I'd have to do some searching, but I read an article about this time last year talking about a guy who has figured it out on paper, but thinks it will take at least a decade for him to actually do it. I had always thought that you could move forward in time but not backwards. This guy says you can move backwards, but it you change something, when you return to your time it is not changed. Rather, when you made the change a second reality split off and you never see the change in your reality.

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Neither am I the means to any end others may wish to accomplish. I am not a tool for their use. I am not a servant of their needs. I am not a bandage for their wounds. I am not a sacrifice on their altars. ... I owe nothing to my brothers, nor do I gather debts from them. I ask no one to live for me, nor do I live for others. I covet no mans soul, nor is my soul theirs to covet.

Ayn Rand, Anthem.