Where is everybody?
So we know that there are lots of potentially habitable planets out there, but now we come onto the last four parameters of the Drake equation, which can really be shortened to two factors: what percentage of habitable planets will develop intelligent life that can communicate with us, and how long do these civilizations last? If we are assuming that there is no intelligent alien life out there, then there are three things that could be preventing it: either life never starts, life never develops to intelligence, or intelligent life doesn’t exist f or long enough for us to have found it. Drake initially assumed that so long as a planet was habitable, it was inevitable that not only life, but intelligence would eventually arise, yet perhaps both these assumptions are wrong. It is possible that there are certain barriers that all lifeforms have to get through in order to advance towards intelligence, and that many of these alien species (indeed, maybe all of them) have been unable to pass through, either hitting evolutionary dead-ends or going extinct. This is called the Great Filter theory, 5 but just where these filters are is the topic of much debate. Perhaps the very emergence of living matter from non-living matter (called abiogenesis) is a filter: it is simply too difficult for life to even begin, and thus no other living organism has emerged on any other planet, despite them being hospitable to life. Other potential filters could include the evolution from simple prokaryotic cells to complex eukaryotic cells, the jump from single-celled to multicellular life, or even the emergence of intelligence itself. However, there is a muchmore worrying possibility, which is that the Great Filter is in fact ahead of our species, not behind as in the previous examples. It might be that all advanced species are destined to wipe themselves out, be it through weapons of mass destruction, runaway nanobot technology, malevolent AI, self- made super viruses… the list of events that could wipe us out is certainly very extensive. Even if humanity is not destined to destro y itself, it’s possible that events out of our control— super volcanoes, gamma ray bursts, solar flares andmore —wipe out every advanced species before they’re able to survive long enough to interact with other civilizations. In a strange twist, it would be better for humanity if we never found any alien lifeforms, as this would suggest that we have already passed the Great Filter, being the first (and potentially last) species to do so. A second explanation for the Fermi paradox is that there is alien life out there, but they are simply not interested in communicating with us. Consider, for example, what happens when you see an anthill. At best, you marvel for a moment at how much these tiny, simple creatures have accomplished, before continuing home to your skyscraper-filled city. At worst, you kick the hill over. But at no point do you stop to discuss Mozart’s finest symphony or attempt to explain general relativity. Similarly, perhaps there are advanced alien empires out there who have received our calls, considered for a moment how endearing it is that the apes have finally cracked radio waves, before returning home to derive the entirety of quantum gravity from scratch. Even if the aliens are not dismissive of our efforts to communicate, theymight be intentionally isolating the Earth to allow it to develop naturally, or to allow us to grow out of our violent ways before communicating with us. However, this theory — called the zoo hypothesis — seems unlikely in my opinion, as it requires every single alien individual to never reach out to us, which seems improbable in a civilization of billions or even trillions of beings.
A final possibility as to why we have found no evidence of aliens is that they do exist but cannot communicate with us. Humans tend to view the universe with us as the central characters, and thus when we imagine alien lifeforms, we naturally think of vaguely humanoid creatures with similar
5 Davis 2018.
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