Sorry for taking so long on this; I forgot to check back using a browser that can see red envelopes (I usually read lesswrong with elinks).
I think if nanotech does what its greatest enthusiasts expect, the minimum size of the industrial base will be in the 1-10 ton range. However, if we're assuming that level of nanotech, anyone who wants will be able to launch their own expedition, personally, without any particular help other than downloading GNU/Spaceship. If nanotech works as advertised, it turns construction into a programming project.
Also, if we limit ourselves to predictions made in the 50s with no assumptions of new science, I think we'll find that the predictions are reasonable, technically, and the main reason we don't have nuclear cars and basement reactors now involve politics. Molecular manufacturing probably cannot be contained this way, since it doesn't require a limited resource that's easy to detect from a distance.
Others have defined singleton, so I assume you're happy with that. :)
Re: Nanotech That's exactly my point: if nanotech performs as advertised by its starriest-eyed advocates, then interstellar colonization can be done with small payloads and energy is cheap enough that they can be launched easily. That is a very big "if," and not one we can shrug off or assume in advance as the underlying principle of all our models.
What if nanotech turns out to have many of the same limits as its closest natural analogue, biological cells? Biotech is great for doing chemistry, but not so great for assembling industrial machinery ...
We have a sample of one modern human civilization, but there are some hints on how likely it was to happen.
Major types of hints are:
Data for:
Data against:
To me it looks like life, animals with nervous systems, Upper Paleolithic-style Homo, language, and behavioral modernity were all extremely unlikely events (notice how far ago they are - vaguely ~3.5bln, ~600mln, ~3mln, ~200k or ~600k, ~50k years ago) - except perhaps language and behavioral modernity might have been linked with each other, if language was relatively late (Homo sapiens only) and behavioral modernity more gradual (and its apparent suddenness is an artifact). Once we have behavioral modernity, modern civilization seems almost inevitable. Your interpretation might vary of course, but at least now you have a lot of data to argue for your position, in convenient format.