I look at it like this: You can make up whatever craziness you want in your universe, but the craziness must remain consistent and follow the 'rules' of your un

I look at it like this: You can make up whatever craziness you want in your universe, but the craziness must remain consistent and follow the 'rules' of your universe. You can violate the real laws of physics, so long as you do so consistently within the rules/laws as you've defined them. You can have 'magic', so long as the magic is used consistently. Heck, you can even break your own previously-established rules, so long as you can explain why in way that is consistent with everything else. If you do it with foreshadowing ("there is some theoretical way to make our faster-than-light engine allow us to travel in time, but no one has ever been able to get it to work") then it might even be accepted. If you do it with no notice because it's merely convenient to the plot ("hey, I just came up with this crazy idea to make our ship travel through time, give me 5 minutes to re-route power and connect a widget and we'll be good to go"), then it's stupid.