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 all life on Earth while building more of themselves (a scenario known as ecophagy). 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.
Precautions
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, designing them to require a rare material that would be sprayed on the construction site before their release or simply requiring constant direct control from an external computer. However, it should be noted that there are reasons to believe that nanotechnology might not be capable of creating grey goo at all.
Limitations on grey goo activity
The primary limitation on even arbitrarily sophisticated nanotechnology which could prevent 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 for 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 to compete with it for indigenous resources. However, some proponents of nanotechnology argue that it is possible nanomachines could be developed that are able to outcompete natural life through the use of novel chemical processes that life would be unable to develop via natural evolution due to irreducible complexity.
Some also consider it highly unlikely that an artificial self-replicator could spontaneously evolve in a manner that could present an immediate threat. A traditional response to the grey goo 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?"
Other varieties
Grey goo has several whimsical cousins, differentiated by their colors and raisons d'être. Most of these are not as commonly referred to as grey goo, however, and the definitions are informal:
- 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.
- Black Goo (or 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.
- 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 is humanity. 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. The term originates from the science fiction classic, Soylent Green.
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