.. the lack of evidence that aliens have ever visited Earth only counts if .. there's evidence to be found..
That doesn't make sense, as written.
Is it possible aliens visited 500 million years ago?
So what? If they landed, had a party, swam in lava and then zipped-off, what of it? The same could be said of Leprechauns (from space) and time-travelling super humans from the future.
How about lack of evidence for alien visitation in recent times? For the absence of evidence argument to work, one would have to argue that we could detect evidence of an advanced civilization visiting us .. I don't have a whole lot of faith in our detection skills.
Again, we could also be missing the Elves in their tight underpants; the ballet dancing ninja elephants; the mega-mind moss; the learned lichen.
You are creating a class of indetectables. It's vast. That dilutes it.
I get what you're saying: we can't know; but it's not a useful state. We
know we can't know, so why go there?
Of course, lack of FTL travel wouldn't rule out alien visitation, but it would require the aliens to have a very good reason to come here.
Indeed.
On the other hand, if the hypothetical aliens we're talking about are machine intelligences, the lack of FTL travel might not even matter to them. You can still get around the galaxy at 10% speed of light, if you don't care about long transit times. A machine intelligence could have the whole galaxy mapped out in a pretty short amount of time, with interesting spots (i.e., us) being actively observed.
The lack of side-effects seems to rule this out. There has been no input from the universe into our observations that defies the natural scheme. Unless these machines can move about leaving no trace upon the fabric of reality, they are not there at all.
I suppose we can mention Dark matter and company here. Yeah. I don't know enough to scope it. If these machine exist, they are being super super super stealthy; more than paranoid levels of secret. Strikes me that a machine intelligence which has mapped the universe would be anything but afraid to be seen. I dunno, too much Saberhagen, I suppose.
Someone who had never seen a plane before could still tell you something weird is in the sky and reasonably be able to describe it.
I doubt that! How would they even know what to relate it to? Unless the person they are describing it to has a better idea, there would be little actual communication of worth. For example, how would he know it was a large object very far away, vs. some kind of bird much closer?
UFO believers are better off, epistemologically, than Big Foot believers. The premises for an argument supporting the reasonable possibility of alien visitation aren't really that unbelievable:
What if Bigfoot
is what is in the UFOs?
A) Out of hundreds of billion of planets, technologically advanced civilizations have developed on at least some of them.
B) Some of these civilizations might be curious enough about the galaxy to explore it (or at least create self-replicating devices that could do the exploration).
C) If FTL travel is possible, then it's reasonably possible a curious technologically advanced civilization is observing us.
D) If FTL travel is impossible, then it's reasonably possible that devices of a curious technologically advanced civilization are observing us.
One of the things that always occurs to me at this point is: when? When did all this happen? If it was, say 500 years ago, then are they even alive? If it was 72 thousand years ago, are they still alive? Did they rise-up, burn their of fuels, build their own Internet, suddenly get into space and then burn-out in war? Did their sun play along or did it fail? Are they still alive?
It's quite the assumption that they are long-lived and wise as well as interested in us.
I think Bigfoot with Big eyes is just as likely as alien visitation. They both inhabit the same probability space: not impossible and homeopathically certain.