Has ET evolved to be discreet? An evolutionary tendency for inconspicuous aliens would solve a nagging paradox – and also suggest that we Earthlings should think twice before advertising our own existence.
As physicist Enrico Fermi argued in 1950, unless the evolution of life is unique to Earth, there must be many intelligent species out there. So why have they neither phoned home nor been detected by us?
"It's a real paradox," says Adrian Kent of the Perimeter Institute in Waterloo, Ontario, Canada.
In order to explain the Fermi paradox, Kent turns to natural selection – and suggests that it may favour quiet aliens.
He argues that it's plausible that there is a competition for resources on a cosmic scale, driving an evolutionary process between alien species on different planets. Advanced species, for example, might want to exploit other planets for their own purposes.
If so, the universe would be a violent place, and evolutionary selection may favour the inconspicuous – those who lay low on purpose, or who simply lack the skill or ambition to venture forth or advertise their existence.
"This is an interesting idea," says alien hunter Seth Shostak of the SETI institute in Mountain View, California. "If I let the cosmos know I exist, then I might be subject to extermination."
However, he is wary of assuming a "straitjacket" on the activities of intelligent species, who might not be able to resist the intellectual pull to develop advanced technologies detectable by others.
"If interstellar violence is possible, the bad news is that all societies are required to constrain their endeavours to activities that could never be detected at a distance," says Shostak.
The theory joins a long list of attempts to explain the Fermi paradox, from the suggestion that aliens' communications are indistinguishable from background noise, to calculations concluding that ET just hasn't had enough time to find us.
Kent acknowledges that his hypothesis is speculative. But he also warns that it could have real consequences for the near future: vehicles such as NASA's two Voyager probes, which are hurtling away from the solar system, may alert imperialist aliens to our existence and require retrieval, he says.
He adds that it may not take much for a truly advanced civilisation to wipe us out pretty quickly. "The hyper-advanced aliens might not have to send their interstellar battle fleet to conquer Earth," he notes. "It might only take three bored undergraduate aliens with borrowed lab equipment."
This post originally appeared on New Scientist.