On Earth, civilizations have limited lifetimes.

Roman civilization, for instance, lasted less than a thousand years from the founding of its republic to the fall of its empire (after a long decline). In the New World, Maya civilization spanned roughly two millennia (maybe a little longer depending on when you date its beginning). In the late Bronze Age, the Greek Mycenaean civilization lasted a mere five centuries or so. As for American civilization (as in the United States of), at the rate things are going it won’t last even that long.

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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