If we stick to what we know, speed of light as effective maximum, I agree entirely that two cultures are very unlikely to physically encounter each other, at least not as a matter of significance since they'd probably have bumped into each other's broadcasts way before that. Personally I assume that if there are aliens out there that are psychologically comprehensible and achieve and maintain foreseeable plausibble tech, they probably tend to know about most every race within the galaxy within a few years (minus signal lag) of them starting to use radios regularly, FM especially.



Well, the problem here is that while we've all read and watched plenty of seemingly plausible books and movies that have people falling back into the Dark Age, the idea of this happening for any prolonged time seems unlikely when you actually examine it. Albeit the Gilligan's Island view of constructing a radio from a coconut is absurd but barring some all-encompassing global taboo against tech it would be very hard to lose it once you have it. Newton or Da Vinci types, and plenty of lesser known and celebrated sorts, would not have a huge problem picking up a modern science text and reading it, and even a taboo would be rough since any group that didn't accept it would be in a wonderful position to economically, militarily, and cultural crush the opposition. So I don't think a culture would spend too long Dark age after collapsing, and in terms of keeping broadcast going. I don't know that a single person could realistically send to other planets as a hobby without the kinda resources available in a tech culture of our scale, but for example NASA transmission Beattle's music at Polaris was only at 20 kW and while you would need something as bigger than SKA to pick it up it's over 400 light years away, and that's only at 20 KW, in theory someone could power that up from a wood-fired generator in their own backyard. You don't really need the consent and assistance of your entire planet to do stuff like that, so I think even if we assume cultures flare out then drop back to some semi-Luddite level or cheerful solar/wind only population 1 billion or so I don't think they'd lack the resources or inclination, on average to keep an ear out or even give shout outs. One planet might, but unless its the norm it wouldn't matter.



I generally like Clarke's stuff though it's a touch dry, Rendezvous with Rama and the 2001 books being his only stuff I've really read though I'm sure I've read that short story before, will have to read it tonight. But I do prefer the more optimistic sci-fi although Alastair Reynolds, in spite of the doom and gloom borderline horror flavor, is the current SF writer I think has the best realism in his work. Traditionally I'm Asimov, Herbert, Niven, Card, etc fan and tend to prefer the human-only stories just because I tend to find the aliens improbable caricatures.