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.

This led de Vladar to apply the mathematics of the prisoner's dilemma to SETI (International Journal of Astrobiology (IJA), doi.org/jx7). In the classic version of the prisoner's dilemma, each selfishly rats on the other. But as we do not know the character of any aliens out there, and as it is difficult to put a value on the benefits to science, culture and technology of finding an advanced civilisation, de Vladar varied the reward of finding aliens and the cost of hostile aliens finding us. The result was a range of optimal broadcasting strategies. "It's not about whether to do it or not, but how often," says de Vladar.

One intriguing insight was that as you scale up the rewards placed on finding aliens, you can scale down the frequency of broadcasts, while keeping the expected benefit to Earthlings the same. Being able to keep broadcasts to a minimum is good news, because they come with costs - rigging our planet with transmitters won't come cheap - and risk catastrophic penalties, such as interstellar war.

Seth Shostak of the SETI Institute in Mountain View, California, says that game theory is a good approach but that there are too many unknowns. Perhaps aliens are not actively broadcasting because they don't need to. Shostak has recently shown that a civilisation even slightly more advanced than ours could use its sun as a "gravitational lens". Such a lens could detect the lights of New York City from up to 500 light years away, once the light has had time to travel that far (IJA, doi.org/jx8). And there are certainly alien star systems that are closer to us than that.

Earth has also been accidently leaking radio and TV signals for the past century, which may have already been picked up. "Any society at least a few centuries beyond the invention of radio will recognise that deliberate transmissions are not the way they will be found," says Shostak. Quick, turn off those lights!

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Quelle:NewScientist