In Physics, a thought experiment or Gedankenexperiment is a common means of exploring the implications of scientific assumptions without resorting to actual apparatus. Einstein's development of Relativity employed thought experiments about light signals exchanged between observers whose relative speed was a significant fraction of the speed of light, not easily attained by any real observer or (at that time) measuring device. The first conscious crafting of something like a thought experiment in fiction may be the novel Die Wahlverwandtschaften ["Elective Affinities"] (1809 2vols; trans R Dillon Boylan as Elective Affinities 1854) by Wolfgang von Goethe (1749-1832), where the chemical theory that some species "tend" to combine provides an excuse for an induced ménage a quatre whose participants are hidden away in a country house, which Goethe describes as a "chemical retort" where their interactions can be observed in isolation. In science fiction, any story which uses sf devices to expound or illuminate a scientific principle may be described as a thought experiment.
Such stories range from the didactic whimsy of George Gamow's Mr Tompkins in Wonderland; Or, Stories of C, G & H (stories 1938-1939 Discovery; 1939 chap) – dealing with Relativity, Gravity and quantum physics – to Hal Clement's less obtrusive exposition of high gravity in Mission of Gravity (April-July 1953 Astounding; cut 1954; text restored with additions and one added story, as coll 1978) and of variously chaotic weather and quasi-weather systems in Close to Critical (May-July 1958 Astounding; 1964), Star Light (June-September 1970 Analog; 1971) and Still River (1987). A K Dewdney's The Planiverse: Computer Contact with a Two-Dimensional World (1984) downplays storytelling in favour of gleefully imagined Flatland science – the design, for example, of a workable two-dimensional steam engine. John E Stith's Redshift Rendezvous (1990) develops one of the Mr Tompkins conceits with a version of Hyperspace whose very low speed of light makes Relativity effects an aspect of everyday life; Stephen Baxter's Raft (September/October 1989 Interzone; much exp 1991) outdoes Mission of Gravity with its depiction of an Alternate Cosmos whose gravitational constant is so large that humans can sense one anothers' pull and Stars are only a mile or two across.
In Mathematics, sf thought experiments can give a sense of the vastness of numbers. The "infinite monkey theorem" whereby – given infinite time – a randomly typing monkey or team of monkeys will eventually generate the complete works of Shakespeare is an old favourite. This features briefly as an actual if unsuccessful physical experiment within an Oxford college in Edmund Crispin's detective tale The Case of the Gilded Fly (1944; vt Obsequies at Oxford 1945); is amusingly dramatized in R A Lafferty's "Been a Long, Long Time" (December 1970 Fantastic), whose action stretches over an almost unimaginable Time Abyss; and is spoofed in Russell Maloney's "Inflexible Logic" (February 1940 The New Yorker). Similarly vast and hard to comprehend is the titular Library of Jorge Luis Borges' "The Library of Babel" (in El jardín de senderos que se bifurcan, coll 1942), which in effect contains all possible outcomes of the monkeys' stochastic efforts. Everything is there and nothing can be found (see Information Theory), even when the hypothetical library is digitized and made computer-searchable as in David Langford's "The Net of Babel" (February 1995 Interzone).
Sociological and cultural-anthropological thought experiments in sf are very numerous and diverse. Unusual human societies – of which Jack Vance was a particularly prolific creator – are regularly encountered on remote Islands, Space Habitats, and planets settled during the Colonization of Other Worlds.
A "real" thought experiment which has fascinated many sf authors is the Schrödinger's Cat paradox. This dramatizes the quantum-physics concept that the wave function representing a physical system is a superposition of possible outcomes which does not "collapse" into a particular state until observed. Should the cat in the box, whose death may have been triggered by an unpredictable quantum event (radioactive decay whose detection releases Poison gas), be regarded as in a mixed state – neither alive nor dead until the box is opened by an observer? Sf explorations of the theme include Douglas Adams's whimsical Dirk Gently's Holistic Detective Agency (1987), Greg Bear's "Schrödinger's Plague" (29 March 1982 Analog), George Alec Effinger's "Schrödinger's Kitten" (September 1988 Omni), Ursula Le Guin's "Schrödinger's Cat" (in Universe 5, anth 1974, ed Terry Carr), F Gwynplaine MacIntyre's "Schrödinger's Cat-Sitter" (July/August 2001 Analog) and Rudy Rucker's "Schrödinger's Cat" (30 March 1981 Analog). Robert Anton Wilson's 1979-1981 Schrödinger's Cat trilogy deals with a labyrinth of Parallel Worlds. A large "Schrödinger cat box" is used for randomly delayed human execution in Dan Simmon's Endymion (1996). In a chapter of Flatterland: Like Flatland Only More So (2001), Ian Stewart examines and debunks the seeming paradox, explaining that the cat/box quantum system is sufficiently large that any mixed state would almost instantaneously collapse or "decohere".
"Maxwell's Demon" was propounded by the physicist James Clerk Maxwell (1831-1879) as a thought experiment in thermodynamics, whose second law states that Entropy (disorder) must always increase. The tiny imaginary demon acts as gatekeeper between two gas-filled chambers, allowing fast (hot) molecules to pass in one direction but not the other so that in apparent violation of thermodynamics one chamber grows ever hotter and the other ever colder. (A more sophisticated analysis shows that the overall entropy must in fact increase, because to distinguish fast from slow molecules the demon in effect needs a flashlight whose energy output adds more entropy to the system than the selection process removes. John Lowell dramatized this argument in comic verse as "The Diabolical Heating Scheme of J C Maxwell" [in More Random Walks in Science, anth 1982, ed Robert L Weber].) Such demons are harnessed in Joseph Samachson's "A Feast of Demons" (March 1958 Galaxy) as by William Morrison, which features a demonstration in which one side of a vat of liquid spontaneously freezes while the other boils, and goes on to postulate entropic and anti-entropic demons which respectively cause accelerated ageing and Rejuvenation. Larry Niven also actualizes Maxwell's Demon – as a joke – in the fantasy vignette "Unfinished Story No. 1" (in All the Myriad Ways, coll 1971).
Notable twenty-first-century examples of thought-experiment sf include Ted Chiang's "Exhalation" (in Eclipse 2, anth 2008, ed Jonathan Strahan), with its melancholy evocation of how a mechanical sentience in a nearly-sealed Pocket Universe might deduce the nature of Entropy, and Greg Egan's Incandescence (2008), demonstrating in unsparing detail how low-tech dwellers in a closed environment where conventional astronomical observations are unavailable could nevertheless reason their way to general Relativity. Such ambitious sf thought experiments can convey an austere, almost purely intellectual Sense of Wonder.
A relevant anthology is Gedanken Fictions: Stories on Themes in Science, Technology, and Society (anth 2000) edited by Thomas A Easton. [DRL]
see also: Feminism; Iconoclasm; Metaphysics; Satire.
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