The Fermi Paradox, in brief, raises the question of why we have yet to observe evidence of extraterrestrial life. The simplest explanation would of course be that such life is extremely rare compared to just how mind-rendingly, jaw-droppingly, eye-wateringly, tooth-shatteringly huge the universe is, how hard it is to generate signals detectable across light-years of space, and what a small part of its lifespan we've been witness to, but many creators prefer to introduce a more interesting reason into their universes.
This can serve two functions. On the one hand, it can provide the author with some excuse for referencing alien cultures, providing alien archaeological sites, and similar without actually introducing a viable alien culture. On the other, it can allow for the creation of interstellar societies that would otherwise appear implausible, especially when they've been so active in our own backyard lately that we would presumably have noticed them in reality.
Common explanations include:
- Disappearing or becoming difficult to observe (at least at interstellar distances) is a natural outgrowth of technological development.
- Intelligent, technological civilisations will inevitably destroy themselves.
- There's a race of aliens, generally of the Sufficiently Advanced variety, who are somehow ensuring that civilizations don't see each other.
- All the spacefaring aliens are involved in a cosmic conspiracy to remain hidden from Earth, possibly because we aren't judged to be advanced enough to know about them, or because we absolutely scare the crap out of them for some reason.
- The weirdness censor strikes again. The reason we haven't seen any explanation of aliens is that we're overlooking the obvious in our quest to exaggerate our own importance and significance.
- Similar to the former. We cannot recognize their signals— for example, we might be mistaking them for background noise, and/or they're too faint to be detected by our (current) equipment, they may be employing very hard to detect directional methods such as lasersnote , or even using a medium we haven't developed yet like beaming signals through hyperspace or quantum entanglement or something.
- All the other aliens are in a pre-sentient, pre-technological, or even incompatible state of development. No Radio Transmitter = No Radio Transmissions.
- We're first. Yay us. The story may be set thousands or millions of years in the future and focus on humanity's descendants encountering something.
- We're last. All of the others have long since Ascended to a Higher Plane of Existence. All that's left on their planets, if anything is left at all, are ruins. Essentially, we're the ones who are lagging behind.
Contrast Absent Aliens, in which the simple explanation is taken as correct.
- The Data Integration Entities from Haruhi Suzumiya are, according to Nagato, undetectable through our normal means. Given that they lack physical form altogether, and seem to exist only as patterns of information, this is perfectly plausible.
- Tengen Toppa Gurren Lagann falls into the third category: the Anti-Spirals found a way to isolate the Spiral races on their planets, and only after the Earthlings defeat them, they discover literally billions of sapient species out there.
- In the Ultimate Galactus Trilogy by Warren Ellis, Reed Richards ponders this very question. The answer seems to be that the sky isn't filled with signals from alien cultures because Galactus ate them. It prefers planets inhabited by intelligent life, and consumes them. There are ''some'' intelligent aliens in the Ultimate universe, but far less than anyone would expect, and they're discreet because they don't want to attract Galactus' attention. Earth seems to be the only planet that ever successfully resisted it.
- In an old comic of Strange Tales, humanity advances to the point where we can send spaceships past pluto only to find an invisible, invincible, indestructible wall surrounding our entire solar system. All efforts by humanity fail to get past the wall, which leads the main protagonist to the conclusion that the wall is in fact a giant FISHBOWL to keep humanity in the solar system because we couldn't live anywhere else. The questions is: who put it there?
- Discussed in The Next Frontier, when Bill Kerman talks about some of the real-life technical difficulties of detecting anything even vaguely recognisable as artificial radio broadcasts at interstellar distances before alluding to "The Bleep", something not unlike the "Wow!" signal emanating from a star eight light years away. On another occasion Jeb mentions a couple of less conventional options like searching for the drive plumes of Generation Ships or the distinctive particle burst that happens whenever an Alcubierre Drive shuts down. Then it's rendered academic because they've spotted evidence of terraforming in the solar system they're charting.
- In Marooned In Realtime by Vernor Vinge, when species attain The Singularity, they uniformly vanish from the universe, at least from the perspective of everyone else.
- In Accelerando by Charles Stross, species all seem to build Matrioshka brains, to which they upload their personalities. The uploaded people aren't interested in interstellar travel because there's no way for them to get enough bandwidth. Everyone else is in hiding from the uploads and therefore not likely to meet each other. (It turns out that something entirely different is happening in the next galaxy over, explaining some cosmological quirks that Real Life science has noticed.)
- In Alastair Reynolds' Revelation Space universe, there's a race of ancient aliens dubbed The Inhibitors (a.k.a. the "wolves") who have for billions of years devoted their existence to exterminating any intelligent life that leaves its planetary system and tries founding interstellar colonies. They have a meaningful reason for it, even if it is very morally questionable. As a result, all the other aliens in the galaxy seem to be either extinct or in very clever hiding.
- In Spin by Robert Charles Wilson, planets with intelligent inhabitants get enclosed by Sufficiently Advanced Aliens in "Spin membranes", which substantially slow down time on those planets to the point where their inhabitants have no good way to make detailed off-planet observations.
- In one of the stories in Sherlock Holmes in Orbit, reviving Holmes after he went over the Reichenbach Falls put Earth out of quantum synch with the rest of the universe.
- In the Priscilla Hutchins series by Jack McDevitt, the galaxy seems to naturally produce things called "omega clouds" every 8000 years or so, strange clouds that are sent out in a wave from the galactic center and can look for and track excessive, clearly artificial regularity on planets — in other words, square and rectangular buildings and constructions of other shapes that are too regular and precise to appear in nature — and crash into them, sometimes catastrophically, to eliminate societies that have advanced much beyond the Stone Age. Some characters theorize that an omega cloud may have inspired the legends of Sodom and Gomorrah when it destroyed an early Earth civilization, since the relative timing fits with when the clouds should have last been through.
- "They're Made Out of Meat" by Terry Bisson suggests another possible reason: the aliens found us and were too weirded out to establish contact. Can be read here.
- The Berserker series by Fred Saberhagen has organic life threatened by the eponymous Killer Robots, who were the Doomsday Device released by the losing side of an intergalactic war. Only a few scrappy (not The Scrappy) species manage to survive, including Earth-Descended, or E.D., life.
- The Honor Harrington novels state that humanity has, in the two millennia they have been tooling around the galaxy, encountered approximately twelve alien races. However, though there is at least one extinct race that achieved inter-stellar flight, the technology level of the other species is never mentioned. Alien races show up only as flavoring (the Sphinxian treecat, shown in almost every story) and a minor background element in two stories. For all practical intents and purposes, it is an all-human galaxy.
- In The Metamorphosis of Prime Intellect by Roger Williams, there were hundreds of alien worlds containing intelligent life at the time of The Change, but when the title AI became omnipotent, its interpretation of the first law led it to freeze them in stasis so that they would never become technologically advanced enough to pose a threat to humanity.
- Addressed in More Information Than You Require, as seen in this excerpt. Hodgman's theory is that Enrico Fermi himself was an alien trying to deflect suspicion. Why else would he demand his only reward be two healthy sperm whales?
- In "The Fermi Paradox Is Our Business Model" by Charlie Jane Anders, all intelligent life-forms that evolve in the galaxy inevitably destroy themselves. They were all seeded by a race of traders who swoop in and pick up all the technology they have lying around after they go extinct.
- In Diaspora by Greg Egan, quite a few advanced civilizations developed in the Milky Way, but they all ran away into other universes to escape an imminent galactic natural disaster.
- The "Noahs" of Carrera's Legions are only known of indirectly because of a few found artifacts and their molding of Terra Nova into its present state, which includes species from Earth's distant past and some plants that were apparently genetically engineered to prevent the rise of intelligent life on the planet by being poisonous to creatures with greater mental capacity.
- In Blood Music, the protagonist theorizes that The Singularity of self-aware biological computers that is overtaking/assimilating human civilization happens with alien civilizations; why explore beyond one's own solar system when a speck of dust is a world unto itself?
- Sylvia Engdahl's Enchantress from the Stars and The Far Side of Evil have an invisible alien as the protagonist. The mature sentient species have found that immature sentient species will suffer severe psychological scarring and never live up to their potential should they find out there are more advanced species out there, and so take great pains to stay invisible.
- In Star Trek, the Prime Directive prohibits the Federation from interfering with aliens until they've discovered interstellar travel; according to Star Trek: First Contact, the Vulcans had a similar rule before the Federation was founded, which presumably "explains" why we haven't seen any Vulcans yet in real life.
- In Doctor Who, the Shadow Proclamation, basically a galactic police force, has rules about how alien life forms are not allowed to make contact or interfere with worlds without a minimum amount of technology — not that most of the aliens seem to care about that, as they really like trying to take over/destroy/control Earth for some reason. It was this rule and groups like UNIT and Torchwood which kept the public from learning about alien existence until around 2008 when the Daleks invaded and physically teleported the Earth.
- In the old Lucasarts game Zak McKracken and the Alien Mindbenders, the Skolarians claim (in a recording) that they left for another world millennia ago, though not before littering Earth and Mars with hidden artefacts should the Caponians attempt to invade. The Caponians, on the other hand, mostly hide with the aid of nose glasses and tall hats. Since they're successfully dumbing down Earth's population significantly, they actually manage to pull it off... until the heroes interfere, that is.
- The Mass Effect series explains why humanity wasn't contacted before we opened the Mass Relay. The Galatic Council bans opening new Relays to unknown locations, after carelessly doing so in the past caused them to encounter the Rachni, leading to a subsequent Bug War that threatened all life in the Galaxy before the Rachni were wiped out. It's later discovered that the reason there are oddly so few Alien civilisations is because the Reapers harvest all space-faring races every 50,000 years..
- Dead Space has no sign of alien civilizations despite there being some pretty hefty evidence that intelligent extraterrestrial life does (or did) exist. Dead Space 3 finally reveals why: the Brethren Moons, the ultimate form of Necromorph, ate them all. The Marker was also created by them, to ensure that humanity would follow and become their next meal.
- Last Res0rt has a few planets with civilizations they're waiting to see "grow up" before they walk in and help them the rest of the way; it's implied that Earth was let in only a couple millenia ago (specifically 2012, according to the RPG), and Adharia's planet, Sevru, is still a few centuries off. However it's also implied that the Talmi were only recognized as a sapient species a few centuries ago. Also, most of them are Jewish.
- In The Inexplicable Adventures of Bob!, Earth is located in the Nemesite Empire, but the Nemesites have designated our world as a nature preserve and have tried to leave us fairly undisturbed, just like animals in a national park. It's recently been revealed that another reason is that the Nemesites had a very, very messy First Contact with Earth's pre-human dragon civilization, and so they're practicing a loose Alien Non-Interference Clause approach with us.
- Sam Starfall (a highly visible alien), in Freefall, has an interesting explanation:
Sam Starfall: There is no Fermi Paradox! Every time space-faring aliens make it to Earth, the cows get them!
- xkcd # 638 - "The Search" is the current page image, and has a good metaphor for #6.
- A fantasy version in the case of the Adar from Tales from My D&D Campaign. With their innate mastery of psionics, this race was once a great power in the land. But a couple of millennia back, they sent out messages to all the other races saying, in these exact words, "We are tired of the gods interfering. We are leaving now." And... they left. As far as anyone can tell, every last Adar, and even most of their cities, simply vanished into thin air. Noone has the slightest idea where they went or what they are now doing.
- An episode of South Park explained that we don't see aliens because all life on Earth was put there by them as part of one really long-running alien reality TV show called — you guessed it — "Earth".
- One of the alternate Earths of Rick and Morty has Earth being ruled by squirrels. And yes we do get to see it briefly.