Man arrested at large hadron collider

[Thyrzul]

If the world was yet to be destroyed when he did travel back, he could have stopped it back than.

Not really. If he was in the middle of an unstoppable chain reaction, which would eventually lead to the destruction of the world, it is logical to get back in time to a point this chain reaction could be prevented. Of course if you have the neccessary device to do so.

The real issue is the action-reaction. If the time travel occured for the sole reason to prevent something that have happened prior to the time travel, then preventing it would erase the reason of the time travel. Thus time travel doesn't occur, and there is nobody to prevent that thing from happening. So it happens, giving a reason for time travel. The cycle is endless, and theoretically is only possible in case of at least 2 parallel universes, one for each different result.

Edit: Perfect example: Time Machine (film from 2000 I guess)

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