In 1974 we sent the Arecibo radio message towards Messier 13, a globular cluster near the edge of the Milky Way, made up of a few hundred thousand stars. The message was mostly symbolic; we weren’t really expecting a reply. Yet surely other civilisations out there are doing the same thing. So, why haven’t we heard anything? What if the silence from the stars is a hint that we shouldn’t be so outgoing? What if aliens are deliberately keeping quiet for fear that they might be destroyed?
