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Gray goo (also spelled grey 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 that has been called ecophagy ("eating the environment"). The original idea assumed machines were designed to have this capability, while popularizations have assumed that machines might somehow gain this capability by accident.
Self-replicating machines of the macroscopic variety were originally described by mathematician John von Neumann, and are sometimes referred to as von Neumann machines or clanking replicators. The term gray goo was coined by nanotechnology pioneer Eric Drexler in his 1986 book Engines of Creation. In 2004 he stated, "I wish I had never used the term 'gray goo'." Engines of Creation mentions "gray goo" in two paragraphs and a note, while the popularized idea of gray goo was first publicized in a mass-circulation magazine, Omni, in November 1986.
The term was first used by molecular nanotechnology pioneer Eric Drexler in his book Engines of Creation (1986). In Chapter 4, Engines Of Abundance, Drexler illustrates both exponential growth and inherent limits (not gray goo) by describing nanomachines that can function only if given special raw materials:
Imagine such a replicator floating in a bottle of chemicals, making copies of itself…the first replicator assembles a copy in one thousand seconds, the two replicators then build two more in the next thousand seconds, the four build another four, and the eight build another eight. At the end of ten hours, there are not thirty-six new replicators, but over 68 billion. In less than a day, they would weigh a ton; in less than two days, they would outweigh the Earth; in another four hours, they would exceed the mass of the Sun and all the planets combined — if the bottle of chemicals hadn't run dry long before.
In a History Channel broadcast, a contrasting idea (a kind of gray goo) is referred to in a futuristic doomsday scenario: "In a common practice, billions of nanobots are released to clean up an oil spill off the coast of Louisiana. However, due to a programming error, the nanobots devour all carbon based objects, instead of just the hydrocarbons of the oil. The nanobots destroy everything, all the while, replicating themselves. Within days, the planet is turned to dust." 
Drexler describes gray goo in Chapter 11 of Engines Of Creation:
Early assembler-based replicators could beat the most advanced modern organisms. 'Plants' with 'leaves' no more efficient than today's solar cells could out-compete real plants, crowding the biosphere with an inedible foliage. Tough, omnivorous 'bacteria' could out-compete real bacteria: they could spread like blowing pollen, replicate swiftly, and reduce the biosphere to dust in a matter of days. Dangerous replicators could easily be too tough, small, and rapidly spreading to stop — at least if we made no preparation. We have trouble enough controlling viruses and fruit flies.
Drexler notes that the geometric growth made possible by self-replication is inherently limited by the availability of suitable raw materials.
Drexler used the term "gray goo" not to indicate color or texture, but to emphasize the difference between "superiority" in terms of human values and "superiority" in terms of competitive success:
Though masses of uncontrolled replicators need not be grey or gooey, the term "grey goo" emphasizes that replicators able to obliterate life might be less inspiring than a single species of crabgrass. They might be "superior" in an evolutionary sense, but this need not make them valuable.
Bill Joy, one of the founders of Sun Microsystems, discussed some of the problems with pursuing this technology in his now-famous 2000 article in Wired magazine, titled "Why the Future Doesn't Need Us". In direct response to Joy's concerns, the first quantitative technical analysis of the ecophagy scenario was published in 2000 by nanomedicine pioneer Robert Freitas.
Risks and precautions
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. None of these measures would prevent a party from creating a weaponized grey goo, were such a thing possible.
Prince Charles called upon the British Royal Society to investigate the "enormous environmental and social risks" of nanotechnology in a planned report, leading to much media commentary on gray goo. The Royal Society's report on nanoscience was released on 29 July 2004, and declared the possibility of self-replicating machines to lie too far in the future to be of concern to regulators.
More recent analysis in the paper titled Safe Exponential Manufacturing from the Institute of Physics (co-written by Chris Phoenix, Director of Research of the Center for Responsible Nanotechnology, and Eric Drexler), shows that the danger of grey goo is far less likely than originally thought. However, other long-term major risks to society and the environment from nanotechnology have been identified. Drexler has made a somewhat public effort to retract his grey goo hypothesis, in an effort to focus the debate on more realistic threats associated with knowledge-enabled nanoterrorism and other misuses.
In Safe Exponential Manufacturing, which was published in a 2004 issue of Nanotechnology, it was suggested that creating manufacturing systems with the ability to self-replicate by the use of their own energy sources would not be needed. The Foresight Institute also recommended embedding controls in the molecular machines. These controls would be able to prevent anyone from purposely abusing nanotechnology, and therefore avoid the grey goo scenario.
Ethics and chaos
Grey goo is a useful construct for considering low-probability, high-impact outcomes from emerging technologies. Thus, it is a useful tool in the ethics of technology. Daniel A. Vallero  applied it as a worst-case scenario thought experiment for technologists contemplating possible risks from advancing a technology. This requires that a decision tree or event tree include even extremely low probability events if such events may have an extremely negative and irreversibile consequence, i.e. application of the precautionary principle. Dianne Irving admonishes that "any error in science will have a rippling effect....". Vallero adapted this reference to chaos theory to emerging technologies, wherein slight permutations of initial conditions can lead to unforeseen and profoundly negative downstream effects, for which the technologist and the new technology's proponents must be held accountable.
Grey goo nanobots need a source of energy to drive their replication. For efficiency reasons, the energy would likely come from oxidation and other chemical reactions on the organic matter itself—a process which in organic life is known as digestion—rather than from an external power source.
In such a scenario, grey goo replication is self-limiting. The more organic material that the grey goo consumes, the less remains available for further consumption. After exhausting available organic material within a local area, grey goo would experience a population crash in that area, slowing or ending its outward spread.
Some organic organisms may prove more resistant than others to grey goo. As with all environmental stresses, natural selection would favor their survival, and amplification of their resistance traits.
If grey goo nanobots could also evolve through the course of their replications, they might gain the ability to consume one another, as an additional source of energy to drive replication. Further from there, they could evolve resistance to consumption by grey goo nanobots. Under such evolutionary pressure, grey goo nanobots would become subject to speciation, inter-species competition, and specialization to occupy ecological niches. However, engineered machines like those which Dexler has proposed are fundamentally simpler and less susceptible to evolution than biological systems.
- The Invincible, a 1964 science fiction novel with intrigue centered on self-configuring nanobotic swarms
- Moonseed, a 1998 science fiction novel where the Earth faces danger from a self-replicating nanobot swarm.
- Self-reconfiguring modular robot
- Programmable matter
- Utility fog
- The Blob
- "Benderama", a Futurama episode about the issue
- Replicator (Stargate) A self-replicating synthetic life-form in the television series Stargate SG-1.
- Tasty Planet, a game released in 2006 by Dingo Games centers around a gray goo eating the universe, starting at the atomic level and progressing to the cosmic level.
- "Grey Goo is a Small Issue". Center for Responsible Nanotechnology. 2003-12-14. Retrieved 2009-12-28.
- "Nanotechnology pioneer slays "grey goo" myths". Nanotechnology. Institute of Physics. 2006-07-06. Retrieved 2009-12-28.
- Freitas Jr., Robert A. (2000-04-00). "Some Limits to Global Ecophagy by Biovorous Nanoreplicators, with Public Policy Recommendations". Retrieved 2009-12-28.
- Joseph, Lawrence E. (2007). Apocalypse 2012. New York: Broadway. p. 6. ISBN 978-0-7679-2448-1.
- Giles, Jim (2004). "Nanotech takes small step towards burying 'grey goo'". Nature 429 (6992): 591. doi:10.1038/429591b. PMID 15190320.
- "Modern Marvels: Doomsday Tech DVD". History Channel. 2004-12-28. Retrieved 2013-07-28.
- Drexler, K. Eric (1992). Nanosystems: molecular machinery, manufacturing, and computation. Wiley. ISBN 978-0-471-57518-4.
- "Nanoscience and nanotechnologies: opportunities and uncertainties". The Royal Society. Retrieved 2011-08-23.
- "Leading nanotech experts put 'grey goo' in perspective" (Press release). Center for Responsible Nanotechnology. 9 June 2004. Retrieved 2006-06-17.
- "Current Results of Our Research". Center for Responsible Nanotechnology. Retrieved 2006-06-17.
- Rincon, Paul (2004-06-09). "Nanotech guru turns back on 'goo'". BBC News. Retrieved 2012-03-30.
- Phoenix, Chris; Eric Drexler (August 2004). "Safe Exponential Manufacturing". Nanotechnology 15 (8): 869–72. doi:10.1088/0957-4484/15/8/001.
- "Foresight Guidelines for Responsible Nanotechnology Development". Foresight Institute and IMM. Retrieved 2012-05-07.
- Vallero, Daniel (2007). Biomedical Ethics for Engineers: Ethics and Decision Making in Biomedical and Biosystem Engineering. Academic Press. ISBN 9780080476100.
- Irving, Dianne (October 28, 1999). "Science, the formation of conscience and moral decision theory". Proceedings of the Guadalupan Appeal: The Dignity and Status of the Human Embryo, Mexico City, Mexico.
- Some Limits to Global Ecophagy by Biovorous Nanoreplicators, with Public Policy Recommendations
- Safe exponential manufacturing Paper critical of "grey goo," summarized in article Nanotechnology pioneer slays "grey goo" myths
- Online edition of the Royal Society's report Nanoscience and nanotechnologies: opportunities and uncertainties
- Goo and Paste Directory
- UK Government & Royal Society commission on Nanotechnology and Nanoscience
- Nanotechnology: Drexler and Smalley make the case for and against 'molecular assemblers' (Richard Smalley argues that laws of chemistry imply it will be impossible to ever create "self-replicating nanobots" whose abilities to assemble molecules are significantly different than those of biological self-replicators. Some pro-nanobot responses to Smalley's argument can be found at Debate About Assemblers — Smalley Rebuttal, The Drexler-Smalley debate on molecular assembly, Of Chemistry, Nanobots, and Policy, and Is the Revolution Real?)
- Nanotechnology and the Grey Goo Problem
- Be Amazing! – Humorous animation that involves a grey goo scenario