A couple of years ago, there was a conversation between some friends of mine about a science fiction story that went something like this:
Person A: You know what problems in worldbuilding are like? They’re like when you’re cycling along, and suddenly hit a patch of stony ground, and end up flipping over your handlebars.
Person B: Hmm, I think I’ve got better suspension than you.
Person C: I’ve got a hovercraft!
[Dialogue slightly edited from original post following an account from someone who was there]
I wasn’t present at this conversation, but since then “the hovercraft of disbelief” has been helpful shorthand to denote one person having a different threshold of disbelief from another. For instance, when I posted a couple of weeks ago the video of someone skydiving from low orbit, Russell Letson commented to say ‘my first response is “How the hell did she avoid burning up on re-entry?” Sense of wonder can be overcome by skepticism, and maybe one of the distinctions between the core readerships for SF and fantasy is that ordering (or perhaps triggering-level) of responses.’  The same question had occurred to me, a bit, but was overtaken by primal isn’t-this-cool-ness. Which I suppose is an obvious point: different people will have their hovercrafts (to extend the metaphor) floating at different altitudes over the ground. Reader expectations play a part too: if you read a story in Analog, by and large, you expect more in the way of rigorous scientific worldbuilding than if it’s in Lady Churchill’s Rosebud Wristlet. And I do agree with Russell that one of the things reading in genre sf does is to prime you to ask these kinds of questions.
Case in point. Martin Amis’s short story “Straight Fiction” (1995, collected in Heavy Water and Other Stories) is about a world where almost everyone is gay and an embattled minority is straight. So, for instance, movie stars get outed as “Totally Het”, there are samizdat erotic novels called things like Breeders, and Amis gets to have fun with ideas like this:
“Every winter Cleve reread half of Jane Austen. Three novels, one in November, one in December, one in January. Every spring he reread the other half. This was January and this was Pride and Prejudice.
“‘Yeah,’ he said. ‘For like the ninth time. What I can’t get over is – every time I read it I’m on the edge of my seat, rooting for Elizabeth and Mr Darcy. You know – *will* Elizabeth finally make it with Charlotte Lucas? *Will* Mr Darcy finally get it on with Mr Bingley? I mean, I know everything’s going to turn out fine. But I still suffer. It’s ridiculous.'” (208)
The problem I ran into was the elementary one: where do all the babies come from? “Straight fiction” is set in more-or-less contemporary New York, in a world where, until recently, you could be arrested or worse for public displays of heterosexual affection. Amis seems to recognise this is a problem, and provides various different accounts of where all the straight people have gone: San Francisco is referred to as a place where as many as two in five people are heterosexual; and the protagonist Cleve “learns” from a news magazine that there might be as many as two and a half million straights in greater New York City. One assumes that gay couples use surrogacy techniques as they do in our world, but none are shown doing so. A straight pregnancy is depicted, but it seems as rare and strange to Cleve as a green sky. And if only a minority are having children, how did New York get to be as populous as it is? How do the demographics work? Are those children brought up in straight families socially disadvantaged? There must surely be a huge number of them. Did none of Amis’s characters come from straight families? Amis could, I suppose, be making a point about social erasure of groups that majority culture isn’t concerned about, but his response to the question of children is so twitchy and partial that I doubt it. Amis’s response would doubtless be that he’s not interested in that kind of question, that he wants to explore the satiric potential of his central conceit. And perhaps the sorts of questions I ask above are the sort that a science fiction reader would tend to ask. (Amis has another story in the same collection, “Career Move”, riffing off exactly the same kind of inversion: in this world, poets are the ones who get flown to Hollywood and have their work made into huge-budget epics, whereas screenplay-writers get their work published in micro-circulation magazines run by catastrophic alcoholics. Again, there’s no sense of how this inversion works, how the poems actually make money.)
There’s a separate issue with the Amis. On the surface, he seems to be depicting a mostly-gay New York as a benign and idyllic place. But at times, he seems to be deriving his comedy from playing up to stereotypes of gay life: “So Cleve, who had not had sex at the gym, blew Kico in the front hall and then set about making dinner: a Gorgonzola souffle to be followed by the Parma-ham confit with pomegranate, papaw, papaya, and pomelo” (205) The gay men who are depicted – and the story focuses almost entirely on them rather than women – are gym-obsessed, tank-top-wearing and, of course, have moustaches.
Maybe I’m living up to the stereotype of sf-reader-as-overly-literal-irony-free-zone. But one of the things that genre sf has built up over its history is a huge armory of technique to do with worldbuilding – how to convey information naturalistically, how to make a world at least seem consistent. (Because it’s all scenery flats in the end.) As a reader you build up a matching set of skills. There are different writerly approaches to the issue (Ursula Le Guin, Larry Niven) just as there are different degrees of readerly tolerance: I myself find the political issue with “Straight Fiction” – the stereotyping of gay culture – far more of a problem than the worldbuilding issue. And worldbuilding can escape the shackles of literalism – probably the most prominent practitioner/advocate of this is M John Harrison. The weird thing about the Amis story, in particular given the moral and intellectual rigour of Time’s Arrow, is that it clearly knows there’s a vacancy in its ideas and it just doesn’t care.
 Admittedly, Gregory Benford weighed in later to note that “[The video] appears to be based on my short story, “Orbitfall,” published in Baen’s Universe about half a year ago. Indeed, avoiding burnup on reentry places demands on the suit. But as well, in the film and in “Orbitfall,” she isn’t in orbit. The falling velocity is thus much less (about 10% of orbital velocity).”