GRAVITY

   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 delightfulaccount of the attractions of weightlessness was given by Fritz LEIBER in "The Beat Cluster" (1961); a more straightforward introduction iscontained in Arthur C. CLARKE's Islands in the Sky (1952). In Bob SHAW's THE RAGGED ASTRONAUTS (1986) the most difficult part of interplanetarytravel by BALLOON (no free fall here) between two mutually orbiting planets only 5000 miles (8000km) or so apart, and with a common atmosphere, 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 or SPACE HABITAT is to spin the shipabout an axis to generate a 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 CaptiveUniverse (1969). Few writers apart from Clarke mention the Coriolis force, a sideways force on a moving object which also results from a spinning system, and makes things tend to move in circles; it might be a severe disadvantage in a very large spinning spaceship. 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.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 (1954). 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" (1957), James BLISH's "Bridge" (1952) - the story which describes thedevelopment of spindizzies - and Arthur C. Clarke's "A Meeting with Medusa" (1971), from which was developed The Medusa Encounter (1990) byClarke and Paul PREUSS.Much stronger gravitational forces than these can be expected near the very massive but small objects composed of collapsed matter (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 "Neutron Star" (1966) and "There is a Tide" (1968). A collapsing star of sufficientmass (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 tosf writers, and is discussed in some detail in that entry. 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 fulfilmentthan to science, but does offer a technological explanation for the behaviour summarized in the title.Gravity as a theme has naturally been in the main the province of HARD-SF writers like Hal Clement and Larry Niven. Working very much in their tradition are the physicist Robert L. FORWARD,who has written 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 (1991) is set in an ALTERNATE UNIVERSE 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 are described with elan.

   TSu/PN