The primary difficulty of interstellar communication is finding common ground between ourselves and other intelligent entities about which we can know nothing with absolute certainty. This common ground would be the basis for a universal language that could be understood by any intelligence, whether in the Milky Way, Andromeda, or beyond the cosmic horizon. To the best of our knowledge, the laws of physics are the same throughout the universe, which suggests that the facts of science may serve as a basis for mutual understanding between humans and an extraterrestrial intelligence. One key set of scientific facts presents an intriguing question. If aliens were to visit Earth and learn about its inhabitants, would they be surprised that such a wide variety of species all share a common genetic code?

When the Arecibo message was sent into space in 1974 – blasting the most powerful signal ever broadcast deep into the universe – it was a pioneering attempt to reach out to aliens, wherever they might be. When a response came back in 2001, it was a hoax that showed just how much some people hope we can actually communicate with extraterrestrials. But that fake reply might actually be the message we ever get back from aliens. That's because some of the greatest minds on Earth fear rather than hope for contact with extraterrestrial life. Because the real reply to the Arecibo message might actually be something far more terrifying.