Where are they?
We now believe that extrasolar planets are common throughout the Universe, forming as a natural part of the evolution of stars. Given abundant planets, the chances of us detecting intelligent life like our own, existing elsewhere in the Galaxy, are fairly high, - chances evaluated by Frank Drake in 1961 through the equation that now bears his name:
(image source: Wikipedia)
Many of the terms in this equation are unknown or highly speculative, and estimates place the number of communicating intelligences in the Galaxy anywhere between near-zero and tens of millions. Most assumptions, though, place the numbers well above one, leading to a question that was attributed to Enrico Fermi in 1950, even before the Drake Equation framed the argument: “If life is common in the Galaxy, where is everybody?”.
Fermi’s Paradox - the lack of alien contact despite the a Universe that should be teeming with life - has excited the interest of scientists as well as philosophers, and some of the many possible solutions have been explored both by popular science writers and the writers of science fiction.
Aliens amongst us
Perhaps the most straightforward possible solution to the Fermi Paradox is the one which stands least scrutiny from archaeologists, historians, scientists or other evidence-based analysis: that aliens have indeed been in contact with humanity.
The most common science fiction trope in this category suggests that alien contact underlies key elements of the mythology of a variety of cultures . Many such stories hover on the boundary of science fiction and fantasy, with the alien ‘divinities’ too far beyond human understanding for scientific explanation. Examples here include the original Star Trek episode “Who Mourns for Adonais?” (TV 1967), in which the ancient Greek pantheon are shown to be powerful aliens, and the Marvel comics’ Thor (comics: 1962 onward, movies: 2011 onwards), in which the Norse pantheon are alien Asgardians. The Asgard also put in an appearance, alongside other ancient mythologies, in the more science-fictional Stargate universe (film and TV, 1994-2018), while biblical stories of giants walking the Earth are explained through alien contact in Sylvain Neuvel’s Themis Files (novels 2016-2018) and a more eclectic mythological and theological assortment can be found in Battlestar Galactica (TV 1978-1980, 2003-2012).
Added to these and many other ancient-god science fictions can be found an equal or larger number of other stories of more contemporary alien contact, ranging from the benign lone alien of ET: The Extra-Terrestrial (film 1982, dir: Spielberg), to the insidious infiltrations of Invasion of the Body Snatchers (film 1956, 1978), V (TV 1984) and The Puppet Masters (novel, Heinlein 1951). Often, in such cases, the aliens are emblematic of human nations or ideologies, and reflect more on the fears of contemporary culture than the nature of the alien species in question.
A Scarcity of Aliens
Accepting the evidence (or rather lack of evidence) for actual alien contact, perhaps the next simplest solution to the Fermi Paradox can be found in science fiction narratives in which intelligent alien life is intrinsically scarce. In terms of the Drake Equation, this would occur if any of fp, ne, fl and fi are very low - either planets don’t easily form, or can’t support life, or life doesn’t emerge, or it never becomes intelligent, or some combination of these factors exists.
The theory that planets were scarce was commonly held into the 1930s and beyond, largely owing to the scientific suggestion that they may be formed through head-on collisions between stars - an intrinsically rare event. This theory was communicated to the public through popular science works and continued to influence science fiction for decades thereafter, even as scientific theories of planet formation shifted towards alternate and more abundant channels.
Many authors make the assumption that life in the Universe is rare - often without articulating it. Most of the works of Isaac Asimov, including his epic Robots/Foundation series (novels, begun in the 1940s and continuing to the 1980s) are entirely lacking in sentient alien life. The author actually commented on this, noting that he felt inadequate to the challenge of writing alien viewpoints. Similarly Frank Herbert’s Dune (novels, 1965-1985) features a galactic empire in which alien life is scarce and limited to non-sentient beings such as the sandworms. Television series such as Firefly (TV 2002) and Red Dwarf (TV 1988-1999) imply or state that all the life encountered by the characters had its origins on Earth (in the case of Red Dwarf diversifying by genetic engineering).
An interesting example of scarce aliens in a galaxy-spanning empire can be found in Dan Simmons’ Hyperion Cantos (novels, 1989-1997). Here humans have spread widely between planets and encountered only a few sentient alien species: two of which are driven to extinction by human colonisation, and one which solely colonises gas giants and therefore does not come into conflict with human requirements. This is a universe in which life is both intrinsically rare and highly vulnerable to extinction.
Such extinction provides another category of solutions to the Fermi Paradox in science fiction - this time focussing on the final L, or lifetime of the intelligent civilisation. The premise here is that while intelligent life might form readily enough, if its duration is typically short the chances of another civilisation being at an equivalent stage to ours (and hence able to communicate at the same time as we are able to listen) would be small.
Here again, science fiction divides into at least two main tropes. Through the 1950s and 60s and into the 1970s, while humanity was poised on the brink of self-annihilation through nuclear warfare, authors tended to assume that any civilisation advanced enough for space travel or interstellar communications would also develop nuclear energy and be likely to destroy itself in a matter of years or decades. Examples of such catastrophism in which humanity annihilates itself or destroys its civilisation can be found for example in blockbuster films such as On the Beach (film 1959), The Day the Earth Caught Fire (film 1961), Planet of the Apes (film 1973), and in literature such as When the Earth Died (novel, Mannheim 1950), The Tide Went Out (novel, Maine 1958), A Canticle for Leibowitz (novel, Miller 1959) and The Postman (novel, Brin 1985), amongst many others. The very real risk of such accidental self-annihilation on Earth inspired a group of scientists to devise the Doomsday Clock to promote awareness of such risks, and also inspired (a somewhat overblown) debate around the safety of particle physics before the activation of the Large Hadron Collider experiment. In recent decades, however, the shift of emphasis in apocalyptic fiction has been from nuclear annihilation to anthropogenic climate change in the form of cli-fi. Again the imminent mass extinction on Earth has arisen as a consequence of an intelligent civilisation’s rise towards interstellar communication and, if an inevitable part of that rise, could itself be a plausible answer to the Fermi Paradox.
A less obvious but perhaps even more interesting subcategory of such stories are those where the attempt at space flight is accidentally but directly responsible for the extinction - as was the case, for example, in Alfred Bester’s 1941 short story “Adam and No Eve”. Here the protagonist develops a power source which will make him the first astronaut. Unfortunately it does so by disintegrating iron atoms, and its exhaust sets in motion a chain reaction which destroys the iron that is integral to the Earth’s crust and to the haemoglobin in our blood. The first astronaut returns to land on a world he himself has destroyed!
The second main category of extinction explanations also places the responsibility for the extinction of intelligent life on space travel - but this time not as a result of an accident. It raises the possibility that one or more early civilisations in the Galaxy may have been hostile and deliberately acted to suppress any subsequent intelligences which attempt to leave or communicate beyond their host planet. This is most often accomplished in science fiction through the use of self-replicating machines . These would reprocess the material they encounter in planetary systems into more versions of themselves, allowing a geometric growth in their population and the volume of space they could probe.
Theoretically, such machines could be benign and act as monitors of arising species (as in 2001: A Space Odyssey, film: 1968, dir Kubrick, novel: Clarke 1968), but in the context of the Fermi Paradox they are more often portrayed as destroying civilisations they encounter - as is the case for the Replicators of the Stargate franchise, the Inhibitors in the Revelation Space series (novels, Reynolds 2000-2003, 2021), the xenophobic races in The Forge of God and The Anvil of Stars (novels, Bear 1987, 1993), and the Berserker series of Fred Saberhagen (novels and stories, 1963-2005). The possibility that such hostile races (whether machine or organic) exist has raised the question of whether humanity should advertise its presence in the Universe even if we have the technical ability to do so. Scientists including Stephen Hawking have argued that, given the risks, the answer to this should be no. Similar discussions on other planets could explain why we have heard no one calling out while listening to the skies.
A Different Perspective
This concept, that aliens could exist but be hiding, highlights a term in the Drake equation that offers a different perspective on the Fermi Paradox. fc denotes the fraction of civilisations that develop the capacity for communication, but it could also be considered as the fraction of extant civilisations that choose to communicate. Science fiction has hypothesised a variety of reasons why intelligent alien races may choose not to communicate with humanity. Again (setting aside the idea of races hiding for their own protection), we can divide these into two main tropes: humanity may either be too primitive for communication, or simply not interesting enough.
In the former category falls stories in which aliens will not contact any race until it leaves its own Solar System. This is the premise behind the influential Prime Directive of the Star Trek universe, first articulated in 1966, which prohibits interference with less advanced races. It’s also at the root of science fictions where humanity is seen as too violent to join a galactic society, or lacks the ability to hear what interstellar communications do exist - whether technological, biological or mental - or attract attention to themselves. In addition to Star Trek, interesting examples of such non-interference directives include Star Maker (novel, Stapledon 1937), “The Hazing” (short story, Asimov 1942), The Tomorrow People (TV 1973) and various examples from the Doctor Who (TV 1963-1989, 2005-present), Babylon 5 (TV 1993-1997) and Stargate franchises. The physicist and space colonisation advocate Gerard K O'Neill articulated the basic premise clearly in his 1976 popular science book The High Frontier:
the same characteristics which render a civilisation immune to intellectual decay and stagnation, if there be such characteristics, are accompanied by a repugnance to inflict harm on others, in particular to other "emerging" civilisations more primitive than its own, In that case, "They may be out there, but they're kind enough to keep quiet." (O'Neill, The High Frontier, 1978 Corgi edition, pg 191)
It’s worth noting that where the non-interference principle appears in science fiction it is often being broken - whether by humans escaping their homeworld or by aliens visiting it. Sometimes it's clear that such breaches are accidental - here it’s worth noting Roadside Picnic (novel, Strugatsky & Strugatsky 1978) and the Xeelee sequence (novels, Aldiss 1991-2018) amongst others in which aliens don’t even appear to have noticed the humans on their way past.
The second category - a mere disinterest in us on the part of what aliens exist - has always presented a challenge to the self-identity of most human cultures, which tend to centre themselves and their philosophy in the Universe. While examples do exist of science fiction in which humans declare themselves to alien races who shrug and ignore them, these tend to be in the minority - at least in part due to the fact that this makes for a less interesting story than active engagement. One possible example are the Organians who first appeared in the Star Trek episode “Errand of Mercy” - initially appearing to be a low technology race and baffling the crew of the Enterprise by their total lack of engagement, before proving to be highly advanced and simply not interested in getting involved in the conflicts of others. Nonetheless, despite the rarity of such examples, the possibility remains that rather than being unique in the Galaxy in existing as a technological culture, humanity is instead unique in its desire to make contact with others. The Fermi Paradox may boil down not to why aliens haven’t visited Earth but instead to why they have no desire to do so.
Until and unless a viable method of interstellar travel is devised, with the requisite extension beyond Einsteinian physics, it is unlikely that we will ever know the true solution to the Fermi Paradox. Its many aspects will doubtless provide fertile material for both science and science fiction speculations for some time to come.
“The Fermi Paradox”, Elizabeth Stanway, Cosmic Stories blog, April 2022.
 This hypothesis was sensationalised by hotelier and convicted fraudster Erich von Daniken in his best-selling Chariots of the Gods (1968). This series of books presented a highly selective assortment of archaeological results, often distorting their descriptions or omitting key contextual information, and has been thoroughly debunked by many qualified authors. However, its influence still lingers in the popular consciousness. [Return to text]
 Often such self-replicating machines are described as “Von Neumann probes” or “Von Neumann machines” in reference to the self-building universal computers hypothesised by physicist-engineer John van Neumann in the 1940s, and developed conceptually by others including physicist Freeman Dyson. [Return to text]