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The
grandfather paradox. You travel back through time and kill your
own grandfather, (even if only by accident). So you end up not being
born. But if you're not born, you can't go back in time to kill
your grandfather. The only sure thing is that you're going to end
up with a headache.
But
of course people have come up with all sorts of ways around that.
One way is the multiple time lines idea - that any decision will
result in a split into different time lines. Thus you can go back
in history, but even if you don't change anything, you can't go
back to the future from whence you came.
Many
prefer the more stable approach, where the timeline can be bumped
around, but largely goes in the same general direction unless there
are some really big changes.
The
grandfather paradox can be resolved by simply saying that once you
have come back in time, your link to your past is effectively severed
and you are a separate entity, so no matter what you do to your
ancestry, you still exist.
Is real
time travel possible? Yes it is possible, but not in any useful
way.
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