Imagine your existence in time as a line from birth farthest to the left and into the future to the right. This line stretches into infinity - in both the past and future directions.
Imagine that a dot is moving along this line from the past to the future. It can only go from past to future, and cannot move faster than any other dot.
Now since you’ve stepped back away from the line, you can jump back in at any point - or into any future point.
Since there is a speed limit along the line, your actions in a past create a new future that proceeds up the line - but never can catch up with future events - since they’ve already ocurred for that dot 250 years ahead. Parallel time - but not really parallel. Time exists as a complete entity - infinitely from beginning to end. You could always jump back to point a, witness and/or interact to affect historical events or take the market by storm. Jump back again to point a - and everything will still be the same as it was on the first jump. Jump ahead - and it would be as if everything you just did had not happened. No stocks, and Obama is still president.
Hence - movies like the new ‘Star Trek’(2009)with 2 Spocks existing within the same time might make more sense than ‘Terminator’, where a “time event dot” could somehow speed along the line and instantly change all time.
...and yes - I did stay at a Holiday Inn last night.
I think I need more coffee.
If “time travel” into the past is indeed feasible, it’s already happened and is part of history. If I, or you, have done it, we’ve done it and are destined to do so at that future time when we did so.
Time travel into the future implies a preexisting future, and thus also demands predestination.