From: hal@finney.org
Date: Fri Aug 17 2001 - 18:24:37 MDT
Robert writes, to Scientific American, about Whitesides' article
at http://www.sciam.com/2001/0901issue/0901whitesides.html:
> First, Dr. Whitesides states,
> "The flagellar motor does not act by using electric current
> to generate moving magnetic fields; instead it uses the
> decomposition of ATP to cause changes in the shape of the
> molecules that, when combined with a sophisticated molecular
> ratchet, makes the protein shaft revolve."
>
> Unfortunately this is incorrect. As [1, pgs 447-450, 2] document,
> the flagellum is a hydrogen ion driven motor. It only uses
> ATP to perform its protein export functions.
Robert is right, but I don't think this small technical error
invalidates Whitesides' argument. His lead-in sentence was "The
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.
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.
(Coincidentally I found an article pointing out another connection:
the ATP synthase engine is apparently evolved from the flagellar motor,
including a rotor and everything,
http://www.mhhe.com/biosci/genbio/olc_linkedcontent/raven_updates/08update_text.html).
> Second, Dr. Whitesides attempts to make the case against
> nanosubmarines as "ships approximately 100 nanometers in scale",
> perhaps because the National Science Foundation is defining
> "nanotechnology" at that scale. To the best of my knowledge
> no one who is serious about the feasibility of molecular
> nanotechnology has ever suggested that nanobots would be
> that size. People who are familiar with the literature know
> that 100 nm is the size of a nanoassembler arm [3, pg 401 &
> 4, pg 14] and the existant serious design studies for
> nanobots [5,6,7] describe them as ~1 micron in size, approximately
> the same size as a typical bacteria or mitochondria.
> That would be 1000 times the volume and mass of the
> nanosubmarines that Dr. Whitesides imagines.
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.
These issues are considered at length in Nanomedicine. Whitesides is
right that the sub would have to use something like a helical screw
similar in concept to the bacterial flagellum rather than a conventional
propellor, as the effective viscosity of the fluids are enormous at that
size scale.
However he is wrong about the Brownian effects since the subs would be
much larger than he assumes. Also, Freitas suggests that the nanobots
would be able to travel at 1 cm/sec (100 times faster than the 100-200
microns per second of sperm). This is fast enough to negotiate
capillaries and small blood vessels without much difficulty.
> Finally, his arguments regarding the locomotion, detection abilities
> and energy sources for nanobots demonstrate a lack of knowledge
> of the large body of work that has been done on such topics [8].
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.
> While I could write a more extended critique, I think my
> point is clear. If Scientific American wants a "scientific"
> critique of the applications of molecular nanotechnology
> they should seek out writers who have read the literature.
> The editors should also have articles containing information
> regarding microbiology or medicine reviewed by experts
> in those fields, particularly when they are written by
> someone whose primary expertise is chemistry. An essay
> providing some other examples of why it is dangerous
> to place too high a value on scientists speaking outside
> their areas of expertise may be found in [9].
I'm not sure this article of yours,
http://www.aeiveos.com/~bradbury/Critiques/WSOT.html, is a good idea
to bring into the discussion. You start off criticizing Stephen
Hawking for expressing some very common beliefs about hypothetical
extra-terrestrials, and you offer your own theory that ETs might not
seek to expand their territory. As you know from discussions here,
very few people find this argument compelling. It's not IMO a very
good example of why scientists can't be trusted outside their field,
since it's not a case where most people will agree that the scientist
was wrong. (The second example in your article, Drake's opinions on
biological longevity, works better.)
I also think that this criticism of Whitesides is not quite on point;
as a chemist he is presumably speaking within his field on a number of
the issues where he was nevertheless wrong (such as the grabbing-atom
problem). Nanobot locomation, detection capabilities and energy flow are
not medical issues. Freitas discusses them only to set the groundwork
for his later analyses. I'm not sure what field they fall into, but
chemistry is probably about as close as you'll get today.
> Scientific American should resist attempts to fan the
> animosities between the believers and disbelievers in
> molecular nanotechnology. Why? Because if, and I realize
> its a big if, the believers happen to be right, and the
> misconstructed arguments of the disbelievers happen to
> be wrong, the costs in premature deaths due to delays in
> the development of the full vision outlined by Drexler [10]
> are in excess of 53 million human lives -- *each* year.
> That is greater than the combined military and civilian
> losses in all of World War II. Think about that.
SciAm is publishing both sides of the issue, a traditional approach to
getting at the truth. I don't think it is fair to characterize this as
fanning animosities, or to ask them to stop. Consider an AIDS skeptic
making a similar appeal to stop printing articles claiming that HIV
causes AIDS, with an emotional point about the number of people dying
from the disesase every year. It's unfortunate that people have to die,
but the solution to that is to seek the truth. That may inherently
involve exposing controversies to the light of day, so that people can
respond openly to criticism.
Hal
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