The search for extraterrestrial intelligence (SETI) is slowly evolving from a fringe endeavor to a more mainstream one thanks to improvements in the capability of astronomical surveys, detector sensitivity, and greater philanthropic financial support. Still, because of the vastness of the universe and the scarcity of resources, scientists must develop strategies around where, when, and how to discover alien civilizations.

Much of SETI involves trying to receive signals broadcast by other civilizations. However, it could be that every civilization in the universe has decided that transmitting messages for other civilizations to receive is unwise or dangerous, but that listening for messages sent by others is a safe and worthwhile pursuit.

This “SETI Paradox” would leave all SETI efforts doomed to failure because for any civilization’s SETI efforts to succeed, some other civilization must engage in messaging extraterrestrial intelligence (METI). One important question is how two civilizations should coordinate their efforts to discover each other, given that they are not certain about each other’s existence. Another is which civilization should send a message, and which should listen.

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