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 &#x27;rules&#x27; of your universe.<p>You can violate the real laws of physics, so long as you do so consistently within the rules&#x2F;laws as you&#x27;ve defined them. You can have &#x27;magic&#x27;, so long as the magic is used consistently.<p>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 (&quot;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&quot;) then it might even be accepted. If you do it with no notice because it&#x27;s merely convenient to the plot (&quot;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&#x27;ll be good to go&quot;), then it&#x27;s stupid.