The Gray Goo Problem

March 20, 2001 by Robert A. Freitas Jr.

In Eric Drexler’s classic “grey goo” scenario, out-of-control nanotech replicators wipe out all life on Earth. This paper by Robert A. Freitas Jr. was the first quantitative technical analysis of this catastrophic scenario, also offering possible solutions. It was written in part as an answer to Bill Joy’s recent concerns.

Research Scientist, Zyvex

Originally published April 2000 as “Some Limits to Global Ecophagy by Biovorous Nanoreplicators, with Public Policy Recommendations.” Excerpted version published on March 20, 2001.


The maximum rate of global ecophagy by biovorous self-replicating nanorobots is fundamentally restricted by the replicative strategy employed; by the maximum dispersal velocity of mobile replicators; by operational energy and chemical element requirements; by the homeostatic resistance of biological ecologies to ecophagy; by ecophagic thermal pollution limits (ETPL); and most importantly by our determination and readiness to stop them.

Assuming current and foreseeable energy-dissipative designs requiring ~100 MJ/kg for chemical transformations (most likely for biovorous systems), ecophagy that proceeds slowly enough to add ~4°C to global warming (near the current threshold for immediate climatological detection) will require ~20 months to run to completion; faster ecophagic devices run hotter, allowing quicker detection by policing authorities. All ecophagic scenarios examined appear to permit early detection by vigilant monitoring, thus enabling rapid deployment of effective defensive instrumentalities.


Recent discussions [1] of the possible dangers posed by future technologies such as artificial intelligence, genetic engineering and molecular nanotechnology have made it clear that an intensive theoretical analysis of the major classes of environmental risks of molecular nanotechnology (MNT) is warranted. No systematic assessment of the risks and limitations of MNT-based technologies has yet been attempted. This paper represents a first effort to begin this analytical process in a quantitative fashion.

Perhaps the earliest-recognized and best-known danger of molecular nanotechnology is the risk that self-replicating nanorobots capable of functioning autonomously in the natural environment could quickly convert that natural environment (e.g., “biomass”) into replicas of themselves (e.g., “nanomass”) on a global basis, a scenario usually referred to as the “gray goo problem” but perhaps more properly termed “global ecophagy.”

As Drexler first warned in Engines of Creation [2]:

“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 make no preparation. We have trouble enough controlling viruses and fruit flies.

Among the cognoscenti of nanotechnology, this threat has become known as the “gray goo problem.” Though masses of uncontrolled replicators need not be gray or gooey, the term “gray 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.

The gray goo threat makes one thing perfectly clear: We cannot afford certain kinds of accidents with replicating assemblers.

Gray goo would surely be a depressing ending to our human adventure on Earth, far worse than mere fire or ice, and one that could stem from a simple laboratory accident.

Lederberg [3] notes that the microbial world is evolving at a fast pace, and suggests that our survival may depend upon embracing a “more microbial point of view.” The emergence of new infectious agents such as HIV and Ebola demonstrates that we have as yet little knowledge of how natural or technological disruptions to the environment might trigger mutations in known organisms or unknown extant organisms [81], producing a limited form of “green goo” [92].

However, biovorous nanorobots capable of comprehensive ecophagy will not be easy to build and their design will require exquisite attention to numerous complex specifications and operational challenges. Such biovores can emerge only after a lengthy period of purposeful focused effort, or as a result of deliberate experiments aimed at creating general-purpose artificial life, perhaps by employing genetic algorithms, and are highly unlikely to arise solely by accident.

The Ecophagic Threat

Classical molecular nanotechnology [2], 4] envisions nanomachines predominantly composed of carbon-rich diamondoid materials. Other useful nanochemistries might employ aluminum-rich sapphire (Al2O3) materials, boron-rich (BN) or titanium-rich (TiC) materials, and the like. TiC has one the highest possible operating temperatures allowed for commonplace materials (melting point ~3410°K [5]), and while diamond can scratch TiC, TiC can be used to melt diamond.

However, atoms of Al, Ti and B are far more abundant in the Earth’s crust (81,300 ppm, 4400 ppm and 3 ppm, respectively [5]) than in biomass, e.g., the human body (0.1 ppm, 0 ppm, and 0.03 ppm [6]), reducing the direct threat of ecophagy by such systems. On the other hand, carbon is a thousand times less abundant in crustal rocks (320 ppm, mostly carbonates) than in the biosphere (~230,000 ppm).

Furthermore, conversion of the lithosphere into nanomachinery is not a primary concern because ordinary rocks typically contain relatively scarce sources of energy. For instance, natural radioactive isotopes present in crustal rocks vary greatly as a function of the geological composition and history of a region, but generally range from 0.15-1.40 mGy/yr [7], giving a raw power density of 0.28-2.6 ×10-7 W/m3 assuming crustal rocks of approximately mean terrestrial density (5522 kg/m3 [5]).

This is quite insufficient to power nanorobots capable of significant activities; current nanomachine designs typically require power densities on the order of 105-109 W/m3 to achieve effective results [6]. (Biological systems typically operate at 102-106 W/m3 [6].) Solar power is not readily available below the surface, and the mean geothermal heat flow is only 0.05 W/m2 at the surface [6], just a tiny fraction of solar insolation.

Hypothesized crustal abiotic highly-reduced petroleum reserves [16] probably could not energize significant replicator nanomass growth due to the anoxic environment deep underground, although potentially large geobacterial populations have been described [1016] and in principle some unusual though highly limited bacterial energy sources could also be tapped by nanorobots.

For example, some anaerobic bacteria use metals (instead of oxygen) as electron-acceptors [13], with iron present in minerals such as pyroxene or olivine being converted to iron in a more oxidized state in magnetic minerals such as magnetite and maghemite, and using geochemically produced hydrogen to reduce CO2 to methane [11]. Underground bacteria in the Antrim Shale deposit produce 1.2 ×107 m3/day of natural gas (methane) by consuming the 370 MY-old remains of ancient algae [17].

Bioremediation experiments have also been done by Envirogen and others in which pollution-eating bacteria are purposely injected into the ground to metabolize organic toxins; in field tests it has proven difficult to get the bacteria to move through underground aquifers, because the negatively-charged cells tend to adhere to positively charged iron oxides in the soil [18].

However, the primary ecophagic concern is that runaway nanorobotic replicators or “replibots” will convert the entire surface biosphere (the ecology of all living things on the surface of the Earth) into alternative or artificial materials of some type–especially, materials like themselves, e.g., more self-replicating nanorobots.

Since advanced nanorobots might be constructed predominantly of carbon-rich diamondoid materials [4], and since ~12% of all atoms in the human body (representative of biology generally) are carbon atoms [6], or ~23% by weight, the global biological carbon inventory may support the self-manufacture of a final mass of replicating diamondoid nanorobots on the order of ~0.23 Mbio, where Mbio is the total global biomass.

Unlike almost any other natural material, biomass can serve both as a source of carbon and as a source of power for nanomachine replication. Ecophagic nanorobots would regard living things as environmental carbon accumulators, and biomass as a valuable ore to be mined for carbon and energy. Of course, biosystems from which all carbon has been extracted can no longer be alive but would instead become lifeless chemical sludge.

Additional Scenarios

Four related scenarios which may lead indirectly to global ecophagy have been identified and are described below. In all cases, early detection appears feasible with advance preparation, and adequate defenses are readily conceived using molecular nanotechnologies of comparable sophistication.

Gray Plankton

The existence of 1-2 ×1016 kg [24] of global undersea carbon storage on continental margins as CH4 clathrates and a like amount (3.8 ×1016 kg) of seawater-dissolved carbon as CO2 represent a carbon inventory more than an order of magnitude larger than in the global biomass. Methane and CO2 can in principle be combined to form free carbon and water, plus 0.5 MJ/kg C of free energy. (Some researchers are studying the possibility of reducing greenhouse gas accumulations by storing liquid [44] or solid [45] CO2 on the ocean floor, which could potentially enable seabed replibots to more easily metabolize methane sources.)

Oxygen could also be imported from the surface in pressurized microtanks via buoyancy transport, with the conversion of carbon clathrates to nanomass taking place on the seabed below. The subsequent colonization of the land-based carbon-rich ecology by a large and hungry seabed-grown replicator population is the “gray plankton” scenario. (Phytoplankton, 1-200 microns in size, are the particles most responsible for the variable optical properties of oceanic water because of the strong absorption of these cells in the blue and red portions of the optical spectrum [37].)

If not largely confined to the sea floor during most of their replication cycle, the natural cell/device ratio could increase by many orders of magnitude, requiring a more diligent census effort. Census-taking nanorobots can alternatively be used to identify, disable, knapsack or destroy the gray plankton devices.

Gray Dust (Aerovores)

Traditional diamondoid nanomachinery designs [4] have employed 8 primary chemical elements, along with the associated atmospheric abundances [46] of each element. (Silicon is present in air as particulate dust which may be taken as ~28% Si for crustal rock [5], with a global average dust concentration of ~0.0025 mg/m3). The requirement for elements that are relatively rare in the atmosphere greatly constrains the potential nanomass and growth rate of airborne replicators.

However, note that at least one of the classical designs exceeds 91% CHON by weight. Although it would be very difficult, it is at least theoretically possible that replicators could be constructed almost solely of CHON, in which case such devices could replicate relatively rapidly using only atmospheric resources, powered by sunlight. A worldwide blanket of airborne replicating dust or “aerovores” that blots out all sunlight has been called the “gray dust” scenario [47]. (There have already been numerous experimental aerial releases of recombinant bacteria [48].)

The most efficient cleanup strategy appears to be the use of air-dropped non-self-replicating nanorobots equipped with prehensile microdragnets.

Alternative airborne or ground-based atmospheric filtration configurations that could permit more rapid filtering are readily envisioned. For example, since drag power varies as the square of the velocity, then by increasing mesh volume 10,000-fold while decreasing airflow velocity 100-fold, total drag power remains unchanged but whole-atmosphere turnover proceeds 100-fold faster, e.g., ~15 minutes.

Gray Lichens

Colonies of symbiotic algae and fungi known as lichens (which some have called a form of sub-aerial biofilm) are among the first plants to grow on bare stone, helping in soil formation by slowly etching the rock [55]. Lithobiontic microbial communities such as crustose saxicolous lichens penetrate mineral surfaces up to depths of 1 cm using a complex dissolution, selective transport, and recrystallization process sometimes termed “biological weathering” [56].

Colonies of epilithic (living on rock surfaces) microscopic bacteria produce a 10 micron thick patina on desert rocks (called “desert varnish” [57]) consisting of trace amounts of Mn and Fe oxides that help to provide protection from heat and UV radiation [5759].

In theory, replicating nanorobots could be made almost entirely of nondiamondoid materials including noncarbon chemical elements found in great abundance in rock such as silicon, aluminum, iron, titanium and oxygen. The subsequent ecophagic destruction of land-based biology by a maliciously programmed noncarbon epilithic replicator population that has grown into a significant nanomass is the “gray lichen” scenario.

Continuous direct census sampling of the Earth’s land surfaces will almost certainly allow early detection, since mineralogical nanorobots should be easily distinguishable from inert rock particles and from organic microbes in the top 3-8 cm of soil.

Malicious Ecophagy

More difficult scenarios involve ecophagic attacks that are launched not to convert biomass to nanomass, but rather primarily to destroy biomass. The optimal malicious ecophagic attack strategy appears to involve a two-phase process.

In the first phase, initial seed replibots are widely distributed in the vicinity of the target biomass, replicating with maximum stealth up to some critical population size by consuming local environmental substrate to build nanomass. In the second phase, the now-large replibot population ceases replication and exclusively undertakes its primary destructive purpose. More generally, this strategy may be described as Build/Destroy.

During the Build phase of the malicious “badbots,” and assuming technological equivalence, defensive “goodbots” enjoy at least three important tactical advantages over their adversaries:

1. Preparation–defensive agencies can manufacture and position in advance overwhelming quantities of (ideally, non-self-replicating) defensive instrumentalities, e.g., goodbots, which can immediately be deployed at the first sign of trouble, with minimal additional risk to the environment;

2. Efficiency–while badbots must simultaneously replicate and defend themselves against attack (either actively or by maintaining stealth), goodbots may concentrate exclusively on attacking badbots (e.g., because of their large numerical superiority in an early deployment) and thus enjoy lower operational overhead and higher efficiency in achieving their purpose, all else equal; and

3. Leverage–in terms of materials, energy, time and sophistication, fewer resources are generally required to confine, disable, or destroy a complex machine than are required to build or replicate the same complex machine from scratch (e.g., one small bomb can destroy a large bomb-making factory; one small missile can sink a large ship).

It is most advantageous to engage a malicious ecophagic threat while it is still in its Build phase. This requires foresight and a commitment to extensive surveillance by the defensive authorities.

Conclusions and Public Policy Recommendations

The smallest plausible biovorous nanoreplicator has a molecular weight of ~1 gigadalton and a minimum replication time of perhaps ~100 seconds, in theory permitting global ecophagy to be completed in as few as ~104 seconds. However, such rapid replication creates an immediately detectable thermal signature enabling effective defensive policing instrumentalities to be promptly deployed before significant damage to the ecology can occur.

Such defensive instrumentalities will generate their own thermal pollution during defensive operations. This should not significantly limit the defense strategy because knapsacking, disabling or destroying a working nanoreplicator should consume far less energy than is consumed by a nanoreplicator during a single replication cycle, hence such defensive operations are effectively endothermic.

Ecophagy that proceeds near the current threshold for immediate climatological detection, adding perhaps ~4°C to global warming, may require ~20 months to run to completion, which is plenty of advance warning to mount an effective defense.

Ecophagy that progresses slowly enough to evade easy detection by thermal monitoring alone would require many years to run to completion, could still be detected by direct in situ surveillance, and may be at least partially offset by increased biomass growth rates due to natural homeostatic compensation mechanisms inherent in the terrestrial ecology.

Ecophagy accomplished indirectly by a replibot population pre-grown on nonbiological substrate may be avoided by diligent thermal monitoring and direct census sampling of relevant terrestrial niches to search for growing, possibly dangerous, pre-ecophagous nanorobot populations.

Specific public policy recommendations suggested by the results of the present analysis include:

1. An immediate international moratorium on all artificial life experiments implemented as nonbiological hardware. In this context, “artificial life” is defined as autonomous foraging replicators, excluding purely biological implementations (already covered by NIH guidelines [65] tacitly accepted worldwide) and also excluding software simulations which are essential preparatory work and should continue. Alternative “inherently safe” replication strategies such as the broadcast architecture [66] are already well-known.

2. Continuous comprehensive infrared surveillance of Earth’s surface by geostationary satellites, both to monitor the current biomass inventory and to detect (and then investigate) any rapidly-developing artificial hotspots. This could be an extension of current or proposed Earth-monitoring systems (e.g., NASA’s Earth Observing System [67]and disease remote-sensing programs [93]) originally intended to understand and predict global warming, changes in land use, and so forth–initially using non-nanoscale technologies. Other methods of detection are feasible and further research is required to identify and properly evaluate the full range of alternatives.

3. Initiating a long-term research program designed to acquire the knowledge and capability needed to counteract ecophagic replicators, including scenario-building and threat analysis with numerical simulations, measure/countermeasure analysis, theory and design of global monitoring systems capable of fast detection and response, IFF (Identification Friend or Foe) discrimination protocols, and eventually the design of relevant nanorobotic systemic defensive capabilities and infrastructure.

A related long-term recommendation is to initiate a global system of comprehensive in situ ecosphere surveillance, potentially including possible nanorobot activity signatures (e.g. changes in greenhouse gas concentrations), multispectral surface imaging to detect disguised signatures, and direct local nanorobot census sampling on land, sea, and air, as warranted by the pace of development of new MNT capabilities.


The author thanks Robert J. Bradbury, J. Storrs Hall, James Logajan, Markus Krummenacker, Thomas McKendree, Ralph C. Merkle, Christopher J. Phoenix, Tihamer Toth-Fejel, James R. Von Ehr II, and Eliezer S. Yudkowsky for helpful comments on earlier versions of this manuscript; J. S. Hall for the word “aerovore”; and R. J. Bradbury for preparing the hypertext version of this document.


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