The force of gravity is the most inescapable and unvarying fact of terrestrial life, and when writers first sent characters into Spaceships and on to other planets the phenomenon of low gravity, or of no gravity at all, figured prominently among the wonders of space. Many early authors did not realize that complete weightlessness is a consequence of free fall, but this soon became a fact to be taken for granted in describing Space Flight, and now few writers bother to emphasize it. A delightful account of the attractions of weightlessness was given by Fritz Leiber in "The Beat Cluster" (October 1961 Galaxy); a more straightforward introduction is contained in Arthur C Clarke's Islands in the Sky (1952). In Bob Shaw's The Ragged Astronauts (1986) the most difficult part of interplanetary travel by Balloon (no free fall here) between two mutually orbiting planets only 5000 miles (8000 km) or so apart, and with a common atmosphere (this being an Alternate Cosmos), is the transition of the weightless zone where the two gravitic pulls cancel out.
Weightlessness in practice is more likely to be a nuisance than anything else. The favoured method of providing "artificial gravity" in a spaceship, Space Station or Space Habitat is to spin the construction about an axis to generate a perceived centrifugal force acting outward from the axis, so that the vessel's wall becomes the "floor". The visual paradoxes associated with a "gravity" that acts outwards on the inside of a hollow object were exploited in the film 2001: A Space Odyssey (1968), in Arthur C Clarke's Rendezvous with Rama (1973), and in Harry Harrison's Captive Universe (1969). Few writers apart from Clarke consider the additional space-station complications caused by the Coriolis force, an effective sideways force on a moving object which also results from a spinning system, and makes things tend to move along curved paths relative to a rotating ship or planet. This might be a severe disadvantage in a fast-spinning spaceship. Robert M H Carter's "Rotating Frame-Up" (in Pulsar 2, coll 1979, ed George Hay) uses just such a setting for a somewhat contrived "murder mystery" involving Coriolis deflection of a thrown weapon. The Coriolis force is not encountered if the gravity is provided by a constant linear acceleration, nor if the problem is solved outside known science by having recourse to gravity generators such as Spindizzies or the artificial-gravity grids in James White's Sector General stories. Larry Niven, in "A Kind of Murder" (April 1974 Analog) characteristically considers Coriolis force as a complicating factor in Matter Transmission between different latitudes of Earth.
Centrifugal force also comes into play on rapidly rotating planets, where it combines with the force of gravity to define the direction of the vertical. Since the surface of a planet tends to be generally at right angles to the combined centrifugal and gravitational forces, the centrifugal force can be treated for most practical purposes as a part of the gravity, having the effect of decreasing the gravity at the equator (where it is already likely to be lower because of the shape of the planet), as in Hal Clement's Mission of Gravity (April-July 1953 Astounding; cut 1954; text restored with additions and one added story, as coll 1978). This novel tells of the very high gravity on the massive, rapidly rotating, discus-shaped planet of Mesklin, and of the effect of these conditions on the psychology of the planet's intelligent lifeforms. In our solar system high gravity, nowhere near as extreme as Mesklin's, can be found on Jupiter; this is described in Poul Anderson's "Call Me Joe" (April 1957 Astounding), James Blish's "Bridge" (February 1952 Astounding) – the story which describes the development of spindizzies – and Arthur C Clarke's "A Meeting with Medusa" (December 1971 Playboy), from which was developed The Medusa Encounter (1990) by Clarke and Paul Preuss. Human adaptation to high-gravity planets is frequently considered in Colonization of Other Worlds scenarios, and massively muscled people from such worlds became something of a Cliché – a well-known example being the Valerians, mighty-thewed colonists of a 3g world, who appear in E E "Doc" Smith's Galactic Patrol (September 1937-February 1938 Astounding; 1950) and later Lensman novels. A darker view of such adaptation, and the divisions which it would inevitably cause, is presented in Robert Silverberg's "Misfit" (December 1957 Super-Science Fiction) and Harry Harrison's "Heavy Duty" (May 1970 Analog) as by Hank Dempsey.
Much stronger gravitational forces than these can be expected near the very massive but small objects composed of collapsed matter (see Neutron Stars; Physics). Not just the gravitational field's overall strength is important: the variations in its strength between different locations can exert forces even on an object in free fall. These are called "tidal forces": the tides on Earth, caused by the difference between the Moon's gravitational pull on opposite sides of Earth, provide the most familiar example. Tidal forces feature in Larry Niven's stories "Neutron Star" (October 1966 If) – where they are strong enough to kill – and "There is a Tide" (July 1968 Galaxy). A collapsing star of sufficient mass (about three times that of the Sun) would pass through the neutron-star stage to become a Black Hole – some high-gravity stories of the 1970s and 1980s are discussed under that heading – and there has been a large amount of sf set around (or even within) such venues.
The wish for a method of manipulating gravity has been a rich source of Imaginary Science. Indeed Antigravity has been something of a Philosopher's Stone to sf writers, and is discussed in some detail in that entry (see also Tractor Beam and Pressor Beam). The attraction of antigravitational themes grows from a kind of resentment at the inescapable restraints gravity imposes on us in the real world. Cecelia Holland deals in rather cavalier manner with gravity in Floating Worlds (1976), the worlds of the title being cities floating above Saturn and Uranus. David Gerrold's Space Skimmers (1972) exploits an imaginary gravitic effect (using gravity as a kind of point applied to a surface) which yields an attractive spaceship designed as if by M C Escher. Walkers on the Sky (1976) by David J Lake owes more to wish fulfilment than to science, but does offer a technological explanation for the behaviour summarized in the title.
Gravity control also suggests the possibility of locally increasing gravity. This yields a Weapon in E E "Doc" Smith's First Lensman (1950), where a fortress is surrounded by an ultra-high gravity field which traps and crushes attacking Spaceships; a portable "gravity mine" deals similarly with a fast-moving assassin in James H Schmitz's A Tale of Two Clocks (1962; vt Legacy 1979). Local gravity concentrations (in translation, "mosquito mange") are deadly hazards in "Piknik na obochine" (1972 Avrora; trans as Roadside Picnic in Roadside Picnic/Tale of the Troika, coll 1977) by Arkady and Boris Strugatski; in Piers Anthony's Macroscope (1969; cut 1972) the low gravity of Neptune's moon Triton (see Outer Planets) is focused to provide a region of normal Earth gravity for the convenience of visitors.
Gravity as a theme has naturally been in the main the province of Hard-SF writers like Hal Clement and Larry Niven. Writers who have worked very much in their tradition are the physicist Robert L Forward, who wrote two interesting novels about a lifeform living in intensely high-gravity conditions on the surface of a neutron star – Dragon's Egg (1980) and its sequel Starquake! (1985) – and Stephen Baxter, whose Raft (September/October 1989 Interzone; exp 1991) is set in an Alternate Cosmos where gravity, instead of being (to simplify) the weakest of the fundamental forces, as it is in our Universe, is one of the strongest. The results, including the fact that the mutual gravitic attraction between human bodies is perceptible to ordinary senses, are described with élan. [TSu/PN/DRL]
see also: Cosmology.
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