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Debate About Assemblers — Smalley Rebuttal


On Physics, Fundamentals, and Nanorobots:
A Rebuttal to Smalley’s Assertion that Self-Replicating Mechanical Nanorobots Are Simply Not Possible

K. Eric Drexler, Ph.D.; David Forrest, ScD.; Robert A. Freitas Jr., J.D.; J. Storrs Hall, Ph.D.; Neil Jacobstein, M.S.; Tom McKendree Ph.D.; Ralph Merkle Ph.D.; Christine Peterson

©Institute for Molecular Manufacturing, 2001

The September 2001 issue of Scientific American was devoted to nanotechnology. It had an article by Nobelist Richard E. Smalley [1] in which he said: “Self-replicating, mechanical nanobots are simply not possible in our world.” In support of this remarkable assertion, two principal objections are advanced: the “fat fingers” problem and the “sticky fingers” problem. “Both these problems are fundamental,” Smalley writes, “and neither can be avoided.” Here we show that neither problem represents a fundamental barrier that will prevent the construction of self-replicating, mechanical nanorobots. Further, Smalley’s objections refer to designs that he himself proposed, not the designs of Drexler, whom he attempted to refute.

The “Fat Fingers” Problem

Smalley has previously stated [2] that: “Chemistry is the concerted motion of at least 10 atoms.” As this excluded a large part of well-known chemistry, it is not surprising that he now more circumspectly [1] says: “In an ordinary chemical reaction five to 15 atoms near the reaction site engage in an intricate three-dimensional waltz that is carried out in a cramped region of space measuring no more than a nanometer on each side.”

The importance of the number of reactants lies in Smalley’s argument [1] that “There just isn’t enough room in the nanometer-size reaction region to accommodate all the fingers of all the manipulators necessary to have complete control of the chemistry.” Having trimmed the required number of fat fingers from ten to five, Smalley’s argument is still on shaky ground. The argument collapses when we observe that chemical reactions often involve two reactants, such as in the controlled vacuum conditions used by the scanning tunneling microscope (STM). Two reactants can be brought together with controlled trajectories if one reactant is bound to a substrate and the second reactant is positioned and moved by a single “finger” as has already been done experimentally. For example, Ho and Lee [3] physically bound a CO molecule to an iron atom on a silver substrate using an STM. Other approaches are also possible, Brenner et al. [9] provided a molecular dynamics simulation of the hydrogen abstraction reaction from a diamond substrate.

As noted elsewhere [4], if steric constraints near the tool tip make it unexpectedly difficult to manipulate particular individual atoms or small molecules with sufficient reliability, a simple alternative is to rely upon conventional solution or gas phase chemistry for the bulk synthesis of nanoparts consisting of 10-100 atoms. These much larger nanoparts can then be bound to a positional device and assembled into larger (molecularly precise) structures without further significant steric constraints. This is the approach taken by the ribosome in the synthesis of proteins. Individual amino acids are sequentially assembled into an atomically precise polypeptide without the need to manipulate individual atoms. “Atomically precise” is a description of the precision of the final product, not a description of the manufacturing method. Complete control of every aspect of a chemical reaction is not actually required to build a nanorobot. Effective control that delivers a precise product is what is necessary. The “fat fingers” problem is not a fundamental barrier to the development of molecular assemblers or the nanorobots they enable.

The “Sticky Fingers” Problem

Smalley also advances the “sticky fingers” problem [1], which is the claim that: “…the atoms of the manipulator hands will adhere to the atom that is being moved. So it will often be impossible to release this minuscule building block in precisely the right spot….these problems are fundamental….” The existence of some reactions that don’t work still leaves plenty of room for reactions that do. To argue that the “sticky fingers” problem is a fundamental barrier to building mechanical assemblers and nanorobots, Smalley must show that no set of reactions exists which allows the synthesis of a useful range of precise molecular structures. To consider but one approach, application of a voltage between a manipulator tool and the workpiece can cause the target atom or moiety to move to the desired position. Again, Ho and Lee [3] have provided an experimental existence proof.

For a biological example, consider the ribosome. This ubiquitous biological molecular assembler suffers from neither the “fat finger” nor the “sticky finger” problem. If, as Smalley argues, both problems are “fundamental,” then why would they prevent the development of mechanical assemblers and not biological assemblers? If the class of molecular structures known as proteins can be synthesized using positional techniques, then why would we expect there to be no other classes of molecular structures that can be synthesized using positional techniques? Upon observing experimentally that polymers such as proteins can be synthesized under programmatic control, what convincing evidence do we have that the programmatic synthesis of stiffer polycyclic structures such as diamond is “fundamentally” impossible, and that mechanical assemblers will never be built?

More directly, we can examine both those reactions that have been proposed specifically for use in a mechanical assembler and those reactions that take place using an SPM (Scanning Probe Microscope). Nanosystems [4] discusses the mechanosynthetic reactions that might be used to synthesize some diamondoid structures of interest. Generally, these involve a single “finger,” i.e., a probe tip with a functionalized end that would cause a site-specific reaction on a growing molecular work-piece. Merkle [5] discusses several reactions which involve two, three, and even four reactants bound to the tips of molecular tools. A large and growing literature of research work with SPMs — both theoretical [4-12] and experimental [3, 13-16] — supports the feasibility of site specific reactions involving a reactive tip structure interacting with a surface or with a molecule on a surface. In addition, SPMs are just the beginning of a growing list of tools for manipulating atoms and molecules.

Reductio ad Absurdum

One curious aspect of Smalley’s comments is his stated replication time for a billion-atom “nanobot”: one second. To obtain this blindingly fast replication speed, he adopts a 1 GHz atomic placement frequency — approximately one atom positioned every nanosecond. But Drexler’s Nanosystems, the canonical textbook in this field, proposed a 1 MHz atomic placement frequency for mature molecular assembler systems with a corresponding replication time of ~103 seconds [4, section 14.3] — figures that are commonly adopted in the supporting literature [17]. Other proposals assume replication times up to ~105 seconds (~28 hours) for earlier and more primitive molecular assembler systems [18, 19]. Smalley’s proposal of a one second replication time thus appears inconsistent with the extant literature, permitting him erroneously to extrapolate the replicative manufacture of a population of ~1018 nanorobots in just 60 seconds. This in turn leads to a rather obvious fallacy: Allowing ~500 zJ/bond and 1 bond/atom in Smalley’s billion-atom molecular assembler, the power draw of a device population that occupies ~0.01 m3 (~1027 atoms) during the last second of operation is ~1 GW, giving a power density of ~1011 W/m3 which for diamond materials implies a mean autocatalytic graphitization decomposition time [20] in vacuum of ~30 millisec, much briefer than the posited 1-second replication time — clearly a problem.

In rhetoric, this approach might be called “reductio ad absurdum” or perhaps a “strawman.” In a serious scientific discussion, a discrepancy of three orders of magnitude between what has been proposed in the literature and what is criticized suggests at best an inadequate grasp of the proposal.

Smalley’s article is accompanied by this categorical statement: “How soon will we see the nanometer-scale robots envisaged by K. Eric Drexler and other molecular nanotechnologists? The simple answer is never.”

As Arthur C. Clarke once observed [21]: “When a distinguished but elderly scientist states that something is possible, he is almost certainly right. When he states that something is impossible, he is very probably wrong.”

In spite of Clarke’s observation, we recommend that rather than focus on issues of credentials, age, cabals, or orthodoxy, we simply stick to the scientific facts and their implications for molecular nanotechnology. The most direct way of evaluating the feasibility of engineering molecular assemblers and nanorobots is to conduct serious design and engineering efforts, along with detailed modeling and analysis. This would be the fastest way to reveal any hypothetical “fundamental problem” that would make non-biological molecular assemblers infeasible to build.

There are two possibilities: either molecular assemblers and the nanorobots they enable are theoretically feasible within the framework of well-understood existing physical laws, or they are not. A substantial and growing body of work strongly supports their feasibility. There are many worthy molecular systems engineering challenges to overcome, but thus far, there has been no credible argument that these devices are infeasible. Molecular nanotechnology assemblers would change society’s relationship to molecules and matter as fundamentally as the computer changed our relationship to bits and information. If they are feasible, we need to focus research on development pathways and safeguards.

References

1. Richard E. Smalley, “Of chemistry, love, and nanobots,” Scientific American 285(September 2001):76-77.

2. Robert F. Service, “Is nanotechnology dangerous?” Science 290(24 November 2000):1526-1527.

3. Wilson Ho, Hyojune Lee, “Single bond formation and characterization with a scanning tunneling microscope,” Science 286(26 November 1999):1719-1722; http://www.physics.uci.edu/~wilsonho/stm-iets.html

4. K. Eric Drexler, Nanosystems: Molecular Machinery, Manufacturing, and Computation, John Wiley & Sons, New York, 1992, Chapter 8.

5. Ralph C. Merkle, “A proposed ‘metabolism’ for a hydrocarbon assembler,” Nanotechnology 8(1997):149-162; http://www.zyvex.com/nanotech/hydroCarbonMetabolism.html

6. Charles B. Musgrave, Jason K. Perry, Ralph C. Merkle, William A. Goddard III, “Theoretical studies of a hydrogen abstraction tool for nanotechnology,” Nanotechnology 2(1991):187-195; http://www.zyvex.com/nanotech/Habs/Habs.html

7. Michael Page, Donald W. Brenner, “Hydrogen abstraction from a diamond surface: Ab initio quantum chemical study using constrained isobutane as a model,” J. Am. Chem. Soc. 113(1991):3270-3274.

8. Susan B. Sinnott, Richard J. Colton, Carter T. White, Donald W. Brenner, “Surface patterning by atomically-controlled chemical forces: molecular dynamics simulations,” Surf. Sci. 316(1994):L1055-L1060.

9. D.W. Brenner, S.B. Sinnott, J.A. Harrison, O.A. Shenderova, “Simulated engineering of nanostructures,” Nanotechnology 7(1996):161-167; http://www.zyvex.com/nanotech/nano4/brennerPaper.pdf

10. S.P. Walch, W.A. Goddard III, R.C. Merkle, “Theoretical studies of reactions on diamond surfaces,” Fifth Foresight Conference on Molecular Nanotechnology, 1997; http://www.foresight.org/Conferences/MNT05/Abstracts/Walcabst.html

11. Stephen P. Walch, Ralph C. Merkle, “Theoretical studies of diamond mechanosynthesis reactions,” Nanotechnology 9(1998):285-296.

12. Fedor N. Dzegilenko, Deepak Srivastava, Subhash Saini, “Simulations of carbon nanotube tip assisted mechano-chemical reactions on a diamond surface,” Nanotechnology 9(December 1998):325-330.

13. J.W. Lyding, K. Hess, G.C. Abeln, D.S. Thompson, J.S. Moore, M.C. Hersam, E.T. Foley, J. Lee, Z. Chen, S.T. Hwang, H. Choi, P.H. Avouris, I.C. Kizilyalli, “UHV-STM nanofabrication and hydrogen/deuterium desorption from silicon surfaces: implications for CMOS technology,” Appl. Surf. Sci. 130(1998):221-230.

14. E.T. Foley, A.F. Kam, J.W. Lyding, P.H. Avouris, P. H. (1998), “Cryogenic UHV-STM study of hydrogen and deuterium desorption from Si(100),” Phys. Rev. Lett. 80(1998):1336-1339.

15. M.C. Hersam, G.C. Abeln, J.W. Lyding, “An approach for efficiently locating and electrically contacting nanostructures fabricated via UHV-STM lithography on Si(100),” Microelectronic Engineering 47(1999):235-.

16. L.J. Lauhon, W. Ho, “Inducing and observing the abstraction of a single hydrogen atom in bimolecular reaction with a scanning tunneling microscope,” J. Phys. Chem. 105(2000):3987-3992.

17. Robert A. Freitas Jr., Nanomedicine, Volume I: Basic Capabilities, Landes Bioscience, Georgetown, TX, 1999, Section 2.4.2.

18. Ralph C. Merkle, “Casing an assembler,” Nanotechnology 10(1999):315-322; http://www.zyvex.com/nanotech/casing

19. J. Storrs Hall, “Architectural considerations for self-replicating manufacturing systems,” Nanotechnology 10(1999):323-330; http://www.foresight.org/Conferences/MNT6/Papers/Hall/index.html

20. Robert A. Freitas Jr., Nanomedicine, Volume I: Basic Capabilities, Landes Bioscience, Georgetown, TX, 1999, Sections 6.5.3 and 9.3.5.3.4.

21. Arthur C. Clarke, Profiles of the Future, Harper & Row, Publishers, New York, 1962.

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