we would already be here.

when i was little i remember my dad telling me something that i couldnt wrap my brain around at the time, and took me a little while to comprehend. he said that time travel could not be possible because if it were, we would already be here. and once i understood that concept i fully agreed. but now another question is smacking at my brain. what if that is what ufo's are? what if thats us from the future? or atleast some of them are? hmmmmmmmmm.... i'm sure i'm not the first to ponder this. opens up a big old can of wormies in my dusty head....anyone?