[JoeP]

One thing that I sometimes think about (until the aliens suppress it) is timescales of communication and thought. Dragar's link raises the question of aliens being greatly different sizes from us (even sf that allows non-humanoid non-bipedal forms doesn't often venture to much bigger or much smaller scales).

But what if they think and develop and communicate on the microsecond timescale? What if they exist and communicate over thousands of years? We would be much less likely to even spot their communications let alone recognise them as possibly intelligent.

I don't know if it holds as a physical principle, but there's a correlation between speed of living and size, on earth - smaller animals have shorter lives and faster heartbeats. Whale songs are much longer than human songs, I fancy. So maybe massive scale organisms could live for millions of years and communicate over thousands of years (always assuming they lived in an environment where that was adaptive) - and then two-way communication and even travel between stars could be feasible for them.

There are so many ways in which intelligent lifeforms could be completely unlike us.