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."

Jones also noted that radio and laser signals would be easier and cheaper means of transmitting messages across interstellar space.

The challenges of building a self-replicating "von Neumann Machine" are enormous. Such a machine would need to mine materials from the environment, process these materials into usable substances, manufacture complex parts and then assemble those parts into another machine. But there are other challenges for starships based on this technology, according to Jones.

"These tasks are common to all types of self-replicating machines. But a von Neumann machine designed to fly through deep space needs a propulsion system, a good power source and a communications system. It also has to survive in the extreme environment of space and possibly function for centuries or even longer." Such engineering challenges could prove too difficult to overcome.

Jones also noted that malicious or badly designed self-replicating machines could run amok, destroying everything around them. Scientists working in the field of nanotechnology have already considered this prospect. In one doomsday situation, self-reproducing robots built at the nanoscale eat up the entire Earth, turning it into a lifeless ball of "gray goo" consisting entirely of nanomachines.

"Extraterrestrials could develop the same fears of self-replicating machines being dangerous, and avoid deploying them", he notes.

Jones thus suggests that the infamous Sagan-Tipler debate is probably irrelevant, as von Neumann machines are unlikely to be deployed by an extraterrestrial civilization.

"Scientists working on the Search for Extraterrestrial Intelligence (SETI) shouldn't place much faith in the idea of self-replicating starships", he advises. "The absence of evidence for these devices is not evidence of the absence of extraterrestrial intelligence."

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