I was inspired to continue this series a while back, when pompe linked to an interesting preprint of a paper at arXiv, Milan M. Cirkovic and Robert J. Bradbury's "Galactic Gradients, Postbiological Evolution and the Apparent Failure of SETI." As pompe notes, while transhumanism and cognate philosophies are too often overused, they aren't overused in this paper. Cirkovic and Bradbury suggest that future civilizations will be more notable for their efficient usage of existing resources than for exorbitant expenditures of said, that the Fermi Paradox might be answered not by predicting the catastrophic end of most every sophont species but by expecting advanced civilizations to be so efficient as to avoid exorbitant waste.
In other words, the authors argue that SETI enthusiasts should forget about detecting extraterrestrial technological civilizations by their Dyson spheres, or the hard-radiation trails left by the high-speed starships that link planetary systems. Rather, look for point sources of electromagnetic radiation out on the fringes of the galaxies or in the depths of interstellar place, in the places where ambient temperatures are low enough to allow for truly high-speed computing necessary to run complex virtual-space civilizations. Expansion, in this paradigm, is rare phenomenon for any advanced civilization.
In contrast to the usually assumed model of expanding "colonial empire" from human history (which confronts us with the gravest form of Fermi’s paradox), the present picture would rather use a model of a "city-state"--if anything from the human history is to be even remotely analogous to the generic pathway of ATCs, which is itself a doubtful proposition. It is too often forgotten (both among SETI proponents, as well as the contact pessimists) that colonial expansion has been an exception, rather than the rule in human history so far; our Western-centric attitude should not blind us into accepting a wrong model for civilizational behavior. Countless city-states, be they in ancient Greece, pre-Aryan India, Babylonia, medieval Italy, Germany or Russia, pre-Incan Andes or Mayan Mexico, have all together much longer and stronger traditions than imperial powers, of which there are no more than two dozen examples altogether, from Assyria to the USA. Even in the cases where cities and other smaller organizational units have been peacefully or otherwise incorporated into a larger whole, this was often regarded as optimization of resources and management, and clear limits to growth have been set in advance; examples in this respect range from Achaean League, to Hansa, to Swiss Confederation, to China after Ch’in unification. It is exactly this understanding of limits (or resources and communication) which made the longevity of civilizations like the Chinese, or organizations like the Roman Catholic Church so prominent in the human history so far. Vice versa, it was disregard for these limits which contributed to downfalls of all historical empires.
What does this imply for space colonies? Nothing very cheery, I fear.
Let's assume that my arguments from 2003 about the ultimate peripheralization of space colonies for the foreseeable future are correct. Further, let's assume that barring the development of biotechnologies and/or nanotechnologies capable of plunging humanity directly into the Land of Cockaigne, there will always be limits on the physical expansion of civilizations. Even at our current levels of technology and wealth, the terraforming of Mars may just be within our civilization's horizon. Barring much more economic growth and technological advancement, however, the settlement of the Alpha Centauri trinary most certainly is not. And when human civilization, mono- or bi- or multi-planetary as it may be, reaches the point where it can achieve that task, will it necessarily be interested? It's worth noting that the settlement of Manchuria took place six and a half centuries after Beijing, on the Manchurian frontier, was selected as China's capital, and two centuries after the Manchus conquered China, and then only because Manchuria was threatened by the Russians and Japanese.
What reason could possibly justify the expense of venturing off-Earth to found offshoot societies, absent direct threat or a compelling need? Ideology of some sort would seem to be required, that or obvious and immediate economic benefit. Most of the possibilities that I can think of--lunar and Mercurian helium-3, solar energy, research into non-Earthly biospheres--could be just as easily achieved by outposts ultimately dependent on Mother Earth, while cultural-separatist settlements may not necessarily have extended lifespans. Without cornucopian technologies, Earth may forever remain the only world of humanity.