You guys HAVE to read
Flatland if you haven't ...
I like the theory that "timetravel" is really the traveling between close parallel universes.
It couples with the theory if each 'cusp' (decision point) creating a branch point - assume that each possible decision was made, and each result is it's own universe (remember, infinite parallel universes). Some, of course, have already ended from poor decisions, etc.
Anyway, travelling back in time and changing something that "already happened" would not be a paradox of the same sort in that you were actually generating a new sheave of universes based on that change, and your presense there.
In the universe you are 'from' you would not be making that change.
So, going back and killing your grandfather would not make you vanish (POOF!) and not create a paradox of the first sort where you could not have done that because as soon as you do you are not present to do it (which would actually simply prevent you from doing the paradoxical thing in the first place and cleans up the problem in another way in another theory).
There is a lot of good theory floating about on various possible scenarios, but one thing in common to most is that a 'future' would not change suddenly if a traveller went back and changed something in the past - that thread is already set, and that change did not happen. The "change" observed by the traveller is at most his switching tracks to one of the futures where that cusp resolved in that manner.
Sort of a "you can never go home again" played out completely.
One other final point, is that in most of these thories it is impossible to tell which one is actually operative, because the "subjective" results are all the same, and the "how" of it can only really be determined by an "outside observer" able to see the entire working of the actions, and by definition in most theories that position is not available ... /ubbthreads/images/graemlins/crazy.gif
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