Sending messages into deep space could be the best way for Earthlings to find extraterrestrial (地球以外的) intelligence, but it carries a risk: alerting unfriendly aliens to our presence. Game theory may provide a way to deal with this situation.

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 starts. 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 he betrays (出卖) a partner who remains silent, with the silent partner getting a maximum sentence. If they both betray each other, each gets a medium sentence. By contrast, if both stay silent, both get minimum sentences---the best overall result.

De Vladar reasoned that the SETI dilemma is essentially the same, but reversed. Mutual betrayal for prisoners is equivalent to mutual silence for aliens. And while a selfish prisoner stops keeping silent, a selfish civilization 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. In the classic version of the prisoner’s dilemma, each selfishly betrays 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 civilization, de Vladar varied the reward of finding aliens and the cost of unfriendly aliens finding us. The result was a range of best broadcasting strategies. “It’s not about whether to do it or not, but how often,” says Vladar.

One interesting idea was that as you increase the rewards placed on finding aliens, you can decrease 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--- equipping our planet with transmitters won’t come cheap---and risk terrible disasters, such as interstellar (星际的) wars.

【小题1】According to the passage, which of the following can be classified as “passive” SETI?

A．Concealing our presence from aliens.	B．Listening for signs of technology elsewhere.
C．Broadcasting to distant stars.	D．Detecting the existence of aliens.

【小题2】According to de Vladar, ________ in the SETI is equivalent to mutual silence for prisoners.

A．both civilizations sending messages	B．people on the earth sending messages
C．both civilizations keeping silent	D．people on the earth keeping silent

【小题3】According to the passage, which of the following might benefit Earthlings best in the SETI?

A．Equipping the earth with the most advanced transmitters.

B．Preparing for the disasters such as interstellar wars.

C．Sending to space as few messages as possible.

D．Decreasing the rewards for finding aliens.

【小题4】Which of the following best sums up the passage?

A．Earthlings are likely to be attacked by extraterrestrial intelligence.

B．Scientists are looking for better approaches to finding aliens.

C．Earthlings should be more active in broadcasting to aliens.

D．Decision to contact aliens is a game-theory dilemma.