“Who are we?” That impossible question opened the 2015 public letter announcing a well-heeled SETI project called Breakthrough Listen. Dozens of people—scientists, astronauts, and also a producer, a chess champ, and a soprano—signed the note, which kicked off a $100 million effort by Russian billionaire Yuri Milner to catch signals from alien civilizations. That quest, Milner and the signatories hoped, would answer that existential query.