Infra digging

摘要

Those who look for intelligent life on other planets usually confine their efforts to listening for radio signals from stars that are, on the cosmic scale of things, fairly close by. The resounding silence so far does not deter enthusiasts, who point out that there are 100 billion stars in Earth's home galaxy, the Milky Way, almost all of which have not yet been examined. He who lives by the billion, however, may also die by it. For another number measured in billions is the Milky Way's age in years. Habitable planets have probably been present in it for at least 10 billion years, which is more than twice the span of Earth's existence. And a spacefaring civilisation, even one relying on craft travelling at far below the speed of light, would be able to colonise the entire galaxy in a few hundred million years. It therefore follows that if intelligent, technologically capable life forms had emerged elsewhere in the Milky Way, they would probably have done so long enough ago that they would, by now, be everywhere-which evidently they are not. This line of reasoning suggests humans really are the only intelligent life in this particular galaxy.