This is a chapter from Alex Ross's groundbreaking history of twentieth-century classical music, 'The Rest is Noise'. Further extracts are available as digital shorts, accompanying the London Southbank festival programme. After Paul McCartney listened to the electronic layering and looping of Stockhausen, the Beatles used the same effects on Revolver's 'Tomorrow Never Knows' and put an image of the composer on Sgt. Pepper's Lonely Hearts Club Band. For a split second during 'Revolution 9' the final chords of Sibelius's Seventh Symphony can be heard. Even the most jaded veteran of twentieth century music must have been startled by the influence of the post-war avant-garde on the psychedelic generation. Now a major festival running throughout 2013 at London's Southbank, The Rest is Noise is an intricate commentary not just on the sounds that defined the century, but on art's troublesome dance with politics, social and cultural change. Alex Ross is the New Yorker's music critic, and the winner of the Guardian First Book Award and the National Book Critics Circle Award for The Rest is Noise, which was also shortlisted for the Samuel Johnson and Pulitzer prizes for non-fiction.