There can be confusion in the usages of the terms "alternate worlds" and "alternate realities"; in essence, the latter term embraces the former, which is more a product of sf than of fantasy. An Alternate World may be looked upon as a form of AR, but there are other alternatives to our mundane Reality than parallel Earths, many rooted in mystical ideas, like those of Theosophy or the notion of the Astral Plane, rather than in science. A classic example of an AR is the Australian Aboriginal Dreamtime, which not only was a period in the past but is still contiguous and isomorphic with the mundane world.
While certainly the Playground of quantum physics has been rich territory for sf writers in search of alternate-world ideas, fantasy writers have played in it too, producing tales that draw on the alternate-world imagery yet envisage a quite different relationship between the other reality (or realities) and our own. For example, in Gene Wolfe's There Are Doors (1988), what can be regarded in the most superficial sense as an alternate world – a grim place into and from which the narrator stumbles repeatedly and uncontrollably – is in fact a place without any real past; whether it is a reality born of the narrator's deteriorating Perception of the world about him, or whether it has come into existence in response to the demands of his bitter unhappiness with his life – or even whether it has always existed, history-less – is a matter left open to the reader's interpretation; what is certain is that it is a reality other than our own which cannot be related to it by any simple, any quasi-scientific, explanation. The tale thus uses, in short, the imagery but not the mechanism, and thus conveys a completely different impression of what is actually the circumstance.
Most tales of ARs are, for obvious reasons, Crosshatches: most often the focus of the tale is some form of interaction between the alternate reality and our own, even if only to the extent that a protagonist may skip between the two. This is a clear difference between alternate-reality and alternate-world stories, in that many distinguished examples of the latter exist in which there is no such cross-linkage – as in SS-GB (1978) by Len Deighton and Fatherland (1992) by Robert Harris, where the Nazis won World War II, or Promises to Keep (1988) by George Bernau, where Kennedy (or a President very like him) survives the Dallas assassination. Otherwise, an AR that is quite unlike our own and has no cross-linkage with it is almost certainly not an AR at all but a Secondary World.
ARs pervade many novels whose concerns are about other things besides, as in Mark Helprin's Winter's Tale (1983). Yet some focus on the topic. The protagonist of Lisa Tuttle's Lost Futures (1992) strays through a diversity of existences, enduring all sorts of "wrong" lives, before discovering that the true reality – the real world – is not the one she seemed to start from, which was ours. In Dream Science (1990) by Thomas Palmer ARs come about because reality is a far more complex construct than we perceive: the fullness of reality is made up of a Russian-doll-like set of concentric shells, each of which is a reality that has the same status as our own. In Daughter of the Bear King by Eleanor Arnason a woman discovers that, at first in her Dreams and then in her waking life, she dwells in an entrancing AR. In Ken Grimwood's Replay (1986) the diverse realities, each sparked by changes of history, are sequential rather than concurrent, so that the tale thereby once more depends on a view of reality (specifically, in this instance, the nature of Time) that is far more complex than what we perceive.
One special category of AR tale is where the other reality is a created one; that is, a character in one reality has created a Secondary World of some type, and that secondary world comes to have a reality of its own, and possibly also an accompanying history that stretches back further than the moment of creation. Paul Kearney's The Way to Babylon (1992) is an example of a basic version of this theme: a fantasy writer discovers the existence, in some skewed direction, of the Fantasyland which he has created as the setting for his novels. A similar effect is brought about by a haunted room on a failed detective-story writer in The Further Adventures of Captain Gregory Dangerfield (1973) by Jeremy Lloyd: each different adventure-land created by the room's previous resident, a more successful writer, becomes, for a while, a reality experienced by the protagonist. God Game (1986) by Andrew M Greeley (1928-2013) sophisticates such ideas a little: the protagonist has created his world through the medium of a marvellously clever interactive computer Game, whose semi-free-willed denizens recognize their own origins and his – for he is, though unworshipped, quite literally their God. In Ralph Bakshi's movie Cool World (1992) the created reality is the somewhat arbitrarily constructed world of a Comics artist; its previous history not only predates the act of creation but has affected our world in that earlier time. John Grant's The World (1992) takes a metafictional view: the created reality in question is the setting of his earlier novel, Albion (1991), and has come to have existence through the publication of that book. The reality within Pictures (which serve as a Portal to it) has been a frequent source of fascination: "The Hall Bedroom" (1905) by Mary Wilkins Freeman is one example; in James Branch Cabell's "The Delta of Radegonde" (1924) a lover enters by Magic a picture of the long-dead Queen Radegonde, and is welcomed by her; and entering pictures is the premise of Ian Watson's anthology Pictures at an Exhibition (anth 1981), which thus features AR stories like David Langford's "Transcends All Wit", set in Dürer's etching Melencolia; the protagonist of Stephen King's Rose Madder (1995) discovers an AR (and herself) through venturing into a picture. Movies like The Purple Rose of Cairo (1984) and Last Action Hero (1993) credit the world behind the cinema screen as being a created AR. Adventures of Baron Munchausen (1989) provides examples of scenes presented on the theatrical stage becoming full-blown realities, at least for a while.
There are also, of course, innumerable tales – from Lewis Carroll's Through the Looking-glass (1871) onwards – of the worlds that exist on the far side of Mirrors. These can be viewed as secondary-world stories except (normally) where the mirror-reality begins to interact with our own. Also, almost all Posthumous Fantasies are by definition tales of ARs, in that they posit the existence of some unseen plane of reality. But by far the majority of AR stories do not require physiological or physical Portals, like death or the surface of a mirror: if there are portals, they generally lie in the mind. [JG]