It's non-trivial based on our current understanding of physics, but we're so primitive we've barely even left our own planet, so I don't put much faith in our understanding of physics being complete. We still haven't reconciled General Relativity and Quantum Mechanics. The problem with the Ancient Alien theory is that there just isn't any good *supporting* evidence. It does make for a nice-sounding though farfetched hypothesis as to how humans might have invented various religions, but again other than crazy religious texts making wild claims about supernatural phenomena, and trying to explain it away with this ET stuff, there just isn't any evidence. But still, it's more plausible than that supernatural stuff (gods, demons, angels, etc.) actually being real. Biological beings from another planet with highly advanced technology not understood by primitive peoples is still more believable than what major religions claim is real. So, the way I see it, while I don't really believe either one, the guy who spouts ancient alien theories is less of a crackpot than the people who go to church or mosque.