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Criticism of criticism

Research in molecular nanotechnology requires a design perspective because it aims to describe workable systems. It is easy to describe unworkable systems, and criticisms of a critic's own bad design have on occasion been presented as if they were criticisms of molecular nanotechnology as a whole. Some examples: assuming the use of flexible molecules, then warning that they will have no stable shape; assuming the manipulation of unbound reactive atoms, then warning that they will react and bond to the manipulator; assuming the use of materials with unstable surfaces, then warning that the surfaces will change; assuming that reactive gases permeate nanosystems, then warning that reactions will occur; assuming that nanomachines must "see," then warning that light waves are too long and x-rays too energetic; assuming that nanomachines swim from point to point, then warning that Brownian motion makes such navigation impossible; assuming that nanomachines dissipate enormous power in a small volume, then warning of overheating; and so on, and so forth. These observations constitute not criticisms, but rediscoveries of elementary engineering constraints.

Use of tenses

In ordinary discourse, "will be" suggests a prediction, while "would be" suggests a conditional prediction. Using these future-tense expressions is inappropriate when discussing the time-independent possibilities inherent in physical law.

In speaking of spacecraft trajectories to Pluto, for example, to say that they "will be" is to predict the future of spaceflight; to say that they "would be" is to remind readers of the uncertainties of budgets and life. Both phrases distract from the analysis of celestial mechanics and engineering trade-offs. The present tense is more serviceable: One can say that as-yet unrealized spacecraft trajectories to Pluto "are of two kinds, direct and gravity assisted," and then analyze their properties without distraction. Similarly, one can say that as-yet unrealized nanomachines of diamondoid structure "are typically stiffer and more stable than folded proteins." Much of the discussion in this volume is cast in this timeless present tense; this is not intended to imply that devices like those discussed in Parts I and II presently exist.

Citations and apologies

It is much easier to grasp and apply the main results of a field than it is to provide a balanced guide to the recent work, omitting no useful citations. I am sure that my discussions of chemistry and protein engineering, for example, omit papers fully as valuable as the best included. I apologize to authors I have slighted.

Less forgivable are those instances (which I cannot yet identify) in which I may have rederived some result that should be attributed to an earlier author, perhaps well known in some specialty. In interdisciplinary research, one cannot spend a professional lifetime immersed in a single literature, and such failures of attribution become likely—mathematics often yields results more easily than does a library. Any such lapses brought to my attention will be corrected in future editions; their most likely locations are Chapters 5, 6, and 7.

Aside from these lapses, material presented without citation falls into two categories that I trust are distinct. First, well-known principles and results from established fields—physics, statistical mechanics, chemistry—are used without citing Newton, Boltzmann, Pauling, or their kin. Second, the designs, concepts, and analytical results that are both specific to nanomechanical systems and not attributed to someone else are to the best of my knowledge original contributions, many presented for the first time in this volume.

It also seems necessary to apologize for doing theoretical work in a world where experimental gains are often so hard-won. If this theoretician's description of possibilities seems to make light of experimental difficulties, I can only plead that it would soon become tedious to say, at every turn, that laboratory work is difficult, and that the hard work is yet to be done.

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