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The idea of self-replicating spacecraft has been applied—in theory—to several distinct "tasks". 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.

Contents 1 Theory 2 Implications for Fermi's paradox 3 Applications for self-replicating spacecraft 3.1 Von Neumann probes 3.2 Autonomous synthetic life 3.3 Berserkers 3.4 Replicating "seeder" ships 4 Examples in fiction 4.1 Von Neumann probes 4.2 Berserkers 4.3 Replicating "seeder" ships 5 References 6 See also

[edit] Theory

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 patterns 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".

The first quantitative engineering analysis of such a spacecraft was published in 1980 by Robert Freitas,^[1] in which the non-replicating Project Daedalus design was modified to include all subsystems necessary for self-replication. The design's strategy was to use the probe to deliver a "seed" factory with a mass of about 443 tons to a distant site, have the seed factory replicate many copies of itself there to increase its total manufacturing capacity, over a 500 year period, and then use the resulting automated industrial complex to construct more probes with a single seed factory on board each.

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.^[2]

[edit] Implications for Fermi's paradox

In 1981, Frank Tipler^[3] 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^[4] 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 destroy any von Neumann probes found as soon as they were detected. As Robert Freitas^[5] has pointed out the assumed rapacity of von Neumann probes described by both sides of the debate are unlikely in reality, and more modestly reproducing systems are unlikely to be observable in their effects on our Solar System and the rest of the Galaxy.

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, etc.

A simple workaround exists to avoid the over-replication scenario. Radio transmitters, or other means of wireless communication, could be used by probes programmed not to replicate beyond a certain density (such as five probes per cubic parsec) or arbitrary limit (such as ten million within one century). One problem with this defence against uncontrolled replication is that it would only require a single probe to malfunction and begin unrestricted reproduction for the entire approach to fail - essentially a technological cancer.

[edit] 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.

[edit] Von Neumann probes

A von Neumann probe is a self-replicating spacecraft designed to investigate its target system and transmit information about it back to its system of origin.^[6] The concept is named after Hungarian American mathematician and physicist John von Neumann, who rigorously studied the concept of self-replicating machines that he called "Universal Assemblers" and which are 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.

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).

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", Dyson conceived the Astrochicken to explore and operate within our own planetary system, and not explore interstellar space.

Oxford based philosopher Nick Bostrom discusses the idea that future powerful superintelligences, will create efficient cost effective space travel and interstellar Von Neumann probes (self-reproducing) ^[7].

[edit] Autonomous synthetic life

Similar to a von Neumann probe, the only intelligent beings capable of traveling the huge distances of space are synthetic and self replicating life forms. These life forms are not single probes, but groups of life forms who are teaming together to achieve their goals in space.

[edit] Berserkers

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. For instance, a von Neumann ship designed for terraforming processes - mining a planet's surface and adjusting its atmosphere to more human-friendly conditions - might malfunction and attack inhabited planets, killing their inhabitants in the process of changing the planetary environment, and then self-replicating and dispatching more ships to attack other planets.

[edit] 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 species 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.

[edit] Examples in fiction

[edit] 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.

• In The Third Millennium: A History of the World AD 2000-3000 by Brian Stableford and David Langford (published by Alfred A. Knopf, Inc. 1985) humanity sends cycle-limited Von Neumann probes out to the nearest stars to do open-ended exploration and to announce humanity's existence to whomever might encounter them.

• In Von Neumann's War by John Ringo and Travis S. Taylor (published by Baen Books 2007) Von Neumann probes arrive in the solar system, moving in from the outer planets, converting all metals into gigantic structures. Eventually, they arrive on Earth, wiping out much of the population before they are fought to a standstill. Interestingly, the Von Neumann probes in this book are not inherently malevolent, they are utterly indifferent to human efforts to resist them, concentrating solely on the metal of our civilization.

• In Spin by Robert Charles Wilson, earth is veiled by a temporal field. Humanity tries to understand and escape this field by using Von Neumann probes. It is later revealed that the field itself was generated by Von Neumann probes from another civilization, and that a competition for resources had taken place between earth's and aliens probes.

• Larry Niven frequently refers to Von Neumann probes in many of his works. In his 1997 book Destiny's Road, Von Neumann machines are scattered throughout the human colony world Destiny and its moon Quicksilver in order to build and maintain technology and to make up for the lack of the resident humans' technical knowledge; the Von Neumann machines primarily construct a stretchable fabric cloth capable of acting as a solar collector which serves as the humans' primary energy source. The Von Neumann machines also engage in ecological maintenance and other exploratory work.

• See also Alexander Kluge, The Devil's Blind Spot (New Directions; 2004.)

[edit] Berserkers

• 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".
  • Science Fiction author Larry Niven later borrowed this notion in his short story A Teardrop Falls
• 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, they would immediately switch to "resource extraction" mode and attempt to dismantle the target ship for raw materials. While dealing with the Slylandro Probe menace is a side-quest, it is virtually impossible to win without getting rid of the probes, as their numbers will increase exponentially until the player is constantly menaced by them.
• 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.
• Also from Alastair Reynolds's books, the "Greenfly" terraforming machines are another form of berserker machines. For unknown reasons, but probably an error in their programming, they destroy planets and turn them into trillions of domes filled with vegetation - after all, their purpose is to produce a habitable environment for humans, however in doing so they inadvertently decimate the human race. By AD 10 000, they have wiped out most of the Galaxy.
• 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 were 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.
• In Gregory Benford's Galactic Center Saga series, an antagonist berserker machine race is encountered by Earth, first as a probe in In the Ocean of Night, and then in an attack in Across the Sea of Suns. The berserker machines do not seek to completely eradicate a race if merely throwing it into a primitive low technological state will do as they did to the EMs encountered in Across the Sea of Suns. The alien machine Watchers would not be considered von Neumann machines themselves, but the collective machine race could.
• 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. (there is now evidence in a more recent episode of Star Gate Atlantis that the Ancients may be the original creators of the Replicators and Reece may have just been able to activate and control some)
• 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, experiments to create more autonomous drones than the ones used by player's ships accidentally created 'rogue drones' which form hives in certain parts of space and are used extensively in missions as difficult opponents.
• 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 attached to the apical points. The mothership is a larger version of the probes. In the 2008 expansion A Murder of Crows, Kerberos Productions also introduces the VN Berserker, which is either an upgrade of or a construction by the previously-mentioned von Neumann motherships/probes. The VN Berserker will, after wearing down any planetary defenses, destroy all colonists on any planet it attacks.
• In the X Computer Game Series, the Xenon are a malevolent race of artificially intelligent machines descended from terraforming ships sent out by humans to prepare worlds for eventual colonization. They are continual antagonists in the X-Universe.
• In the comic Transmetropolitan a character mentions "Von Neumann rectal infestations" which are apparently caused by "Shit-ticks that build more shit-ticks that build more shit-ticks".
• In the computer game FreeSpace 2 there is a race called the Shivans which seem to have behaviour befitting a race of Berserker probes. Whether they actually are berserkers or not is unknown as their internal motives are never explained , only extrapolated from their behaviour as seen from their opponents point of view.

[edit] Replicating "seeder" ships

• In David Brin's short story collection, The River of Time (1986), the short story "Lungfish" prominently features von Neumann probes.^[8] 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.
• In The Songs of Distant Earth by Arthur C. Clarke, humanity on a future Earth facing imminent destruction creates automated seedships that act as fire and forget lifeboats aimed at distant, habitable worlds. Upon landing, the ship begins to create new humans from stored genetic information, and an onboard computer system raises and trains the first few generations of new inhabitants. The massive ships are then broken down and used as building materials by their "children".
• Stephen Baxter's novel Manifold: Space starts with the discovery of alien self-replicating machines active within the Solar system.
• Code of the Lifemaker by James P. Hogan describes the evolution of a society of humanoid-like robots who inhabit Saturn's moon Titan. The sentient machines are descended from an unmanned factory ship that was to be self replicating, but suffered radiation damage and went off course, eventually landing on Titan around 1,000,000 B.C..
• On the Stargate Atlantis episode "Remnants", the Atlantis team finds an ancient probe that they later learn was launched by a now-extinct, technologically advanced race in order to seed new worlds and re-propagate their species.

[edit] References

[edit] See also

• Astrochicken
• Bracewell probe
• Embryo space colonization
• Interstellar travel
• Space colonization
• Self-replicating machine

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