possibility of it discovering you in the future, also forcing it to make the choice. Destroying it insures that it doesn't get to make that decision. Since any civilization that has made it this far is very likely to at least be concerned with its own survival, likely above all else, it will always choose the option with the lowest risk to itself, regardless of potential benefits. In this case, the choice being to destroy it.
One thing to note is that this hypothesis necessarily assumes that there is a way to attack another civilization without giving your own location away to third parties. If this weren't the case, choosing the third option would bring the highest risk of destruction, rather than the lowest, making this whole scenario fall apart at the seams. As such, I will be assuming that this is the case for the duration of this writeup.
Clear-Cutting the Forest (A.K.A Why it is Dumb)
The criticisms of the idea here come in two general forms: the logic of the idea failing, particularly when real-world considerations are taken into account, and observational evidence. As such, I will be presenting them in two separate sections.
Logical Failures
Lets establish what must be true for a Dark Forest scenario to make even a modicum of sense.
Observational ability of a given civilization can never be good enough to image the surface of a planet at or further than the distance of the nearest civilization to them. The ability to do so would imply that the observed civilization would be able to do the same to you, and the fact that your planet wasn't wiped out before your civilization even emerged would run contrary to what the hypothesis says. This would have to be a hard physical limit, too, since civilizations operating under the Dark Forest model would have a strong incentive to improve observational capabilities as much as physically possible. (note: there are already proposals for telescopes that would be able to do exactly this lolololol)
Due to having to avoid being detected, civilizations under this scenario are placed under pretty severe energy constraints. Enough so that, in any model of the universe that at all matches reality, meaningful force projection even to the nearest star system against a peer opponent should be impossible.
In spite of that, a civilization must have the ability to reliably completely wipe out an opponent in a single attack, as even a single self-sustaining population will be able to rebuild eventually, and you will be unable to verify whether or not it has been destroyed due to aforementioned limited observational capabilities.
But also, a single attack can't be powerful enough to singlehandedly wipe out an opponent either, since being able to do so would allow a civilization to slowly clear out every star system in a large volume around it, inhabited or otherwise, allowing expansion. Oops!
There is a minimum civilization density below which a Dark Forest scenario is impossible. Below this density, a given civilization will be able to expand without worry, since by the time the first attack came in, what might have been a fatal blow to an energy-constrained civilization would be ineffectual to a civilization that has been on an exponential growth curve ever since the attack was launched. Never mind that the star systems around it would have already been seeded by that point as well. If you were to ask me for an estimate for what it is, I'd say that it would take around 1k years from 1st radio transmission to scaling that much (honestly a bit on the conservative side but w/e), so around 500 light years average distance between civs (lower if launching an attack at near lightspeed is impossible).
There is a much higher minimum density that is necessary as well. Since self-replicating spacecraft of any kind (note that this doesn't have to be killbot swarm of any kind, a self-sufficient crewed ship would work just as well) will take any unoccupied star systems and fill them with at least somewhat aligned civilizations; once there is a critical mass, each of them would be able to expand freely, since unaligned civilizations would be outnumbered, especially if they were destroying them/allying* with them as they were being encountered. To prevent this, density must be high enough that the growth of such would be capped (btw I'm pretty sure that density is 0.5 lololol literally every 2nd star system wwwwwww).
There is also a maximum density. If density is high enough, a civilization will know that another is in a given star system without observing it by virtue of, well, there being a star system there. Since everyone knowing where everyone else is, under this paradigm, everyone would be launching attacks everywhere as frequently as possible until they were fairly certain a random attack wouldn't hit anything.
You will note that the maximum density (lets say 0.1) is well below the minimum (0.5, 0.2 being extremely generous). It gets even worse if you consider the real max is around 50ly average distance (more on that in the next section).
The minimum density problem also presents a bigger structural problem: Why didn't the first aliens to arise just settle everything before a Dark Forest paradigm was cemented. The obvious rebuttal to this is that those civilizations thought they were in a dark forest too, but, considering that the universe being filled to the brim with aliens now would mean that intelligent life began to emerge basically the moment conditions began allowing for it, aliens emerging in that timeframe would realize that they literally couldn't have formed not too long before and put together that they were at least a little early.
Quite frankly, I could keep going for a while. I could examine the truly absurd form an attack would have to take, for example, or go into depth on how an alliance-forming strategy would work and how it would break the board', so to speak. However, I don't think I need to; I've made my point.
*To put it succinctly, if an alien civilization contacts your own, and specifically your own, the destroy option is no longer a safe option unless your certain that you can wipe them out before they can deploy a retaliatory strike. From there, diplomacy would ensue.
Observational Evidence
Even if we were to accept the Dark Forest Hypothesis as fact, there are still things we'd expect to see in our night skies that we just... don't.
Okay, so for this first point, I have to take a slight detour, so bear with me for a little: Why is it expected that every civilization will act perfectly rationally? After all, we already have one example of a civilization not acting in accordance with how they should in a Dark Forest paradigm, what with blasting radio waves everywhere and all. I think anyone who keeps up with the news knows that even nations don't act perfectly rationally. Are we really to believe that there are no alien species that, for example, put proselytization over their own survival? Or take the gamble and broadcast regardless of risk? Or any other number of things? Are we really expected to just believe that humans are the Odd Ones Out? If so, here's a heads up: any Fermi Paradox solution that requires all spacefaring civilizations everywhere to act in the same extremely specific way is not a good one.
And what of smaller groups? Individuals? The only thing that could keep tight enough control to prevent a signal from ever leaking is a singleton ASI. Speaking of, even if everyone is rationally prioritizing their own survival, there may be specific circumstances where broadcasting would actually help. Something like the Cosmic Virus comes to mind.
That being said, lets get to the first real point: where are all the exploding aliens? Even at the lowest densities given prior, there should still be so many aliens just in the galaxy that there would be random fuckups relatively frequently, which should lead to very visible destruction. Or at the very least some aliens being very loud before suddenly cutting out.
Second point, perhaps the most important one: How the fuck do we still exist? Pretty much every civilization even somewhat near us should have been able to see that our planet had at least a very high chance of having life well before we existed. Under the Dark Forest paradigm, that enough is good reason to destroy it, just in case intelligence develops there in the future. It's not like they wouldn't be able to see it, either. Hell, we would be able to see it with our current observational capabilities.
And even if that wasn't enough, the radio waves we've been putting out for over a century certainly would be. That puts a minimum distance between us and the nearest alien civilization at ~50 light years, and growing.
Postamble
If you have been following the Fermi Paradox for a while now, you'll have noticed that this particular 'solution' has somewhat exploded in popularity in recent years, particularly on social media. So, if it's such a bad hypothesis, why is this the case.
To start with, it originated from a book series, so that's likely where the first few were convinced. Then, perhaps a post on some social media or another would go viral, showing it to a lot of other people. Now, for many of them, they just... wouldn't think very hard about it, go 'oh, I guess that's why we don't see any aliens', and then continue not thinking about it. There's also the fact that it's often sold as being the logical conclusion of game theory; a lot of people will stop thinking about it right there and go 'oh, I guess it's confirmed by math lol'.
There are also people who are more interested in the subject that subscribe to this idea, though, so that can't explain all of it. However, having interest doesn't mean having working critical thinking skills (go into any John Michael Godier comments section to see what I mean).
There are also peoples' personal outlooks to consider. For example, social media is composed of mainly teens. Teens tend to love edgy ideas, and the Dark Forest Hypotheses is certainly edgy. But my main point is that people are growing increasingly pessimistic about the world, and the Dark Forest certainly fits such a worldview well.
Also, here's my most conspiratorial thought on the subject: I lowkey think that it owes part of it's popularity to being the subject of a stealth advertising campaign. Seriously, go anywhere on social media involving aliens, and you'll see randos in the comments just completely ignoring the content of the post they are commenting on and just talking about the dark forest. You'll see shit like 'ah... le Dork Forest... no wonder we dont see any aliens dot dot dot'. Is there any reasonable explanation for this besides an advertising campaign?