The tropes and themes of SF are increasingly mainstream, or as Seed puts it, there is a "realignment of novelistic genres so that it is no longer assumed that science fiction is marginal". Even fantasy is becoming trendy and, according to China Miéville, can be a "subversive, radical" genre. Uniquely, SF authors have occupied a liminal terrain between "the empirical and the extraordinary", a wonderfully liberating space in which they are free to let their minds wander through alternate histories, post-nuclear landscapes, feminist utopias and dystopian cities, to name but a few of the genre's many themes, including (but not solely, as some misguided souls believe) the impact of science and technology on society. SF is, in Seed's neat formulation, an "embodied thought experiment", writing in the "what if" mode; or, as Samuel Delany puts it, "subjunctivity". From space operas to cyberspace, this brief yet thorough introduction emphasises SF's rich diversity: more than a single genre, it is "a mode or field where different genres and subgenres intersect".