If you can imagine it, it can become reality in science fiction. Unlike other genres, there is no necessity of tying these concepts with fact either, because anything is possible, if you believe as they say. However, you still need to add an element of reality in which a human (or alien) is affected in some way. There always needs to be a character with whom the reader can identify, empathize and/or care about.
You will also need to decide upon the basic setting for the story, so you might have to know a thing or two about space, planets, black holes, etc. You can also do research to uncover the latest information available on possible space and time travel, what the temperature, gravity or other factors are like on different planets and so on. Remember that readers of science fiction want to escape to an imaginary world, so make it sensational! Then don’t just describe this new world; work it into the story through the words, actions reactions and senses of the characters.
There also seems to be a specific type of language used in sci-fi novels that you don’t find elsewhere so read a few similar books before attempting your own. As well, the plot of a sci-fi book is more about stopping some action or creature, the victory of good over evil or the completion of a journey rather.
Finally, most sci-fi books include alien species – some human-like and/or some way out there creatures. If you decide on aliens that can interact with humans, you immediately bring into play the actions and reactions of both. In this way, you can also include such issues as cultural differences, moral values, differing intellects, etc. The views of both humans and aliens must stem from some genetic and biological source. For instance, humans don’t eat other humans. It is just not part of their belief system or even desire to do so. However, aliens may have been raised to believe that eating their enemies is acceptable to keep them strong and virile. On a simpler note, humans often greet each other with a handshake, whereas aliens might bow or at the other extreme, draw a weapon.
Basically, science fiction writing sounds like a lot of fun, if you follow the basic rules of the genre. Maybe, we should all try it just once to see what it’s like!