Overall name for the theories of space-time Physics formulated by Albert Einstein (1869-1955): special relativity, proposed in 1905, and general relativity, proposed in 1916. Both theories, though initially controversial, have been exhaustively tested by experiment.
In special relativity it is axiomatic that there is no absolute or "privileged" frame of reference against which velocity can be measured: only the relative velocity of any given two bodies is meaningful. A second axiom is that, counterintuitively, the velocity of light is the same irrespective of the frame of reference in which it is measured. A light beam projected forwards from a moving train travels no faster than one shone in the same direction (or any other direction) from a stationary trackside lamp. The logical consequence, emerging from undergraduate-level calculations, is that – although relativistic physics is almost indistinguishable from classical Newtonian physics when relative velocities are no greater than found in everyday life – increasing distortions of mass, length and Time are observed as velocities reach a significant fraction of c (the speed of light, roughly 3 x 108 metres per second). Measured length shrinks towards zero, an effect formerly known as the Lorentz or Lorentz-Fitzgerald contraction; measured mass tends towards infinity as the relative velocity approaches c; Time Distortion likewise tends to infinity, with each of two observers moving at high relative velocity seeing time pass more slowly for the other. This effect is known as time dilation.
Meanwhile, the velocity c cannot be attained or exceeded by adding lesser velocities – as in Edward Page Mitchell's "The Tachypomp" (January 1874 The Sun anon) with its extravagant plan to attain unlimited speed by running a succession of railway trains on top of one another; Spaceship equivalents have been proposed by several hopeful authors. Alas, the axioms of special relativity lead to a nonlinear addition rule which always yields a sum less than c.
The notorious "twins paradox" imagines twin brothers of whom one stays on Earth while the other makes a round trip by relativistic Spaceship to another star, and despite the apparent symmetry of Time Distortion returns home significantly younger than his Earthbound brother. The seeming paradox can be explained, though not simply, in terms of the accelerations and reversals experienced only by the space-travelling brother. Robert A Heinlein dramatizes this situation in Time for the Stars (1956), with the relativistically untenable addition of constant Faster Than Light contact between the twins via Telepathy. Such simultaneity makes it impossible for each twin to observe the other as time-slowed: Heinlein settles for the more comfortable asymmetry of having the Earth-twin perceived by his brother as living faster (and, conversely, the space-twin slower), eliminating the paradox at some cost to the physics.
Further sf stories which play with relativistic time-dilation effects include L Ron Hubbard's earnest Return to Tomorrow (February-March 1950 Astounding as "To the Stars"; 1954), Vladislav Krapivin's "Meeting my Brother" (trans 1966), Larry Niven's A World Out of Time (fixup 1976), Tom Shippey's "Not Absolute" (in Andromeda 3, anth 1978, ed Peter Weston) as by Tom Allen, George Turner's Beloved Son (1978), Mary Doria Russell's The Sparrow (1996) and Gene Wolfe's Book of the Short Sun (1999-2001). Such effects are taken to spectacular extremes in Poul Anderson's Tau Zero (June-August 1967 Galaxy as "To Outlive Eternity"; exp 1970), whose protagonists become Flying Dutchmen aboard a damaged relativistic Starship that can only accelerate, at one stage smashing through galaxies in an eyeblink as a car might encounter a swarm of bees; ultimately they outlive the Universe and see the birth of its successor (> Cosmology). Even more expansive is Frederik Pohl's and Jack Williamson's The Singers of Time (1991).
Very many sf stories use relativistic time dilation for one-way Time Travel into the future. Joe Haldeman's The Forever War (June 1972-January 1975 Analog; fixup 1974) makes particularly effective use of the effect as a metaphor for the culture shock of US soldiers returning from Vietnam. The constraints of relativity are not generally popular in Space Opera, when Faster-than-Light drives predominate – but notable exceptions include Alastair Reynolds's Revelation Space (2000) and its successors in the Inhibitors sequence, where considerable effort goes into orchestrating the movements of Sublight ships over many years of interstellar flight towards significant encounters.
The elementary changes have now been rung, but there is probably further scope for intriguing time-dilation plots. One such is Redshift Rendezvous (1990) by John E Stith, set on a starship in a version of Hyperspace wherein the velocity of light is so low (22mph or 35kph) that its passage is visible, and relativistic phenomena are obvious at walking speed. This echoes the instructive Thought Experiments with changed universal constants which are dramatized in George Gamow's Mr Tompkins in Wonderland (coll 1939 chap).
Some memorable imagery attempting to envisage real relativistic visual effects (mistakenly, as it later turned out, because based on an overly simplistic scientific paper) can be found in Frederik Pohl's "The Gold at the Starbow's End" (March 1972 Analog; exp vt Starburst 1982).
General relativity is a far more profound and complex theory which marries special relativity with Newtonian Gravity to describe the deep geometry of space-time and so form the basis of modern Cosmology. Its most science-fictionally interesting prediction is the existence of Black Holes. Greg Egan's Incandescence (2008) is a tour-de-force of Conceptual Breakthrough by an enclave of human-descended beings orbiting a Black Hole, who lack Technology and cannot make conventional astronomical observations but nevertheless contrive to deduce the nature of their situation and the detailed general-relativistic space-time geometry governing the local orbital mechanics. [DRL/BS]
see also: Communications; Dimensions; Antigravity; Elements; Imaginary Science; Space Warp; Tachyons.
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