Why the Dark Forest Theory is (probably) wrong

Should we hide from potentially hostile aliens and stop trying to communicate with them? Well, it’s too late for that

Ever since we realised that those bright dots on the night sky are all solar systems, we have been wondering if we are alone in the universe. In fact, it is not so far-fetched to believe that the evolutionary process, to which we owe our existence, also took place on many other planets in our galaxy and beyond. However, since the inception of SETI (Search for Extraterrestrial Life) in the early 1960s, no radio signals of artificial origin (other than our own) have been detected so far, despite considerable technological efforts, including the deployment of large radio telescopes. So where is everybody?

The Dark Forest (by Darkmoon_Art on Pixabay.com)

This question summarises the core problem of the so called Fermi paradox: in 1950, the physicist Enrico Fermi assumed that the evolution on Earth is not exceptional in the universe and given the age of our universe (13.8 billion years), there should be many other civilisations out there, many of which might be technologically more advanced than our civilisation. Some civilisations might even be so old that they should have managed to spread beyond their own planet and colonise other solar systems. But so far we have not been able to find proof of intelligent alien life.

Before discussing why we have so far been unable to detect any intelligent alien species, let’s get an idea first about the possible number of alien civilisations in our galaxy with which we should be able to communicate. A rough estimate (with many uncertainties) is provided by the so called Drake Equation. The equation is probabilistic, so it can’t deliver a precise result — the variation between a possible upper and lower limit depends on the assumptions about its factors. We can apply each factor in the equation as a kind of filter — by adding additional filters, the total number gets smaller and smaller.

The factors are:

1. the average rate of star formation in our galaxy;
2. the fraction of those stars that have planets;
3. the average number of planets which can potentially support life (per star that have planets);
4. the fraction of planets that could support life and actually develop life at some point;
5. the fraction of planets with life that actually go on to develop intelligent life;
6. the fraction of civilisations that develop a technology that releases detectable signs of their existence into space;
7. the length of time for which such civilisations release detectable signals into space.

Depending on the assumptions for each of the factors, a very wide bandwidth of the possible number of civilisations with which we should be able to come into contact can be established: from 0 (rare Earth hypothesis) to about 15.6 million (for details, see the section “Range of results” in this article).

In June 2020, astronomers from the University of Nottingham reported the possible existence of over 30 intelligent alien civilisations in our Milky Way galaxy, based on astrophysical estimations (here for details).

Based on the Drake Equation and its many factors, there is a large variety of possible explanations for the Fermi Paradox. Just to name a few:

• Factor 5 might be much lower than in early estimations, so that the formation of intelligent life is nearly impossible, so we are alone at least in our galaxy;
• There is no other civilisation in our galaxy that actually developed the technology to release detectable signals into space because all civilisations at some point in their history, annihilate themselves (e.g. through weapons of mass destruction);
• We are in an area of the Milky Way galaxy that is too remote from other civilisations and those civilisations have not reached the technological level to release detectable signals in time for those signals to reach us (yet).

Among the many possible explanations for the Fermi Paradox, there is one that has become quite popular in recent years — the so called Dark Forest Theory. It was popularised in Liu Cixin’s “Trisolaris” novel trilogy whose second part bears the title “The Dark Forest”. This hypothesis finds the explanation for the Fermi Paradox in factor number 6 and it boils down to this: Although there is possibly a large number of technologically advanced civilisations in our galaxy that have the ability to release detectable signals into space, they choose not to. Why? Because revealing their existence can have potentially devastating consequences. Technologically more advanced civilisations might preemptively annihilate other civilisations to prevent those other civilisations from striking first.

The Dark Forest hypothesis has a foundation in Game Theory: since alien species are not able to effectively communicate with one another (given the long distance between them and different languages, means of communication and cultural concepts), it makes sense to assume the worst case scenario, that is, the other civilisation will annihilate us on the first encounter. However, it is impossible to tell with certainty if the other civilisation is militarily superior or inferior to our own, so the best strategy is to remain silent in the first place and not reveal our existence to other civilisations.

In practical terms, it might not even be necessary to conquer the solar system of the other civilisation if the attacking civilisation is advanced enough — it could suffice to send a military probe which might or might not arrive in time (even if it takes several hundred Earth years or longer) before the other civilisation can reach a technological level that is advanced enough to allow an effective defence against the attacking probe.

If human history is any guide, more dominant cultures have indeed conquered or destroyed technologically inferior cultures — voluntary or involuntary, with or without prior communication attempts. The Aztec and Inca empires were first weakened by diseases brought by the Europeans and then destroyed through warfare. Native Americans were killed by the thousands, deprived or their land and locked up in reservations. A similar fate befell the Australian Aborigines and many other indigenous populations. So it seems that the bigger fish always swallows the smaller fish.

From the point of view of the Dark Forest hypothesis, all voluntary or involuntary communication attempts with other civilisations are foolish, because revealing our existence might lead to our annihilation. What seems to be impossible on the scale of a planet like Earth — hiding from conquerers — might very well be possible in the vastness of space.

Indeed, the Dark Forest hypothesis has gained some traction even among some reputable scientists such as Stephen Hawking (see here). They point out that given the age of our galaxy, there might be alien civilisations that are at least a billion years old and might have the technological capability to reach and destroy us — from their point of view, after all, we are just as primitive to them as cockroaches are to us.

Well, it is too late for that because the first radio communication signals were in fact emitted in the late 19th century, so roughly 120 years ago and from World War II, our radio communication activities really began to explode. It is unlikely that terrestrial radio signals will make an alien civilisation aware of our existence and location because of the non-directional nature and weakness of those signals however, there have been numerous attempts (since the 1970s) to send directed high-energy signals to other star systems (you can find a very good overview about CETI here). These messages typically contained mathematical and other scientific information like atomic numbers, the human genome, depictions of our solar system and home planet and a depiction of the human body. So according to the Dark Forest hypothesis, that was a very bad idea. Maybe the deadly probe that is destined to wipe us out, is already on its way… Or is it?

In order to show why the Dark Forest hypothesis is not the most likely explanation for the Fermi Paradox, I will make a couple of assumptions that my main argument is based upon:

1. All life forms, including alien life forms (intelligent or primitive), consist of individuals that are part of a group. The relationships among the group members can be anything from very primitive (bacteria) to highly complex (like human societies).
2. Intelligent life can only come into existence after a long development process called evolution. Evolution might not follow the same rules in the whole universe, but it is safe to assume that a more intelligent species takes longer to develop than a primitive species.
3. All intelligent lifeforms, that exist as individuals and organise in groups, will organise into factions once the total population grows above a certain threshold. Separate groups start to form and move into different geographical areas. Individuals can belong to several factions on different levels in complex societies.
4. Intelligent species can be distinguished from non-intelligent species by the following characteristics, that must all be fulfilled: complex language (not necessarily a phonetic language), the capability of planning and executing coordinated actions (based on considering several outcomes and scenarios for future events) and the ability to accumulate and archive knowledge and be able to pass on that knowledge to later generations.
5. Intelligent species are able to create technology, improve technology and pass on technology from one generation to the next.

Even if some of these assumptions are too strong to apply to all possible intelligent alien species, I strongly believe that if intelligent alien civilisations exist, these characteristics will be true for most of them.

On Earth, the most important factions mentioned in assumption 3 are called countries nowadays. If our history has taught us one thing, it is that forming a world government that all countries obey to is very difficult. Why should states give up their military power to a world government? The international system has an anarchical structure rather than a hierarchical one — the organisational principle that best describes the internal organisation of states is in fact hierarchical. The United Nations are not a world government but rather a forum to coordinate actions of individual states for better outcomes for all and to prevent crises and wars. The success of the United Nations has at best been very limited so far. This is because the states in the international system are sovereign.

So if we apply these assumptions to alien civilisations, it turns out that deciding to remain silent and not emit any signals into outer space (although the technology exists) is not possible over a long period of time. Simply because there will always be individuals and groups who think differently. Applied to our planet: Suppose the European Union ban all CETI activities (communication with extraterrestrial intelligence) and all member states comply. Why should all other states do the same?

The fundamental flaw in the Dark Forrest hypothesis is a very naive simplification that Star Trek fans know all too well: the “one culture, one ruler” simplification. Whenever the starship Enterprise visits a planet (first contact or other) there is usually some kind of planetary government or even a single ruler that can make decisions for the whole planet. In defence of Star Trek: to fit in the 50 something minutes long episodes, some simplifications are necessary. And yes, there are some storylines that are a little more complex but these oversimplifications are made very frequently in several of the Star Trek TV series.

The Dark Forrest hypothesis implicitly assumes that an alien civilisation acts as one entity with a coherent behaviour over a very long period of time. That is simply not very likely — and even if it applies to one species (maybe some kind of alien queen that somehow controls all other individuals), it does definitely not apply to all species. Yes, there might be a world government one day that can make all factions or countries comply to a possible CETI ban. But on the way to that future, there will be many more attempts to communicate with alien civilisations.

So where is everybody? If the Dark Forrest hypothesis is not the most likely explanation for the Fermi Paradox, then what is the explanation? I believe that in our galaxy there are probably thousands, maybe millions of worlds with alien lifeforms. However, there are probably very, very few — maybe even none besides Earth — with intelligent alien lifeforms. And even if there are other civilisations out there — maybe their natural and technological evolution took much longer or will take much longer until they will be capable to emit and receive signals. Some alien civilisations might not constantly send signals strong enough into outer space so that we can pick them up after hundreds or thousands of Earth years. Moreover, they might have sent the few signals into the wrong direction (there are in fact a lot of wrong directions from our perspective).

Let’s put our SETI activities into perspective: we have been technologically capable of receiving signals from outer space only for a few decades — the first radio telescope was built only in 1932 (details: here). Compared with the entire 200,000 years of existence of homo sapiens, 89 years are a very short time period. Even 200,000 years is a very short time period compared with the age of Earth, which is roughly 4.5 billion years old… Given the vastness of space and the long time signals need to travel through space to reach us, can we really conclude after using radio telescopes only for 89 years, that alien civilisations don’t exist (or that they are hiding) because we haven’t picked up their signals yet? Maybe we just need to wait a couple of centuries longer and in the meantime make sure that we don’t destroy Earth and, at the same time, ourselves. It would be a pity if humans go extinct a few decades before the first alien signals will pass through our solar system.