-Now! What do we want? Time travel! When do we want it?-

That’s right you guys, it’s finally time travel time. This paradox is all about what happens when you go back in time and try to add something. You could use this paradox with objects, people, whatever, but I’m going to start with an idea. Let’s say that you really like Thomas Edison’s work on the light bulb. You like it so much, in fact, that when you get a time machine for your 225th birthday, you decide you want in on the action. You want to be a part of history, give the guy a helping hand. So you google Edison’s best design for a light bulb and print it out. Then, you travel back in time to 1879 and ever-so-casually place that design on Edison’s desk. You travel back to your present, and everything seems fine… until you realize something. You got that image from google, which got it from Edison’s workbook. But, thanks to your time travel escapade, Edison really got it from you. So if you got it from Edison, and Edison got it from you… who drew the design?
(Obviously, it was drawn by the Time Elves, but you’re not really supposed to know about those, so…)
It seems like the idea came out of nowhere. Maybe it caused itself? Can ideas do that? This is where the name of the paradox comes from. It seems that the idea has pulled itself into existence like someone rising in the air by pulling up on their own shoelaces.
It only gets crazier when you make it about a physical object. Imagine that you have a watch that you’ve had for as long as you can remember. One day in the future, you decide that you don’t want it any more. You could give it away to a friend of yours, but it means a lot to you. You’d rather give it to yourself. So, you travel back to when you were very young, and give your past self the watch. Now, though, the same problem arises. Where did the watch come from? You got it from future you, who has had it ever since that moment. You got it from you got it from you. And nowhere in that cycle was the watch made. There was no metal mined out of the ground, no gears put together, nothing. It appears that matter has been created out of nothing, which according to thermodynamics, simply can’t happen.
This is often used as evidence that time travel can’t happen, since it would allow something this crazy to happen. I don’t know nearly enough about physics to speak to that, but I do know that many time travel books and movies have one of these hidden in the plot. TV tropes has a page about this, with lots of examples, but they call them ‘Stable time loops’. Check them out: http://tvtropes.org/pmwiki/pmwiki.php/Main/StableTimeLoop
And, by the way, if you were wondering why I didn’t post yesterday, I was kidnapped by the Time Elves. (:

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