Kinematic Self-Replicating Machines
© 2004 Robert A. Freitas Jr. and Ralph C. Merkle. All Rights Reserved.
5.1.9.D Replicator Structure
D1. Replicator Physicality. This factor is interdependent with substrate physicality (see C1).
D2. Replicator Naturalicity. The relative advantages and limitations of “soft” vs. “hard” machines in growth and replication have been discussed by Drexler , Freitas , and Sosic and Johnson .
D3. Replicator Technology Level or Granularity. One interesting unconventional alternative is the quantum-dot-based “programmable matter” of McCarthy , which would involve physical substances made of artificial atoms whose elemental character is capable of being electronically switched.
D4. Replicator Physical Dimensionality. Tool-tip AFM-based systems are 0-D; early Penrose blocks (Section 3.3, Figure 3.3) are 1-D; etc. This item refers to the physical dimensionality of the constructing, assembling, or manufacturing process, and not to the kinematic manipulator degrees of freedom described in H5. For “temporal dimensionality” of replicators, see L8 (heterochrony).
D5. Replicator Scale. Because of their superior speed of operation and thus their ability to build more finely-grained structures composed of vastly greater numbers of parts in a given volume per unit of time, microscale replicators constructed from nanoscale components should be capable of far greater replicative complexity than macroscale replicators constructed of macroscale parts. But replicators can be built at either scale.
D6. Replicator Mass. This quantity is a continuous measure.
D7. Replicator Boundary. Compartmentation is regarded by many as essential for the origin of life  and successful replication. Compartmentation can occur via a medium that limits macromolecular diffusion more than small molecule diffusion (e.g., the interior of a gel matrix, or a micro- or nano-porous rock) , or via an emulsion in which small aqueous compartments in a non-aqueous matrix house replicating molecules , or via membranes or walls enclosing the replicator machinery . Interestingly, Barricelli  also predicted that his virtual symbioorganisms would need to develop means of controlling their local environment (such as an external membrane) if they were to evolve past a certain level of complexity.
D8. Product Export through Boundary. “In typical applications, the products of molecular manufacturing must be delivered to an external environment without permitting back contamination of the eutactic internal environment” of the assembler . “Product” here may include both replicas and non-self objects manufactured by the original replicator.
D9. Replicator Structural Fixity. Internal components may be fixed or mobile within the replicator boundary, or they (most likely active subunits) may be free to exit the replicator boundary, possibly to return later. Replicators may fragment into non-growing entities that are considerably smaller than the whole viable entity, and these fragments may come together at a later time to form a viable organism. Alternatively, a replicator might fragment, with each disparate component incapable of growth but the population of components still capable of achieving success . Some plant RNA viruses exhibit just such a pattern , with each particle packaging separate RNA and sometimes three separate particles needed to establish an infection, though there are as yet no examples of this strategy among the prokaryotes. Size, shape, and morphology of a replicator composed of a facultative aggregate of active subunits may change according to the environmental and social status of the aggregate. One example of such an organism is Myxococcus xanthus , whose life cycle is carefully controlled by cell density and nutrient levels, and consists in turn of tiny forms, actively moving large forms, and huge social formations producing mushroom-like fruiting bodies . It is also proposed  that nanobacteria may react to stress by becoming social and forming communities, or by “hibernating” for extensive periods waiting for suitable conditions permitting growth, or by forming and shedding units resembling viruses – an elementary system of tiny units performing special tasks. In this theory, only when united and surrounded by membrane, thus closing the compartment, would the putative nanobacteria resemble present forms of bacteria .
D10. Replicator Particity. Is the replicator composed of discrete parts, or is it constructed as a unitary block with no individual parts whatsoever?
D11. Process Materials Storage. Where are process materials stored?
D12. Replicator Computer. Does the replicator possess an onboard computer (e.g., central processing unit (CPU) and general-purpose memory? Note that any CPU contains memory, and any memory requires a CPU (or at least logic).
D13. Replicator Contiguity. Are the various components of a replicator contiguous in physical space, or not? Notes Friedman : “There is an implicit assumption in much of the literature that the self-replicating machines must consist of a single complex assembly (that’s almost what we see in nature if we ignore the complementary activity of the sexes.) In the engineering world, it appears abundantly more practical to envision a large, distributed set of cooperating elements, such as a modern industrial factory. Then it becomes interesting and practical to ask questions such as: ‘What is the simplest factory that can manufacture a useful product and also manufacture a complete replication of itself (with and without human intervention)?’ Or: ‘What is the simplest factory which can self-replicate autotrophically (with and without human intervention)?’ For the cases permitting human intervention at least we have an existence proof: The set of all factories in the United States can produce useful products and they – as a system – clearly have the ability to produce more of their own kind (or where else did they come from?). So far, there is no existence proof of the case without human intervention – except the biological analogy.” See also I4.
Last updated on 1 August 2005