For more than 50 years, we’ve been eavesdropping on the cosmos, searching for transmissions that would reveal the existence of intelligent, extraterrestrial life.

To date, nobody’s bothered to call.

Is it something we said?

One possibility is that we’re alone, or at the very least, the only intelligent civilization with a Zip code in the Milky Way galaxy. (Read "The Hunt for Life Beyond Earth.")

And then, there’s the morbid alternative: Intelligent life periodically emerges on other worlds, but has an unfortunate tendency to self-destruct.

Sadly, it’s not implausible, given the devastation we’ve wrought during our relatively brief span as the dominant species on this planet.

That’s why a trio of scientists recently published a guide to help astronomers detect alien apocalypses—whether