View source for Self-replicating machine

{{Short description|Device able to make copies of itself}}
{{Use dmy dates|date=August 2022|cs1-dates=y}}
[[File:Self-replicating machine.png|300px|thumb|A simple form of machine self-replication]]

A '''self-replicating machine''' is a type of [[autonomous robot]] that is capable of reproducing itself autonomously using raw materials found in the environment, thus exhibiting [[self-replication]] in a way analogous to that found in [[nature]]. The concept of self-replicating machines has been advanced and examined by [[Homer Jacobson]], [[Edward F. Moore]], [[Freeman Dyson]], [[John von Neumann]], [[Konrad Zuse]]<ref name="Böttiger_2011"/><ref name="Eibisch_2016"/> and in more recent times by [[K. Eric Drexler]] in his book on [[nanotechnology]], ''[[Engines of Creation]]'' (coining the term '''clanking replicator ''' for such machines) and by [[Robert Freitas]] and [[Ralph Merkle]] in their review ''Kinematic Self-Replicating Machines''<ref name="Freitas 2004">{{cite book| last=Freitas |first=Robert A.|author2=Ralph C. Merkle | title=Kinematic Self-Replicating Machines| date=2004| publisher=Landes Bioscience| location=Georgetown, Texas| isbn=978-1-57059-690-2| url=http://www.MolecularAssembler.com/KSRM.htm}}</ref> which provided the first comprehensive analysis of the entire replicator design space. The future development of such technology is an integral part of several plans involving the mining of [[Natural satellite|moon]]s and [[asteroid]] belts for ore and other materials, the creation of lunar factories, and even the construction of [[solar power satellite]]s in space. The [[von Neumann probe]]<ref>{{cite web|url=http://www.MolecularAssembler.com/KSRM/3.11.htm#p3 |title=3.11 Freitas Interstellar Probe Replicator (1979-1980) |publisher=Molecularassembler.com |date=2005-08-01 |access-date=2009-09-16}}</ref> is one theoretical example of such a machine. Von Neumann also worked on what he called the [[Von Neumann universal constructor|universal constructor]], a self-replicating machine that would be able to evolve and which he formalized in a [[cellular automaton|cellular automata]] environment. Notably, [[Von Neumann universal constructor|Von Neumann's Self-Reproducing Automata scheme]] posited that open-ended evolution requires inherited information to be copied and passed to offspring separately from the self-replicating machine, an insight that preceded the discovery of the structure of the DNA molecule by [[James Watson|Watson]] and [[Francis Crick|Crick]] and how it is separately translated and replicated in the cell.<ref name="Rocha1998">{{Citation| date=1998| first=Luis M.| last=Rocha| title=Evolutionary Systems| chapter=Selected Self-Organization and the Semiotics of Evolutionary Systems|pages=341–358|publisher=Springer, Dordrecht| doi=10.1007/978-94-017-1510-2_25| isbn=978-90-481-5103-5}}</ref><ref name="Brenner2012">{{Citation|journal=Nature| title=Life's code script| date=2012| first=Sydney| last=Brenner|volume=482| issue=7386|pages=461|url=https://www.nature.com/articles/482461a|doi=10.1038/482461a|pmid=22358811| s2cid=205070101}}</ref>

A self-replicating machine is an artificial [[self-replication|self-replicating]] system that relies on conventional large-scale technology and automation. The concept, first proposed by Von Neumann no later than the 1940s, has attracted a range of different approaches involving various types of technology. Certain idiosyncratic terms are occasionally found in the literature. For example, the term clanking replicator was once used by Drexler<ref name="EoC">{{cite web| url=http://www.e-drexler.com/d/06/00/EOC/EOC_Chapter_4.html#section01of03| title=Engines of Abundance (Chapter 4) Clanking Replicators| work=Engines of Creation| first=K. Eric| last=Drexler| date=1986}}</ref> to distinguish macroscale replicating systems from the microscopic [[nanorobot]]s or "[[assembler (nanotechnology)|assemblers]]" that [[nanotechnology]] may make possible, but the term is informal and is rarely used by others in popular or technical discussions. Replicators have also been called "von Neumann machines" after John von Neumann, who first rigorously studied the idea. However, the term "von Neumann machine" is less specific and also refers to a completely unrelated [[Von Neumann architecture|computer architecture]] that von Neumann proposed and so its use is discouraged where accuracy is important.<ref name="Freitas 2004"/> Von Neumann  used the term [[Von Neumann universal constructor|universal constructor]] to describe such self-replicating machines.

Historians of [[machine tool]]s, even before the [[numerical control]] era, sometimes figuratively said that machine tools were a unique class of machines because they have the ability to "reproduce themselves"<ref name="Colvin1947pp6-7">{{Harvnb|Colvin|1947|pp=6–7}}.</ref> by copying all of their parts. Implicit in these discussions is that a human would direct the cutting processes (later planning and programming the machines), and would then assemble the parts. The same is true for [[RepRap project|RepRaps]], which are another class of machines sometimes mentioned in reference to such non-autonomous "self-replication". In contrast, machines that are ''truly autonomously'' self-replicating (like [[Molecular machine#Biological|biological machines]]) are the main subject discussed here.

== History ==
The general concept of artificial machines capable of producing copies of themselves dates back at least several hundred years. An early reference is an anecdote regarding the philosopher [[René Descartes]], who suggested to Queen [[Christina of Sweden]] that the human body could be regarded as a machine; she responded by pointing to a clock and ordering "see to it that it reproduces offspring."<ref>{{cite journal|journal=Scientific American| title=Build Your Own Replicator |volume=285|date=August 2001| pages=38–39| first1=Moshe| last1= Sipper |first2=James A. |last2=Reggia | issue=2 |url=https://www.jstor.org/stable/26059294 |doi=10.1038/scientificamerican0801-34 | jstor=26059294 | pmid=11478000 |bibcode=2001SciAm.285b..34S}}<!-- total article: pg 34-43, original title: Go Forth and Replicate --> Several other variations on this anecdotal response also exist.</ref> Several other variations on this anecdotal response also exist. [[Samuel Butler (novelist)|Samuel Butler]] proposed in his 1872 novel ''[[Erewhon]]'' that machines were already capable of reproducing themselves but it was man who made them do so,<ref>{{cite book|url=http://www.MolecularAssembler.com/KSRM/1.htm#p5 |title=Kinematic Self-Replicating Machines |author=Robert A. Freitas Jr. |author2=Ralph C. Merkle|publisher=Landes Bioscience |page=5 |date=2004}}</ref>  and added that ''"machines which reproduce machinery do not reproduce machines after their own kind"''.<ref>{{cite web|author=Samuel Butler |url=https://nzetc.victoria.ac.nz/tm/scholarly/tei-ButErew.html |title=Erewhon, Chapter 24, The book Of the Machines |publisher=Nzetc.org |access-date=2009-09-16}}</ref> In [[George Eliot|George Eliot's]] 1879 book ''[[Impressions of Theophrastus Such]]'', a series of essays that she wrote in the character of a fictional scholar named Theophrastus, the essay "Shadows of the Coming Race" speculated about self-replicating machines, with Theophrastus asking "how do I know that they may not be ultimately made to carry, or may not in themselves evolve, conditions of self-supply, self-repair, and reproduction".<ref>{{cite web|author=George Eliot |url=http://www.online-literature.com/george_eliot/theophrastus-such/17/ |title=Impressions of Theophrastus Such, Chapter 17, Shadows of the Coming Race |publisher=online-literature.com |access-date=2017-08-25}}</ref>

In 1802 [[William Paley]] formulated the first known [[teleological argument]] depicting machines producing other machines,<ref>{{cite book|url=http://www.MolecularAssembler.com/KSRM/1.htm#p11 |title=Kinematic Self-Replicating Machines |author=Robert A. Freitas Jr. |author2=Ralph C. Merkle|publisher=Landes Bioscience |page=11 |date=2004}}</ref> suggesting that the [[watchmaker argument|question of who originally made a watch]] was rendered moot if it were demonstrated that the watch was able to manufacture a copy of itself.<ref>{{cite book| first=William| last= Paley| title= Natural Theology: or Evidences of the Existence and Attributes of the Deity, Collected from the Appearances of Nature| chapter=Chapter i, Section 1| publisher= E. Goodale| date= 1802| isbn=978-0-576-29166-8}}; [http://www.hti.umich.edu/cgi/p/pd-modeng/pd-modeng-idx?type=HTML&rgn=TEI.2&byte=53049319 (12th Edition, 1809)]{{dead link|date=December 2017 |bot=InternetArchiveBot |fix-attempted=yes }} See also: {{cite book| editor=Michael Ruse| title=Philosophy of Biology| url=https://archive.org/details/philosophybiolog00hull_136| url-access=limited| date=1998| pages=[https://archive.org/details/philosophybiolog00hull_136/page/n43 36]–40}}; {{cite journal| first=Richard| last= Lenski| title=Twice as Natural| journal=Nature| volume= 414|date=15 November 2001| page=255| doi=10.1038/35104715| pmid=11713507| issue=6861| bibcode= 2001Natur.414..255L| s2cid= 205023396| doi-access=free}}</ref> Scientific study of self-reproducing machines was anticipated by [[John Bernal]] as early as 1929<ref>{{cite web| url=http://www.cscs.umich.edu/~crshalizi/Bernal/| last=Bernal| first= John Desmond| title=The World, the Flesh and the Devil: An Enquiry into the Future of the Three Enemies of the Rational Soul| date=1929}}</ref> and by mathematicians such as [[Stephen Kleene]] who began developing [[recursion theory]] in the 1930s.<ref>{{cite book|url=http://www.MolecularAssembler.com/KSRM/1.htm#p14 |title=Kinematic Self-Replicating Machines |author=Robert A. Freitas Jr. |author2=Ralph C. Merkle|publisher=Landes Bioscience |page=14 |date=2004}}</ref> Much of this latter work was motivated by interest in information processing and algorithms rather than physical implementation of such a system, however. In the course of the 1950s, suggestions of several increasingly simple mechanical systems capable of self-reproduction were made — notably by [[Lionel Penrose]].<ref>Lionel Penrose: [http://www.cba.mit.edu/events/03.11.ASE/docs/Penrose.pdf Self-reproducing machines], Scientific American, vol 200, June 1959, pp 105-114</ref><ref>{{cite web|url=https://www.scientificamerican.com/article/go-forth-and-replicate-2008-02/|title=Go Forth and Replicate|publisher=Scientific American|date=February 1, 2008|access-date=March 13, 2021}}</ref>

=== Von Neumann's kinematic model ===
A detailed conceptual proposal for a [[Von Neumann universal constructor|self-replicating machine]] was first put forward by mathematician [[John von Neumann]] in lectures delivered in 1948 and 1949, when he proposed a [[kinematic]] model of [[Von Neumann universal constructor|self-reproducing automata]] as a [[thought experiment]].<ref name="TSRA">{{Citation| url=https://archive.org/details/theoryofselfrepr00vonn_0| title=''Theory of Self-Reproducing Automata.''| author1=von Neumann, John| author2=Burks, Arthur W.| date=1966| publisher=University of Illinois Press| format=Scanned book online| access-date=2017-02-28}}</ref><ref>{{cite web|url=http://www.MolecularAssembler.com/KSRM/2.1.htm |title=2.1 Von Neumann's Contributions |publisher=Molecularassembler.com |access-date=2009-09-16}}</ref> Von Neumann's concept of a physical self-replicating machine was dealt with only abstractly, with the hypothetical machine using a "sea" or stockroom of spare parts as its source of raw materials. The machine had a program stored on a memory tape that directed it to retrieve parts from this "sea" using a manipulator, assemble them into a duplicate of itself, and then copy the contents of its memory tape into the empty duplicate's. The machine was envisioned as consisting of as few as eight different types of components; four logic elements that send and receive stimuli and four mechanical elements used to provide a structural skeleton and mobility. While qualitatively sound, von Neumann was evidently dissatisfied with this model of a self-replicating machine due to the difficulty of analyzing it with mathematical rigor. He went on to instead develop an even more abstract model self-replicator based on [[cellular automata]].<ref>{{cite web|url=http://www.MolecularAssembler.com/KSRM/2.1.3.htm |title=2.1.3 The Cellular Automaton (CA) Model of Machine Replication |publisher=Molecularassembler.com |access-date=2009-09-16}}</ref> His original kinematic concept remained obscure until it was popularized in a 1955 issue of ''[[Scientific American]]''.<ref>{{cite journal| last=Kemeny| first=John G.| title=Man Viewed as a Machine| journal=Scientific American| volume=192|date=April 1955| issue=4| pages=58–67| doi=10.1038/scientificamerican0455-58| bibcode=1955SciAm.192d..58K}}</ref>

Von Neummann's goal for his [[Von Neumann universal constructor|self-reproducing automata theory]], as specified in his lectures at the University of Illinois in 1949,<ref name="TSRA"/> was to design a machine whose complexity could grow automatically akin to biological organisms under [[Natural Selection|natural selection]]. He asked what is the ''threshold of complexity'' that must be crossed for machines to be able to evolve.<ref name="Rocha1998"/> His answer was to design an abstract machine which, when run, would replicate itself. Notably, his design implies that open-ended evolution requires inherited information to be copied and passed to offspring separately from the self-replicating machine, an insight that preceded the discovery of the structure of the DNA molecule by [[James Watson|Watson]] and [[Francis Crick|Crick]] and how it is separately translated and replicated in the cell.<ref name="Rocha1998"/><ref name="Brenner2012"/>

=== Moore's artificial living plants ===
In 1956 mathematician [[Edward F. Moore]] proposed the first known suggestion for a practical real-world self-replicating machine, also published in ''Scientific American''.<ref>{{cite journal| journal=Scientific American| first=Edward F.| last=Moore|date=October 1956| volume=195| title=Artificial Living Plants| issue=4| pages=118–126| doi=10.1038/scientificamerican1056-118| bibcode=1956SciAm.195d.118M}}</ref><ref>{{cite web|url=http://www.MolecularAssembler.com/KSRM/3.1.htm |title=3.1 Moore Artificial Living Plants (1956) |publisher=Molecularassembler.com |access-date=2009-09-16}}</ref> Moore's "artificial living plants" were proposed as machines able to use air, water and soil as sources of raw materials and to draw its energy from sunlight via a [[solar cell|solar battery]] or a [[steam engine]]. He chose the seashore as an initial habitat for such machines, giving them easy access to the chemicals in seawater, and suggested that later generations of the machine could be designed to float freely on the ocean's surface as self-replicating factory barges or to be placed in barren desert terrain that was otherwise useless for industrial purposes. The self-replicators would be "harvested" for their component parts, to be used by humanity in other non-replicating machines.

=== Dyson's replicating systems ===
The next major development of the concept of self-replicating machines was a series of thought experiments proposed by physicist [[Freeman Dyson]] in his 1970 Vanuxem Lecture.<ref>{{cite speech |author=Freeman J. Dyson |title=The twenty-first century |event=Vanuxem Lecture |location=Princeton University |date=26 February 1970}}</ref><ref>{{cite web|url=http://www.MolecularAssembler.com/KSRM/3.6.htm |title=3.6 Dyson Terraforming Replicators (1970, 1979) |publisher=Molecularassembler.com |date=2005-08-01 |access-date=2009-09-16}}</ref> He proposed three large-scale applications of machine replicators. First was to send a self-replicating system to [[Saturn]]'s moon [[Enceladus (moon)|Enceladus]], which in addition to producing copies of itself would also be programmed to manufacture and launch [[solar sail]]-propelled cargo spacecraft. These spacecraft would carry blocks of Enceladean ice to [[Mars]], where they would be used to [[Terraforming of Mars|terraform the planet]]. His second proposal was a solar-powered factory system designed for a terrestrial desert environment, and his third was an "industrial development kit" based on this replicator that could be sold to developing countries to provide them with as much industrial capacity as desired. When Dyson revised and reprinted his lecture in 1979 he added proposals for a modified version of Moore's seagoing artificial living plants that was designed to distill and store fresh water for human use<ref>{{cite book| title=Chapter 18: Thought Experiments| work=Disturbing the Universe| first=Freeman J.| last=Dyson| date=1979| publisher=Harper and Row| location=New York| pages=194–204}}</ref> and the "[[Astrochicken]]."

=== ''Advanced Automation for Space Missions'' ===
[[File:Advanced Automation for Space Missions figure 5-19.jpg|thumb|right|An artist's conception of a "self-growing" robotic lunar factory]]

In 1980, inspired by a 1979 "New Directions Workshop" held at Wood's Hole, [[NASA]] conducted a joint summer study with [[ASEE]] entitled [[s:Advanced Automation for Space Missions|Advanced Automation for Space Missions]] to produce a detailed proposal for self-replicating factories to develop [[Moon|lunar]] resources without requiring additional launches or human workers on-site. The study was conducted at [[Santa Clara University]] and ran from June 23 to August 29, with the final report published in 1982.<ref>{{cite book| url=http://en.wikisource.org/wiki/Advanced_Automation_for_Space_Missions |editor=[[Robert Freitas]], William P. Gilbreath| title=Advanced Automation for Space Missions| date=1982| id=NASA Conference Publication CP-2255 (N83-15348)}}</ref> The proposed system would have been capable of [[exponential growth|exponentially increasing]] productive capacity and the design could be modified to build self-replicating probes to explore the galaxy.

The reference design included small computer-controlled electric carts running on rails inside the factory, mobile "paving machines" that used large parabolic mirrors to focus sunlight on lunar [[regolith]] to melt and sinter it into a hard surface suitable for building on, and robotic front-end loaders for [[strip mining]]. Raw lunar regolith would be refined by a variety of techniques, primarily [[hydrofluoric acid]] [[Leaching (metallurgy)|leaching]]. Large transports with a variety of manipulator arms and tools were proposed as the constructors that would put together new factories from parts and assemblies produced by its parent.

Power would be provided by a "canopy" of solar cells supported on pillars. The other machinery would be placed under the canopy.

A "[[casting]] [[robot]]" would use sculpting tools and templates to make [[plaster]] [[molding (process)|molds]]. Plaster was selected because the molds are easy to make, can make precise parts with good surface finishes, and the plaster can be easily recycled afterward using an oven to bake the water back out. The robot would then cast most of the parts either from nonconductive molten rock ([[basalt]]) or purified metals. A carbon dioxide [[laser cutting]] and welding system was also included.

A more speculative, more complex microchip fabricator was specified to produce the computer and electronic systems, but the designers also said that it might prove practical to ship the chips from Earth as if they were "vitamins."

A 2004 study supported by NASA's Institute for Advanced Concepts took this idea further.<ref>{{cite web|url=http://www.niac.usra.edu/studies/883Toth-Fejel.html|title=Modeling Kinematic Cellular Automata: An Approach to Self-Replication|last=Toth-Fejel|first=Tihamer|date=2004|publisher=NASA Institute for Advanced Concepts}}</ref> Some experts are beginning to consider self-replicating machines for [[asteroid mining]].

Much of the design study was concerned with a simple, flexible chemical system for processing the ores, and the differences between the ratio of elements needed by the replicator, and the ratios available in lunar [[regolith]]. The element that most limited the growth rate was [[chlorine]], needed to process regolith for [[aluminium]]. Chlorine is very rare in lunar regolith.

=== Lackner-Wendt Auxon replicators ===
In 1995, inspired by Dyson's 1970 suggestion of seeding uninhabited deserts on Earth with self-replicating machines for industrial development, [[Klaus Lackner]] and Christopher Wendt developed a more detailed outline for such a system.<ref>{{cite journal| last=Lackner| first=Klaus S.|author2=Christopher H. Wendt | title=Exponential growth of large self-replicating machine systems| journal=Mathl. Comput. Modelling| volume=21| date=1995| issue=10| pages=55–81| doi=10.1016/0895-7177(95)00071-9| doi-access=free}}</ref><ref>Lackner, Klaus S., and Wendt, Christopher H., "Self-reproducing machine systems for global scale projects," Document LA-UR-93-2886, 4th International Conference and Exposition on Engineering, Construction and Operations in Space/Conference and Exposition/Demonstrations on Robotic for Challenging Environments, Albuquerque, New Mexico, 26 February – 3 March 1994</ref><ref>{{cite web|url=http://www.MolecularAssembler.com/KSRM/3.15.htm |title=3.15 |publisher=Molecularassembler.com |date=2005-08-01 |access-date=2009-09-16}}</ref> They proposed a colony of cooperating mobile robots 10–30&nbsp;cm in size running on a grid of electrified ceramic tracks around stationary manufacturing equipment and fields of solar cells. Their proposal didn't include a complete analysis of the system's material requirements, but described a novel method for extracting the ten most common chemical elements found in raw desert topsoil (Na, Fe, Mg, Si, Ca, Ti, Al, C, O<sub>2</sub> and H<sub>2</sub>) using a high-temperature carbothermic process. This proposal was popularized in [[Discover (magazine)|''Discover'' magazine]], featuring solar-powered desalination equipment used to irrigate the desert in which the system was based.<ref>{{cite journal| journal=Discover|date=October 1995| first=Thomas| last=Bass| title=Robot, build thyself| pages=64–72 |url=https://www.discovermagazine.com/technology/robot-build-thyself}}</ref> They named their machines "Auxons", from the [[Greek language|Greek]] word ''auxein'' which means "to grow".

== Recent work ==
=== NIAC studies on self-replicating systems ===
In the spirit of the 1980 "Advanced Automation for Space Missions" study, the [[NASA Institute for Advanced Concepts]] began several studies of self-replicating system design in 2002 and 2003. Four phase I grants were awarded:

* [[Hod Lipson]] ([[Cornell University]]), "Autonomous Self-Extending Machines for Accelerating Space Exploration"<ref>{{cite web| url=http://www.niac.usra.edu/files/studies/final_report/737Lipson.pdf| title=Autonomous Self-Extending Machines for Accelerating Space Exploration| first=Hod| last= Lipson|author2=Evan Malone | access-date=2007-01-04}}</ref>
* [[Gregory Chirikjian]] ([[Johns Hopkins University]]), "Architecture for Unmanned Self-Replicating Lunar Factories"<ref>{{cite web| url=http://www.niac.usra.edu/files/studies/final_report/880Chirikjian.pdf| title=An Architecture for Self-Replicating Lunar Factories| first=Gregory S.| last= Chirikjian|date=April 26, 2004| access-date=2007-01-04}}</ref>
* [[Paul Todd (biophysicist)|Paul Todd]] (Space Hardware Optimization Technology Inc.), "Robotic Lunar Ecopoiesis"<ref>{{cite web| last= Todd| first= Paul| title=Final Progress Report on Robotic Lunar Ecopoiesis Test Bed
| date=30 April 2004| access-date=2007-01-04| url=http://www.niac.usra.edu/files/studies/final_report/884Todd.pdf}} (phase I report)</ref><ref>{{cite web| url=http://www.niac.usra.edu/files/studies/final_report/918Todd.pdf| title=Robotic Lunar Ecopoiesis Test Bed| date=July 6, 2006| access-date=2007-01-04| first=Paul| last=Todd}} (phase II report)</ref>
* [[Tihamer Toth-Fejel]] ([[General Dynamics]]), "Modeling Kinematic Cellular Automata: An Approach to Self-Replication"<ref>{{cite web| url=http://www.niac.usra.edu/files/studies/final_report/883Toth-Fejel.pdf | title=Modeling Kinematic Cellular Automata| first= Tihamer| last= Toth-Fejel |author2=Robert Freitas |author3=Matt Moses|date=April 30, 2004| access-date=2007-01-04}}</ref><ref>{{cite web|url=http://www.MolecularAssembler.com/KSRM/3.25.4.htm |title=3.25.4 Toth-Fejel Kinematic Cellular Automata (2003-2004) |publisher=Molecularassembler.com |access-date=2009-09-16}}</ref> The study concluded that complexity of the development was equal to that of a Pentium 4, and promoted a design based on cellular automata.

=== Bootstrapping self-replicating factories in space ===
In 2012, NASA researchers Metzger, Muscatello, Mueller, and Mantovani argued for a so-called "bootstrapping approach" to start self-replicating factories in space.<ref>{{cite journal | last1=Metzger | first1=Philip | last2=Muscatello | first2=Anthony | last3=Mueller | first3=Robert | last4=Mantovani | first4=James | title=Affordable, Rapid Bootstrapping of the Space Industry and Solar System Civilization | journal=Journal of Aerospace Engineering | volume=26 | issue=1 | pages=18–29 | date= January 2013 | language=en | doi=10.1061/(ASCE)AS.1943-5525.0000236 | arxiv=1612.03238 | s2cid=53336745 }}</ref> They developed this concept on the basis of [[In situ resource utilization|In Situ Resource Utilization (ISRU)]] technologies that NASA has been developing to "live off the land" on the Moon or Mars. Their modeling showed that in just 20 to 40 years this industry could become self-sufficient then grow to large size, enabling greater exploration in space as well as providing benefits back to Earth. In 2014, [[Thomas Kalil]] of the White House [[Office of Science and Technology Policy]] published on the White House blog an interview with Metzger on bootstrapping solar system civilization through self-replicating space industry.<ref>{{cite web|url=https://obamawhitehouse.archives.gov/blog/2014/10/14/bootstrapping-solar-system-civilization |title=Bootstrapping a Solar System Civilization |date=2014-10-14 |via=[[NARA|National Archives]] |work=[[whitehouse.gov]] |access-date=2016-12-09}}</ref> Kalil requested the public submit ideas for how "the Administration, the private sector, philanthropists, the research community, and storytellers can further these goals." Kalil connected this concept to what former NASA Chief technologist [[Mason Peck]] has dubbed "Massless Exploration", the ability to make everything in space so that you do not need to launch it from Earth. Peck has said, "...all the mass we need to explore the solar system is already in space. It's just in the wrong shape."<ref>{{cite web|url=http://www.pri.org/stories/2015-01-05/exciting-new-ideas-space-technology-are-getting-short-changed-congress |title=Exciting new ideas in space technology are getting short-changed by Congress |first=Adam |last=Wernick |publisher=PRI.org |date=2015-01-15 |access-date=2016-12-09}}</ref> In 2016, Metzger argued that fully self-replicating industry can be started over several decades by astronauts at a lunar outpost for a total cost (outpost plus starting the industry) of about a third of the space budgets of the [[International Space Station]] partner nations, and that this industry would solve Earth's energy and environmental problems in addition to providing massless exploration.<ref>{{cite journal | last=Metzger | first=Philip | title=Space Development and Space Science Together, an Historic Opportunity | journal=Space Policy | volume=37 | issue=2 | pages=77–91 | date= August 2016 | language=en | doi=10.1016/j.spacepol.2016.08.004 | arxiv=1609.00737| bibcode=2016SpPol..37...77M | s2cid=118612272 }}</ref>

=== New York University artificial DNA tile motifs ===
In 2011, a team of scientists at [[New York University]] created a structure called 'BTX' (bent triple helix) based around three double helix molecules, each made from a short strand of DNA.  Treating each group of three double-helices as a code letter, they can (in principle) build up self-replicating structures that encode large quantities of information.<ref>{{cite web |url=https://www.sciencedaily.com/releases/2011/10/111012132651.htm |title=Self-Replication Process Holds Promise for Production of New Materials |date=17 October 2011 |publisher=Science Daily |access-date=2011-10-14}}</ref><ref>{{cite journal|title=Self-replication of information-bearing nanoscale patterns|journal=Nature|volume=478|issue=7368|doi=10.1038/nature10500|pmid = 21993758|pmc=3192504|pages=225–228|bibcode=2011Natur.478..225W|date=2011|last1=Wang|first1=Tong|last2=Sha|first2=Ruojie|last3=Dreyfus|first3=Rémi|last4=Leunissen|first4=Mirjam E.|last5=Maass|first5=Corinna|last6=Pine|first6=David J.|last7=Chaikin|first7=Paul M.|last8=Seeman|first8=Nadrian C.}}</ref>

=== Self-replication of magnetic polymers ===
In 2001, Jarle Breivik at [[University of Oslo]] created a system of magnetic building blocks, which in response to temperature fluctuations, spontaneously form self-replicating polymers.<ref>{{cite journal|title=Self-Organization of Template-Replicating Polymers and the Spontaneous Rise of Genetic Information|journal=Entropy|volume=3|issue=4|pages=273–279| publisher=Entroy |doi=10.3390/e3040273|date=2001|last1=Breivik|first1=Jarle|bibcode=2001Entrp...3..273B|doi-access=free}}</ref>

=== Self-replication of neural circuits ===
In 1968, [[Zellig Harris]] wrote that "the metalanguage is in the language,"<ref>{{cite book
|title=Mathematical Structures of Language|page=17|last=Harris|first=Zellig|date=1968|publisher=John Wiley and Son|location= New York, NY}}</ref> suggesting that self-replication is part of language. In 1977 [[Niklaus Wirth]] formalized this proposition by publishing a self-replicating [[deterministic context-free grammar]].<ref>{{cite journal|last1=Wirth|first1=Niklaus|title= What can we do about the unnecessary diversity of notation for syntactic definitions?|journal=Commun. ACM|volume=20|issue=11|pages=822–823|doi=10.1145/359863.359883|date=1977|s2cid=35182224|doi-access=free}}</ref> Adding to it probabilities, [[Bertrand du Castel]] published in 2015 a self-replicating [[stochastic grammar]] and presented a mapping of that grammar to [[neural networks]], thereby presenting a [[Conceptual model|model]] for a self-replicating neural circuit.<ref>{{cite journal | last=du Castel | first=Bertrand | title=Pattern activation/recognition theory of mind | journal=Frontiers in Computational Neuroscience | volume=9 | pages=90 | date=2015-07-15 | issn=1662-5188 | pmid=26236228 | pmc=4502584 | doi=10.3389/fncom.2015.00090 |ref=neuroscience| doi-access=free }}</ref>

=== Harvard Wyss Institute ===
November 29, 2021 a team at Harvard Wyss Institute built the first living robots that can reproduce.<ref>{{cite web |url=https://wyss.harvard.edu/news/team-builds-first-living-robots-that-can-reproduce/ |title = Team builds first living robots—that can reproduce| date=29 November 2021 }}</ref>

== Self-replicating spacecraft ==
{{Main article|Self-replicating spacecraft}}

The idea of an automated spacecraft capable of constructing copies of itself was first proposed in scientific literature in 1974 by [[Michael A. Arbib]],<ref>{{cite web|url=http://www.MolecularAssembler.com/KSRM/3.11.htm#p1 |title=3.11 |publisher=Molecularassembler.com |date=2005-08-01 |access-date=2009-09-16}}</ref><ref>{{cite book| first=Michael A.| last=Arbib| title=The Likelihood of the Evolution of Communicating Intelligences on Other Planets| work= Interstellar Communication: Scientific Perspectives| editor=Cyril Ponnamperuma, A. G. W. Cameron| location=Boston| publisher=Houghton Mifflin Company| date=1974| pages=59–78}}</ref> but the concept had appeared earlier in [[science fiction]] such as the 1967 novel ''[[Berserker (Saberhagen)|Berserker]]'' by [[Fred Saberhagen]] or the 1950 novellette trilogy ''[[The Voyage of the Space Beagle]]'' by [[A. E. van Vogt]]. The first quantitative engineering analysis of a self-replicating spacecraft was published in 1980 by [[Robert Freitas]],<ref>{{cite journal| first=Robert A. Jr. | last=Freitas| title=A Self-Reproducing Interstellar Probe| journal=[[Journal of the British Interplanetary Society]]| volume=33|date=July 1980| pages=251–264| url=http://www.rfreitas.com/Astro/ReproJBISJuly1980.htm | access-date = 2008-10-01|bibcode = 1980JBIS...33..251F }}</ref> 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, and then use the resulting automated industrial complex to construct more probes with a single seed factory on board each.

== Prospects for implementation ==
As the use of industrial automation has expanded over time, some factories have begun to approach a semblance of self-sufficiency that is suggestive of self-replicating machines.<ref>{{cite web|url=http://www.MolecularAssembler.com/KSRM/3.7.htm |title=3.7 Self-Replicating Automated Industrial Factory (1973-present) |publisher=Molecularassembler.com |date=2005-08-01 |access-date=2009-09-16}}</ref> However, such factories are unlikely to achieve "full closure"<ref>{{cite web|url=http://www.MolecularAssembler.com/KSRM/5.6.htm |title=5.6 Closure Theory and Closure Engineering |publisher=Molecularassembler.com |date=2005-08-01 |access-date=2009-09-16}}</ref> until the cost and flexibility of automated machinery comes close to that of human labour and the manufacture of spare parts and other components locally becomes more economical than transporting them from elsewhere. As [[Samuel Butler (novelist)|Samuel Butler]] has pointed out in ''[[Erewhon]]'', replication of partially closed universal machine tool factories is already possible. Since safety is a primary goal of all legislative consideration of regulation of such development, future development efforts may be limited to systems which lack either control, matter, or energy closure. Fully capable machine replicators are most useful for developing resources in dangerous environments which are not easily reached by existing transportation systems (such as [[outer space]]).

An artificial replicator can be considered to be a form of [[artificial life]]. Depending on its design, it might be subject to [[evolution]] over an extended period of time.<ref>{{cite web|url=http://www.MolecularAssembler.com/KSRM/5.1.9.L.htm |title=5.1.9.L Evolvability |publisher=Molecularassembler.com |date=2005-08-01 |access-date=2009-09-16}}</ref> However, with robust [[error correction]], and the possibility of external intervention, the common [[science fiction]] scenario of robotic life run amok will remain extremely unlikely for the foreseeable future.<ref>{{cite web|url=http://www.MolecularAssembler.com/KSRM/5.11.htm |title=5.11 Replicators and Public Safety |publisher=Molecularassembler.com |access-date=2009-09-16}}</ref>

== Other sources ==
{{See also|Self-replicating machines in fiction}}
* A number of patents have been granted for self-replicating machine concepts.<ref>{{cite web|url=http://www.MolecularAssembler.com/KSRM/3.16.htm |title=3.16 The Collins Patents on Reproductive Mechanics (1997-1998) |publisher=Molecularassembler.com |date=2005-08-01 |access-date=2009-09-16}}</ref> {{US patent|5659477}} "Self reproducing fundamental fabricating machines (F-Units)" Inventor: Collins; Charles M. (Burke, Va.) (August 1997), {{US patent|5764518}} " Self reproducing fundamental fabricating machine system" Inventor: Collins; Charles M. (Burke, Va.)(June 1998); and Collins' PCT patent WO 96/20453:<ref>{{cite web |author=WIPO |url=https://patents.google.com/patent/WO1996020453A1/en?q=f-units&assignee=collins+charles+m&oq=f-units+collins+charles+m |title=(WO/1996/020453) SELF REPRODUCING FUNDAMENTAL FABRICATING MACHINES (F-UNITS) |publisher=Wipo.int |access-date=2009-09-16 |archive-date=2019-11-01 |archive-url=https://web.archive.org/web/20191101185001/https://patents.google.com/patent/WO1996020453A1/en?q=f-units&assignee=collins+charles+m&oq=f-units+collins+charles+m |url-status=dead }}</ref> "Method and system for self-replicating manufacturing stations" Inventors: Merkle; Ralph C. (Sunnyvale, Calif.), Parker; Eric G. (Wylie, Tex.), Skidmore; George D. (Plano, Tex.) (January 2003).
*Macroscopic replicators are mentioned briefly in the fourth chapter of [[K. Eric Drexler|K. Eric Drexler's]] 1986 book ''[[Engines of Creation]]''.<ref name="EoC"/>
* In 1995, [[Nick Szabo]] proposed a challenge to build a macroscale replicator from [[Lego]] robot kits and similar basic parts.<ref>{{cite web| url=http://www.lucifer.com/~sean/N-FX/macro.html| archive-url=https://web.archive.org/web/20060307220916/http://www.lucifer.com/~sean/N-FX/macro.html| archive-date=2006-03-07| title=Macroscale Replicator| first=Nick| last=Szabo| access-date=2007-03-07}}</ref> Szabo wrote that this approach was easier than previous proposals for macroscale replicators, but successfully predicted that even this method would not lead to a macroscale replicator within ten years.
* In 2004, [[Robert Freitas]] and [[Ralph Merkle]] published the first comprehensive review of the field of self-replication (from which much of the material in this article is derived, with permission of the authors), in their book ''[[Kinematic Self-Replicating Machines (book)|Kinematic Self-Replicating Machines]]'', which includes 3000+ literature references.<ref name="Freitas 2004"/> This book included a new molecular assembler design,<ref>{{cite web|url=http://www.MolecularAssembler.com/KSRM/4.11.3.htm |title=4.11.3 Merkle-Freitas Hydrocarbon Molecular Assembler (2000-2003) |publisher=Molecularassembler.com |date=2005-08-01 |access-date=2009-09-16}}</ref> a primer on the mathematics of replication,<ref>{{cite web|url=http://www.MolecularAssembler.com/KSRM/5.9.htm |title=5.9 Brief Mathematical Primer on Self-Replicating Systems |publisher=Molecularassembler.com |date=2005-08-01 |access-date=2009-09-16}}</ref> and the first comprehensive analysis of the entire replicator design space.<ref>{{cite web|url=http://www.MolecularAssembler.com/KSRM/5.1.9.htm |title=5.1.9 Freitas-Merkle Map of the Kinematic Replicator Design Space (2003-2004) |publisher=Molecularassembler.com |date=2005-08-01 |access-date=2009-09-16}}</ref>

== See also ==
{{colbegin}}
* [[Autopoiesis]]
* [[Grey goo|Grey goo scenario]]
* [[Self-reconfiguring modular robot]]
* [[AI takeover]]
* [[3D printing]]
* [[Computer virus]]
* [[Computer worm]]
* [[Ecophagy]]
* [[Existential risk from advanced artificial intelligence]]
* [[Astrochicken]]
* [[Lights out (manufacturing)]]
* [[Nanorobotics]]
* [[Spiegelman's Monster]]
* [[Self-replicating machines in fiction]]
* [[Self-replicating spacecraft]]
* [[RepRap project]]
* [[Molecubes|Self-reconfiguring and self-reproducing molecube robots]]
* [[Quine (computing)|Quine]]
{{colend}}

== References ==
{{reflist|refs=
<ref name="Eibisch_2016">{{cite book |title=Selbstreproduzierende Maschinen: Konrad Zuses Montagestraße SRS 72 und ihr Kontext |language=de |location=Mountain View, California, USA |publication-place=Wiesbaden, Germany |author-first=Nora |author-last=Eibisch |publisher=[[Springer Vieweg]] / [[Springer Fachmedien Wiesbaden GmbH]] |series=Research |date=2016 |type=Thesis |isbn=978-3-658-12941-5 |doi=10.1007/978-3-658-12942-2 |url=https://books.google.com/books?id=nluhCwAAQBAJ}} (272+4 pages)</ref>
<ref name="Böttiger_2011">{{cite book |title=Konrad Zuse: Erfinder, Unternehmer, Philosoph und Künstler |language=de |chapter=Der Philosoph - Die technische Keimzelle |author-first=Helmut |author-last=Böttiger |author-link=:de:Helmut Böttiger (Autor, 1940) |editor-first=Adolf |editor-last=Rabenseifner |date=2011-10-26 |edition=1 |isbn=978-3-86568-743-2 |publisher=[[Michael Imhof Verlag]] |location=Petersberg, Germany |pages=69–75 [70–74]}} (128 pages)</ref>
}}

== Further reading ==
* {{Colvin1947}}
* M. Sipper, [https://drive.google.com/file/d/0B6G3tbmMcpR4MkpqY3EyN3k0djQ/view?usp=sharing Fifty years of research on self-replication: An overview], Artificial Life, vol. 4, no. 3, pp.&nbsp;237–257, Summer 1998.
* [[Freeman Dyson]] expanded upon Neumann's automata theories, and advanced a biotechnology-inspired theory. See [[Astrochicken]].
* The first technical design study of a self-replicating interstellar probe was published in a [http://www.rfreitas.com/Astro/ReproJBISJuly1980.htm 1980 paper] by [[Robert Freitas]].
* Clanking replicators are also mentioned briefly in the [http://www.foresight.org/EOC/EOC_Chapter_4.html#section01of03 fourth chapter] of [[K. Eric Drexler]]'s 1986 book ''[[Engines of Creation]]''.
* Article about a proposed clanking replicator system to be used for developing Earthly deserts in the October 1995 [[Discover Magazine]], featuring forests of solar panels that powered desalination equipment to irrigate the land.
* In 1995, [[Nick Szabo]] [https://web.archive.org/web/20060307220916/http://www.lucifer.com/~sean/N-FX/macro.html proposed] a challenge to build a macroscale replicator from Lego(tm) robot kits and similar basic parts.  Szabo wrote that this approach was easier than previous proposals for macroscale replicators, but successfully predicted that even this method would not lead to a macroscale replicator within ten years.
* In 1998, [[Chris Phoenix (nanotechnologist)|Chris Phoenix]] [https://groups.google.com/groups?hl=en&selm=6f0nui%248ih%241%40news.nanospace.com suggested] a general idea for a macroscale replicator on the sci.nanotech [[usenet|newsgroup]], operating in a pool of [[ultraviolet]]-cured liquid [[plastic]], selectively solidifying the plastic to form solid parts. Computation could be done by [[fluidic logic]]. Power for the process could be supplied by a pressurized source of the liquid.
* In 2001, [[Peter Ward (paleontologist)|Peter Ward]] mentioned an escaped clanking replicator destroying the human race in his book ''[[Future Evolution]]''.
* In 2004, General Dynamics completed a [http://www.niac.usra.edu/files/studies/final_report/883Toth-Fejel.pdf study] for NASA's Institute for Advanced Concepts.  It concluded that complexity of the development was equal to that of a Pentium 4, and promoted a design based on cellular automata.
* In 2004, Robert Freitas and [[Ralph Merkle]] published the first comprehensive review of the field of self-replication, in their book [http://www.MolecularAssembler.com/KSRM.htm Kinematic Self-Replicating Machines], which includes 3000+ literature references.
* In 2005, [[Adrian Bowyer]] of the [[University of Bath]] started the [[RepRap]] project to develop a [[rapid prototyping]] machine that would be able to replicate itself, making such machines cheap enough for people to buy and use in their homes.  The project is releasing material under the [[GNU GPL]]. [https://web.archive.org/web/20070112200308/http://staff.bath.ac.uk/ensab/replicator/]
* In 2015, advances in [[graphene]] and [[silicene]] suggested that it could form the basis for a neural network with densities comparable to the human brain if integrated with [[silicon carbide]] based nanoscale CPUs containing [[memristors]].
The power source might be [[sun|solar]] or possibly [[radioisotope]] based given that new liquid based compounds can generate substantial power from radioactive decay.

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