Some time ago, I wrote a little think-piece on here about dumb time travel in fiction, calling out some of the more ridiculous things movies, books and games do with it that make absolutely no sense. Well, whilst I stand by every word of it, the offenders listed there do deserve an apology. It's not that they aren't dumb, it's that most of the others are too.
Oddly enough, what led me to this realisation is actually one of my favourite repeat offenders, the DC TV-verse. Without veering too far into spoiler territory, recent plots in the DC shows are revolving quite a lot around time travel, and one of the noted side-effects has been that a character who previously had a daughter now instead has a son. This development sent me down a rabbit hole of thinking that led to some pretty startling conclusions.
One quite common trope (I hate that word, but it's useful here) in time-travel stories is a character having to make sure that they get born. Maybe they stop someone killing one of their ancestors, or persuade their future parents to get back together after a breakup. This is usually presented as a pre-destination paradox- the result of an event (e.g. Marty) goes back in time and causes the event (e.g. Marty's parents getting busy). What isn't addressed is how incredibly, astronomically difficult that is. I'm going to do that really pretentious italicised quotes thing now, because I haven't done it before and it looks cool:
"I can't wait to get back to Deep Space 9 and see your face when you find out that I never existed!"
Julian Bashir, DS9, 'Trials and Tribble-ations'
Julian doesn't know the half of it. In 'Trials and Tribble-ations', a sub-plot features Bashir wondering if he's supposed to be his own great-grandfather. Leaving aside for a moment the effects a time-loop might have on genetic diversity, as a doctor Julian should know something very important, which most of these stories (with the notable exception, presumably by pure luck, of the DC shows) ignore. In order to make sure a baby 'A' is produced by parents 'B' and 'C', we don't just need them to fall in love and produce a child- we need them to do it at exactly the right moment.
"When your parents combined their DNA, the odds of them producing someone with your precise genetic pattern were ten million to one. Add in the odds of your parents meeting and bothering to procreate in the first place, and the odds of your existence are along the lines of drawing three straight Imperial Courts in an honest game of Vedran whist! If you overcame those odds once, who's to say you can't do it again? "
Tyr Anasazi, Andromeda, 'The Widening Gyre'
Now Tyr gets it, or most of it. As most of us know, the process of fertilisation involves millions of sperm attempting a Death Star Trench Run to be the first to reach and penetrate an egg. Unlike Rebel fighters, however, these things are constantly being created and re-absorbed by the body, and so every one is subtly different. Even the most minute change in circumstances might result in a different sperm remembering to use The Force and... ok, that metaphor has stretched past breaking point. Where I'm going with this is that even a few seconds difference in the time of conception will probably lead to a different resulting person. Of course, they'll be a very similar person in many respects, carrying most of the same genetic markers and so on, but even two 'identical' twins will generally go on to lead different lives. Our baby will be more like a brother or sister to the potential person who should have been born if our hapless time traveller hadn't spilt coffee in the lap of his great-grandfather and made him miss the train home. (Not actually 'our' baby, of course. I mean, I'm flattered, but I don't think of you that way.)
Once again, this whole thing probably comes under the heading of Thinking About It Too Much, but it's certainly an interesting thing to consider. So many stories revolve around the idea that time can take a certain amount of punishment and just bounce back as if nothing had happened, or maybe you come back to find your dog never died and now your uncle speaks Portugese, but as we've seen, go more than one generation and the chances that any of the same people even exist start to become pretty remote. It's probably the sort of thing that gets Richard Dawkins very excited and makes him write books that make Buddhists want to punch him. I feel a Nightwish segue coming on..