As Tim Urban says, ‘We’d estimate that there are 1 billion Earth-like planets and 100,000 intelligent civilizations in our galaxy.’ So there must be aliens. Why haven’t we heard from them? (Read about the Fermi Paradox here.)
This blog is a list of answers. I’ve summed up each one in a line and dropped a relevant quote afterwards, and mostly linked to an article not a paper. Some of the ideas overlap or are sub-categories of each other. I tried to avoid an endless list, but still give different perspectives.
There is no paradox. ‘We only know 0.0012699565% of the galaxy.’
There is no paradox. ‘Based upon the current state of astrobiological knowledge, there’s a 53 to 99.6 percent chance we are the only civilization in this galaxy and a 39 to 85 percent chance we are the only one in the observable universe.’
There is no paradox. ‘Natural variability will mean that sometimes galaxies will be settled, but often not.’
There is no paradox. ‘Fermi was questioning the feasibility of interstellar travel—nobody thought he was questioning the possible existence of extraterrestrial civilizations.’
There is no paradox. ‘There is no logical contradiction between the statement “E.T. might exist elsewhere” and the statement “E.T. is not here” because nobody knows that travel between the stars is possible in the first place.’
There is no paradox. All life, including us, is distributed through the Universe by space dust.
We are too far away from anyone else to be found because we are in a supervoid.
We’re too remote to notice we live in an otherwise colonised galaxy, like Inuits who didn’t know America had been colonised.
They are too far away because they live at the extreme edge of the universe. ‘Machine-based civilizations, with their massive supercomputers, will have huge problems managing their heat waste. They’ll have to set up camp where it’s super cool.’
They don’t exist: we are living in a computer simulation. (Alternatively, they are running the simulation, I suppose).
They already destroyed themselves through something like climate change.
The aliens can’t exist because it’s too outlandish they haven’t found us yet. ‘If a civilization on Planet X were similar to ours and were able to survive all the way to Type III level, the natural thought is that they’d probably have mastered inter-stellar travel by now, possibly even colonizing the entire galaxy.’
No aliens would have the resources to colonise a billion stars.
The aliens gave up looking or are morally against trying to colonise us.’Habitable planets lacking technical civilizations will frequently be encountered by spacefaring civilizations. It is not clear what their response will be…Perhaps strict injunctions against colonization of populated but pre-technical planets are in effect in some Codex Galactica. But we are in no position to judge extraterrestrial ethics. Perhaps attempts are made to colonize every habitable planet…A whole spectrum of intermediate cases can also be imagined.’
No other life exists because of the great filter.’ This is a candidate because it took about a billion years of Earth’s existence to finally happen, and because we have tried extensively to replicate that event in labs and have never been able to do it.’
Earth is unique. ‘Though there may be many Earth-like planets, the particular conditions on Earth are exceptionally friendly to life.’
We are the first ones to be this advanced and will therefore inadvertently wipe out other forms of life. ‘We are the first to arrive at the stage. And, most likely, will be the last to leave.’
Every time the universe advances enough for intelligence to evolve, it then wipes it out. ‘A possible regulatory mechanism that can account for this is the frequency of gamma-ray bursts — super-cataclysmic events that can literally sterilize large swaths of the galaxy.’
We missed the aliens. They visited before we evolved. (See also: ‘NASA conspiracy? Space agency’s Spirit rover destroys ‘alien dinosaur skull’ on Mars.’)
The aliens are already here. ‘“The click beetles in my backyard don’t notice that they’re surrounded by intelligent beings — namely my neighbors and me,” Shostak said, “but we’re here, nonetheless.”’
We are a civilisation seeded here by wealthy alien philanthropists. ‘If you end up with 100 successfully seeded solar systems for each very advanced civilization, the resulting odds suggest that we are indeed the result of a seed.’
We wouldn’t know life from other planets if we saw it. ‘I predict that, if a form of life is ever discovered in another part of the universe, however outlandish and weirdly alien that form of life may be in detail, it will be found to resemble life on 4 Earth in one key respect: it will have evolved by some kind of Darwinian natural selection’. Dawkins
We don’t matter enough to be invaded yet. ‘It’s an inefficient use of resources to exterminate all emerging intelligences, maybe because most die out on their own. But past a certain point, the super beings make their move.’
We haven’t been visited because the other advanced life forms destroyed themselves (think nuclear war).
We are too boring for other civilisations to be interested in, for friendly reasons for otherwise.’ Imagine a life form whose brain power is to ours as ours is to a chimpanzee’s. To such a species, our highest mental achievements would be trivial.’
We are interesting to them, but in a niche way, and are living in a cosmic zoo. ‘Our relationship to ETI might be similar to the relationship of Eciton burchelli to mankind: E.O. Wilson knows and cares, but Bill Clinton doesn’t.’
We don’t have the right technology to receive their signals.
We’re expecting results too quickly.’ Nobody has visited because they’re all too far away; it takes time to evolve a species intelligent enough to invent interstellar travel, and time for that species to spread across so many worlds.’
They will never find us/it’s going to take an eternity. ‘He found that even if the alien ships could hurtle through space at a tenth of the speed of light, or 30,000km a second, – Nasa’s current Cassini mission to Saturn is plodding along at 32km a second – it would take 10bn years, roughly half the age of the universe, to explore just 4% of the galaxy.’
Most species don’t want to go into space. (We do and we’re quite bad at it.)
We don’t seem to have got much beyond this list of categories of answers from ‘Extraterrestrial Intelligence: Where is Everybody?’ by John A. Ball, which was written in 1985.
There are no other civilizations.
Other civilizations exist, but they’re very primitive.
Other civilizations exist at about our level of development. They suspect that we might be here, and they might like to talk with us.
Advanced civilizations exist and they know we’re here. They would like to talk with us if they could just attract our attention.
They know we’re here, but they don’t care; they’re ignoring us.
They exist, we are of some interest to them and a few of their scientists are discreetly studying us.
They exist, we are interesting to them, and they are studying us in some detail but inconspicuously.
They exist, they are studying us and occasionally even dabbling in our affairs (UFOs).
We are an experiment in their laboratory.
I’m naturally sceptical, and it might be bad news if there are aliens, but as Tyler Cowen says, ‘when you run all the arguments through your mind, is it not possible to come away with an estimate of at least a one-in-a-thousand chance that alien visitations are a real thing? ‘
p.s. After I finished this post, I found this list of ‘50 solutions to the Fermi Paradox‘ (based on a book about the same thing by physicist Stephen Webb). That list is not all brilliant (mine is closer to what I wanted to read but couldn’t find) but it is fairly comprehensive without too much chatter. I haven’t read the book.