Did Smalley deliver a killer blow to Drexlerian MNT?

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, Smalleys 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 doesnt 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, whats 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 isnt straightforward you need to choose different conditions or different targets. You dont need an impossible number of fingers to guide the system through configuration space for the same reason that you dont 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).

Drexler and the nanosubmarines

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.

Making me a better human

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.

Drexler responds

This morning brought a somewhat tetchy email from K. Eric Drexler, not entirely happy about my article in Physics World, The future of nanotechnology. There were three main complaints:

1. That he, Drexler, could not be held responsible for the “ridiculous artist’s concepts” that have become associated with his work. Thus my criticism of the nanosubmarine illustration isn’t a fair criticism of MNT. Actually, I have some sympathy with his predicament on this, in that I’m sure that the elementary errors that show up in the particularly silly image I chose wouldn’t be there if Drexler had had anything to do with it. Nonetheless, my criticism of these images does make one important point very clear – you shouldn’t expect macroscopic engineering design concepts to apply to directly to the nanoworld. Is this a fair criticism of MNT? I think it is – to quote from the preface of Nanosystems; “Molecular manufacturing applies the principles of mechanical engineering to chemistry”.

2. Next he argues that my statement that “Strong surface forces may make the moving parts of a NEMS device stick together and seize up” reflects a lack of study of the appropriate section of Nanosystems, chapter 10, which argues that very low friction is to be expected between atomically smooth diamond surfaces. It’s worth noting first of all that this statement in my article isn’t actually directed at MNT at all, but at top-down NEMS. Nonetheless, I do believe that the discussion in Nanosystems does substantially underestimate the problems of friction and dissipation at the nanoscale. This is a rather technical discussion, which I will enlarge on at a later time.

3. Finally, he objects that I have not proved my central contention, that biology is highly optimised for the nanoscale, pointing out that biology hasn’t been able to explore the space of non-aqueous molecular machine systems. This gets to the heart of the argument of Soft Machines. A crucial, though obvious, point, is that it only makes sense to talk about optimisation in the context of a particular environment, and what is optimised for ambient operation at 300 K in the presence of water is not the same as what is optimised for ultra-high vacuum at a temperature of 3 K. I wouldn’t exclude the possibility that MNT would work at 3 K in UHV, but I think that what works in ambient conditions is much more interesting, if only because medicine is likely to be such an important application of nanotechnology.