Science Fair Project Encyclopedia
Von Neumann probe
A von Neumann probe is a fictional or hypothetical self-replicating machine, specifically designed for space exploration or colonisation. It is named after John von Neumann, a Hungarian-born American mathematician and physicist who was the first scientist to rigorously study the concept of self-replicating machines, although von Neumann himself never actually proposed or discussed the use of replicating machines for this purpose during his lifetime.
The probe would be launched to a neighbouring star-system. Upon its arrival it would immediately seek out raw materials (asteroids, moons, gas giants, etc.) to create replicas of itself. Once it had created a sufficient number of replica probes, the original probe to enter the system would go about exploring the star-system. 
If a self-replicating probe only finds evidence of primitive life (such as an unstable, savage civilization) they might simply lie dormant on the moon, silently waiting for it to evolve into a stable higher civilization. After waiting quietly for several millennia, they may be activated when the emerging civilization is advanced enough to set up a lunar colony. Physicist Paul Davies of the University of Adelaide has even raised the possibility of a self-replicating probe resting on our own Moon, left over from a previous visit in our system aeons ago.
Meanwhile the duplicate probes would make their way to nearby star-systems and repeat the process, creating an exponentially increasing fleet of probes. It has been theorized that using self-replicating probes, a galaxy could be explored in as little as half a million years, although it might be destroyed in the process. 
A variant of the Von Neumann probe is the berserker probe, a self-replicating machine that has been deliberately (or inadvertantly) designed to sterilize the entire galaxy so that life cannot emerge, and/or destroy all existing life in its path as the probes spread out at an exponentially increasing rate across the galaxy. Another subtle variant exists in the form of the astrochicken, which are space-faring and self-replicating automata.
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. Originally, the film began 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 Iain Banks' novel Excession, Hegemonising swarms are described as a form of Outside Context Problem. An example of a 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 reprogrammed by The Culture and become Evangelical Hegemonising Swarm Objects.
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