Eugene writes:
> On Fri, 17 Aug 2001 hal@finney.org wrote:
>
> > Robert is right, but I don't think this small technical error
> > invalidates Whitesides' argument. His lead-in sentence was "The
>
> What is the argument? I don't have the URL.
If you look earlier at the message you are quoting you see:
> Robert writes, to Scientific American, about Whitesides' article
> at http://www.sciam.com/2001/0901issue/0901whitesides.html:
> > similarity between flagellar and electrical motors is, however,
> > largely illusory." While he was wrong about ATP, his basic point was
> > correct that unlike macro-scale motors, the flagellar motor is not
> > magnetic. It is apparently electrostatic, and/or may rely on changes
> > in shapes of proteins; the details are not known.
>
> What's the argument?
Well, the specific point is that Whitesides said that the flagellum
is powered by ATP, and Robert is pointing out that it is powered by a
proton gradient instead (some cells apparently use sodium ion gradient).
Whitesides' larger point is quoted mostly above, that the similarity
between the flagellar and electric motors is "largely illusory" because
electric motors are magnetic. Everything about their design is oriented
towards organizing the magnetic fields to induce rotary motion. However
magnetic fields don't work that well at the nanoscale and do not drive
the flagellar motor. Hence his claim that they are not really all that
similar in their operating principles.
> > Also, the proton gradient itself is presumably maintained by molecular
> > pumps powered by ATP, so there is a sense in which Whitesides'
> > statement can be interpreted as being correct.
>
> Again, what is the argument?
Just that in a sense Whitesides' claim that the motor was powered by ATP
was correct in the ultimate sense, if ATP maintains the proton gradient
that drives the motor.
> > I think it would be relevant here to list the specific criticisms that
> > Whitesides made based on this misconception of submarine size, in
> > order to show that they would not be relevant. It seemed that the
> > main point made by Whitesides with regards to the size of the
> > submarine was the difficulty in navigation due to Brownian motion.
>
> Well, bacteria manage to navigate nicely using flagellar propulsion within
> animal cells, so I don't see why an array of electrostatic motors with
> bucky or sapphire fibers sticking out wouldn't make for a rather speedy
> nanosub.
Freitas points out that bacteria smaller than about 0.6 microns don't
swim, below that size "locomotion has no apparent benefit" (9.4.2.4
of Nanomedicine). So Whitesides' error was in misunderstanding the
intended size of nanobots; his reasoning would be correct for the 0.1
micron size he was describing.
> > Your references rely pretty heavily on Freitas. Another article I like
> > is Merkle's hydrocarbon metabolism,
> > http://www.zyvex.com/nanotech/hydroCarbonMetabolism.html, which is the
> > best article I've seen to address the "grabbing atoms" issue. He shows
> > specific molecular structures for reactive tools which can be used to
> > add or remove hydrogen and carbon atoms, as well as to build more of
> > the tools.
>
> The problem with this is that you use reactive moieties attached to rigid
> rods which imply bulk (which he sidesteps by showing them as quadratic
> blocks), which have to touch down at a surface (one hemisphere of space is
> hence unaccessible). Several of the moieties much be present at the site
> simultaneously, which is difficult to steric hindrance which he refers to
> as the fat fingers problem. Plus you have to cycle the rods, retracting,
> recycling and protruding them again in a rapid oscillation, which implies
> you have to minimize bulk, or be slow and burn a lot of juice.
I don't think that the rigid rods would have to be all that bulky.
Merkle is just showing the end structures and they are quite thin.
You don't have to get too fat to have strong rods, and in fact the tools
used by Merkle don't seem to require huge forces anyway.
As for speed, that's not really an issue at this level of design.
Merkle's main point was to show that you could come up with tools that
would do the job.
> This is unelegant. What would make more sense is a nanolithoprinter, using
> a flexible hollow (e.g. graphene) duct to transport monomers and a head
> with an aperture aligning them and activating them with a current pulse,
> at least initially (when you've got a cluster bristling with radicals you
> can just firehose your allenes at it, and they will stick). That may not
> give you atomic control, but when do you really need atomic control? I see
> no showstoppers even if you can deposit only a tiny subset of all possible
> structures, as long as the repertoire is sufficiently rich.
The point of Merkle's analysis was to show how you could have a tool
set that could build carbon structures, and, most importantly, also
build more copies of the tools. This is the crucial element necessary
for self replication and to my knowledge no other paper has showed a
specific design that could accomplish this.
> Once you have got your computronium, you can burn ridiculous amounts of
> simulation time to fit the optimal paradigm, and optimize it into
> ridiculous dexterity, here you're only constrained by raw physics, and
> local physics looks so far extremely friendly to hackers.
"Computronium" is not likely to be an early product of nanotech IMO.
It seems to be a very advanced material. To build mass quantities
of computronium you must have already solved all of the construction
problems, so it will not work as a design strategy to rely on this kind
of massive computation to solve the low level problems. (OTOH it may be
possible to build sufficiently massive computers using bulk technology.)
Hal
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