Category: Radical nanotechnology and MNT

The current edition of “Physics World” carries a letter from K. Eric Drexler, written in response to my article in the August edition, “The future of nanotechnology“. There is also a response from me to Drexler’s letter. Since the letters section of Physics World is not published online, I reproduce the letters here. The text here is as the authors wrote them; they’ve been lightly edited to conform with Physics World house style in the printed version.

From Dr K.Eric Drexler to Physics World.

I applaud Physics World for drawing attention to the emerging field of artificial molecular machine systems. Their enormous productive potential is illustrated in biology and nature, where we observe molecular machine systems constructing molecular machinery, electronics, and digital information storage systems at rates measured in billions of tons per year. To understand the future potential of fabrication technologies (the foundation of all physical technology) we must examine the productive potential of artificial molecular machine systems. This field of enquiry has been a focus of my research since (Drexler 1981), which explored directions first suggested by (Feynman 1959).

I was surprised to find that Professor Richard Jones, in describing ???flaws in Drexler???s vision,??? ignores my physical analysis of productive molecular machine systems. He instead criticizes the implied hydrodynamics of an artist???s fantastic conception of a ???nanosubmarine??? ??? a conception not based on my work. It is, I think, important that scientific criticisms address the scientific literature, not artistic fantasies.

Professor Jones then offers a discussion of nanoscale surface forces, thermal motion, and friction that could easily leave readers with the impression that these present dire problems, which he implies have been ignored. But ignoring surface forces or thermal motion in molecular engineering would be like ignoring gravity or air in aeronautics, and physical analysis shows that well-designed molecular bearing interfaces can have friction coefficients far lower than those in conventional machines. These issues (and many others) are analyzed in depth, using the methods of applied physics, in Chapters 3, 5, and 10 of my book Nanosystems (Drexler 1992). Professor Jones fails to cite this work, noting instead only my earlier, popular book written for a general audience.

I agree with Professor Jones regarding the importance of molecular machine systems and the value of learning from and imitating biological systems at this stage of the development of our field. Where we part company is in our judgment of the future potential of the field, of the feasibility of molecular machine systems that are as far from the biological model as a jet aircraft is from a bird, or a telescope is from an eye. I invite your readers to examine the physical analysis that supports this understanding of non-biological productive molecular machine systems, and to disregard the myths that have sprung up around it. (One persistent myth bizarrely equates productive molecular machines with gooey nanomonsters, and then declares these to be impossible contraptions that grab and juggle atoms using fat, sticky fingers.)

There are many interesting research questions to address and technology thresholds to cross before we arrive at advanced artificial molecular machine systems. The sooner we focus on the real physics and engineering issues, building on the published literature, the sooner progress can be made. To focus on artist???s conceptions and myths does a disservice to the community.

K. Eric Drexler, PhD
Molecular Engineering Research Institute

K E Drexler 1981 Molecular Engineering: An approach to the development of general capabilities for molecular manipulation. Proc. Nat. Acad. Sci. (USA) 78:5275???5278
K E Drexler 1992 Nanosystems: Molecular Machinery, Manufacturing, and Computation (New York Wiley/Interscience)
R Feynman 1959 There???s Plenty of Room at the Bottom, in D Gilbert (ed) 1961 Miniaturization (New York Reinhold)

From Dr Richard A.L. Jones to Physics World.

I am pleased that Dr Drexler finds so much to agree with in my article. Our goals are the same; our research aims to understand how to make molecular scale machines and devices. Where we differ is how best to achieve that goal. The article was necessarily very brief in its discussion of surface forces, friction and thermal motion, and my book [1] contains a much fuller discussion, which does explicitly refer to Drexler???s book ???Nanosystems???. No-one who has read ???Nanosystems??? could imagine that Drexler is unaware of these problems, and it was not my intention in the article to imply that he was. Absurd images like the nanosubmarine illustration I used are widely circulated in popular writings about Drexlerian nanotechnology; they well illustrate the point that na??ve extrapolations of macro-scale engineering to the nanoscale won???t work, but I???m happy to agree that Drexler???s own views are considerably more sophisticated than this. The point I was making was that the approach Drexler describes in detail in Nanosystems, (which he himself describes in the words: ???molecular manufacturing applies the principles of mechanical engineering to chemistry???), works within a paradigm established in macroscopic engineering and seeks to find ways to engineer around the special features of the nanoworld. In contrast to this, the design principles adopted by cell biology turn these special features to advantage and actively exploit them, using concepts such as self-assembly and molecular shape change that have no analogue in macroscopic engineering. Again, Dr Drexler and I are in agreement that in the short term biomimetic nanotechnology will be very fruitful and should be strongly pursued. We differ about the likely long term trajectory of the technology, but here, experiment will decide. Such is the unpredictable nature of the development of technology that I rather suspect that the outcome will surprise us both.

[1] Soft Machines, R.A.L. Jones, OUP (2004)

The most high profile opponent of Drexlerian nanotechnology (MNT) is certainly Richard Smalley; he’s a brilliant chemist who commands a great deal of attention because of his Nobel prize, and his polemics are certainly entertainingly written. He has a handy way with a soundbite, too, and his phrases “fat fingers” and sticky fingers” have become a shorthand expression of the scientific case against MNT. On the other hand, as I discussed below in the context of the Betterhumans article, I don’t think that the now-famous exchange between Smalley and Drexler delivered the killer blow against MNT that sceptics were hoping for.

For my part, I am one of those sceptics; I’m convinced that the MNT project as laid out in Nanosystems will be very much more difficult than many of its supporters think, and that other approaches will be more fruitful. The argument for this is covered in my book Soft Machines. But, on the other hand, I’m not convinced that a central part of Smalley’s argument is actually correct. In fact, Smalley???s line of reasoning if taken to its conclusion would imply not only that MNT was impossible, but that conventional chemistry is impossible too.

The key concept is the idea of an energy hypersurface embedded in a many-dimensional hyperspace, the dimensions corresponding to the degrees of freedom of the participating atoms in the reaction. Smalley argues that this space is so vast that it would be impossible for a robot arm or arms to guide the reaction along the correct path from reactants to products. This seems plausible enough on first sight ??? until one pauses to ask, what in an ordinary chemical reaction guides the system through this complex space? The fact that ordinary chemistry works ??? one can put a collection of reactants in a flask, apply some heat, and remove the key products (hopefully this will be your desired product in a respectable yield, with maybe some unwanted products of side-reactions as well) ??? tells us that in many cases the topography of the hypersurface is actually rather simple. The initial state of the reaction corresponds to a deep free energy minimum, the product of each reaction corresponds to another, similarly deep minimum, and connecting these two wells is a valley; this leads over a saddle-point, like a mountain pass, that defines the transition state. A few side-valleys correspond to the side-reactions. Given this simple topography, the system doesn???t need a guide to find its way through the landscape; it is strongly constrained to take the valley route over the mountain pass, with the probability of it taking an excursion to climb a nearby mountain being negligible. This insight is the fundamental justification of the basic theory of reaction kinetics that every undergraduate chemist learns. Elementary textbooks feature graphs with energy on one axis, and a ???reaction coordinate??? along the other; the graph shows a low energy starting point, a low energy finishing point, and an energy barrier in between. This plot encapsulates the implicit, and almost always correct, assumption that out of all the myriad of possible paths the system could take through the hyperspace of configuration space the only one that matters is the easy way, along the valley and over the pass.

So if in ordinary chemistry the system can navigate its own way through hyperspace, what???s different in the world of Drexlerian mechanochemistry? Constraining the system by having the reaction take place on a surface and spatially localising one of the reactants will simplify the structure of the hyperspace by reducing the number of degrees of freedom. This makes life easier, not harder ??? surfaces of any kind generally have a strong tendency to have a catalytic effect ??? but nonetheless, the same basic considerations apply. Given a sensible starting point and a sensible desired product (i.e. one defined by a free energy minimum) chemistry teaches us that it is quite reasonable to hope for a topographically straightforward path through the energy landscape. As Drexler says, if the pathway isn???t straightforward you need to choose different conditions or different targets. You don???t need an impossible number of fingers to guide the system through configuration space for the same reason that you don???t need fingers in conventional chemistry, the structure of configuration space itself guides the way the system searches it.

This is a technical and rather abstract argument. As always, the real test is experimental. There’s some powerful food for thought in the report on a Royal Society Discussion Meeting “‘Organizing atoms: manipulation of matter on the sub-10 nm scale'” which was published in the June 15 issue of Philosophical Transactions. Perhaps the most impressive example of a chemical reaction induced by physically moving individual reactants into place with an STM is the synthesis of biphenyl from two iodobenzene molecules (Hla et al, PRL 85 2777 (2001)). To use their concluding words “In conclusion, we have demonstrated that by employing the STM tip as an engineering tool on the atomic scale all
steps of a chemical reaction can be induced: Chemical reactants can be prepared, brought together mechanically, and finally welded together chemically. ” Two caveats need to be added: firstly, the work was done at very low temperature (20 K) presumably so the molecules didn’t run around too much as a result of Brownian motion. Secondly, the reaction wasn’t induced simply by putting fragments together into physical proximity; the chemical state of the reactants had to be manipulated by the injection and withdrawal of electrons from the STM tip.

Nonetheless, I rather suspect that this is exactly the sort of reaction that one would say wasn’t possible on the basis of Smalley’s argument.

(Links in this post probably need subscriptions).

I wrote below about Drexler’s unhappiness that I had illustrated my article in Physics World with a particularly
silly image of a nanosubmarine. He wrote that could not be held responsible for the “ridiculous artists concepts” that have become associated with his work, and thus my criticism of the nanosubmarine illustration wasn’t a fair criticism of MNT. I’m quite sure that if Drexler had been directly involved in the production of images like these, then they would be much more physically plausible. But I wonder if the supporters of Drexler have been as quick to seek correction when these images are used in connection with articles that are positive about MNT? The particular image I chose is very widely circulated, as it appears on the Microsoft Encarta online encyclopedia with the caption “Nanobot computers of the future” . Many readers – particularly high school students – will regard this source as authoritative, and it is perhaps a pity that this image remains unchallenged there.

The neutral onlooker might also find it puzzling that exactly the same image appears on the website of the Foresight Institute, of which Drexler is Founder and Chairman Emeritus. Of course, Drexler can’t be held responsible for everything on this large website, particularly given that he has no executive role. But the casual browser must surely be forgiven for thinking that images on the Foresight website carried some kind of endorsement from the Foresight Institute, and thus by extension from its Board chairman.

But the issue of the use of imaginative images is far from black and white. I gave a talk at a conference in May in which I made similar criticisms of this kind of image, and I was surprised to be taken to task about it by a prominent member of the UK nanobusiness community. His argument was that I should consider the image as a metaphor, and if the public found it easier to understand the image of a nanobot vacuum cleaner sucking up cholesterol deposits than a more realistic picture of, say, an anti-cholesterol drug wrapped up in an advanced nanoscale drug-delivery device like a liposome, then the imaginative image served a valuable purpose. Perhaps I’m too literal minded to buy this argument. The message must surely be that visual metaphors are very powerful, but if not used carefully they can rebound in unexpected and unwelcome ways.

An interesting article on the Better Humans website, Unraveling the Big Debate over Small Machines, quotes me, and adds that my position on nanotechnology isn’t very different to Drexler’s. This is at first sight rather puzzling since my recent article in Physics World, The Future of Nanotechnology, and indeed my book Soft Machines, have been read by many people, including Drexler himself, as attacks on the Drexlerian position. Indeed, I would say myself that my views are actually pretty similar to those of MNT arch-sceptic George Whitesides, though I possibly express them a bit more politely, and with a little less self-confidence.

But on reflection, I find this rather a welcome perception. Perhaps it does mean that a space is growing on both sides of the debate for some rather more nuanced positions than we’ve seen in the past. The Better Humans article gives a lot of attention to the Drexler-Smalley debate. It seems to me that we need to move on from this. MNT sceptics need to recognise that Smalley did not deliver the knock-out punch that they were hoping for. This was brought home to me in Santa Barbara this week in a conversation with an old friend who teaches a sophomore class in nanotechnology at the University of Pennsylvania. She’d set her class the task of studying the debate and deciding which side they thought had prevailed; an overwhelming majority favoured Drexler. So a reasonable sample of educated and intelligent young people was not convinced by Smalley. On the other hand, I think that MNT devotees are wrong to think that this means there are now no rational grounds for scepticism about MNT. While the possibility of some kind of radical nanotechnology is proved by the existence of biological nanomachines, the question of what the best approach to making synthetic nanomachines is is by no means decided. My book Soft Machines argues that MNT has many more disadvantages and potential difficulties than some of its supporters admit, and it will be interesting to see whether its arguments prove more convincing than Smalley’s.