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A von Neumann probe is a specific example of a hypothetical concept based on the work of Hungarian-born American mathematician and physicist John von Neumann. Von Neumann rigorously studied the concept of self-replicating machines that he called "Universal Assemblers", which are most often referred to as von Neumann machines. While von Neumann never applied his work to the idea of spacecraft, theoreticians since then have done so. The idea of self-replicating spacecraft has been applied—in theory—to several distinct "tasks", and the particular variant of this idea applied to the idea of space exploration is known as a von Neumann probe. Other variants include the Berserker and an automated seeder ship.
In theory, a self-replicating spacecraft could be sent to a neighbouring star-system, where it would seek out raw materials (extracted from asteroids, moons, gas giants, etc.) to create replicas of itself. These replicas would then be sent out to other star systems, repeating the process in an exponentially increasing pattern. The original "parent" probe could then pursue its primary purpose within the star system. This mission varies widely depending on the variant of self-replicating starship proposed.
Given this pattern, and its similarity to the reproduction patterns1 of bacteria, it has been pointed out that von Neumann machines might be considered a form of life. In his short story, "Lungfish" (see Examples in fiction below), David Brin touches on this idea, pointing out that self-replicating machines launched by different species might actually compete with one another (in a Darwinistic fashion) for raw material, or even have conflicting missions. Given enough variety of "species" they might even form a type of ecology, or - should they also have a form of artificial intelligence - a society. They may even mutate with untold thousands of "generations".
It has been theorized that a self-replicating starship utilizing relatively conventional theoretical methods of interstellar travel (i.e. no exotic faster-than-light propulsion such as "warp drive", and speeds limited to an "average cruising speed" of 0.1c.) could spread throughout a galaxy the size of the Milky Way in as little as half a million years.
Implications for Fermi's Paradox
In 1981, Frank Tipler put forth an argument that extraterrestrial intelligences do not exist based on Von Neumann probes. Given even a moderate rate of replication and the history of the galaxy, such probes should already be common throughout space and thus, we should have already encountered them. Because we haven't, this shows that extraterrestrial intelligences do not exist. This is thus a resolution to the Fermi Paradox -- that is, the question of why we haven't already encountered extraterrestrial intelligence if it's common throughout the universe.
A response came from Carl Sagan and William Newman. Now known as Sagan's Response, it pointed out that in fact Tipler had underestimated the rate of replication, and that Von Neumann probes should have already started to consume most of the mass in the galaxy. Any intelligent race would therefore, Sagan and Newman reasoned, not design Von Neumann probes in the first place, and would try to wipe out any Von Neumann probes found as soon as they were detected.
Another objection to the prevalence of Von Neumann probes is that civilizations of the type that could potentially create such devices may have inherently short lifetimes, and self-destruct before so advanced a stage is reached, through such events as biological or nuclear warfare, nanoterrorism, resource exhaustion, ecological catastrophe, pandemics due to antibiotic resistance, et cetera.
Applications for self-replicating spacecraft
The details of the mission of self-replicating starships can vary widely from proposal to proposal, and the only common trait is the self-replicating nature.
Von Neumann probes
In the classic von Neumann probe variant, a probe would investigate its target system and transmit information about it back to its system of origin. If a self-replicating probe finds evidence of primitive life (or a primitive, low level culture) it might be programmed to lie dormant, silently observe, attempt to make contact (this variant is known as a Bracewell probe), or even interfere with or guide the evolution of life in some way.
Physicist Paul Davies of the University of Adelaide has even raised the possibility of a probe resting on our own Moon, having arrived at some point in Earth's ancient prehistory and remained to monitor Earth (see Bracewell probe and Sentinel hypothesis).
An interesting variant idea on the interstellar von Neumann probe idea is that of the "Astrochicken", proposed by Freeman Dyson. While it has the common traits of self-replication, exploration, and communication with its "home base", the Astrochicken is meant to explore and "live" within a single planetary system (Dyson's concept assumed our own), and not explore interstellar space. It is interesting to speculate about the usefulness of such an automata, combined with its interstellar cousin, who might use such "local" replicators as a means of exploring its target system.
A more frightening variant of the self-replicating starship is the Berserker. Unlike the benign probe concept, Berserkers are programmed to seek out and exterminate lifeforms and life-bearing exoplanets whenever they are encountered.
The name is derived from a series of novels by Fred Saberhagen which feature an ongoing war between humanity and such machines (see: Berserker). Saberhagen points out (through one of his characters) that the Berserker warships in his novels are not von Neumann machines themselves, but the larger complex of Berserker machines - including automated shipyards - do constitute a von Neumann machine. This again brings up the concept of an ecology of von Neumann machines, or even a von Neumann hive entity.
It is speculated that Berserkers could be created and launched by a xenophobic civilization (see Anvil of Stars, by Greg Bear, in Examples in fiction below) or could theoretically "mutate" from a more benign probe.
Replicating "seeder" ships
Yet another variant on the idea of the self-replicating starship is that of the "seeder" ship. Such starships might store the genetic patterns of lifeforms from their home world, perhaps even of the race which created it. Upon finding a habitable exoplanet, or even one that might be terraformed, it would try to replicate such lifeforms - either from stored embryos (see: embryo space colonization) or from stored information using molecular nanotechnology to "build" zygotes with varying genetic information from local raw materials.
Such ships might be terraforming vessels, preparing colony worlds for later colonization by other vessels, or - should they be programmed to recreate, raise, and educate individuals of the species that created it - self-replicating colonizers themselves.
As a side note, this pattern of terraforming and colonization need not be "automated". Manned interstellar colony ships could follow a similar pattern - and might be considered a sort of a combined von Neumann probe/seeder ship in which replication can be performed by the living inhabitants. Some proponents of space habitats suggest that planets would be entirely unnecessary to a civilization using this approach..
Examples in fiction
Von Neumann probes
The monoliths in Arthur C. Clarke's book and Stanley Kubrick's film 2001: A Space Odyssey are self-replicating probes, though the artifacts in "The Sentinel", Clarke's original short story upon which 2001 was based, were not. The film was to begin with a series of scientists explaining how probes like these would be the most efficient method of exploring outer space. Kubrick cut the opening segment from his film at the last minute, however, and these monoliths became almost mystical entities.
See also Alexander Kluge, The Devil's Blind Spot (New Directions; 2004.)
In the science fiction short story collection Berserker by Fred Saberhagen, a series of short stories include accounts of battles fought against extremely destructive Berserker machines. This and subsequent books set in the same fictional universe are the origin of the term "Berserker probe".
In the computer game Star Control II, the Slylandro Probe is an out-of-control self-replicating probe that attacks starships of other races. They were not originally intended to be a berserker probe. They sought out intelligent life for peaceful contact but due to a programming error would immediately switch to "resource extraction" mode and attempt to dismantle the target ship.
In Iain Banks's novel Excession, hegemonising swarms are described as a form of Outside Context Problem. An example of an "Aggressive Hegemonising Swarm Object" is given as an uncontrolled self-replicating probe with the goal of turning all matter into copies of itself. After causing great damage, they are somehow transformed using unspecified techniques by The Zetetic Elench and become "Evangelical Hegemonising Swarm Objects".
The Inhibitors from Alastair Reynolds' Revelation Space series are self-replicating machines whose purpose is to inhibit the development of intelligent star-faring cultures. They are dormant for extreme periods of time until they detect the presence of a space-faring culture and proceed to exterminate it even to the point of sterilizing entire planets. They are very difficult to destroy as they seem to have faced any type of weapon ever devised and only need a short time to 'remember' the necessary counter-measures.
Mantrid Drones from the science fiction television series Lexx were an extremely aggressive type of self-replicating Berserker machine, eventually converting the majority of the matter in the universe into copies of themselves in the course of their quest to thoroughly exterminate humanity.
Greg Bear's novel The Forge of God deals directly with the concept of "Berserker" von Neumann probes and their consequences. The idea is further explored in the novel's sequel, Anvil of Stars, which explores the reaction other civilizations have to the creation and release of Berserkers.
The 1995 Babylon 5 television series episode "A Day in the Strife" featured an encounter with a "berserker probe" that evaluated the scientific advancement of its target and detonated a powerful explosive if it was high enough to pose a threat. There was no evidence that it was capable of self-replication, however, and both its selectivity and limited method of attack meant it was not intended to wipe out all life as with a traditional Berserker.
On Stargate SG-1 the Replicators were a vicious race of insect-like robots that were originally created by an android named Reece to serve as toys. They grew beyond her control and began evolving, eventually spreading throughout at least two galaxies. In addition to ordinary autonomous evolution they were able to analyze and incorporate new technologies they encountered into themselves, ultimately making them one of the most advanced "races" known. In the Justice League Unlimited episode "Dark Heart", an alien weapon based on this same idea lands on Earth.
In the Homeworld: Cataclysm video game, a bio-mechanical virus called Beast has the ability to alter organic and mechanic material to suit its needs, and the ships infected become self-replicating hubs for the virus.
In the Sci-Fi MMO, EVE Online, drones used by the various civilizations in the star cluster where the game takes place are occasionally left adrift having been left behind by the ships that have launched them. In the story of the game, some continue to perform their tasks, and to create additional drones in order to perpetuate their task. These eventually form hives with battleship sized hive mothers, and are used extensively in missions as difficult opponents. Two Star Trek movies, Star Trek: The Movie and Star Trek IV: The Voyage Home deal with destructive "Berserker-like" probes. V'ger, actually a reconstructed N.A.S.A. Voyager, has been literally gathering planets to bring home to its maker. The fictional story line is that it fell in a worm hole and was found by a computer/cyborg race (Roddenbery did even allude that it was possibly the Borg) who re-equipped it to carry out its mission. The second probe, unnamed, appearently came to communicate with whales, all of which in the future are extinct, thus it attacks the Earth. The second probe's plot was quite weaker, and not much was given for its underlying purpose of contact with the whales. A TOS episode called "Doomsday Machine" also introduced a threat that was theorized to be a berserker probe of a vanished civilization. However, none of these three probes showed any evidence of being capable or interested in self-replication.
In the computer game Sword of the Stars, the player may randomly encounter "Von Neumann". A Von Neumann mothership appears along with smaller Von Neumann probes, which attack and consume the player's ships. The probes then return to the mothership, returning the consumed material. If probes are destroyed, the mothership will create new ones. If all the player's ships are destroyed, the Von Neumann probes will reduce the planets resource levels before leaving. The probes appear as blue octahedrons, with small spheres attatched to the apical points. The mothership is a larger version of the probes.
Replicating "seeder" ships
In David Brin's short story collection, The River of Time (1986), the short story "Lungfish" prominently features von Neumann probes. Not only does he explore the concept of the probes themselves, but indirectly explores the ideas of competition between different designs of probes, evolution of von Neumann probes in the face of such competition, and the development of a type of ecology between von Neumann probes. One of the vessels mentioned is clearly a Seeder type.
The trilogy of albums which conclude the comic book series Storm by Don Lawrence (starting with Chronicles of Pandarve 11: The Von Neumann machine) is based on self-replicating conscious machines containing the sum of all human knowledge employed to rebuild human society throughout the universe in case of disaster on Earth. The probe malfunctions and although new probes are built, they do not separate from the motherprobe which eventually results in a cluster of malfunctioning probes so big that it can absorb entire moons.
- "Extraterrestrial Beings Do Not Exist", Quarterly Journal of the Royal Astronomical Society, vol. 21, number 267 (1981)
- Sagan, Carl and Newman, William: "The Solipsist Approach to Extraterrestrial Intelligence", Quarterly Journal of the Royal Astronomical Society, vol. 24, number 113 (1983)
- Star Trek Encyclopedia article: Star Trek The Movie
- The complete text of the story has been placed on the author's website at http://www.davidbrin.com/lungfish1.html
- Boyce, Chris. Extraterrestrial Encounter: A Personal Perspective. London: David & Charles, Newton Abbot (1979).
- von Tiesenhausen, G., and Darbro, W. A. "Self-Replicating Systems," NASA Technical Memorandum 78304. Washington, D.C.: National Aeronautics and Space Administration (1980).
- Freitas Jr., Robert A. "A Self-Reproducing Interstellar Probe," Journal of the British Interplanetary Society, 33, 251-264 (1980).
- Valdes, F., and Freitas, R. A. "Comparison of Reproducing and Non-Reproducing Starprobe Strategies for Galactic Exploration," Journal of the British Interplanetary Society, 33, 402-408 (1980).