There's a theory about time travel, it's often used in debates in an attempt to prove that a
journey through time simply isn't possible. It's called "The Grandfather Paradox". The theory is this - if I could travel back in
time, and I traveled to a year where my grandfather was a young man, and say, I got into an argument and killed him - how could my father be born? How could I then be born...and
then go back in time?
Marty McFly faced a similar time travel paradox in Back to the Future. Only his situation was a bit different - a chance encounter with his mother prevented her from falling in love with his father at the right time. You know the rest.
Problem is I think time is a lot less flexible than time travel movies, tv shows and books make it out to be. I can't prove it...but consider this. We have a time traveler, his name is Glenn. From the moment his time travel machine deposits him in the past, he's already changing things. He's bumped into someone, he's bought a drink at a corner store, he's taken a break on a park bench. All of these things don't seem like very much, do they? But I believe they are. I believe every one of Glenn's actions would snowball out of control.
Take that person he bumped into, Mike. Mike is now 2 seconds later getting to the corner. The first time, without the bump, he would have crossed the light at the last second. Now, after a split second's hesitation, he's decided to stop and wait for the next walk signal. That means he's an extra minute crossing the street. Because of that, Mike see's a sales sign in a store he wouldn't have noticed before, because previously his view would have been blocked by a passerby. He goes into the store, he buys a shirt. He's now 22 minutes behind. By the time he gets home, he's late for dinner. Part of the conversation at dinner is about the great deal he got on the shirt, and it leads to other topics that would not have been discussed the first time around. Dinner gets extended. Putting the kids to bed doesn't happen until 40 minutes later than usual. Mike and his wife had been planning to have another child, but as a result of things going later, they do not try that night. Their son is not born. But they try a few nights later and fast forward nine months and they have a daughter.
There was a TV show in 2008 called Journeyman that dealt specifically with this situation. In the show, a San Francisco newspaper reporter (played by Kevin McKidd) finds himself involuntarily traveling through time in each episode. After one time travel adventure, he returns home to find his son Zach has been replaced by a daughter Caroline. As a parent I could only imagine the horror...that, and he has to deal with the fact that nobody remembers Zach! (By the way, Journeyman was a great show. IMDB has it rated as 8.1, which is high for IMDB. It only lasted one season, and deserved better).
Now think about Mike again and his story. He was late getting home. But what about the sales person he interacted with, when he bought the shirt? Or the person he struck up a conversation with, in line? The course of history has changed for these people too, and for all the people they interacted with.
Now think about the time traveler, Glenn. Mike is only just one of the people he interacted with, and look at the changes. What about all the other people Glenn interacted with?
This is why I believe the future would be quite a different place if you change even one very minor thing in the past. The Grandfather
Paradox is a very specific example of an extreme action causing an extreme result, but I believe any change to the past, no matter how small, would have serious repercussions. In other
words, a Time Traveler would have already been changing history long before he got to grandpa's house.