Science fiction has frequently considered self-replicating robots that travel across space, making copies of themselves as they spread throughout the galaxy. Such devices are formally known as "von Neumann machines" after the Hungarian-born mathematician John von Neumann, who performed theoretical research into their operations. In theory, such devices could gradually explore the entire galaxy over a period of several hundreds of thousands of years, and we could expect one of them to be no more than a few tens of light years away from any star system.
During the 1980s, the American physicist Frank Tipler claimed that extraterrestrials would build these self-replicating starships, and we should have spotted one of them in nearby space. The fact that none have ever been found suggested that extraterrestrials did not exist, at least in Tipler's view.
The American astronomer Carl Sagan answered Tipler's call for a debate on the subject, suggesting that self-replicating robots would not be as common as Tipler had suggested. But neither Tipler nor Sagan addressed a significant question: Is it really practical to build these devices?
An Australian space analyst suggests that self-replicating starships are probably impossible to build or impractical. Speaking at the 2017 International Astronautical Congress in Adelaide, Dr Morris Jones considered the challenges faced in designing such a machine. He concluded that even if an advanced extraterrestrial civilization could build such a device, they could have reasons not to do so.
"There are easier ways of exploring the galaxy", observed Jones. "Robot interstellar probes can be built with less technology."
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