Tuesday, January 27, 2009

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.


James Polley said...

I lost you right around "If you could change something in the past".

It's fantasy. You can't change the past; your ramblings are just as irrational as storylines where people are almost the same but in a slightly different line of work.

(You're right though. About the only time-travel premise that hasn't made me switch off my brain in self-defence was in the movie "Prime", where they don't so much travel in time as jump into a parallel timeline which happens to be delayed by X hours.

Or. Something. It's a slightly confussing movie. Hard to be sure.)

The Barefoot Bum said...

It's called dramatic license.

The purpose of "time travel" in most literature is not to examine the physics of time travel, but rather to introduce what the author really wants to talk about: how things might be different.

James Polley is kind of correct: If you're going to quibble about dramatic license, then you might as well go whole hog and simply dismiss all fiction.

(Of course, there are contexts and circumstances where even dramatic license fails. Even with relaxed standards of factual accuracy, writing fiction is not trivially easy, and dramatic license is not a free pass.)

DBB said...

Time travel is theoretically possible. And there's nothing in physics that says you can't change the past. You may just end up moving to a different quantum universe.

As for dramatic license - I thought I acknowledged that, in not so many words, that it is useful as an interesting plot device where you can use the same actors.

I don't begrudge dramatic license. The point is, it seems to ONLY be used that way, even where it doesn't have to be. But then that kind of gets me where I'm raw - science as portrayed in popular media. They get it so wrong so much of the time, and usually not as dramatic license, but just because they are too lazy or stupid to get it right. Hell, I've seen them get it wrong as part of the storyline where the storyline would have been vastly improved and more interesting had they bothered to get it right.

I guess I could forgive the time travel thing more if science in general wasn't handled so poorly.

Silver Hexagon said...

I know you posted this seven years ago, but i was thinking about the same thing today, while showering. I was thinking about the show CONTINUUM, and just make this spoiler free, some shit that happens in the future shouldn't of happened anymore. circumstances changed drastically enoguh to where people should not exist. Of course i couldn't figure out why the show bugged me the way it did, but somehow in the back of my mind i knew. (btw it was an awesome show regardless)

I had asked my gf what she thought about it....my time travel and the ONE sperm idea, and just like the two comments you got....she dismissed it by saying im overthinking FICTION. and shes a bigger comic book nerd than me. and I think that's the problem we can see in most people. they have this kneejerk reaction to where things get way too real and complicated that they rather not think of it. Either way, you are not alone. I share your same concerns when I realized how fragile one's existence really is.

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