> 2 - Aliens are everywhere, [...] As a teenager I read Heinlein's 'Goldfish Bowl' [1] and it was the best explanation apart from the one from Stanislaw Lem I

> 2 - Aliens are everywhere, [...] As a teenager I read Heinlein's 'Goldfish Bowl' [1] and it was the best explanation apart from the one from Stanislaw Lem I had heard for the lack of aliens. Namely that: you can't 'see' them because they're just too different. Heinlein doesn't even make it clear if the higher intelligence in his short story are aliens or from Earth. I think Lem's explanation (don't remember in which book/short story of his) was that the window where a civilisation is not advanced enough to learn something from aliens but not too advanced to care is just sth. like 3000 years. And therefore, even if this level of development was reached by two proximate civilizations, e.g. even on neighboring planets in the same solar system -- if those windows didn't overlap, chances for communication happening would essentially be zero. [1] https://en.m.wikipedia.org/wiki/Goldfish_Bowl https://en.m.wikipedia.org/wiki/Goldfish_Bowl