Grey goo (alternatively spelled gray goo) is a hypothetical
end-of-the-world 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").
Self-replicating machines of the macroscopic variety were originally described by mathematician John von Neumann, and are sometimes referred to as von Neumann machines. The term grey goo was coined by nanotechnology pioneer Eric Drexler in his 1986 book Engines of Creation, stating that "we cannot afford certain types of accidents." In 2004 he stated "I wish I had never used the term 'grey goo'
Drexler more recently conceded that there is no need to build anything that even resembles a potential runaway replicator. This would avoid the problem entirely. In a paper in the journal Nanotechnology, he argues that self-replicating machines are needlessly complex and inefficient. His 1992 technical book on advanced nanotechnologies Nanosystems: Molecular Machinery, Manufacturing, and Computation describes manufacturing systems that are desktop-scale factories with specialized machines in fixed locations and conveyor belts to move parts from place to place. Popular culture, however, remains focused on imagined scenarios derived from his older ideas. None of these measures would prevent a party creating a weaponized grey goo, were such a thing possible.