Well, first and foremost the obvious reason is: It’s a movie. If you want to get pedantic on paradoxes, then you may as well kiss almost every time travel movie ever goodbye.

But here’s my real explanation:

I don’t even consider such situations paradoxes. Time is a river. Sometimes that river is diverted (time travel/alternate realities). If you add a specific amount of energy to the water (say with an electric pump, which would be analogous to the actions a time traveler uses to affect the past), you can divert a river so that it flows upstream and runs back into itself, affecting itself earlier in the stream and changing the ultimate outcome.

Eventually this will create a backlog of water, as the river is now carrying its normal load, plus what you pumped upstream. This backlog will overpower the energy required to divert it back, and it will no longer be able to all be diverted back. This is analogous to the self-correcting aspect of time. Another analogy: Eventually the diversion mechanisms (the electric pump, if you will) will erode away due to affects of time and mechanical failure, leaving no trace that it ever happened in the first place. But the flow of the river (time) was changed nonetheless — even though the original cause is no longer visible.

Eventually we will have a river that looks the same as it did before it was ever affected by the water pump (time travel). Time (and the river) has corrected itself, and events that “cannot” happen have happened in such a way so as to affect things, but as to leave no direct evidence. Corrective alternate realities only exist until the corrections are made in the past. But the system was affected, and is not the same as it would have been without the original diversion.

[A CGI animation custom-made for me would really help explain this!]

One example is the movie Timecrimes .. After everything happened in this movie, the paradoxical timeline actually no longer exists. How did he go back if he was never there in the first place? Because he did. And then he didn’t, because the timeline was fixed. But anyone sitting outside the timeline — say, in the 5th dimension — would have been able to watch these events occur. But only while they were happening. Timecrimes is not paradoxical at all; people who look at it that way are thinking [at least] one dimension lower than they should be.

My brain hurts.

Mood: yummed out
Music: Pixies – Into The White