When I work on a story, I've got three main concerns. 1: It has to be strong in terms of plot and action, because that keeps my publishers happy. 2: It has to have some thematic depth, something that fleshes it out and makes it more round and whole (because that makes me happy). 3: It has to have a certain level of plausibility.
By 'certain level of plausibility', I mean: it might contain an idea or concept entirely fantastic and/or impossible within our current understanding of the laws of physics and the universe as we know it (force fields, FTL etc), but contained within that idea or concept is the notion that our understanding of the universe is sufficiently incomplete that new paradigms might, maybe, some day make the apparently impossible possible, or at least potentially achievable. When you live in a universe where once-impossible things like atomic bombs and space travel have since become everyday, where the instantaneous teleportation of information is well researched, and where concepts such as multiple alternate universes are regarded as entirely plausible by people incredibly better qualified than me to hold such opinions, the act of suspension of disbelief becomes rather more easily achievable. In fact, for me to say that something is either possible or impossible when I am unqualified to hold any such opinion (by dint of not being a physicist or a scientist) strikes me as entirely foolish.
Yes, personal force fields that let you hit a mountain and the mountain breaks (as in Stealing Light) are completely fantastic, but try and tell me Hugh Everett's Many Worlds theory is any less bizarre (or his theory of quantum immortality, for that matter). And yes, I am aware there's vastly more evidence for Everett's notion of multiple universes than there is or likely ever will be for personal force fields. But that's one of the fun things about being a science fiction writer.
Now to my primary concern here: it has on occasion been suggested to me that science fiction is a branch of fantasy. I disagree, and here's why.
Science Fiction is largely driven by the idea that the universe and its workings are still largely unknown. The human mind is driven to enquire into its nature in order to understand it. That understanding, though incomplete, is increasing. Science Fiction, then, is a form of writing that allows one to speculate on the apparently impossible, given that history is littered with incidents in which the impossible has been shown to be, in fact, possible. It's this latter point that allows for the willing suspension of disbelief on the part of a reader (or this reader, anyway).
This is not to say that all things impossible will eventually prove to be possible; but the incompleteness of our knowledge allows some of us to freely speculate, and to imagine the impossible made possible without feeling too guilty about it. This is particularly rewarding for those who, like me, lack any scientific training and yet have a fascination for the achievements of modern science. Given that scientific history is primarily composed of a series of paradigms giving way to each other as successive generations of enquiring minds throw up new questions with the aid of constantly improving tools, one might feel further justified in speculating on a world in which the current paradigms could perhaps also be superseded.
If we take 'fantasy' in its purest form - which to me usually means elves, magic swords and orcs - I think it's fair to say these are things which neither exist nor are going to exist, ever. One might speculate as to the (im)possibility of faster-than-light travel, time travel or alternate realities; no one to my knowledge has ever speculated on the possibility of finding elves, orcs or magic swords any time soon. Let's be clear: I do not say this to denigrate fantasy in any way, and I also acknowledge that as definitions go, it's an appallingly crude one. However, it is very much my contention that a work of 'fantasy' in this particular sense offers a fundamentally different form of experience from that provided by the majority of works of science fiction. Let me repeat: this is not the same thing as suggesting either form is intrinsically superior or inferior to the other; it is not to suggest a lack of equivalence in the writing skills necessary to produce either. I merely want to make clear my opinion that the suggestion one is somehow a branch of the other is, to my mind, incorrect.
I know there are probably countless examples and counter-examples. I don't lay any claims to impartiality in my reading tastes either, because I've generally avoided books with elves, orcs and magic swords like the plague throughout my life. I have never in my life picked up a book with a dragon on the cover and thought, 'gee, I really want to read that'. So why do I feel the need to make this distinction?
Because I get a bit annoyed when people tell me I write fantasy in such a way that the implication is (to my ears, anyway) that a work of fiction containing aliens and space craft is somehow the same as one featuring magic swords, elves and dragons. They're different forms, with different aims and different appeals. One might as well say science fiction shares many traits with, say, the detective genre, because both are bodies of fiction. It's a statement that is equally, on analysis, both true and untrue.
For me, sf and fantasy (within the very narrow definitions I've used here) merely are what they are; different things with different aims living in different boxes, that sometimes share the same audience - but not always.