The late cosmologist Stephen Hawking said that if humankind receives a signal from extraterrestrials, we should stay quiet. "Meeting an advanced civilization could be like Native Americans encountering Columbus. That didn't turn out so well," he said in his 2016 documentary Stephen Hawking's Favorite Places. If we let the aliens know we're here, he argued, we might be inviting an interstellar armada that could wipe out life on Earth.
Hawking overlooked an essential point: it's too late to hide. For two billion years Earth's microbial life has been making itself known to the universe through changes to our planet's atmosphere. In the next twenty years, humankind will have space-based observatories capable of detecting extraterrestrial life by studying the chemical composition of atmospheres covering exoplanets. Advanced extraterrestrials will have even greater capacities.
Similarly, for the past century Earth has been leaking accidental radio and television signals into space, letting eavesdropping aliens know that we have rudimentary technology. Broadcasts of I Love Lucy have been our interstellar emissaries for decades.
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