We evolved at a relatively late stage compared to the age of the universe and there are hundreds of billions of planets orbiting stars in our galaxy. One would therefore think it likely that intelligent life has also evolved on one of these other planets. Why then do our telescopes not observe any evidence for life elsewhere? This is the Fermi Paradox and the “Great Filter” argument offers an explanation. I will set out the logic of the paradox before explaining the Great Filter argument and its consequences. Importantly, the Great Filter argument implies that if we discover aliens, then this would be very alarming for the future prospects of our species.
Fermi Paradox construction:
- There are approximately 250 billion stars in our galaxy, each has a high probability of hosting orbiting planets that could harbour intelligent life like our own.
- The universe is 13.8 billion years old.
- Our planet is about 4.5 billion years old.
- We evolved into our cognitively-advanced state just 70,000 years ago and yet we’re on the cusp of being able to travel to, and populate, other planets.
- Within a few thousand years, taking very conservative assumptions about technological advancement, we should be starting to significantly populate other stars in the galaxy.
- From (4) and (5) we can say that it takes around 100,000 years for an intelligent species similar to our own to populate the galaxy. This is negligible compared to the age of the universe.
- Our satellites have seen zero evidence of intelligent life elsewhere in our galaxy!
- Given (1) and (6), why has another intelligent species – originating on another planet in our galaxy – not already done this? In other words, why is (7) true? This is the “Fermi Paradox”.
Why is (6) true? Why would an intelligent species want to spread out and populate the galaxy?
This argument only postulates the existence of some Great Filter between a planet being formed and a galaxy-exploring civilisation. It’s certainly possible that a filter could be attributed to the motivations of a species. Perhaps an overwhelming number of species don’t share our species’ desire to explore and spread out as we do, or perhaps in the future we too will recognise (maybe due to philosophical insights) that colonising the galaxy is not a “good” thing to do and our motivations will revert inwards.
Another possibility, and I don’t think this is unlikely, is that at some point in a species’ technological advancement, the technology it has created becomes so addictive and irresistible that its advancement ceases. If virtual reality games truly become fully immersive, then I can certainly imagine a world where people will spend more and more time in a virtual world, which is safer (no physical harm or crime), more varied (one can spend every hour in a different virtual country or world) and more environmentally friendly (no-one has to emit ‘real’ emissions in a virtual world).
However, here are four reasons why “intelligent species” might want to colonise:
- All known life on Earth has a natural tendency to want to disperse and explore. This makes sense from an evolutionary perspective. It only takes a few individuals of one species to want to explore and spread out and they will be awarded with a greater number of descendants. This is because if they populate an area with less competition for resources, land, mating etc, then they’ll be able to spend more time having more children which are likely to survive for longer. From a Darwinian perspective, this group that were “more exploratory” will become the dominant one over time. The mutation that encourages exploration is therefore selected for through natural selection, which is why almost all wild species are naturally exploratory. Domesticated animals are the exception – but that’s because over thousands of years we’ve acted against natural selection and selectively bred for obedience and against curiosity!
- Even a world full of people that just play video games all day might want to spread out to build and run better computers, or seek to mitigate against local disasters (volcanoes, asteroids etc).
- A lot of current philosophical ethical theories would view more human lives as better (including most religious ones). This would imply that colonising the galaxy and enabling our species’ population to grow is something we ought to do. This being said, I’ve thought about this quite a lot… I think the question “should we populate the galaxy” is a harder one that some people acknowledge, but I won’t expand on this further here.
- At some point stars run out of hydrogen in their core and evolves by expanding into a red giant. This will completely change the conditions of a solar system and render most planets inhabitable. For example, our star, the sun, is about 5 billion years old and it has a life expectancy of about 10 billion years. If we don’t get beyond our solar system, our species will go extinct in 5 billion years. This is a long time. However, species in other solar systems might have less time which would encourage them to colonise other planets just for self-preservation.
Great Filter Argument:
To answer (8), we must postulate the existence of a “Great Filter” that must exist somewhere along the timeline from a planet being first created (point 3 for us) and a species populating the galaxy (point 6 for us). This filter must be overwhelmingly hard (almost impossible) for a “species’ evolutionary path” to pass through otherwise (7) would be false.
There exists two options:
- We’re lucky and the Great Filer has come before us in our evolutionary history. This would mean that we’re the first species to pass through it and reach our state of technological advancement in the galaxy.
- The Great Filter lies ahead of us and we’re very likely to go extinct before we populate the galaxy. Many people suggest there could be a “Doomsday technology” that, when a species discovers it, invariably means they tend to wipe themselves out (this could be nuclear weapons, or super intelligent AI, or something else that we’re bound to discover in the future).
For the first option, there already exists lots of evolutionary hurdles which could qualify as a Great Filter that have all been incredibly hard for our species to jump (formation of single-cell life, going from single-cell to multi-cell life, sexual reproduction, evolution of big brains and intelligence etc). We must hope that the Great Filter is one of these, otherwise, the Great Filter lies ahead of us – in which case, we’re likely to go extinct before we populate the galaxy.
The existence of the Great Filter makes it unlikely that we will ever observe intelligent life because, if no other intelligent life exists in our galaxy, the next closest galaxy is 2.5 million light years away. I don’t expect our species to ever travel that sort of distance (with current laws of physics – there’s potential for wormholes, which would allow such travel, and are permitted in General Relativity, but sadly no-one has ever observed anything to suggest their existence).
The important consequence of this argument is not just that it’s unlikely we’ll ever observe intelligent life, it also says that we should hope not to see other life in our galaxy. If we do find a form of life that’s evolved independently and is advanced (say a dog), then this makes it less likely the Great Filter lies behind us. This would be very bad news because it would suggest that the Great Filter must be ahead of us and therefore increases the likelihood that we’ll go extinct before we populate the galaxy.
Does this mean no aliens?
No! This argument is about the existence of intelligent life in our galaxy; it’s very likely that there are aliens out there somewhere. The observable universe hosts about 100 billion galaxies, and each of these hosts hundreds of billions of stars with planets. The argument set out above doesn’t apply in this case because (7) cannot be confirmed or falsified – we can’t see planets in other galaxies (yet). Also, although the Great Filter might be strong, it might not be strong enough to overcome the odds of one species on the ~1022 planets in the observable universe surviving through the filter.
Furthermore, our latest cosmological models suggest that the universe is infinite, which means (by definition) that the Great Filter is irrelevant (because any non-zero number x infinity = infinity) and there must be infinitely many other intelligent species out there. In this case, we are left with the conclusion that there are aliens out there, it’s just we’ll probably never meet them.