Grey goo

Grey goo refers, usually in a science fictional context, to a hypothetical human extinction event involving nanotechnology, in which out-of-control self-replicating robots (Von Neumann machines) consume the Earth while building more of themselves. In a worst-case scenario, all of the matter in the Galaxy could be turned into goo (with "goo" meaning a large mass of replicating nanomachines lacking large-scale structure, which may or may not actually appear goo-like), killing the Galaxy's residents. The disaster could result from an accidental mutation in a self-replicating nanomachine used for other purposes, or possibly from a deliberate doomsday device.

Assuming a nanotechnological replicator is capable of causing a grey goo disaster, safety precautions might include programming them to stop reproducing after a certain number of generations, or designing them to require a rare material that would be sprayed on the construction site before their release. However, it should be noted that it is unlikely that nanotechnology will be capable of creating grey goo at all.

The primary limitation on even arbitrarily sophsiticated nanotechnology which prevents a runaway grey goo reaction is the lack of a sufficient source of energy. A nanomachine wouldn't be able to get much energy out of eating inorganic matter such as rocks because, aside from a few exceptions (coal, for example) it's mostly well-oxidized and sitting in a free-energy minimum.

This means that the nanobots would be competing with natural life forms for organic matter or sunlight, life forms which have been evolving for over four billion years to optimize their ability to compete for these resources. If the nanomachine is itself composed of organic molecules, then it might even find itself being preyed upon by preexisting bacteria and other natural life forms. If they are built of inorganic compounds or make much use of elements that are not generally found in living matter, then they will need to use much of their metabolic output to fighting entropy as they purify (reduce sand to silicon, for instance) and synthesize the necessary building blocks. Grey goo may only be possible in an environment which lacks indigenous life.

A traditional response to the grey goo (or ecophagy) scenario in nanotechnology discussions:

"How likely is it that your car could spontaneously mutate into a wild car, run off road and live in the forest off of tree sap?"

Grey goo has several cousins, differentiated by their colors and raisons d'�tre. Most of these are not as commonly referred to as grey goo, however, and the definitions are informal:

Red Goo is goo unleashed intentionally by terrorists, a doomsday weapon, or a private individual who wishes to commit suicide with a bang.
Khaki Goo is goo intended by the military to wipe out somebody else's continent, planet, etc.
Golden Goo is the backfiring of a get-rich-quick scheme to assemble gold or other economically valuable substance. The details are left to the imagination.
Blue Goo is goo deliberately released in order to stop some other type of grey goo. It might well be the only solution to such a disaster, and would hopefully be better controlled than the original goo.
Pink Goo refers to humankind. It replicates relatively slowly, but will nevertheless fill any amount of space given enough time. Some people think that allowing the entire Galaxy to get filled with Pink Goo would be the ultimate crime, to be stopped at any cost.
Green Goo is goo deliberately released, for example by ecoterrorists, in order to stop the spread of Pink Goo, either by sterilization or simply by digesting the pink goo. Some form of this, along with an antidote available to the selected few, has been suggested as a strategy for achieving zero population growth.

Bibliography

Greg Bear, Blood Music, the classical sci-fi novel about ecophagy. One can argue it is a Green Goo-type. 1985 but still young.