Sending messages into deep space could be the best way for Earthlings to find extraterrestrial intelligence, but it carries a grave risk: alerting hostile aliens to our presence. Game theory may provide a way to navigate this dilemma.
So far the search for extraterrestrial intelligence (SETI) has mostly been restricted to listening for signs of technology elsewhere. Only a few attempts have been made to broadcast messages towards distant stars. Many scientists are against such "active" SETI for fear of revealing our presence. If all aliens feel the same way then no one will be broadcasting, and the chance of detecting each other is limited.
To weigh up the potential losses and gains, Harold de Vladar of the Institute of Science and Technology Austria in Klosterneuburg turned to the prisoner's dilemma, a game-theory problem in which two prisoners choose between admitting their shared crime or keeping quiet, with different sentences depending on what they say. An individual prisoner gets off scot free if they rat on a partner who remains silent, with the silent partner getting a maximum sentence. If they both rat on each other, each gets a medium sentence. By contrast, if both stay silent, both get token sentences - the best overall result.
De Vladar reasoned that the SETI dilemma is essentially the same, but reversed. Mutual silence for prisoners is equivalent to mutual broadcasting for aliens, giving the best results for both civilisations. And while a selfish prisoner rats, a selfish civilisation is silent, waiting for someone else to take the risk of waving "Over here!" at the rest of the universe.
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