Silence is imaginary, because the world never stops making noise. A sound is a disruption of the air, and it doesn’t so much die as recede until it subsides beneath the level of the world’s random noise and can no longer be recovered, like a face that is lost in a crowd. In past times, people sometimes thought that all sounds that ever existed were still present, hovering like ghosts. Guglielmo Marconi, who sent the first radio message, in 1902, believed that with a microphone that was sufficiently sensitive he could hear Jesus delivering the Sermon on the Mount, and in 1925 a writer for the Washington Post speculated that a radio was capable of broadcasting the voices of the dead.