Paradoxically speaking

In one of my SF novels, after killing all the baddies and saving the universe, my heroes return home the day before they left. There – sorted.

   ‘But you can’t do that.’ One of my friends announced proudly.

   ‘Why?’

   ‘Because you’ll create a paradox.’

   ‘Huh?’

   'Well it will create an infinite loop where your heroes leave, do their stuff and return the day before they go, only to go again the next day and the next, forever.’ etc etc, ad nauseum.

   ‘Well they create a new quantum timeline when they return,’ I sighed relieved to bring the “Many Worlds” theory into a conversation for the first time in my life.

   ‘But what if the new timeline isn’t exactly the same as the one they left? You might have flying buses or wives that are man-eating raptors.’ This was all becoming a bit extreme now. And, anyway, my wife would make a man eating raptor look like a girl when she gets upset.

   And that got me thinking.

   Within the Second Law of  Thermodynamics, as my primate brain understands it; Note LAW and not a theory. So apparently it’s proven even though we’d all have to survive until the end of time to test it; it states that after all the black holes finally coalesce and eat the universe, and long, much longer after that, when even the photons have decayed, there won’t be anything left; not time, not space. Bugger all in fact.

   Yet, if we listen to the String Theory, alternate dimensions or branes hang fluttering like washing from a galactic line until a particularly spirited solar wind makes them touch. Lo, the energy expended creates another big bang.

    You can’t have it both ways. Either time ends or it doesn’t. So what I’m coming to in a rather roundabout way is. And even if all I’ve said above is a complete load of old nonsense; provided we the writers attempt to keep science within the bounds of silliness, is it really necessary to get it completely right. And provided we give them a good read – does it really matter?

Any thoughts?