The Fermi Paradox – not a true logical Paradox – denotes the apparent contradiction between plausibility arguments for the existence of numerous Alien civilizations (based on the age of the universe, estimated numbers of potentially life-supporting planets in the habitable zones of suitable stars, and so on, encapsulated in a formula known as the Drake equation) and the lack of hard evidence for any such extraterrestrial culture, past or present. "Where are they?" asked pioneer nuclear physicist Enrico Fermi (1901-1954), posing what has also been termed Fermi's Question. Science fiction offers a wide variety of answers including the following:

1. The Drake equation is flawed or has been misinterpreted or fed with incorrect values. We are indeed unique and alone, as in Stephen Baxter's Time: Manifold 1 (1999; vt Manifold: Time 1999). One bizarre explanation is that our solar system is a Pocket Universe and the distant stars are illusory, as in Philip José Farmer's Behind the Walls of Terra (1970; rev 1982). Baxter's "Touching Centauri" (in Phase Space, coll 2002) posits that the solar system is a limited Computer simulation whose programming breaks down catastrophically at the first attempted interaction across interstellar distances.

2. Intelligent life arises so rarely, or the resulting civilizations are so short-lived (owing to the inevitability of War, unwise research into Physics, development of malign AIs, or simple exhaustion of local resources), that the odds of meaningful encounter across interstellar gulfs and Time Abysses are vanishingly small. This argument is central to John McLoughlin's Toolmaker Koan (1987). In Harry Harrison's "Final Encounter" (April 1964 Galaxy) the "aliens" discovered by humanity's long galactic search prove to be our own much-changed kinfolk (see Pantropy).

3. The universe is a highly dangerous place in which any civilization drawing attention to itself is liable to be snuffed out by some equivalent of Fred Saberhagen's Berserkers, or by a wide variety of extraterrestrial predators, from the Martians of H G Wells's The War of the Worlds (1898) to the Crackers of Stephen Baxter's Space: Manifold 2 (2000; vt Manifold: Space 2000) or the Trisolarians of Liu Cixin's Santi (May-December 2006 Kehuan Shijie; 2007; trans Ken Liu as The Three-Body Problem 2014). The watchful Secret Masters in Eric Frank Russell's Sentinels from Space (November 1951 Startling as "The Star Watchers"; exp 1953; vt Sentinels of Space 1954 dos) are guarding Earth and other worlds from premature First Contact with the dangerous "Denebs". In Charles Stross's Laundry universe, thought and computation are increasingly likely (as population grows) to attract the deadly attention of Cthulhu Mythos entities; the Fermi Paradox is explicitly referenced in the series novel The Nightmare Stacks (2016). David Brin's Existence (fixup 2012) features many rival alien emissaries who have long staked out the solar system but keep a low profile owing to memories of past Berserker attacks. In Liu Cixin's Hei'an Senlin ["Dark Forest"] (2007; trans Joel Martinsen as The Dark Forest 2015), humanity counters the coming Trisolarian Invasion (see above) with a credible threat to broadcast the location of the invaders' home system across the "dark forest" of the galaxy, thus attracting dangerous attention. Much more rarely, a benevolent intervention removes civilizations from the galactic stage as in Robert Charles Wilson's Spin (2005).

4. Aliens are indeed frequent visitors to our solar system but choose to conceal themselves, as in countless UFO scenarios and the many stories in which humanity is not yet deemed fit to be told of and initiated into galactic society. Examples of the latter trope include Robert A Heinlein's Have Space Suit – Will Travel (August-October 1958 F&SF; 1958), Lloyd Biggle Jr's All the Colors of Darkness (1963), Clifford D Simak's Way Station (June-August 1963 Galaxy as "Here Gather the Stars"; 1963), Robert Silverberg's Those Who Watch (1967) and Frederik Pohl's Narabedla Ltd (1988).

5. A variation of 4 sees humanity abhorred and shunned. Edmond Hamilton's "The Accursed Galaxy" (July 1935 Astounding) posits that everything else in the expanding universe is fleeing our galaxy because it carries the hideous infection of life. Similarly, Alien intelligences with a different physical substrate find us just too unbearably icky in Terry Bisson's "They're Made Out of Meat" (April 1991 Omni). Our unwittingly destructive habit of observing things and collapsing their multi-valued quantum wave functions leads to Earth being screened off from the universe at large in Greg Egan's Quarantine (1992).

6. Galactic Communications transmissions fill all space but humanity detects only empty silence because we lack the preferred Technology of Ansibles, Dirac Communicators, Ultrawave receivers or the like. An unprecedently powerful radio telescope suffices to pick up the transmissions from Andromeda in Fred Hoyle's and John Elliot's A for Andromeda (1961), whose never-seen senders tempt us with blueprints for advanced Technology whose ultimate aim is to replace humanity with recreated Andromedans. Piers Anthony's Macroscope (1969; cut 1972) proposes photon-like particles called macrons which are ideal carriers for interstellar and intergalactic data-traffic but can be sensed only by the titular Invention – whereupon the transformingly information-rich messages prove to be booby-trapped with Basilisk imagery. In this class of contact, mere passive listening proves hazardous even without drawing attention to oneself as in 3 above.

7. Galactic society may thrive in other parts of the Galactic Lens but Earth is or has been unfortunately situated. In Poul Anderson's Brain Wave (September 1953 Space Science Fiction as "The Escape", first instalment only before magazine ceased publication; 1954) our solar system moves at last out of an Intelligence-inhibiting region of space; in Daniel F Galouye's The Lost Perception (1966; vt A Scourge of Screamers 1968) Earth is similarly emerging from the Stygum Field which blocks a sense regarded by advanced extraterrestrials as more important than sight; Vernor Vinge's A Fire Upon the Deep (1992) places us in the galaxy's "Slow Zone" where Faster Than Light travel and genuine AIs are close to impossible – thus the real galactic action is farther out from the core in the "Beyond", where such restrictions no longer apply.

8. The Aliens are so very different that they do not recognize us, nor we them, as sentient beings; the gap of incomprehension may or may not be ultimately bridged by a Conceptual Breakthrough leading to genuine First Contact. Examples here include Frederik Pohl's Slave Ship (1957), Brian N Ball's Sundog (1965), George R R Martin's "Guardians" (October 1981 Analog) and Stanisław Lem's Fiasko (1986; trans as Fiasco 1987).

Further and weirder explanations have been proposed. Anthologies with the Fermi Paradox as their theme include Is Anybody Out There? (anth 2010) edited by Nick Gevers and Marty Halpern, and Paradox (anth 2014) edited by Ian Whates. [DRL]

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