> More than 20 years ago, I corresponded with famous UFO researcher Stanton T. Friedman. His central claim was that “the evidence is overwhelming that some UFOs are extraterrestrial spacecraft”.

I don't believe in UFOs. But if I had to believe in UFOs, this would be my position:

We have had modern humans for 500 000 years [1]. If it takes 10 000 years to make it from stone age to space age, we have theoretically had time to make that 50 times over. Maybe there was a previous version of a human civilization [2], then wiped out by ice ages or something, but a small number of highly advanced humans have survived and keep hiding from us. I think this could be somewhat more plausible than interstellar travel.

[1] Or maybe 1 million https://news.ycombinator.com/item?id=45510582

[2] https://en.wikipedia.org/wiki/Silurian_hypothesis

The largest obstacle to this theory being taken seriously is the lack of evidence in long term records, such as ice cores.

Our current age will show up in future ice cores as a massive spike; we affect CO2, Methane, Sulfates, and probably a lot more. Additionally we produce and have produced various synthetic compounds that will remain detectable in the environment for hundreds of thousands of years, if not millions.

In order to circumvent this lack of evidence such a society would have had to had a very small footprint, taken very specific industrial steps, and had a focus on research that wasn't exploited. This is highly unlikely - most of our lessons have actually been learned from exploitation, and most of our research facilities require astounding amounts of labour to construct. This isn't even touching on the social improbability of maintaining such a society.

what do you think the cambrian explosion was