Robin Milner, Susan Stepney. Nanotechnology -- Computer Science opportunities and challenges

Submission by the UK Computing Research Committee to the Nanotechnology Working Group of the Royal Society and the Royal Academy of Engineering August 2003


Advances in technology repeatedly allow software to permeate the design of artefacts at lower and lower levels. This occurred long ago with microprogramming, when computers were stand-alone; more recently it occurred with programmable networks (e.g. routers); it is now occurring with wireless networks. Nanotechnology will lead to a new dramatic step of this kind. Although it will include applications that require little computational input, it will also provide opportunities for exploitation that involve complex computational interactions among structured populations of nanoscopic agents. The more sophisticated these structures and interactions, the more computer science (CS) research will be involved.

This submission is arranged as follows: Section 1 discusses in more detail the character of the applications where CS is relevant. Section 2 outlines three possible kinds of relevant pragmatic theory that computer scientists can expect to study. Section 3 surveys how these theories may develop as generalisations and refinements of current trends in CS. Section 4 discusses how CS is relevant to the issues for safety and dependability that are raised by nanotechnology.

Two main points emerge. First, nanotechnology presents research challenges that will lead to a greatly enriched and more general science of computation, which is a scientific goal of intrinsic value. Second, safety and dependability will present unprecedented demands; the science will be responsible not only for robust design to meet these demands, but for robust analysis to show they have been met.

1. Background and context

Nanotechnology is the design, development and use of devices on the nanometre (atomic) scale. In this document we are not so much concerned with nano-scale artefacts that take the current trend of miniaturisation a few orders of magnitude further. Rather we are interested in those active physical nano-devices that themselves manipulate the world at their nano-scale in order to manufacture macroscopic artefacts. This is Drexler's [D86][D92] vision of nano-scale assemblers that build (assemble) macroscopic artefacts. (Such assemblers are often known as nanites or nanobots .)

In order for nanites to build macroscopic objects in useful timescales, there needs to be a vast number of them. A starting population of a few nanites assembles more of their kind, which then assemble more, with exponentially growing numbers. Once they exist in sufficient numbers, they can build, or become, the macroscopic artefact. This view of nanotechnology promises many awe-inspiring possibilities.

Some argue that such a technology is too good to be true, or at least question the detail of Drexler's predictions. But one should note that there is no conclusive counter-argument to them; indeed, proteins and their associated cellular machinery routinely assemble macroscopic artefacts, or, to use more biological terminology, they grow organisms . This submission confines itself to computational structures that will be relevant whenever some technology for sophisticated populations of nanites is achieved, even if not all that has been predicted.

In principle it is possible for nanites to assemble any physical artefact, by carefully controlled placement of every component atom (possibly requiring the use of much scaffolding). But in general this is infeasible: in the worst case it could need the global control and choreography of the behaviour of every individual nanite. A more feasible approach is to exploit mainly local cooperation between suitably-programmed neighbouring nanites, possibly mediated by their shared local environment (which also more closely mirrors the way biological organisms grow).

In order for nanotechnology to be possible, the initial nanites must be fabricated somehow. This complex engineering problem requires collaborative research by physicists, chemists, engineers, and biologists. To the extent that the nanites need to be programmed to perform their assembly tasks, computer science (CS) also has a crucial role in this interdisciplinary challenge. We need to develop capabilities to design, program and control complex networks of nanites, so that they safely and dependably build the desired artefacts, and so that they do not accidentally build un desired ones.

Initial CS research needs to focus on potential ways of designing and assembling artefacts in ways that can be described in terms of predominately local interactions, that is, in terms of the emergent properties of vast numbers of cooperating nanites . This requires analysis of emergent behaviour; given the orders of magnitude involved, this can be done only with a hierarchy of computational models, explaining the assembly at many different levels of abstraction.

2. Required CS advances

What CS theory and practice do we need in order to be able to design, program and control networks of nanites?

Emergent properties

We need a pragmatic theory of emergent properties .

In much the same way that an organism is an emergent property of its genes and proteins, the assembled artefact will be an emergent property of the assembling nanites and their programming. In general, this problem is computationally irreducible, that is, there is no "short cuts" to understanding or prediction, beyond watching the behaviour unfold. Thus reasoning about the precise behaviour of arbitrary networks with a number of nodes comparable to the number of cells in the human body (~10 13 ) is (currently) well beyond the state of the art. However, inability to solve the general problem, in principle or in practice, does not prevent exploration of large classes of specific and interesting problems (consider the theoretical impossibility of the general Halting Problem versus the many useful proofs of specific program termination, for example). So we merely need a sufficient theory, one that enables us to design nanites to build the many artefacts of interest, and to analyse them for safety and dependability. Certain classes of useful emergent properties may well be tractable to reasoning. For example, many organisms contain emergent hierarchical branching structures, such as arteries, lungs, nervous systems, and, of course, prototypical tree branches. Such emergent structures are particularly straightforward to "program", as evidenced by L-systems [PrL].

Growth and Development

We need a pragmatic theory of development and growth.

A population of nanites first "grows" a vastly larger population, then "grows" the artefact in question. Again, we need a sufficient theory of growth -- to enable us to reason about structures that are the result of a growth process.

Biological insights from embryology and development will be fruitful here, and the relevant ideas need to be abstracted and adapted for nanite assemblers. This "artificial development" also has its own properties: for example, the use of scaffolding will probably be much more important.

Research here will also feed back into biology. Which features of biological organisms are consequences of growth in general, and which are consequences of "wet-ware" growth, and so are different in assembled hardware? What constraints are there in the growth process: is it possible to "grow" a cooked steak ab initio , or must if first be grown raw (isolated, or as part of a cow), and then chemically modified?

Complex Networks

We need a pragmatic theory of dynamic, heterogeneous, unstructured, open networks [Ste].

Dynamic : the network is not in steady state or equilibrium, but is far from equilibrium, governed by attractors and trajectories. If the nanites were in equilibrium, they would not be assembling anything: biological systems in equilibrium are dead. Swarm networks may offer insights to this kind of dynamics [BDT])

Heterogeneous : the nodes (nanites), the connections (neighbourhoods), and the communications (messages) can be of many different types, including higher order types (for example, a message might communicate the instructions for a new kind of nanite to be assembled). If it were homo geneous, it would most likely form some unpleasant formless mass.

Unstructured : the network connectivity has no particular regularity: it is not fully regular, or fully connected, or even fully random. Clearly there need to be some kinds of regularity present, but these are likely to be of kinds that cannot be reasoned about in terms of simple averages or mean field notions; they are more likely have fractal structure. Some recent advances in Small World networks offer intriguing new insights [Bar][Wat].

Open (metadynamic) : the structures are unbounded, and the components are not fixed: nodes and connections may come and go; new kinds of nodes and connections may appear. Nodes and connections appear as nanites are born, change as nanites move around, and disappear as scaffolding is removed.

Much current mathematical network theory is restricted to static, homogeneous, structured, closed networks. We need to relax all of these assumptions. Progress and intuition in this highly complex area is likely to be inspired and driven by CS behavioural modelling, simulation, and patterns.

Complex Adaptive Systems

All the CS advances mentioned above would have application well beyond nanotechnology. All are basic requirements for the general area of Complex Adaptive Systems , of which nanotechnology is but one exemplar. Real world examples of CASs include swarms and flocks, ants, immune systems, brains, autocatalytic networks, life, ecologies, and so on. Artificial CASs include complex control systems (industrial plants, Air Traffic Control, etc), eCommerce supply chains and webs, telecoms systems and the Internet, and ubiquitous computing with its hordes of communicating smart devices, economic systems, and so on.

3. Behavioural Modelling

The pragmatic theories for Complex Adaptive Systems, above, must be developed in response to the challenge of nanotechnology, but they need not start from scratch. During the last two or three decades computer scientists have eroded the boundary between programming, which prescribes behaviour of a system, and modelling, which analyses it. This trend arises naturally from a change of emphasis, from stand-alone computers doing one thing at a time to distributed systems -- networks of devices each acting independently, with no centralised control. The resulting computational models are in varying degrees logical, algebraic, non-deterministic, stochastic. They have been effectively used to analyse programming languages and communication disciplines. They have also been applied to computer security, mobile phone systems, behaviour in ant colonies, business processes, and signal transduction in biological cells.

A large system such as the Internet can be modelled at many levels of abstraction , correlated where possible with the structure of the system. At the higher levels, the analysis of agents' behaviour need not depend on the underlying technology used to realise them. A natural research direction is therefore to extrapolate existing CS models to nanosystems where, despite orders of magnitude increase in population size (compared with, say, the Internet), many of the same general principles of emergence and behaviour may apply.

At the lowest levels of abstraction, which may be called embodiment , the analysis of agents' behaviour depends crucially the underlying technology used to realise them. For example, individual nanites are made of only small numbers of atoms, so a one-atom mutation to a nanite -- caused by faults in manufacture, by other nanites, by random impact of cosmic rays -- could have a dramatic effect on behaviour. In order to reason about the kinds of change that mutations might make (to reason about the "adjacent possible" [Kau] of the nanite), it is essential to know the detailed make-up and characteristics of the system undergoing mutation.

Close cooperation is therefore needed among many research disciplines, of which CS is one, in order to understand nanopopulations fully. From the CS viewpoint, the gain (as we said in the introduction) will be a greatly enriched and more general science of computation. We conclude this section by summarising some of the concepts, theories and tools that CS can bring to the cooperation at the outset. We cite only a selection from the large literature.

Stand-alone computation

Before distributed computing systems became the norm, much computing research laid foundations for the models and tools that those systems need. A start was made in establishing the verification of computer programs as an activity in formal logic [Flo][Hoa]. Tools for computer-assisted verification, especially for computer hardware designs [Gor], were pioneered. The status of computer programs as mathematical descriptions of behaviour was established [ScS]. Theories of types began to emerge as a powerful aid to behavioural analysis as well as to programming [Rey]. Even in the 1940s, von Neumann's model of self-reproducing cellular automata anticipated some of the central ideas of nanotechnology [voN].

Abstract machines and process calculi

The first model to capture the complex interplay between non-determinism and concurrency in distributed systems was Petri Nets [Pet]; these nets were designed for information flow in natural as well as man-made systems. In the early eighties, algebraic process calculi [BHR][Mil] were designed to model interactive systems hierarchically, and to model their behaviour abstractly. The Chemical Abstract Machine [BeB] captured the spatial structure of systems. The pi calculus [MPW] and mobile ambient calculus [CaG] made a further step in modelling systems that can reconfigure both their spatial arrangement and their connectivity.

These models have influenced the design of programming and specification languages, for example LOTOS, occam and Ada. They have been developed to model systems stochastically, and to deal with hybrid discrete/continuous systems. Recently their theory has been seen to extend to graphical models that are a priori suitable for populations of agents such as nanites.

Logics and Tools

Allied to algebraic calculi are new forms of mathematical logic, especially modal logics, specially designed to specify the properties that an interactive system should satisfy. Well-known examples are dynamic logic [Pra], temporal logic [Pnu], the temporal logic of actions [LeL] and the mu calculus [Koz]. These logics often have a close link with algebraic calculi; an algebraic term denotes (part of) a system. while a logical formula says (in part) how it should behave. This underlies a successfully applied incremental methodology for system analysis; one verifies more and more properties of more and more parts (even the whole) of a system. Such verification is aided by software tools: model-checkers that can automatically verify properties of fairly complex finite-state systems [CES]; and semi-automated tools that can perform verifications with human guidance [CPS].

4. Safety and dependability

Nanites can disassemble, as well as assemble, structures. This has led to the notion of the so-called "grey goo" problem: nightmare visions of hordes of rogue nanites disassembling the wrong things; disassembling people, or even disassembling the entire planet. It is potentially the ultimate terrorist weapon.

Even if nanites are not deliberately engineered to be destructive, such objects will naturally appear in any replicating swarm of nanites. We are dealing with such vast numbers of nanites that some will spontaneously mutate . Given the three features of reproduction, variation, and selection, some form of evolution will inevitably occur, leading to populations of adjacent possible undesigned nanites. Computer science, allied with biology, is crucial to the task of investigating and understanding these artificial evolutionary processes, and the defences we can design against them.

More generally, our discussion of behavioural modelling makes it clear that dependability -- the quality of a system that justifies its use even in critical conditions -- is already a topic of extensive research in computer science. It involves mathematical analysis, as in the case of program verification and computer security; more widely, it involves making systems aware of, and able to report upon, their behaviour. It cannot exist without good modelling. The modelling of nanopopulations with dependability in mind, given their emergent properties and the inevitability of mutation, offers a huge challenge to CS, which its researchers will welcome.

5. Conclusion

Nanotech assemblers offer the promise of fantastic rewards. Some forms of nano-assemblers may well be exploitable and exploited in many ways without much CS input. Before we can achieve the full promise, however, there are many hard computer science problems to solve, concerning the design of emergent properties, the growth of physical artefacts, the programming and control of nanites, and defences against the grey goo and other safety critical scenarios.


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