You can't call God a Theory because there is ZERO evidence to back it up. Time travel, on the other hand, is allowed by the laws of physics and is therefore a valid theory. We simply lack the means as of yet to prove it. And for all we know it is happening now. Just because nobody has shown up on your front porch and said "Hey man, I'm your great great great great great great grandson!" Doesn't mean it can't happen.
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