Signs of life

摘要

Half a century ago a radio astronomer called Frank Drake thought of a way to calculate the likelihood of establishing contact with aliens. He suggested the following figures should be multiplied: how many stars are formed in the galaxy in a year; what fraction of these have planets and thus form solar systems; the average number of planets per solar system that have the potential to support life; on what percentage of those where it is possible do such biospheres actually form; what percentage of such biospheres give rise to intelligent species; what percentage of intelligent life is able to transmit signals into space; and for how long could such intelligence keeps sending signals.