A few days ago, a high-ranking official at the U.S. National Nanotechnology Initiative told me that statements against "nanobots" on their website had been intended to argue against three-nanometer devices that could build anything.
This is frustrating, because no one has proposed such devices.
A three-nanometer cube would contain a few thousand atoms. This is about the right size for a single component, such as a switch or gear. No one has suggested building an entire robot in such a tiny volume. Even ribosomes, the protein-constructing machinery of cells, are more like 30 nanometers. A mechanical molecular fabrication system might be closer to 100 or 200 nanometers. That's still small enough to be built molecule-by-molecule in a few seconds, but large enough to contain thousands or millions of components.
Nanosystems a few hundred nanometers in size are convenient for several other reasons. They are small enough to be built error-free, and remain error-free for months or years despite background radiation. They are large enough to be handled mechanically with high efficiency and speed. They are smaller than a human cell. They are large enough to contain a complete CPU or other useful package of equipment. So it seems likely that designs for molecular manufacturing products and nanofactories will be based on components of this size.
So much for size. Let's look at the other half of that strawman, the part about "could build anything." There has been a persistent idea that molecular manufacturing proposes, and depends on, devices that can build any desired molecule. In fact, such devices have never been proposed. The idea probably comes from a misinterpretation of a section heading in Eric Drexler's early book, Engines of Creation.
The section in question talked about designing and building a variety of special-purpose devices to build special 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.
Unfortunately, the section was titled "Universal Assemblers." This was misread as referring to a single "universal" assembler, rather than a collective capability of a large number of special-purpose machines. But there is not, and never was, any proposal for a single universal assembler. The phrase has always been plural.
The development of molecular manufacturing theory has in fact moved in the opposite direction. Instead of planning for systems that can do a very broad range of molecular fabrication, the latest designs aim to do just a few reactions. This will make it easier to develop the reactions and analyze the resulting structures.
Another persistent but incorrect idea that has attached itself to molecular manufacturing is the concept of "disassemblers." According to popular belief, tiny nanomachines will be able to take apart anything and turn it into raw materials. In fact, disassemblers, as described in Engines, have a far more mundane purpose: "Assemblers will help engineers synthesize things; their relatives, disassemblers, will help scientists and engineers analyze things." In other words, disassemblers are a research tool, not a source of feedstock.
Without universal assemblers and disassemblers, molecular manufacturing is actually pretty simple. Manufacturing systems built on a 100-nanometer scale would convert simple molecular feedstock into machine parts with fairly simple molecular structure—but, just as simple bricks can be used to build a wide variety of buildings, the simple molecular structure could serve as a backbone for rather intricate shapes. The manufacturing systems as well as their products would be built out of modules a few hundred nanometers in size. These modules would be fastened together to make large systems.
As I explained in my recent 50-page paper, "Molecular Manufacturing: What, Why, and How," recent advances in theory have shown that a planar layout for a nanofactory system can be scaled to any size, producing about a kilogram per square meter per hour. Since the factory would weigh about a kilogram per square meter, and could build a larger factory by extruding it edgewise, manufacturing capacity can be doubled and redoubled as often as desired.
The implications of non-scarce and portable manufacturing capacity, as well as the high performance, rapid fabrication, and low cost of the products, are another topic altogether. In fact, studying and preparing for these implications is the reason that CRN exists.