Six tricks that alien trackers could use

Gallery: Six tricks that alien trackers could use

So far, the search for extraterrestrial intelligence has focused on listening for radio signals deliberately sent our way. But even if alien civilisations are not trying to get our attention, their activities could produce detectable signs. Here are a few things we might detect, most of which are discussed in a recent paper by Richard Carrigan of the Fermi National Accelerator Laboratory in Batavia, Illinois.

Gallery: Six tricks that alien trackers could use