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Gray goo is a theoretical existential risk scenario involving molecular nanotechnology in which out-of-control self-replicating robots consume all matter on Earth while building more of themselves a scenario known as ecophagy (“eating the environment”). The term gray goo was coined by nanotechnology pioneer Eric Drexler in his 1986 book Engines of Creation.
Molecular nanotechnology (MNT) nanofacturing is popularly linked with this idea of swarms of coordinated nanoscale robots working together, a popularization of Eric Drexler’s 1986 discussions of MNT, which he superseded in 1992. In this early proposal, sufficiently capable nanorobots would construct more nanorobots in an artificial environment containing special molecular building blocks.
The fear is that nanomechanical robots, if achieved, and if designed to self-replicate using naturally occurring materials (a difficult task), could consume the entire planet in their hunger for raw materials, or simply crowd out natural life, out-competing it for energy (as happened historically when blue-green algae appeared and outcompeted earlier life forms). Eric Drexler considers an accidental “gray goo” scenario extremely unlikely and says so in later editions of Engines of Creation.
Critics have doubted both the feasibility of self-replicating nanorobots and the feasibility of control if self-replicating nanorobots could be achieved. In any event, since 1992 technical proposals for MNT do not include self-replicating nanorobots, and recent ethical guidelines put forth by MNT advocates prohibit unconstrained self-replication.
In light of this perception of potential danger, the Foresight Institute (founded by Eric Drexler to prepare for the arrival of future technologies) has drafted a set of guidelines for the ethical development of nanotechnology. These include the banning of free-foraging self-replicating pseudo-organisms on the Earth’s surface, at least, and possibly in other places.
The term gray goo is often used in a science fiction context. In the worst postulated scenarios (requiring large, space-capable machines), matter beyond Earth would also 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). The disaster is posited to result from a deliberate doomsday device, or from an accidental mutation in a self-replicating nanomachine used only for other purposes, but designed to operate in a natural environment.