von Neumann probe
A von Neumann machine able to move over interstellar or interplanetary distances and to utilize local materials to build new copies of itself. Von Neumann probes are named after the Hungarian-born American mathematician John von Neumann who, among many other achievements, was the first to develop a mathematical theory of machines that can make exact copies of themselves.
Origin of the idea
The potential advantages of using self-replicating robot spacecraft for galactic exploration was discussed by Chris Boyce in his book Extraterrestrial Encounter: A Personal Perspective (Chartwell Books, New York, pp. 113-124, 1979).1 Boyce, in turn, has said he got the idea from a chapter entitled "The Likelihood of the Evolution of Communicating Intelligences on Other Planets" (ch. 4) written by Michael A. Arbib in the 1974 Ponnamperuma-Cameron book Interstellar Communication: Scientific Perspectives (pp. 63-66). Arbib wrote an even earlier paper, in 1969, in which he discusses self-replicating automata (SRA) based on von Neumann and Burk's seminal paper published in 1966.
How a von Neumann probe might work
Boyce, in his book, envisaged the following scenario. Initially, a von Neumann probe, consisting of an interstellar propulsion system and a universal von Neumann replicator with human-level intelligence, would be launched from the home star toward a neighboring stellar system. Upon arrival it would seek out raw materials, from local sources such as asteroids, and use these to make several copies of itself (including its rocket engines). The copies would then be launched at the next set of neighboring stars. This process would be repeated, over and over again, so that increasing numbers of identical probes would be involved in penetrating ever more remote regions of the Galaxy.
Having dispatched copies of itself, a probe would begin to explore the star system in which it found itself. It would conduct scientific research and transmit the results back to the point of origin. It could also be used as a means of interstellar colonization by constructing an artificial life-sustaining environment and then implanting this with synthesized fertile egg-cells bearing genomes transcribed from the probe's computer memory. Eiseley has suggested that the embryonic individuals of such a colony could be tended by robots, also built by the probe, until they were old enough to function independently. They would then be free to develop their own civilization around the host star.
The great advantage of a von Neumann probe is that, being a universal machine, it can be used for any purpose at its target system depending on the instructions sent out to it from its ultimate creators. Consequently, as the creators made technological advances at home they could reprogram a remote von Neumann probe, for example, to build faster rocket engines for the next generation of probes or more sensitive sensing equipment with which to study its host stellar system. How rapidly the Galaxy could be completely explored and colonized in this way depends on several factors, including the interstellar transit times of the probes (see interstellar travel), the speed at which they reproduce and carry out other tasks within the host systems, and the specific strategy used for interstellar colonization.
Von Neumann probes and extraterrestrial intelligence
Frank Tipler, writing after Boyce et al, and borrowing the idea of von Neumann probes from them, has used the concept as an argument against the existence of intelligence elsewhere in the Galaxy. He proposed a conservative value of 300 million years, or less than 5% of the present age of the Galaxy, for complete galactic colonization. He assumed the von Neumann probe approach to be so logical and economical that it would be commonly adopted by advanced civilizations. According to this view, there should be a significant and obvious presence of such devices within the solar system. Yet, no such presence has been detected. Tipler therefore draws the conclusion that we are the only intelligent race among the Galaxy's several hundred billion stars (Tipler's Argument). The validity of this conclusion has been questioned by, among others, Sagan and Newman (see Sagan's Response).
The idea of using automatically exponentiating systems has been investigated for extraterrestrial mining by von Tiesenhausen and Darbro at NASA's Marshall Spaceflight Center.2
See also Bracewell probes, extraterrestrial probes, and SETA.3, 4
[Thanks to Scot Stride (NASA/JPL) for details of Boyce's and Arbib's writings on this subject, and for pointing out that they have priority over Frank Tipler on the issue of von Neumann probes.]
1. Boyce, Chris. Extraterrestrial Encounter: A Personal Perspective.
London: David & Charles, Newton Abbot (1979).
2. von Tiesenhausen, G., and Darbro, W. A. "Self-Replicating Systems," NASA Technical Memorandum 78304. Washington, D.C.: National Aeronautics and Space Administration (1980).
3. Freitas Jr., Robert A. "A Self-Reproducing Interstellar Probe," Journal of the British Interplanetary Society, 33, 251-264 (1980).
Abstract: Bracewell and Frietas have discussed the possible superiority of interstellar probes in missions of galactic exploration and recently Calder and Boyce have raised the issue of self-organizing machines in related contexts. In this paper a preliminary sketch of a self-reproducing starprobe is presented, with generation time ~103 years given a ~10-fold improvement in current human space/manufacturing technology.
4. 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).
Abstract: Bracewell has suggested the use if nonreproducing messenger probes for interstellar exploration, and Freitas has examined the feasibility of self-reproducing automated devices in the same context. The present paper compares reproducing and nonreproducing strategies for missions of interstellar and galactic discovery. Self-reproducing probes are found to be the method of choice for active exploration programs lasting >106 years, involving searches of >106 target stars to distances > 1000 light-years in the Galactic Disk, and are superior to one-shot "Bracewell probes" for searches of >103 stars to distances > 100 light-years in the Galactic Disk. Any nonreproducing alien probes discovered in the Solar System during the normal course of future SETI research would most likely have been sent by extraterrestrial civilizations located within a approx. 1,000 light-year radius of the Sun, whereas any self-reproducing devices similarly detected probably originated far outside the exploration sphere.