ONE OF THE MAY VIRTUES of science fiction is that it permits the reader the opportunity, not generally available otherwise, of living in the future -- of sampling an alternate reality, coherent and plausible but unfamiliar and provocative. Of course, the degree to which the reader can experience this effect is governed by the skills whereby the writer spins his web of futility out of his knowledge of today's world, and also -- a rarely appreciated fact -- by the reader's own intensity of imagination and ability to transform the symbols on the printed page into living images of the imaginary unimaginable.

John Varley is a youngish West Coast writer who in the past five years has had extraordinary success in giving readers the experience of living in the future. His work is a continuously imagined fabric of clonings, limb replacements, organ transplants, sex changes and other wonders, mostly taken for granted as part of the backdrops of his stories (though he can explain them plausibly and in detail when necessary). His confident understanding of the way things work leads him to attempt audacious concepts -- such as turning the head of a comet into an interplanetary cruise liner used for sightseeing excursions to the sun, or painting the rings of Saturn red, or holding conversations with black holes; and these startlements are developed not as surrealistic japes but as calm, quiet, matter-of-fact plot centers. The Varley method is effectively demonstrated in the first few lines of The Barbie Murders (Berkley, $2.25), a collection of nine splendid short stories:

"There was a bomb on the Leystrasse, level forty-five, right outside the Bagatelle Flower and Gift Shoppe, about a hundred meters down the promenade from Prosperity Plaza.

"'I am a bomb,' the bomb said to passersby. 'I will explode in four hours, five minutes, and seventeen seconds. I have a force equal to fifty thousand English tons of trinitrotoluene.'

"A small knot of people gathered to look at it.

"'I will go off in four hours, four minutes, and thirty-seven seconds.'"

The stories are all like that -- straightforward in style, jaunty and colloquial, briskly told without literary flourishes, marvelously and unfailingly inventive. Several of the stories ("Equinoctial," "Beatnik Bayou," "Lollipop and Tar Baby") are brilliant; all are vigorous and unfailingly astonishing: a fine book from the most accomplished new science fiction writer of recent years.

Less immediately accessible to the novice sf reader is the handsomely printed (though poorly proofread) Their Immortal Hearts: Three Visions of Time (West Coast Poetry Review, 1335 Dartmouth Dr., Reno, Nev. 89509, $5): but these three long stories are far from being esoteric obscurities. Michael Bishop's "Cold War Orphans" is a sensitive and moving Christian variation on the myth of Icarus -- science fiction only by extension and implication, but so well written it will give joy to all but the hardened space-opera fanatic. Barry Malzberg's "Le Croix (The Cross)" is a typical Malzbergian obsessive black comedy probing at the nature of society, in particular the essence of the religious function within a technological world: like much of this writer's darkest work it has wildly funny moments. And Bruce McAllister's lengthy "Their Immortal Hearts," though it begins as conventional sf of the clumsiest, dreariest kind, gradually gains visionary power, turning into a bizarre tour-de-force conveying an elliptical, dazzling view of a barely comprehensible elitist society of the future.

To bring off such effects requires a certain passionate kind of seriousness about the future, and the lack of that is what undoes Justin Leiber's flawed first novel, Beyond Rejection (Ballentine/Del Rey, $2.25). Leiber is the son of sf grand master Fritz Leiber, and to go into competition with such a father is a formidable challenge; but the younger Leiber, a member of the philosophy department of the University of Houston with advanced degrees from Oxford and the University of Chicago, is a formidable challenger. Unfortunately his first attempt at science fiction shows little more than promise. He handles the hardware of sf nicely, but the polt is sketchy and schematic, and the prose is no more than efficient bare-bones stuff. Leiber makes no sustained attempt at letting us live in the future. His ostensibly far-future world is peppered with mild jokes at the expense of today's Houston and San Francisco; he indulges in little nudges in the direction of Joyce, Melville and Fritz Leiber; and he barely touches the implications of his central sf situation, that of implanting stored personalities in "blank" bodies. For anyone else, this light and slender novel might be a passable debut. But -- leaving questions of heredity aside, for it is cruelly unfair to expect the son of a great writer to achieve greatness automatically his first time out -- Justin Leiber is obviously capable of something more than this fluffy divertissement, which seems the work of a playful, highly intelligent man in the grip of terminal facetiousness.

Facetiousness of a more potent kind is the hallmark of Craig Strete, a diabolical young writer of American Indian ancestry whose first collection of short stories, If All Else Fails. . . (Doubleday, $8.95), comes bearing an introduction by Jorge Luis Borges, no less. "I would like to introduce you to a collection of small nightmares of great consequence," says Borges, calling Strete his "most startling discovery" on a recent American trip. Powerful praise; and, almost unexpectedly, Strete lives up to it. The 19 strange and idiosyncratic stories in this explosive and abrasive book -- with titles such as "Who Was the First Oscar to Win Negro?" and "Why Has the Virgin Mary Never Entered the Wigwam of Standing Bear?" -- strike all sorts of terrifying resonances. They remind one of other writers -- the young Ray Bradbury, and Ishmael Reed, and the late and nearly forgotten Cordwainer Smith, and even the Beatles of "A Day in the Life," but those are only superficial resemblances, for Strete is very much sui generis , an angry, bitter, exhilarating writer capable of wild frenzied flights of fantastic comedy. Only a few of his stories can be considered true science fiction, though all have a powerful streak of fantasy. Most draw their energy from Strete's American Indian background, and his narrative method, disconcerting and even frightening to readers accustomed to European storytelling modes, becomes more comfortable to the degree that one is familiar with non-linear native American narrative techniques; but that should be no drawback to the sympathetic and adventurous reader. Here and there Strete and John Varley overlap in material, since both deal in the changes and miracles that render life in the late 20th century so plastic (in all senses of that word). But Strete is the next stage farther out. Both take miracles calmly, but Varley's orthodox storytelling methods tend to downplay their impact. Craig Strete seems to live in the world of the incomprehensible future that John Varley has so brilliantly imagined.