Ralph C. Merkle
3333 Coyote Hill Road
Palo Alto, CA 94304
This is one of a series of Predictive Papers published in the Proceedings of the IEEE.
This paper is available on the web at http://www.zyvex.com/nanotech/IEEEpredictivePaper.html
Watkins' 1962 forecast saw two major developments: the ability to "grow" complex circuits and "construct highly complex three-dimensional circuits entirely within a single crystal of solid."
Rather than add to this plethora of predictions about what we'll be able to build, we'll instead hazard a few guesses about how we'll build them. The forecasts of Watkins and the others can only happen if we develop new and vastly superior ways of making things. What new principles, what new ideas will lay the foundations for this technological revolution?
Watkins' "single crystal of solid" must have the "defects, impurities, vacancies, interstitials, dislocations, precipitates, grain boundaries, etc." in just the right places: the places required for its function. While we want to build it with the same precision that a chemist brings to the synthesis of a molecule, yet it will require something very different from conventional chemistry. Today's SPMs have demonstrated the basic principle, that we can indeed (as Feynman put it) "...arrange the atoms the way we want; the very atoms, all the way down!"
Two things will save us: shrinking the Eiffel tower and making many of them. With smaller size comes faster operation and the possibility of massive parallelism. Very many very small SPM-like devices, rank upon rank of them, each orders of magnitude faster than any of today's SPMs, would give us a combined throughput equal to the task of making tomorrow's computers. Whether we use them to read and write molecular marks on a surface, thereby giving us the ability to make and read billions upon billions of marks per second, or whether we use them to make more complex devices and indeed whole arrays of computers, the principle remains the same. If one is too slow, use two. If two aren't enough, use four. If four aren't enough, use eight....
Have we, though, merely solved one problem by creating another? We wanted to make untold billions of switches, and solved our problem by asking for untold billions of miniature SPM-like devices. But how do we get untold billions of them? Lithography might let us make thousands or perhaps even millions, but untold billions? How can we possibly make so many, and make them cheaply and precisely to boot?
This is not the first time we have been inspired by nature. Birds flying through the air demonstrated that heavier-than-air flight was possible. But when we actually made our own flying machines they were very different from birds. A 747 has no feathers, nor can it perch upon a branch nor flap its wings. Birds do not use jet engines, nor do they have a metal skin. To build airplanes we had to do more than copy the biological world, we had to develop new principles and new approaches more suited to our human capabilities and limitations.
In the same way, biological self replicating systems are both more and less than we want. We want to make, not squishy soft watery cells, but hard crystals with precise impurities and imperfections. We aren't seeking merely to copy nature's example, but to design something new: an artifical self replicating device able to make complex circuits within "...a single crystal of solid." Is this possible? Can we do it?
The idea of a very small computer is one that we are already comfortable with. To this we must add a very small descendant of the SPM to act as the "constructor." Given a molecular constructor able to arrange and rearrange molecular structure in some programmable way, the difficulties of self replication become more pragmatic than conceptual. What does such a system look like? How will thermal noise influence our design? What sort of framework must exist around the constructor if it is to function correctly? People have skin, bacteria have bacterial walls, what protects the computer and constructor from a less than perfect environment? How will it bring raw materials through this "skin"? What sort of "molecular tools" will it use? These and other questions have already been the subject of many papers (see http://www.zyvex.com/nano for further information). They pose no fundamental barriers but instead invite us to create, not just one answer, but whole classes of answers: they invite us into whole new vistas of research.
Watkins foresaw both engineers who would "grow" their computers and also computers made with amazing precision within a three-dimensional crystal of solid. He didn't foresee the merging of these two forecasts. The idea of a self replicating device able to arrange "the very atoms" was first advanced by Eric Drexler, who coined the term "assembler" to describe it. As we are forced to move beyond conventional lithography to some new and remarkably precise, remarkably flexible and remarkably low cost manufacturing technology, what could be more natural than to use self replication to make the many billions of assemblers with which to make the mole quantities of logic gates that future computers will require?
The only self replicating systems we are familiar with are living, and we unconsciously assume that artificial self replicating systems will be similar. But the machines people make bear little resemblance to living systems. The image of a 747 going feral, swooping out of the sky to clutch an unsuspecting horse in its landing gear, seems incongruous. Machines lack the wonderful adaptability of living systems. A 747 requires Jet A, a refined source of energy that is delivered to it by an elaborate system that includes oil fields, pumps, tankers, refineries, fuel lines, and trucks. It can convert this artificially refined fuel into energy using engines that can run on little else. Cut off from refined fuel, airstrips, maintenance crews, spare parts, navigational systems and all the other paraphenalia that keeps it flying and a 747 is just a large piece of scrap metal. A bird, in contrast, can live on berries, seeds, worms, insects, small rodents, fish and bits of bread tossed to it by amused tourists. Its living and remarkably adaptable digestive system can convert all these and more into energy and essential raw materials for power and self repair. It thrives in the complex and ever changing natural world.
It will be challenge enough to design an artificial self replicating system able to function in a controlled artificial environment using a single specific source of highly refined energy, let alone in the wild disarray of the natural world where it would have to adapt to whatever came its way. The fear that we will accidentally destroy the planet seems remote indeed. Yet there is cause for concern. We have a long and bloody history of settling our differences by war: new tools and new technologies will let us make new weapons, giving us new opportunities to express the less civilized aspects of our nature. Peace and global security might happen by accident, but prudence dictates that we take a more active role in preparing for our future.
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