Grey goo, a term coined by nanotechnology pioneer Eric Drexler, refers to a hypothetical end-of-the-world event involving nanotechnology in which out-of-control self-replicating robots consume all life on Earth while building more of themselves (a scenario known as ecophagy). It is usually used in a science fictional context. In a worst-case scenario, all of the matter in the universe 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 universe'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.
It is unclear whether nanotechnology is capable of creating grey goo at all. While the biological matter that composes life releases significant amounts of energy when oxidised, and other sources of energy such as sunlight are available, this energy might not be sufficient for the robots to out-compete existing organic life that already uses those 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. There would be little chemical energy available from 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.
Because of these limitations grey goo may only be possible in an environment which lacks indigenous life to compete with it for resources. However, some proponents of nanotechnology argue that artificial nanomachines might be able to outcompete natural life because they could have irreducibly complex designs that life could not have developed via natural evolution.
Some also consider it unlikely that an artificial self-replicator could spontaneously evolve in a manner that could present an immediate threat.
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 (but see cancer), designing them to require a rare material that would be sprayed on the construction site before their release, or requiring constant direct control from an external computer. Another possibility is to encrypt the memory of the replicators in such a way that any changed copy is overwhelmingly likely to decrypt to nonfunctioning static.
Grey goo has several whimsical cousins, differentiated by their colors and raisons d'Ítre. 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.
- 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 mankind. It replicates relatively slowly, but some people think it will nevertheless fill any amount of space given enough time. In the pink goo worldview the spread of humanity is a catastrophe and space exploration opens up the possibility of the entire galaxy or the universe getting filled up with Pink Goo - the ultimate crime, something 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.
One fanciful depiction of a grey goo crisis was in an episode of the Gargoyles animated series where the protagonists face an advanced form of nanotechnology. They eventually manage to convince it to stop its spread and interest it in human society.
Another cartoon version of "grey goo" is The Germ, an episode of GI Joe written by Roger Slifer.
Psyudomondo U Bacteria, better known as Bacteria X, is stolen by the Crimson Guard, when they double cross Destro. In usual Cobra form, the Crimson Guard drops the Germ and a growth formula that Destro was working on, and it forms a giant blob that's eating everything in its path. The Joes throw everything at it from missiles to insecticide, and even send Airtight into the blob with explosive antibiotics. The blob separates in two after the Joes' attack, and one blob is weakened after going through an apple orchard. The Joes figure it's because of the poison in the apple seeds, so they bombard the blob with apples, which destroys the Germ.
Wil McCarthy's science fiction novel Bloom is set in a future in which a grey goo has overwhelmed the entire inner solar system, with the only remaining colonies of humans surviving in the asteroid belt and on Jupiter's moons.
Greg Bear's novel Blood Music is a classic of the field, depicting a form of grey goo originally derived from human lymphocytes.
The topic of nanotechnology has been tackled by several science fictional television shows.
In the science fiction television series Lexx, self-replicating robot arms called Mantrid drones wind up consuming the mass of an entire universe. Mantrid drones were macroscopic machines, but they apparently used nanotechnology as part of their means of manufacturing new parts for themselves.
Stargate SG-1 Also fought a form of macroscopic self replicating machines. This enemy was known as the Replicators. The basic building block of the Replicators is a 1cm trapezoidal block containing its own power supply and computing/memory capacity. However these blocks could be organized into units as small as 6 legged scout bot to FTL capable star ships of unlimited size. Initally the creation of a synthetic life, due to their immense computing power and hive mind, they quickly became sentient and began executing their agenda of converting the entire universe into replicators. Initally macroscopic, more advanced nanoscopic versions appeared that could mimic humanoid lifeforms.
Walter Jon Williams's novel Aristoi featured a future wherein Earth was consumed and destroyed by runaway nano, referred to as "mataglap", from an Indonesian word meaning "dilated eye" (referring to the look on one's face when they go berserk).
Sierra Entertainment's game Outpost 2 was a Civilization-like game which theme was based on a space colony where a lab exploded, creating a plague that consumed everything in it's path, called 'the Blight'. The game was a RTS, but failed terribly, as review sites rated it as one of the most boring games ever. The game came with a small novel counting the happenings of each level on both sides.
An episode of Cartoon Network's series Justice League Unlimited entitled "Heart of Darkness" pitted the comic book heroes of the DC comics universe against a nanotechnological weapon of mass destruction created by an ancient alien race designed to defeat its enemy by literally devouing the planet from under them.
- Greg Bear, Blood Music, the classic sci-fi novel about ecophagy. Arguably it is a Green or Golden Goo-type.
- Kurt Vonnegut, Cat's Cradle, centering around a grey-goo-like polywater, "ice-nine," converting every drop of water on Earth into a solid. It should be noted that Vonnegut's ice-nine is an alternate form of water, not a life form or nanomachine.