Re: Self-replicating factories [was Re: Our rocky solar system may be rare]
Sun, 12 Sep 1999 14:25:40 -0700

Robert J. Bradbury, <>, writes:
> To:
> On Sat, 11 Sep 1999 wrote:
> >
> > I had commented in my comments on Robin's Great Filter paper:
> >
> > > Even without nanotechnology, Von Neumann type factory operations
> > > would allow asteroid and planetary engineering. If current growth
> > > rates in silicon wafer production continue, we could construct a
> > > Dyson Shell with microprocessors within 300 years.
> >
> > My response:
> >
> > If we extrapolated the use of horses in the 19th century we could build a
> > Dyson Shell out of horseshit in a few hundred years.
> >
> I hope there is a smiley after this Hal. Because in fact you can't build
> strong structural stuff out of horseshit (unless you have a trick I don't
> know about).

Yes, I was being somewhat facetious, but mostly as you surmised I was objecting to your extrapolation of the current exponential growth rate in silicon-wafer microprocessors. Surely it is unreasonable to assume that the growth rate of this technology will continue for 300 years. More likely we will switch to some other technology within decades, even if Drexlerian nanotech can't be built for some reason. So I don't see that this extrapolation is meaningful.

> The main point I was trying to make was to counter Robin's statement that
> we don't know if we can do hard nanotechnology. Robin is correct in that
> there is no "evidence proof" as there is with wet(bio)-nanotechnology.
> However, molecular assembly per se *is* not nessessary for a Dyson Shell
> full of computronium in a short time.

I think Robin was trying to close any loopholes in his argument about filters, by suggesting that it is likely that any technologically advanced civilization would be able to do megascale engineering. To be complete he was raising the possibility that somehow technology would have a limit which makes this kind of transformation impossible; he expressed this by suggesting that nanotech might turn out to be impossible. I think you are right that even if Drexlerian nanotech is not possible, eventually it would be possible to do solar system engineering. We would have to invoke still other technological limitations to bound the ultimate engineering capabilities of civilizations, straining credibility. This is basically consistent with the point Robin was trying to make, which is that this is an unlikely loophole for resolving the Fermi paradox.

> What *is* necessary is self-replicating
> factories to build the parts that (a) dismantle planets; and (b) build the
> Dyson Shell satellite subunits. The NASA 1980 Study that Robert Freitas was
> the co-author of showed that self-replicating factories could be done with
> macroscale components, the problem is that the doubling time is long.
> I'm moderately confident that using MEMS technology and/or nano-imprint
> lithography you could get the building blocks small enough (but still not
> atomic sized) that you could construct a building-block factories factory.
> I think with small universal building blocks, you significantly decrease
> the doubling times to something closer to nanotechnology based doubling
> times.

There is a somewhat crackpottish web page on universal building blocks (no fluids though) at They've actually hand-built some building blocks and hooked them up together. A more plausible set of pages on a similar idea is at, although I think these are supposed to be built using nanotech.