The concept of assemblers was first introduced by K. Eric Drexler to describe nanocontruction machines. These machines could be instructed build structures atom-by-atom or molecule-by-molecule. While assemblers have been widely discussed, nobody has actually made one and not everyone agrees that it would be possible to build an assembler. Drexler thought that the first generation of assemblers would resemble enzymes. Enzymes are catalysts that are often made from proteins. He believes that in the future, it will be possible to make universal assemblers. These are assemblers that can be programmed to construct any stable atomic structure. Here is an excerpt from Drexler's book, Engines of Creation.
K. Eric Drexler
Engines of Creation.
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Universal assemblers
These second-generation nanomachines - built of more than just proteins - will do all that proteins can do, and more. In particular, some will serve as improved devices for assembling molecular structures. Able to tolerate acid or vacuum, freezing or baking, depending on design, enzyme-like second-generation machines will be able to use as "tools" almost any of the reactive molecules used by chemists - but they will wield them with the precision of programmed machines. They will be able to bond atoms together in virtually any stable pattern, adding a few at a time to the surface of a workpiece until a complex structure is complete. Think of such nanomachines as assemblers.
Because assemblers will let us place atoms in almost any reasonable arrangement (as discussed in the Notes), they will let us build almost anything that the laws of nature allow to exist. In particular, they will let us build almost anything we can design - including more assemblers. The consequences of this will be profound, because our crude tools have let us explore only a small part of the range of possibilities that natural law permits. Assemblers will open a world of new technologies.
Advances in the technologies of medicine, space, computation, and production - and warfare - all depend on our ability to arrange atoms. With assemblers, we will be able to remake our world or destroy it. So at this point it seems wise to step back and look at the prospect as clearly as we can, so we can be sure that assemblers and nanotechnology are not a mere futurological mirage.
MECHANOSYNTHETIC REACTIONS As conceived by Drexler, to deposit carbon, a device moves a vinylidenecarbene along a barrier-free path to insert into the strained alkene, twists 90o to break a pi bond, and then pulls to cleave the remaining sigma bond. Source: K. ERIC DREXLER
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Building devices atom by atom like this is called molecular nanotechnology. This approach is championed by the Foresight Institute and the Institute for Molecular Manufacturing. It seems like an appealing idea but most mainstream scientists do not support it. Of the billions of dollars that governments are now investing in nanotechnology research, very little funding is going to molecular nanotechnology. Most scientists have not been pursuaded that it will be possible to make assemblers as Drexler envisions them. Here is what George Whitesides had so say about assemblers.
George Whitesides
The Once and Future Nanomachine, Scientific American, 2001.
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Nanomachines
The assembler, with its pick-and-place pincers, eliminates the many difficulties of fabricating nanomachines and of self-replication by ignoring them: positing a machine that can make any composition and any structure by simply placing atoms one at a time dismisses the most vexing aspects of fabrication. The assembler seems, however, from the vantage of a chemist, to be unworkable. Consider just two of the constraints.
First is the pincers, or jaws, of the assembler. If they are to pick up atoms with any dexterity, they should be smaller than the atoms. But the jaws must be built of atoms and are thus larger than the atom they must pick and place. (Imagine trying to build a fine watch with your fingers, unaided by tools.) Second is the nature of atoms. Atoms, especially carbon atoms, bond strongly to their neighbors. Substantial energy would be needed to pull an atom from its place (a problem for the energy supply) and substantial energy released when it is put in place (a problem of cooling). More important, a carbon atom forms bonds with almost everything. It is difficult to imagine how the jaws of the assembler would be built so that, in pulling the atoms away from their starting material, they would not stick. (Imagine trying to build your watch with parts salvaged from another watch in which all the parts were coated with a particularly sticky glue: if you could separate the pieces at all, they would stick to your fingers.)
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The dream of the assembler holds seductive charm in that it appears to circumvent these myriad difficulties. This charm is illusory: it is more appealing as metaphor than as reality, and less the solution of a problem than the hope for a miracle. Considering the many constraints on the construction and operation of nanomachines, it seems that new systems for building them might ultimately look much like the ancient systems of biology.
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One facet of the ongoing debate about assemblers is the possiblity that assemblers could potentially cause big problems. Drexler was one of the first people to point out the dangers of self-reproducing nanomachines. Here is another excerpt from Engines of Creation.
K. Eric Drexler
Engines of creation.
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Gray Goo
The early transistorized computers soon beat the most advanced vacuum-tube computers because they were based on superior devices. For the same reason, early assembler-based replicators could beat the most advanced modern organisms. "Plants" with "leaves" no more efficient than today's solar cells could out-compete real plants, crowding the biosphere with an inedible foliage. Tough, omnivorous "bacteria" could out-compete real bacteria: they could spread like blowing pollen, replicate swiftly, and reduce the biosphere to dust in a matter of days. Dangerous replicators could easily be too tough, small, and rapidly spreading to stop - at least if we made no preparation. We have trouble enough controlling viruses and fruit flies.
Among the cognoscenti of nanotechnology, this threat has become known as the "gray goo problem." Though masses of uncontrolled replicators need not be gray or gooey, the term "gray goo" emphasizes that replicators able to obliterate life might be less inspiring than a single species of crabgrass. They might be "superior" in an evolutionary sense, but this need not make them valuable. We have evolved to love a world rich in living things, ideas, and diversity, so there is no reason to value gray goo merely because it could spread. Indeed, if we prevent it we will thereby prove our evolutionary superiority.
The gray goo threat makes one thing perfectly clear: we cannot afford certain kinds of accidents with replicating assemblers.
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The press really picked up on this idea. Runaway nanorobots threaten to take over the earth! A huge debate erupted over the dangers of nanotechnology. Some of the discussion can be found in Nanotech Now's Goo and Paste Glossary and the Foresight Guidelines on Molecular Nanotechnology. Governments set up expert panels to try to determine if the public was really in danger. Perhaps the most famous debate was between K. Eric Drexler and Richard Smalley, a Nobel Prize recipient in Chemistry. Much of this debate can be found in the articles: Machine-Phase Nanotechnology by Drexler,
Of Chemistry, Love and Nanobots by Smalley, and Chemical & Engineering News report on the Smalley/Drexler debate. At the end of the debate, Smalley had this to say about gray goo.
In 2004, K. Eric Drexler modified his view on the dangers of self-reproducing nanorobots. While still supporting the idea of assemblers, he does not emphasize their potential dangers so much.
K. Eric Drexler
Safe exponential manufacturing
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Safe exponential manufacturing
Early proposals for artificial nanomachinery focused on small
self-replicating machines, discussing their potential productivity and their
potential destructiveness if abused. In the light of controversy regarding
scenarios based on runaway replication (so-called 'grey goo'), a review of
current thinking regarding nanotechnology-based manufacturing is in order.
Nanotechnology-based fabrication can be thoroughly non-biological and
inherently safe: such systems need have no ability to move about, use
natural resources, or undergo incremental mutation. Moreover,
self-replication is unnecessary: the development and use of highly
productive systems of nanomachinery (nanofactories) need not involve the
construction of autonomous self-replicating nanomachines.
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It seems that we are in no immediate danger from rampaging nanorobots. In fact, the fear was probably misplaced from the start. Simple molecules can form just as great a danger as self reproducing nanorobots. This can best be explained by considering how life arose on earth.
Long ago, the earth was covered with lifeless seas. All the molecules in these seas eventually fell apart due to the random collisions they had with other molecules. Molecules also had a rate of formation. Through the random collisions of their constituents, molecules were formed. The concentration of a particular molecule in the primordial seas depended on the relative abundance of the constituents, the rate of formation, and the rate of decay. Occasionally, a new molecule was introduced in the sea by a lighting strike or other unlikely event. One extra molecule in the ocean could hardly make a difference and this molecule eventually fell apart anyway. Sometimes a catalyst was be produced by an unlikely event. This catalyst increased the rate of formation of the molecule it catalyzed. The concentration of the molecule that was catalyzed then increased. However, eventually the catalyst fell apart as well and the concentrations returned to their original levels. This state of chemical equilibrium continued until, by some unlikely event, an autocatalyst appeared. An autocatalyst catalyzes itself. The appearance of one autocatalytic molecule irreversibly changed the concentration of chemicals in the ocean.
Researchers studying autocatalysis have noted that often autocatalysis involves more than one molecule in an autocatalytic cycle. In such a cycle, molecule A catalyzes molecule B while molecule B catalyzes molecule A. Sometimes many molecules are involved in an autocatalytic cycle. Life itself can be seen as a complex autocatalytic cycle. Evolution is inevitable in a system complex enough to support autocatalytic cycles.
If a new autocatalytic cycle were to appear on earth, the environment could be drastically effected and many forms of life could be endangered. This could happen spontaneously as it did when life first arose on earth or the new autocatalytic cycle could be the result of human exploration with autocatalysts.
An autocatalytic cycle of simple molecules could also cover the earth with a sort of goo just as self-reproducing assemblers could. These molecules would presumably be much simpler to synthesize than an assembler that could be programmed to build any structure imaginable. The more immediate danger lies then not with assemblers but with simple chemical systems.
Assembly by STM
Drexler's assemblers do not exist yet, however, we are able to build structures atom-by-atom. It is possible to use a STM to build certain structures on certain surfaces with atomic precision. One of the leading groups in this field is Don Eigler's group at IBM. They have made a variety of structures by manipulating atoms on surfaces with a low temperature STM. The IBM website http://www.almaden.ibm.com/vis/stm/ contains many beautiful examples of atomic-scale structures made using an STM.
This figure shows different stages in the assembly of a quantum corral. In this case, 48 iron atoms where arranged in a ring on an atomically-smooth copper surface. Reference: M.F. Crommie, C.P. Lutz, D.M. Eigler. Confinement of electrons to quantum corrals on a metal surface. Science 262, 218-220 (1993).
Atomic assembly using an STM is a powerful technique but it is intrinsically slow. It typically takes on the order of one minute to put an atom at a particular site. At this rate it would take about 1017 years to create a gram of material. The prospects of reducing this time to less than the age of the universe are slim. STM assembly is an important technique for fundamental studies but it is not suitable for large-scale manufacturing.
Assemblers in biological systems
It is always useful to look to biology for examples of nanomachines. Probably the closest thing to an assembler in biology is a ribosome or RNA polymerase. RNA polymerase is a motor that moves along DNA and constructs a strand of RNA that holds a copy of the information in the DNA. A ribosome is a structure 20 - 30 nm in diameter that holds onto a single strand of RNA and uses the information in the RNA to construct proteins.
| Protein Data Bank.
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The RNA Factory
RNA polymerase is a huge factory with many moving parts. The one shown here, from PDB entry 1i6h, is from yeast cells. It is composed of a dozen different proteins. Together, they form a machine that surrounds DNA strands, unwinds them, and builds an RNA strand based on the information held inside the DNA. Once the enzyme gets started, RNA polymerase marches confidently along the DNA copying RNA strands thousands of nucleotides long.
As you might expect, RNA polymerase needs to be accurate in its copying of genetic information. To improve its accuracy, it performs a simple proofreading step as it builds an RNA strand. The active site is designed to be able to remove nucleotides as well as add them to the growing strand. The enzyme tends to hover around mismatched nucleotides longer than properly added ones, giving the enzyme time to remove them. This process is somewhat wasteful, since proper nucleotides are also occasionally removed, but this is a small price to pay for creating better RNA transcripts. Overall, RNA polymerase makes an error about once in 10,000 nucleotides added, or about once per RNA strand created.
RNA polymerase from a yeast cell. This molecular motor reads the information in DNA and builds a strand of RNA.
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A ribosome holding a strand of RNA and using it to synthesize a protein. Reference: http://www.hybridmedicalanimation.com/.
Biological nanomachines are complicated three dimensional structures. Below George Whitesides explains how the instructions contained in DNA are transformed into a three-dimensional structure.
George Whitesides
The once and future nanomachine
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Assemblers in cells
The strategy adopted by the cell to make its parts--and thus to replicate and maintain itself--is based on two ideas. The first is to use a single, conceptually straightforward chemical process--polymerization--to create large, linear molecules. The second is to build molecules that spontaneously fold themselves into functional, three-dimensional structures. This two-part strategy does not require a difficult and sophisticated three-dimensional pick-and-place fabrication: it simply strings beads (for example, amino acids) together into a necklace (a polypeptide) and lets the necklace self-assemble into a machine (a protein). Thus, the information for the final, functional, three-dimensional structure is coded in the sequence of the beads. The three most important classes of molecules in the cell--DNA, RNA and proteins--are all made by this strategy; the proteins then make the other molecules in the cell. In many instances proteins also spontaneously associate with other molecules--proteins, nucleic acids, small molecules--to form larger functional structures. As a strategy for building complex, three-dimensional structures, this method of linear synthesis followed by various levels of molecular self-assembly is probably unbeatable for its efficiency.
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Some general considerations about assemblers
It is not immediately evident what will limit the speed of an assembler. Various candidates for the rate-limiting step are the supply of material, the supply of energy, the supply of information, the removal of the dissipated heat, and the removal of waste products. In principle, all possible rate limiting steps should be investigated, however, I will limit myself to the one that seems the most likely to limit the rate and that is the removal of waste heat. This calculation provides an upper limit on the speed.
Let's suppose we are going to use an assembler to construct something using carbon atoms because nanotechnologists like to talk about building diamond structures. The number of carbon atoms in a gram is 0.001/(12×1.66-27) = 5×1022. A gram of material is about a cubic centimeter in size.
Every time an atom is placed, the assembler will have to perform some work. The amount of energy needed to break or form a chemical bond is about 1 eV ~ 10-19 Joules. This energy would be dissipated near the part of the assembler that places the atom. This region cannot be much bigger than an atom so let's say the energy is dissipated in a volume of 1 nm3. A high heat dissipation is about 10 kW/cm3 = 10-17 W/nm3 . This corresponds to 100 × 100 W light bulbs packed into the space of 1 cm3. About 100 atoms per second could be placed by the assembler at this power dissipation rate. This means it would take about 5×1020seconds = 1013 years for one assembler to create a structure with a mass of one gram. Working at 100 atoms/second, an assembler could place 36000 atoms in an hour. That corresponds to a structure about 10 nm on a side. Clearly it is not possible to build something as large as a gram with a single assembler.
We can perform a reality check by comparing this estimate to a biological motor. The motors used in muscles are called myosin. They each deliver a power of about 10-18 W in a volume of about a cubic nanometer. The dissipated power density is similar to the estimation used above. It is unlikely that an assembler that we could design would be much more efficient than this.
In his book, Engines of Creation, K. Eric Drexler estimates that an assembler could place one million atoms per second. Even at this rate it is clear that a single assembler will not be able to produce very much material. What is needed is more assemblers in parallel. They all have to be provided with building material, energy, and instructions. They will all produce waste heat and waste materials. Chemical and or biological systems are a better model of an army of assemblers fabricating something than a single robotic arm building a structure atom-by-atom.
Problems
1. If a procedure were known to make a autocatalytic cycle that might cover the earth with a goo, would someone fabricate the necessary chemicals and introduce them into the environment? In what way is this be different from introducing a computer virus?
2. Estimate how long it would take for a single universal assembler to make a structure that weighs one gram. Estimate the maximum size of a structure that a universal assembler could create in one hour.
3. Many assemblers would have to work in parallel in order to fabricate a significant amount of any product. How could they be given information? How could the building materials be supplied? How could the energy be supplied? How could the waste products be transported away? What would the rate limiting step be?
4. In Star Trek, a device call a replicator was used to create near-perfect
reproductions of small objects by using a molecular pattern stored in computer memory. Most food service on starships is provided by food replicators located throughout the ship. Could this work? Could you use it to produce fuel?
The science of Star Trek.