People would likely fall into two camps. Catastrophists, as one of the camps is called, might well predict the end of humanity as we know it, or at least the end of our current culture. In 2010 Stephen Hawking said that making contact with aliens would be "a little too risky" and compared the event to Columbus arriving in the New World, "which didn't turn out very well for the Native Americans." But millenarian enthusiasts anticipate revelations of rapture: how to cure cancer, solve the energy crisis or win world peace. And if aliens did manage to come to Earth, says Tarter, an admitted enthusiast, "they would likely have outgrown the aggressiveness that has served us so well."
As for a supersecretive government agency or powerful corporation keeping a discovery of alien life a secret, Davies thinks that's unlikely. "I think there's a big misconception in the public that somehow this is all a cloak-and-dagger operation, and it's not," he says. "People are quite open about what they are doing."
Sarah Zielinski is an assistant editor at Smithsonian. She writes the "Surprising Science" blog on Smithsonian.com.