Science fiction is not monolithic. There are, in fact, two major sub-genres that divide sci-fi — hard sci-fi and speculative (or soft) sci-fi — and most of the time, the delineation between the two is pretty clear.
Hard sci-fi is all about scientific accuracy and logic; it might contain technology and science we don't yet have, but it's all stuff that can exist in our current understanding. Speculative sci-fi, meanwhile, plays a little hard and fast with the known rules of the universe to tell exciting and fantastical stories. These are two staunch pillars that hardly ever meet.
