Paradox

I've been thinking about time travel. I've been watching a show called Primeval, which has "rifts" in time in which creatures (dinosaurs mainly) are able to travel into our time. It is an interesting idea but would never work because time travel has a fatal flaw (or at least how I understand it). Before I go into this I want to discuss the Fermi Paradox which kind of goes hand in hand with this one. The Fermi paradox is about aliens, and basically states that if there were aliens there would have been alien contact by now, but since there has been no contact there are no aliens. It isn't exactly a paradox in that wording but it basically explains that it is an either or situation. Time travel is much the same way. Our past so effects the future that if there was time travel we would have seen it by now (maybe). So lets say we can effect the past with time travel. You travel back in time and change whatever it was you went back time to fix. When you come back, you no longer need to change the thing you went back in time for, so you would never have left in the first place to change the past, but if you never left to change the past, it wouldn't get changed. That's fucked up. Well lets say that isn't a problem (I don't know how, but we'll go hypothetical). How can we say we are living in the present then, if in the future we build a time travel device, and we travel back in time, then you have an issue that if we could travel back in time we would have all ready seen time travelers, but since we haven't seen time travelers there is no way to time travel. That theory of course depends on how you look at time, because if we truly are the present then that doesn't work, but this is all very confusing and hypothetical. The other issue is Hitler. I only bring this up because of a comic I read recently, in which a guy is guarding Hitler's room, in the first panel he shoots a guy, and then in the next panel he explains he doesn't mind guarding Hitler, but now he is questioning the guy because of the time travelers that are coming to kill him. Okay well I am rambling. I just wanted to put this idea out there because it was in my head. Perhaps I'll post something a little more thought out in the future, or in the past maybe?