The story Kmar is referring to is "The Sound of Thunder" by Ray Bradbury (I think). The problem with this (and with all 'change the past to change the future' stories) is that you then have made one or more events in the past dependent on a future which hasn't happened yet.
Confused by that last statement? Wait, you haven't heard the good part yet. If Tempus, for example, goes back in time and kills Clark as a baby in Schuster's Field, Utopia doesn't happen, and the chain of events that causes his parents to meet is also broken, therefore he doesn't exist, and he can't be there to expose Clark to Kryponite, so Clark grows up to be Superman and marries Lois and the result is Utopia and Tempus gets bored and goes back in time --
Ouch. My head hurts.
There are a couple of ways to get around that loop in causality. The first is the most obvious, that our universe is actually a determinist universe and there is no free choice for anyone, we're only playing our parts, and any individuality we think we feel is an illusion.
Me, I don't like that one. Besides, it would mean that all my imaginations are a product of natural and mechanical processes, not my own sparkling wit and vivid thought processes.
The second obvious way is to postulate a "branch" universe created by the change in the timestream. This theory says that every choice made anywhere at any time has a possibility of carving out a new path of reality, and that every choice we make in fact does create a new reality.
I'm not real fond of that one either, because no one has shown that there's room for an infinite number of parallel universes in our reality. It's a wonderful fictional concept, but not many scientists embrace it. (Some talk about it because they can't think of any other theory that fits their world view.) But it's the theory that seems to fit the alt-Superman worlds in the actual show and in a lot of the fanfic on this site, and if you were asking me directly, I'd advise you to go with this one.
For more information (assuming you want to subject yourself to it), just type in "chaos theory" in any search engine.
Hope this helps the discussion.
