A Debate About Assembler Design
Post-debate comments by David Forrest, 10 September 2005
Richard Jones, a physicist at U. Sheffield, has challenged the notion that the vision of molecular manufacturing systems proposed by Eric Drexler in Nanosystems offers the best design for productive nanosystems. In various places in his book, Soft Machines, Jones points out three issues that he claims will "scupper the entire enterprise" (p. 159): Brownian motion, thermal noise, and surface forces (p. 161). As an alternative he proposes that biologically-inspired systems that are adapted to and exploit Brownian motion are a better approach to develop molecular machine systems (in fact, he claims this is the only approach that will work). He elaborated on this to some extent in a debate in August 2005, but has not offered any specific alternative designs.
In response, we offer the following points:
- Until Jones proposes a specific design, it is not possible to evaluate whether his philosophy represents a superior alternative.
- With existing design tools, it is harder to design machines with the irregular surface conformations and electrostatic potential distributions found in most biological protein molecules than to design machines based on the regular geometric features of diamondoid materials.1
- As early as 1981, Drexler noted that biological structures can function as machine components, and proposed tailoring protein molecules to serve as more effective molecular machine components.2
- The designs proposed in Nanosystems do in fact exploit Brownian motion (during the sorting and purification operations, p. 374ff.).3
- Stiction is relevant to MEMS--not the designs proposed in Nanosystems, which have engineered atomic surfaces that exploit selective stickiness or provide low friction, as needed.
- A molecular motor based on carbon nanotubes has already been built, refuting the speculations by Jones that non-biological molecular motors based on rigid structures will not work.
- Nested carbon nanotubes have already been shown to have very low friction and they work as efficient bearings, refuting the claim by Jones that non-biological molecular bearings would have high friction and not work.
- A eutactic system, in which the trajectories and orientations of all atoms in the system are precisely controlled and directed to and from reaction sites at high speed, will be intrinsically more efficient than a system that relies on the random motion of molecules for transport and assembly processes. On a more primitive level, this is the principle behind catalysts, which are used to accelerate industrial chemical reactions by guiding molecules together in favorable orientations.
- The legitimate issue of thermal noise has been accounted for in the Nanosystems designs, and also treated by Merkle.
- Regardless of design approach, it is important to develop molecular machine systems with appropriate safeguards. Conformance to a set of guidelines, such as the Foresight Guidelines, and to consensus standards based on these principles, will help to insure safe development and instill public confidence in the process.
1Consider for example the complex shape of the ribosome, a natural molecular assembler, which is composed of two halves that adhere together according to matching geometries and potential energy surfaces.
2In addition to Drexler's first paper on nanotechnology there is a publication that provides a more specific approach to the concept of protein design for the construction of molecular machine components for productive nanosystems.
3Prof. Jones advocates developing systems that take advantage of Brownian motion, which is a good idea. There are certainly tradeoffs involved in terms of performance and reliability if you make the entire system dependent on Brownian motion, but there's not really much to debate--we encourage a diversity of approaches to the development of molecular manufacturing systems. Again, we need to see an actual design.
In chapter 4 of his book, Professor Jones provides a description of Brownian motion, explaining how molecules in liquids and gases are continually jostled about. He also states that one cannot engineer a system that does not contain Brownian motion (page 63). We note again that only part of the system proposed by Drexler in Nanosystems would exclude Brownian motion: the eutactic environment which is sealed from contaminants, and in which the trajectory of every atom is controlled. This is distinguished from the traditional phases of matter--solid, liquid, and gas--by the term "machine-phase."