OK, I suspected it was along those lines. And since this is a more fun/interesting topic, IMHO, than the vast majority we see here, let's play with it a bit further.
>
> 1.) Even if we were just communicating via radio or laser rather than
> meeting them face-to-face, the closest planet with the means to carry
> on a conversation with us is likely to be hundreds -or, more likely,
> thousands- of light years distant, which means the time lag between
> them saying "hullo" and us replying "who the heck are you?" is going
> to be measured in centuries at the very least.
Granted, although the original question was about meeting the aliens, not having a long-distance phone call with them. It is certainly conceivable, at least to me, that we could "meet" extraterrestrial intelligences through means which do not violate the law of fizzix as they is currently knowed. F'rinstance:
1. Assuming it's possible to sustain acceleration for a good long time, Einstein does permit interstellar travel in reasonable shipboard time frames, as long as your travelers don't care that everyone back home will be worm food before they get back there. So someone from "out there" COULD come passing through any day now.
2. There IS still the longer-duration, slower option for interstellar travel which you mentioned. I agree it's less likely to succeed, but I can't call it impossible. Something like Arthur Clarke's "Rama" seems to be the best choice if you're going this route.
3. We could "meet" the aliens, at least in the sense of knowing of their existences and learning something about them, through the same way that we've
already made it possible for other species to "meet" humans - alien artifacts could make it here, regardless of how long it takes to make the journey. Pioneers 10 and 11 and Voyagers 1 and 2 all carry messages intended for whomever or whatever may someday find them. It may have been incredibly optimistic to do that, but they're almost certainly not going to be the last man-made objects to carry information about us to Out There Somewhere. It is at least possible someone else has done the same thing, and that we might find their version. Note that it's also possible that the artifact - which could take as long to get here as it needs to - might itself be intelligent.
>
> 2.) If our world is typical, technological civizations are likely to
> have fairly short working lifetimes before they collapse due to
> overpopulation, industrial pollution, resource depletion,
> environmental collapse, or some combination.
We don't know yet that our "technological civilation" is going to have short working lifetime. It certainly might. Or maybe Gene Roddenberry was right, and we all wind up being amazingly well-adjusted, singing "Kumbaya" as we explore the galaxy in sleek starships. (Or maybe we eventually evolve into the Q....or the Borg.) We just don't know.
On the other hand, you don't have to be as pessimistic about humanity's fate as you were for the "time lag" problem to arise. If technological civilizations DO tend to survive, we have to assume they continue to develop - and in that case, how many years have to pass before a given civilization gets to the point that it simply doesn't care to talk to those who are at the point they were long, long ago? A thousand? Ten thousand?
Of course, all of this moves to a whole different set of questions if it turns out that Einstein was wrong and the Star Trek/Star Wars/Babylon 5/etc. writers were right, and there IS a way around the speed of light limit. Then it becomes more a question of how far you can go in a given period of time, and how many others there are who are roughly at your stage of development within that volume. To be sure, that "roughly at your stage" makes for a very, very tiny percentage...but then on the other side we've got a very, very large number of stars, and presumably planets, out there. I don't think we know even a tenth of what we'd need to know, at this point, to hang a decent estimate on the product of those two numbers (along with a few others; the rest of y'all, see "Drake Equation" for more).
Bob M.