One of the greatest challenges that civilizations face is communicating across vast distances. Over the millennia humans have gotten much better (and faster) at it: signal fires, resonant drums, and fleet-footed messengers gave way to telegraphs, then radios and telephones, and now satellite phones and the internet. Those systems all work well, to some extent, here on Earth. But when we look outward into the universe and imagine a conversation with some other form of intelligent life, we’re faced with radically larger distances, which take mind-bending amounts of time to cross.

How would a system of communication with extraterrestrial intelligence work? In a new paper, published in the International Journal of Astrobiology, Duncan Forgan, an astrophysicist at the University of St. Andrews, proposes an interstellar communication network that would work like a planetary equivalent of smoke signals. It starts with planets passing in front of the stars they orbit.

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