[einhverfr]

I don't think you can dismiss the possibility of an as-yet-undiscovered end-run around these limits. The problem with a critique of a hypothetically more technologically advanced civilization than our own which is based on modern understandings of physics is problematic at best.

There are larger problems with the ET hypothesis, though, and that is why generally, descriptions of beings associated with lights in the sky, although also often associated with knowledge of new technology and with kidnapping, are described in physical details differently by different cultures. Even in somewhere as narrow 10th century Europe, you have at least three different, if you will, species of entity associated with this sort of thing.

So this leaves the ET hypothesis with two bad choices, which are either we are more observant than our ancestors which is patently false, or else there is some intersteller convention somewhere which divvies up cultures for observation and follows them as they move around, and stops when the culture changes sufficiently. That starts to sound very implausible.

[c3d]

What about: a scientific expedition to earth is expensive, so when it arrives here, the ship takes shore for a while, sends scouts, gathers data, and then moves on until another one arrives, possibly from another planet? Is it so far-fetched?

Also, things are always described in a sociological context. What we would call "automobiles" would have been called "chariots of fire" by our elders.

[einhverfr]

You still have near-exact boundaries between culture and portrayal of the entities involved. If you are in Continental Europe in the 8th Century, these are demons or even Satan himself. If you are in Sweden at the same time, it's the dwarves and they are the best metalworkers in the 9 worlds. Folk religion in England at the time still talked about elfs being associated with this sort of thing, and folk religion in Ireland was different yet.

I don't think you can have regional stability and such variation between regions with that hypothesis. I think an anthropological one makes more sense.