Time Travel, Eggs, and Sperm

There's been something that always has bugged me about time travel as depicted in movies, books, and TV shows. It is where you see someone change the past, then you go to the "present" and see all of the same people, but in different roles. Now, this is ok where the change was not that far in the past and did not have ripple effects that would have travelled out far enough to cause a change before those people were born, but it is done routinely with things much further back.

The problem is, if you change even slightly something in the past before a given person was born, if that affects that person's parents at all, that person will very likely never exist. I see this is simple statistics. There are millions of sperm in a man's testicles at any given moment. Only one of those is the one that will make a particular individual. Any tiny, miniscule change, and a different sperm is the one that makes it to the egg, if any make it at all. Maybe all it is is changing the moment of conception back an hour or to a different day. Any slight change like that and voila, you have a different person. Now, on the egg end of it, there is more leeway. Only a single egg (generally) gets released during an ovulation cycle, and there might even be some predictable order to which egg matures on a given cycle, so at least that half of the genetic code has some more stability to it, but even then, you only have a few day window in a given month for a given individual to be born, and if something prevents conception during those few days, there goes the other half of the equation. On top of that, factors inside the woman's body after ejaculation may also play a role in which sperm makes it to the egg, so you have yet more variables to consider.

In all, the chance that a particular sperm will fertilize a particular egg is rather small, and so anything that changes the initial parameters will very likely result in a different individual being born. Multiply this times the population of the planet and add in some significant change in the time stream far enough back, and you have a planet full of people, none of whom match the people who would have been there originally.

Of course, the basic Hollywood reason to avoid this problem is so you can use the same actors and, in the case of certain TV shows, have a bit of fun with them playing different roles. That is a bit of dramatic license that is excusable, particulalry where it really isn't a very serious sci-fi show (like some sort of "It's a wonderful life" flashback). But where it is supposed to be hard sci-fi, it just bugs me.

I've only seen one show actually address it (Journeyman), and it made me very happy. I really liked the show in general - so of course, it got cancelled. Ugh. But at least in one episode, the protagonist time-travelled and changed the past, causing him to miss a "date" with his wife on a particular day, so his son turned into his daughter, because she was concieved on a different day.

Of course, given the "let's use the same actors" phenomenon, I don't expect to see this actually addressed much on TV or in movies. But there's always the written word.