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Wednesday, May 30th, 2007

Time Event
Feeds, Seeds, and Gray Goo
A couple of the books I've read recently illustrated the powers and dangers of nanotechnology. One of the disputes in the field is whether molecular manufacturing can provide exponential production capabilities. MM would let us create a "nanofactory", a machine which builds things atom by atom, capable of producing anything it has the design data for. Exponential production happens when a nanofactory can build a duplicate of itself. Then they could both duplicate themselves, until we have 4, 8, 16, 32, . . . enough nanofactories for every household in the world to have one. That would totally eliminate the world economy as we know it. If there's no limits on what the nanofactories can produce there could be a wave of homemade WMDs that would eliminate the world as we know it.

The Diamond Age: "Feed" Nanofactories
One of the boxes was called the MC. It was built into the wall over the counter. Nell dragged a chair and climbed up to watch as Harv worked at it. The front of the MC was a mediatron, which meant anything that had pictures moving around on it, or sound coming out of it, or both. As Harv poked it with his fingers and spoke to it, little moving pictures danced around. . . . Harv gave it an especially dramatic poke, and then a new mediaglyphic came up, a white circle with a narrow green wedge at the top. The wedge got wider and wider. The MC played a little tune that meant you were supposed to wait. Harv went to the fridge and got himself a juice box and one for Nell too. He looked at the MC disdainfully. "This takes so long, it's ridiculous. . . . We've got a cheap Feed, just a few grams per second. Pathetic." . . . . Finally the music came to a bouncy conclusion just as the white wedge vanished. Harv . . . poked a mediaglyph that was an animated picture of a door swinging open. The MC took to hissing loudly. "Got to release the vacuum" he explained. The sound ended, and the door popped open. Inside the MC, folded up neatly, was Nell's new mattress.
This is a world where even the poorest homes have a nanofactory. There are even public ones free to passerby. Those "matter compilers" have a sharply restricted list of products they'll make. Even if someone hacks the user interface to program one to produce something else he can be blocked by central control. Each nanofactory is hooked up to the "Feed"—a physical line that delivers power and prepared materials. The nanofactory can only work with specially purified streams of individual elements (probably bound into carrier molecules). If an unusual combination of elements is requested the Feed will refuse to deliver them and probably sound an alarm.

As befits the social structure of the book's Neo-Victorians, nanofactories can be given increasing flexibility for higher-class users. An Equity Lord can presumably build anything he has a design for. In that environment production is limited by the availability of designs. So intellectual property dominates the economy (entertainment having an apparently larger share than product designs). New nanofactories can only be created with the permission of the Feed owner and have to be connected to a Feed before they can produce.

A nanofactory can be "off the grid" if it has dedicated infrastructure. A converter is needed to transform raw materials into the purified elements accepted by the nanofactory. If there's no regular power available it has to have a generator of its own. Depending on the robustness of the technology cooling and vibration isolation facilities may also be needed.

Exponential production stops when it runs out of any one of the ingredients needed to produce another duplicate. If the nanofactory can't build a piece of its infrastructure new ones are useless once the existing infrastructure is fully used. New infrastructure can be installed until the nanofactories are processing the available inflows of feed materials and generator fuel. The limit may be the number of people available to set up newly produced nanofactories. That wouldn't stop production but limit it to N per day instead of being exponential. If robotics technology has advanced enough the factories could produce installation and resource extraction robots to keep exponential growth going until the available resources are exhausted.

By this stage "exponential" production is moving so slowly that any interested party can interfere. Visiting a macrotech factory will show you that infrastructure facilities routinely outmass the actual material-processing tools. For a nanofactory to usefully reproduce itself it's going to spend much longer on the infrastructure than on the new nanofactory. Adding robots to the list would make each cycle take even longer. The neighbors will have plenty of time to notice Dr. X transforming his backyard into an industrial complex.

The Diamond Age: "Seed" Manufacturing
Instead of an army of stonemasons and carpenters, the builder was a single man, a portly gray-bearded fellow puffing at a long slender pipe, carrying a leather bag on his belt. Arriving at the center of the building site, he reached into his bag and drew out a great seed the size of an apple and pitched it into the soil. By the time this man had walked back to the spiral road, a tall shaft of gleaming crystal had arisen from the soil and grown far above their heads, gleaming in the sunlight, and branched out like a tree. By the time Princess Nell lost sight of it around the corner, the builder was puffing contentedly and looking at a crystalline vault that nearly covered the lot.
A "Seed" is a self-contained device for creating a single product from raw materials. It's a miniaturized version of the exponentially reproducing nanofactory above. The seed starts with some "assemblers"—nanoscale robots which move around the outside of the product, precisely placing atoms to build the whole thing (a nanofactory can be considered a set of immobile assemblers). The first few assemblers concentrate on making more assemblers until there's enough to build the whole product. Then they all switch tasks and rapidly assemble the product. Once the job is done the assemblers self-destruct, leaving it ready for use.

Like a plant, the seed assemblers have to get their resources from the surrounding soil. There might be a separate set of "gatherer" robots to collect materials and energy, or the assemblers might do it themselves. Carbon would be the most-needed element—most nanotech designs rely on crystalline carbon for their structure (hence the book title). It will still need small amounts of other elements. Making a nanodevice function requires some variations in the size and bonding characteristics of the atoms used to let the parts have the shape and movement needed. This assembler has to find enough of each element to finish its duplicate.

For example, look at a design for a nanoscale gear:

This uses hydrogen, carbon, silicon, nitrogen, phosphorus, oxygen, and sulfur. Running short of any one of those elements keeps the assembler from finishing the gear. Other elements are needed less often, probably in a power-law distribution. As more assemblers are built they'll have to go farther and farther from the seed's location to get the material they need. But until picotechnology is invented the assembler will have to keep searching until it finds the exact atoms it needs.

Energy will also be collected (in the form of organic molecules worth burning) from the soil. Solar power can supplement that but is too diffuse for the rapid production visualized for the seeds.

This concept could be fine for producing a house or car from Iowa topsoil. If you drop that same seed into the Arizona desert the assemblers will crawl out and find . . . lots of silicon dioxide. Very little hydrogen, less carbon, and no energy sources worth mentioning. The assemblers will wander about a bit on solar power, but you won't get a house.

My backyard splits the difference. Under the thin topsoil is red clay. So a seed could build something small, but would run out of energy before finishing a house. If I wanted to grow a car in my backyard I could help the seed by putting down "fertilizer"—molecules rich in energy and needed elements. A bag of charcoal would do nicely. But the more fertilizer I have to put down the less sense it makes to use a seed instead of running a Feed to the site, or making the car at a nanofactory elsewhere and bringing it home.

In the novel Dr. X wanted the Seed technology to bring back the Confucian ideal of a society based on peasant labor. Rather than his people being welfare clients dependent on the output of a distant Feed source, he saw a new peasantry fulfilling everyone's needs by planting seeds. That's not going to involve enough harsh manual labor to induce the virtues extolled by urban poets, even after planting peanuts to restore depleted fields. The Seeds would keep the people safe from single-point feed failures and domination by foreign feed sources.

The Neo-Victorians and their Common Economic Protocol allies opposed developing the Seed because it would let WMDs be made with no intervention from the feed source. But that requires a WMD Seed. Nanofactories are general-purpose tools with a user interface for inputting designs. Any restriction on inputs can be hacked around with time. The Feed has to be constrained to keep nanofactories from building WMDs or other forbidden goods.

A Seed has no user interface. You take it out of the packet, drop it in the ground, and step back. It produces whatever it was programmed to. If the Seed's original creator wanted a decentralized society new Seeds can be part of the product. A house seed can grow a house with a drawer full of new seed packets for cars, clothes, food, toys, etc. If it doesn't make new seeds people have to go to a central source every time they need something, which is just as centrally controlled as the Feed.

Creating a new kind of Seed is harder than creating a new design for a nanofactory to produce. The designer ("Alchemist" in the book) has to create the plan for obtaining raw materials and assembling the product. The design has to be optimized for the soil the seed will be grown in (what mix of elements and energy sources it has).

Anyone with the capabilities of an Alchemist can make a WMD seed. One of these is the nightmare of the Neo-Victorians. A seed can be carried into their secure inner areas without being detected as a dangerous object. Then it could be planted and launch a WMD attack. The only way to stop it would be to detect the seed assembly process and smash it before the WMD is ready, a task roughly as difficult as keeping every dandelion from reaching puffballhood in an entire county, forever.

There's two ways to deal with the threat of WMD seeds. One is ubiquitous surveillance (and sousveillance) as detailed in Brin's Transparent Society. If everyone's being watched terrorists can be caught in the act before doing much harm. Abandoning privacy would destroy many social structures, so it would be a traumatic transition. The alternative is to be very careful with who can become an Alchemist.

Walter Jon Williams called them "Aristoi" in his novel. These geniuses were carefully tested for technical ability and psychological stability. Once they passed the tests they would receive full privileges to use nanotech. These constraints weren't as concerned about using nanotech for WMDs as much as the danger of technical errors creating out-of-control nanotech devices—the "gray goo" problem.

Gray Goo, or Flesh Eating Assemblers
Sanjay was a hollowed-out asteroid, an irregular potato shape set in orbit by a strap-on gravity generator. Another chill ran through Gabriel as he saw that it was covered with what looked to be like dirty-white foam. Slow-motion bubbles rose to the surface, burst, left brief hollows soon filled by more glittering nano. Occasionally there was a scintillation, shining six-sided reflective patterns that formed for only a second in sunlight, like a diffraction halo that formed around a dust speck sitting on a camera lens.
IR scanners showed that the surface was hot. The nano was still active, still working away on something. . . . The Eldest Brother deployed a solar shield, several kilometers wide, between Sanjay and the sun, just in case the nano was absorbing energy from photons. IR readings showed the surface temperature had decreased immediately.
Pan had slowed the stuff down.
[snip other countermeasures]
After the nano had all been destroyed there was precious little left of the asteroid, a little stone spine, elongated and fragile, like a squab bone etched by acid.
From Aristoi by Walter Jon Williams
Gray goo is what happens when a seed goes bad. Instead of assemblers duplicating themselves until they hit the number needed to build the product, they keep duplicating forever. It was named by Erik Drexler: 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.

Some excitable reporters have read descriptions of gray goo and penned lurid scenarios of a puddle of goo converting the whole Earth to assemblers in a matter of days. Exponential growth can work that fast—if it doesn't run into any limits.

Look at that gray puddle out in the woods. Say it's 10 kg of goo and an assembler can duplicate itself in a minute. How much goo will there be a minute from now? Not 20 kg. Only the assemblers on the outside of the puddle have access to new materials and energy. The inside of the puddle is filled with junk: molecules with atoms more common than needed, assemblers inert until they get more energy, others sitting with a replica half finished, waiting on more material. If assemblers could teleport to a virgin field after each replication they could keep growing exponentially.

The gray goo will keep growing but at a rate proportional to the surface area of the puddle in contact with useful raw materials. The goo would expand through the woods until it reached an area lacking carbon or energy. A concrete road or sandy desert would stop it. An assembler optimized for building in forest will have a carbon-based structure. A solid piece of silicon dioxide won't provide anything useful for next generation of assemblers. The same problem hits goo headed straight down. Once it's through the topsoil it'll be short on materials and energy. Bedrock will stop it cold.

All this makes gray goo a slow-acting, easy to contain menace. On a small scale it's like a bacterial infection, where spreading bits of it to a new location is a bigger worry than the initial outbreak. Large gray goo infestations can be dealt with like forest fires. Creating a "backfire" to strip key materials from the path of the outbreak will create a barrier.

A gray goo assembler doesn't have to be carbon-hungry. It could be designed to primarily use silicon or another suitable element. Even if a gray goo assembler is optimized for a particular environment, such as the asteroid above, it may not be able to propagate. Solid rock doesn't have consumable energy for the assemblers. They could work off of solar power but that's diffuse and only available to the surface of the puddle. The goo might be able to eat an asteroid, but it would be slow, small nibbles, not huge gulps.

In the example above of gray goo destroying an asteroid, the author is violating conservation of energy to produce a dramatic scene. The nanobots are effectively boiling away a chunk of solid rock, but they're getting their energy from the sun. If there was enough sunlight to support vaporizing the rock that asteroid would have been destroyed long before the nanobots arrived. They can't supplement solar power with chemical energy because it's stone, which is not burnable—it's the ash of silicon burned in oxygen. Even if the asteroid was a chrondite, made of burnable carbon, there wouldn't be any oxygen for the nanobots to burn it with. So gray goo can't boil away asteroids, not matter how dramatic a scene it makes in the novel.

Current Mood: thoughtful

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