> Even if you had nanomachines, building something big with a chemically powered system might take as long as growing a tree.
Suppose you have a tree-sized vat of milk --- 5 tonnes, say --- which has been boiled to sterilize it and allowed to cool down to 44°, the optimal temperature for Streptococcus thermophilus, where it is being maintained by a thermostat as it is stirred continuously with an impeller. You introduce a single Streptococcus bacterium, two microns in diameter, into the vat. If it happens to survive, how long does it take to convert the entire vat into yogurt, metabolizing most of the lactose into lactic acid?
Our initial seed bacterium is about 4 x 10¯¹⁸ m³ in volume. If we suppose that S. thermophilus doubles its population every 30 minutes under these circumstances, which is quite close to the truth, the answer is about 30 hours, entirely under the strength of the clouds of nanomachines floating around in the milk, despite the absence of external electrical power.
If we instead grant ourselves the luxury of dumping an entire 250-ml cup of seed yogurt into the vat, it only takes a bit over 7 hours.
You will note that this is noticeably faster than the time needed to grow a full-sized tree, even in a 44° rain forest; clearly there was plenty of chemical energy to transform a sufficient fraction of the vat into peptidoglycan to give it structure. This suggests that the tree's biology is not optimized to take advantage of the abundance of chemical resources available in such a quotidian object as a sterilized vat of milk; instead, it arises from a history of substantially more hostile environments, where such rapid growth will merely exhaust the available resources and then allow leaf-cutter ants to harvest your tender, unprotected leaves. There is no particular reason that nanotechnological factories need to operate under such conditions.
This is far from a wild speculation; yogurt-making is a well-understood industrial process, practiced for centuries if not millennia, which can be easily scaled down to tabletop experiments, and often is.
Of course, the gelled mass of bacterial corpses we call yogurt has substantially less macroscopic structure than a tree, and is also mechanically weaker, but that's not primarily a matter of available energy or speed of communication; it's a matter of evolutionary optimization. Organizing an ad-hoc network of 10²⁰ computers into a useful morphogenetic process, maybe one that builds a circulatory system into your house so it can keep getting nutrients until it finishes solidifying, is a nontrivial challenge, but it hardly seems like an insuperable one.
Zyvex's "convergent assembly" proposal provides an alternative that avoids the power problem in a different way and is less exposed to the vagaries of noisy self-replication and emergent morphogenesis.
I agree that most current "nanotechnology" is clearly humbug, sharing nothing with Drexler's (or Smalley's) ideas that it doesn't also share with asbestos or wood smoke.