The only way back time travel could be possible is if nothing changed, as if every action of the person was predetermined... the only movie I can think of that actually did that well was the first terminator... the others don't really follow that logic though.
but think of it this way. how can you do time travel if you can't do teleportation? cause if you suddenly went back 6 months... you'd be in the same place... in space with the earth having moved to the other side of the sun, the sun having rotated arround the milky way's center and the milkyway having drifted somewhere else... or you could end up in the middle of an other star....
An other thing... if your time travel is for some reason a static travel ala time machine movie... then as soon as you start time again, you are both in the place of the current time... and traveling to that time. (when you go back 10 years in a non moving sphere, the sphere has to be in that spot for the 10 years, otherwise there is an inconsistency in the physical occupancy of space....)... but that's also a problem when you step onto the machine... you body is occupying the space you will be occupying again when you are traveling back these seconds.... which makes the principle of a static jump impossible. (i'm bad with words sorry)
In short, if you jump in time, you'll be in the middle of space or objects... but if you travel in time, you have an inconsistancy by being in the same space twice.... which means the only thing you could do is travel forward... which recoups the gravitational effect on time, making time run slower closer to massive objects...
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