^That's where I have problems with it. If you imagine the quantum timeline like a tree, where the timeline splits into two or more to play out every outcome of a quantum event (and human decisions, if they are quantum in nature) then you have an effect in which you have "pruned" a timeline from the tree, and thus should be inaccessible anyway.
If I may explain (and this is original thought, so it's probably bollocks anyway) going back in time is like taking a branch from a tree, curving it back on itself and expecting it to seamlessly fuse with the preceded branches. This would be your personal timeline within the timestream as a whole.
1: Hawking's feedback loop. Essentially you have created a loop in which effects have been taken back in time (via you, your memories etc) to a point preceding their cause. This then changes the cause, and therefore the effect. Which changes the cause. And the effect. And you can see where this is going. So you can imagine that each time the events between arriving in the past and leaving the present play out, they do so differently. This might even create even more loops if the effect causes you to go back to a different time. Eventually the effect will occur that causes the time machine to either never be built or never be used, thus breaking the cycle, and we all continue on none the wiser.
Or so I understand it. It is a paradox, and I might have missed something here, but the result is that paradoxes prevent themselves from occuring, thus rendering this time travel impossible.
2: The other one (I haven't a name). This is the one in which what you did when you went back in time caused you to go back in time in the first place, thus there is no feedback nor changing history. The simplest way to explain the problem is that there is nothing that causes the loop in the first place, it just always existed. But this is (so far as we know) an unnatural state for a timeline to be in, there is no reason to suggest that it would just appear to be that way. Maybe the existence of a time machine would distort the timeline in such a way, but it could not exist if the timeline were not there to begin with. Imagine time as a tree again, branching at each event with multiple outcomes. Tree branches do not spout from the ether and fuse into the nearby tree, which is what this would be like. Branches split as they grow, only this particular split cannot occur without prior and improbable input from another branch which already resulted from this very split. Trees cannot grow this way, and I don't think history can either. Although the loop, once it exists, is stable, there seems to be no way to create it in the first place.
Come to think of it, these are all just reiterations of old paradoxes and proofs against the possibility of time travel. Damn.
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