my theory is that they are beings that are connected to some future timeline that they are trying to preserve, it's why they dont just enslave us given their technology, there may even be multiple factions of them warring over which timeline wins out but i dont thing time travel is possible in the conventional sense (you can't send anything physical to the past), more like, you hook up a machine that can read information sent back from the future, which instructs the machine on how to operate with a machine that's generalized and advanced enough, eventually it can cause a runaway effect similar to the "grey goo" idea sci-fi writers play with, it's why there is so much diversity regarding ufo craft however there must be some original timeline that allowed for the creation of the original machines
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