We've all heard the advice to "write what you know." But what about sci-fi and historical topics? I love books in these genres and the old/new settings and civilizations described. How then can authors write something they know about if they didn't live hundreds of years ago or thousands of years in the future? It's a mystery!
But, if I were to guess. I would say historical research in the first case and research+science+imagination in the latter case. At least that's how I would tackle it. For science fiction, I would imagine all kinds of science gadgets that I wish I had or dreaded having, then give them to some young characters to figure out. I would do the same thing with clothing, environment, and family relationships to name a few. Then, I would start fiddling with a mashup of several of the threads. Conflicts? Characters? Journeys? Resolutions?
I wouldn't start out knowing much about the story, but with a lot of analysis and (for me) outlining. I would know a lot by the time I finished the story. How about you? Do you wonder about writing in the past or future, invisible or unknown? - Q