The previous post ended with an oblique reference to postbiological humans that only require energy rather than material inputs (after manufacture, assuming little need for repair, etc.) Some calcs about calcs for non-carbon based minds.

How efficient could a postbiological civilization be? The current IBM roadrunner does 376 million calculations per watts. If we take my mid-range estimates of computing needs, 10^22 to 10^25 FLOPS, then a single emulation would need 10^13 to 10^16 watts. The total insolation of Earth is about 10^17 watts, so this won't do - there would be space for just a few minds on the entire planet. But current research on zettaflops computing suggest we can do much better. A DARPA exascale study suggests we can do 10^12 flops per watt, which means "just" a dozen Hoover dams per mind. Quantum dot cellular automata could give 10^19 flops per watt, putting the energy needs at 200-2000 watts.

That is between 2 and 20 times the current wattage of a current human. However, we bio-humans get our energy through the inefficient method of having plants collect sunshine (at about 3%) efficiency, then we either harvest them and eat a small part of them (expending a lot of agricultural energy) or have animals eat them (at a few percent efficiency) and finally we eat the result, again with a few percent efficiency. A brain emulation of this type would just need a few square meters of solar panels (plus night-time energy storage). In terms of area and energy required, these postbiological humans would have far smaller material requirements than we do. They could also run slower to save energy. . .

I personally think that this sketch of a post-biological vegetable humanity is one of the most positive possibilities for our future. I think, once the technology is around, it will attract people voluntarily (after all, it gives the chance for immortality, any conceivable lifestyle *and* is green). It is sustainable, since it would use a minuscule amount of resources, energy and area to keep mankind running and would not need great material flows. Just running on renewable energy it could easily last until the sun starts to act up. It is also able to protect itself and the environment from unforeseen threats: going virtual does not mean we completely abandon the physical world (we would be keeping telepresence bodies around for tourism, repair and science - as well as a few small colonies of holdouts of Homo sapiens just in case).

This is taking "green" and "sustainability" seriously.

I don't think romanticised small agrarian communities (or hunter-gatherers) are stable over megayears - as the climate and culture changes, sooner or later someone will get exponential ideas and re-run industrialization. Or they will be wiped out the next time a supervolcano erupts. Similarly any kind of civilization of biological humans will be subjected to environmental challenges (remember, we are running out of interglacial! This is a good reason to save oil) and genetic drift. It would need to maintain itself against the environment, necessitating reasonably big material and energy flows (even if they are efficient) just to have stockpiles. It would need cultural constraints to keep from veering into dangerous areas, degenerating or going posthuman one way or another - and these constraints would need to be upheld globally indefinitely. Some form of Singleton - something with power over humanity's actions that could sustain its power indefinitely - would likely be needed. We do not know if singletons are possible, and I have strong intuitions that most possible singletons are very nasty (Orwell suggested one in 1984 - a stable worldwide tripartite dictatorship system).

Stopping human cultural evolution pretty much destroys humanity.

It doesn't seem like much thought was put into imaging the challenges for post humans. The classic threat is insanity. Living forever with no constraints is boring, forever, after the novelty wears off. Another threat is frustration. Doing anything interesting, like exploring the universe, takes too long at light speeds. There are many more challenges, and many responses to them, but they weren't discussed.