MODES OF OPERATION:
The Nano-constructs operate in two modes - "native", and "fog". In "native" mode, individual foglets move into different positions and perform certain mechanical operations depending on what object it is forming. For example, if it forms part of a table, then it would be motionless and locked. If the object was a fan, then most of the structure would remain locked, and only the foglets between the two parts would need to move. With a suit made of Fog, you might wrestle alligators, cheating a little by having the suit amplify your movements as it protects you from the alligator's teeth.
In "fog" mode, the foglets do not move, but act more like pixels on a television screen - they "pixelate". The foglets vary other properties according to which part of the object they are representing, generally transmitting information and sound. A Fog-filled room would contain 90% air, and surround its occupant with a display screen with 100 micron resolution. Meanwhile, each litre of foglets behind the display would contain about a billion times the processing power of a 286 PC, making possible some pretty impressive virtual reality simulations.
The Utility Fog which is simulating air needs to be impalpable. One would like to be able to walk through a Fog-filled room without the feeling of having been cast into a block of solid Lucite. It is also desire-able to be able to breathe while using the Fog in this way! To this end, the robots representing empty space constantly run a fluid-flow simulation of what the air would be doing if the robots weren't there. Then each robot does what the air it displaces would do in its absence.
How can one breathe when the air is a solid mass of machines? Actually, it isn't really solid: the Foglets only occupy about 10% of the actual volume of the air (they need lots of "elbow room" to move around easily). There's plenty of air left to breathe. As far as physically breathing it, we set up a pressure-sensitive boundary, which translates air motions on one side to Fog motions on the other. It might even be possible to have the Fog continue the air simulation all the way into the lungs.
To understand why we want to fill the air with microscopic robots only to go to so much trouble to make it seem as if they weren't there, consider the advantages of a TV or computer screen over an ordinary picture. Objects on the screen can appear and disappear
at will; they are not constrained by the laws of physics. The whole scene can shift instantly from one apparent locale to another. Completely imaginary constructions, not possible to build in physical reality, could be commonplace. Virtually anything imaginable could be given tangible reality in a Utility Fog environment.
Why not, instead, build a virtual reality machine that produces a purely sensory version of the same apparent world? The Fog acts as a continuous bridge between actual physical reality and virtual reality. The Fog is universal effector as well as a universal sensor. Any (real) object in the Fog environment can be manipulated with an extremely wide array of patterns of pressure, force, and supported, measured, analyzed, weighed, cut, reassembled, or reduced to bacteria-sized pieces and sorted for recycling.
Utility Fog can act as a transparent interface between "cyberspace" and physical reality.
Most currently proposed nanotechnological designs are based on carbon. Carbon is a marvelous atom for structural purposes, forming a crystal (diamond) which is very stiff and strong. However, a Fog built of diamond would have a problem which nanomechanical designs of a more conventional form do not pose: the Fog has so much surface area exposed to the air that if it were largely diamond, especially on the surface, it would amount to a "fuel-air explosive".
Therefore the Foglet is designed so that its structural elements, forming the major component of its mass, are made of aluminum oxide, a refractory compound using common elements. The structural elements form an exoskeleton, which besides being a good mechanical design allows us to have an evacuated interior in which more sensitive nanomechanical components can operate. Of course, any macroscopic ignition source would vaporize the entire Foglet; but as long as more energy is used vaporizing the exoskeleton than is gained burning the carbon-based components inside, the reaction cannot spread.
Each Foglet has twelve arms, arranged as the faces of a dodecahedron. The arms telescope rather than having joints. The arms swivel on a universal joint at the base, and the gripper at the end cart rotate about the arm's axis. Each arm thus has four degrees of freedom, plus opening and closing the gripper. The only load-carrying motor on each axis is the extension/retraction motor. The swivel and rotate axes are weakly driven, able to position the arm in free air hut not drive any kind of load; however, there are load-holding brakes on these axes.
The gripper is a hexagonal structure with three fingers, mounted on alternating faces of the hexagon. Two Foglets "grasp hands" in an interleaved six-finger grip. Since the fingers are designed to match the end of the other arm, this provides a relatively rigid connection; forces are only transmitted axially through the grip.
When at rest, the Foglets form a regular lattice structure. If the bodies of the Foglets are thought of as atoms, it is a "face-centered cubic" crystal formation, where each atom touches 12 other atoms. Consider the arms of the Foglets as the girders of the trusswork of a bridge: they form the configuration known as the "octet truss" invented by Buckminster Fuller in 1956. The spaces bounded by the arms form alternate tetrahedrons and octahedrons, both of which are rigid shapes.
The Fog may he thought of as consisting of layers of Foglets. Tire layers, and the shear planes they define, lie at 4 major angles (corresponding to tire faces of the tetrahredrons and octahedrons) and 3 minor ones (corresponding to tire face-centered cube faces). In each of the 4 major orientations, each Foglet uses six arms to hold its neighbors in the layer; layers are thus a 2-dimensionally rigid fabric of equilateral triangles. In face-centered mode, the layers work out to he square grids, and are thins not rigid, a slight disadvantage. Most Fog motion is organized in
layers; layers slide by passing each other down hand-over-hand in bucket brigade fashion. At any instant, roughly half the arms will lie lurked between layers when they are in motion.
The Fog moves an object by setting up a seed-shaped zone around it. The Foglets in the zone move with the object, forming a fairing which makes the motions around it smoother. If the object is moving fast, the Fog around its path will compress to let it go by. The air does not have time to move in the Fog matrix and so the motion is fairly efficient. For slower motions, efficiency is not so important, but if we wish to prevent slow-moving high-pressure areas from interfering with other airflow operations, we can enclose the object's zone in a self-contained convection cell which moves Foglets from in front to behind it.
Each moving layer of robots is similarly passing the next layer along, So each layer adds another increment of the velocity difference of adjacent layers. Motors for arm extension can run at a gigahertz, and be geared down by a factor of 100 to the main screw in the arm. This will have a pitch of about a micron, giving a linear extension/retraction rate of about 10 meters per second. We can estimate the inter-layer shear rate at this velocity; the foglets are essentially pulling them selves along. Thus for a 100-micron interlayer distance Fog can sustain a 100 meter-per-second shear per millimeter of thickness.
The atomically-precise crystals of the Foglets' structural members will have a tensile strength of at least 100,000 psi (i.e. high for steel but low for the materials, including some fairly refractory ceramics, used in modern "high-tech" composites). At arms length of 100 microns, the Fog will occupy 10% of the volume of the air but has structural efficiency of only about 1% in any given direction.
Thus Utility Fog as a bulk material will have a density (specific gravity) of 0.2; for comparison, balsa wood is about 0.15 and cork is about 0.25. Fog will have a tensile strength of only 1000 psi; this is about the same as low-density polyethylene (solid, not foam). The material properties arising from the lattice structure are more or less isotropic; the one exception is that when Fog is flowing, tensile strength perpendicular to the shear plane is cut roughly in half.
Without altering the lattice connectivity, Fog can contract by up to about 40% in any linear dimension, reducing its overall volume (and increasing its density) by a factor of five. (This is of course done by retracting all arms but not letting go.) In this state the fog has the density of water. An even denser state can be attained by forming two interpenetrating lattices and retracting; at this point its density and strength would both be similar to ivory or Corian structural plastic, at specific gravity of 2 and about 6000 psi. Such high density Fog would have time useful property of being waterproof (which ordinary Fog is not), but it cannot flow and takes much longer to change configuration. Selective application of this technique allows Fog to simulate shapes and flow fields to a precision considerably greater than 100 microns.
An appropriate mass of Utility Fog can be programmed to simulate most of the physical properties of any macroscopic object (including air and water), to roughly the same precision those properties are measured by human senses. The major exceptions are taste, smell, and transparency. The latter can be overcome with holographic "eyephones" if a person is to be completely embedded in Fog.
GENERAL PROPERTIES AND USES
As well as forming an extension of the senses and muscles of individual people, the Fog can act as a generalized infrastructure for society at large. Fog City need have no permanent buildings of concrete, no roads of asphalt, no cars, trucks, or busses. It can look like a park, or a forest, or if the population is sufficiently whimsical, ancient Rome one day and Emerald City the next.
It will be more efficient to build dedicated machines for long distance energy and information propagation, and physical transport. For local use, and interface to the worldwide networks, the Fog is ideal for all of these functions. It can act as shelter, clothing, telephone, computer, and automobile. It will be almost any common household object, appearing from nowhere when needed (and disappearing afterwards). It gains a certain efficiency from this extreme of polymorphism; consider the number of hardcopy photographs necessary to store all the images one sees on a television or computer screen. With Utility Fog we can have one "display" and keep all our physical possessions on disk.
Another item of infrastructure that will become increasingly important in the future is information processing. Nanotechnology will allow us to build some really monster computers. Although each Foglet will possess a comparatively small processor—which is to say the power of a current-day supercomputer—there are about 16 million Foglets to a cubic inch. When those Foglets are not doing anything else, i.e. when they are simulating the interior of a solid object or air that nothing is passing through at the moment, they can be used as a computing resource (with the caveats below).
LIMITATIONS OF UTILITY FOG CAPABILITY
When discussing something as far outside of everyday experience as the Utility Fog, it is a good idea to delineate both sides of the boundary. The Fog is capable of so many literally amazing things, we will point out a few of the things it isn't capable of: Anything requiring hard metal (cold steel). For example, Fog couldn't simulate a drill bit cutting through hardwood. It would be able to cut the hole, but the process would be better described as intelligent sandpaper.
• Anything requiring both high strength and low volume. A parachute could not be made of Fog (unless, of course, all the air were filled with Fog, in which case one could simply fly).
• Anything requiring high heat. A Fog fire blazing merrily away on Fog logs in a fireplace would feel warm on the skin a few feet away; it would feel the same to a hand inserted into the "flame".
• Anything requiring molecular manipulation or chemical transformation. Foglets are simply on the wrong scale to play with atoms. In particular, they cannot reproduce themselves. On the other hand, they can do things like prepare food the same way a hit-man cook does—by mixing, stirring, and using special-purpose devices that were designed for theta to use.
• Fog cannot simulate food, or anything else that is destined to be broken down chemically. Eating it would be like eating the same amount of sand or sawdust.
• Fog can simulate air to the touch hut not to the eyes. The best indications are that it would look like heavy fog. Thus the Fog would need to support a pair of holographic goggles in front of the eyes of an embedded user. Such goggles are clearly within the capabilities of the same level of nanotechnology as is needed for the Fog, but are beyond the scope of this paper.
OTHER DESIRABLE LIMITATIONS
In 1611, William Shakespeare wrote his final play, "The Tempest." 445 years later, an obscure science fiction writer named W. J. Stuart updated the Tempest's plot into a story called "Forbidden Planet," and created a modern myth.
Forbidden Planet, more precisely the movie version, has become the classic cautionary tale for arty scenario in which people become too powerful and control their environment too easily. In the story, the Krell are an ancient, wise, and highly advanced civilization. They perfect an enormous and powerful machine, capable of projecting objects and forces anywhere in any form, upon the mental commands of any Krell. The machine works "not wisely but too well," manifesting all the deeply buried subconscious desires of the Krell to destroy each other.
Utility Fog will provide humans with powers that, approximate those of the fictional Krell machine. Luckily, we have centuries of literary tradition to guide us around the pitfalls of hubris made reality. We must study this tradition, or we may be doomed to repeat if a truth that is by no means limited to the Utility Fog, or indeed to nanotechnology in general.
The first thing we can do is to require fully conscious, unequivocal commands for the Fog to take any action. Beyond that, we can try to suggest some of the protocols that may be useful in managing the Fog in a situation where humans are interacting in close physical proximity. Even if we have solved the problem of translating one's individual wishes, however expressed, into the quadrillions of sets of instructions to individual Foglets to accomplish what one desired, the problem of who gets to control which Foglets is probably a much more contentious one.
We can physicalize the psychological concept of "personal space". The Foglets within some distance of each person would be under that person's exclusive control; personal spaces could not merge except by mutual consent. This single protocol could prevent most crimes of violence in our hypothetical Fog City.
A corollary point is that physically perpetrated theft would be impossible in a Fog world. It would still be possible by informational means, i.e. fraud, hacking, etc; but the Fog could be programmed to put ownership on the level of a physical law. Not that it really makes any sense to think of stealing a fog-mode object, anyway. Ownership and control of the Fog need not be any more complex than the bundles of rights currently associated with everything from land to corporate stock.