Unscrambling the Egg

I've been watching "Timeless" on E4 for the past few weeks. It's reasonably entertaining, though it is utter nonsense, of course. But then, so is the rest of this post, so be warned...

While watching the show yesterday, I was wondering about the classic sci-fi debate: if you travel forward in time, do you go to the future, or do you go only to a future - that is, are the events that you see what will happen, or only something that might happen?

I promptly answered myself: due to Uncertainty, you really should travel only to a future, rather than the future. God does indeed play dice with the universe, and so if you roll it twice you'll get different results.

But then it occurred to me: why is travelling back in time any different? After all, the equations work both ways around. So, why should travelling backwards take you to the past, rather than to a past?

And, equally, it's worth noting that if you travel back in time and then forward again, you almost certainly can't get back to where you came from - that second journey forward will take you to a future from the point you left, not the future. In TV shows about time travel, the characters are generally very concerned about not changing the future. But those concerns are misplaced - surely, even if they do nothing at all while in the past, they'll still wind up somewhen different?

(As a simple example of this, consider that if you travel to a point before your own life, the odds of returning to a future in which you even exist are incredibly remote. After all, your existence hinges on one particular egg, out of millions, being released in ovulation in order to meet one of millions of sperm cells at just the right time. The odds against that being repeated are vanishingly small... and even that only provides for the existence of a genetically-identical person - there's considerably more to "you" than that!)

Bottom line: if you ever invent a time machine, don't ever ever use it!