We have no way of knowing how an alien civilisation will act. This is one of the biggest stumbling blocks of the Search for Extraterrestrial Intelligence. After all, it’s very easy to explain away the lack of contact with alien life (Fermi’s Paradox) by simply saying “well, they don’t want to talk to us”, or “they’re not allowed to because of the Prime Directive“. These sorts of arguments are the weakest solutions to Fermi’s Paradox, because they rely on knowledge we don’t have.
So how do we solve this conundrum? Sadly, we can’t – at least until we make first contact, that is. So in the meantime, we are forced to play let’s pretend, and speculate on how alien civilisations will behave. But we can still be sensible, and rein in our wilder ideas. Ideally, our educated guesses should have a basis in something biological – not too Earth-centric, but in processes that we think must occur regardless of where life arises.
So Jonathan Starling and I turned to the concept of symbiosis. Symbiosis describes the relationship between two different species. These relationships can be broadly categorised as
- Parasitical – one species (the parasite) uses the other (the host) to its own advantage, having a negative effect on the host.
- Mutualist – both species benefit from the interaction.
- Commensalist – both species interact, and one benefits, but the host is not affected positively or negatively
We decided to model the interaction between a civilisation and its host planet in the same way, using my computer models of civilisation growth in the Milky Way, to see how this would affect the number of communicating species in the Galaxy. Our simulated civilisations can either feast on their planets, destroying it as a deadly virus kills its host, or they can work with their environment, in a slower but more rewarding growth.
But what happens when the virus can jump from one host to the next? If a deadly virus has plenty of new hosts within close distance, it can kill its host and jump to the next without killing itself. But if there are not enough new hosts, then the virus will die.
We added interplanetary and interstellar colonisation into the model to see how different species behaviours are transmitted into the Milky Way at large. Does it pay to be a parasite, sucking your planet dry and moving on? Or do better behaved, less aggressive civilisations win the day?
The number of hosts/planets available to a civilisation will depend on how “near” they seem, which depends on how quickly the civilisation can travel. A fast ship will be able to cross larger distances and “infect” planets more easily. We ran the model several times, increasing the maximum colonisation speed from Voyager’s velocity, 10x Voyager’s velocity to 100x Voyager’s velocity.
These top speeds we are setting for these colonising species aren’t particularly high: Voyager is currently travelling at about a hundred thousandth of the speed of light. A clever probe might be able to use gravitational slingshots to boost up to nearer a hundredth of the speed of light.
We found that if the civilisations can only colonise slowly, then it doesn’t pay to be a parasite. As you can see from the graph, mutualists (blue) tend to do better than the parasites (red).
As we increase the velocity, we increase the number of available hosts, making it less costly to be a deadly parasite, until at 100x Voyager’s velocity, parasites dominate the Galaxy, colonising and destroying planets that would have hosted benevolent civilisations, before the good guys could even pick up tools. This is analogous to invasions of species on Earth – sometimes, a species will enter a local ecosystem from outside and simply out-eat and out-breed its competitors. In the UK, the grey squirrel’s dominance of the native red squirrel is a classic example.
So what does this mean for the real world? Sadly, not as much as we would like. This work is a speculative exercise in civilisation behaviour, a somewhat contrived numerical gedankenexperiment in a sandbox. It doesn’t solve the problem of our ignorance of other civilisations’ behaviour. It describes a (most likely fictional) Milky Way where civilisations develop one behaviour type very early in their existence, and don’t learn from their experiences. The survival of the human race has depended (and will depend) on our ability to understand and move on from our mistakes!
But, the message of this experiment is important. Once a parasitical civilisation passes the technological barriers to interstellar colonisation – at a speed that is relatively modest – then it will make its presence felt very quickly. In this scenario, we must solve Fermi’s Paradox with one of the following possibilities:
i) Interstellar travel is impossible or very difficult;
ii) Parasitical civilisations have a very short lifetime – they either destroy their host and themselves, or they change their behaviour and stop being parasites
iii) Parasitical civilisations are not that common in the first place
Ultimately, Jonathan and I wanted to use this experiment to reflect on humanity’s relationship with the Earth. Are we parasites, commensalists or mutualists? I’d love to know your thoughts in the comments below.