Ever since K. Eric Drexler published his 1987 nonfiction book Engine of Creation, science fiction has been alive with speculation about "nanotechnology"--the future construction and application of molecule-sized machinery capable of manipulating matter at the level of individual atoms. One of the ideas Drexler proposed was "smart materials" made of tiny, programmable machines rather than normal, inert molecules. To some extent, a laptop computer's liquid crystal display (LCD) meets this definition, but Drexler's proposal included future advances such as "paint" which could spread itself automatically, change color and texture on command, etc., and even "smart walls" in which the windows and doors could be moved around as easily as picture frames.

Still, science fiction often falls back on nanotechnology as an almost magical plot device, capable not only of changing texture or shape, but of building permanent structures from "seeds" and adding exponentially (or even instantly) to its own substance. While in principle this may be possible, the nanomachines which make up "smart matter" will need an energy source (probably specialized chemical fuels) and a source of raw materials (probably specialized chemical feedstocks), so as in biology, delivery and distribution of "nutrients" becomes a primary function of the system. Any other activity has to take a lower priority, fitting in where time and resources permit.

If nanotech applications were needed outside the laboratory, they'd have to resemble plants or molds, drawing energy and nutrients from the immediate environment (perhaps through elaborate systems of "leaves" or "roots") in order to feed themselves. In nature, these metabolic processes are slow, although bamboo and mushrooms can sometimes grow at naked-eye speeds, and a typical acre of forest or grassland does contain a great deal of harvestable chemical energy, as the rapid release of a brush or forest fire will demonstrate. But while it may be possible to build nanomachines with faster-than-natural metabolism, this heightens the danger of a runaway replication or "gray goo" or "bloom" event--the ultimate nanotech nightmare.

Utility fog may free us all

With such difficulties in mind, Dr. J. Storrs Hall of Rutgers University built on Drexler's concepts in the early 1990s by proposing a "utility fog" of 12-armed, dust-sized silicon micromachines ("foglets," as seen at right in an image courtesy of J. Storrs Hall) capable of joining hands in a variety of configurations to create a programmable substance which could assume any shape, and vary its density from that of vapor or cobwebs to something as solid as wood, plastic or even porous cement. This is not only the ultimate modeling clay and 3-D entertainment system (holodeck, anyone?), but potentially an important structural component, safety system and furnishing for homes, factories and vehicles.

Utility fog is a more achievable technology than Drexler's nanotech, since its key components are roughly a thousand times larger, and since primitive micro-electro-mechanical systems, or MEMS, can already be etched from silicon, including gears, pumps, electric motors and even complex devices like microscopic steam engines. Unfortunately, today's MEMS don't last long. They're made of silicon because the electronics industry has given us a number of advanced techniques for shaping that particular substance on the micro scale. But silicon is highly susceptible to an atomic friction force called "stiction," which tends to jam and erode very small moving parts. If this problem can be overcome, or alternative materials found, the technology to build a primitive foglet could be in our hands within a decade.

Still, the journey from foglet to utility fog could be a long one indeed. There'd be enormous challenges in communication, power distribution and especially the intelligent coordination of activity between thousands or millions of foglets. Not to mention getting their little arms to grab reliably, without breaking off.

Fortunately, silicon offers another exciting possibility in its role as a semiconductor. Most materials are either conductors, which permit the free flow of electrons, or insulators, which don't. Semiconductors are insulators which are capable of conducting electrons within a certain narrow energy band--a very useful property in the field of electronics.

Silicon's electrical properties are fixed by the laws of physics, but through "doping," the carefully controlled introduction of impurities, its crystals can be tuned so that, for example, room-temperature electrons have a good chance of jumping up into the conduction band when a voltage is applied. Silicon doped with electron donor atoms such as phosphorus becomes an "N" or negative-type semiconductor, through which electrons can travel more easily. Doping with electron borrowers like aluminum produces a "P" or positive material, which conducts "holes," or spaces where an electron isn't. A "P" layer adjacent to an "N" layer creates a P-N junction, a kind of electrical "valve" or "gate" which permits electrons to flow easily in one direction but not the other.

This effect is used in electronic components such as diodes, light-emitting diodes and transistors, but from a programmable matter standpoint the most interesting applications occur when an N layer is sandwiched between two Ps. This creates a kind of "trap" which attracts electrons into the middle layer and doesn't let them out. This is a useful trick all by itself, but if the P layer is thin enough--about 10 nanometers or 0.000001 millimeters or 50 atoms high--we enter the realm of quantum mechanics, and the electrons trapped in the P layer begin to do something strange and potentially wonderful: along the vertical dimension they start behaving as waves rather than particles.

Such devices, known as quantum wells, are easy and cheap to produce, and find practical use in computers, fiber-optic networks and those $10 keychain laser pointers everyone seems to have these days. But even that isn't the exciting part. A quantum well confines electrons in a two-dimensional layer, like the meat inside a sandwich. But if the meat and top bread layers are sliced away, leaving a narrow stripe of P-N sandwich on top of a sheet of P bread, the electrons take on wavelike behavior along an additional axis. This structure is called a "quantum wire." Finally, etching away the sides of the stripe to leave a tiny square of meat and bread resting on the lower slice, we can produce a "quantum dot" (as seen below) which confines the electrons in all three dimensions, forcing them to behave as standing waves, or probability density functions, or strangely shaped clouds of diffuse electric charge.

Quantum dots could create Camelot

There is one other place where electrons are known to behave this way: in atoms. Electrons which are part of an atom will arrange themselves into negatively charged "orbitals" which constrain and define their positions around the positively charged nucleus. These orbitals, and the electrons which partially or completely fill them, are what determine the chemical properties of an atom, i.e., what other sorts of atoms it can bind to, and how strongly.

The thing that's wonderful and important and shocking about a quantum dot is that the electrons trapped in it will arrange themselves just as though they were part of an atom, even though there's no atomic nucleus for them to surround. Which atom they emulate depends on the number of excess electrons trapped inside. What's more, the electrons in two adjacent quantum dots will interact just as they would in two real atoms placed at the same distance, meaning the two dots can share electrons between them, meaning they can form genuine chemical bonds.

Now we'll take it a step further: quantum dots needn't be formed by etching squares out of a quantum well. Since like charges repel one another (and opposites attract), electrons in the well can instead be confined by rings or squares of negatively charged material on the upper "P" layer, a kind of electrostatic fence or corral atop the quantum well sandwich. This is better because it lets us adjust the quantum dot's characteristics simply by varying the voltage on the fence. Thus, we can not only form "chemical" bonds between dots, but also turn the bonds on and off as electrons are pumped in and out.

That's virtual chemistry, baby! Programmed changes in shape and texture are all well and fine, but not really so different from what we can achieve manually, with a machine shop or even a simple potter's wheel. But here is something entirely new: a material capable of changing its very substance. And that's not all; quantum dots can also be made in various shapes, offering possibilities beyond the symmetries of normal atoms. And where cranky nuclear forces limit the number of electrons in a stable atom to 92, quantum dots can hold many times that number--electrons by the hundreds, forming gigantic new orbitals classical chemists could never have imagined.

In short, these controllable quantum dots--what Yale University's Mark Reed calls "designer atoms"--can not only mimic every atom on the periodic table, but can quite easily produce atom-like structures with properties that don't occur in nature. For example, with many more electrons to share than even the faithful carbon atom, such designer atoms may be capable of forming superstrong chemical bonds, to produce materials much tougher than diamond, the hardest natural substance. Other possibilities include dramatic changes in the reflection and absorption of light, and the conduction of electricity. (Just for starters, think of an indestructible, 100% efficient solar energy cell.)

So picture this: a lattice of crystalline silicon, superfine threads much thinner than a human hair crisscrossing to form a translucent structure like microscopic basket wicker. Except that with the application of electrical currents, the spaces between the threads can be filled with "atoms" of any desired species, producing a virtual substance with the mass of wickered silicon, but the chemical, physical and electrical properties of some new, hybrid material. And changeable at the flip of a bit, to a nearly infinite variety of other materials!

In contrast to the biology-like limitations of nanotechnology and the fragile complexity of utility fog, such a material (which I have elsewhere dubbed "wellstone") would be capable of dramatic and instantaneous changes more akin to magic than to technology as we've previously known it. So watch out: if this trick does turn out to be technologically feasible, then the high-tech, high-gloss future of Star Trek may slip from our fingers, to be supplanted by something more like the mists and dreams and enchantments of Camelot. Or even--God help us all--Olympus!

Wil McCarthy is a rocket guidance engineer, robot designer, science fiction author and occasional aquanaut. He has contributed to three interplanetary spacecraft, five communication and weather satellites, a line of landmine-clearing robots, and some other "really cool stuff" he can't tell us about. His short fiction has graced the pages of Analog, Asimov's, Science Fiction Age and other major publications, and his novel-length works include Aggressor Six, the New York Times notable Bloom, and The Collapsium.