Late to the discussion, it seems to me the evidence offer is meager compared to the claims made.
And "non-human bodies" is a particulary wishy-washy expression. A dead cat is a non-human body.
And moving the story explanation from "maybe it wasn'T aliens, maybe they were dimensional travelers" doesn't make the claim more plausible. We can't travel interstellar distances now, not even without requiring faster-than-light travel, and even less do we have a clue how to travel beyond our spacetime into "some other dimensions".
I guess it would explain one thing - why there is no evidence that we had technological leaps that couldn't be explained without an external source. Because it would be as if an F-22 crashed in the stone age. Humans would have no idea what they're dealing with, the best thing we'd manage to get from that wreck is probably better spear and arrow-tips. (Unless an F22 is made only from carbon-composites or something, not sure what you make from that?)
There is a bit of a difference here. He is saying he can't tell them in an unclassified hearing. He also told them this under oath. Whether he is right about what he saw is entirely debatable, but it isn't like he's just the "Aliens" meme guy. There are very real consequences if he is just blowing smoke.
How much risk is there really? At what point can you prove beyond reasonable doubt that he lied, if he did? Does he listen the people that he has his information from? Does he listen the locations the wrecks or non-human bodies were stored? Does he offer anything that can be verified by an investigation?
UngainlyTitan said:
I mostly agree where I think it breaks down is if; it is possible to live in space habitats by extracting base materials and energy requirements from the easily available materials in asteroids and cometary cores. While I agree that this is a big if, I am also not aware of any real showstoppers in the current science.
I think the problem for interstellar travel is deep space beyond the Oort Cloud. It has a really low density, so you might need to bring everything you need along for the ride to the next star system.
So, what is the smallest ecosystem you can build that can support human life for likely centuries?
We unfortunately have only one ecosystem available to us, that wasn't build, and it has supported human life for the last hundreds of thousands of years. It's really difficult to make guesstimates if you have a sample of one. We had experiments with stuff like Biosphere 2 that showed us the limits of what we can do now, but the experiment did only last long enough for us to show some of the pitfalls. And our "experiments" with Earth show that we can still damage our ecosystem to degrees that we aren't certain we can fix and what the fallout will be (ugly almost certainly, but just "mass migration + maybe some resource wars"-ugly, or is it "naughty word everything we need to survive is dying off".