Reply #45: Different "worldlines" is the phrase he uses.

[Occulus]

As he describes it, the present would be that fluid, if it were normal practise to send people back- say, as tourists.

However, the problem you're describing still would not happen, because the entire past, and everything that event affects, would be changed, including how you remember it.

And that's the key part. Our perception of the 'present' wouldn't change, only our memories of the events affected by the changes made in the past. Those memories would, by necessity, jibe with our 'current' perception of reality.

Douglas Adams described it best when one of his characters- I think it's Reg from Dirk Gently's Holistic Detective Agency- said something to the effect of the space/time continuum being very muchlike a piece of badly-laid wallpaper. Push down a bubble in one spot, and another one pops up somewhere else.

Basically, were time travel possible, we would never know our pasts had been changed, because the moment the past was changed, we would remember all the events along the 'new' timeline. Our 'present' would remain consistent with the past and we would never know the difference.