h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Time cannot be changed as some say. It cannot be altered at all in fact.

Every thought and action that you do or have done or will do has already happened.

Think of time as a line and by traveling through it you would go to a different snapshot of it. A different point of the line. You couldn't change what has happened because it happened already.

If you try to kill hitler you will fail because he killed himself...or so we believe it may have been you that killed him.

you cannot cause a temporal paradox because you cannot change the past

you can affect it true but anything you do to it would not affect the future as the future had already occured

for more on this see the only movie that ever did time travel correctly

'Twelve Monkeys'

it was very enlightening

what if everything that can happen does or has happened. Each choice we make has a different consequence. what if every choice we can make is made and this results in parallel existences. If time travel is possible, maybe the timeline can't change because any changes you make have already happened on a parallel dimension. so if you travel back in time and change something, when you return to the present everything will still be the same because that event has already resulted in the branching creation of a parallel world.