In writing fantasy, you don't even try to base mechanics on current theory. Fantasy is predominantly about relationships, where the mechanisms of action are use

In writing fantasy, you don't even try to base mechanics on current theory. Fantasy is predominantly about relationships, where the mechanisms of action are useful for furthering character colour and the plot but not in themselves key plot points. In sci-fi, the mechanisms tend to be at least plausible developments of current theory. In some cases, they even form the basis of the work rather than a mere plot point. Compare Greg Egan's oeuvre, where in many cases the mechanism is the point of the work rather than a plot device.