The recent comments by Alec Broers in his Reith Lecture about the feasibility or otherwise of the Drexlerian flavour of molecular nanotechnology have sparked off a debate that seems to have picked up some of the character of the British general election campaign (Liar! Vampire!! Drunkard!!! ). See here for Howard Lovy’s take, here for TNTlogs view. All of this prompted an intervention by Drexler himself (channeled through Howard Lovy), which was treated with less than total respect by TNTlog. Meanwhile, Howard Lovy visited Soft Machines to tell us that “when it comes to being blatantly political, you scientists are just as clumsy about it as any corrupt city politician I’ve covered in my career. The only difference is that you (I don’t mean you, personally) can sound incredibly smart while you lie and distort to get your way.” Time, I think (as a politician would say), to return to the issues.
Philip Moriarty, in his comment on Drexler’s letter, makes, as usual, some very important points about the practicalities of mechanosynthesis. Here I want to look at what I think is the strongest argument that supporters of radical nanotechnologies have, the argument that the very existence of the amazing contrivances of cell biology shows us that radical nanotechnology must be possible. I’ve written on this theme often before (for example here), but it’s so important it’s worth returning to.
In Drexler’s own words, in this essay for the AAAS, “Biology shows that molecular machines can exist, can be programmed with genetic data, and can build more molecular machines”. This argument is clearly absolutely correct, and Drexler deserves credit for highlighting this important idea in his book Engines of Creation. But we need to pursue the argument a little bit further than the proponents of molecular manufacturing generally take it.
Cell biology shows us that it is possible to make sophisticated molecular machines that can operate, in some circumstances, with atomic precision, and which can replicate themselves. What it does not show is that the approach to making molecular machines outlined in Drexler’s book Nanosystems, an approach that Drexler describes in that book as “the principles of mechanical engineering applied to chemistry”, will work. The crucial point is that the molecular machines of biology work on very different principles to those used by our macroscopic products of mechanical engineering. This is much clearer now than it was when Engines of Creation was written, because in the ensuing 20 years there’s been spectacular progress in structural biology and single molecule biophysics; this progress has unravelled the operating details of many biological molecular machines and has allowed us to understand much more deeply the design philosophy that underlies them. I’ve tried to explain this design philosophy in my book Soft Machines; for a much more technical account, with full mathematical and physical details, the excellent textbook by Phil Nelson, Biological Physics: Energy, Information, Life, is the place to go.
Where Drexler takes the argument next is to say that, if nature can achieve such marvelous devices using materials whose properties, constrained by the accidents of evolution, are far from optimal, and using essentially random design principles, then how much more effective will our synthetic nano-machines be. We can use hard, stiff materials like diamond, rather than the soft, wet and jelly-like components of biology, and we can use the rationally designed products of a mechanical engineering approach rather than the ramshackle and jury-rigged contrivances of biology. In Drexler’s own words, we can expect “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”.
There’s something wrong with this argument, though. The shortcomings of biological design are very obvious at the macroscopic scale – 747s are more effective at flying than crows, and, like many over-40 year olds, I can personally testify to the inadequacy of the tendon arrangements in the knee-joint. But the smaller we go in biology, the better things seem to work. My favourite example of this is ATP-synthase. This remarkable nanoscale machine is an energy conversion device that is shared by living creatures as different as bacteria and elephants (and indeed, ourselves). It converts the chemical energy of a hydrogen ion gradient, first into mechanical energy of rotation, and then into chemical energy again, in the form of the energy molecular ATP, and it does this with an efficiency approaching 100%.
Why does biology work so well at the nanoscale? I think the reason is related to the by now well-known fact that physics looks very different on the nanoscale than it does at the macroscale. In the environment we live in – with temperatures around 300 K and a lot of water around – what dominates the physics of the nanoscale is ubiquitous Brownian motion (the continuous jostling of everything by thermal motion), strong surface forces (which tend to make most things stick together), and, in water, the complete dominance of viscosity over inertia, making water behave at the nanoscale in the way molasses behaves on human scales. The kind of nanotechnology biology uses exploits these peculiarly nanoscale phenomena. It uses design principles which are completely unknown in the macroscopic world of mechanical engineering. These principles include self-assembly, in which strong surface forces and Brownian motion combine to allow complex structures to form spontaneously from their component parts. The lack of stiffness of biological molecules, and the importance of Brownian motion in continuously buffeting them, is exploited in the principle of molecular shape change as a mechanism for doing mechanical work in the molecular motors that make our muscles function. These biological nanomachines are exquisitely optimised for the nanoscale world in which they operate.
It’s important to be clear that I’m not accusing Drexler of failing to appreciate the importance of nanoscale phenomena like Brownian motion; they’re treated in some detail in Nanosystems. But the mechanical engineering approach to nanotechnology – the Nanosystems approach – treats these phenomena as problems to be engineered around. Biology doesn’t engineer around them, though, it’s found ways of exploiting them.
My view, then, is that the mechanical engineering approach to nanotechnology that underlies MNT is less likely to succeed than an approach that seeks to emulate the design principles of nature. MNT is working against the grain of nanoscale physics, while the biological approach – the soft, wet, and flexible approach, works with the grain of the way the nanoscale works. Appealing to biology to prove the possibility of radical nanotechnology of some kind is absolutely legitimate, but the logic of this argument doesn’t lead to MNT.