Why haven’t we been contacted by alien civilizations? Maybe because most have extinguished themselves, as we likely will

space
Credit: Geoff Marcy

For some reason, civilization is not a self-perpetuating state of affairs on this planet. And perhaps not on other planets, either. In fact, limits to civilization lifetimes may explain why extraterrestrial aliens have not yet communicated with Earthlings. A new analysis suggests that the entire Milky Way galaxy currently houses only a few dozen worlds equipped with sufficiently sophisticated technology to send us a message. They are probably scattered at such great distances that any signals sent our way haven’t had time to get here. And by the time a signal arrives, there may be nobody here around to hear it.

“We may imagine a galaxy in which intelligent life is widespread, but communication unlikely,” write Tom Westby and Christopher Conselice in the June 10 Astrophysical Journal.

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With the strictest set of assumptions, assuming 100 years as the average CETI life span computes to only 36 communicating civilizations in the galaxy today. If so, far more movies have been made on Earth about alien civilizations than there actually are alien civilizations.

“The lifetime of civilizations in our galaxy is a big unknown … and is by far the most important factor in the CETI equation,” Westby and Conselice note. “It is clear that … very long lifetimes are needed for … the galaxy to contain even a few possible active contemporary civilizations.”

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