The Grandfather Paradox - if
you go back in time and kill
your grandfather, then you
would have never been born
to kill your grandfather , so
you would be alive to kill your
grandfather et.
To go back or forwards in time, you
would have to displace matter, but if
your current self didn't already exist
in that time then you would have to
s suddenly take up take up space
that you didn't before.