Gordon D. Pusch wrote:
> Mr. Marlow's book and interviews _GROSSLY_ overstate the maximum possible
> rate that rogue nanobots can replicate. Based on extremely general physical
> principles, such as the conservation of energy and the limits of available
> energy vs. the characteristic energies of chemical bonds, Dr. Robert Frietas
> has shown that the maximum possible "biomass to nanobot" conversion rate of
> self-replicating nanobots cannot possibly be greatly larger than that of
> already-existing self-replicating organic lifeforms such as bacteria,
Um, I don't think he said that. He said that waste heat was "a major
restriction." He cited Drexler's estimate of 100 MJ/kg of final
product. And he also assumed that the goo would be working slowly to
avoid detection. Under this assumption, it would indeed take 40 months
(20 in the final cycle). If it's working at the same efficiency but
just under the boiling point of water, it'd be 50 times as fast.
But Freitas also pointed out that Drexler says that energy dissipation
may in theory be as low as 0.1 MJ/kg for carefully planned reactions.
Thermal-limited speed decreases in inverse proportion to efficiency, so
this would allow conversion to happen 1000 times as fast.
Natural enzyme chemistries are closer to 100 MJ/kg; plants fix biomass
at 38 MJ/kg, and plants have evolved for eons for efficiency. And so
even a moderately more efficient 10 MJ/kg would allow a factor of 10
speedup on top of the 50.
In my opinion, plants should not be taken as the gold standard for
efficiency. They have to raise significant amounts of water; resist
parasite attack; use photons in a wide energy range; and maintain
metabolism overnight. So I think 10 MJ/kg cannot be ruled out.
So a moderately efficient goo, not trying to avoid detection, could in
theory convert the biosphere in 2.44 days. Note that at the _start_ of
this period, according to the scenario, the biosphere would already be
near-boiling, and the damage we care about would already be done. But
note also that this scenario requires global dispersal of the initial
goo-bots, which, I think, most goo designs (including Marlow's) do not
adequately take into account. The real issue is not how quickly goo can
work. And how quickly it reaches its thermal limits is totally
irrelevant, since it'll reach the thermal limits of living biomass
first. The important question is whether nanobots or larger nano-built
weapons can access the globe without being intercepted. If so, anyone
with an enemy is toast.
> In particular, exponential growth of a nanite infestation cannot possibly be
> sustained past the earliest, initial stage of the infestation, due to energy
> and resource limitations, as well as production of waste heat by the nanobots;
> hence, late growth can only occur at polynomial rather than exponential
> rates, and at the surface of the expanding colony rather than throughout
> its volume. Therefore, the response time required to deal with a rogue
> nanotech infestation will _NOT_ be on the order of "minutes" as Mr. Marlow
> claims, but rather on the order of days or weeks --- i.e., comparable to
> the response time currently required to deal with an epidemic outbreak of a
> typical infectious disease.
A gray goo will be very hard to build. It'll need a metabolism, a
fabricator, a control computer with full blueprints, and an
environmental shell with chemical/mechanical interfaces... all in a very
small package. Note that this means that there's no way a nanofactory
can accidentally mutate into a gray goo. If gray goo happens, it will
Long before goo is designable, it'll be possible to design UAV's
(unmanned aerial vehicles; think cruise missiles) that can spread it
very quickly. But why bother spreading goo? With much simpler
robotics/avionics, you can just kill the person (or city) you want to
kill; no need to destroy the world in the process.
If we ever get to the point where script kiddies can release dangerous
gray goo, we're probably doomed--since it'll surely be harder to stop
goo than to stop slow-moving slow-thinking meat robots from pushing the
wrong buttons. But we will have much more severe dangers to deal with
before that point. Like nano-arms races with weapons much more rapidly
destructive than gray goo--and much more controllable, hence easier to
> In short, Mr. Marlow has paid too much attention to the overblown and
> scientifically inaccurate claims of doomsday science fiction writers
> (many of whom are scientifically illiterate luddites), and too little
> attention to the physical limitations imposed on _ALL_ forms of nanotech
> by real-world physical laws.
I'd feel more comfortable with this assertion if I didn't think you had
misread Freitas' paper. Again, the 20 month final cycle time was
apparently the most optimistic one he could come up with--not anywhere
close to the physical limits. The least optimistic scenarios make it
clear that, without a very rapid response prepared, a widely distributed
goo would be extremely bad news for any biomass within the contaminated
Even though John Marlow has his nanobots doing impossible things--like
rapidly eating low-energy materials, and building things too fast--I
don't think it would change the book much if he had written it to
conform to reasonable physical limitations. Less chemistry, more
micro-scale robotics... and you'd get much the same impact.
Another question is how quickly advanced nanotech designs could be
developed. I think he's very over-optimistic there, but he invokes a
wildcard that I can't say is absolutely impossible.
In any event, the book actually scared me--and I've been thinking about
this stuff for a long time, and I know all about doing the math on
physical limitations, and I spotted lots of places where things couldn't
work the way he wrote them. But the strength of the story is that it
doesn't really depend on the technological details. Make a powerful
enough technology--and diamondoid molecular manufacturing certainly
qualifies--and you're opening the door to all sorts of human-invoked
I think gray goo is not a big concern, at least initially--but only
because other, more dangerous possibilities must be survived first.
Chris Phoenix cphoe...@CRNano.org
Director of Research
Center for Responsible Nanotechnology http://CRNano.org